INFLUENCE OF TEMPERING ON IMPACT BEHAVIOR OF QUENCHED AND TEMPERED STEELS WITH LOW HARDENABILITY

The influence of tempering on the impact behaviour and strength of que nched and tempered C-Mn-Al steel flanges made to the ASTM A350 LF2 spe cification has been determined. Specimens of Al containing forged flan ges were heated to 950 degrees 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 degrees 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 temperatu re from 600 to 700 degrees C continued to I educe the strength, the im pact behaviour hardly changed. For the oil quenched steel (major phase being ferrite), this insensitivity of the impact behaviour to the hig her tempering temperature range is believed to be a result of the dete rioration in impact behaviour, resulting from the coarsening of the ca rbides which is largely offset by the reduction in yield stress; the l atter being expected to improve the impact behaviour. For the ice wate r quenched steel (major phase being bainite), the impact behaviour was very poor after quenching, probably as a result of the presence of so me martensite. Tempering at 600 degrees C considerably improved the im pact performance but again, tempering in the range 600-700 degrees C h ad little influence on the toughness, although there was a considerabl e fall in strength. This is a result of a gradual transition from a la th to a recrystallised bainitic ferrite grain structure; the removal o f low angle boundaries reducing strength but having little influence o il impact behaviour. The expected improvement in impact behaviour owin g to removal of dislocations and reduction in dispersion strengthening was counteracted by the coarsening of the grain boundary carbides. Ex cept for the untempered ice water condition, the impact requirement fo r the specification was always met. (C) 1997 The Institute of Material s.