Carburizing of Duplex Stainless Steel (DSS) Under Compression Superplastic Deformation
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TRODUCTION
CONVENTIONAL surface hardening via diffusion methods such as carburizing, boronizing and nitriding (regardless under solid, liquid or gas medium) can be considered as a static process. This is due to the fact that there is no force element introduced into the system during the process which will affect the shape and dimension of the substrate. Hard layer surface is basically via through thermo-chemical reaction, whereby heat is required to enhance the diffusion of hardening atoms. Generally, the layer thickness exhibits timetemperature dependence. Studies have shown that superplastic boronizing, whereby boronizing is conducted concurrently with superplastic deformation of the substrate, could provide faster rate and superior mechanical properties compared with conventional methods.[1,2] The introduction of a force leading to substrate plastic deformation is considered to be the novelty of the process. However, previous studies are carried out under tensile and it is felt that a more practical study under compression mode is warranted.[3] On the other hand, mechanics of superplasticity of metals and alloys has been studied mainly by means of a tensile test in which a true constant strain or a constant displacement rate is imposed, and the steady-state flow stress is recorded. The optimal superplastic deformation temperature, strain rate, maximum elongation, and flow stress, as well as strain rate sensitivity of the material are obtained through experiment.[3] However, in practical applications, there are few cases similar to the simple tension test but many involve compressively formed products. In this experiment, a NOR WAHIDA AHAMAD, Ph.D. Student, and ISWADI JAUHARI, Senior Lecturer, are with the Department of Mechanical Engineering, Faculty of Engineering, University of Malaya, Lembah Pantai, 50603 Kuala Lumpur, Malaysia. Contact e-mail: [email protected] Manuscript submitted June 21, 2011. Article published online September 1, 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A
different approach of SPC under compression method was studied. This method was selected because it is beneficial to find the optimal parameters of superplastic deformation compare with tension tests. In addition, compression tests have the advantage of avoiding the problem of necking particularly at high strain rates. The surface properties and kinetics of SPC of DSS under compression mode have been studied, and the results show that the carburizing process is highly accelerated by the concurrent superplastic deformation.[4] This process is conducted by SPC under a fixed strain rate condition. In the present work, carburizing of DSS is carried out in order to understand the carburizing process behaviour under superplastic deformation. The effects of deformation temperature and strain rate on the carburizing process are investigated. The results are compared with those from conventional carburizing (CC) and nonsuperplastic carburizing (NSPC). It is expected that this study will provide a fresh insight on carburizing process development an
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