Effect of prior austenite carbon partitioning on martensite hardening variation in a low alloy ferrite-martensite dual phase steel

The aim of this research work is to investigate in detail the carbon partitioning within prior austenite developed during austenite to ferrite phase transformation, and consequently its relation to the martensite hardening variation in a low alloy ferrite-martensite dual phase (DP) steel. For this purpose, a wide variety of ferrite-martensite DP samples with different volume fractions of ferrite and martensite have been developed using step quenching heat treatment processes at 600 °C for various holding times after being austenitized at 860 °C for 60 min. Both spot and line scan energy dispersive X-ray spectroscopy for carbon analyses have been used in conjunction with nanoindentation tests to follow the variation of carbon partitioning within prior austenite areas and consequently the associated martensite hardening response in the DP specimens. Experimental results showed that the martensite hardening behavior was quite variable in the ferrite-martensite DP samples and even within a specific martensite area within a specific DP microstructure. A higher level and also a more scattered nanohardness were observed for martensite in the DP samples treated at 600 °C for longer holding times. These results were rationalized due to the variation of carbon partitioning within the prior austenite area developed during isothermal holding in the ferrite-austenite DP region. Longer isothermal holding times were associated with more carbon redistribution within prior austenite as a consequence of more ferrite formation, which resulted in the formation of harder martensite with a more scattered hardening response. Furthermore, compared to the central locations of martensite area, those nearer to the ferrite-martensite interfaces contained higher carbon concentration and consequently higher hardening responses.