Duplex stainless steel is generally divided into four types: Cr18, Cr21, Cr25 and super duplex stainless steel with 25% to 26%. UNS S31803 is a standard duplex stainless steel, Cr21 type, accounting for more than 80% of the total duplex stainless steel. In the production of UNS S31803 is widely used, especially in the application of many operation medium acid, nitrogen, acetophenone equipment, the medium is highly toxic and explosive, temperature is generally high and equipment, close to 200 degrees, so the weld requirement is very high. Based on the wide application of UNS S31803 in production and the poor use of media in contact, it is of great practical significance to choose UNS S31803 as the welding base material to study its weldability.
The composition characteristics of UNS S31803: the mass fraction of C in 0.02% ~ 0.03%, which belongs to the ultra low carbon; the mass fraction of Cr in 23% ~ 26%, belongs to the high concentration of Cr; its organizational features: two phase (austenite and ferrite) each accounted for about 50%; performance characteristics: because both ultra low carbon and iron at room temperature the ferrite and austenite, high Cr content and add features of Mo, N and other elements, so it has a higher resistance to stress corrosion, crevice corrosion resistance, Cl- corrosion, intergranular corrosion resistance, high temperature resistance and sediment erosion ability, has good mechanical properties and low thermal expansion properties, good solderability and low cost etc..
In the process of welding, the microstructure of the heat affected zone of the welded metal box has undergone a series of changes under the action of thermal cycling. The microstructure of duplex stainless steel is composed of ferrite at high temperature, while austenite is precipitated during cooling. UNS S31803 welding process should pay attention to the following three aspects:
(1) welding crack. The Creq/Nieq of UNS S31803 is higher, the content of Cr is 23%, and the content of Ni is 6.5%, that is duplex stainless steel UNS S31803 Creq/Nieq=3.54. Therefore, the weld of UNS S31803 belongs to the ferrite solidification zone, and has certain sensitivity to solidification cracks. Therefore, in the welding process, it is necessary to use appropriate welding materials, welding methods and control welding process parameters, so that the welding seam has a good ability to resist solidification cracks. Cold crack is related to the delayed crack susceptibility of hydrogen, but the cold crack can be avoided if the welding material is used correctly.
(2) embrittlement of heat affected zone. In the welding process, the weld heat affected zone is in a fast cooling non-equilibrium state, and more ferrite is retained after cooling so as to increase the corrosion tendency and the susceptibility to hydrogen induced cracking. Therefore, the main problem of UNS S31803 welding is the heat affected zone. The main problem of heat affected zone is the loss of corrosion resistance, toughness loss and post weld crack. In order to avoid these problems, the welding process should consider reducing the material at a temperature range of hot time.
(3) stomatal sensitivity. The formation of pores depends on the welding parameters and alloy composition. In order to prevent porosity and reduce porosity sensitivity, the nitrogen content in the weld should be controlled during the welding process, so as to strengthen the protection of the welding seam and improve the permeability of the slag.
Source: China Duplex Stainless Steel Pipe Manufacturer – Yaang Pipe Industry Co., Limited (www.yaang.com)