Heat treatment is the cooperating application of heating and cooling of a metal component (without melting), resulting in alterations of condition or properties.
Heat treatments can be used to improve or alter the properties of metal parts, to control their microstructure in precise ways, to deliver properties that the ‘native’ state of the material cannot otherwise achieve.
Heat treatment is also used on metallic components to alter the surface properties, to a limited depth. The diffusion of Carbon and Nitrogen and other materials into the microstructure of the surface layer of the component can have significant effects in that region in enhancing desirable properties without altering the behavior of the bulk of the part.
This is generally done to provide improved surface hardness, corrosion and fatigue resistance and improved wear properties for bearing surfaces.
I. What is Heat Treatment?
Heat treatment is a process used to alter the physical, structural and chemical properties of materials through the application of heat and sometimes adsorbed chemicals and alloying agents.
- Annealing or normalizing involves heating a material to a specific temperature, holding it at that temperature for a period of time, and then slowly cooling it back to ambient. This process helps to reduce internal stress, increase ductility, and improve machinability by causing crystal boundaries and micro-dislocations to diminish. The result is large, regular and low stress crystal formations, in a variety of metals. Annealing is generally used for the process of slowed, oven controlled cooling, where normalizing involves return to ambient temperature and a less rigorous process.
- Quenching involves heating a material to a specific (generally high) temperature and then rapidly cooling it in a liquid or gas. This process hardens the material by freezing the fine-grained microstructure present in hot material, before larger, more pliable crystal growth can occur.
- Tempering involves heating a previously quenched material to a lower temperature and holding it for a specific time. This process reduces the brittleness of the material by allowing some blurring growth at the crystal boundaries, without losing the fine grain structure achieved in quenching.
- Nitriding requires the material to be heated to a specific temperature in a controlled atmosphere of nitrogen gas or a bath of molten salt containing nitrogen. The nitrogen diffuses into the surface of the material, forming nitrides that increase the hardness and wear resistance of the surface layer. The process can also improve the fatigue strength and corrosion resistance of the material.
- Carburizing involves heating the material to a specific temperature in the presence of a carbon-rich gas, such as methane or propane. The carbon atoms from the gas diffuse into the surface of the material, forming carbides that increase the hardness and wear resistance, plus fatigue strength and corrosion resistance of the surface layer. This family of processes includes carbontriding, nitrocarburizing case hardening and flame hardening.
- Other surface diffusion processes include Boriding, Titanium-carbon diffusion, and Toyota diffusion, which all use the same approach of hot immersion/exposure to diffuse alloying agents into the surface layer of the metal.
II. Heat Treatment Process Steps
In most cases, heat treatments are closely defined steps that must be adhered to precisely. There is a long history of accidental discovery of benefits in metallurgy, and the diffusion hardening processes are a leading case in this.
Examples such as the famed Japanese and Toledo sword forging processes, both of which used wooden anvils, initially out of necessity, introduced Carbon into the steels as they were worked, improving properties.
Legendary swordmakers developed a variety of techniques, even swearing by the benefits of quenching in the urine of a red haired virgin (nitriding using urea compounds).
The heating stage of heat treatment is generally performed at high temperatures, but short of the melting point of the metal. The imposition of heat dissolves all crystalline structures in the metal and returns it to a close to amorphous composition, with minimum to zero crystallinity.
It is critically important to heat slowly and uniformly. Cocalized and uneven heating can result in distortions and fracturing, as hotter parts expand relative to colder regions.
Ideal heating takes place either in an induction furnace, under a gas torch in a heat retaining fire-brick well, or more traditionally by immersion in air blown charcoal, to apply heat steadily and uniformly.
Holding is also referred to as soaking, and this is the stage when the component is allowed to remain at a steady temperature to give time for the atomic and macro level structural changes to take place.
The soak time is very dependent on the mass and thickness of the metal part – a larger mass and thicker section part should receive a long soak period, as deep internal structures must fully dissolve to complete the crystalline alteration of the structure.
With an appropriate soak time completed, the part must be cooled. Cooling falls into two categories – normalizing and quenching.
Normalizing allows a slow cooling process that gives time for maximum crystal growth. This results in the largest possible crystals in the material to develop, rendering the metal more ductile and malleable because the slip-planes that allow crystalline displacement are thereby larger and more uniform.
This process is primarily used to improve the cold working properties of a material, to allow the follow-on forming process to be completed with the minimum amount of internal microfracturing and with lower effort.
Quenching achieves the opposite effect to normalizing, in that the sudden removal of heat by immersion in a lower temperature medium such as water, oil or a gas stream serves to apply a controlled freezing to the hot microstructure.
This prevents the growth of large crystals and results in increased stiffness, strength and hardness and reduced ductility.
III. Benefits of Heat Treatment of Metals
The benefits of heat treatment of metals are extensive in that the impact on a range of properties can be significant and valuable.
- Heat treatment can increase the hardness of metals, making them more resistant to wear and abrasion. Bearing steels and edged tools benefit from this aspect by allowing longer functional life under potentially wearing conditions
- Heat treatment can improve the toughness of metals, making them more resistant to cracking or breaking under stress. Screwdriver bits are a good example of a part where the increased toughness dramatically improves the functional lifespan of the product
- Heat treatment can improve the strength and elastic limit of metals, making them more resistant to deformation or failure under large distorting loads. Springs are a good example of the increased elasticity that results from heat treatment.
- Heat treatment can improve the ductility of metals, making them more malleable and easier to shape without cracking or breaking. Normalization (the common term for thai treatment in steels) and annealing (the term used in Aluminum and other metals) is an imperative step in improving workability by bending, compression and stretching.
- Heat treatment can improve the machinability of metals, making them easier to cut, drill, or machine. Normalized steels are significantly pre free cutting than when, for example, they are cold rolled and work hardened.
- Heat treatment can improve the corrosion resistance of metals, making them more resistant to rust and other forms of corrosion by;
- Formation of oxide layers such as blueing of steel or the reinforcement of the Chromium oxide film on stainless steels.
- Reduction of impurities in metals, such as sulfur and phosphorus, which can promote corrosion.
- Change in microstructure, making metals more resistant to corrosion as crystal boundaries can act as barriers to oxidation travel.
- Precipitation hardening can increase the strength and corrosion resistance of metals by creating a more homogeneous structure and reducing the potential for corrosion at grain boundaries.
- Improved dimensional stability of metals often results from heat treatment, making them less prone to warping or distortion under stress.
IV. Types of Heat Treatment and Their Purposes in Machining
There are various advantages that accrue through heat treatment that can improve aspects of machining processes.
This is not always about making the machining processes easier – it can make them harder (or more limited in the tools available) but improve the accuracy of the result.
For example, hardening processes can impose distortions onto components that cannot be tolerated, so it can be beneficial to perform the hardening BEFORE machining, imposing the need to use EDM and wire cutting for machining of the hardened materials.
Annealing is a heat treatment process in which a metal is heated to a specific temperature and then cooled slowly, generally in a temperature controlled process.
During annealing, the metal is heated to a temperature below its melting point but above its glass transition temperature.
In this temperature range, the crystalline structure begins to dissolve, and dislocations and other defects within the metal begin to diffuse and reform. The metal is then cooled slowly, allowing crystal growth to maximize and allowing the new crystal structure to stabilize.
- Softening the metal, making it easier to shape or work with, and reducing the risk of cracking or other damage when cold working.
- Improved ductility, making it more malleable and less likely to break or crack under stress.
- Reduced internal stresses within the metal, which can lead to improved dimensional stability and reduced risk of warping or distortion.
- Improving machinability, making them easier to cut, drill, or machine.
Normalizing is essentially identical to annealing, except the cooling takes place in open atmosphere at room temperature. In some cases this can limit the ductility gains that are achieved, compared with the more controlled process of annealing.
Hardening is heat treatment used to increase the hardness and strength of some metals – primarily alloys containing a high proportion of Iron or Titanium.
The process involves heating the metal to a specific temperature, holding it at that temperature for a specified time, and then cooling it rapidly by quenching it in a liquid such as water, oil, or brine or gas stream.
At high temperatures, the metal atoms become more mobile, and dislocations and other defects within the metal begin to move and heal while hot. When the metal is cooled rapidly, the atoms become “frozen” in place, trapping the new structure in place and creating a harder and stronger metal.
A secondary effect that applies to alloys with solute-rich precipitate regions is that these regions tend to dissolve into the mass, making a more regular and uniform constituency.
There are several types of hardening processes, depending on the nature of the material being heat treated:
- Martensitic hardening of steel and Titanium alloys involves heating the metal to a temperature above its critical point, holding it there for a short time, and then rapidly quenching it. This process creates a very hard and brittle metal, which generally will require tempering to reduce its brittleness and make it usable.
- Austenitic hardening involves heating the metal to a temperature below its critical point (but above the glass transition temperature), holding it there for a longer time, and then rapidly quenching it. This process creates a harder and stronger metal than the annealed form, and less brittle than the martensitic form.
- Precipitation hardening involves heating the metal to a specific temperature, holding it there for a specified time, and then cooling it slowly. This first dissolves step any solute rich precipitate regions and then, during cooling, condenses the precipitates in a highly distributed array through the microstructure. This utilizes the presence of the precipitate regions as controlled stress in the microstructure that can result in significant increase in strength and hardness. This can also be referred to as age hardening, when the temperature is lower and the process is much more slowly executed.
Closely related to precipitation hardening is age hardening, which also involves the slow precipitation of solute alloying agents to stress the lattice and increase strength and hardness.
This applies most commonly to Aluminum and Titanium alloys. In artificial aging, the amount of precipitate and effect on material lattice structure aging are influenced by time and temperature. A range of precipitates can result, when aging is performed at different temperatures.
Internal stresses build up in metal components as they are worked. This includes the effects known as work hardening, but can also manifest as distortion, cracking, and eventual failure of the component.
Stress relieving involves heating the metal to a point lower than the critical temperature, but high enough to allow the metal to slowly and uniformly relax and release the residual stresses. The metal is held at this temperature, potentially for hours, depending on the thickness and composition of the alloy. It is allowed to cool slowly to room temperature.
The stress relieving process is appropriate to steel, aluminum, and copper alloys. It is widely used after welding or machining operations, to prevent distortion or cracking. It will often improve the dimensional stability of the part, in long service.
In tempering, a metal part is heated to a temperature that is below its critical temperature but high enough to allow the diffusion of Carbon atoms or other alloying agents.
This helps to stabilize the metal’s microstructure and relieve residual stress at crystal boundaries. The metal is then held at this temperature for a certain amount of time before being cooled down slowly.
The temperature and duration of the tempering process will vary depending on the specific metal being treated, as well as the desired mechanical properties. For example, tempering low Carbon steel at 400-600°C for 1-2 hours can increase its toughness and ductility, while still maintaining some of its worked strength.
Carburization increases the Carbon content of a low-carbon steel or iron-based materials by hot diffusion. Exposing the material to a carbon-rich atmosphere (of methan or less often Carbon containing solids) at temperatures from 900-1100°C for a long period (hours to days) allows development of a high Carbon ‘skin’ whose depth is time, alloy and temperature dependent. This is also referred to as case hardening
This renders low cost materials such as low Carbon steel or Iron as suitable for applications such as gears, bearings, and other mechanical components that require high strength and durability. It can also improve the corrosion resistance of some materials by forming a protective layer of carbides on the surface.
V. What Metals Are Suitable for Heat Treating?
Various metals and alloys are able to be heat treated to improve their mechanical properties, but the specific heat treatment processes used and their effectiveness will depend on the composition of the material and the desired outcome.
Some of the more commonly heat-treated metals:
- Steels are the most commonly heat treated metal, due to their widespread use and the wide range of property changes that can be achieved. Steels can commonly be heat treated by various methods, including annealing, tempering, quenching, carburizing, nitriding, and precipitation hardening
- Stainless steels are widely heat treated by annealing, to improve their workability. They are also stress relieved and increasing use of specialist alloys is seeing dramatic increases in strength by precipitation hardening
- Aluminum alloys can be heat-treated to improve their strength, toughness, and corrosion resistance. Suitable processes include solution heat treatment, quenching, aging, and precipitation hardening.
- Copper alloys are heat treated to improve their strength and more often their ductility. Heat treatment processes include annealing, solution heat treatment, and precipitation hardening
- Titanium alloys are often heat-treated to improve their strength and toughness. Heat treatment processes include annealing, solution heat treatment, and aging
- Nickel-based alloys can be heat-treated to improve their high-temperature strength and resistance to corrosion and oxidation. Heat treatment processes include annealing, solution heat treatment, and precipitation hardening
Heat treatment is a criticall group of treatments for various metals. Significant benefits accrue in material properties, product life expectancy, ductility/malleability/hardness/toughness, depending on the process and the specific characteristics of the alloys being used.
The history of heat treatment is as old as the history of metallurgy and this has lead to great depth of study and understanding which has long been applied to increment the performance of the metals and alloys in common use.