This paper has been concerned to investigate in details the effect of tempering heat treatment on mechanical properties of 35CHGSA heat treatable low alloy steel under ferrite–martensite dual-phase (DP) microstruc- tures. For this purpose, two sets of ferrite–martensite DP samples containing 6% volume fraction of ferrite have been developed using step-quench heat treatment processes at 720°C for 5 min holding times with the subsequent water quenching after being austenitized at 900°C for 15 min. In comparison to the first set of fresh ferrite–martensite DP samples (marked FDP), the finalized tempering heat treatment has been carried out at 500°C for 60 min only for the second set of tempered ferrite–martensite DP (marked TDP) samples in order to optimize the strength–ductility combination. Light and electron microscopes have been used in conjunction with hardness and tensile tests to assess the structure–property relationships of FDP and TDP heat-treated samples. The experimental results demonstrate that the mechanical properties of FDP heat-treated samples are significantly increased after tempering heat treatment. The product of tensile strength multiple total elon- gation has been significantly increased from 2.4 (FDP) to 15.8% GPa (TDP). Moreover, the absorbed impact energy is sharply increased from 3.5 to 12 J corresponding to the FDP and TDP marked samples, respectively. These results are rationalized to the fact that the tempering heat treatment modifies the individual mechanical behavior of ferrite and martensite microphases through influencing the ferrite and martensite hardening vari- ations. In addition to general softening of martensite hardening from 796 to 459HV10g after tempering heat treatment, the average ferrite hardness has been surprisingly decreased from 280 (FDP-marked samples) to 207HV5g (TDP-marked samples).

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