MONTHLY TECHNICAL ARTICLE

AMCO-TA-110

April 2016

In-Situ Metallography as Reliable

Inspection Technique: Advantages &

Limitations

FOREWORD

AMCO Saudi Arabia is an autonomous and independent Consulting Company with the objectives of best Metallurgical

and Lifting Equipment Services to Saudi Arabian Oil and Gas, Petrochemical, Power Generations, Fertilizers,

Refineries, Manufacturing, Construction, Manufacturing, Defense and Automobile Industries.

Our specialization is: Plant Life Assessment /Extension, Failure Investigation, Asset Integrity Management, Boiler

Inspection, Boiler Tube Condition Assessment, Tube Failure Analysis, RCM Studies, RAM Studies, Single Point of

Failure(SPOF) Studies, Plant Cycling, Cost Analysis, Plant Benchmarking, Crack Assessment, Risk Based

Inspection/Maintenance, Probabilistic Assessment, Fitness-for-Service Assessment, Conditional Assessment, Plan

Reliability Studies, Vibration Analysis, Condition Monitoring, Stress Analysis, Support and guidance in Plant Operation

and Maintenance, Advice in weld repairs, Support with Materials, Inspection and Monitoring; Corrosion and oxidation

issues, Technology Development, Finite Element Analysis, Stress Analysis, P91 Steel Assessment, Metallography,

SEM/EDS Analysis, Contamination Analysis, Plant Mechanical Improvement Studies with years’ experience around the

globe. The AMCO Monthly Article are offered within the following areas:

i. Plant Life Management

ii. Lifting Inspection

iii. Fitness for Service

iv. Risk Based Inspection/ Maintenance

v. Advance Materials

vi. Reliability Engineering

vii. Qualification, Quality and Safety Methodology

viii. Materials Technology

ix. Pipelines and Risers

x. Asset Operation

xi. Quality Control/Assurance

xii. Corrosion and Erosion

xiii. Inspection and NDT testing

xiv. Microstructures and damage mechanisms

xv. Operations and Maintenance

xvi. Vibration and Condition Monitoring

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About the Authors

Engr. Mohammad Hussain Turi

Lead strategist and authority figure in charge of asset integrity & reliability

management, life assessment, inspection activities, failure investigation,

risk management, and continuous process improvement studies for plant

operators. In order to achieve client requirements, my team works on

undertaking improvement initiatives in terms of technology, strategies, and

other initiatives with regards to material selection, RBI, corrosion

monitoring and control activities amongst others in the Operations and

maintenance side of business. Mentor, direct and lead a team of 50 direct and indirect reports.

Have a deep understanding of mechanical issues in a plant and can discuss and agree on

solutions to prevent re-occurrence in the future. He has actively participated in ongoing

improvement of equipment health and extension of component lives through the use of

engineered solutions, current and developing technologies and integrated computerized

maintenance systems

Engr. Owais Manzoor Malik

Engr. Owais Manzoor is a graduate Engineer in Metallurgy & Materials,

encompassing more than 6 years of experience. He has served in upstream

and downstream sector (oil & gas) in Pakistan and Middle East. Owais

specializes in microstructural characterization of materials, Lab analysis,

damage assessment and corrosion failure investigations. He possesses

substantial and demonstrable Oil & Gas materials knowledge and intrinsic

expertise including macro and micro examination and fractographic

analysis. He has performed several failure analysis of the in service components in

Petrochemical, Power generation and Oil and Gas sector. His interest lies in determining the root

cause of the failed components with special emphasis on their metallurgical properties.

He has hands-on experience in performing innumerable site (In-situ) metallography to evaluate

nondestructively the conformity of microstructure of equipments. The ability to determine the

detrimental phases in the microstructural analysis of carbon steels, duplex stainless steels and

Inconel material are his trade mark. He has been the part of corrosion monitoring and testing

team, working in the field and Laboratory. These technical traits enable him to conduct the

fitness for service studies of varied equipment.

In-Situ Metallography as reliable inspection technique

INTRODUCTION

Non-destructive metallography of surfaces makes it possible to analyses a material microstructure and a

surface condition as well as various surface damages occurring due to overloading or improper

tribological circumstances. Machine or tool parts inspected are not damaged during the surface

preparation. It is also required that the replica applied is strong and elastic enough not to get damaged

when removed. As the replica does not damage the machine part and does not chemically affect its

condition, the machine part can further operate if the inspection performed confirms its quality. The way

of preparing the machine part, i.e., the area to be inspected depends on the requirements or objectives of

the examination to be performed and on the accessibility of the area to be inspected. The inspection of the

surface condition or microstructure can be performed in two ways.

AMCO’s Portable grinder, Polisher and microscope for onsite analysis

If the part to be examined and its environment provide enough room for the surface

preparation and observation, then the surface inspection can be performed directly with

an optical microscope. For this purpose adapted optical microscopes are available.

In the opposite case, a replica is produced, which is then observed with a light or

electronic microscope in a laboratory.

Replication Process

Plastic replication is used principally for reproducing surface features such as creep cavities, cracks, and

microstructure features. It involves placing a coating of a resin on the prepared surface to be examined,

which, after hardening, is backed with a softened cellulose acetate. The resulting film can then be stripped

off and examined by a scanning electron microscope combined with energy-dispersive x-ray analysis.

The general principles of the technique are depicted in Fig below. The specimen is etched to highlight the

particles of interest in relief on the surface. A carbon coating is applied, and the replica is then stripped

off, perhaps using a second etch, carrying with it many of the second-phase particles. The use of a

suitable etchant will preserve the number, shape, and distribution of particles in the replica.

A metallurgical specimen is generally first polished flat to facilitate lifting the replica, then etched. The

chemical or electrolytic etchant selected should remove the matrix but not attack any particles of interest.

The etched layer should be shallow so that particles are exposed but not removed. Metallurgical

considerations dictate etchant choice; a standard textbook should be consulted. After etching, the

specimens are washed and dried. Replication should begin as soon as possible to avoid the deposition of

airborne dust, which would contaminate the replica.

Replication Process [2]

The surface preparation is similar to that used in the general

metallographic analysis with the optical microscope. In case the

machine part has already been fine-ground or polished because of

functional requirements, it is needed to etch it for the microstructure

analysis. It is different, however, with the analysis of the surface

condition due to tribological conditions, in which case the surface

shall only be cleaned and a replica shall be made for subsequent

observation.

A small piece of cellulose acetate, 3 to 5mm thick and approximately

three times the size of the area to be replicated, is held by tweezers

and softened on one side only. Apply acetone with an eyedropper,

allow setting for approximately 30 seconds, and then shake to remove

excess acetone.

Polished and Etched surface

Surface

The soften side is then pressed firmly against the area with the eraser end of a pencil or tip of the fingers

and helped several seconds. Care must be taken that the acetate is not allowed to slip or a smudged replica

will be obtained. Let the replica remain in this position until it hardens, usually 10 to 15 minutes,

depending upon the size of the replica. Once it has hardened it can be stripped with tweezers. At this point

the stripped replica should be put into a separate container (usually a small Plastic box, or taped to a glass

slide with double-sided tape. Several Replicas should be made of each area and should be labeled

appropriately

For an analysis of topographical features of the surface of a machine or tool part the surface shall be

thoroughly cleaned, without any preliminary mechanical treatment. The preparation of both simple and

exacting surfaces may be made easier with the application of admissible chemical media for the

elimination of color, grease, and other impurities.

One finds commercially available various types of replicas made of different materials. The materials

available are practical for application and permit the production of a replica of the surface concerned in a

few minutes.

Applications

It has been proven that In-Situ Metallography (REPLICAS) is a very powerful technique for addressing

many metallurgical problems, particularly when conducting failure investigations. There are many

applications for the use of this technique. It can be employed as a non-destructive method for identifying

materials and their microstructures, without having to remove or cut out samples for chemical analysis

i.e., to distinguish between cast iron, cast steel and any other type of metallic material etc. The technique

can also be employed as a quality control tool, in order to verify whether the heat treatment of various

components or equipment is in the normalized, annealed, or quenched and tempered condition.

The identification and sizing of surface defects or cracks in the Parent Material, HAZ and

the Weld Deposit.

To establish the cracking characteristics / morphology of the components and of welds.

Example: Cracked Turbine Casings.

The microscopic assessment of the microstructural degradation of high temperature

materials in service i.e. establishing the micro voiding and the creep status of the

components / equipment material and the welds.

To characterise the present microstructural condition of a material that has operated at

high temperatures, short term or long term overheating can also be identified.

To identify and characterise cracks in-situ - prior to them being ground, excavated or cut

out.

To establish the soundness of material prior to repair welding. (Creep Exposed/Exhausted

Material).

To identify the degree of graphitisation on carbon steels normally used in the older

industrial boilers and plants, which are still operating.

The determination and possible quantification of retained austenite, in hardenable

materials.

It is a non-destructive and very cost effective technique. No samples are cut out.

It is employed for the microscopic examination of the microstructures of large castings

and forgings.

It is quick and results can be made available within hours of taking the replicas. This trait

has thus made this technique very attractive, especially during plant shut down’s and

failures.

Damage discernible through replica metallography

Spheroidization is a change in the microstructure of steels after exposure in the (440°C to 760°C) range,

where the carbide phases in carbon steels are unstable and may agglomerate from their normal plate-like

form to a spheroidal form, or from small, finely dispersed carbides in low alloy steels like 1Cr-0.5Mo to

large agglomerated carbides. Spheroidization may cause a loss in strength and/or creep resistance.

(a) Unaffected microstructure (b) Spherodization of the pearlite phase in carbon steel [2]

Creep and Stress Rupture

At high temperatures, metal components can slowly and continuously deform under load below the yield stress. This time dependent deformation of stressed components is known as creep. Deformation leads to damage that may eventually lead to a rupture. Creep voids typically show up at the grain boundaries and in later stages form fissures and then cracks.

Creep Failure of Heater Tube [3]

Carburization

Carbon is absorbed into a material at elevated temperature while in contact with a carbonaceous material

or carburizing environment.

Carburized condition [4]

Carburization can result in the loss of high temperature creep ductility, loss of ambient temperature

mechanical properties (specifically toughness/ductility), loss of weldability, and corrosion resistance [3]

Sensitization

Stainless steels rely on their chromium content to prevent corrosion. Generally, about 12% Cr is needed

for this task. One example of unintentional microstructural alteration that has serious consequences for a

stainless steel is the result of exposure to temperatures in the range of 425 to 870 °C (800 to 1600 °F).

During exposure, chromium carbides form at the grain boundaries and deplete the regions near the

boundaries of chromium. The longer the exposure, the greater the depletion of chromium, until eventually

the level drops below 12% locally, and corrosion along grain boundaries can result. The process of

chromium loss is called sensitization.

Microstructure of an AISI 316 stainless steel showing severe sensitization [5]

Micro-crack Formation.

Replica Metallography can be a useful technique to detect the presence of micro crack which are not

evident under visual inspection. The limitation of the replica technique makes the technique applicable for

the detection of only surface initiated micro cracks. Therefore other NDE techniques must be utilized

along to give the clear picture of the cracks depths and sizes.

Intergranular micro cracks in SS 304 clad material

Sigma Phase Embrittlement

The phenomenon of embrittlement in austenitic stainless steel welds exposed to high temperature is

accelerated by the presence of delta ferrite. The composition of the filler material must be optimised to

ensure that there is some delta ferrite present in the weld metal (typically >3%).

However, delta ferrite transforms to intermetallic phases, notably sigma phase, faster than austenite either

during high temperature service or during postweld heat treatment (PWHT). Sigma phase is an

intermetallic with an approximate chemical formula FeCr and, as with most intermetallics, it is very

brittle and hence has a deleterious effect upon mechanical properties. It has been shown that, for a variety

of iron-chromium-nickel alloys, Charpy toughness drops off exponentially with increasing sigma phase

content.[1] The more delta ferrite a nominally austenitic stainless steel has, the more susceptible it will be

to sigma phase formation. To avoid significant embrittlement it is typically desirable to limit the delta

ferrite content in the original microstructure to below 10%.

Sigma phase embrittlement is a metallurgical change that is not readily apparent, and can only be

confirmed through metallographic examination and impact testing [6].

Sigma Phase in SS 312 Weld after high temperature ageing

Limitations

The replication process involves extraction of the duplicated microstructure from the

component’s surface. The examination of the microstructure and coming to precise conclusion

during analyses part is equally important. Therefore high level of expertise is required from

Metallurgists performing the analysis.

The technique is limited to analysis of surface damages. Other techniques like UT, RT and

utilized in combination to provide the clear picture of the subsurface damage.

References:

1. Atlas of Microstructures of Industrial Alloys, Vol 7, Metals Handbook, 8th ed., American Society for Metals, 1972.

2. The Heat Treaters Guide—Standard Practices and Procedures for Steel, American Society for

Metals, 1982 Heat Treating, Vol 4, ASM Handbook, ASM International, 1991

3. API 571 , Damage Mechanisms affecting fixed Equipments in refinery (2011)

4. G. Krauss, Principles of Heat Treatment, 2nd ed., ASM International, 1993

5. Properties and Selection: Irons, Steels, and High-Performance Alloys, Vol 1, ASM Handbook, ASM International, 1990

6. L.E. Samuels, Optical Microscopy of Carbon Steels, American Society for Metals, 1980

7. H. Thielsch, Defects and Failures in Pressure Vessels and Piping, Reinhold Publishing, 1965

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