Hot Forging of C45 using PVD (Ti,Al)N/g-Al 2 O 3 Coated Dies K. Bobzin 1) , G. Hirt 2) , F. Springorum 3) , U. Zitz 3) , F. Steinhof 3) , N. Bagcivan 1) , R. Baadjou 2) , M. Ewering 1) * , and P. Immich 4) ** 1) Surface Engineering Institute, RWTH Aachen University, Augustinerbach 4-22, 52062 Aachen, Germany 2) Institute of Metal Forming, RWTH Aachen University, Intzestraße 4, 52056 Aachen, Germany 3) Hammerwerk Fridingen, Hammerwerkstraße, 78567 Fridingen, Germany 4) LMT Fette GmbH, Grabauer Str. 24, 21493 Schwarzenbek, Germany *Corresponding author, E-mail: ewering@iot.rwth-aachen.de **Corresponding author, E-mail: p.immich@fette.com The process of hot forging with permanent moulds is a challenge in respect to the very high thermal, mechanical and tribological loads on tools. Ensuring sufficient lifetime application of protective films can be beneficial. Initial screening experiments using PVD coated compression plates show that one of the metastable phases of alumina, the g-phase, exhibits high strength and toughness and fulfils the requirements for a protective coating. The next important step in the development towards an industrial application is the implementation on complex tool shapes and verification in real forming experiments. After coating deposition using an industrial coating unit, coated dies were tested in forming experiments under industrial conditions. The forming experiments show an improvement of the wear resistance after 1000 forming cycles for the coated dies compared to the uncoated dies. Keywords: PVD, tool coatings, aluminium oxide, g-Al 2 O 3 , hot forging, plasma nitriding, hot working steel Submitted on 26 February 2010, accepted on 25 May 2010 Introduction Hot forging of steel using uncoated but pre-treated tool steels is state-of-the-art in industrial mass production. Lifetime enhancement of hot forging dies is reaching R&D focus. Due to the high forming temperature range of 1100 8C to 1400 8C and high mechanical loads, a sufficient surface protection is needed when using conventional die materials. Investigations on conven- tional PVD (Physical Vapour Deposition) ceramic coat- ings, especially TiN [1], multilayer TiAlN [2], TiBN [3] and CrN based coatings [4], show that the lifetime of the tools can be enhanced. Up to now, only nitride and boride based coatings were investigated for a possible hot forging die protection. A new integral approach by the Aachen Collaborative Research Centre (SFB) 289 ‘‘Forming of Metals in the Semi-Solid State and their Properties’’, based on performance-related screening tests, was chosen: Dedicated coatings adapted for their particular application were developed and analysed with respect to the expected load types. Furthermore, the combination of substrate and coating was considered, as the success of a coating application depends not only on the coating properties but also on the properties of the whole compound. Within the SFB 289, metastable oxides showed a good combination of hot hardness, wear resistance, and chemical stability. In particular, g-Al 2 O 3 synthesised by PVD technology is one of these promising candidates [5, 6]. The PVD technology enables the deposition of crystalline Al 2 O 3 at comparatively low temperatures, where hot working steel (like 1.2343 or 1.2367) can be used as tool material without loss of hardness [7]. Here, the integral approach is elucidated using the example of hot forging of steel. First of all, the substrate was examined with regard to the high application temper- atures. In a second step, the development and analysis of a (Ti,Al)N/g-Al 2 O 3 coating for its use in hot forging is described. Finally, different substrate and coating combi- nations were tested in industrial application. Experimental Setup Substrate investigations. Annealing experiments were performed to simulate the impact of the high forming temperatures on the substrate hardness. The purpose of this was to obtain a general understanding of die substrate behaviour during forming operations, especially in surface near regions where the impact of high forming temperature and mechanical loads are the most significant failure mechanisms. As substrate material 1.2367 (X38CrMoV5-3) was used and vacuum heat treated with an austenitising temperature of 1078 8C. After a holding time of 30 min the substrate material was quenched under controlled conditions and then tempered 3 times at 580 8C and once at 620 8C to reach the required hardness of HRC 48. In a next step, uncoated 1.2367 HRC 48 and uncoated plasma nitrided 1.2367 HRC 48 hot working steels were annealed for one hour at different temperatures (RT, 500, 700 and 900 8C) in air. The annealing time of 1 hour was chosen to simulate the heat flow during the forging process: Due to the later used forging setup 6 parts per minute were forged. The heat transfer of every part into the tool was around 3.8 seconds during the hot forging step. Forging of 1000 parts therefore results in a heat input duration of DOI: 10.1002/srin.201000031 steel research int. 81 (2010) No. 7 www.steelresearch-journal.com ß 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 603