Heat-Treating Furnace

an industrial furnace for the heat treatment or thermochemical treatment of metal articles. Heat-treating furnaces are divided according to mode of operation into batch furnaces (tank, compartment, and aerodynamic holding furnaces) and continuous-operation furnaces (induction heating plants and continuous, drawing, and patenting furnaces).

Continuous and drawing furnaces are most widely used in the metallurgical industry for the heat treatment of rolled products. Hot-rolled sheets are hardened, normalized, and tempered by passing through roller-hearth furnaces. Cold-rolled coils of steel strip are annealed in both drawing and bell furnaces. Drawing furnaces are used for the heat treatment of strips of carbon steel, stainless steel, and nonferrous metals and for the thermochemical treatment of electrical steel strip and preparation of the strip for the application of various coatings, such as zinc or aluminum. Standardized roller products are treated in roller-hearth and car-bottom furnaces. Pipes are treated in roller-hearth, fast-heating sectional, walking hearth, and car-bottom furnaces. Rods and coiled wire are treated in roller-hearth furnaces; bell furnaces are used for small batches. Hardening of wire in a lead bath, as well as galvanizing, is done in patenting furnaces. The heat treatment of railroad wheels and rims is accomplished in updraft furnaces, and sometimes in rotary-ring furnaces.

In the machine-building industry, batch furnaces are used for individual or small series production, whereas continuous-operation furnaces are used for large series production and mass production. Foundries and heat-treatment shops of machine-building plants make extensive use of roller-hearth furnaces. Updraft and pit furnaces are used in heavy machine-building plants for the treatment of large articles. Bell and elevator furnaces are finding increasing use in machine-building plants that conduct multiple heat-treatment operations in a controlled atmosphere. Pusher, car-bottom, roller-hearth, walking hearth, and sometimes rotary-ring and rotary furnaces are most suitable for continuous heat treatment in large series production. In the mass production of motor vehicles, tractors, and bearings, extensive use is made of combined flow quenching-tempering, normalizing-tempering, nitrogen case-hardening, and carburizing units. Tank furnaces are used where especially uniform and rapid heating is required, and also for case-hardening thin surfaces or for heating without oxidation or carburization of small parts. Particularly precise, rapid, and specific heat-treatment cycles for heavy parts are carried out in induction furnaces. Aerodynamics holding furnaces are suitable for the treatment of large and complex articles made of light metals in cases where high precision is desired, mainly in the aviation industry. To provide high-precision heat treatment of metal, many furnaces are equipped with electric heating.

The development of methods for the heat treatment of metals by combustion of gaseous fuel—for example, radiant-tube heating, jet heating, and the use of forced circulation—means that almost all types of heat-treating furnaces can be operated with gas firing; this is especially important since a high-quality fuel, natural gas, is available to most plants.

Modern heat-treatment sequences, especially thermochemical treatment, are extremely complex. They show excellent promise, however, for in-line units or continuous lines using several compartments or continuous-operation furnaces. Thermochemical treatment and, to an increasing extent, heat treatment are carried out in controlled atmospheres, for which continuous-operation furnaces are most feasible. Batch furnaces in the machine-building industry are being improved by the use of controlled atmospheres and forced circulation in conjunction with mechanization of operations and materials handling.