Duplex grades production, worldwide, represent nowadays about 300,000 tonnes about 1% of the total Stainless Steel production. DSS products mainly are plates, tubes, bars, wires, coils. For those productions weight savings are often obtained taking advantage of their high mechanical properties. Most of the applications concerns highly corrosion resistance properties encountered in process industry. (Chemical, petrochemical, off-shore, chemical tankers, pulp and paper industry, pollution control equipments (Flue Gas Desulfurization), desalination, sea- water applications, see Figures 3-5 from Google picture). In those applications DSS have always been very competitive compared to 317 and 904 grades due to their reduced Nickel content and high mechanical properties compared to austenitic which make it possible to reduce thicknesses.

Figures 3-5 are the main application for DSS [1] .

According to Michael Mcguire [2] , DSS has much better corrosion resistance performance compared to other stainless steel. Usually pitting resistance ability for stainless steel is evaluated by PREN = %Cr + 3.3(%Mo) +

1.65(%W) + 16(%N), PREN, short of Pitting Resistance Equivalent Number, a measurement of the corrosion resistance of stainless steel. that is why DSS has much better pitting resistance capability than other stainless steel. PREN value for stainless steel. See Table 1.

Figure 6 and Figure 7 [2] shows DSS grade such as S32750, S31803 have much higher critical pitting temperature and stress corrosion cracking temperature than S30403 and S31603 in chlorine containing solution.

First, element contents exert obvious impact on duplex stainless steel’s deformation ability. Generally, the contents of Si, Mo, Cr, V, Ti increase the α phase amount and that of C, N, Ni, Cu, Mn contribute more to γ phase amount. It is reported that whenever the γ percentage is more than 20% and less than 80%, cracks is more likely to occur (see Figure 8).

According to schaeffler’s test, the element contents have direct influence on duplex stainless steel’s microstructure, determining the percentages of A(γ), F(α) and M (martensite). See Figure 9.

But S32304, S32205 and S32507 both have the roughly 50% α and 50% γ, so they have the worse deformation ability in stainless steel. It’s a big challenge to make them easy to process.

If temperature exceeds 1300˚C, only one phase of α is maintained, theoretically it’s good for deformation, but its negative aspect is the grain size of α will grow up rapidly and oppositely it make the thermoplasticity of duplex stainless steel drop dramatically and present fragile behaviour. So it’s harmful to increase the temperature too high. On the other hand, if the temperature do not climb to 1000˚C, so many types of brittle phases such as σ, χ, R will appear with worst plasticity to make it difficult to deform. As a result it’s appropriate to choose a temperature between 1050˚C and 1200˚C when being hot worked.

Chi-Shang Huang, Chia-Chang Shih [3] 2005 found that σ phase precipitates at about 600˚C - 1000˚C, of which it is a phase rich in chromium and molybdenum. And it not only will cause substantial damage to ductility and toughness of steel material, but it will also undermine corrosion resistance of DSS. Figure 10(a) and Figure 10(b) are the photograph taken by Optical Microscope and Scanning Electron Microscope.

σ phase is harmful to DSS because it is fragile and vulnerable to corrode. But N can decrease the σ phase amount effectively, see Figure 11. It can be found that the more the nitrogen content is, the less is the σ phase content. This is because nitrogen is an austenitic stabilizing element. It will directly increase the austenitic phase content and decrease the percentage of ferritic phase in DSS, thereby indirectly leaving less room for σ phase

after precipitation to develop. Consequently, the addition of nitrogen content does help slow down the precipitation rate of σ phase.

Henrik Sieurin, Rolf Sandström 2005 [4] found that the volume fraction of σ phase is related to temperature and time, see Figure 12. The precipitation starts at around 920˚C. The maximum of σ phase precipitation is found around 850˚C. No sigma phase appears below 650˚C. The longer stayed at 850˚C, the more of the σ phase precipitation [5] [6] .

T.H. Chen, J.R. Yang 2001 [7] and L.H. Chiu, W.C. Hsieh, C.H. Wu 2003 [8] found that the σ phase amount precipitated change dramatically with the cooling rate. In order to avoid σ phase precipitation, the cooling rate should approximately be in the range more than 0.25 K∙S⁻¹. See Figure 13 and Figure 14.

J.-O. NILSSON, P. KANGAS, T. KARLSSON, and A. WILSON [9] got the Volume percentage of intermetallic phase through the approach of ISO 9042, found the precipitation of intermetallic phase has significant influence on impact toughness, as well as observed the Cr2N and σ articles through backscattered image by EPMA and distribution of element N, Cr, Mo through X-ray when DSS aged at 900˚C/72h. See Table 2, Figure 15 and Figure 16 [10] .

They [9] also observed the backscatter electron micrograph by SEM when DSS aged for 72 h at various temperatures. Bright one is σ article and dark is Cr₂N. See Figure 17.

The phase diagram was calculated through Thermo-Calc [9] shows the stability areas of liquid, austenite, ferrite, Cr₂N, σ phase, and M₂₃C₆. See Figure 18.

Also the phase fractions of liquid, austenite, ferrite, Cr₂N, σ phase, and M₂₃C₆ [9] is calculated by Thermo- Calc. See Figure 19.

According to Michael Mcguire [2] , DSS should implement annealing process before the end usages to achieve the equilibrium composition within both the ferrite and austenite and rebalance the volume fraction of the ferrite and austenite as well as dissolve the unwanted intermetallic phase. See Table 3.

It’s important to consider the welding parameters such as Filler, heat input, shielding gas, preheat or not, max interpass, etc. See Table 4 for recommendation [11] .