Heat Treating 101

What Is Heat Treating?

Heat treating is a process in which metal is heated and cooled under tight controls to optimize its mechanical properties, such as strength and ductility, for the use it is likely to see while in service. Heat treatment does not normally change a material's physical properties (density, melting point, or thermal conductivity). Sometimes heat treating is used to correct mechanical properties that were induced during a previous manufacturing process such as forging, machining, or to simply improve the final properties. The primary mechanical properties that can be altered with heat treating include the following:

• Strength—The maximum stress a material can sustain.
• Toughness—The ability to absorb energy and deform prior to failure.
• Fatigue Strength—The maximum stress sustained for a specified number of cycles.
• Hardness—Ability to resist indentation or abrasion.
• Corrosion Resistance—Ability to withstand chemical attack.

Heat treating describes a family of processes where the micro-structural changes within a metal occur solely by the application and removal of heat. This heat can be applied and removed at many different rates and temperature levels to produce a variety of crystalline and chemical changes within a metal. Typically, heat treating processes follow three basic steps:

1. Heating the material to a specific temperature level.
2. Holding the temperature level for a set amount of time.
3. Cooling slowly or rapidly (quench) according to a prescribed recipe.

The maximum heat treating temperature can be as high as 2,400°F (just under the melting point of steel), but is seldom over 2,000°F, and dwell times may range from seconds to as long as 60 hours or more. Some materials are cooled slowly in a furnace, while others must be quenched when removed from a furnace to achieve the desired material properties.

The following time-temperature chart illustrates the cooling cycle for hardening a typical steel material followed by a tempering cycle to soften the metal. It is necessary to heat above the Ac₃ temperature of a metal to achieve one type of microstructure (austenite or face-centered-cubic) and cool rapidly through the M_s and M_f temperatures to transform much of the austenite to martensite (body-centered-cubic) microstructure which is much harder. Tempering reduces the brittleness in hardened steel by removing the internal strains caused by the sudden cooling in the quenching bath. The volume of martensite is locked in and thus does not change much at all while reheated to tempering temperatures.

Heat Treating Business Segments

Two different types of companies engage in heat treating processes: commercial and captive. These two segments are quite different in their operations, motivations, and processing needs.

The commercial heat treaters are sometimes called job shops since heat treating is their only business and they perform this service for many different manufacturers that have decided to outsource this process step. Commercial heat treaters are identified by the North American Industry Classification System as NAICS 332811 (SIC 3398). A majority of these firms are small privately owned businesses and have a customer base that is primarily regional to reduce the cost of transporting the parts to and from the customer's plant. As such, commercial heat treaters typically serve a wide variety of customers and processing needs.

Captive heat treaters are found in manufacturing facilities that do their heat treating in-house. They tend to be more automated and specialized than commercial heat treaters and are often characterized by medium to high volume production of a limited variety of parts. Captive heat treaters are not easy to identify as there NAICS classification can fall anywhere between NAICS 331 to 339, however, captive facilities outnumber commercial facilities by 10 to 1. Captive heat treaters tend to have their furnaces and equipment under-utilized, which has given rise to more outsourcing of this process step when fast turn-around or strict process control is not as important.

Heat Treating Processes

Heat treating processes are accomplished with a myriad of different types of equipment, energy forms, temperature profiles, and environments to effect the desired changes in microstructure within a metal part. In fact, the number of ways to achieve the desired mechanical properties and the nomenclature associated with it can be quite intimidating to the novice. Resources have been noted below that can help you be more informed about heat treating terminology.

In general, heat treating processes can be applied to a manufactured part in one of two ways, either through thickness or selective heat treating. When the entire volume of a part is heat treated at the same time it is called through heat treating. Through heat treating is the most common mode of operation among the major process classifications and the specific through heating processes of annealing and stress relieving are by far the most predominate of all heat treating processes. Heat treating that is applied to only a portion of the part (normally the surface) is called selective heat treating. This selective type of processing reduces the cost of heating an entire part when it is not necessary or provides different mechanical properties to different areas of a part.

Heat treating processes originated as part of the steel manufacturing process as steel makers realized that they could reduce the brittleness of the parts they produced by reheating (or annealing) them. Since then, many different heat treating processes have been developed for alloys and their material properties—first by trial and error, and now by computer models. Several primary heat treating families have evolved as the science has progressed:

• 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 and/or atmospheres.
• Tempering—Increases toughness for parts subjected to high wear and stress applications.
• Stress Relieving—Relatively low temperature heating to release stresses and reduce strain.
• Solution Heat Treating—Achieves soluble alloy constituents prior to aging.
• Aging—Low temperature heating of a solution-treated metal to precipitate out alloy constituents in order to achieve an increase in hardness and strength with a decrease in ductility.

Heat Treating Furnaces

Furnaces are broadly classified as either batch or continuous. Batch furnaces can only treat a single unit or individual batch at one time, while continuous furnaces have an automatic conveying system to continuously move the load to be heat treated through the furnace.

Another way to classify furnaces is according to their heating method. Direct-fired furnaces burn natural gas and the flames provide convection and radiation heat transfer to the parts. For parts that can be damaged by the presence of combustion gases, the load can be heated indirectly by muffle or radiant tube furnaces. Using electric heating elements, which generate no combustion gases, may also protect parts. Vacuum furnaces are built within internal vacuum chambers to prevent the part from reacting with the air or atmosphere. Fluidized-bed furnaces heat a part by immersing it in a stream of heated particles (often aluminum oxide) and combustion gases.

Heat treating furnaces are designed in many shapes and sizes to accommodate a wide variety of materials and processes. These furnaces are tightly built and 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 also desired. Some furnaces even have integrated quench systems and atmosphere controls as standard equipment.

Furnace Energy Sources
Most heat treating furnaces are fuel-fired, although electric heated furnaces have made significant inroads especially with higher temperature applications above 2,000°F. The main advantages and disadvantages of both energy sources are detailed as follows:

• Fuel-Fired Advantages
• Flexibility to switch between various fuels with a simple orifice change.
• Ability to recover heat from the exhaust gases with a recuperator device.
• Faster heat-up times and generally lower energy costs than electric heated furnaces.
• Heat transfer by both convection and radiation.
• Fuel-Fired Disadvantages
• Requires ventilation system for exhaust gases that is often expensive.
• Control systems needed to maintain optimum combustion efficiency.
• Efficiency is lower because of up the stack losses.
• Potential for explosion or fire hazard if used improperly.
• Electric Heated Advantages
• Systems environmentally cleaner at the point of use than fuel-fired furnaces.
• Plant environment is often cooler due to the absence of major exhaust systems.
• Quieter operation due to the absence of combustion noise and blowers.
• More uniform heat pattern from electric element grid.
• No make-up air system required to affect building air pressure.
• Relatively higher energy efficiency.
• Electric Heated Disadvantages
• Limits in flexibility of energy zone control.
• Difficult to change heating capacity.
• Typically higher initial equipment capital costs.
• Demand charges for electric capacity during peak periods.
• Generally higher operating costs depending upon local energy rates.
• Electric elements need to be replaced to maintain peak efficiency.

Heat Treating Atmospheres
All heat treating processes are operated with some sort of an atmosphere or physical environment where the heating cycle takes place. The main purposes of heat treating atmospheres are to protect the part from oxidation, promote alloying on the surface of the part, or to simply act as a heat transfer medium.

The most common atmospheres that are used in heat treating are:

• Air—Most common, but allows oxidation to take place and scale to develop.
• Endothermic Gas—Completely reacted gas for surface protection or alloying.
• Exothermic Gas—Unreacted gas for protection of low carbon steel or copper brazing.
• Elemental Gas—N, Ar, He used for protection of stainless steel and high or exotic alloys.
• Dissociated Ammonia—High purity atmosphere of hydrogen and nitrogen.
• Vacuum—Used for complete surface protection from contaminating atmospheres.
• Salt bath—Primarily for heat transfer medium but waste disposal can be a problem.
• Aluminum Oxide—Primarily for heat transfer medium and clean alternative to salt.
• Foil—Used for protection around parts when atmospheres are not available.