Role Of Each Alloying Element in A335 P91

① Carbon is the element with the most obvious solid solution strengthening effect in steel. As the carbon content increases, the short-term strength of the steel increases, and the plasticity and toughness decrease. For martensitic steels such as P91, the increase in carbon content will Accelerate the spheroidization and aggregation rate of carbides, accelerate the redistribution of alloy elements, and reduce the weldability, corrosion resistance and oxidation resistance of steel. Therefore, heat-resistant steel generally hopes to reduce the carbon content, but if the carbon content is too low, the steel's Intensity will be reduced. Compared with 12Cr1MoV steel, P91 steel has a 20% lower carbon content, which is determined by comprehensively considering the above factors.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

②P91 steel contains trace amounts of nitrogen, and the role of nitrogen is reflected in two aspects. On the one hand, it plays the role of solid solution strengthening. The solubility of nitrogen in steel at room temperature is very small. During the welding heating and post-weld heat treatment processes in the post-weld heat affected zone of P91 steel, the solid solution and precipitation processes of VN will occur successively: During welding heating The nitrogen content of the austenite structure formed in the heat affected zone increases due to the dissolution of VN. After that, the degree of supersaturation in the normal temperature structure increases, and fine VN precipitates during the subsequent post-weld heat treatment, which increases the structural stability. , improving the lasting strength value of the heat affected zone. On the other hand, P91 steel also contains a small amount of A1, and nitrogen can form A1N with it. A1N only dissolves into the matrix in large quantities above 1100°C, and re-precipitates at lower temperatures, which can achieve a better dispersion strengthening effect.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

③ Adding chromium is mainly to improve the oxidation resistance and corrosion resistance of heat-resistant steel. When the chromium content is less than 5%, violent oxidation begins at 600°C, and when the chromium content reaches 5%, it has good oxidation resistance. 12Cr1MoV steel has good oxidation resistance below 580℃, with a corrosion depth of 0.05 mm/a. Its performance begins to deteriorate at 600℃, with a corrosion depth of 0.13 mm/a. The chromium content of P91 is increased to about 9%, and the service temperature can reach 650°C. The main measure is to dissolve more chromium in the matrix.
④ Vanadium and niobium are both strong carbide-forming elements. When added, they can form fine and stable alloy carbides with carbon, which have a strong dispersion strengthening effect.
⑤The main purpose of adding molybdenum is to improve the thermal strength of steel and play the role of solid solution strengthening.