SPRINGBACK BEHAVIOR AND SURFACE QUALITY OF AUSTENITIC

STAINLESS STEEL PLATE DURING BENDING PROCESS

MOHD SHAHRIMAN BIN SUTAN

A project report submitted in partial fulfilment of

the requirements for the award of the degree of Master of

Engineering (Mechanical - Advanced Manufacturing Technology)

Faculty of Mechanical Engineering

Universiti Teknologi Malaysia

JUNE 2014

iv

To my beloved mother and father

Hj. Sutan B. Hj.Abd Hamid

Hjh. Kasmah Bt. Ismail

My beloved wife & daughter

Azrina Bt. Zakaria

Airis Zahrah Soffiya Bt. Mohd Shahriman

v

ACKNOWLEDGEMENT

First and foremost, I would like thank Allah S.W.T for the blessing and the

strength he gave me complete this thesis. Zillions of thanks to the my respected

supervisor, Prof. Dr. Safian Sharif for his guidance, advise, critics and input throughtout

the process of completing this thesis. He never accepted less than my best efforts. His

patience and encouragement has opened up my eyes to make him as my inspiration. Haji

Shamsudin Man from GMI for being the co-supervisor and also for his

encouragement, guidance, advices and motivation. Without their continued support

and interest, this thesis would not been a success.

To my beloved family escpecially my wife, words alone cannot express what I

owe them for their encouragement and whose endless love and support have made me

strong throughtout the years of my study.

I would like also to express my sincere thanks to Majlis Amanah Rakyat

(MARA) for funding my study and Director of KKTMBP for her undivided

encouragement and consideration. My superiors Mr Azmin Ariffin, Mr. Halimudin

Isa and Mr. Asmar Suid in KKTM Balik Pulau should also be recognised for their

supports. My sincere appreciation extends to all my colleages and others who have

provided assistance at various occasions. Their views and advises are useful

indeed.Technicians at UiTM Permatang Pauh and Malaysian-Spanish Institute also

deserve special thanks for their assistance in supplying the relevant technical support

and literatures. Unfortunately, it is not possible to list all of them in this limited

space. I am also indebted to those who had helped me either directly or indirectly.

Finally I am really grateful to Universiti Teknologi Malaysia , especially to Faculty

of Mechanical Engineering for providing me with all the knowledge until I

completed this thesis.

vi

ABSTRACT

This project presents the design development and fabrication of bending dies

and spring-back in behaviour and surface quality bending process on a Austenitic

Stainless Steel 316 plate by means of stamping machine. A bending die had been

designed in 2D and 3D using AutoCad and Solidwork. In order to find spring-back

in bending, a ‘‘V’’ shaped die is designed and fabricated. The aims of this study is to

investigate how much can 316L stainless steel sheet resile at various angle and to

bring forward spring-back graphic to field of knowledge. Spring-back is a common

phenomenon in sheet metal forming, caused by the elastic relocation of the internal

stresses during unloading. It has been recognized that spring-back is essential for

the design of tools used in sheet metal forming operations. Therefore, in this

study the subject of bending dies and spring-back in bending process has been

investigated rolling direction is one of the factors to control the spring-back. In this

study two directions (along & across bending) of the rolling direction were analyzed

using optical microscope to check the surface quality. A bending die was fabricated

using standard procedure and machining. The special design 316L plates were used

as the strip and stamped using Press Machine. Most of the components were

fabricated using the various facilities at KKTM Balik Pulau, Pulau Pinang. Hardened

Steel punches with various angles (60°, 90° and 110°) were used. The upper surface

bending was collected and analysed using microscope. The minimum angle and

surface quality of the plates were then recorded.

vii

ABSTRACT

Projek ini membentangkan acuan tekan dan ‘spring-back’ dalam proses

lenturan ke atas plat ‘Austenitic Stainless Steel 316’ dengan cara menggunakan

mesin tekan. Perbezaan penumbuk keluli keras di mana saiz ketebalan besi 316L

yang digunakan adalah 1 mm. Lukisan 2D dan 3D acuan lenturan telah direka

menggunakan perisian AutoCad dan Solidwork. Untuk mendapatkan ‘spring-back’

di dalam lenturan, acuan berbentuk, ‘‘V’’ telah direka dengan tujuan untuk

mengetahui bahan-bahan logam lembaran keluli boleh berubah dalam pelbagai

sudut. ‘Spring-back’ adalah fenomena biasa dalam lembaran logam membentuk,

berpunca daripada daya dalaman yang dialihkan. Ianya telah diiktiraf bahawa

‘spring-back’ adalah perlu untuk digunakan dalam lembaran logam bagi operasi

membentuk. Oleh itu dalam kajian ini subjek acuan lenturan dan ‘spring-back’

dalam proses melengkung telah dikaji. Arah gelekan merupakan salah satu factor

untuk mengawal ‘spring-back’. Dalam kajian ini dua arah iaitu sepanjang lenturan

dan melalui lenturan telah dianalisis menggunakan mikroskop untuk memeriksa

kualiti permukaan plat tersebut. Dalam proses ini, acuan lenturan akan dihasilkan

mengikut prosedur standard dan pemesinan. Plat 316L dengan reka bentuk khas

telah digunakan sebagai kepingan dan ditekan menggunakan mesin penekan.

Kebanyakan komponen telah dihasilkan menggunakan pelbagai mesin dan alatan

yang terdapat di KKTM Balik Pulau, Pulau Pinang. Penumbuk keluli keras

menggunakan tiga sudut iaitu (60°, 90° and 110°) telah digunakan. Lentur

permukaan atas dikumpulkan dan dianalisis menggunakan optical mikroskop. Sudut

dan permukaan minimum kualiti plat kemudiannya direkodkan.

viii

TABLE OF CONTENT

CHAPTER TITLE PAGE

DECLARATION OF SUPERVISOR ii

DEDICATION OF AUTHOR iii

DEDICATION iv

ACKNOWLEDGEMENTS v

ABSTRACT vi

ABSTRAK vii

TABLE OF CONTENT viii

LIST OF TABLES xii

LIST OF FIGURES xiv

LIST OF SYMBOLS xvii

LIST OF APPENDICES xviii

1 INTRODUCTION

1.1 Project Background 1

1.2 Problem Statement 3

1.3 Research Question 4

1.4 Project Objective 4

1.5 Project Scope 5

1.6 Significance of findings 5

ix

2 LITERATURE REVIEW

2.1 Stamping Press 6

2.1.1 Hydraulic presses 7

2.2 Basic of Bending 7

2.2.1 Bending 8

2.2.2 Type of Bending 10

2.2.2.1 Air Bending 10

2.2.2.2 Bottoming 11

2.2.2.3 Coining 12

2.2.2.4 V Bending 12

2.2.2.5 U Die Bending 13

2.2.2.6 Wiping Die Bending 13

2.2.2.8 Rotary Bending 14

2.3 Bend Allowance 15

2.4 Bending stress 17

2.5 Spring-back 19

2.6 Spring-back Compensation Methods 21

2.6.3 Calculation Spring-back 24

2.6.4 Factors Influencing Spring-back 26

2.7 Effect of Bending in Rolling Direction 26

2.7.1 Bend across Rolling Direction 28

2.7.2 Bend along Rolling Direction 28

2.7.3 The Importance of Plain Direction 29

2.8 Bone Plate 29

2.8.1 Type of Material 30

2.8.1.1 Metals 30

2.8.1.2 Polycaprolactone - (PCL) 31

2.9 Stainless Steel 32

2.9.1 Austenitic 34

2.9.2 Ferritic 35

2.9.3 Martensitic 36

2.10 Stainless/Austenitic Steel 316L (AISI 31600) 38

2.11 Physical Properties 39

x

2.12 Mechanical Properties of 316L 39

3 RESEARCH METHODOLOGY

3.1 Bending Die Design 40

3.2 Die Fabrication & Machining 42

3.3 Experimental Setup 46

3.4 Experimental procedure and data collection 47

3.5 Analysis of Surface Quality On Bending Surface 48

3.6 Die fabrication and materials used 49

3.7 Materials and fabrication 50

3.8 Different Plain Direction Strip 56

3.9 Experimental procedure and data collection 57

3.9.1 Dependent parameters 57

3.9.2 Surface Quality analysis on bending angle 58

4 RESULT AND DISCUSSION

4.1 Introduction 61

4.2 Complete Drawing of 2D And 3D Bending Die

Design 61

4.3 Material Preparation & Cost 64

4.4 Force Calculation 66

4.5 Material properties of strip 67

4.6 Die Fabrication and Experiment 67

4.7 Experiment Result and Discussion 69

4.7.1 Experiment 1 for Bending Angle vs

Spring-back in Bend Across Direction 71

4.7.2 Experiment 2 for Bending Angle vs

Spring-back in Bend Along Direction 76

4.8 Analysis Result of the Rolling Direction 81

4.9 Comparison Along Band Vs. Across Bend

Rolling Direction 89

xi

5 CONCLUSIONS 91

6 RECOMMENDATION 92

REFERENCES 93

APPENDICES 96

xii

LIST OF TABLES

TABLE NO. TITLE PAGE

2.1 Comparison of Mechanical Property 32

2.2 (Part A) Comparative Properties of Stainless

Steel Families 37

2.3 (Part B) Comparative Properties of Stainless

Steel Families 37

2.4 Physical Properties of SS 316L 39

2.5 Mechanical Properties of 316L Stainless Steels 39

3.1 CNC Milling Machine Specification (DMU 50) 43

3.2 Okamoto Grinding Surface Specification 44

3.3 EDM Wire Cut Specification 45

4.1 Bill of material bending set 64

4.2 Total Cost for Material for Die Set 65

4.3 Material SS 316L Properties 67

4.4 Experiment Result For Bending Angle Across

Direction 71

4.5 Experiment Result For Bending Angle Along

Direction 76

4.6 Experiment Result For Different Material 82

4.7(a) Result Surface Quality Along Bend With Dwell

Time 5 Sec (100x mag) 81

4.7(b) Result Surface Quality Along Bend With Dwell

Time 10 Sec (100x mag) 82

4.7(c) Result Surface Quality Along Bend With Dwell

Time 15 Sec (100x mag) 83

xiii

4.7(d) Result Surface Quality Along Bend With Dwell

Time 15 Sec (400x mag) 84

4.8(a) Result Surface Quality Across Bend With Dwell

Time 5 Sec (400x mag) 85

4.8(b) Result Surface QualityAcross Bend With Dwell

Time 10 Sec (400x mag) 86

4.8(c) Result Surface QualityAcross Bend With Dwell

Time 15 Sec (400x mag) 87

xiv

LIST OF FIGURES

FIGURE NO. TITLE PAGE

1.1 Bone Plate 2

2.1 Bending Diagrams 9

2.2 Air Bending 11

2.3 Bottoming Bend 11

2.4 Coining Bending 12

2.5 V-Bending 12

2.6 U-Bending 13

2.7 Wiping Die Bending 13

2.8 Double Die Bending 14

2.9 Rotary Bending 15

2.10 Neutral Axes 15

2.11 Bend Allowance & Bend Deduction 17

2.12 Bending Stresses 18

2.13 Bending 18

2.14 Spring-Back 20

2.15 Spring-Back Phenomena 22

2.16 Stress-Strain Diagram 23

2.17 Molecular Displacement Caused By Bending 24

2.18 Rolling Direction at Strip SS 316L before Bending 27

2.19 Bend across Rolling Direction 27

2.20 Bend along Rolling Direction 28

2.21 Type Bone Plates 29

2.22 Families of Stainless 33

2.23 The Austenitic Stainless Steels 34

xv

2.24 Families of Ferrite Stainless Steels 35

2.25. The Families of Martensitic Stainless Steels 36

3.1 Process flow chart of project implementation 41

3.2 CNC Milling Machine (Heidenhain DMU 50) 42

3.3 Grinding Machine (OkamotoACC-16 32DX) 43

3.4 EDM Wire Cut Machine (Fanuc a-1iC) 44

3.5 Hydraulic Stamping Machines 46

3.6 Raw material for the bending die assembly 49

3.7 Surface Grinding Operation 51

3.8 EDM Wire Cut Operation 51

3.9 CNC Milling Machine Setting (3 Axes) 52

3.10 Drilling Operation at Top Plate & Lower Plate 52

3.11 Shear Cutters For Cutting Strip Material 53

3.12 Strip Plate by Rolling Direction 53

3.13 Tapping Process 54

3.14 Lower & Upper Plate Bending Die Set 54

3.15 Complete Die Set 55

3.16 (A) And (B) The effect elongated inclusion

(stringers) on cracking in the sheets as a function

of the direction of bending with respect to the

original rolling direction. 56

3.17 Strip Plate Different Rolling Direction 56

3.18 (A) Rolling Direction vs. (B) Bending Angle 57

3.19 Mitutoyo Profile Projectors 58

3.20 Reading Angle & DRO 59

3.21 Strip Product 60°, 90° & 110° 59

3.22 High Euromac Camscope (MSI) 60

4.1 2D Drawing Bending Die (Autocad) 62

4.2 3D Drawing Bending Die (Solidwork) 63

4.3 Complete Assembled Bending Die Set 68

4.4 Spring-back for bending across direction (5sec) 72

4.5 Spring-back for bending across direction (10sec) 73

4.6 Spring-back for bending across direction (55sec) 74

xvi

4.7 Comparison of spring-back vs bending angle at

different dweel time in across direction 75

4.8 Spring-back for bending along direction (5sec) 77

4.9 Spring-back for bending along direction (10sec) 78

4.10 Spring-back for bending along direction (15sec) 80

4.11 Comparison Bending Vs Spring-Back In Bend Along

Direction (15sec) 79

4.12 Rolling Direction Vs. Springback Angle For Dwell

Time 5 Sec 89

4.13 Rolling Direction Vs. Springback Angle For Dwell

Time 10 Sec 90

4.14 Rolling Direction Vs. Springback Angle For Dwell

Time 15 Sec 90

xvii

LIST OF SYMBOLS

D, d - diameter

F - Force

g - Gravity = 9.81 m/s

I - Moment of Inertia

l - Length

m - Mass

N - Rotational velocity

P - Pressure

Q - Volumetric flow-rate

r - Radius

T - Torque

V - Velocity

w - Angular velocity

x - Displacement

z - Height

θ - Angle

ρ - Density

αf - Final sheet angle

Rf – Radius

αi – Die angle

Ri – Die Radius

t – Thickness

TS – Tensile Strength

xviii

LIST OF APPENDICES

APPENDIX TITLE PAGE

1 Stamping Process 96

2 Open & Close Die 97

3 Gantt chart Project Proposal 98

4 Gantt Chart Final Master Project 99

5 2D Drawing Bending Die 100

6 Isometric 3D Drawing Bending Die 101

7 Bill of Material 3D Drawing Bending Die 102

8 Exploded View 3D Drawing Bending Die 103

1

CHAPTER 1

INTRODUCTION

1.1 Project Background

Sheet metal stamping plays a major role in many industries today. Sheet

metals have a wide range of application in industry and commonly used for

automobiles, household goods, electronics and medical devices (Ozgu Tekaslan et al.

2008). Spring-back can be minimized by means of die design however it cannot be

eliminated totally. To reduce spring-back, compression to minimum is one of the

most important problems in die design (Ozgu Tekaslan et al. 2006). As part

components get smaller and tolerances get tighter, the dimensional accuracy of a

stamped part becomes a crucial factor in determining the overall quality of the part.

In most, if not all, sheet metal forming processes, spring-back is the major problem

faced (Y.E. Ling et al. 2005).

Stainless steel 316L (SS316L) is widely use in various products and is the

common material for the manufacture especially implant bone plate because of

corrosion resistance, heat resistance, heat treatment, welding and machining. Bone

plates (Figures 1.1) are surgical tools, which are used to assist in the healing of

broken and fractured bones.

2

Figures 1.1 Bone plate (www.gpc-medical.com)

At present, the bone plates are produced by CNC Milling machining,

whereby several jigs, fixture and cutting processes are used to produce the bone

plates. In order to minimize the machining time and cost, stamping process is much

faster to produce the bone plates compared to milling and drilling.

Before that, an investigation must be made to observe and verify that

stamping processes are suitable for SS316L in terms of its upper surface & back

surface, and in order to minimize the spring-back amount after stamping processes.

3

1.2 Problem Statement

Some of the issues related to bending or stamping process are highlighted as

follows:

1. What is the optimum bending angle and dwell time setting to minimize

spring-back issue in achieving the desired angle?

2. The spring-back problems such as increased tolerances and variability in the

subsequent forming operations, in assembly, and in the final part. These

effects typically degrade the appearance and quality of the products being

manufactured - Robert H. Wagoner et.al. (2013)

3. Data from previous studies on bending especially on material type 316L is

still limited. Mostly used material type include Stainless Steel, Brass, Cooper,

Galvanized & Aluminium – Zafer Tekıner (2003)

Recently, in medical industry many companies are trying to form the bone

plate in single stamping operation while keeping the material costs and the scrap

down by using 316L plate. The driver for usage is 316L in the medical industry

results from customers demand, high accident rate, lower price than titanium plat and

effective production.

Spring-back causes the following problems in sheet-metal forming:

1) The assembly of the sheet metal components becomes problematic

thereby increasing the assembly time and reducing the productivity.

2) Rolling direction affects the spring-back as the strength of the sheet-metal

is different in various directions i.e. 316L sheet metal.

3) In medical industry different punch corner radius is used for different

bending operations which in turn affects the spring-back in components.

4) A wide range of thickness are used in sheet-metal components which again

affects the spring-back.

4

However, spring-back characteristic of SS316L has not been investigated

widely and very little information is available about its behaviour during bending

operations.

2.12 Research Question

1. What is the minimum angle after done bending process?

2. What is the measured angle for the spring-back after bending process?

3. Are there any crack occurs on that angle?

1.4 Project Objective

In the view of above mentioned facts the spring-back has been analysed in V-

bending process with the following objectives:

1. To design and fabricate bending dies with various angle when bending SS

316L.

2. To study the effect of bending parameters on the spring-back and bending

rolling direction of SS 316L plate.

5

1.5 Project Scope

1. Design bending die using AutoCAD and Solid Work.

2. Stamping process (bending) using Hydraulic Press Machine will be

employed.

3. To studied spring-back at angle 60°, 90°& 110° and dwell time (5s, 10s

& 15s) in bending process.

4. Austenitic Stainless Steel 316L plate will be used as strip material

(0.5mm).

5. Harden Steel punches will be used as bending punches.

6. To analyse bending plate based on along and across the rolling direction

using optical microscope.

1.6 Significance of findings

The significance of this project is to find out whether sheet metal stamping

(bending) can be applied to a Austenitic Stainless Steel 316L plate with the desired

quality to be used in the bone plates manufacturing industry.

93

REFERENCES

A.Albut,G.Brabie. (2006). The Influence Of The Rolling Direction Of The Joined

Steel Sheet on the Springback Intensity in the case of angle-shape Parts Made

From Tailor Welded Strips. Archieves of Civil And Mechanichal Engineering

Vol 6 Issue 3 , 5-12.

Ayers, R. (1984). SHAPESET: A Process to Reduce Sidewall Curl Spring-back in

High Strength Steel Rails. Journal of Applied Metalworking 3 , 127.

Chuantao Wang,Gary Kinzel And Taylan Altan. (1193). Mathematical Modeling of

Plane-strain Bending of Sheet and Plate. Journal of Material Processing

Technology , 279-304.

Dae-Kwei. Leu. (1997). A Simplified Approach for evaluating Bendability and

spring-back in Plastic Bending of Anisotropic Sheets. Journal of Material

Processing Technology 66 , 9-17.

Semig Benli, Sami Aksoy, Hasan Havitcioglu, Mumin Kucuk (2008) Evaluation of

bone plate with low-stiffness material in terms of stress distribution. Journal

of Biomechanic 41, 3229-3235

Sun P ,Ferreira JA, Gracio JJ. (2006). Close loop control of a hydraulic press for

springback analysis. Journal Material Process Technology 177 , 377-381.

Gardiner.F. (1957). The Springback of metals. Transaction of the ASME 77 , 1-9.

Johnson.W. (1981). Spring-back after the biaxial elastic-plastic pure bending of

rectangular plate-I. International Journal of Mechanical Sciences 23 , 619-

630.

94

Li X, Yang Y, Wang Y, Bao J, Shunping L. (2002). Effect of the Material Hardening

Mode on the Spring-back Simulation Accuracy of V-Free Bending. Journal

Material Process Technology 123 , 209-211.

Liu, Y. (1988). The Effect of Restraining Force on Shape Deviations in Flanged

Channels. Jornal of Material Technology 110 , 389.

M. G. (2009). Formability and Bend Testing. Parkland Blvd: Inc, Brush Welman.

Ostergaard, D. (2001). Basic Diemaking. New-York,New York Colombus,Ohio

Woodland,California Peoria,Illinios: Glencoe/McGraw-Hill.

Ozgu Tekaslan, Ulvi Seker and Ahmet Ozdemir. (2006). Determining springback

amount of steel sheet metal has 0.5 mm thickness in bending dies. Material

and Design 27 , 55-61.

Ozgur Tekaslan,Nedim Gerger,Ulvi Seker. (2008). Determination od springback of

stainless steel sheet metal in "V" bending dies,. Material and design 29 ,

1043-1050.

R.Hill. (1950). The mathematical Theory of Plasticity. Oxford London.

Rosochwski. (2001). Die Compensation Procedure to Negate Die Deflection and

Component Spring-back. Journal of Material Processing Technology 115 ,

187-191.

Tekiner, Z. (2004). An Experimental Study on the Examination of Springback of

Sheet Metal with Several Thickness and Properties in Bending Dies. Journal

of Material Processing Technology 145 , 109-117.

VK.Ganesh. (2005). Biomechanics of bone-fracture fixation by stiffness-graded

plates in comparison with stainless-steel plate. Biomed Eng Online.

Wenner, A. L. (1983). On Work Hardening and Spring-back in Plane Straining Draw

Forming. Journal of Applied Metalworking 2 , 277.

Woo, D. (1959). The Engineer 208 , 135.

95

Y.C.Chen,H.Fuji,Y.Kitagawa. (2012). Banded structure and its distribution in

friction stir processing of 316L austenitic stainless steel. Journal of Nuclear

Material 420 , 497-500.

Y.E.Ling, H.P.Lee and B.T.Cheok. (2005). Finite element analysis of springback in

L-Bending of sheet metal. Material Processing Technology 168 , 296-302.

GMI Syllabus Note Book, 2014

http://web.mit.edu/3.082/www/team1s/background/materials.html (June 2014)

http://classes.engr.oregonstate.edu/mime/winter2010/ie337001/Laboratories/7.Met

al%20Forming_bending-1.pdf (June 2014)

http://www.ciri.org.nz/bendworks/bending.pdf (June 2014)

http://www.custompartnet.com/wu/sheet-metal-forming#bending (June 2014)

http://www.pa-international.com/ (June 2014)

http://www.aws.org/w/a/wj/1998/11/kotecki (June 2014)

http://www.azom.com/article.aspx?ArticleID=470 (June 2014)

http://www.castlemetalsuk.com/stainless-steel/grade-316l-uns-s31600-1-4404-sheet-

plate-bar/ (June 2014)

http://homepage.ntlworld.com/jeff.burrill/ABC_of_bending/Three%20Kinds%20of%

20Bending%20Explained.htm (June 2014)

http://sheetmetal.me/category/sheetmetal/page/5/ (June 2014)

http://www.schwebel.com/userfiles/files/Fractures%281024%29.pdf (June 2014)

http://www.gpc-medical.com/ (June 2014)

http://www.thefabricator.com/article/bending/bending-basics-the-fundamentals-of-

heavy-bending (June 2014)