The basic manufacturing process of thick wall P22 seamless alloy pipe is: melting, blank making, extrusion and annealing heat treatment.
(1) Smelting P91 steel has high alloy content. How to complete the alloying of steel in a short time is the key of smelting process. P22 seamless alloy pipe is smelted by 50t power electric arc furnace + 5T medium frequency induction furnace – VOD furnace refining – LF furnace refining – VD vacuum treatment. Two medium frequency induction furnaces are used to melt alloy materials. After tapping of 5OT electric furnace, the alloy melted in medium frequency furnace is mixed into the ladle. In the process of alloying, hydrogen is mixed to solve the problems of chemical composition control and liquid steel homogenization of low-carbon high alloy steel.
(2) Since the 360MN vertical extruder was put into operation, the traditional forging hole striking method is no longer applicable to the production of seamless tubes. P91, as a low-carbon high alloy material, has high crack sensitivity. Especially in the case of large specification, extrusion forming should be used instead of free forging. Seamless alloy tube extrusion is different from free forging. It is a pressure processing method to apply pressure on one end of the ingot placed in the extrusion cylinder to form it through the die hole. In extrusion deformation, it has a stronger three-dimensional compressive stress state than rolling, so that the deformed metal can exert its greater plasticity. Compared with free forging, this method can improve the internal structure of forging billet, and the yield is much higher than that of forging Keng hole production process. At the same time, this method has large deformation force, forming, and severe deformation, which leads to the further increase of metal temperature on the basis of the original heating temperature, resulting in the high final extrusion temperature.
(3) Whether the cooling process of annealed white spot sensitive ingot after melting and pouring is correct is the premise of product quality, and the secondary cooling after extrusion is also the key. Generally, P91 seamless tube billet adopts 700-770℃ and 600-670℃ stage isothermal annealing, so as to extrude the microstructure stress and residual stress generated during the microstructure transformation of seamless tube billet and prevent white spots during cooling.
(4) For heat treatment, the extruded seamless tube blank needs to be welded and assembled into the main steam pipe, hot reheat steam pipe and corresponding high-temperature header of large capacity thermal power unit. In addition to welding and elbow, it is necessary to maintain the required performance index in the harsh environment with steam temperature of 580-625℃ and pressure greater than 25MPa. Seamless steel tubes for high pressure boilers (GB5310-2008) meet the mechanical property requirements at room temperature.
In order to meet the final technical requirements of products and obtain good performance indexes, alloy seamless tube blanks need heat treatment after annealing. Considering the high element content of P91 alloy and the slow decomposition of austenite, combined with the material characteristics of the product, high-temperature austenitizing and air cooling quenching are adopted for heat treatment, and high-temperature tempering is adopted after obtaining martensite structure, so that the structure is transformed into tempered sorbite to obtain high comprehensive properties.