Heat Treating Overview
Application
and Background
The
Basics of Heat Treat
The
Value of Heat Treat
Heat
Treat Methods
Heat
Treating Equipment
Heat
Treat Chemistry
Heat
Treat Aluminum
Flame
Heat Treating
Gas Technologies
Heat
Treat Batch
Heat Treat Continuous
Heat Treat Atmosphere Generators
Flame Heat Treating
Vacuum
Furnace
Application and Background
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.
The Basics of 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.
The Value Of Heat Treating
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.
Heat Treat Methods
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.
Carburize
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
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.
Carbonitride
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".
Pit Carburize
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.
Stress Relieve
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.
Annealing
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
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.
Duplex Treatments
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.
Heat Treating Equipment
Batch Furnaces
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
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
Atmosphere Generators
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.