久久久久国产一级毛片高清板_成人国产内射综合_日韩成人在线观看_久久久久免费看成人影片

as a kind of common metal products, steel spade is widely used in agriculture, construction and other industries. the quality of steel spade depends on the production process and heat treatment process of steel spade. the heat treatment process of steel includes annealing, normalizing, quenching and tempering and surface heat treatment. tempering includes high temperature tempering, medium temperature tempering and low temperature tempering. tempering is to reheat the quenched steel to a certain temperature and then cool it by a certain method. its purpose is to eliminate the internal stress produced by quenching, reduce the hardness and brittleness, so as to achieve the expected mechanical properties.