When it comes to stainless steel heat treatment, methods will vary according to the metal’s end use and the properties necessary for the application. Sometimes this involves the metal undergoing several types of heat treatment, with methods for certain superalloys going through as many as six steps to optimize their properties.
Heat-treating stainless steel is done for a variety of reasons. Some heat treatment methods increase hardness or softness, dielectric properties, or heat conductivity. Others relieve stresses caused by cold working processes applied earlier in processing. Still, others affect the chemistry of the steel. For those looking for the best heat treatment for their specific application, it’s important to understand the advantages and disadvantages of each when it comes to heat-treating stainless steel.
9 Stainless Steel Heat Treatment Methods
Depending on the metal’s purpose, heat treatment methods can make stainless steel more durable, more ductile, harder, stronger, or otherwise alter its properties. Heat-treating steel is often done in different stages during stainless steel fabrication to ensure it performs appropriately for the application. Treating stainless steel with heat is important in producing products that include spacecraft, cutting implements like axes and saws, computers, and aircraft, along with piping and other equipment used by the petrochemical industry.
Here are some of the more commonly used heat treatment methods for stainless steel:
Aging
Also known as precipitation hardening, aging increases the malleability of stainless steel. The process disperses particles uniformly within the metal’s grain structure, altering its properties. Aging is normally used after the metal has already been exposed to higher temperatures in another type of heat treatment. Aging methods utilize midlevel temperatures during this process, generally between 480-620°C (896-1148°F), though temperatures above 550°C (1022°F) give certain aged stainless steels better resistance to stress corrosion cracking. Aged stainless steels have moderate to good corrosion resistance, depending on their exposed conditions.
Hardening
To harden stainless steel, heat must first transform it into a solution. Treatment methods for hardening stainless steel vary depending on the exact stainless variety, but generally, heating helps strengthen the metal, increasing its hardness. This makes the steel less malleable and more brittle, so it may require tempering to mitigate the brittleness resulting from the hardening process.
Hardening stainless steel involves slowly increasing the temperature, followed by rapid cooling. Referred to as quenching, this rapid cooling process helps prevent warping or cracking. Immersion and soaking the metal in brine, oil, or water for a specified period cause the stainless steel to harden. The time it takes to cool differs depending on the type of steel, with added alloys allowing slower cooling rates.
Using alloyed stainless steel decreases the risk of warping or cracking, though hardness also depends on the carbon content. Hardness increases with a carbon content of up to .80 percent, though beyond that proportion it increases wear-resistance rather than hardness. By adding alloys to stainless steel, it improves the carbon’s capacity to strengthen and harden the metal. The stainless steel hardening process requires the metal’s temperature to cools under a second to 538°C (1000°F). Alloys increase the efficacy of carbon in the metal, allowing slower quenching times to achieve the desired hardness.
Typically, stainless steels are quenched in oil. As quenching results in high stress within the metal, stainless steel is often tempered after hardening. Once a stainless steel component reaches a temperature of 93°C (200°F), it should be removed from the quenching bath and air-cooled. The metal will likely crack if it’s not removed from the bath at that temperature.
Tempering
Iron-based alloys that are very hard are also brittle. This includes stainless steels that undergo a hardening process, which tends to make the metal too brittle. A tempering process is often used to reduce this brittleness and strengthen the metal, along with relieving stress internally. This involves heating the steel to a temperature lower than the critical point, the temperature at which the metal’s physical properties and crystal structure would become altered.
In a nutshell, tempering involves heating the steel under the temperature at which it hardens, holding it at this temperature for a stipulated period, and then cooling the metal. A simple rule applies to tempering and how it increases tensile strength: when tempered up to 232°C (450°F), tensile strength increases and decreases beyond that temperature. For stainless steel parts, tempering should occur once they’ve reached room temperature to prevent cracking later. If tempering at above 510°C (950°F), the stainless steel should be cooled at a relatively rapid rate until the temperature falls below 400°C (750°F). This will keep the metal from becoming brittle at 475°C (885°F).
Many heat treatment methods include a tempering process to reduce hardness while developing other physical properties within the stainless steel. This softens the metal somewhat, though this is controlled by the temperature at which it’s tempered. Presetting the tempering temperature allows metal fabricators to decide how strong and hard the steel will be. Once removed from the tempering oven, the tempered stainless steel is cooled in still air. For most steels, the time it takes to cool after tempering usually does not affect the metal.
Annealing
There are multiple types of annealing techniques, including full annealing, final annealing, partial annealing and recrystallization annealing. Yet all these annealing processes are essentially just the opposite of hardening. Annealing softens stainless steel, reducing the chance of fracturing while increasing its workability, relieving stress within the metal’s structure and improving the metal’s grain structures.
To produce the highest ductility within the metal during the annealing process, the metal must be heated slowly to above its critical temperature. Normally this means temperatures need to be above 1040°C (1900°F). However, to attain a finer grain, certain types of stainless steel are heated to as low as 1010°C (1850°F). After heating, the metal must be slowly cooled, either in the oven once it’s been turned off or by covering it with an insulating material. This causes the steel to re-crystalize, altering the properties of the metal.
Case Hardening
This specific hardening process hardens the exterior of metal while keeping the interior soft. Since hardening causes stainless steel to become brittle, case hardening is used to ensure a certain amount of flexibility while creating a durable outer layer. These heat treatment methods normally include adding carbon or nitrogen to the steel, causing reactions between solids and gases at the metal’s surface. Generally, case hardening increases strength and wear resistance, resulting in hardened, higher-strength surfaces and a core that’s softer and more ductile.
The two general methods for case hardening are divided into those that involve quenching and those that don’t. Heat treatment methods involving quenching include low-pressure carburizing, gas carburizing, and carbon nitriding. Gas nitriding and ferritic nitrocarburizing don’t utilize this step. Case hardening protects the outer layer from wear and is commonly used for stainless steel shafts. When a bearing is mounted to a shaft, it can damage its surface and make it wear more quickly. Case hardening protects the surface while providing the properties necessary for a shaft to handle stress from fatigue.
Carburization
As per its description, carburization involves heating stainless steel and another material that releases carbon as it decomposes. The metal surface hardens as it absorbs the released carbon, resulting in an inner core that’s softer than its exterior. The depth of the hard casing depends on the amount of carbon diffused into the steel’s surface, the temperature to which the metal is exposed, and how long it’s heated. Two common heat treatment methods for carburizing stainless steel are gas and low-pressure carburizing.
Gas carburization uses a gas that reverses or inhibits oxidation on stainless steel surfaces. This is produced with an endothermic gas generator, which heats air and natural gas to form a mixture that’s approximately one part carbon monoxide, two parts hydrogen, and two parts nitrogen. This mixture is carbon-neutral, so it won’t increase or decrease carbon on the metal’s surface, though additional natural gas or propane are used to increase surface carbon. The process generally reaches temperatures from 843-954°C (1550-1750°F), with the hours necessary for carburization varying considerably depending on equipment, other processes involved and the type of stainless steel used.
Also known as vacuum carburization, low-pressure carburizing is done in a vacuum furnace that uses a hydrocarbon gas like acetylene or propane under partial pressure to diffuse carbon into the metal’s surface. Low-pressure carburization normally speeds carburizing times when compared to other methods. This method works best when hardening high-alloy stainless steels, as this heat treatment method has a slower cooling rate. During carburization, the process also allows for higher temperatures reaching as high as 1038°C (1900°F).
Carbonitriding
Like carburization, carbonitriding also involves diffusing a metal’s surface with carbon to case harden stainless steel, though it also diffuses nitrogen gas into the surface. The process is mostly done with low-alloy steels with minimal carbon content, producing a shallower hard casing of the metal than carbonization. It enhances wear and corrosion resistance while improving a stainless steel part’s ability to withstand stress without breaking. Typically, this heat treatment method produces bearings, gears, pistons, rollers, shafts, and levers for hydraulic, mechanical, and pneumatic systems.
Temperatures for carbonitriding usually fall between 788-871°C (1450-1600°F). The same equipment used in carburization can be used for atmosphere carbonitriding, with the oven’s atmosphere usually utilizing endothermic gas and adding ammonia from which the nitrogen is derived. Hydrocarbons and ammonia are added for vacuum carbonitriding using the same equipment used for low-pressure carburizing.
Stress Relieving
Commonly used as a heat-treating technique for fabricating items like accumulators, air bottles, or boiler parts, stress relieving heats the stainless steel to just below its critical point. This heat treatment method relieves stresses on stainless steel components caused by processes earlier during fabrication, such as straightening, rolling, machining or forming. It helps reduce the chance of stress corrosion cracking while improving dimensional stability and notch toughness. Though full annealing at around 1080°C (1975°F) can also remove these stresses, it’s not practical for more complex or larger workpieces.
Stress relieving for austenitic stainless steels typically involves heating to 425-925°C (800-1700°F). Usually, heated steel for an hour at 870°C (1600°F) will relieve around 85 percent of residual stress. However, this temperature range can lead to the metal experiencing what’s known as chromium carbide precipitation in austenitic stainless steel. Known as sensitization, it depletes chromium in a narrow zone along the grain boundary, reducing corrosion resistance in the steel.
Normalization
Like the annealing process, normalization makes stainless steel tougher and more ductile. The process does this by heating the steel until it transforms at critical temperatures. Normalizing metal removes internal stresses caused by other heat treatment methods, as well as casting, forging, forming, machining or welding. Uncontrolled stress can cause metal failure, so often the metal is normalized prior to hardening.
Normalizing differs from annealing in that the process can only be done with ferrous metals such as stainless steel. However, normalizing the metal happens at a higher temperature than with annealing. The critical point at which normalization occurs is from 750-980 °C (1320-1796 °F), though the exact temperature at which stainless steel transforms depends on its carbon content.
Once it’s transformed, the metal is removed from the oven and air-cooled rather than cooled in the oven as per the annealing process. Additionally, the rate of cooling is determined by the steel’s mass, which partly determines its hardness. The thinner the workpiece, the quicker it cools and the harder the metal becomes.
This is unlike the annealing process, where hardness isn’t determined by mass. Normalized stainless steel additionally has higher strength than annealed steel. The metal also has excellent ductile properties and withstands impact better, so it is usually preferred for tougher stainless steel applications.
Heat Treatments by Arthur Harris
When it comes to stainless steel heat treatment, Arthur Harris & Company’s methods are done only after careful analysis of the best application process. To do this, we evaluate the properties of specific steel grades our customers require to handle the stresses the metal will undergo. This ensures the stainless steel performs according to the customer’s specifications.
Our company utilizes a wide array of heat-treating techniques, including annealing, carbonitriding, carburizing, case hardening, water quenching, etc. Arthur Harris can perform secondary operations, such as cleaning, imprinting, labeling, straightening, and vacuum brazing. Contact us today to learn more about our heat treatment capabilities!