MORPHOLOGY AND TRIBOLOGICAL CHARACTERISTICS OF STEEL

Abstract

Of the many failure modes associated with mechanical systems, wear presents a unique challenge to the designers and developers of such systems. It is therefore, necessary to develop a comprehensive understanding of wear including its materials aspects. The present investigation aims to correlate the microstructural features of a material with dry sliding wear: Plain carbon steel of composition 0.13% carbon has been subjected to . annealing and normalising from a holding temperature of 8400C to result in ferrite-pearlite microstructures containing coarse and fine pearlites respectively. Intercritical annealing at the same temperature has also been carried out to result in ferrite-martensite microstructure which, when tempered, results in ferrite-tempered martensite structure. 0.13% carbon steel with these four different microstructures - ferrite-coarse pearlite, ferrite-fine pearlite, ferrite-martensite and ferrite-tempered martensite, have been tested for friction and wear under dry sliding condition using pin-on-disc wear testing machine at loads of 2.5, 3.5, 4.5 and 5.5 kg and at a constant sliding speed of 1 m/s Three replicating tests have been carried out under each condition. The friction coefficient does not change significantly when the ferritic-pearlitic structure changes from annealed coarse pearlite to normalised fine pearlite structure. There is also hardly any change in coefficient of friction when the microstructure changes from ferritic-pearlitic to ferritic-martensitic. By tempering the martensite in ferritic martensitic structure, no change in friction could be observed. Thus, the coefficient of friction changes only marginally with changes in microstructure as attained in the present study. The accumulated volume loss at relatively low loads is observed to be the highest for a microstructure containing tempered martensite but for structures containing coarse pearlite or fine pearlite'have lower and almost similar volume loss. At higher loads, ferrite-fine pearlite and 111 ferrite-martensite structures have higher and almost similar accumulated volume loss whereas ferrite-coarse pearlite structure has significantly lower wear loss. Highest accumulated volume loss is observed in ferrite-martensite and ferrite-tempered -martensite structures. The variation of volume loss with sliding distance shows two linear segments with distinctly different slopes. The slope of the second segment is generally lower than that of the first segment. It may be attributed. to the formation of thicker oxide layer covering a wider area of sliding surface at a larger sliding distance corresponding to the second segment, reducing its wear rate as reflected in its slope. The overall wear rates observed, in general, are lower for the ferrite-coarse pearlite structure than that • for the structure containing fine pearlite at higher loads of 4.5 and 5.5 kg whereas at lower loads, the ferrite-fine pearlite structure generally results in lower overall wear rate compared to that in ferrite-coarse pearlite structure. The wear rate observed for intercritically annealed and tempered steel was highest at low as well as at high loads. The average wear coefficient calculated for both the segment was minimum for ferrite-coarse pearlite structure and highest for the-ferrite-tempered martensite structure: The present study indicates that the wear resistance of 0.13% carbon steel decreases in the following order with microstructures under dry sliding wear - the coarse pearlite, fine pearlite, martensite and tempered martensite each in association with ferrite