欧美成人午夜一区二区三区_欧美成人影音_欧美成熟美妇乱_欧美厨房性猛交xx乱大交_欧美大bbwbbwbbwvideos_欧美大尺度做爰无遮挡_欧美大色_欧美岛国大片_欧美动漫一区二区三区_欧美高清不卡_欧美高清在线_欧美国产黄_欧美国产禁国产网站cc_欧美国产亚洲自拍_欧美韩精品_欧美韩日国产_欧美黑寡妇黄网站免费_欧美黑丝少妇_欧美黄色主播_欧美激情3p_欧美激情办公室videoshd_欧美激情图片一区二区_欧美极品中文字幕_欧美经典大片_欧美精品日韩www.p站_欧美精品天堂_欧美久久伊人_欧美卡一卡二卡三_欧美来月经时猛交_欧美乱操视频

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.