August 2000 Ž . Materials Letters 45 2000 32–38 www.elsevier.comrlocatermatlet Sintering of injection molded M2 high-speed steel Z.Y. Liu, N.H. Loh ) , K.A. Khor, S.B. Tor School of Mechanical and Production Engineering, Nanyang Technological UniÕersity, Nanyang AÕenue, Singapore 639798, Singapore Received 28 October 1999; received in revised form 29 January 2000; accepted 29 January 2000 Abstract Ž . The sintering of injection molded M2 high speed steel HSS was studied. The sintering parameters investigated were: sintering atmosphere, heating rate, sintering temperature and sintering time. Mechanical properties and SEM microstructures were used to determine an optimum sintering schedule. It was found that sintering in vacuum is better than in nitrogen as densification is faster in the former. In vacuum sintering, effective sintering took place in the range 1190–12108C, whereas in nitrogen atmosphere, effective sintering occurred form 12508C to 12708C. A heating rate of 108Crmin is preferred since a higher heating rate of 308Crmin resulted in distortion. Near full density was obtained within 10 min after reaching an optimum sintering temperature of 12108C. However, the maximum ultimate tensile strength of 752.6 MPa was obtained after 2 h of sintering in a vacuum. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Powder injection molding; High speed steel; Sintering; Tool steel; Supersolidus liquid phase sintering 1. Introduction Ž . Sintering of high speed steels HSS has been extensively studied in conventional powder metal- Ž . w x lurgy PM 1–4 . The most promising advantage of PM lies in cost saving when compared to the wrought production route. However, due to composition com- plexity, sintering practice is extremely sensitive to sintering parameters. The densification process, mi- crostructure development and composition controls are the major problems to be solved. In addition, the Ž sintering window temperature region in which opti- . mum sintering is realized is very narrow, about 38C for M2 HSS and no more than 108C for other grades w x 5–7 . Composition segregation and consolidation heterogeneity is also associated with non-uniform ) Corresponding author. Fax: q 65-791-1859. Ž . E-mail address: mnhloh@ntu.edu.sg N.H. Loh . sintering and distortion. These problems made indus- trial production of HSS very difficult. Ž . Powder Injection Molding PIM has been well developed and is used industrially for materials such as Fe–Ni alloy, Fe–Co alloy, tungsten heavy alloy and stainless steel. However, PIM of HSS was just w x developed in recent years 8–10 . Literature on the sintering of injection molded M2 HSS were surpris- ingly scarce and limited work was conducted on the w x sintering kinetics of this particular steel grade 11,12 . w x Ruzi-Roman et al. 12 found that transverse rupture strength, impact strength and hardness properties are better in the PIM HSS than the conventional PM steel. This is due to its better microstructure that gives a better carbide distribution, as well as higher quantity of carbides. w x According to past work 13–15 , various amounts of liquid were evident during the sintering of M2 HSS. This kind of liquid differed from the liquid that 00167-577Xr00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. Ž . PII: S0167-577X 00 00070-7