Carburizing of Duplex Stainless Steel (DSS) Under Compression Superplastic Deformation
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- Ahamad, N.W. & Jauhari, I. Metall and Mat Trans A (2012) 43: 5115. doi:10.1007/s11661-012-1357-4
A new surface carburizing technique which combines superplastic deformation with superplastic carburizing (SPC) is introduced. SPC was conducted on duplex stainless steel under compression mode at a fixed 0.5 height reduction strain rates ranging from 6.25 × 10−5 to 1 × 10−3 s−1 and temperature ranging from 1173 K to 1248 K (900 °C to 975 °C). The results are compared with those from conventional and non-superplastic carburizing. The results show that thick hard carburized layers are formed at a much faster rate compared with the other two processes. A more gradual hardness transition from the surface to the substrate is also obtained. The highest carburized layer thickness and surface hardness are attained under SPC process at 1248 K (975 °C) and 6.25 × 10−5 s−1 with a value of (218.3 ± 0.5) μm and (1581.0 ± 5.0) HV respectively. Other than that, SPC also has the highest scratch resistance.
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 time-temperature 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. 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. However, in practical applications, there are few cases similar to the simple tension test but many involve compressively formed products. In this experiment, 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. 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 non-superplastic carburizing (NSPC). It is expected that this study will provide a fresh insight on carburizing process development and the application of DSS in industries.
2 Materials and Experimental Procedures
Chemical Composition of Duplex Stainless Steel (JIS SUS329J1) in Weight Percent
The Carburized Layer Thickness and the Surface Hardness Values for SPC Process
Temperature [K (°C)]
Strain Rate (s−1)
Carburized Layer Thickness (μm)
Surface Hardness (HV)
1 × 10−4
1 × 10−3
2 × 10−4
1 × 10−4
6.25 × 10−5
NSPC was conducted using the same procedure as SPC at 1248 K (975 °C) and 6.25 × 10−5 s−1.
Several techniques were used to characterise the samples. To investigate the microstructure of the carburizing layer, optical and scanning electron microscopy tests were conducted. X-ray diffraction (XRD) of the specimens before and after SPC was carried out. Phase identification was made by X-ray diffraction using Philips X’Pert MPD PW3040 with Cu-kα radiation at 1.54056 Å X-ray wavelength. The surface and layer hardness were determined using Vickers hardness tests at a load of 25 gf. Scratch test was also performed on the samples using microscratch tester with 1000 mN load at 300 μm constant distance.
3 Results and Discussion
3.1 Flow Stress Behaviour
3.2 XRD Analyses
3.3 Structural Observations
3.3.1 Microstructure analyses
3.3.2 Carburized microstructure and layer thickness
From these results, it shows how superplastic deformation has managed to produce a carburized layer with thickness much higher than that of the other methods. The carburized layer thickness has differences although carburized at the same temperature and time condition. It gives the interpretation that a different controlling the diffusion process of the carbon atoms in the DSS at each carburizing method. In the CC method, the self and grain boundary diffusion is expected to be the control mechanism. On the other hand, under the NSPC, the conventional creep deformation mechanism where deformation occurs through stress-induced diffusional flow that elongates the grains. It is expected to take place since the microstructure of the as-received DSS is considered coarse about 10 μm. Meanwhile, it is well understood that superplastic deformation occurs mainly from the grain boundary sliding and slipping of the fine grains. Thus, it is expected here that the same grains were to have been transporting the carbon atoms from the surface to the inner part of the substrate through the said grain boundary sliding and slipping. As a result, a much faster diffusion rate of carbon was obtained through the SPC method. The NSPC shows thicker carburized layer as compared to the CC owing to the stress-induced diffusional flow. It is a well known fact that simultaneous deformation could enhance diffusion coefficient through recrystallization.[11–13] However the studies diffusion is restricted to the elements originally exits in the material. In this study we have been able to show that the diffusion of outsider element (element which is originally not exits in the material) can also be enhanced through simultaneous deformation, and through superplastic deformation the diffusion process can be further enhanced.
3.3.3 Hardness profile
Although showing thicker carburized layer than CC which as explained previously, NSPC gives lower surface hardness. The amount of carbon exist on the surface can be reflected by the surface hardness. This means that the amount of carbon atoms diffused into the DSS layer under NSPC is lesser than CC. It is again indicates that the grain size of the material does have an effect on the surface hardness which is similar as the formation of carburized layer. Therefore, the smaller the grain size, the harder the carburized layer formed. This is due to the higher value of grain boundaries available on the surface for fine size grains compared to the coarse size grains which promotes more diffusion of carbon atoms at the surface area. As a result, more carbon atoms are available at the surface area of the fine grain microstructure DSS resulting in a high amount of carbon atoms which in return increases the surface hardness. Although NSPC has coarse grain size, it could be diffused further the carbon atoms into the substrate however the diffused amount is lesser.
Meanwhile, under SPC, the carbon atoms are diffused further into the substrate and the diffused amount is considerably higher than the other two carburizing processes. This clearly indicates that the superplastic deformation effect under the compression process not only increase the carbon layer thickness but also improves the surface hardness of DSS as well.
3.3.4 Scratch test
In this study, a new method for carburizing DSS, called superplastic carburizing under compression mode has been carried out successfully. In this method, the SPC is strongly affected by temperature and strain rate. The layer thickness and the surface hardness for the SPC process are (281.3 ± 0.5) μm and (1581.0 ± 5.0) HV respectively at 1248 K (975 °C) and 133 min/6 × 10−5 s−1 strain rate. This shows that a thicker and harder carburized layer is attained on the surface of the substrate and also the highest scratch resistance compared with the conventional and non-superplastic carburizing method. The diffusion of carbon atoms into the substrate is highly accelerated via grain boundary sliding and slipping mechanisms. This finding will certainly enhance the potential of SPC process for more explorations.
The authors graciously acknowledge University Malaya for funding this research under the High Impact Research (HIR) (Grant No: J-16001-73804).