Application of metallography in failure investigations: some case studies

Abstract

Metallography is a tool to know the internal condition of the material and no investigation can be initiated without this. It can be both nondestruc¬tive i.e., in-situ metallography, as well as destructive i.e.. cutting the relevant parts from the site to undertake investigation in the laboratory. In both the cases grinding, polishing and etching to reveal the microstructures remain the same. The only difference is the special tools and portable optical microscopes are used in the in-situ techniques to obtain replicas for laboratory use. Two types of etching are used to reveal macro and microstructures. The macro structures are viewed at 5 to 10 x, which reveal flow line, cold rolling fibres, welding zones, pores, dendrites, coarse grains etc. whereas microstructures reveal the micro constituents, pearlite, martensite, inclusions, precipitates, carbides, creep voids etc. at magnification of 50 to 1500 x. Metallography alone gives lot of information regarding the deviations of microstructures which are indication of the causes of failures. Some typical case studies are described in the present paper where metallography plays very important role, like : (i) Improper weld deposits to salvage the undersize rail axle, which led to premature failure, (ii) improper welding joints and lack of post weld heat treatment, which gave stress on the joint of cover dome of pressure vessel tank, which caused failure, (iii) revealing of tool marking and sulphide inclusions in the steel isolation valve of LPG storage tank, which led to premature failure, (iv) sensitization and stress corrosion cracking in S.S. cover dome of centrifuge used in the manufacture of NG explosive, which caused premature failure, (v) Improper heat treatment and forging were indicated by the microstructures which led to the premature failure of mining dragline sleeve. These are only a few investigations carried out in the laboratory in the recent past which justify that without metallography it is not possible to diagonise the causes of premature failures.