Optimizing Morphology of Primary Carbides and Mechanical Properties during Processing of Cast-Cold Work AISI D2-Steel Press Forming Dies

Abstract

Cold work tool steels are used for punching, cutting, forming, cold forging, cold extrusion, cold rolling, etc. Those steels have higher hard phases like carbides and plate martensite compared to other steel alloys, but the everlasting challenge is the enough strength to toughness relation, without immediate fracture. Carbides provide the wear resistance, acting as hard precipitates inside the structure of steels. Variation in size and fraction of carbides, depending on manufacturing route, alloy content, hot working and heat treatment, will give the tool steel the desired mechanical properties. However, conventional cast tooling is the major production route used, which could be followed by a process called electro slag refining where not only the nonmetallic inclusion content is lowered but also the texture of steel is modified. Conventional casting of tool and die steels has a lower production cost per unit than the conventional processing routes. The disadvantage though is a more heterogeneous material, due to segregation of primary or leduburitic carbide net cells. Experimental trials were made to control the distribution and morphology of primary carbides in the matrix of martensite by spheroidizing them through molten metal treatment by using FeSiMgRe ferroalloy additions and heat treatment processes. Types and distributions of different carbides in the matrix of cast D2-steel were detected using X-ray diffraction and SEM/EDX units. Series of carbides like (FeCr)7C3, (FeCr)3C and (FeCr)4C were detected. In all cases, the optimum tempering conditions were found to be (300 °C, 5 h, air cooling), which in turn insured hardness of about 59–63HRC for cast AISI-D2 steel. The fracture index for that hard matrix was Kic = 70–60 MPa √m, and impact energy = 54–35 J was achieved.