INFLUENCE OF INTERCRITICAL ANNEALING ON STRENGTH AND IMPACT BEHAVIOR OF STEELS

Intercritical annealing as a method of improving the impact behaviour of normalised C-Mn-Nb-Al steels has been examined. The steels were hea ted to 920-degrees-C and cooled directly to 730-degrees-C or cooled to room temperature and then heated to 730-degrees-C. Plates were held a t 730-degrees-C for times in the range 15-900 min and their strengths and Charpy impact properties determined. Results were compared with th ose for plates which were normalised. In all tests the cooling rate wa s 7 K min-1. For 0.1% C steels, containing 0.56 and 1% Mn, significant improvements in impact behaviour occurred (the impact transition temp erature was decreased by 20-30 K), this effect being more pronounced o n heating to 730-degrees-C than on cooling from 920-degrees-C to 730-d egrees-C. Significant decreases in impact transition temperature occur red after as short an intercritical annealing time as 15 min. Increasi ng the Mn content to 1.49% introduced high C martensite into the struc ture, leading to poor impact performance and low strengths after holdi ng for long times at 730-degrees-C. However, by reducing the C content or the cooling rate, martensite formation was prevented so that impro ved impact behaviour was obtained on intercritical annealing. Increasi ng the intercritical annealing temperature to 760-degrees-C again gave improved impact performance on heating to 760-degrees-C, but on cooli ng the improvements were small. Refining the ferrite grain size also r esulted in greater improvements in impact performance on intercritical annealing. Generally strength was little influenced by this heat trea tment as the grain size remained constant. It has been shown that thes e improvements in impact behaviour can be related to the refinement of the grain boundary carbides that occurs on intercritical annealing. T his treatment allows Mn to segregate to the ferrite boundaries and its principal effect is to decrease the transformation temperature for ca rbide formation.