Heat Treating Overview
Practically nothing can be manufactured without heat treating, a process in which metal is heated and cooled under tight controls to improve its properties, performance and durability. Heat treating can soften metal, to improve formability. It can make parts harder, to improve strength. It can put a hard surface on relatively soft components, to increase abrasion resistance. It can create a corrosion-resistant skin, to protect parts that would otherwise corrode. And, it can toughen brittle products.
Heat treated parts are essential to the operation of automobiles, aircraft, spacecraft, computers and heavy equipment of every kind. Saws, axes, cutting tools, bearings, gears, axles, fasteners, camshafts and crankshafts all depend on heat treating.
Although iron and steel account for the vast majority of heat treated materials, alloys of aluminum, copper, magnesium, nickel and titanium may also be heat treated.
Heat treating processes require three basic steps:
- Heating to a specified temperature
- Holding at that temperature for the appropriate amount of time
- Cooling according to prescribed methods
Temperatures may range as high as 2,400F and time at temperature may vary from a few seconds to as many as 60 hours or more.
Some materials are cooled slowly in the furnace, but others must be cooled quickly, or quenched. Certain cryogenic processes require treatment at -120 F or lower. Quenching media include water, brine, oils, polymer solutions, molten salts, molten metals and gases. Each has specific characteristics that make it idea for certain applications. However, 90 percent of parts are quenched in water, oil, gases or polymers.
Heat treating adds about $15 billion per year in value to metal products by imparting specific properties that are required if parts are to function successfully.
It is very closely linked to the manufacture of steel products: about 80 percent of heat treated parts are made of steel. These include steel mill output such as bar and tube, as well as parts that have been cast, forged, welded, machined, rolled, stamped, drawn or extruded.
It is also a vital step in the manufacture of nonferrous products. For example, aluminum alloy automotive castings are heat treated to improve hardness and strength; brass and bronze items are heat treated to increase strength and prevent cracking; titanium alloy structures are heat treated to improve strength at high temperatures.
The Induction Process (Electric)
This process consists of heating a small area of a part using induced electric currents. Hardening occurs when the part is rapidly cooled from a temperature above the transformation range; using quenchants such as water, oil, etc.
Also see Flame Heat Treating for a natural gas technology for surface hardening.
This is a surface or case hardening process normally applied to low-carbon steel alloys. During processing, carbon is diffused into the surface of the parts at elevated temperatures. Hardening occurs to this “carburized case” by quenching in oil from above the transformation range resulting in a hard surface for wear resistance and a soft core for ductility. Typical case depths achieved range from .020″ to .050″.
Nitriding is the introduction of nascent nitrogen into the surface of specific ferrous alloys by holding the alloys at relatively low temperature (975 – 1,050F) for an extended period (24 – 72 Hrs). This is a direct conversion process requiring no quenching to produce a hard wear-resistant case.
Similar to carburizing except carbon and nitrogen are diffused into the surface of the parts. The nitrogen addition increases hardenability of the steel allowing a lower alloy, less expensive steel to be used. Typical case depths achieved range from .005″ to.030″.
This is a carburizing process typically used for long, thin parts. Parts are suspended in a deep pit-type furnace for the carburizing process. This processing will minimize distortion compared to laying the parts horizontally into a furnace. This process produces similar case depths as with carburizing.
Quench & Temper
This process is for hardening medium carbon alloy steel. This consists of heating the parts to a temperature above the transformation range and rapid cooling to room temperature, usually using an oil quench. This may be performed in air or in a controlled atmosphere to protect the part’s surface. Parts are then reheated to a low temperature to temper to the desired final hardness range. Quench & temper of medium carbon alloy steels increases both strength and hardness.
Vacuum Heat Treat
This hardening process is conducted in a vacuum furnace. Parts are elevated in temperature and quenched in oil, polymer or air. When properly conducted, there is no change to the chemical analysis of the surface of the parts eliminating the need for cleaning and reducing the tendency to crack during hardening. This process is usually used to harden higher alloy tool steels.
See Vacuum Furnace for more information.
This is a process using controlled heating and cooling to relieve machining or welding stress from large parts or weldments. Time and temperature relationships are developed based on prior hardness requirements or by the size and complexity of weldments. Stress relieving will minimize part distortion during subsequent heat treatment or while in service.
This process consists of heating to a temperature above or slightly below the transformation range followed by slow, controlled cooling. Annealing is used to develop a soft easily machined structure. Annealing is usually followed by some type of hardening process after machining. There are three types of Annealing, Full Annealing, Process Annealing and Spheroidizing.
Normalizing is similar to Annealing, but the material (steel) is heated to a higher temperature followed by slow cooling in air. The higher temperature affects the crystal pattern and evens out the carbon content of the material. This is often needed are shaping and cold-forming steel where the crystal structure is stretched and misshaped.
Ion plus induction to produce deep heating characteristics with very high surface wear resistance.
Heat Treat Chemistry
For more information on what actually happens in the heat treating process, see Heat Treating Chemistry.
A batch furnace does one load at a time. It is generally loaded and unloaded by hand and has minimal automated controls. These furnaces come in all sizes with process time lasting from hours to days. Loads can be small baskets loaded by hand, up to large “car bottom” furnaces loaded by cranes and forklifts.
For more information on Batch Furnaces
Continuous furnaces have conveyor belts, ‘walking beams’, rotary screws, or other automated means of moving parts through the furnace. They may still by loaded and unloaded by hand, but process time is more often measured in minutes than hours. These types are furnaces are designed for high production where thousands of the same parts are made the same way.
For more information on Continuous Furnaces
Certain types of heat treating requires the absence of oxygen (protective atmosphere) and/or the presence of certain other gases (ions or carburization) to improve the hardening process. Atmosphere Generators are on-site units, separate of heat treat furnace that use natural gas as a source to produce the gases needed by the heat treat furnace. Often a central atmosphere generator will produce gases used by several heat treat furnaces.
For more information on Atmosphere Generators
Source: TechPro, DTE Energy 2001.
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