When the heat source extends the weldment moving in the welding process, the temperature at a certain point of the weldment is low and high, and after reaching the maximum value, the heat source has a thermal effect on the base. Metals in welding process. Welding process is also a process of non-uniform heating and cooling or special heat treatment. Therefore, both welding heat treatment and welding heat treatment are heat treatment processes. The welding grain size can be controlled by controlling the welding heat treatment in the welding process, and the heat treatment can refine the grain size and reduce the welding residual stress.  The following is the effect of heat treatment control on the toughness of SA335-P91 material.


SA335-P91 steel is a modified 9Cr-1Mo martensitic steel containing V, Nb and other alloying elements. It has good resistance to high temperature oxidation and creep. AC1 is 830-850 degree C and AC3 is 900-940. C, MS = 370 C, Mf = 220 C.


The heating temperature of the coarse grained zone in the weld and heat affected zone exceeds 1100 C, and the grain growth is faster. The longer the residence time is, the coarser the grain is and the more brittle the coarse grain structure is.  Therefore, in order to solve this problem, it is necessary to control the energy (E) of the welding line. Low linear energy, short high temperature residence time, small grain size, greatly improve the toughness of welded joints.


The final martensitic transformation line of SA335-P91 steel is Mf = 220 C, that is to say, when the welding temperature reaches 220 C, the martensitic transformation in the welding structure is completed. In the process of manual arc welding, the upper limit of heat treatment temperature is above the starting line of martensite transformation, and the lower limit of heat treatment temperature is above the end line of martensite transformation, which makes the microstructure of welding become single martensite.


During field heat treatment, there is an error between the measured temperature of thermocouple and the welding temperature. There are also errors between the thermometer of the heat treatment machine and the actual temperature measured.  Therefore, it is an important task to adjust the error between the temperature of heat treatment machine and the actual temperature measured before work.  Obviously, the range of heat treatment used should be minimized. It is better to place another high precision thermometer as a reference. In addition, the position of the thermocouple is also very important. The thermocouple is placed at 20 mm on both sides of the weld to compare the interlayer temperatures of all welds at one level, so as to minimize the error between each weld.


The initial martensite transformation line of SA335P91 steel is 370 C, which means that martensite transformation begins to occur in residual austenite in the welding structure when the welding temperature is lower than 370 C. During the heat treatment, it is controlled at 370 C. In the following, considering the working environment of the welding machine, the actual temperature is usually lower. When the temperature exceeds 300 C, the welding operation is stopped immediately, and when the lower temperature limit is reduced to 200 C, the temperature continues to work.