SPECIAL ISSUE ARTICLE Microstructural and mechanical characterisation of friction stir welded 15-5PH steel T. Weinberger, N. Enzinger and H. Cerjak* Martensitic precipitation hardening steels are characterised by high strength which is achieved by a martensitic matrix and precipitates. The material also shows a good ductility and toughness if properly heat treated. But welding of these steel types is often problematic and requires a special procedure (e.g. post-weld heat treatment) in order to achieve satisfactory results. In this contribution, the solid state welding process – friction stir welding was used to weld 15-5PH and the results of the investigations are shown. The butt welds for 2?6 mm thick steel sheets have been carried out at Institute for Materials Science and Welding at Graz University of Technology using tungsten based tools, different welding speeds and tool rotational rates. Temperature measurements using thermocouples have been performed on the advancing and retreating sides of the weld. Detailed microstructural observations were performed for base material, heat affected zone, thermomechanically affected zone and stir zone. The appearance of retained austenite, which reduces the strength of the material, has been studied for the distinct regions of the friction stir weld. A quantitative spot analysis by energy dispersive spectroscopy was performed to identify tool remanents in the stir zone of the weld. For further characterisation, hardness profiles of the weld have been created. Tensile tests and surface fracture analysis using scanning electron microscopy have been performed. Welds with low energy input have shown better results than welds with high energy input. Additionally, effects of post-weld heat treatment on microstructure and properties of the joint have been analysed. Keywords: Friction stir welding, Martensitic precipitation hardening steels Introduction Martensitic precipitation hardened (PH) stainless steels like 15-5PH are widely used as structural material because of their balanced combination of good mechan- ical properties (high strength, relatively good ductility) and excellent corrosion resistance. This type of steel has been used for a variety of applications including oil field valve parts, chemical process equipment, air craft fittings and fasteners, etc. 1–3 The 15-5PH steel is a martensite stainless steel containing approximately 2?5–5?0 wt-%Cu and is strengthened by the precipitation of submicro- scopic copper rich particles in the martensite matrix. 4 The martenistic PH steels have the M F temperature just above room temperature resulting in a complete transformation to martensite upon cooling from solu- tion–treatment temperature. The highest values of strength and hardness correspond with the lowest ageing temperatures due to the steel structure containing fine, coherent with matrix, body centred cubic copper rich precipitates. Increasing the ageing temperature causes consequential loss of coherency between steel matrix and copper precipitates lattices, growth of the precipitates and their lattice type transition to face centred cubic. 4,5 Several studies have been published reporting the microstructure and mechanical properties of fusion welded PH steels. 6–8 Generally this grades of steel show a good weldability but overageing in the heat affected zone (HAZ) takes place and softens the material. Therefore, weldments of PH steels usually are not put into service in the as welded condition. In order to obtain weld properties approximating those of the base material, post-weld heat treatment (PWHT) is necessary. 1 Friction stir welding (FSW) is a solid state welding process and was invented by TWI in 1991. 9 First, the process was used for alloys with low melting tempera- ture like aluminium and magnesium. Second, FSW of different steel types was performed and outstanding properties were achieved. 10,11 It is expected that the lower heat input of this process leads to less Graz University of Technology, Institute for Materials Science and Welding, Austria, JOIN – Network of Excellence for Joining, Austria *Corresponding author, email horst.cerjak@tugraz.at ß 2009 Institute of Materials, Minerals and Mining Published by Maney on behalf of the Institute Received 30 September 2008; accepted 22 December 2008 DOI 10.1179/136217109X406956 Science and Technology of Welding and Joining 2009 VOL 14 NO 3 210