Friday, June 04, 2010

Paresh Haribhakti of TCR to present a paper at the National Workshop on Failure Analysis of Heat treated Components ( FAHC - 2010) being held at IIT Bombay on June 14th, 2010

Mr. Paresh Haribhakti will present his paper on approaches of Failure investigation of heat treated components through microstructure assessment.

Heat treatment of components plays an important role in industrial equipments. It is done for various reasons the main reasons are normally to achieve the required mechanical properties and enhance metallurgical integrity of the component. Improper heat treatment done could certainly hampered the desired performance of the component. Often it leads to premature failures. It may even introduced defects that would reduce the life of component of render it unfit for further use. There are varieties of heat treatments carried out on metallic engineering components to achieve desired mechanical properties and enhance service life. Improved design and engineering practices rise in production cost have compelled industry to ought for higher production volumes to reduce the cost of heat treatment as well as rejection by way of achieving more perfection in the practices. One of the important aspects to achieve this goal is by  conducting root cause metallurgical investigation for the defects observed during heat treatment. Root cause investigation approach can only be obtained with perfection by an experienced metallurgist who has basic knowledge of heat treatment and practices, Knowledge regarding field applications and with the sound background of physical metallurgy. When metal fails, it leaves behind evidences in terms of micro structural changes. The very purpose of heat treatment is to bring about desired changes in the microstructure of the component. Any abnormal conditions during heat treatment will reflect concomitant changes in terms of microstructure.

Following are the various damage mechanism that are observed due to faulty heat treatment.

1.    Decarburization  : depletion of carbon takes place at the surface due to highly oxidizing furnace atmosphere. Following microstructure indicates surface decarburization in medium carbon low alloy steel. In an engineering component surface decarburization is indicated by low amount of ferrite at the edges.

2.    Improper distribution of second phase in microstructure: following microstructure indicates presence of un-tempered martensite ( low temperature tempered martensite)  and its bands on account of the poor quality of tempering done and basic raw materials conditions. 

3.    Presence of harmful phases like retained austenite : presence of retained austenite especially in surface hardened components, beyond permissible levels will drastically reduce fatigue as well as wear resistance. Often it may as well lead to premature failures. Following is the microstructure of 17CrNiMo6 case carburized steel in quenched and tempered condition showing excessive amount of retained austenite.

4.    Problem related to quality of quenching: Appropriate quenching based on hardenability is important to achieve the desired microstructure. Slag quenching will more often result in to undesirable high temperature transformation phases. On the other hand severe quenching will invariably result in quench cracking and failure of the component.

5.    Problem arising due to improper raw material: The quality of raw material has a direct influence on the ultimate heat treatment quality. Higher level of impurities, tramp elements, and more  inclusions, coarse grained structure, micro segregation faulty deoxidation and teeming practice adversely influence the heat treatment quality.

6.    Cracking due to improper volume change : if the component shape is quite intricate having wide variations in section thickness it would introduce large internal stresses on account of anomalous volume changes during quenching eventually it may lead to cracking. 

7.    Quench cracking: improper choice of quenchant that is not based on hardenability considerations, changes in sections and mass effect invariably results in quench  cracking.

8.    Cracking due to low temperature melting phases: on account of micro segregation of certain impurities some intermetallic compounds having low melting point are formed They melt at the heat treatment temperature resulting in grain boundary weakening that leads to inter granular cracking. In free cutting steels having high sulfur does have this kind of problem when it is not properly balanced with manganese levels.

9.    cracking in the surface case hardening : Often surface cracking results due to improper case hardening done or grinding carried out that precedes the final heat treatment.