A large part of the service life of the chain is due to heat treatment, Therefore, the company continuously introduces advanced heat treatment equipment and develops new heat treatment process; the ultimate load of chain mainly depends on the heat treatment process of chain piece; the initial elongation time and wear resistance (service life) of chain mainly depend on the heat treatment process of sleeve and pin shaft; according to barrel theory, the quality of the whole finished chain depends on the part with the lowest quality Therefore, we strictly control the number of parts, carburizing or quenching and tempering time to ensure the uniformity of the quality of each part, so that the surface hardness and internal toughness of each part can reach a balanced optimal value, so as to achieve the quality stability of the finished product chain.