High-temperature deformation behaviour of duplex stainless steel with hard boronised layer

摘要

In order to understand the high-temperature deformation behaviour of alloy having hard surface layer,thermo-mechanically treated duplex stainless steel(DSS)is boronised for 0.75-6 h at 1223 K and subsequently deformed under compression mode at the same temperature under strain rate condition of 1×10^(-3),2×10^(-4) and 6×10^(-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 2.74μm.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 20-40μm 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.