Influence of tempering on impact behaviour of quenched and tempered steels with low hardenability

The influence of tempering on the impact behaviour and strength of quenched and tempered C-Mn-Al steel flanges made to the ASTM A350 LF2 specification has been determined. Specimens of Al containing forged flanges were heated to 950°C, followed by quenching in oil or ice water to produce a variety of commercially obtainable microstructures, from fine grained ferrite to almost fully bainitic. After quenching, the blocks were tempered at 600, 650, and 700°C and the Charpy V notch impact transition curves and tensile data obtained. Tempering the as quenched steels caused a marked reduction in strength and the impact behaviour improved. Although increasing the tempering temperature from 600 to 700°C continued to reduce the strength, the impact behaviour hardly changed. For the oil quenched steel (major phase being ferrite), this insensitivity of the impact behaviour to the higher tempering temperature range is believed to be a result of the deterioration in impact behaviour, resulting from the coarsening of the carbides which is largely offset by the reduction in yield stress; the latter being expected to improve the impact behaviour. For the ice water quenched steel (major phase being bainite), the impact behaviour was very poor after quenching, probably as a result of the presence of some martensite. Tempering at 600°C considerably improved the impact performance but again, tempering in the range 600–700°C had little influence on the toughness, although there was a considerable fall in strength. This is a result of a gradual transition from a lath to a recrystallised bainitic ferrite grain structure; the rempval of low angle boundaries reducing strength but having little influence on impact behaviour. The expected improvement in impact behaviour owing to removal of dislocations and reduction in dispersion strengthening was counteracted by the coarsening of the grain boundary carbides. Except for the untempered ice water condition, the impact requirement for the specification was always met.