Duplex stainless steel application brief

Duplex stainless steel application brief

First and second generation duplex stainless steel

The application of the first generation of duplex stainless steel began in the 1930s, and its typical representatives are Type 329 (S32900) and Uranus 50 (S32404). Due to the high content of Cr and Mo in the alloy, this kind of material has excellent pitting resistance. The weldability of the first generation duplex stainless steel is poor. After welding, the austro-iron pair at the joint and in the heat affected zone will deviate from the original two-phase ratio of the base material seriously, resulting in the reduction of toughness and corrosion resistance. Although the properties can be recovered by post-welding heat treatment, the application of first-generation duplex stainless steel is limited by its poor weldability.

In the 1970s, the welding performance of duplex stainless steel was greatly improved by the quantitative addition of N to the alloy system. This is mainly due to the application of the argon-oxygen decarbonization process (AOD), which can add N in a more economical way and its content control is more precise. Although N was initially added because it was a cheaper austenitic forming element than Ni, engineers soon discovered other benefits of N, such as increased material strength, improved auro-iron ratio required for rapid recovery after welding, and increased local corrosion resistance. With the increase of N content, the transition temperature of ferrite to austenite increases, and the ideal austenite two-phase equilibrium can be obtained in the heat affected zone (HAZ) even after multiple thermal cycles. The beneficial effect of N enables duplex stainless steel to be used directly in the post-welding state, which greatly promotes the development of the second generation of duplex stainless steel. Precise addition and control of nitrogen content in the composite system has become the most important feature of the second generation duplex stainless steel, and the typical nitrogen content is in the range of 0.15-0.40%.

Classification of duplex stainless steel
In 1969, in order to evaluate the corrosion Resistance of duplex stainless steel in chlorine-containing media, metallurgists introduced the concept of Chromium Pitting Resistance Equivalent Number (PREN). The most common formula for calculating PREN is:

PREN = %Cr + 3.3[%Mo + 0.5(%W)] + 16[%N]

The empirical relationship is derived from the statistical regression of a large number of corrosion test results. The content of Cr, Mo, W and N in the alloy was used to characterize the relative ability of stainless steel to resist pitting and crevice corrosion. Duplex stainless steel can be divided into four categories according to the degree of alloying (see ISO17781 for details).

Saving duplex stainless steel Lean DSS (24≤PREN≤30)

Duplex stainless steel Standard DSS (30<PREN<40)

Super Duplex Stainless Steel Super DSS (40≤PREN<48)

Super Super Duplex Stainless Steel Hyper DSS (48≤PREN≤55)

Saving duplex stainless steel is characterized by a low Ni and/or Mo content. In order to compensate for the low austenitic phase ratio caused by low Ni content, it is necessary to increase the content of N and Mn in the alloy system. The economical DSS has very good strength and corrosion resistance, and is suitable for structural bearing materials, tank materials, and service environments where resistance to chloride stress corrosion cracking is required.

Standard DSS usually contains 22-25% Cr and 2-3% Mo and is by far the most widely used duplex stainless steel. Among them, 2205 alloy (S32205) is the workhorse in standard duplex stainless steel and is by far the most widely used of all second-generation duplex stainless steels. Product forms cover almost all metal material forms.

Super duplex stainless steel usually has a high Cr, Mo and N(or W) content to ensure a PREN of more than 40. This kind of duplex stainless steel is roughly equivalent to 6%Mo super austenitic stainless steel in terms of resistance to chloride pitting, and is widely used in the chemical industry, pollution control, Marine environment, acid leaching mining and other areas with harsh corrosive environments.

The super duplex stainless steels represented by S32707 and S33207 are designed for acidic and chloride-containing corrosion environments. Although the increase in Cr and Mo content further improves the corrosion resistance of the alloy, it also makes the super duplex stainless steel have a stronger tendency to second phase precipitation, which also means that more stringent control is required during processing. At present, this type of duplex stainless steel is only available for seamless pipes and tubes.