Age Hardening in Martensitic/Bainitic Matrices in a Copper-Bearing Steel

Abstract

In order to understand the influence of matrix microstructures on age-hardening behavior in a copper-bearing steel, NAK 80, the phase transformation of austenite (during a variety of continuous cooling treatments) and the consequent precipitation of copper particles (during isothermal aging) were investigated by dilatometry, optical metallography, hardness measurement, transmission electron microscopy, and field-emission-gun transmission electron microscopy (FEG-TEM). It was found that at a wide range of cooling rates (about from 30 to 0.3°C/s) after austenitization at 900°C for 15 min, the steel produces a mixture of martensite and bainite. Three different pre-treated specimens, which had been continuously cooled at 120, 5 and 1°C/s, respectively, were studied in order to determine their response to copper age-hardening. The results show that the general level of peak hardness for the fully martensitic specimen is the lowest when compared with those of the other two specimens, which contained a mixture of nearly equal volume fractions of martensite and bainite. The findings in this work show that tempering martensite during aging drastically hinders the hardening of copper precipitates.