Characteristics of 100Cr6 bearing steel after thixoforming process performed with prototype device Ł. ROGAL 1 , J. DUTKIEWICZ 1 , T. CZEPPE 1 , J. BONARSKI 1 , B. OLSZOWSKA-SOBIERAJ 2 1. Institute of Metallurgy and Materials Science of the Polish Academy of Sciences, 30-059 Kraków, 25 Reymonta St. Poland; 2. Faculty of Foundry Engineering, University of Science and Technology, 30-059 Kraków, 23 Reymonta St. Poland Received 13 May 2010; Accepted 12 July 2010 Abstract: A device based on a high pressure die-casting(HPDC) machine was constructed, with a capacity to produce thixo-casts from steel. After inductive heating to the required semi-solid temperature range, the samples were transported in a protective argon atmosphere to a cylinder of modified HPDC and injected into a pre-heated die. Bearing steel 100Cr6 (after forging) was used as the feedstock material. The metallographic analysis of the steel showed a homogenous structure within a whole volume characterized by a grain size between 2 µm and 8 µm. Differential scanning calorimetry (DSC) analysis allowed to estimate the process temperature for thixoforming at 1 390 ˚C which was attributed to about 45% of a liquid phase. With such technological parameters after the semi-solid processing the microstructure consisted of globular grains of size from 15 µm to 40 µm. The microstructure of globules indicated predominant presence of martensite (about 76% volume fraction) with residual austenite between plates. X-ray analysis confirmed that the thixo-cast samples contain mainly martensite and residual austenite. The hardness of the initial forged sample was HRC 27 and after the thixoforming process it was HRC 60, which confirms the presence of martensite and carbides. The thixo-casts were subjected to low tempering, at temperature of 150 ˚C for a period of 2 h, in order to remove stresses created during the process of thixoforming and to increase plasticity. The hardness after tempering was HRC 62.3. Additionally, in order to identify phases present in the thixo-casts, transmission electron microscopy (TEM) was carried out. Key words: SSM; semi-solid metal; thixoforming; 100Cr6 steel 1 Introduction Thixoforming technology or semi-solid processing (SSP) is a method of forming parts from the solidus-liquidus range. In this process the thixotropic flow behaviour of metal in the solidus-liquidus range is used. The behaviour of Pb-Sn alloys was discovered by Spencer and all in 1970[1−3]. In order to obtain appropriate flow behaviour of the semi-solid slurry, a microstructure of an alloy prior to forming should consist preferably of solid metal spheroids in a liquid matrix[4]. The liquid fraction in the thixoforming processes should be within the range of 30%−60%. The essential factor influencing the final structure and properties of an alloy to be shaped in the semi-solid state is a method of the globular structure preparation. Many processes have been developed to produce the non-dendritic spherical microstructure, e.g. mechanical stirring, magnetohydrodynamic stirring (MHD), strain induced melt activated (SIMA) recrystallization and partial melting (RAP) and spray forming [5−6]. The thixoforming technology enables the production of near-net shape products for various industrial applications. For Al and Mg alloys, this technology has been used for a serial production of responsible parts in the automobile industry and aerospace engineering[7−10]. Currently, several research centres all over the world are conducting experiments on the flow behaviour of thixotropic iron alloys in the semi-solid state[11−13]. Until now, several steels have been qualified as suitable for the thixoforming process. Research has centered on alloys with a wide enough semi-solid range. It has been estimated that the steels X210CrW12, HS6-5-2, 100Cr6 and C38 are the best for the thixoforming process[14−16]. The research has been conducted on the modification of existing steels to make them more suitable for the thixoforming process and the right heat treatment of thixo-casts. Concurrently, the problems of thixo-casts manufacturing technology have Corresponding author: Ł. ROGAL; Tel/fax: +48-12-2952861; E-mail: lukaszrogal@wp.pl Trans. Nonferrous Met. Soc. China 20(2010) s1033-s1036