无套内谢少妇毛片A片樱花,波多野42部无码喷潮在线,成熟妇人A片免费看网站,女警乳链乳环蒂环铃铛调教,无码少妇高潮喷水A片免费,欧美性猛交XXXX乱大交极品,欧美性猛交XXXX乱大交3,蜜桃视频无码区在线观看,丁香啪啪综合成人亚洲,色噜噜噜色噜噜噜色琪琪

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.