Unlocking Corrosion Resistance: The Power of S31254 Stainless Steel in Industrial Applications

Performance of S31254

S31254 is a new type of high-alloy austenitic stainless steel. Its chemical composition is as follows: Carbon ≤ 0.2%; Silicon ≤ 0.8%; Manganese ≤ 1.0%; Sulfur ≤ 0.01%; Phosphorus ≤ 0.03%; Chromium 19.5% ~ 20.5%; Nickel 17.5% ~ 18.5%; Molybdenum 6.0% ~ 6.5%; Copper 0.5% ~ 1.0%; Nitrogen 0.18% ~ 0.22%. Its mechanical properties are: Tensile strength Rm ≥ 650 MPa; Yield strength Rpo2 ≥ 310 MPa; Elongation As ≥ 40%; Brinell hardness HB = 223. Due to the high molybdenum content in S31254, it has high resistance to pitting corrosion and crevice corrosion, making it suitable for environments containing halide ions such as seawater; it also has good resistance to uniform corrosion, especially in acids containing halides, superior to ordinary stainless steels; unlike ordinary stainless steels, it has excellent resistance to localized corrosion in terms of chemical composition. It has good pitting corrosion resistance (PI ≥ 40) under conditions of seawater, aeration, crevice, and low-velocity flushing, showing good stress corrosion resistance, making it an alternative material to high-cost nickel-based alloys and titanium; it also has better high-temperature resistance or corrosion resistance; additionally, from the classification of stainless steels, it exhibits a stable austenitic microstructure.

Welding Characteristics of S31254

It has good resistance to intergranular corrosion. According to the "chromium-depleted zone" theory, when the carbon content of austenitic stainless steel is higher than 0.08%, during heating, supersaturated carbon can precipitate along grain boundaries in the form of Cr23C6, leading to intergranular chromium depletion, thereby reducing the intergranular corrosion resistance of austenitic stainless steel; however, when the carbon content of austenitic stainless steel is lower than 0.02% ~ 0.03%, all carbon is dissolved in the austenite, so even between 450°C and 850°C, no "chromium-depleted zone" intergranular corrosion will occur. Since the carbon content of high-alloy austenitic stainless steel S31254 is ≤ 0.2%, intergranular corrosion can be basically eliminated by using smaller welding heat input and accelerating cooling rate during welding. Compared with other stainless steels, austenitic stainless steels have higher sensitivity to thermal cracking, mainly due to the following characteristics: (1) Austenitic stainless steels have a low thermal conductivity and a large coefficient of linear expansion. Under local welding heating and cooling conditions, the welding joint stays at high temperature for a longer time, and the welding metal and the nearby zone bear higher tensile stress at high temperature, forming larger tensile stress during cooling. The tensile stress during the solidification process of the weld metal is a necessary condition for solidification cracking. 

(2) Austenitic stainless steels are prone to forming columnar crystal weld structures with strong directional characteristics, and some harmful impurity elements are prone to forming low-melting-point liquid films between crystals, leading to weld solidification cracking. To prevent thermal cracking in austenitic stainless steel welds, the following measures can be taken: strict control of harmful impurities such as sulfur and phosphorus in materials; use of welding wires that can form duplex-phase structures in welds to disrupt the directional orientation of austenitic stainless steel crystals and reduce the segregation of low-melting-point eutectics between crystals; use of low-line energy and narrow-welding techniques to increase the cooling rate of the weld pool; use of argon arc welding with a heavy-duty tungsten electrode, with large arc energy and good cooling effect of the pipe airflow, small heat-affected zone, and well-formed internal welds without splatter and slag; to accelerate the cooling rate of the weld edge, water forced cooling can be used after welding; for welding between heat exchange tubes and tube sheets, stress grooves can be set on the tube bridge to reduce or even eliminate welding stress.

Use of S31254 Steel Pipe

The medium in the mother II heater tube bundle contains a high concentration of EHCO, which is a highly corrosive medium. The N-thermal composite has a service life of 20 carbon steel pipes, which is short, so equipment updates are frequent, seriously affecting production capacity; if anti-corrosion 20 carbon steel pipes are selected for the heat exchange tubes, the thermal conductivity of the anti-corrosion layer is too low, seriously reducing the heat exchange effect; if titanium pipes are selected for the heat exchange tubes, the service life is long and the effect is good, but the price of titanium pipes is three times that of S31254 steel pipes. S31254 has excellent resistance to pitting corrosion and crevice corrosion for titanium and austenitic stainless steels. Hole stress gang 1 has excellent resistance to uniform corrosion. Its welded joints have good intergranular corrosion resistance. Effective welding process elements can prevent hot cracking in welds, so S31254 steel is used as heat exchange tubes.

Because the shell medium is saturated steam, the heat exchange tubes should withstand vibration and fatigue loads, so the connection between the heat exchange tubes and the tube sheet should adopt expansion welding and sealing welding connection forms, and the hardness of the heat exchange tube material should be lower than that of the tube sheet material. S22053 steel is a dual-phase stainless steel with good resistance to Cl- corrosion, with a Brinell hardness value of 293 (HB), which is greater than the Brinell hardness value of S31254 steel pipe, 223 (HB), and good welding performance. Therefore, it is used. S22053 steel forgings are used as tube sheet materials, and stress grooves are opened on the tube bridge between the tube sheet holes. The shell box is made of Q345R steel plate, with the inner surface shot blasted and rust removed, and baked with phenolic alkyd varnish.

In the design of the mother heater, S31254 pipes are used as heat exchange tubes, S22053 forgings are used as tube sheet materials, and the inner surface of the shell box is coated with baked phenolic alkyd varnish to effectively prevent corrosion of the high concentration Cl-, HCO mother liquor II medium on the equipment. Practice has proved that using S31254 high-alloy austenitic stainless steel instead of high-cost nickel-based alloys and titanium to prevent corrosion of Cl-, HCO is an effective method to extend the service life of equipment, improve production capacity, and reduce manufacturing costs.