NES 738 Metals and Corrosion Guide Category 3

Ministry of Defence Defence Standard 02-738 (NES 738)

Issue 1 Publication Date 01 April 2000

Incorporating NES 738 Category 3

Issue 2 Publication Date August 1992

Metals And Corrosion Guide

DStanDStan is now the publishing authority for all Maritime Standards (formerly NESs). Any reference to any other publishing authority throughout this standard should be ignored.Any queries regarding this or any other Defence Standard should be referred to the DStan Helpdesk as detailed at the back of this document.

AMENDMENT RECORD

Amd No Date Text Affected Signature and Date

REVISION NOTE

This standard is raised to Issue 1 to update its content.

HISTORICAL RECORD

This standard supersedes the following:

Naval Engineering Standard (NES) 738 Issue 2 dated August 1992.

Ministry of Defence

Naval Engineering Standard

NES 738 Issue 2 (Reformatted) August 1992

METALS AND CORROSION GUIDE

This NES Supersedes

NES 738 ISSUE 1

Record of Amendments

AMDT INSERTED BY DATE

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2

3

4

5

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7

8

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NAVAL ENGINEERING STANDARD 738

ISSUE 2 (REFORMATTED)

METALS AND CORROSION GUIDE

The issue and use of this Standard

is authorized for use in MOD contracts

by MOD(PE) Sea Systems and

the Naval Support Command

ECROWN COPYRIGHT

Published by:

Director of Naval ArchitectureProcurement Executive, Ministry of DefenceSea Systems, Foxhill, Bath BA1 5AB

NES 738Issue 2 (Reformatted)

iii

SCOPE

1. This NES is a guide to the use of preferred metallic materials for use in selected applicationsin Surface Ships and Submarines. General information is given on the properties and selectionof metals, metal processing and finishing, testing, failure modes, and corrosion.

2. It does not give detailed properties of any metal or alloy for which reference must be made tothe relevant material specification or data sheet.

3. It is not an authority for departure from the material specified on drawings, etc, for whichestablished concession procedures are to be followed.


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v

FOREWORD

Sponsorship

1. ThisNaval Engineering Standard (NES) is sponsored by theProcurement Executive, Ministryof Defence, Director Naval Architecture (Submarines) (DNA(SM)), Section NA 115.

2. It is to be applied as required by any Ministry of Defence contract to provide generalinformation on the use of preferred metallic materials for use in selected applications inSurface Ships and Submarines and on the corrosion and marine fouling to which they aresubjected. It is applicable to Ships Systems and Equipment and Weapon Systems andEquipment.

3. If it is found to be technically unsuitable for any particular requirement the Sponsor is to beinformed in writing of the circumstances with a copy to Director Naval Architecture (SurfaceShips) (DNA(SS)), Section NA 145 for Ship Systems and Equipment.

4. Any user of this NES either within MOD or in industry may propose an amendment to it.Proposals for amendments which are:

a. not directly applicable to a particular contract are to be made to the Sponsor of theNES;

b. directly applicable to a particular contract are to be dealt with using existing proceduresor as specified in the contract.

5. No alteration may be made to this NES except by the issue of an authorized amendment.

6. Unless otherwise stated, reference in this NES to approval, approved, authorized or similarterms means by the Ministry of Defence in writing.

7. Any significant amendments that may be made to this NES at a later date will be indicatedby a vertical sideline. Deletions will be indicated by 000 appearing at the end of the lineintervals.

8. This NES has been reissued to reflect changes in nomenclature, departmental reorganizationand technical changes.

Conditions of Release

General

9. This Naval Engineering Standard (NES) has been prepared for the use of the Crown and ofits contractors in the execution of contracts for the Crown. The Crown hereby excludes allliability (other than liability for death or personal injury) whatsoever and howsoever arising(including but without limitation, negligence on the part of the Crown, its servants or agents)for any loss or damage however caused where the NES is used for any other purpose.

10. This document is Crown Copyright and the information herein may be subject to Crown orthird party rights. It is not to be released, reproduced or published without written permissionof the MOD.

11. The Crown reserves the right to amend or modify the contents of this NES without consultingor informing any holder.


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MOD Tender or Contract Process

12. ThisNES is the property of the Crown and unless otherwise authorized in writing by theMODmust be returned on completion of the contract, or submission of the tender, in connectionwith which it is issued.

13. When this NES is used in connection with aMOD tender or contract, the user is to ensure thathe is in possession of the appropriate version of each document, including related documents,relevant to each particular tender or contract. Enquiries in this connection may be made ofthe local MOD(PE) Quality Assurance Representative or the Authority named in the tenderor contract.

14. When NES are incorporated into MOD contracts, users are responsible for their correctapplication and for complying with contracts and any other statutory requirements.Compliance with an NES does not of itself confer immunity from legal obligations.

Related Documents

15. In the tender and procurement processes the related documents listed in each section andAnnex A can be obtained as follows:

a. British Standards British Standards Institution,389 Chiswick High Road,London W4 4AL

b. Defence Standards Directorate of Standardization and Safety Policy,Stan 1, Kentigern House, 65 Brown Street,Glasgow G2 8EX

c. Naval Engineering Standards CSE3a, CSE Llangennech, Llanelli,Dyfed SA14 8YP

d. Other documents Tender or Contract Sponsor to advise.

Note: Tender or Contract Sponsor can advise in cases of difficulty.

16. All applications to Ministry Establishments for related documents are to quote the relevantMOD Invitation to Tender or Contract Number and date, together with the sponsoringDirectorate and the Tender or Contract Sponsor.

17. Prime Contractors are responsible for supplying their subcontractors with relevantdocumentation, including specifications, standards and drawings.

Health and Safety

Warning

18. ThisNESmay call for the use of processes, substances and/or procedures thatmay be injuriousto health if adequate precautions are not taken. It refers only to technical suitability and inno way absolves either the supplier or the user from statutory obligations relating to healthand safety at any stage of manufacture or use. Where attention is drawn to hazards, thosequoted may not necessarily be exhaustive.


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CONTENTSPage No

TITLE PAGE i. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

SCOPE iii. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

FOREWORD v. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Sponsorship v. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Conditions of Release v. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

General v. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .MOD Tender or Contract Process vi. . . . . . . . . . . . . . . . . . . . . . . . . .

Related Documents vi. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Health and Safety vi. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Warning vi. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

CONTENTS vii. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

SECTION 1. METALLIC MATERIALS 1.1. . . . . . . . . . . . . . . . . . . .

SECTION 2. ENVIRONMENTAL CONDITIONS 2.1. . . . . . . . . . . .

SECTION 3. PROPERTIES AND SELECTION OF METALLICMATERIALS 3.1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3.1 Physical Properties 3.1. . . . . . . . . . . . . . . . . . . . . . . . . . .3.2 Mechanical Properties 3.1. . . . . . . . . . . . . . . . . . . . . . . .3.3 Tensile and Shear Properties 3.1. . . . . . . . . . . . . . . . . . .

FIGURE 3.1 TYPICAL STRESS-STRAINCURVES 3.2. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3.4 Hardness and Abrasion 3.3. . . . . . . . . . . . . . . . . . . . . . .3.5 Notch Toughness 3.3. . . . . . . . . . . . . . . . . . . . . . . . . . . . .3.6 Creep 3.4. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3.7 Stress Relaxation 3.4. . . . . . . . . . . . . . . . . . . . . . . . . . . . .

FIGURE 3.2 STRESS RELAXATION BEHAVIOURFOR TWO BOLTING MATERIALS 3.5. . . . . . . . . . . .

3.8 Fatigue 3.5. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3.9 Corrosion Fatigue 3.5. . . . . . . . . . . . . . . . . . . . . . . . . . . .3.10 Chemical Properties 3.6. . . . . . . . . . . . . . . . . . . . . . . . . .3.11 Stress and Corrosion 3.6. . . . . . . . . . . . . . . . . . . . . . . . .3.12 Selection of Metals 3.6. . . . . . . . . . . . . . . . . . . . . . . . . . .

FIGURE 3.3 SOME FACTORS IN THESELECTION OF A METAL 3.7. . . . . . . . . . . . . . . . . . .

3.13 Toxicity of Metals 3.8. . . . . . . . . . . . . . . . . . . . . . . . . . . .

SECTION 4. MODES OF FAILURE 4.1. . . . . . . . . . . . . . . . . . . . . . .4.1 Plastic Collapse 4.1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4.2 Brittle Fracture 4.1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

FIGURE 4.1 FRACTURE APPEARANCE IN THECHARPY TEST 4.2. . . . . . . . . . . . . . . . . . . . . . . . . . . . .

4.3 Fatigue and Corrosion Fatigue 4.3. . . . . . . . . . . . . . . . .FIGURE 4.2 TYPICAL SN CURVE FORUNWELDED STEEL 4.4. . . . . . . . . . . . . . . . . . . . . . . . .


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FIGURE 4.3 TYPICAL SN CURVE FOR MANYNON-FERROUS ALLOYS 4.4. . . . . . . . . . . . . . . . . . . .

4.4 Corrosion 4.5. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4.5 Stress Corrosion and Stress Corrosion Cracking 4.5. .4.6 Low Energy Ductile Tearing 4.5. . . . . . . . . . . . . . . . . . .

SECTION 5. METAL FORMS AND PROCESSES 5.1. . . . . . . . . . .5.1 Cast Metals 5.1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5.2 Advantages and Disadvantages of Castings 5.2. . . . . . .5.3 Wrought Products 5.2. . . . . . . . . . . . . . . . . . . . . . . . . . .5.4 Working Processes 5.2. . . . . . . . . . . . . . . . . . . . . . . . . . .5.5 Extrusion 5.3. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5.6 Powder Metallurgy 5.3. . . . . . . . . . . . . . . . . . . . . . . . . . .5.7 Clad Metals 5.4. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

SECTION 6. SHAPING OF METALS 6.1. . . . . . . . . . . . . . . . . . . . . .6.1 Cold Forming 6.1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6.2 Hot Forming 6.1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6.3 Shaping Weldments 6.1. . . . . . . . . . . . . . . . . . . . . . . . . .6.4 Spinning 6.1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6.5 Machining 6.2. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

SECTION 7. HEAT TREATMENT 7.1. . . . . . . . . . . . . . . . . . . . . . . . .7.1 Steel 7.1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7.2 Annealing 7.1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7.3 Normalizing 7.1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7.4 Quenching 7.1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7.5 Interrupted Quenching 7.1. . . . . . . . . . . . . . . . . . . . . . .7.6 Tempering 7.2. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7.7 Flame Hardening 7.2. . . . . . . . . . . . . . . . . . . . . . . . . . . .7.8 Induction Hardening 7.2. . . . . . . . . . . . . . . . . . . . . . . . .7.9 Stress Relief 7.2. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7.10 Non-Ferrous Alloys 7.2. . . . . . . . . . . . . . . . . . . . . . . . . . .7.11 Solution Treatment and Precipitation Hardening 7.2. .

SECTION 8. SURFACE TREATMENT 8.1. . . . . . . . . . . . . . . . . . . . .8.1 Surface Hardening 8.1. . . . . . . . . . . . . . . . . . . . . . . . . . .8.2 Carburizing 8.1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8.3 Pack Carburizing 8.1. . . . . . . . . . . . . . . . . . . . . . . . . . . .8.4 Liquid Carburizing 8.1. . . . . . . . . . . . . . . . . . . . . . . . . .8.5 Gas Carburizing 8.1. . . . . . . . . . . . . . . . . . . . . . . . . . . . .8.6 Steels for Carburizing 8.1. . . . . . . . . . . . . . . . . . . . . . . .8.7 Nitriding 8.2. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8.8 Ion-Nitriding 8.2. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8.9 Steels for Nitriding 8.2. . . . . . . . . . . . . . . . . . . . . . . . . . .8.10 Carbon-nitriding 8.2. . . . . . . . . . . . . . . . . . . . . . . . . . . . .8.11 Nitriding v Carburizing 8.2. . . . . . . . . . . . . . . . . . . . . . .8.12 Patented Processes 8.2. . . . . . . . . . . . . . . . . . . . . . . . . . .8.13 Surface Coatings 8.2. . . . . . . . . . . . . . . . . . . . . . . . . . . . .


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8.14 Hot Dipped Coatings 8.3. . . . . . . . . . . . . . . . . . . . . . . . .8.15 Electroplated Coatings 8.3. . . . . . . . . . . . . . . . . . . . . . . .8.16 Electroless Plating 8.4. . . . . . . . . . . . . . . . . . . . . . . . . . . .8.17 Sherardizing 8.4. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8.18 Calorizing 8.4. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8.19 Ion-Plating 8.4. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8.20 Phosphating 8.4. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8.21 Anodizing 8.4. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8.22 Metal Spraying 8.5. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

SECTION 9. TESTING AND QUALITY ASSURANCE 9.1. . . . . . .9.1 Strength and Ductility 9.1. . . . . . . . . . . . . . . . . . . . . . . .9.2 Hardness 9.2. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

FIGURE 9.1 CHARPY V-NOTCH TRANSITIONCURVES FOR DIFFERENT STEELS 9.2. . . . . . . . . .

9.3 Notch Toughness 9.3. . . . . . . . . . . . . . . . . . . . . . . . . . . . .9.4 Production Testing of Notch Toughness 9.3. . . . . . . . . .9.5 Laboratory Testing of Notch Toughness 9.3. . . . . . . . . .9.6 Drop Weight Test 9.4. . . . . . . . . . . . . . . . . . . . . . . . . . . .9.7 Bulge Explosion Test 9.4. . . . . . . . . . . . . . . . . . . . . . . . . .

FIGURE 9.2 DROP WEIGHT TEST 9.4. . . . . . . . . . . .FIGURE 9.3 BULGE EXPLOSION TEST 9.5. . . . . . .

9.8 Crack Arrest Test 9.5. . . . . . . . . . . . . . . . . . . . . . . . . . . .9.9 Isothermal Crack Arrest Test 9.5. . . . . . . . . . . . . . . . . .9.10 Wide Plate Test 9.5. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9.11 Dynamic Tear Test 9.6. . . . . . . . . . . . . . . . . . . . . . . . . . .

FIGURE 9.4 CRACK ARREST TEST ANDTRANSITION CURVE 9.6. . . . . . . . . . . . . . . . . . . . . . .FIGURE 9.5 WIDE PLATE TEST 9.7. . . . . . . . . . . . . .FIGURE 9.6 DYNAMIC TEAR TEST 9.7. . . . . . . . . . .

9.12 Fracture Mechanics Test 9.7. . . . . . . . . . . . . . . . . . . . . .FIGURE 9.7 FRACTURE MECHANICSSPECIMENS 9.9. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .FIGURE 9.8 FIELD OF USE OF KIc AND COD 9.10.

9.13 Material Cleanness 9.10. . . . . . . . . . . . . . . . . . . . . . . . . . .9.14 Creep and Stress Rupture 9.11. . . . . . . . . . . . . . . . . . . . .9.15 Fatigue Limit 9.11. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9.16 Corrosion Fatigue and Stress Corrosion Cracking 9.12.9.17 Welding and Brazing Tests 9.12. . . . . . . . . . . . . . . . . . . .9.18 Non-Destructive Examination 9.12. . . . . . . . . . . . . . . . . .9.19 Ultrasonics 9.12. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9.20 Eddy Current 9.12. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9.21 Radiography 9.12. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9.22 Magnetic Particle 9.12. . . . . . . . . . . . . . . . . . . . . . . . . . . .9.23 Dye Penetrant 9.13. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9.24 Quality Assurance 9.13. . . . . . . . . . . . . . . . . . . . . . . . . . . .


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SECTION 10. CARBON AND LOW ALLOY STEELS 10.1. . . . . . . . .10.1 Limitations of Plain Carbon Steels 10.1. . . . . . . . . . . . . .10.2 Low Alloy Steels 10.2. . . . . . . . . . . . . . . . . . . . . . . . . . . . .10.3 Steels for Naval Use 10.2. . . . . . . . . . . . . . . . . . . . . . . . . .

TABLE 10.1 STEEL PLATES AND SECTIONS FORNAVAL USE 10.3. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .TABLE 10.2 STEEL TUBES AND BARS FORNAVAL USE 10.4. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .TABLE 10.3 STEEL FORGINGS FOR NAVALUSE 10.5. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .TABLE 10.4 STEEL CASTING FOR NAVALUSE 10.6. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

SECTION 11. STAINLESS STEEL 11.1. . . . . . . . . . . . . . . . . . . . . . . . . .11.1 Austenitic 11.1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11.2 Ferritic 11.1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11.3 Martensitic 11.1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

TABLE 11.1 TYPICAL PROPERTIES OF TYPE 410MARTENSITIC STAINLESS STEEL AFTERVARYING HEAT TREATMENTS 11.2. . . . . . . . . . . . . .

11.4 Duplex (Austenitic-Ferritic) 11.2. . . . . . . . . . . . . . . . . . .11.5 Precipitation Hardening 11.2. . . . . . . . . . . . . . . . . . . . . .11.6 Corrosion Resistance 11.2. . . . . . . . . . . . . . . . . . . . . . . . .11.7 Stress Corrosion Cracking 11.3. . . . . . . . . . . . . . . . . . . . .11.8 Carbide Precipitation 11.3. . . . . . . . . . . . . . . . . . . . . . . . .11.9 Stainless Steels for Naval Use 11.3. . . . . . . . . . . . . . . . . .

TABLE 11.2 WROUGHT STAINLESS STEELSFOR NAVAL USE 11.4. . . . . . . . . . . . . . . . . . . . . . . . . . . .TABLE 11.3 CAST STAINLESS STEELS FORNAVAL USE 11.5. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

SECTION 12. CAST IRONS (GREY FLAKE AND DUCTILEIRONS) 12.1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .TABLE 12.1 CAST IRONS FOR NAVAL USE 12.2. . .

SECTION 13. COPPER AND COPPER ALLOYS 13.1. . . . . . . . . . . . .TABLE 13.1 CAST COPPER-BASED ALLOYSFOR NAVAL USE 13.3. . . . . . . . . . . . . . . . . . . . . . . . . . . .TABLE 13.2 GUIDE TO THE USE OF CASTCOPPER-BASED ALLOYS 13.5. . . . . . . . . . . . . . . . . . .TABLE 13.3 KEY TO PROPERTIES AND USES OFCOPPER BASED ALLOYS LISTED IN TABLE 13.2AND TABLE 13.5 13.6. . . . . . . . . . . . . . . . . . . . . . . . . . . .TABLE 13.4 WROUGHT COPPER-BASED ALLOYFOR NAVAL USE 13.7. . . . . . . . . . . . . . . . . . . . . . . . . . . .TABLE 13.5 GUIDE TO THE USE OF WROUGHTCOPPER-BASED ALLOYS 13.10. . . . . . . . . . . . . . . . . . .

SECTION 14. NICKEL AND NICKEL ALLOYS 14.1. . . . . . . . . . . . . .14.1 NA 13 14.1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14.2 NA 18 14.1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14.3 NA 21 14.1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14.4 Corrosion Resistance 14.1. . . . . . . . . . . . . . . . . . . . . . . . .

TABLE 14.1 WROUGHT NICKEL-BASEDALLOYS FOR NAVAL USE 14.2. . . . . . . . . . . . . . . . . . .


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SECTION 15. ALUMINIUM AND ALUMINIUM ALLOYS 15.1. . . . .TABLE 15.1 WROUGHT AND CAST ALUMINIUMALLOYS FOR NAVAL USE 15.2. . . . . . . . . . . . . . . . . . .

SECTION 16. TITANIUM 16.1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

SECTION 17. DAMPING ALLOYS 17.1. . . . . . . . . . . . . . . . . . . . . . . . .

SECTION 18. BEARING ALLOYS 18.1. . . . . . . . . . . . . . . . . . . . . . . . .TABLE 18.1 PLAIN BEARING ALLOYS FORNAVAL USE 18.2. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

SECTION 19. MISCELLANEOUS ALLOYS 19.1. . . . . . . . . . . . . . . . .19.1 Shape Memory Effect (SME) Alloys 19.1. . . . . . . . . . . . .19.2 Lead and Zinc 19.1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

TABLE 19.1 NAVAL USES OF LEAD AND ZINC 19.1

SECTION 20. CORROSION 20.1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .20.1 Mechanisms of Corrosion 20.1. . . . . . . . . . . . . . . . . . . . .20.2 Bimetallic Corrosion 20.1. . . . . . . . . . . . . . . . . . . . . . . . . .

TABLE 20.1 POTENTIALS IN SEA WATERAGAINST A SILVER/SILVER CHLORIDEELECTRODE 20.2. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

20.3 Formation of Surface Films 20.3. . . . . . . . . . . . . . . . . . . .20.4 Protective Films 20.3. . . . . . . . . . . . . . . . . . . . . . . . . . . . .20.5 Selective Phase Corrosion 20.4. . . . . . . . . . . . . . . . . . . . .20.6 Crevice Corrosion 20.4. . . . . . . . . . . . . . . . . . . . . . . . . . . .20.7 Impingement Corrosion 20.4. . . . . . . . . . . . . . . . . . . . . . .20.8 Cavitation 20.5. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .20.9 Hot Spot Corrosion 20.5. . . . . . . . . . . . . . . . . . . . . . . . . . .20.10 Pitting 20.5. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .20.11 Exfoliation of Aluminium Alloy 20.6. . . . . . . . . . . . . . . .20.12 Harbour and Estuarine Waters 20.6. . . . . . . . . . . . . . . . .

SECTION 21. CATHODIC PROTECTION 21.1. . . . . . . . . . . . . . . . . . .21.1 Sacrificial Anode System 21.1. . . . . . . . . . . . . . . . . . . . . .21.2 Materials 21.1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .21.3 Impressed Current Cathodic Protection (ICCP)

Systems 21.1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

SECTION 22. MARINE FOULING 22.1. . . . . . . . . . . . . . . . . . . . . . . . .22.1 Outer Bottom Fouling 22.1. . . . . . . . . . . . . . . . . . . . . . . .22.2 Fouling of Sea Water Systems 22.1. . . . . . . . . . . . . . . . . .22.3 Fouling by Bacteria and Fungi 22.1. . . . . . . . . . . . . . . . .

SECTION 23. ANTIFOULING METHODS 23.1. . . . . . . . . . . . . . . . . .

SECTION 24. HOT GAS CORROSION 24.1. . . . . . . . . . . . . . . . . . . . .

SECTION 25. FRETTING CORROSION 25.1. . . . . . . . . . . . . . . . . . . .

SECTION 26. LUBRICATING OILS 26.1. . . . . . . . . . . . . . . . . . . . . . . .

ANNEX A. RELATED DOCUMENTS A.1. . . . . . . . . . . . . . . . . . . .

ANNEX B. DEFINITIONS AND ABBREVIATIONS B.1. . . . . . . .

ALPHABETICAL INDEX


xii


1.1

1. METALLIC MATERIALS

a. The wide range of metals and alloys used in Surface Ships and Submarines islisted in Sections 10. to 19. of this NES. In generic terms they cover carbon andlow alloy steels, stainless steels, cast irons, copper and copper alloys, nickel andnickel alloys, aluminium and aluminium alloys, titanium and titanium alloys,plus various other metals for specific use.


1.2


2.1

2. ENVIRONMENTAL CONDITIONS

a. Metallic materials must be selected with the right properties in addition tostrength so that components and structures operate effectively in theirparticular environment without failure. The environments include sea water,salt laden atmosphere, steam, oil, exhaust gases, refrigerants, sewage, etc. Somematerials must operate under cryogenic conditions and others must withstandvery high temperature. Other induced conditions thatmay have to be toleratedare noise, shock, vibration, electromagnetic pulse (EMP) and transientradiation effects on electronics (TREE).


2.2


3.1

3. PROPERTIES AND SELECTION OF METALLIC MATERIALS

a. The properties of metallic materials may be divided into:

(1) Physical properties .

(2) Mechanical properties.

(3) Chemical properties.

3.1 Physical Properties

a. These properties are intrinsic to the material and those of principal engineeringinterest are:

(1) Density.

(2) Specific heat.

(3) Thermal expansion.

(4) Melting point.

(5) Thermal conductivity.

(6) Electrical conductivity and resistivity.

(7) Magnetic permeabilityis a measure of the ease with which a magneticfield will pass through a substance. Ferromagnetic materials have a highpermeability and paramagnetic materials a low permeability.

(8) Dampingcertain metals and alloys have a damping capacity which canlead to a reduction in vibration and noise. They achieve this capacity bytheir ability to dissipate elastic strain energy as heat, see Section 17.

3.2 Mechanical Properties

a. The way metals respond to externally applied force is controlled by theirmechanical properties. The properties can be divided into tensile and shearproperties, hardness and abrasion resistance, notch ductility, creep propertiesand fatigue properties.

3.3 Tensile and Shear Properties

a. Tensile and shear properties are used in determining the safe loading to beplaced on a component or structure. The properties associated with tensile andshear loading revolve around the elastic constants E andN, and the relationshipbetween stress and strain. Most metals are utilized under elastic conditions,which means that when the metal is deformed the deformation is notpermanent and it returns to its original shape upon removal of the load. To takeaccount of local overloading it is also necessary to have a measure of the metalsductility. Initially, ductility is determined from percentage elongation andreduction of area in the tensile test, but a further property of notch ductility isconsidered later. The tensile and shear properties concerned are:

(1) Limit of proportionality (N/mm2)that part of the stress/strain curvewhere strain is proportional to stress and is represented by a straight line.The limit of proportionality is the upper point of the straight line. SeeFIGURE 3.1.


3.2

FIGURE 3.1 TYPICAL STRESS-STRAIN CURVES


3.3

(2) Elastic Limit (N/mm2)closely related to the limit of proportionality andis the maximum stress to which a material can be subjected withoutcausing permanent strain. It is not necessarily coincident with (a). Amaterial can show a slight extension on the stress/strain curve and stillreturn to its original length on unloading. See FIGURE 3.1.

(3) Yield stress (N/mm2)the stress marking the onset of plasticdeformation.

(4) Proof Stress (N/mm2)usually defined as 0.1% or 0.2% proof stress andis the stress required to produce a permanent elongation of 0.1% or 0.2Xof the original gauge length in the tensile test.

(5) Ultimate Tensile Strength (N/mm2)the stress corresponding tomaximum load prior to failure in the tensile test.

(6) Elongation (%)measured in the tensile test and expressed as apercentage of the original gauge length after fracture.

(7) Reduction of Area (%)expressed as a percentage of the originalcross-sectional area in the tensile test and indicates the extent of neckingof the specimen prior to failure.

(8) Shear Strength (N/mm2)the stress corresponding to maximum load inshear prior to failure.

(9) Elastic Modulus (Youngs Modulus)the modulus of elasticity for puretension, denoted by the letter E, and is the ratio of tensile stress to tensilestrain within the limit of proportionality.

(10) Shear Modulussometimes known as the Modulus of Rigidity anddenoted by the letter N, expresses the ratio of shear stress to shear strainunder elastic conditions.

(11) Poissons Ratioas a specimen elongates in the elastic region itscross-sectional area decreases, the relationship of the transverse strain tothe tensile strain is known as Poissons Ratio.

b. Unless otherwise stated, the tests to determine the above properties are carriedout at room temperature or at a nominal 20 C. Tensile strength generallydecreases as temperature increases.

3.4 Hardness and Abrasion

a. Hardness is specifically a measure of resistance to penetration.

b. High hardness may be required to resist wear or abrasion. In somecircumstances therefore, hardness is a desirable property and in othersundesirable since it can represent a loss of ductility. An example of this conflictis white cast iron which is not employed for structural parts because of itsexcessive brittleness, but it is used for resisting wear by abrasion.

c. Hardness can indicate the yield strength of a material. It is not representativeof ductility although generally the higher the hardness the lower the ductility.Hardness can, therefore, be an approximate guide to the condition of a metalafter heat treatment or after hot or cold working. (See Clause 9.2a.)

3.5 Notch Toughness

a. Notch toughness is ameasure of the resistance of ametal to brittle fracture. (SeeClause 4.2a.)


3.4

b. Brittle fractures frequently originate at a notch. In a normal tensile test ametalmay exhibit a high level of ductility, but in the presence of a notch, failure mayoccur well within the normal working level of stress. The notch may take theform of an abrupt discontinuity in the structure, ormaybe a flaw such as a crackin a weld or an inclusion in a casting. In the presence of such a notch a metalwith inadequate notch toughness can fail prematurely.

c. Risk of failure in way of a notch is increased with an increase in the rate ofloading, eg shock loading.

d. Notch toughness is a property which is also highly temperature-dependent.Many metals and particularly steel exhibit a transition behaviour. Thetransition behaviour is a function of metals with a body centred cubic (bcc)crystal structure. Austenitic steels which have face centred cubic (fcc) crystalstructure do not show this characteristic. As the temperature is lowered thereis a sudden fall off in notch toughness. The temperatures over which thistransition occurs vary for different metals and this is an important factor inchoosing metals to operate at low temperatures. Testing for adequate notchtoughness is described in Section 9. of this NES.

3.6 Creep

a. Creep is a time-dependent property of ametal which is of particular significancefor those metals stressed at elevated temperatures eg, in steam systems. Atconstant stress, deformation takes place slowly over a period of time, ie, creepoccurs. Ultimately failure may result or the deformation may reachunacceptable limits. In some metals creep occurs at room temperature.

b. Creep occurs in three stages. The primary stage is when upon application of theload extension occurs asa result of elastic strain and some plastic strain; the rateof creep then gradually decreases. The secondary stage follows: creep rate is ata minimum and extension occurs at a uniform rate. The tertiary stage thentakes place when the creep rate increases markedly leading to eventual rupture.Most components spend their life in the second stage of creep.

3.7 Stress Relaxation

a. Stress relaxation is the property of some materials whereby a slow decrease instress occurs at constant strain. This can be important for applications such asfasteners and rolled tubes in tubeplates. Typical stress relaxation curves for twosteels are shown in FIGURE 3.2.


3.5

FIGURE 3.2 STRESS RELAXATION BEHAVIOUR FOR TWO BOLTING MATERIALS

3.8 Fatigue

a. Most failures of engineering components that are subjected to alternatingstresses can be traced to fatigue. Fracture occurs at working stresses very muchlower than the ultimate strength of the metal. Few engineering areas escape:fatigue failures have occured in ships, bridges, aircraft and machinery. Theprocess begins with the formation of a small crack usually at a point of stressconcentration. Under further repeated loading the crack slowly spreads in adirection normal to the direction of principal tensile stress until fracture occurs.The final fracture may be ductile tearing or brittle fracture. Failure occurs morerapidly where stresses fluctuate between compression and tension than wherethe fluctuating stress is all tensile. When a design is subject to varying loads itis necessary therefore to know the fatigue properties of the metal in question.

3.9 Corrosion Fatigue

a. Fatigue life is shortened still further if the component is working in a corrosiveenvironment. See Clause 3.11a.


3.6

3.10 Chemical Properties

a. The chemical properties of a metallic material which are of prime concern toship design are its corrosion characteristics. Corrosion occurs by a number ofdifferent mechanisms. It can occur by direct chemical action when the metalenters into a chemical reaction with other elements such as oxygen, chlorine orsulphur; or it can occur by probably the most common mechanism of corrosion,namely, electrochemical or galvanic action. Corrosion is dealt with in moredetail in Section 20. of this NES.

b. Metals vary considerably in their resistance to corrosion and it is necessary tocarry out tests under simulated service conditions before selecting previouslyuntried materials.

3.11 Stress and Corrosion

a. The combined effects of stress and corrosion manifest themselves in thephenomena of stress corrosion cracking and corrosion fatigue. In each case thecombined effect is to shorten life. Metallic materials vary in their resistance tostress corrosion cracking and corrosion fatigue. Testing is necessary undersimulated service conditions.

3.12 Selection of Metals

a. A metallic material is completely defined by its physical, mechanical andchemical properties. The first stage in selecting a metal or an alloy for aparticular purpose is to examine these properties in relation to fitness forservice. The requirements for service must therefore be defined. Strength inrelation to weight is frequently examined first. Other factors may or may notbe so well defined, such as a level of notch toughness required. This in turn isrelated to operating temperatures and whether the loading is static or by shock.Some of the factors involved are illustrated in FIGURE 3.3.

NES738

Issue2(Reform

atted)

3.7

FIGURE3.3

SOMEFA

CTORSIN

THESELECTIONOFAMETA

L


3.8

b. Service requirements will determine the materials to be used. Further factorshave now to be considered. What do we need to do with the material: has it tobe machined, cold or hot worked, cast, welded, or brazed? The ease or otherwiseof fabrication may further reduce the field of choice. Cost could be a governingfactor, not only the first cost ofmetal but also the subsequent cost of fabrication.

c. Experience of use of the metal must also be sought. Risk increases whereexperience is scant.

d. Finally the availability of the metal in the form, shapes and sizes that arerequired has to be investigated. Asmuch as possible is to be found out about themanufacturing route of the metal and the other uses being made of it.

3.13 Toxicity of Metals

a. Many metals and their oxides are toxic if inhaled in a finely divided form. Thehazard may be a metal powder or as a metal fume from a working process suchas brazing, welding or metal spraying. Of particular risk are beryllium andtellurium followed by cadmium, lead, zinc, chromium etc. Guidance on the useof hazardous metals is issued by the Health and Safety Executive.


4.1

4. MODES OF FAILURE

a. Over the years much data has been collected on engineering failures and thishas gradually led to a better understanding ofmetals and their properties. Mostfailures can be ascribed to one or more of the following mechanisms.

(1) Plastic collapse.

(2) Brittle fracture.

(3) Fatigue and corrosion fatigue.

(4) Corrosion.

(5) Stress corrosion and stress corrosion cracking.

(6) Low energy ductile tearing.

4.1 Plastic Collapse

a. Failure by plastic collapse can occur as a result of overloading ormanufacturingerrors or from inadequate or inaccurate data on which the design was based.

4.2 Brittle Fracture

a. Brittle fracture is a rare event in a large monolithic steel structure but it canresult in total loss. Some of the more notable failures have been the American-built liberty ships inWorldWar II, and the offshore drilling rig, Sea-Gem. Brittlefailure of minor items such as fasteners, chain cables, etc can have potentiallyserious results.

b. The characteristic of a typical brittle fracture is that it occurs without warningand with little or no previous deformation and propagates at a very high speed.The nominal stressmay bewell below the yield strength of the steel. The surfaceappearance of the fracture is crystalline arising from the fracture of themajorityof crystals by cleavage. Some crystals separate by shear and a measure of thebrittleness of the fracture is the percentage of the fracture surface that iscrystalline. The edges of the fracture may have shear lips or the fracture maybe entirely flat indicating extreme brittleness. The occurrence of differentappearances of the fracture in the Charpy test, see Clause 9.4b., is illustratedin FIGURE 4.1. Brittle fractures in plate bear characteristic chevron patternswhich invariably point to the origin of the fracture.

c. The principal factors determining whether brittle fracture occurs are:

(1) Fracture toughness or notch toughness of the metal.

(2) Stress concentration and notch effects.

(3) Service temperature.

(4) Rate of loading.

(5) Size effect.

d. The effect of (1) to (4) above is described in Clauses 3.5a. to 3.5d. The size effectrelates both to the thickness of plates and sections and to the overall size of thestructure. Brittle behaviour is more likely to occur in heavy structures madefrom thick plate and sections rather than in structures of lighter scantlings.This is partly due to metallurgical effects such as segregation and the difficultyof obtaining uniform cooling, and partly due to geometric and stress conditions.


4.2

e. Avoidance of brittle fracture depends upon design, material selection, andcontrol over fabrication and acceptance.

(1) DesignAscertain, as accurately as possible, the service temperature andthe character of the loading, eg static, fluctuating, impact etc. Section sizeincreases the hazard, therefore keep thicknesses to a minimum. Avoiddesigning-in stress concentrations, taking particular care of details. Payspecial attention to minor fittings attached to main strength members.Arrange joints to ensure easy access for welding and inspection.

(2) Material SelectionEnsure that the steel has adequate notch ductilitywith a transition range well below the lowest operating temperature.Check that the steel has been tested in accordance with Section 9. of thisNES.

(3) FabricationThe origins of many brittle fractures have been traced towelds: the quality of welding is therefore very important. The welds ofminor fittings are as important as the structure on which they are made.The weld metal must have adequate strength and notch toughness andmust have been tested as vigorously as the basematerial. Inspection mustbe equally of a high standard. A satisfactory quality assurance proceduremust be agreed to cover weld procedure, welder qualifications, inspectionand repair methods. Care must also be taken to ensure that cold workingor heat treatment does not significantly impair the notch toughness of thestructure; this in turn requires that the effects of these treatments onnotch toughness be known. Avoid all uncontrolled welding.

FIGURE 4.1 FRACTURE APPEARANCE IN THE CHARPY TEST


4.3

4.3 Fatigue and Corrosion Fatigue

a. One of the most common causes of failure in metals is by fatigue. Underfrequently repeated stresses metals fail by fracture well below their yieldstrength. If the stresses are continually reversed failure occurs even earlier.Failure by fatigue is instantly recognizable by the appearance of the fracture.Two distinct zones are evident, a smooth area due to minute rubbing of thesurfaces together which may show striations or lines of arrest, and a rough orcrystalline area where final failure has occurred by ductile or brittle fracture.

b. Fatigue fractures are usually initiated at stress concentrations whichmay resultfrom a design error such as an abrupt change of section or from a surface flaw,such as a chisel mark or weld reinforcement. The mechanism of failure is thata small crack forms at the stress concentration and slowly spreads with repeatedloading. The crack will extend in a direction normal to the direction of the maintensile stress.

c. Welding can introduce various stress concentrations from which fatiguecracking can be initiated. These can be planar defects, such as lack of fusion orpenetration, or more likely surface discontinuities, such as the toe of welds orweld ripples. The fatigue life of welded items comprises mainly the time spentin propagating the fracture. For unwelded items a significant portion of life isalso spent in nucleating the fracture. Since the rate of crack propagation doesnot vary much with the UTS of the material the fatigue life of welded items isnot increased by using stronger material.

d. Tensile residual stresses occur in and around welds following contraction oncooling and can affect fatigue life. The result of residual tensile stresses is toalter the point in the stress range at which the applied stress acts. Appliedcompressive stresses can effectively become tensile and fatigue failure of weldeditems can occur under compressive loading. Stress relief can extend the lifeunder compressive loading but has little effect if the loading is tensile since therate of crack propagation does not significantly depend on mean stress.

e. The method of testing to determine the fatigue life of metals is described inSection 9. The number of cycles, N, before failure is plotted against the appliedstress, S, usually as log N and log S. A typical curve for steel is shown inFIGURE 4.2. From it, it will be seen that the allowable stress falls with increasein the number of cycles. The point where the curve becomes asymptotic to theabscissa is known as the fatigue limit. FIGURE 4.3 shows a typical SN curvefor most nonferrous alloys; it is also representative of some steels undercorrosion fatigue conditions. Where there is no clear fatigue limit an endurancelimit is used instead, eg for aluminium alloys. This use of SN data for fatiguedesign is now being replaced by crack propagation data whereby the incrementin crack length per cycle (da/dN) is plotted against a range of stress intensities(K).


4.4

FIGURE 4.2 TYPICAL SN CURVE FOR UNWELDED STEEL

FIGURE 4.3 TYPICAL SN CURVE FOR MANY NON-FERROUS ALLOYS

f. Fatigue strength or endurance limit can be markedly lowered by a corrosiveenvironment. If the highly stressed root of a crack is in a corrosive medium thenthe corrosion rate increases due to the stress, and the overall fracture rate alsoincreases.

g. To avoid failure by fatigue or corrosion fatigue it is necessary to know the cyclicpattern of stress and the environmental medium, and also to have appropriatetest data. In many cases accurate information is not available, nominal designstresses have to be used and an estimate made of the number of cycles of stress.In applying available test data to welded structures and components the dataused is not to be based on information derived from polished unweldedspecimens. Similarly, results of tests in air may seriously overestimate the lifeof an item which operates in sea water. In the end it may be necessary to basea judgement on the test results available and experience of metals in similarcircumstances in real life situations.

h. Where there is clear evidence of a fatigue problem there are a number ofmeasures which can be taken to avoid premature failure. First and foremost is


4.5

the avoidance or removal of stress raisers. Geometric discontinuities must bedesigned out. Even a minor fitting can be a stress raiser if carelessly placed ona strength member. Machining or fine grinding welds or other rough surfacescan remove or reduce stress concentration, but such operations may be limitedor prohibited altogether by cost. Stress relief can assist under compressiveloading. Shot and hammer peening can introduce compressive residual stresswhich will assist in tensile loading (peening of welds is, however, prohibited, seeNES 706). Where the stress range is low cathodic protection will increase thefatigue strength compared with free corrosion. The effect is uncertain at highstress ranges. Specialist advice must be sought before introducing anypreventive measures other than improving the design.

4.4 Corrosion

a. Themechanism of various forms of corrosion and their preventive measures arecovered in Section 20. of this NES. A brief note on corrosion is included in thissection since it accounts for some costly failures in metals.

b. By a programmed system of surveys, dockings and refits, the replacement ofcorroded items is generally effected before in-service failures occur. The cost isvery large. Hull structural plating and sections suffer from general corrosion,pitting corrosion, and localized galvanic attack. Serious corrosion occursparticularly in bilges and all places where water can collect. Sea-water systemscan suffer from impingement attack and cavitation resulting from localturbulence. Any dissimilar metal combination in the presence of an electrolyte(sea water) will suffer galvanic attack. Condensers, and heat exchangers maysuffer from tube and tube plate failure due to corrosion and erosion. Internalpitting can affect steam and feed water pipes. Stainless steels fail by creviceattack in themarine environment and selective phase corrosion can affect somecast copper alloys.

c. The selection of metals to avoid or minimize corrosion is covered in Section 20.

4.5 Stress Corrosion and Stress Corrosion Cracking

a. Although not a widespread problem, failure from stress corrosion can occurwhere both stress, internal or external, and a corrosive environment arepresent. The corrosion is specific to the material and its environment, and thestress, whether imposed or residual internal, has to be tensile. The mechanismdiffers according to the material and the environment but failure would notoccur if either stress or corrosion were absent.

4.6 Low Energy Ductile Tearing

a. The fact that a material fails in a shear mode when tested at its servicetemperature is not always a guarantee of safety. Failure by low energy ductiletearing is possible and suspect materials are to be assessed by the J-integralmethod, see Clauses 9.12a.9.12d.


4.6


5.1

5. METAL FORMS AND PROCESSES

a. With the exception of a relatively small number of sintered products, metals areused either in the cast or wrought form.

5.1 Cast Metals

a. Metals may be cast by a number of different methods. These include:

(1) Sand castings.

(2) Shell moulding.

(3) Gravity die and pressure die.

(4) Centrifugal.

(5) Investment.

(6) Continuous.

b. Sand casting is the best known method. Foundry sand is rammed around awooden pattern which is contained within a moulding box. The moulding boxis separated in two halves and the pattern is withdrawn leaving a cavity intowhich the metal can be poured. Sand casting is a highly skilled process andselection of a modern foundry with appropriate experience in the alloyconcerned is vital to achieve good quality components free of defects.

c. Shell moulding requires a clay-free sand to which is added a thermosettingbonding agent such as phenol or urea formaldehyde. A metal pattern is made,the thermosetting mix is poured on to the heated pattern to form a shell. Theshells are made in parts and clipped together to receive the molten charge. Inthe casting operation, the shells are usually backed by sand or by metal shot.Surface finish and dimensional accuracy are better than with sand castings.

d. Die casting is mainly confined to zinc and aluminium alloys with some use ofmagnesium alloys and low melting brasses. In gravity die casting, the metal isfed by gravity into the die and in pressure diecasting it is forced into the dieunder considerable pressure. A run of castings is required to justify the cost ofthe die and provided the design is satisfactory output can be high. Dimensionalaccuracy and surface finish are very good: fine screw threads may be accuratelyincorporated into the casting. Not all light alloys can be die cast because of theirhigh shrinkage characteristics which leads to cracking.

e. Centrifugal casting employs a metallic mould without cores which is spun athigh speed. Centrifugal force flings the metal to the surface of the mould. Theresulting casting is usually of uniform wall thickness with a fine-grain outersurface. The product is superior to that produced by sand castings but theprocess is only applicable to symmetrical shapes. It is used for producing pipes,cylindrical sleeves, hollow shafts etc.

f. Investment casting is used for the production of small items requiring closetolerances. Non-machinable and non-forgeable alloys can be cast to finisheddimensions by this process. The item to be cast is made of a low melting pointmaterial, originally beeswax, enclosed in amould which is heated to remove thewax, and then the molten metal is run in.

g. Continuous casting is used for producing solid and hollow bar, pipe and billetin a continuous process. Themoltenmetal passes through a cooled die emergingas a just solidified bar or tube.


5.2

5.2 Advantages and Disadvantages of Castings

a. The main advantage of castings is that large and intricate shapes can beproduced generally in one piece more economically than by any other process.Batch production of castings of smaller items is frequently cheaper than bymanufacture from wrought material. Ability to achieve the required shapewithout expensive forging and machining is where the principal savings lie.

b. An inherent weakness of casting is that it can produce the least predictable ofmetallurgical structures. The characteristics of the solidification process leadto inherent weaknesses arising from shrinkage, segregation, porosity and lowhot strength. Despite increasing quality assurance measures, the castingprocess, particularly sand casting, is fraught with difficulties. Competentdesign can, however, considerably reduce the problems and the defects. Closeco-operation is essential from the earliest concept stage between the designer,pattern maker and foundry man. It should be noted that the tensile strengthspecified for castings usually relates to standard, separately cast bars (keelbars), and that the actual tensile strength at any point within a casting mayvary depending on the local microstructure.

5.3 Wrought Products

a. Wrought products are produced mainly by hot processes although coldprocesses are necessary for some materials. In the first stage the metal is castas an ingot. Subsequent working of the ingot produces plastic flow andimpurities are elongated in the direction of working. Elongation of impuritiesand crystalline structure contributes to the directional properties found inwrought products. Ductility, impact, and fatigue properties are greater in theprincipal direction of working. Yield and tensile strength are less affected. Forcertain materials, by control of working temperature, it is possible to producea fine grain structure. Worked material has a more uniform structure and is ofgreater density and of higher strength than cast material of similarcomposition.

b. Wrought material is not without its defects. The original ingot can suffer frompipes, segregation, blowholes, inclusions and cracks. Primary piping may becropped from the ingot but if a secondary pipe exists it could ultimately appearas centre-line porosity in a bar product or as laminations in a plate. Blowholesmay be welded up by the working process or remain as a defect although alteredin shape. Inclusions will be elongated and generally well dispersed. They can,however, appear as stringers and are then more serious. Internal cracks andbursts can result from forging and various surface defects can result from theworking process. Blooms for forging made from continuously cast metal willcontain fewer defects than those from cast ingots.

c. Control measures are possible. These startwith thematerial production processand control over impurities. Other defects such as piping, porosity, etc can bereduced by the method used in the melt or casting of the ingot and subsequentcontrol of the cropping process to ensure defective material is removed from thetop and bottom of the ingot. Thereafter working can produce directionality ofproperties or conversely a more isotropic condition if required. Measures canalso be taken to produce a satisfactory surface finish.

5.4 Working Processes

a. The principal working processes are rolling, forging, upsetting, drawing andextrusion. From the ingot stage hot rolling produces semi-finished productssuch as blooms and slabs. Subsequently hot rolling is used for the reduction ofthese products to plate, sheet, sections and rods.


5.3

b. Cold working is used, where required, as a finishing process. Its purpose is to:

(1) increase strength, also known as temper;

(2) provide a cleaner, smoother finish;

(3) meet closer, dimensional tolerances;

(4) straighten the product.

A combination of cold working and precipitation hardening in suitable alloyscan increase strength and hardness to an extent which is not possible witheither process singly.

c. In addition to finishing processes, cold working is also used for drawing.Section, tubes and wire can be drawn cold through lubricated dies. The drawingprocess relies on the high ductility of the metal being drawn.

d. Most forging is now power assisted although hand forging is still practised bysmiths. Where large numbers of similar articles are to be produced, drop orclosed die forging may be used. By this method heated bars or billets are forgedbetween dies; the hammer is raised by power and allowed to fall under gravity.Modifications of this process use a power-assisted hammer or hydraulic press.A further development is high-energy rate forging which utilizes pneumaticpower.

e. Hot pressing may be used for items of simple geometry. The hammer is replacedby a hydraulic ram and the heated material is gradually squeezed into shape bythe static pressure of the ram. One of the advantages of this process is that themetal is worked in depth and not primarily in the surface layer, as with hammerforging.

f. Upset forging or heading is used in the production of bolts, rivets and similaritems. The end of the bar is heated and the head is forged in a single operationin a machine. Dependent on the size of the item and the material, cold headingmay be used instead of hot forging.

5.5 Extrusion

a. The extrusion process may be direct or indirect. In the direct process metal isforced through a die, and in the indirect process the die is forced into the metalthereby extruding the required shape through the die. The ram in each case ispowered hydraulically. Metals that can be extruded include aluminium, brassesand ferrous alloys. The extrusion process is usually carried out hot, up to 500 Cfor aluminium alloys, 800 C for brasses and 1250 C for steels, but certain alloysmay also be extruded cold.

5.6 Powder Metallurgy

a. Sintering of metal powders is a means by which homogeneous alloys may beproduced frommetals which are not soluble in the liquid state or which, becauseof a wide range of melting points or very high melting points, are difficult toproduce commercially. The metals are reduced to fine powders, mixed andcompressed in a hardened steel die. The pressure is high enough to produce adegree of cold welding between the metal particles. The compressed mass isheated to a suitable temperature, below the melting point, at which sinteringtakes place. Sintering is the conversion to a homogeneous alloy by grain growthacross the cold welds between metal particles.


5.4

b. Powder metallurgy can also be used to obtain an even distribution of aninsoluble constituent in a metallic matrix. Graphite can be evenly distributedin a bearing alloy. Cermets can be produced. Tungsten carbide alloyed withtitanium carbide and cobalt is used for cutting tools. A range of hightemperature alloys can also be made by these methods.

5.7 Clad Metals

a. Cladmetals can be produced by a number of processes. Composite plates can bemade by rolling two metals together to effect bonding by pressure welding. Thecladmetal can also be bonded to the parent plate byan explosive process. Amoreexpensive way is to clad by fusion welding, a process which is more appropriatefor castings and forgings. The principal naval application by fusion welding isto obtain a corrosion resistant surface on a steel strength member. Claddingmay also be carried out onnon-critical, low fatigue, applications by plugweldingsheet material to the component. This procedure can be used for items such assea-tubes and muffler tanks.


6.1

6. SHAPING OF METALS

6.1 Cold Forming

a. The most economical method of shaping metals is by cold forming which maybe by rolling, bending or pressing. The metal being formed must have adequateductility and its properties must not be seriously impaired by the formingprocess. The extent of cold work must not produce cracking.

b. Cold working produces strain hardening leading to an increase in strength andalso a reduction in notch ductility for those metals exhibiting a transitionbehaviour. For this reason it is important in structural steels to limit theamount of cold work. Permitted strain limits for ship structural steels are givenin NES 706 and NES 770, Part 1.

c. Stress relieving, normalizing, or process annealing may be permitted to relieveinternal stresses produced by cold work or to restore complete plasticity. Theheat treatments permitted for structural steels are defined in NES 706 andNES 770, Part 1. For other metals reference is to be made to the relevant DataSheet in Def Stan 012 or specialist advice is to be sought.

6.2 Hot Forming

a. Where the required deformation is such that the strain limit on cold work willbe exceeded it will be necessary to work the metal in the hot condition. It isimportant to question the effect of hot work on themetal concerned. If themetalis in the heat treated condition prior to forming then subsequent heattreatment, such as quenching and tempering, or normalizing will be necessary.Alternatively, the metal may have been purchased in a cold worked, hardenedcondition tomeet design strength requirements. The extra strength induced bycold work will be lost on hot working as the metal reverts to a softened state.

b. The working temperatures and particularly the finishing temperature will havea crucial effect on the structure of themetal and on its properties. If theworkingtemperatures are not clearly specified then specialist advice must be sought.Temperatures for structural steels are defined inNES 706 andNES 770, Part 1.

6.3 Shaping Weldments

a. If the item to be worked has been fabricated by welding then the effect of thework on the weld metal must be considered. Normally mild steel weldmentsmay be worked hot or cold but where a steel needs to be quenched and temperedafter forming then the weld metal will need to be cut out and the joint rewelded.See NES 706 and NES 770, Part 1.

6.4 Spinning

a. A particular method of working metals which is suitable for dome ends ofpressure vessels and similar shapes is by spinning. The plates are continuallyspun while a forming tool eases the metal into shape. For small items in lowstrengthmetals, spinningmay be carried out cold, but for a steel pressure vesselit will need to be carried out hot to achieve the necessary plasticity. Therequirements for cold or hot working the metal or weld metal concerned will beequally applicable to spinning.


6.2

6.5 Machining

a. The ease with which metals can be machined is a function of their mechanicalproperties and metallurgical structure. Ductile metals tend to spread and arenot as easily machined as harder and more brittle metals. The difference isshown in the turnings: ductile metals produce continuous coils of turningswhereas in themore brittle butmore easilymachined metals the turnings breakoff in small chips.

b. Finely dispersed inclusions assist chip forming and the compositions of metalscan be modified to produce such inclusions. The addition of sulphur to steelscontaining manganese creates manganese sulphide inclusions. These are the socalled sulphur bearing free machining steels. Other free machining steelscontain lead which exists as microscopic globules in the steel structure. Theadvantage of lead is that it has little effect on the other mechanical propertiesof the steel; manganese and sulphur can considerably reduce notch ductility.Other treated metals are stainless steels with added selenium or molybdenum,and copper alloys and nickel silver with lead additions.

c. The aim of free machining metals is to reduce machining costs. Free cuttingsteels owe much of their development to their cost advantage in automaticscrew cutting. These steels may be unsuitable for steam or sea waterapplications and specialist advice is to be sought.


7.1

7. HEAT TREATMENT

a. The heat treatment of metal alloys is carried out in order to produce the desiredproperties for the service intended. In addition to mechanical properties thiscould also mean a stress-free condition or enhanced corrosion resistance. Thealteration of properties is obtained in the solid state by properly controlledheating and cooling and is directly related to changes produced in themicrostructure. Alloying additions play a vital role in the transformation ofmicrostructure.

7.1 Steel

a. The properties of steel may be altered with relative ease by heat treatment andthis is one reason why steel is so useful. The critical factors are the temperatureto which it is heated, the time it is held at that temperature, and initially, therate atwhich it is cooled. The temperatures atwhich transformation takes placein the solid state are called critical temperatures or critical points and will varywith the particular composition of the alloy. In steel we are mainly concernedwith the upper and lower critical points.

7.2 Annealing

a. Annealing of steel is carried out principally to soften the alloy and to improveductility. It also relieves the internal stresses caused by previous treatments.The workpiece is heated to 40 C above the upper critical point and heldsufficiently long at that temperature, dependent on its size, to ensure athorough soak, and then cooled very slowly, preferably in the furnace. This issometimes known as full annealing. A slightly different operation is processannealing; here the steel is heated just above the lower critical point,550 C650 C, and then cooled. Its purpose is to remove hardening effectsproduced by cold work and is used extensively in the production of sheet andwire. The rate of cooling in process annealing is not so critical since reliance isplaced on temperature to partially soften the steel and relieve internal stresses.

7.3 Normalizing

a. Normalizing is carried out to refine grain structure and improve mechanicalproperties and also to reduce alloy segregation in forgings and castings. Theworkpiece is heated to 40 C above the upper critical point and cooled in still air.This results in less ductility than the full anneal, a small increase in hardness,and probably better impact properties. Steels are normalized before hardeningand tempering.

7.4 Quenching

b. Quenching is carried out in order to harden steel and is normally followed bytempering. The workpiece is heated to 40 C above the upper critical point andthen quenched in some medium to achieve the desired cooling rate. The coolingrate influences the degree of hardness that will be achieved and is in turncontrolled by the selection of the correct quenching medium. Various fluids areused and include water, brine, oil, oilwater emulsion, and air. Where extremelyrapid cooling is required water spray may be used. The rate which achievesmaximum hardness is known as the critical cooling rate.

7.5 Interrupted Quenching

a. Where high surface hardness is required with minimum internal stress it isnecessary in certain steels, particularly if large items have to be treated, to carryout interrupted quenching. The initial rapid quench in water is followed by aslower quench in oil. High surface hardness is obtained by the rapid quench andthe slower quench reduces internal hardness and reduces internal stress.


7.2

7.6 Tempering

a. The object of tempering is to increase the ductility of a quenched steel at theexpense of hardness, yield and UTS. The steel is heated to a temperature belowthe lower critical point usually in the region 550 C650 C and then cooled ata pre-determined rate. Steels for submarine pressure hulls have for many yearsbeen quenched and tempered. By this method very tough steels of medium tohigh strength have been produced.

7.7 Flame Hardening

a. This process produces a hard surface on medium carbon steels while leaving asofter and tougher core. The surface of the workpiece is rapidly heated byoxyacetylene torch or other high temperature flame followed immediately by arapid quench. For some applications it is possible to attach a quenching sprayto the torch. Flame hardening relies on the fact that heat is applied very rapidly,building up a high thermal gradient and raising the surface temperature abovethe upper critical point prior to the rapid quench.

7.8 Induction Hardening

a. Where components are large and it is impossible to heat the whole surface atonce with a torch then induction heating is the preferred alternative. The objectto be surface hardened is enclosed in a coil through which a high frequencycurrent is passed. The resultant induced current on the workpiece raises thesurface temperature above the upper critical point. The frequency of thecurrent determines the depth of hardening. Coils can be specially shaped to theworkpiece being treated and can incorporate a spray so that a rapid quenchfollows the heating.

7.9 Stress Relief

a. Thermal stress relievingmay be carried out on castings, welded fabrications andrepairs, and on severely cold worked items to relieve internal stress. The processinvolves heating the item to a temperature below the lower critical point, approx575 C, holding at that temperature for one hour per inch of thickness and thencooling in still air.

7.10 Non-Ferrous Alloys

a. Annealing of non-ferrous alloys is carried out to deliberately soften the metalafter it has been hardened by work or other process. Annealing temperaturesdiffer for different alloys and for any particular alloy the degree of softening willbe time and temperature dependent.

7.11 Solution Treatment and Precipitation Hardening

a. Some non-ferrous alloys, in particular aluminium alloys, are amenable tosolution treatment and subsequent precipitation hardening. The alloy is heatedto a temperature which, as the name suggests, is sufficient for the differentmetallic phases present to dissolve and form a solid solution. If the alloy isquenched from this temperature the phases remain in solution and the alloy isductile and in the solution-treated condition.

b. Subsequent precipitation treatment, sometimes called ageing, which usuallyinvolves heat treatment at a much lower temperature than the solutiontreatment temperature but which can also, with some alloys, occur at roomtemperature over a long period of time, results in the precipitation of phasesfrom the solution to critical sites in the lattice structure resulting in hardeningof the alloy. The ductility will decrease and the strength will increase.


7.3

c. Care must be exercised when welding, or heating for any reason, precipitationhardened alloys as the mechanical properties may be impaired and onlypartially recoverable by further heat-treatment.

d. Precipitation hardening does not apply only to non-ferrous alloys; it also appliesto certain steels when tempered after quenching and to precipitation hardeningstainless steels.


7.4


8.1

8. SURFACE TREATMENT

a. The surface treatments covered in this section are the surface hardening ofsteels and the application of coatings to both ferrous and non-ferrous alloys inorder to improve corrosion, heat and wear resistance.

8.1 Surface Hardening

a. Many moving components used in engineering such as cams, gears and shaftsrequire a surface that is resistant to hard wear and must also possess a toughinterior. Surface hardening by heat treatment is described in Clauses 7.7a. to7.8a.; other means of surface hardening are by carburizing, nitriding,ion-nitriding, or carbon-nitriding.

8.2 Carburizing

a. Surface hardening by carburizing is achieved by the introduction of carbon intothe surface layer of steel. The process consists of surrounding the componentwith carburizing material and heating to produce a carbon-enriched layer. Thecarburizing material may be either solid, liquid or gaseous.

8.3 Pack Carburizing

a. In this process the steel component is packed with charcoal in a heat-resistingbox and the temperature raised to 875 C925 C. The depth of hardness isdependent on time and temperature, of which time is the most important incontrolling the depth of carbon penetration.

8.4 Liquid Carburizing

a. The liquid used in this carburizing process is a cyanide-rich bath of fused saltscomprising up to 50% sodium cyanide together with sodium carbonate andsodium or barium chloride. The molten salts are held at a temperature of870 C950 C and the steel components to be hardened are lowered into thebath in wire baskets. The process is ideal for small parts requiring shallowhardening.

8.5 Gas Carburizing

a. Gas carburizing is carried out in a controlled-atmosphere furnace where theatmosphere is fed with methane or propane diluted by a carrier gas. Thetemperatures used are mostly in the range 930 C955 C although hightemperature gas carburizing at 1095 C is also possible. The process is the mostcontrollable of the carburizing methods as not only can the temperature beaccurately controlled but the composition of the carburizing atmosphere can bemonitored and the carbon potential controlled.

b. After carburizing, further heat treatment of the steel is required to toughen thecore and for some applications to produce the required hardness in the surface.The heat treatment required will depend on the carburizing temperature used,the composition of core and case, and the properties required for the componentto function properly. Quenching and tempering or interrupted quenching (seeSection 7.) will be required.

8.6 Steels for Carburizing

a. A wide variety of steels are used for carburizing. Low carbon steels up to 0.2% Cprovide a ductile core. Steels with a higher carbon content or lowalloy steelswillbe used if greater strength is required. The advantage of the low alloy steels isthat the toughness of the core is retained despite the increase in hardness.


8.2

8.7 Nitriding

a. Surface hardening of steel by nitriding is achieved by heating the component incontact with a nitrogeneous agent. Nascent nitrogen is released at the surfaceof the component and combines with elements in the steel to form nitrides. Thenitriding agent used is ammonia gas which breaks down in the furnace torelease single atoms of nitrogen. The temperatures used are lower than thosefor carburizing and are in the range 500 C565 C.

b. Hardness is achieved by the creation of nitrides: subsequent heat treatment isnot required. The component can be heat treated to obtain the required coreproperties, final machined and then nitrided.

8.8 Ion-Nitriding

a. In this process the steel workpiece is held in a chamber containing nitrogen atvery low pressures (110 mbar). The gas is ionized using the workpiece as thecathode, positive ions bombard the workpiece and raise the surfacetemperatures to that required for nitriding. The process is more controllableand has a higher output than the gas nitriding process.

8.9 Steels for Nitriding

a. Nitriding can be used to surface harden many steels but where a high surfacehardness is required steels of special composition are necessary. The steelsmustcontain those elements such as aluminium, chromium, vanadium, titanium,tungsten and molybdenum, which form hard stable nitrides. Steels of specialcomposition for nitriding are produced and are known as nitro-alloys.

8.10 Carbon-nitriding

a. Carbon-nitriding is amodification of the gas carburizing process by the additionof ammonia gas to the furnace atmosphere. Both carbon and nitrogen arereleased to be absorbed in the surface of theworkpiece. The steelmust beheatedto 800 C875 C for the carbon to be absorbed. At higher temperaturesabsorption of nitrogen is reduced. By control of temperatures and the amountof ammonia gas added the relative absorption of carbon and nitrogen can beregulated.

8.11 Nitriding v Carburizing

a. In addition to the very high hardness that is possible, nitrided steels have theadvantages of being more corrosion resistant with greater resistance to fatiguethan carburized steels. Nitrided steels are also better at elevated temperatures.No quenching is required after nitriding and items can be machine finishedbefore treatment. The disadvantage with nitriding is the high cost of capitalequipment which makes the process economical only where large numbers ofitems have to be treated. Carburizing can produce a much deeper, tougher casebut due to distortion considerable grinding of the hardened surface is necessary.Main propulsion gears in MOD ships are now carburized after earlier failureswith nitrided gears.

8.12 Patented Processes

a. There are several patented processes for surface hardening of steel, some ofwhich confer improved properties, eg Sulphanizing and Tufftriding.

8.13 Surface Coatings

a. The function of surface coatings is to improve the corrosion, heat, or wearresistance of metals. Coatings may be metallic or non-metallic and may be hotdipped, electroplated, sprayed, or produced by chemical action.


8.3

8.14 Hot Dipped Coatings

a. The coatings used in the hot dip process are zinc and tin. These have relativelylow melting points enabling steel products to be dipped in molten baths of thecoatings. The component must be cleaned by degreasing and acid picking priorto dipping. Fluxes are used to assist in obtaining a good bond between coatingand base metal.

(1) Zinc coating, or galvanizing, provides excellent corrosion resistance.Protection is afforded by the oxide surface on the zinc. Scratches or scoreson the coating result in local sacrificial corrosion of the zinc but the basemetal is still protected. Zinc coating is relatively soft and easily abraded.

(2) Tin provides a nontoxic coating which is extensively used in the foodprocessing industry for canning all types of food. It is resistant to foodacids but offers no protection once scratched. Tin-plate is easily solderedand is ideal for the fabrication of containers. It ismore expensive thanzinccoated sheet.

8.15 Electroplated Coatings

a. Components to be plated are made the cathode in an electrolytic cell. Theelectrolyte contains a salt of the metal to be deposited. The anode is sometimesa non-reactive conductor but more often it is made from the metal to bedeposited. By this lattermethod the concentration of themetal in the electrolyteis maintained at the expense of the anode. Degreasing and pickling are anessential forerunner to electroplating. Metals that may be deposited includecopper, nickel, chromium, cadmium, zinc and tin (Def Stans 038, 0310,0319 and 0320).

(1) Copper is mainly used as an undercoat for nickel and chromium plating,and for copper coated wire.

(2) Nickel coatings provide excellent corrosion resistance and are sufficientlyhard not to be easily damaged. Nickel provides a sound base forsubsequent chromium plating. It can also be used for restoring worn steelparts.

(3) Chromium plating provides a very hard, wear resistant finish. It is usedindustrially on gauges, taps, drills, etc and can also be used for restoringworn surfaces. A low temperature heat treatment is required after platingto avoid hydrogen embrittlement. The chromium layer is porous sowherecorrosion resistance is required it must be backed by nickel plating.

(4) Zinc plating is widely used for fasteners and similar articles whereuniformity and control of thickness of film is important and in this respectthe process is preferred to hot-dipping.

(5) Restrictions in the use of cadmium: cadmium and its corrosion productsare a potential health hazard to personnel and contribute to toxicpollution. The need for safe disposal of waste cadmium and its corrosionproducts and effluent from electroplating plants is of considerableimportance. Sea Systems Controllerate policy is that cadmium may onlybe specified and used where there is no acceptable alternative. Cadmiumand cadmium plated components which can be satisfactorily replaced areto be eliminated from new and existing equipment, as stocks of sparecomponents are used up.


8.4

8.16 Electroless Plating

a. Electroless plating involves the deposition of an alloy coating, eg 90% nickel,10% phosphorus, from a chemical solution without the use of an externalelectropotential. Commercial processes are Kanigen and Fescolising (Def Stan035).

8.17 Sherardizing

a. Sherardizing is a process of coating articles with zinc that is similar in manyrespects to carburizing. The articles are slowly tumbled in a drum containingzinc powder heated to 370 C. Thinner and more uniform coatings of zinc arepossible than with hot dip galvanizing which makes the process particularlysuitable for threaded items. Degreasing and pickling are required prior tocoating.

8.18 Calorizing

a. This is an aluminizing process which is also similar to carburizing. The itemsto be coated are tumbled in a mixture of aluminium powder, aluminium oxideand aluminium chloride at 815 C980 C. The resultant coating is heat andcorrosion resistant and is used for coating ferrous turbine blades, furnace partsand similar items and for items in contact with flue gases containing sulphur.

8.19 Ion-Plating

a. The article to be plated is contained in a chamber with argon at very lowpressure, 2N/m2. The argon is ionized by a high dc voltage and the cathode isbombarded by positive argon ions. This has a cleaning effect on the surface ofthe cathode and is known as sputter cleaning or ion scrubbing. The coatingmetal is contained in a tungsten boat in the chamber and is separately heated.In the near vacuum conditions in the chamber the metal vaporizes readily anda small percentage of the vapour is ionized. Positive ions of metal stream to thecathode and plate it.

8.20 Phosphating

a. Phosphate coatings are produced on the surface of steel by dipping, brushing,or spraying with phosphoric acid (Def Stan 0311). The resultant coating isthin and offers only limited protection against corrosion, but its surface is roughand is an excellent key for a subsequent paint system. Commercial processesbased on phosphating are:

(1) Parkerizinguses phosphoric acid plus iron and manganese phosphate.

(2) Bonderizingphosphoric acid plus a catalyst.

8.21 Anodizing

a. Anodizing, or anodic oxidation, is an electrolytic process for providingaluminium with a thick protective oxide film. The article must be chemicallyclean before anodizing. Thorough mechanical cleaning or polishing is followedby degreasing or electrolytic cleaning. The article to be anodized is then madethe anode in an electrolytic cell containing a solution of chromic, sulphuric, oroxalic acid. When the current is flowing oxygen forms on the anode andcombines with the aluminium. The layer of aluminium oxide so formed growsoutward from the surface. The oxide film is spongy and requires sealing beforethe article goes into service. Various sealing treatments are available. Prior tosealing the coating will readily accept a dye if required.


8.5

8.22 Metal Spraying

a. Metal sprayingmethods are defined inDef Stan 036. Awide variety of coatingsare available for restoring worn parts and for improving wear resistance andcorrosion resistance. Other coatings improve heat resistance or provideelectrical conduction or electrical resistance. Metal spraying also has a wideapplication in view of its portability and flexibility. Critical parts of steel shipstructure can be sprayed with zinc or aluminium to give sacrificial protectionfrom corrosion. Wear and corrosion resistant ceramic materials are alsothermally sprayed.

b. All the metals used for coating by hot dipping and electroplating, and manymore besides, can be deposited by metal spraying. Sprayed coatings are porouswhich is an advantage for certain coatings used as oil-lubricated bearings, butin a corrosive environment coatings require sealing.


8.6


9.1

9. TESTING AND QUALITY ASSURANCE

a. Tests used onmetallic materials to ensure that the required properties are beingachieved may be classified as:

(1) Laboratory and type tests used initially when introducing a newmaterialinto service.

(2) Smaller scale test and non-destructive examination, aimed at ensuringthat essential properties are met in production runs.

b. Metals and alloys are required to comply with defined compositions and tomeetmechanical and other type tests. Exceptionally metallurgical examination ofthe microstructure is also required. Chemical analysis to determine that thecomposition is within the required tolerances is the first test in the productionroute.

c. Mechanical and other properties that are tested are:

(1) Strength and ductility.

(2) Hardness.

(3) Notch toughness.

(4) Material cleanness.

(5) Fatigue limit.

(6) Creep.

(7) Corrosion resistance.

(8) Corrosion fatigue limit.

(9) Susceptibility to stress corrosion cracking.

(10) Ease of welding or brazing.

9.1 Strength and Ductility

a. Strength and ductility are measured by the tensile test in accordance with BSEN 10 002.1. The results are influenced by the shape and size of test specimen,the rate of loading under test, and the elastic compliance of the testingmachine.By standardizing tests we arrive at meaningful and comparable results.

b. The tensile test is suitable for laboratory and production use. From it aredetermined:

(1) Ultimate tensile strength.

(2) Yield or proof stress.

(3) Elongation.

(4) Reduction of area.

c. The tensile properties for steel plate are often only determined for one directionin a plate. There are, however, three principal directions affecting properties ina rolled plate, these are:


9.2

(1) Longitudinal or direction of rolling.

(2) Transverse to rolling.

(3) Short transverse, ie, through the thickness.

d. The difference in the tensile test results for longitudinal and transversedirection may not be great and will depend on the degree of cross-rolling of theplate inmanufacture. Most structural steels have a dramatic fall-off in ductilityin the short transverse direction. The fall-off is directly attributable to the steelmaking process and in particular with the degree of inclusion in the steel, ie thematerial cleanness.

e. Production tests will be limited to longitudinal or both longitudinal andtransverse tensile tests. Normally the thicknesses of plates do not lendthemselves to tensile testing on a production basis in the short transversedirection, but on occasions such tests are specified. Small Hounsfield tensilespecimens are one of the tests used for this purpose.

9.2 Hardness

a. Hardness, although specifically a measure of resistance to penetration is also ameasure of resistance to deformation. For any given metal, hardness relates tothe condition of metal eg, caused by heat treatment, and can be used as ameasure that treatments have been correctly carried out. It is also a useful toolin examining the heat affected zones adjacent to welds where high hardnesslevels, ie high strength and low ductility, may be undesirable. There are variousmethods of measuring hardness all of which employ a hardened ball or pyramidand then relate the size of the indentation to a standard scale.

b. Hardness values may indicate the strength of a metal but do not represent theductility and more particularly the notch ductility (notch toughness) of themetal.

FIGURE 9.1 CHARPY V-NOTCH TRANSITION CURVESFOR DIFFERENT STEELS


9.3

9.3 Notch Toughness

a. Notch toughness is a measure of the resistance of a metal to fracture in thepresence of a notch. Brittle fracture is a form of fracture that occurs suddenlyunder a loadwhich is not sufficient to result in general yielding across thewholeof the fractured section. Design concern may be for:

(1) Freedom from brittle fracture under all operating conditions particularlyin extreme cold weather.

(2) Maximum fracture resistance under explosive attack.

b. In warship construction geometric and metallurgical notches and some cra