P91 steel is a martensitic heat-resistant steel filled with Pearlite Heat-resistant Steel and austenitic heat-resistant steel between the temperature regions of new soda water pipeline steel. The highest temperature is 650 C. In fact, the original 9Cr-1Mo steel is based on the addition of V, Nb, N and other strengthening elements, resulting in the deterioration of the new steel.


Weldability of P91
P91 steel can be arc welded according to existing methods, including through argon welding (TIG). The selection of electrode and flux should be as close as possible to the chemical composition of the weld and base metal so that the creep and durability of the weld metal are the same or better than that of the base metal. Because the steel is insensitive to hot cracks, no cracks will appear before preheating to 150-200 C, and welding with various steels, such as P22 steel (pearlite heat-resistant steel), X20CrMoV121 (martensite heat steel) and TP304H (austenitic heat-resistant steel). The welding of P91 steel is introduced below.


A) For welding of P91 steel and P91 steel, 9Cr-1Mo (T9) or improved 9Cr-1Mo (T91) electrode is selected. The wall thickness of the latter is more than 25 mm and the preheating temperature is 200 C. After welding, it is slowly cooled to room temperature, and then tempered at a temperature higher than 730 degree C.
B) P91 steel is welded with 10CrMo910, the welding material is matched with 10CrMo910, and it should be cooled in air for 2 hours after release at 730 (?) C, because the two parts of the welding site belong to decarburization zone. If 10CrMo910 electrode is used for welding, the carbon content of the welding metal will be high enough to meet the requirement of durability strength.
C) P91 steel and P22 steel can be welded with 2.25Cr-1Mo electrode, preheated at 200 C, slowly cooled to room temperature after welding, then tempered at 700-725 C, or 5Cr1-1Mo electrode can be welded on the side of P91 steel, and then welded with P22 steel.
D) P91 steel is welded with X20CrMoV121. Two kinds of matching welding materials are used, such as P91 steel electrode, preheating temperature 250 C, slow cooling to 80 ~ 100 C after welding, tempering temperature above 750 C.
E) P91 steel and TP304H are welded with Inconel 182Ni-based alloy electrode. The preheating temperature is 200 C. After welding, it is tempered at 700-730 C to room temperature.


The heat treatment temperature after welding depends on the thickness of pipe wall. Small diameter pipe is treated for 0.5 h and large diameter pipe is treated for 1 h every 25 mm. Because of the high creep strength of P91, under the same conditions, the thickness of pipe wall is thinner than that of P22 steel, and the metal content of filling weld is relatively small, but P91 steel is sensitive to mode IV crack of weld. Therefore, it is necessary to minimize the system stress of P91 steel. Pipes with wall thickness over 12.5 mm are required to be cooled to more than 100 C after welding.