Effect of tempering time on microstructure, mechanical, and electrochemical properties of quenched–partitioned–tempered Advanced High Strength Steel (AHSS)

Abstract

The quenched–partitioned–tempered (QPT) steel is one of the novel steel in third–generation advanced high strength steels. Herein, an attempt has been made to investigate the microstructure, mechanical, and electrochemical properties of QPT steel as a function of tempering time. Microstructural analysis reveals that a high volume fraction of martensite with low fractions of retained austenite and bainite were achieved after 30 s of tempering. Mechanical tests validated that this triplex composite microstructure resulted in an 18% improvement in Vickers hardness, 73% in tensile strength (Rm) with little reduction of 12% elongation (εT) in comparison to non–heat treated steel sample. Tempering for 90 s increased the volume fractions of stable retained austenite, bainite and decreased the fractions of martensite by carbon diffusion to austenite. Further increase in tempering time stabilized retained austenite at an expense of bainite and martensite phases, resulting in reduced hardness and improved strength and elongation. Electrochemical analysis of the QPT heat–treated samples performed in 3.5% NaCl solution showed that the high corrosion rate was yielded from the steel sample tempered for 30 s, while other samples tempered for 90 to 150 s showed much lower corrosion rate than non–heat–treated steel sample.