Ball of convection furnace (hereinafter referred to as the ball of the furnace) from the furnace, furnace, lid, gas protection system, cooling water circulation system and the cabinet composed of several parts，Can be used for low, medium carbon steel and alloy steel wire ball annealing and recrystallization annealing. Furnace main features of the ball are:
① using electric heating, high precision temperature control, heat treatment process is automatic;
②Bottom convection fan installed in the whole process of heat treatment to achieve a stable circulation heat transfer, temperature uniformity is good, the design temperature control accuracy ± 5 ℃, the actual reach ± 3 ℃；
③Ammonia decomposition as a protective gas or nitrogen gas, achieved no oxidation, no decarburization heat, less wear and tear of metal；
④Load of optional flexible according to production requirements, for different varieties, quantities, production organization to facilitate；
⑤Pit structure, can be buried underground, can not adapt to the high elevation of the old crane foundation conditions;
⑥Easy to operate, safe and reliable。
    Furnace equipment, the ball has been in the drawing, cold heading, widely used in the fastener industry, is globalization, and then annealed the best upgrade equipment.

　　Recrystallization is hypoeutectoid Ac3 steel heated to more than 30 ~ 50 ℃, holding a certain time to make it fully austenitized and then cooled slowly, most of the fully annealed steel is heated, the temperature is 780 ~ 880 ℃. This annealing is mainly used for sub-eutectoid steel casting, forging.
    Recrystallization purpose：
⑴Eliminate the stress, tissue defects, overheating defect；
⑵Improve the organization, grain refinement；
⑶Reduce the hardness, ease of machining；
⑷Improve the plasticity and toughness.

　　Annealing process commonly used is a ball isothermal annealing, the commonly used ball annealing temperature above Ac1 20 ~ 50 ℃, the appropriate time to stay at this temperature, the slowly cooled to a temperature just below Ar1 and stay longer Time,Such as: 45 # steel Ac1724 ℃ Ar1682 ℃, the microstructure transformation completed, and then with the furnace cooling to 500 ~ 550 ℃, baked air cooling.
    The purpose of spheroidizing：
⑴Reduce the hardness of the workpiece, to facilitate machine processing, and organizational preparation for the quenching to do;
⑵The eutectoid eutectoid steel, steel, or get the ball off of (granular) pearlite and so on.

　　Wire in the absence of controlled atmosphere furnace heated, because of the furnace containing CO, CO2, H2, N2, H2O, O2, and CH4 and other gases react with the steel surface will make steel surface oxidation and decarbonization. CO, CO2, H2O, etc. are oxidizing atmosphere, and it can react with the iron in steel, so steel crunchy oxide formed on the surface, a phenomenon known as oxidation.
　　Oxide formation is not only consumed metal, and increased consumption of acid pickling. To this end, the heat treatment should be to minimize the amount of surface oxide formation. CO2, H2O, O2 and H2 such as energy and carbon steel surface, forming gas to the surface of the carbon is burned wire, a phenomenon known as decarburization. Decarbonization of the steel surface resulting in lower carbon content, resulting in decreased surface hardness. Surface decarburization, the inner surface diffusion of carbon Pianxiang. This allows decarburization layer gradually deepened, the longer the heating, decarburization deeper. Strong oxidizing atmosphere in the heating surface decarburization and oxidation will occur simultaneously.

　　Overheating is when the heating temperature is above Ac3, the excessive grain growth of steel, which led to weakening the binding force between grains, resulting in deterioration of mechanical properties of steel, this defect is called overheating. 　　The degree of grain growth and the heating temperature and time. The higher the temperature, the longer the grain to grow larger when the heating near the solidus, the grain growth and rapid overheating. Overheated wire showing coarse grains in the fracture, the cause is: wire the heating temperature is higher, in the soaking time is too long; Another reason is that the chemical composition of steel, the heating of the sensitivity of steel, such as manganese steel will increase Overheating tendencies, especially aluminum increased thermal sensitivity, and tungsten, molybdenum can promote grain growth of steel, silicon can promote grain refinement. Thermal annealing method can be saved, can be properly controlled to prevent overheating of the furnace air temperature and reduce excess capacity.
        Over-burning heating temperature is too high, too long residence time at high temperature, especially in the strong oxidation medium heat, not only grain growth of metal, steel, grain boundary concentration of impurities in the oxidation, or melting, seriously damaged The connection between the grains, thus making a hot wire in the drawing will occur when the rupture or crack, and even touched down, over burning metal wire fracture has lost luster, and its causes are:
⑴ Wire heating temperature is too high, a long time.
⑵ Stronger oxidizing atmosphere furnace, the more easy to produce through burning。
⑶Low carbon steel, carbon steel had the possibility of burning a small, alloy tool steel, high carbon tool steel have a greater risk than burning, particularly in copper over the steel most likely burn.
   Steel can not save over burning, burning is hot off the further development, to prevent overheating to prevent excessive burning.