ORIGINAL PAPER High-temperature deformation behaviour of duplex stainless steel with hard boronised layer N.M. Sultan 1 • I.Jauhari 1 • R.Saidan 1 • M.F.M. Sabri 1 Received: 3 December 2019 / Revised: 23 March 2020 / Accepted: 1 April 2020 / Published online: 26 January 2021 Ó China Iron and Steel Research Institute Group 2021 Abstract In order to understand the high-temperature deformation behaviour of alloy having hard surface layer, thermo-mechani- cally treated duplex stainless steel (DSS) is boronised for 0.75–6 h at 1223 K and subsequently deformed under com- pression mode at the same temperature under strain rate condition of 1 9 10 -3 ,2 9 10 -4 and 6 9 10 -5 s -1 until strain of 0.4. The substrate microstructure is almost isotropic with grain size after boronising with layer thickness between 1.61 and 2.74 lm. X-ray diffraction results confirm the formation of boride on DSS surface. The surface hardness of DSS increases from 387 to 1000–2400 HV after boronising. Uniform boronised layer with thickness of 20–40 lm is formed at DSS surface. Compression results show that the flow stress of the deformation increases with the strain rate and boronising time. For the boronised samples, the flow stress range is between 5 and 89 MPa. To determine the actual effect of the boronised layer on the flow stress, the results are also compared with those from un-boronised samples having similar microstructure. The results suggest that at a constant grain size, even with the hardest layer, the effect of hard surface layer on the flow stress almost could be negligible when the deformation rate is slow, but at faster deformation rate, even in the layer with the least hardness, the flow stress shows a significant increase. It is also observed that the hard boride surface disintegration could be avoided at a sufficiently low deformation flow stress that could be attributed to superplasticity. Keywords High-temperature deformation Á Superplasticity Á Duplex stainless steel Á Hard boride layer Á Surface integrity Á Grain size effect Á Strain rate effect 1 Introduction Wear-resistant surface coatings [1–6] and surface harden- ing [7–17] are implemented as the main objective to increase object service life [18], thus minimising mainte- nance and operating cost [19]. Case boronising offers a simple effective surface hardening process of steels [20]. In the process, boron atoms are diffused into the surface of metal component creating hard boronised layer [21] that has successfully preserved various components from wear damages including hip-joint implants [22], valve seal [23], ball bearing [24], and tools and dies for hot forging [25]. The common boronised surface in these applications is in the hardness range of 800–1200 HV [26]. Boronised layer is basically partitioned to boride and diffusion zone layer. Boride layer is reported to be prone to disintegration when undergoes deformation [27]. It is most likely due to residual stresses in the layer owing to its thermal expansion coefficient inequality [28]. Many reports have attempted to control boronising parameter in order to curb boride layer formation [29]. This is because the brittle characteristic of the boride layer may cause problem in the subsequent forming step. However, by doing this, the excellent surface hardness of boride layer would be sacrificed. Duplex stainless steels (DSSs) are the most recent family of stainless steels with two-phase microstructure consisting of ferrite and austenite grains [30, 31]. DSS is used for their good mechanical properties and excellent corrosion resistance properties, especially stress corrosion cracking resistance [32–35]. Additionally, a fine & I.Jauhari iswadi@um.edu.my 1 Department of Mechanical Engineering, Faculty of Engineering, University Malaya, Kuala Lumpur 50603, Malaysia 123 J. Iron Steel Res. Int. (2021) 28:244–253 https://doi.org/10.1007/s42243-020-00544-4