Heating of Metals

There are three types of metal forming with respect to work temperature: cold forming, thermomechanical treatment and hot forming. The main type is hot forming, or hot plastic metal working. As we know, cold metal working causes strain hardening (sometimes it's favourable effect), decrease in plasticity and, as a result, cracks in billets appear. During hot plastic working metal has high plasticity, it does not undergo structural changes and strain hardening, capacity of forming machines and deformation forces required are less as compared with cold forming. But hot forming causes a scale formation, which leads to metal losses and dimensional accuracy decrease.

Heating temperatures for steel are (Fig. 4.6) by 100…200°C below solidus line AE and by 25…50°C above the line GSK, that is hot forming is carried out when steel has homogeneus austenite structure (hypereutectoid steel has structure: austenite + carbides).

Fig. 4.6. Heating temperatures for hot plastic metal working

According to a source of heat a distinction is made between flame (gas or oil) reverberatory furnaces and electrical resistance reverberatoryfurnaces. According to a working principle a distinction is made between bath and continuous furnaces (Fig. 4.7).

Flame furnaces are used for big ingots, both electrical induction and contact heating devices are used for small billets.

Fig. 4.7. Chamber (bath) (a) and continuous (b) heating furnaces: 1 – bottom; 2 – billet; 3 – gas burner;

4 – lid; 5 – unload hole; 6 – flue; 7 – pusher; I – preheating zone (600…800°C);

II – zone of maximum heating (1250…1350°C); III – Standing zone (1200…1300°C)

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