Sabita Ghosh

Influence of pre-straining on mechanical properties of HSLA steel by using ball indentation technique

Abstract Ball indentation technique has been employed to explore the possibilities for revealing the effect of pre-strain on mechanical properties of two groups of high-strength low-alloy steels. These steels were pre-strained up to 5%. Results are presented here for 1, 3, 4, and 5% of pre-strain effect on these steels. The dependence of yield strength, ultimate tensile strength, strength coefficient, strain-hardening component and true stress–strain curves on pre-straining effect are available from the ball indentation test, and they are validated with the standard mechanical test results. It is found that the increment of the pre-straining effect results in an increment of the yield strength and ultimate tensile strength values, while strength coefficient and strain-hardening component values are decreased. The existence of Luders bands in the conventional stress–strain curves is also revealed in ball indentation test by the presence of the Luders strain surrounding the indentation profile. In the conve...

Fatigue failure of a boiler feed pump rotor shaft

This paper deals with a detailed failure investigation of a rotor shaft in a boiler feed pump of a thermal power plant. The investigation mainly included chemical analysis, microscopy, fractography, hardness measurement and residual stress measurement. Analysis revealed that the metallization processes was primarily responsible for the failure. There were a large number of defects at the interface where the metallization process was terminated near the fillet. The crack had initiated from one or more such defect areas and then further propagated by fatigue. The mode of fracture was fatigue and the presence of oxides at the base of the cavities at the interface regions indicated that surface preparation was not made properly prior to metallization.

Investigation of sub-surface cracks in continuous cast billets

Sub-surface cracks in the continuous cast product of steel were studied to understand the causes of such defects and to suggest remedial measures. Even after hot rolling the cracks were not welded. The diameter of the as cast billets were 225 mm and most of the cracks were located around l cm away from the edges. The cracks were visible with the naked eye at transverse sections. The cracks were not continuous but scattered. The objective of this work was to identify the root cause of the formation of the sub-surface defects on the concast product and to ascertain whether it was due to any material related problem or due to any other cause such as processing. The existing cracks were opened through mechanical testing. The fracture surfaces were observed by SEM. Metallography and SEM observations revealed that the cracks were generated due to preferential segregation of sulphur, Cu and subsequent formation of various types of sulphide inclusions. The presence of a higher amount of Cu, particularly in the cracked region is of great concern. Cu is very harmful in steel if it exceeds the tolerable limit, particularly with the presence of high amount of S. The sulphide inclusions are found at the existing cracked regions. Inclusions at the crack is indicative of a correlation between inclusions and cracks. The inclusions act as a second phase and may influence the solidification of the matrix by restriction in directional solidification, differential contraction and even acting as a source of stress concentration—hence the appearance of cracks. The location-specific segregation of the inclusions is however surprising. Had there been dispersion of inclusions, probably the cracks would not have resulted.