Effect of on-site heat treatment on microstructure and toughness of SA335-P91 material

When the heat source extension weldment moves during the welding process, the temperature at a certain point on the weldment is low and high, and after reaching the maximum value, it is high and low, and the heat source has a thermal effect on the base metal during the welding process. The welding process is also a process of uneven heating and cooling, or a special heat treatment. Therefore, the welding heat process and the welding heat treatment are both heat treatment processes. The size of the weld crystal grains can be controlled by controlling the welding heat process during the welding process, and the heat treatment can refine the grains and reduce the welding residual stress. The following is the effect of heat treatment control on the toughness of SA335-P91 materials.

SA335-P91 steel is a modified 9Cr-1Mo martensitic steel with V, Nb and other alloying elements. It has good resistance to high temperature oxidation and creep resistance. AC1 is 830-850°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 above 1100°C, the coarser the grain coarsening and the greater the brittleness of the coarse-grained structure. So to solve this problem, you must control the welding line energy (E). The line energy is small, the high temperature residence time is short, the crystal grains are small, and the toughness of the welded joint is greatly improved.

The final line of martensitic transformation of SA335-P91 steel is Mf=220°C, that is to say, when the weld temperature reaches 220°C, the martensite transformation in the weld structure is finished. In the manual arc welding process, the upper limit of the heat treatment temperature is controlled above the martensite transformation start line, and the lower limit of the heat treatment temperature is controlled above the martensite transformation end line, so that the weld microstructure becomes a single martensite.

There is an error between the measured temperature of the thermocouple and the temperature of the weld during the on-site heat treatment. There is also an error between the temperature gauge of the heat treatment machine and the actual temperature measured. Therefore, it is an important task to adjust the error between the temperature of the heat treatment machine and the measured actual temperature before work. It is clear that the range of heat treatment used is to be minimized. It is better to place another high-precision thermometer as a reference. In addition, the placement of the thermocouple position is also very important. The thermocouple arrangement was placed 20 mm on both sides of the weld, so that the interlayer temperatures of all welds were compared at one level, minimizing the error between each weld.

The initial transformation line of SA335P91 steel martensite is 370°C, which means that when the weld temperature reaches below 370°C, the retained austenite in the weld structure begins to undergo martensite transformation. During the heat treatment, it is controlled at 370°C. In the following, considering the working environment of the welder, the actual temperature is generally low. When the temperature exceeds 300° C, the welding operation is stopped immediately, and when the lower temperature limit is lowered to 200° C, the temperature continues to work.