Age hardening in martensitic/bainitic matrices in a copper-bearing steel
Journal
Materials Transactions, JIM
Journal Volume
41
Journal Issue
10
Pages
1312-1321
Date Issued
2000
Author(s)
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.
Type
journal article