The effect of dynamic strain aging on room temperature mechanical properties of high martensite dual phase (HMDP) steel

AISI 4340 steel bars were heated at 900°C for one hour, annealed at 738°C for different durations and oil-quenched in order to obtain dual phase steels with different ferrite volume fractions. A 3% prestrain at the temperature range of 150–450°C was then imposed to the samples, and room temperature tensile tests were carried out, afterwards. Results indicate that the maximum values for both yield and ultimate tensile strength would exist for the samples pre-strained at the temperature range of 250–300°C. Also, a sudden drop of the ductility was observed at the mentioned temperature range. The observed behavior might be attributed to the occurrence of dynamic strain aging taken place at this temperature range. It was also shown that the ferrite volume fraction has not had any remarkable influence on the strength behavior of the samples, yet its increase was shown to have affected the work hardening behavior of the samples. Fractography analyses were also performed, showing the featureless fracture surface of the samples prestrain at the range of 250–300°C, confirming the occurrence of dynamic strain aging at this condition.