Abstract
Fracture toughness testpieces made from tool steels M2, T1, A2, and S1 were given various heat treatments in an attempt to establish the requirements for maximum toughness without undue loss of hardness. The three point bend, crack opening displacement method was used to obtain values of plane strain fracture toughness KIc. The required fatigue precrack was initiated by sparking the root of the machined notch with an electric marking pen. Steels M2 and T1 gave KIc values in the range 20–28 MN m−3/2 and heat treatment had little effect. Steels S1 and A2 showed, respectively, marked and slight improvements in toughness when the hardness levels were reduced to about 520 HV. Steel S1 gave the highest toughness recorded (48 MN m−3/2). All toughness values for steel A2 were lower than expected. This was considered to result from the rapid cooling imposed by the nitrogen gas quench on the very small specimens employed in the work.
MST/1159
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