Heat Treatment 101

Introduced in 1924, the "Two-In-One" Furnace could be used for both heat treating and melting a variety of metals

A brief history of heat treating reads much like a brief history of the world. Metallurgical knowledge is a key attribute all the great advanced civilizations. Although the fundamental mechanisms that make heat treatment valuable have only been discovered and investigated in recent centuries, the resultant knowledge has spawned whole new classes of materials and industries to manufacture them.

The value of heat treatment resides in the control of material properties. Consider all of the different items manufactured from steel. Some items are very hard and resistant to wear, such as a wood or metal file. Others applications demand impact resistance, such as the head of a sledgehammer. Manipulating the microstructure of the metal after it is partially or wholly formed into a tool, properties such as hardness, wear resistance and toughness can be further enhanced in the final product. This manipulation of the microstructure that is accomplished by changes in part temperature is commonly referred to as heat treating or heat treatment.

In practice, each item requiring heat treatment brings a host of variables to the process, some of which are difficult to quantify, such as residual stress, variation of alloy chemistry and grain size. Further complicating matters, there are different heat treatments for different types of metal and different alloys of the same base metal.

Basic Processes

Probably the most widely used and the oldest heat treatment is quenching.

Rotary Retory equipped with Whirl-A-Way quench for annealing coin blanks

Quenching involves heating the metal item above a temperature which causes a change in microstructure, then rapidly cooling to force an unusual change in the microstructure not found when slow cooling.

In steels, this process manipulates the location of carbon in the microstructure; at high temperatures steel possesses a crystal structure called austenite. Austenite can hold a larger amount of carbon than the room temperature crystal structure called ferrite. Rapidly cooling the steel from temperatures conducive to austenite (the transformation temperature) will produce a new, different crystal structure called martensite.

Martensite crystals are longer in one dimension than the other two; during the change to martensite this fact can promote distortion of the item being quenched; in severe cases, cracking may occur. The benefit to the material properties of the item are a higher yield strength and higher hardness. Typical transformation temperatures for steel depend heavily on the specific alloying elements used; for simple carbon steel at 0.77% carbon content, the austenite transformation temperature is 1341°F/727°C. Several nonferrous alloys can also be hardened by quenching; a similar set of microstructural changes occurs, though the metallurgical terminology changes. Some of these alloys include aluminum bronzes, nickel aluminum bronzes, and certain brasses.

Age Hardening

Solution Heat Treat and Age Hardening aluminum castings in a roller hearth furnace

Age hardening processes typically start with a quench, but in this instance the material is softer with a lower yield point after the quench (also referred to as solution heat treatment). Following near net shape forming, a final heat treatment to a moderate temperature ages the material. In reality, a second phase with a different crystal structure precipitates throughout the metal item; this second phase modifies the material properties of the item, increasing the hardness and yield strength.The most common age hardening alloys are probably those of aluminum; but there are a wide variety of steels, stainless steels, copper and titanium alloys, to name a few, that are processed by age hardening.

Stress Relieving and Annealing

Roller Hearth Annealing Furnace for steel tubing

During the course of fabrication of metal items, whether ferrous or nonferrous, stresses accumulate within the metal due to temperature differences within the part, such as occur during casting; also from deformation due to rolling and forging; and material removal processes such as milling or buffing. These stresses, called residual stresses, can cause the item to fail prematurely or distort beyond repair during later forming operations. Consequently, some heat treatments are intended to remove the history of stress accumulated from past operations upon the item of interest. The most thorough of these is annealing.

Annealing occurs at or near the transformation temperature mentioned in regards to quenching; however, in this case, the metal is slowly cooled to room temperature. The result is a microstructure that is totally free from stress, consisting of the crystal structure that is stable at room temperature, ferrite in the case of steel.

Partial annealing, as the name implies, does a less complete job of removing stress and provides a less uniform microstructure; due to the fact that lower temperatures and shorter times are used for the process.

Stress relieving is conducted at even lower temperatures, and has the more limited goal of only dealing with residual stresses. The benefit of partial annealing and stress relieving lies in the amount of allowable distortion; if an item is already near net shape, the risk of distortion may be too great for complete annealing to be conducted.

Tempering

Vacuum Retort Tempering Furnace for commercial heat treating

Tempering is a process conducted after a quench, usually intended to promote greater ductility and toughness. Tempering typically occurs in the range of temperatures where stress relieving or partial annealing are conducted. This process may be conducted multiple times to obtain the exact material properties required for a given application.

 

 

 

Brazing¹

CAB Brazing Furnace for automotive radiators

The American Welding Society defines brazing as a group of welding processes which produces coalescence of materials by heating to suitable temperature and using a filler metal having a liquidus above 840°F (450°C) and below solidus of the base metal. The filler metal is distributed between the closely fitting surfaces of the joint by capillary attraction. For brazing to occur, the following three criteria must be met:

 

• Parts must be joined without melting the base metal
• Brazing filler metal (BFM) must have a liquidus temperature above 840°F (450°C)
• The BFM must wet the base metal surfaces and be drawn into or held in the joint by capillary attraction

 

Carburizing & Carbonitriding²

Pit Carburizing Furnace for wind turbine gears

Carburizing is the absorption and diffusion of carbon into solid ferrous alloys by heating to a temperature above Ac₃, in contact with a suitable carbonaceous material. A form of case hardening that produces a carbon gradient extending inward from the surface, enabling the surface layer to be hardened either by quenching directly from the carburizing temperature or by cooling to room temperature, the reaustenitizing and quenching.

Carbonitriding is a case hardening process in which a suitable ferrous material is heated above the lower transformation temperature in a gaseous atmosphere of such composition as to cause simultaneous absorption of carbon and nitrogen the the surface and, by diffusion, create a concentration gradient. The process is completed by cooling at a rate that produces the desired properties in the work.

Castings

Metal items formed by casting have a distinct microstructure from wrought products. Typically in wrought products, the microstructure has a directional bias; as a plate or bar is rolled down to size, the grains in the metal are stretched out in one direction, and are very narrow through the bar or plate thickness. Cast material usually has a more uniform grain structure. As a casting solidifies, insulation and chilling can be used to influence the microstructure to assure uniform or beneficial properties in the finished product. All of the heat treatments applied to wrought products can be applied equally as well to castings.

Equipment

Various types of furnaces are used for heat treating, depending on the specific alloy requirements. Usually surface finish determines the general furnace type; if exposure to oxygen is not detrimental, an open furnace or fluidized bed furnace may be used. If a material, such as titanium, is particularly sensitive to oxygen or nitrogen, a vacuum furnace may be used in order to provide the highest possible cleanliness.

For most common alloys, some measure of atmosphere control is used to limit any damage to the surface of a work piece.The type, size and quantity of pieces usually determines the specific kind of furnace to be used. For small ferrous pieces, a mesh belt furnace may most appropriate.

For larger pieces or greater throughput a cast link or roller hearth furnace may be used. Aluminum pieces may be treated in box furnaces or pusher type continuous furnaces, with or without atmosphere control.

Items requiring hardening by quenching may be processed in an integral quench furnace using oil or water; a molten salt bath may be used if a higher finish temperature is desired. Vacuum furnaces can also harden by the use of high pressure gas quenching.

Heat treatment is an integral part of the fabrication of most metallic products. It has expanded with humanity's knowledge to encompass a huge body of practice, theory and experience. As history has repeatedly demonstrated, it is certain that the art of metal heat treating will continue to evolve along with the science of metallurgy to produce quality metal products.For more information on heat treating and metallurgy resources contact the American Society of Metals (ASM) or Industrial Heating Magazine.

This article was originally written by Craig Klingler, MS, PE for Diecasting Times, 2001;edited for the web by Beth Ryan R1 4/26/06,
R2 10/26/06 ¹ Added definition of brazing from Issues in Brazing, Janusz Szczurek, Dallas Airmotive, Inc., Dallas, TX USA and Janusz Kowalewski, SECO/WARWICK, Meadville, PA USA²Added definitions of carburizing and carbonitriding from ASM Glossary of Terms
1/10/2008 updated source code

The latest information on heat treatment should be obtained from the company before any reliance is placed on the enclosed since changes may occur due to process and metallurgical improvements.

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