Microstructure and Phase Transformation Characteristics of Non-quenched & Tempered and Quenched & Tempered Prehardened Steels for Large Section Plastic Moulds

The hardness and microstructure of the 460 mm×800 mm×3 200 mm non-quenched & tempered prehardened (SWFT) steel and the 500 mm×800 mm×2 900 mm quenched & tempered prehardened 3Cr2MnNiMo steel for plastic moulds were compared and studied.The continuous cooling transformation curves of the two steels were measured.The results show that the two steels both had good hardenability,which strongly put off the appearance of massive proecutectoid ferrite.The surface microstructure of 3Cr2MnNiMo steel was tempered martensite and center microstructure was tempered bainite,which reflected that the tempering process adjusted the uniformity of its hardness.While the broad bainitic transformation zone made the microstructure of SWFT steel obtained uniform bainite through the whole mould.