Failure Analysis: MidTest - Marking Schema

A process engineer responsible for the operation of petrochemical plant noticed a significant pressure drop which indicates that the boiler tubes might have experienced some damage problem. Primary witness report by inspection engineers found that the water content in steam tanks was reduced by as much as 15%. There are the some pitting marks were observed at tensioned side at the bend area of the boiler tube (refer Figure 1) results the vaporizing out of water vapour. The tube was then removed and sends to your consultancy firm for further investigation.

To initiate the failure investigation, you have to make a site visit to that petrochemical plant to get some background information. Identify the important information needed from plant engineers that helps you getting some primary clues. [30 marks]

1. Location, 2. name, 3. Part No, 4. Description, 5. Manufacturer or fabricator 6. Function of item 7. Maintenance procedures 8. Maintenance history 9. Cleaning procedures 10.Cleaning history 11.Operational procedures (temp./ pressure etc)

12. Type of materials (Chemical composition) 13. Operational documentation (standard, temp./ pressure etc) 14. Service deviations 15. Normal stress orientations 16. Extent of incident 17. Precipitating events 18. Drawings 19. Photographs 20. Previous inspections reports, 21. Opinions of related personnel

ii) Considering chemical composition of boiler tube as shown in Table 1,

a) What your hypothesis regarding the root cause of the tube failure [25 marks]

1. Residual stress resulted from bending works 2. Corrosion attacks originate from water, believed resulted from application of untreated water 3. Lower content of Mo (lower pitting index no.) contributes to pitting corrosion 4. All of the above factors contribute to the stress corrosion cracking (SCC).

b) Calculate the tube pitting index number [20 marks]

Pitting Index (PI) = wt.% Cr + (3.3)(wt.% Mo) = 17.585 + 3.3(0.396) = 18.89

c) Recommend the new materials to prevent the same future failure. [25 marks]

Further upgrades are out of the realm of the stainless steels and into high-alloys steel. Eg Pitting resistance metals such as Ø high-molybdenum SS Ø Incoloys, Ø Inconels, or Ø Hastelloys

Replica-Q14

What is the qualitative thermal degradation indexes considered in metallographic analysis?

• Tendency to pearlite/ bainite spheroidisation

• Coarsening of precipitates in the ferritic matrix and at grain boundaries,

• Broadening of denuded zones (no precipitates) along grain boundaries.

Replica-Q13

What is Creep or creed deformation?

Creep is a phenomenom of a sample to deform against time at particular temperature. Creep is the tendency of a solid material to slowly move or deform permanently under the influence of stresses. It occurs as a result of long term exposure to levels of stress that are below the yield strength or ultimate strength of the material. Creep is more severe in materials that are subjected to heat for long periods, and near the melting point

Replica-Q12

What is Creep or creed deformation?

Creep is a phenomenom of a sample to deform against time at particular temperature. Creep is the tendency of a solid material to slowly move or deform permanently under the influence of stresses. It occurs as a result of long term exposure to levels of stress that are below the yield strength or ultimate strength of the material. Creep is more severe in materials that are subjected to heat for long periods, and near the melting point

Replica-Q11

What are the deterioration mechanisms that could be detected by using in-situ metallography

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• graphitization

• spheroidazation of pearlite

• creep

• thermal fatigue

• hydrogen attack

• carburization

• grain boundary oxidization

• embrittlement of microstructure etc.

Replica-Q10

Why microstructural crack analysis is important?

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• Possible to make a correct determination of crack type.

• Important to determine whether the crack is the original defect or caused by service conditions or damage.

• Once the crack type is identified, the proper corrective action, such as

• Eliminating a corrosive environment

• Reducing stress levels

• Re-examine the services condition & make a preventive actions

Replica-Q9

An inspection engineer responsible for the operation of petrochemical plant boiler observed a significant pressure drop which indicates that the boiler tubes might have experienced some damage problem. Preliminary report by process engineer states that the tubes were operated since 15 year ago. For temporary period, they decide to implements the fitness for service inspection to certify that boiler system component was fully ready to operate reliably.

1. Taking into consideration the microstructural condition of boiler system component in the aged petrochemical plant, propose the best assessment technique to certify that boiler system component was fully ready to operate reliably
2. Give your reasons for the above proposed method.
3. Recommend the best interval inspection package on boiler system component that would satisfies the plant engineers.
4. If you decide to perform replica assessment, identify the important information needed from plant engineers for assessment interpretation

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i) The condition assessment shall be implemented by in-situ metallography or replication
assessment method.

ii) This in-situ metallography method will be ensure;

• The fitness for service for the safety of operators/ workers/ contractors and/ or local communities as well as environment

• All equipment operates reliably

• Remaining life of metal structure or component which help the prediction when equipment should be re-inspected for the long-term & continuous productive operations

• Degree of overheating or other damage mechanisms manifested by microstructural changes.

• Assessment of surface flaws to determine if there are any cracks presented on boiler system component which help predicting the root cause of failure resulted by damage mechanism such as creep, fatigue, stress or corrosion.

• Information that supports emerging conditions to ensure optimal use of plant & resources

iii) The best practice of interval inspection is according to Nordtest Method. NT NDT 010: High Temperature Components in Power Plants: Remnant Lifetime Assessment, Replica Inspection. based on Neubauer microstructure evolution as shown in Table below.

Grade/ Class		Microstructure Characteristic	Recommendation Action
0/1		No cavity	No Action
2		Single cavities	Reinspect after 20,000hrs of operation/ inspection interval 3-5 years
3		Coherent cavities	Reinspect after 15,000hrs of operation/ inspection interval 1~1.5 years
4		Microcracks <2mm>	Repair/ Replace of reinspect after 10000 hrs of operation/ inspection interval 0 - 0.5 years
5		Macrocracks >2mm detected by NDT	Immediate Repair/ replace

iv) The important information needed from plant engineers for assessment interpretation are as follows;

• Objective of In-situ metallography; in this case - condition assessment,

• Material of construction with exact specification

• Location of replication with sketch

• Process parameters and design parameters

• Service life of the component at the time of replication.

• Any history of previous failures at the location of replication.

Replica-Q8

What are the Critical Point in Boiler Tubes ?

• Bends

• T-sections

• Reducting section

• Valves

• Rigidly-held pipes

• hangers

Replica-Q7

What are the components that subjected to in-situ metallography?

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• Boiler tubes

• Furnace

• Superheaters

• Steam piping

• Turbine

• Heat exchanger

• Pressure Vessel

Replica-Q6

There are wide differences in the techniques of in-situ metallography (replication technique) against laboratory metallography. These differences get highlighted when in-situ metallography is actually carried out. Together with your experiences in lectures, laboratory works and industrial training, list down on this issue.

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• The nature of metal and alloys under investigation,

• Field conditions,

• Geometry and location of area intended for in-situ metallography,

• Purpose of the in- situ metallography,

• Selection of polishing equipment and method,

• Selection of etching method,

• Technique of replication,

• The details required from the replica,

• Types of microscopy to be used,

• Technique of viewing the replica and

• Interpretation of results.

Replica-Q5

In-situ metallography is used on vessels or structures that have been exposed to temperatures in excess of their design limits, and materials that have been in a corrosive atmosphere. What does in-situ metallography reveal?

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• Displays flow pattern lines indicating hot or cold working

• Grain structure is viewable

• Characteristic structure such as pearlite’s lamellae

• Exposes whether alloys have segregated during solidification

• Microscopic characteristics give insight into macroscopic properties

Replica-Q4

The object of in-situ alloy metallographic method is to allow viewing of the surface microstructure of operating components. Elaborate the application of this non-destructive technique?

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• Remaining life of metal structure or component studies, which help predict when equipment should be re-inspected.

• Damage assessment from creep mechanisms or fire damage to determine if it an item is safe to use after being exposed to fire.

• Heat treatment condition evaluations to assess if the component has been properly heat-treated.

• Composition verifications.

• Classification of cast irons without sacrificing any components.

• Degree of overheating or other damage mechanisms manifested by microstructural changes.

• Assessment of surface flaws to determine if they are cracks, laminations or seams.

• The cause of cracking which help predicting the root cause of failure where it resulted from corrossion, creep, fatigue, stress etc.

• Fitness for service to ensure that all equipment operates with safety and reliably

Replica-Q3

As an NDE technique, in-situ metallography is ideal for assessing the remaining life of power and petrochemical plants. Why this technique is important to industries?

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• To ensure the safety of operators/ workers/ contractors and/ or local communities as well as environment

• To ensure that all equipment operates reliably

• To ensures long-term & continuous productive operations

• To provide information that supports emerging conditions to ensure optimal use of plant & resources

• To be applied for microstructural analysis when examining large components that cannot be easily moved or destructive sample preparation is difficult or not permissible.

Replica-Q2

On field testing, metallography can be applied non-destructively by replication technique. Briefly explain, what is the replication technique and it objective?

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• Replication means, the act or process of duplicating or reproducing something.

• Replication in metallography, is the use of thin plastic films to nondestructively duplicate the microstructure of a component. The film is then examined at high magnifications using metallurgical microscope (or SEM).

• The object of this nondestructive testing method is to allow viewing of the surface microstructure of operating components.

Or the answer could be like this

• The sample is ground, polished, etched, replication & inspected in the field to identify microscopic defects

• Tests metals currently in use, instead of metals being mass produced

• Addresses potential failure problems of industrial equipment

• The objective is to determine the microstructural deterioration & damage of engineering alloys subjected to various environments

Replica-Q1

What is Metallography?

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Metallography is the science and art of preparing a metal surface for analysis by grinding, polishing and etching to reveal it's microstructure constituent

Failure Analysis-Q10

What are objectives of failure analysis and explain how it can be implemented ?

 

There are 3 primary objectives;

i) 

The first objective is to examine the objective evidence presented by the failed components and, from that evidence, determine the failure mechanism.

ii) 

The second objective is to determine the primary cause, or what is commonly called the root cause, of the failure. This is accomplished by examining design and operational issues to determine what specific factor, or factors, was responsible for the failure.

iii) 

The third objective, recommendation of corrective actions that will prevent similar failures, can be accomplished once the root cause of the failure has been identified.

Failure Analysis-Q9

“Microstructural examinations were conducted on boiler tube from local petrochemical plant for assessment on the reusability of the tube. Tube diameter was measured along the tube and it was shows that there is no bulges on the as received tube. The boiler tube was found bending at fire side area (Figure 1). The structure has equated grains consisting of pearlite and ferrite. Microstructure examination revealed that the grain size at the bend was slightly bigger as compared to grains at far from than the bend side area. There is no change in structure of grain at both areas; it means that the temperature applied to the tube was not higher than the limit of permissible temperature of the tube. According to the different in grain size, it means that temperature at the bent area was also different with the straight area of the tube. Temperature gradient should be existed along the tube due to the different in temperature. The bending in the tube was caused by the temperature gradient.”

Figure 1 Schematic diagram shows the condition of bending and metallographic spot on the tube surface marked as R1, R2, R3 & R4

From the statement above, plan your investigation procedures to implement failure analysis on the boiler tube. Give your reason for each procedures

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1) Obtaining background data

• To investigate the basic information that possible to be a primary clues of the root cause analysis.
• These importance information are including its location, manufacturer, function of component, operational condition, maintenance and cleaning history as well as opinions of related personnel

2) Visual examination followed by closed-up photographs by digital camera.

• Since many of the subsequent steps in the evaluation will involve destructive examinations, the as-received photographs provide an archival record of the original condition of the failed component prior to cutting and sectioning
• The as-received photographs also provide an effective place to indicate the locations from which the various samples used in the destructive examination were obtained

3) Chemical composition analysis by using XRF or spark emission spectrometer

• Analysis of the chemical composition of the component is a routine part of a failure analysis
• The analysis is used either to determine if the material meets the specified composition limits (in this case, boiler tube is carbon steel) or to identify an unknown material

4) Dimension measurement by vernier calliper

• The operational condition of boiler tube normally involves a high temperature stress. Thus as-received boiler tubes should have to document their dimension along the tube to investigate the occurrence of bulging on the tubes resulted by its operational condition. However there’re no bulging occur on the sample

5) The straightness of the tubes is carried by rolling the tube on the flat & smooth surface.

• Any bending or uneven on the tube can be identified when unstable rolling is observed resulted by high temperature operational condition that probably hits the one side of the tubes.

6) Determination of failure mechanism & recommendations

• Without identification of the failure mechanism, it is usually impossible to determine the root cause of the failure, which forms the basis for recommendations for immediate action whether the components should be replaced or not and to eliminate or reduce the frequency of future failures.
• Determination of the failure mechanism is considered a critical step in the failure investigation. In this particular case, the failure was caused by the occurrence of temperature gradient on the tube.

Failure Analysis-Q8

Visual observation, optical microscope and scanning electron microscope are three (3) major tools that play an important role in determining the success of a root-caused analysis. Explain the each their roles in determining the (a) ductile and (b) brittle failure

[Pemerhatian mata kasar, mikroskop optik dan mikroskop elektron imbasan adalah tiga (3) alat utama yang memainkan peranan penting dalam menentukan kejayaan analisis punca-penyebab. Terangkan peranan setiap alat tersebut dalam patahan mulur dan rapuh]


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Ductile Fracture	Brittle Fracture
Visual Observation - Necking or plastic deformation - Dull and fibrous fracture surface - Shear lips	Visual Observation - Little/no plastic deformation - Shiny, course, crystalline fracture surface - Chevrons
Optical Microscopic - Plastic distortion of grains - Irregular trans-granular fracture	Optical Microscopic - Minimal deformation - Inter-granular or trans-granular
SEM Microscopic - Micro-voids elongation in direction of load - Singular crack with limited/no branching	SEM Microscopic - Cleavage or inter-granular - Discontinuity or stress riser at origin