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Heat Treatment Furnace manufacturing process and product specification

Heat treating refers to a group of industrial and metalworking process used to alter the physical and chemical properties of a metal. This could be done either by a change in the microstructure or can be done by changing the metal’s chemistry and composition. Some of the commonly treated ferrous metals are steel, cast iron, alloys, stainless steel and tool steel while non-ferrous metals include aluminum, brass, copper and titanium. Heat Treatment Furnace Manufacturers are commonly found in steel mills, iron and steel foundations, metal fabrication, electrical and transport equipment. This article discusses the process of heat treatment furnace manufacturing and other product specifications.

1. Why is heat treatment manufacturing done?
Heat treatment manufacturing is common in all steel and iron, transport and commercial industries for the following reasons –

  • To improve toughness
  • To increase hardness
  • To increase ductility
  • To improve machineability
  • To refine grain structure
  • To remove residual stress
  • To improve wear resistance

2. Procedure
a) Annealing – Metal is heated up to a certain temperature and then cooling it in a controlled manner. This produces a refined microstructure, and is often used to soften a metal for cold working for cold working, or to improve metal properties. In ferrous alloys, the metal is heated beyond the critical temperature and then cooled very slowly, which forms pearlite. In pure metals and alloys, they are heated to a temperature of recrystallization, and are remolded. Non-ferrous alloys are cooled to full precipitation to bring a refined microstructure.

b) Aging –
The alloying elements of a precipitation-hardening alloy would be trapped in a solution, which results in a soft metal by allowing the elements to form intermetallic particles.

c) Quenching –
The metal is cooled at a rapid rate, which helps ferrous metals produce a harder metal and non-ferrous metals produce a softer metal. Afterwards, tempered martensitic steel remains too brittle for application, which is further tempered by heating the steel below the low critical temperature. This gives toughness and ductility to the steel, albeit at the cost of losing some of its yield strength.

d) Cold and cryogenic treating –
Further transformation of the austenite still present in the metal is done by cooling the metal to very low temperatures.

e) Decarburization –
The steel is now heated to alter the carbon content. When steel turns into austenite, oxygen and iron react together to form slag, offering no protection from decarburization. The carbon atoms now react with the slag to produce carbon dioxide and carbon monoxide. The process is usually performed to produce malleable cast iron in a process called “white tempering”.

2. Furnace types
Heat treatment furnaces are categorized into batch furnaces, that are manually loaded and unloaded and continuous furnaces, which have an automatic system providing constant load into the furnace chamber.

a) Car-type furnace –
A car-type furnace has a floor constructed as an insulated movable that loads and unloads for convenience. Using sand seals or solid seals in position, these are used in non-atmosphere processes.

b) Elevator-type furnace –
Similar to a car-type furnace, the car and hearth are raised by means of a motor driven mechanism. They can carry heavy and large loads, and don’t require cranes and other transfer mechanisms.

c) Bell-type furnace –
They come with removable covers called bells. These are lowered over the load by means of a crane. An inner bell is sealed over the load for protection, while an outer bell provides the heat supply.

d) Pit furnaces –
They are constructed in a pit which extends to the floor level. Thus, workpieces can be suspended from fixtures, held in baskets or placed on bases in the furnace. They are suited to heating long tubes, shafts and rods held in a vertical position, thus preventing distortion.

Heat Treatment Furnace Manufacturers ensure that the product steel is corrosion-resistant and can resist common weathering elements. The produced steel should withstand erosion and shock loading, while maintaining its toughness and ductility. The same can be done by other processes such as diffusion of carbon or nitrogen to the surface through flame hardening, induction hardening, nitriding and carbonitriding.

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