Sub-temperature annealing heat treatment process

Sub-temperature normalizing heat treatment

Sub-temperature normalizing refers to the heat treatment process in which hypoeutectoid steel is heated between Ac1 and Ac3 temperature and air-cooled after heat preservation (see Figure). After the hypoeutectoid steel is hot processed, the hardness of the pearlite lamellae is relatively large and the hardness is low. In order to improve its cutting performance, sub-temperature normalizing can be carried out. In actual production, in order to improve the cutting performance of medium carbon alloy steel, the traditional method is to use quenching and tempering or normalizing + high temperature tempering process(SIMUWU RVA type vacuum tempering furnace). Although the cutting performance is improved by O time after treatment, sometimes the effect is not ideal. In this regard, the sub-temperature normalizing process can be used. By controlling the heating temperature, the amount of carbides dissolved into austenite and the homogenization of austenite are suppressed Influence, make the austenite in an unstable state, thereby inhibiting the formation conditions of granular bainite during the cooling process to obtain the ideal machining hardness.

For example, the metallographic structure of 30siMn2MoVA steel that has been normalized and tempered at high temperature has a large carbide dispersion and is accompanied by granular bainite; after the sub-temperature normalizing process is used, the carbide dispersion is reduced, and the granular The bainite is eliminated, the cutting performance is obviously improved, and the processing quality is improved.

Sub-temperature normalizing can also improve the strength and toughness of hypoeutectoid steels containing granular bainite, such as 15 SiMnVTi steel. Sub-temperature normalizing at 770°C can improve the quantity, size and distribution of granular bainite; dislocation martensite replaces twin martensite in it, so that the mechanical properties of 15nVTi steel can fully reach Improved the performance of 441N structural steel.

For ductile iron, partial austenitization or low-carbon austenitization normalizing has been widely used, which is essentially sub-temperature normalizing. For example, partial austenitizing normalizing can obtain broken ferrite, and due to the proper shortening of the holding time, the austenite is low-carbonized, thereby obtaining a significant strengthening and toughening effect. For general rare earth-magnesium spheroidal graphite cast iron, the heating temperature range of austenitizing normalizing for heating up part is 810~870℃; the temperature range of austenitizing for cooling part is 850-940℃.