Influence of high temperature deformation and double tempering on the microstructure of a H21 tool steel M. Nurbanasari a,b,n , P. Tsakiropoulos a , E.J. Palmiere a a Department of Materials Science and Engineering, The University of Sheffield, Sheffield S1 3JD, United Kingdom b Department of Mechanical Engineering, Institut Teknologi Nasional, Bandung, Indonesia article info Article history: Received 22 August 2012 Received in revised form 26 January 2013 Accepted 29 January 2013 Available online 8 February 2013 Keywords: Thermomechanical processing Precipitation Dislocations Electron microscopy abstract An axisymmetric compression test was used to study the effect of hot deformation and double tempering on the microstructure and hardness of a H21 tool steel. The compression tests used a constant true strain rate of 0.01 s 1 , and were performed in the temperature range 1000–1100 1C after austenising at temperatures of either 1100 or 1250 1C. The double tempering was carried out at 650, 750 and 800 1C, with air cooling in between the first and second temper. An overview of the flow curves and the characterisation of microstructures showed no evidence of dynamic recrystallisation. The increase in flow stress with decreasing austenising and deformation temperature was attributed to dislocation movement and the presence of fine and dispersed carbides causing a Zener pinning effect. Without double tempering, the highest hardness was measured after austenising at 1250 1C, followed by deformation at 1000 1C and water quenching (617 HV). No secondary hardening phenomena occurred after double tempering for samples that were first subjected to hot deformation, and the highest double tempered hardness (354 HV) occurred after a double temper at 650 1C following an austenising temperature of 1250 1C and subsequent deformation at temperature of 1000 1C. & 2013 Elsevier B.V. All rights reserved. 1. Introduction The H21 tungsten hot work tool steel is widely used for high working temperature applications, such as die-casting dies and forging dies due to its resistance to deformation at elevated temperature, high hot hardness, and high compressive strength [1]. This tool steel contains a high concentration of carbide forming elements that tends to promote segregation and form brittle eutectic carbide networks, which consequently decreases the toughness [2]. The H21 tool steels are usually used in the heat treated condition. This heat treatment involves austenising to dissolve the carbides, followed by oil quenching which transforms the austenite to martensite. Finally, these steels are tempered to produce a secondary hardening effect, which provides high strength at both ambient and elevated temperatures. Previous work [3,4] has reported that it is difficult to dissolve the carbides during the austenising process due to the thermodynamic stability of the carbides. Thus, the carbide networks are still present after austenisation, and as a consequence, the hot workability of the tool steel is reduced. Earlier investigations [5–7] reported that the controlled thermo- mechanical processing (TMP) is an effective method to control the microstructure, break up the carbide network and optimise the mechanical properties owing to refinement of the carbides and their uniform distribution. Furthermore, through TMP, the austenite grain size will be refined as a result of strain accumulation producing a higher density of nucleation sites and consequently refinement of final microstructure, as reported in [8–10]. However, the carbide forming elements in the tool steels present problems to hot deformation in terms of a narrow hot working temperature range [11]. The lower limit is defined by the type, size and morphology of carbides located along the grain boundaries and within grain interiors. The upper limit is determined by the incipient melting of eutectic carbides and by segregation of low melting point phases along the grain boundaries [4,12]. Though the studies on tool steels are extensive [4,13–17], the study of the effect of combining TMP and a double tempering process on the H21 tool steel is still limited. In this paper, microstructural evolution and variation of hardness during hot deformation and double tempering of a H21 tool steel were investigated. The precipitation behaviour was also studied. It is intended that this information will add to the existing knowledge base regarding the choice of appropriate hot deformation and heat treatment parameters to achieve desired microstructure and properties. Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/msea Materials Science & Engineering A 0921-5093/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2013.01.067 n Corresponding author at: Department of Materials Science and Engineering, The University of Sheffield, Sheffield, S1 3JD, United Kingdom. Tel.: þ44 114 222 5941; fax: þ44 114 222 5943. E-mail addresses: mtq09mn@sheffield.ac.uk, nurbanasarimeilinda@yahoo.com (M. Nurbanasari), p.tsakiropoulos@sheffield.ac.uk (P. Tsakiropoulos), e.j.palmiere@sheffield.ac.uk (E.J. Palmiere). Materials Science & Engineering A 570 (2013) 92–101