TEM Analysis of Precipitates in a Low Alloy Wear Resistant Steel and Correlating with its Corrosion Behaviour

Abstract

Wear-resistant low alloy steels containing elements like Cr, Mo, attain their properties due to precipitation of carbides upon heat treatment. These steels are widely used for components in earthmoving and mining equipment. Even though the mechanical properties and wear behaviour of these steels are studied extensively, the studies on nature of carbide precipitation and their influence on corrosion behaviour are very rare. The present work attempts to correlate the microstructure with corrosion properties of low alloy wear resistant steel. After quenching from hot working temperature of about 900 °C, the alloy is subjected to tempering at two different temperatures i.e. 150 °C & 300 °C for durations of 1 h and 4 h. Two competitive process occur simultaneously effecting the hardness of the alloy during tempering i.e. (i) Softening due to transformation of martensite with depleting carbon (ii) Increase in hardness due to formation of carbide precipitates in the matrix. Transmission electron microscopy (TEM) of as quenched condition revealed fully martensite structure in the alloy. TEM investigation of 150 °C/ 4 h tempering condition, revealed presence of some amount of martensite and fine carbide precipitates in the alloy. TEM micrographs of 300 °C/ 4 h condition revealed relatively coarse carbides in a softer ferrite matrix. The presence of martensite and fine precipitates, resulted in relative improvements in wear and corrosion resistance respectively, for the alloy tempered at 150 °C for 4 h, when compared to the alloy tempered at 300 °C for 4 h.