- Heat treating is a widely used industrial process to optimize the mechanical properties of metal.
- Metal is heated to a set temperature and then cooled according to a prescribed schedule.
- Heat treating processes vary according to equipment type, energy source, temperature profile and environment.
Heating treating is important in a variety of industries. To optimize their mechanical properties, including strength, toughness, fatigue strength, hardness and corrosion resistance, metals are heated and cooled under tight controls. This article provides an overview of various heat treating processes and equipment options, as well as the advantages and disadvantages of different energy sources.
What happens during heat treating?
Heat treating causes micro structural changes within a metal through the application and removal of heat at different rates and temperature levels. This produces a variety of crystalline and chemical changes. Typically, heat treating processes follow three basic steps:
- Heating the material to a specific temperature level
- Holding the temperature level for a set amount of time
- Cooling slowly or rapidly (quench) according to a prescribed schedule
The maximum heat treating temperature is as high as 2,400°F (just under the melting point of steel), but it is seldom more than 2,000°F. Hold times may range from seconds to as long as 60 hours or more. To achieve the desired material properties, some materials are cooled slowly in a furnace while others must be quenched when removed from a furnace. After quenching, tempering (a reheating step) may be required to reduce the brittleness by removing the internal strains caused by the sudden cooling. There is little change to the micro structure when reheated to tempering temperatures.
Heat treating processes
Heat treating processes vary widely according to equipment type, energy source, temperature profile and environment. In general, heat treating processes fall into one of two categories, through and selective. Through heat treating, the most common mode of operation, treats the entire volume of a part at the same time. Selective heat treating is applied to only a portion of a component and can create different mechanical properties to different areas.
Specific heating treating processes include:
Annealing reduces the metal strength to improve machinability and cold working.
Normalizing refines the grain structure and composition of metals for further processing.
Hardening increases hardness and toughness with thermal cycles or atmospheres.
Tempering enhances toughness for parts subjected to high wear and stress applications.
Stress relieving is low temperature heating to release stress and reduce strain.
Solution heat treating achieves soluble alloy constituents prior to aging.
Aging is a low temperature heating to increase hardness and strength while decreasing ductility.
Heat treating furnaces
Furnaces are broadly classified as either batch or continuous. Batch furnaces can only treat a single unit or batch at one time while continuous furnaces have an automatic conveying system to move the load through the furnace.
A variety of heating methods are used. Direct-fired furnaces burn natural gas and the flames provide convection and radiation heat transfer to the parts. For components that may be damaged by combustion gas, the load can be treated indirectly with muffle or tube furnaces, as well as those using electric heating elements. Vacuum furnaces are built within internal vacuum chambers to prevent parts from reacting with air or the atmosphere. Fluidized-bed furnaces heat a part by immersing it in a stream of heated particles and combustion gases.
Heat treating furnaces come in many shapes and sizes to accommodate a variety of materials and processes. These furnaces are well-insulated to prevent atmospheres from escaping or air from infiltrating. Precise temperature controls and efficient transport mechanisms (for moving product through the furnace) are common features. Some furnaces have integrated quench systems and atmosphere controls.
Furnace energy sources
Most heat treating furnaces are fueled by natural gas, although electric units are often used for applications requiring temperatures of more than 2,000°F.
Advantages of natural gas furnaces include:
- Flexibility to switch between various fuels
- Ability to recover heat from exhaust gases
- Faster heat-up times and generally lower energy costs than electric furnaces
- Heat transfer by both convection and radiation
Despite these advantages, natural gas systems require ventilation, which is often expensive. Control systems are needed to optimize combustion efficiency, and there is a potential for explosion or fire hazard if the equipment is used improperly.
Electric units provide a number of benefits over natural gas furnaces for certain applications, including:
- Environmentally cleaner at the point of use
- Cooler plant environment due to the absence of exhaust systems
- Quieter operation
- More uniform heat pattern
- No make-up air system required
- Higher energy efficiency
Electric systems typically cost more to install and may have higher operating costs, depending on local electric rates and demand charges during peak periods. Electric heating has limitations in the flexibility of zone control and it may be difficult to adjust heating capacity.
Heat treating atmospheres
All heat treating processes operate with an atmosphere or physical environment where the heating cycle protects the part from oxidation, promotes alloying on the surface of the part or acts as a heat transfer medium.
Common atmospheres used in heat treating are air, endothermic, exothermic and elemental gases, dissociated ammonia, vacuum, salt bath, aluminum oxide and foil.
Manufacturers of heat treating furnaces continue to upgrade their controls and improve efficiency. Organizations like the Center for Heat Treating Excellence (CHTE) at Worcester Polytechnic Institute work with industry to conduct research in modeling and process optimization.