Evaluation of tempered martensite embrittlement in AlSl 151341 steel

Abstract

Tempered martensite embrittlement (TME) is char60 acterized by a drop in the impact toughness after tempering in a range of temperatures from 250 to 450 °C for short times (1 or 2 h) in martensitic steels. Both intergranular and transgranular fracture are associated with TME. The former is attributed to the formation of carbides on the prior austenite grain boundaries already weakened by segregation of ~4o c impurities during austenization [1]. The latter is >, assumed to be caused by (i) the thermal decomposition of inter-lath films of retained austenite and the o associated precipitation of inter-lath cementite [2] or (ii) the formatinn of coarse intra-lath carbides [3] that contribute to causing cleavage or ductile fracture. 0 Boron is con:tmonly used to increase the harden0 ability of steel. This effect is associated with the kinetics of the boron migration to the austenite grain boundaries. The result is a delay of ferrite nucleation [4] but there is no conclusive information about the influence of boron on TME. A commercial AISI 15B41 steel with composition 100 (wt%) 0.42 C, 1.37 Mn, 0.26 Si, 0.04 Ti, 40 ppm B, 72 ppm N, 0.39 Cr, 0.030 S, 0.023 P, 0.030 A1 was used. Standard Charpy V-notch (CVN) tests were run after austenitizing at 880°C for 1 h in air, oil quenching and tempering in a range of temperatures +5 50 from 150 to 550 °C for 1 h. The fracture surfaces were examined mainly by scanning electron microscopy (SEM). The amounts a) of different types of fractures were estimated from a. SEM images using a point-counting method. An anomalous drop in toughness related to TME was observed in this steel. 0 Fig. 1 is a plot of the impact toughness change 0 with tempering temperature in room temperature tests. The curve shows a minimum between 300 and 400 °C, which is the characteristic TME trough. This result shows that boron addition is not a remedy for TME in this steel. The intergranular fracture is noticeable only in the TME trough, as shown in Fig. 2. Between 200 and 300 °C the drop in the absorbed energy is associated with the transgranular brittle mechanism of fracture (quasi-cleavage). This shows that the cohesion energy of the austenite grain boundary is smaller than that of the matrix only between 350 and 400 °C, despite the noticeable decrease of toughness between 200 and 300°C. Also, the increase in absorbed