Study of Carbide Evolution During Thermo-Mechanical Processing of AISI D2 Tool Steel D. Bombac, M. Fazarinc, A. Saha Podder, and G. Kugler (Submitted May 21, 2012; in revised form July 17, 2012) The microstructure of a cold-worked tool steel (AISI D2) with various thermo-mechanical treatments was examined in the current study to identify the effects of these treatments on phases. X-ray diffraction was used to identify phases. Microstructural changes such as spheroidization and coarsening of carbides were studied. Thermodynamic calculations were used to verify the results of the differential thermal analysis. It was found that soaking temperature and time have a large influence on dissolution, precipitation, spheroidization, and coalescence of carbides present in the steel. This consequently influences the hot workability and final properties. Keywords AISI D2, carbide coarsening, differential thermody- namic analysis, precipitates, XRD 1. Introduction AISI D2 is one of the most extensively used cold work, high-carbon, high-chromium ledeburitic tool steel. Dissolution of alloying elements and precipitation of carbides results in its high strength, hardness, and wear resistance. The mechanical properties of the AISI D2 steel are mainly determined by the alloying elements, austenite grain size, subgrain size, mar- tensite lath width, dislocation density, and precipitates. The precipitates predominant correlation to the toughness implies their amount, type, size distribution, morphology, stability, and spatial distribution. The steel can have different performances depending on the size, morphology, and distribution of the eutectic carbides, which largely depend on the hot working conditions, the soaking temperature, and the time between each forging cycle (Ref 1–4). Size and amount of precipitates could be therefore controlled with the production process parameters. Solidification begins with austenite formation followed by the eutectic reaction where M 7 C 3 primary eutectic carbides precipitate, thus giving the steel a ledeburitic structure. Since the cast structure of the AISI D2 cold-worked tool steel contains undesirable eutectic carbide networks, normally hot rolling or forging is conducted on the cast ingots to break down the carbide networks (Ref 5–9). The effect of deformation on the breakdown of the carbide networks has been studied previously only by means of laboratory hot compression tests (Ref 6, 9). The chemical composition of AISI D2 steel is based on the Fe-C-Cr alloy system, and therefore before deformation, it needs ramping and soaking. The hot-forming properties of the material are closely related to the type, the distribution, and the amount of the phases present during deformation (Ref 10). The AISI D2 was chosen in the current study because its unique manufacturing process includes long soaking times between the forging cycles. A forging cycle consists of austenitization and multiple forging passes until temperature of the billet drops too low to ensure sufficient hot workability. The literature publications studying ledeburitic tool steels predominantly focus on partial individual factors, i.e., influence of deformation parameters (Ref 2, 4), deformation limits (Ref 2), spheroidization and break down of carbide networks (Ref 6, 9), etc. This study was employed to characterize the microstructural changes and provide a better understanding of carbide spheroidization, coalescence, and coarsening during the thermo-mechanical processing with emphasis on thermody- namic equilibrium calculations. 2. Experimental Procedure Samples of the alloy were taken after four different forging cycles from the open-die forged billet using an on-site guillotine at Slovenian Steel Group—Metal Ravne. The forging cycle is defined as reheating and soaking the billet at high temperatures, and subsequent open-die forging passes. The temperature of the billet was 1160 °C at the start of forging. The forging was continued until the temperature dropped to 900 °C followed by billet reheating to 1160 °C and keeping it at that temperature for 8 h. A number of samples were taken from different stages of the forging process. An as-cast sample was taken from a 16 ton octagonal ingot which was soaked for 8 h at the temperature of 1160 °C. Other samples were cut from the billet at the end of the forging cycle (i.e., at about 900 °C). After the samples were air cooled, with approximate cooling rate 1 °C/s, cylindrical specimens with dimensions of ;5  3 mm were prepared using D. Bombac, M. Fazarinc, and G. Kugler, Faculty of Natural Sciences and Engineering, University of Ljubljana, Askerceva cesta 12, 1000 Ljubljana, Slovenia; and A. Saha Podder, Materials Science and Metallurgy, University of Cambridge, Pembroke Street, Cambridge CB2 3QZ, UK. Contact e-mail: david.bombac@ntf.uni-lj.si. JMEPEG ÓASM International DOI: 10.1007/s11665-012-0340-y 1059-9495/$19.00 Journal of Materials Engineering and Performance Author's personal copy