Strength,toughness,and stiffness of wrought and directly sintered T6 high-speed steel at 20–600°C

The mechanical properties of directly sintered T6 high-speed steel in the temperature range 20–600°C were generally comparable to those of concurrently heat-treated wrought material of similar composition. For the hardness range 860–940 HV30 macroscopic ductility was detected at 200°C and 450°C in the wrought and sintered materials, respectively; failure strains, however, did not exceed 2%. The value of Young's modulus dropped from ∼240 to ∼120 GNm⁻² as the temperature was raised to 600°C, yield strength dropped from 2·2 to 1·0 GNm⁻², but the fracture strengths showed a maximum, ∼2·1 GNm⁻² at ∼400°C for the wrought steel and ∼1·4 GNm⁻² at ∼450°C for the sintered steel. Microcracking preceded yielding and/or failure and was mainly through carbides, which were generally below the critical size to cause catastrophic fracture. The second stage of the failure process involved the linking through the matrix of such microcracks until conditions for fast fracture were satisfied (stage three). A quantitative model for carbide cracking in high-speed steels is absent as is the correlation of fracture strength with fracture toughness via the critical defect size, since, for example, the failure originating zones in wrought samples identified by scanning electron microscopy were generally larger than those predicted by linear elastic fracture mechanics (LEFM). It is suggested that there may be some analogies between failure in monotonic loading of high-speed steels and of ceramics with small defects; the behaviour in fatigue of short cracks in alloys and microscopic crack growth in delayed fracture of ceramics where LEFM analyses developed as a result of studying artificial long cracks appear not to hold.

MST/606