Heat treatments alter the properties of metals and alloys by carefully applying heat and cooling. These improve hardness, strength, and flexibility, while also strengthening the ability to bear resistance impacts. So, their role-play is critical in manufacturing sectors, as they directly impact the metal components’ efficiency. In this article, we will explore the top 11 common types of heat treatments, adopted for diverse benefits.
1. Carburizing Treatment Process
Carburizing is normally used for heat-treating steel. It is carried out at a temperature range of 850-950°C. It is in an atmosphere containing carbon in the form of methane or propane in a gas furnace. Carbon penetrates the steel surface and forms a case depth from 0.5 mm to 2.5 mm, which is based on the treatment time. It has been found that the surface hardness produced is between 55 and 65 HRC while the core hardness remains untreated. Carburizing treatment is effective for low-carbon steels. They have a carbon percentage of less than 0.25%. Enhances the fatigue strength as well as wear characteristics for parts such as gears and crankshafts.
2. Nitriding Treatment Process
Nitriding treatment is carried out at comparatively lower temperatures ranging from 500°C to 590°C (932°F-1094°F). In nitrogen-bearing environments, the common being ammonia gas. The nitrogen also goes in as a nitride in alloy steels. It produces surface hardness up to 1000 VHN (Vickers hardness number), with case depth between 0.2 mm to 0.7 mm. As compared with carburizing, nitriding doesn’t involve quenching. Therefore, it forms a hard surface layer without distortion. The surface roughness of nitrided surfaces is generally brought down to Ra 0.1 – 0.4 μm. As such, it’s ideal for the production of wear-resistant parts, camshafts and valves.
3. Hardening and Tempering Treatment Process
In hardening heat treatment, the steel is heated to its austenitizing temperature (which ranges from 800 °C to 950°C for most of the alloys). Then cooled under controlled conditions in water, oil, or air. The process helps converts the microstructure into martensite thus achieving a hardness of up to 65 HRC. However, martensite is significantly hard but extremely brittle. Therefore tempering comes next. In tempering, the steel is heated again to 150°C-600°C to make the steel less brittle. As a result, its hardness is reduced to around 40-55 HRC as per the final use. Tempering also improves the toughness properties of steel alloys. Therefore, widely employed in the manufacturing of industrial tools and automotive parts.
4. Annealing and Stress Relief Treatment
Annealing heat treatment is done by heating metal to 500°C-800°C depending on the alloy type and then cooling it in the furnace to make the metal softer. The process removes any hardness, usually to a level of 150 – 200 HV. Moreover, it removes internal stresses that were induced in the course of manufacturing. Low-temperature carburizing annealing is carried out at comparatively lower temperatures in the range of 400°C-650°C for relieving the residual stresses without altering the mechanical properties appreciably. Annealing treatments enhance machineability, and ductility in subsequent operations in forming.
5. Cryogenics Treatment Process
Cryogenic treatment always requires the materials to be exposed to temperatures ranging from -196 degrees Celsius (-321 degrees Fahrenheit) using liquid nitrogen. The process changes retained austenite into martensite making the microstructure more stable. Cryogenic treatment improves wear resistance by increasing hardness and toughness. They are usually improved by 10-20% than those obtained in conventional heat treatments. So, the process is useful, especially for parts such as cutting tools where reduction of brittleness and improvement of dimensional stability are desirable.
6. Normalizing Treatment Process
Normalizing is performed by heating steel to 50-100 degrees above the upper critical temperature (approximately 850-950 degrees depending on steel grades) and then cooling it by air. This process helps in refining the grain structure and the resulting microstructure is made more uniform. Normalized steel is usually more machinable than spheroid annealed steel and may also show enhanced mechanical properties with a hardness of between 150-250 HB (Brinell hardness). The most frequent use of this heat treatment process is on forgings and castings to obtain a more homogeneous and less stressed component before further working.
7. Cyaniding Treatment Process
The cyaniding heat treatment process is a surface hardening treatment. Usually, it’s carried out in a molten cyanide bath at a temperature range of 850°C-950°C. The carbon and nitrogen from the cyanide salts penetrate the steel to yield a hard external layer with a thickness of 0.25 mm to 0.75 mm. After cyaniding, the parts are tempered in water or oil to get the surface hardness in the range of 55 to 62 HRC. Cyaniding is mostly applied on small parts like fasteners, screws, and gears on which a hard and wear-resistant surface is needed without distorting the part.
8. Case Hardening Treatment Process
Case hardening heat treatment uses carbon and nitrogen to have a hard surface and a soft core. The general temperature of the process ranges from 900°C – 950°C. This treatment makes the surface hardness 58-65 HRC while the inner part has a much lower hardness of about 20-35 HRC. The case depth can range from 0 to 4, with 4 being the deepest level of case hardening. The depth typically varies between 2 mm and 3 mm, depending on the treatment time. Casing hardening is normally used on gears, shafts, and other parts where the surface needs to be hard and wear-resistant while the core needs to be tough and strong to absorb shock.
9. Aluminum Heat Treatment Process
This process is applied to alloying elements in the 2xxx, 6xxx, and 7xxx series. In the case of aluminum alloys, solution heat treatment is done at a temperature range of 450 to 550°C. Then, the material is quenched in water. After that, aging, either natural or artificial, is done to enhance the strength and material hardness. Artificial aging takes place at a temperature range of 120°C to 200°C relying on the alloy type to provide the best hardness and tensile strength. Aluminum parts treatments have enhanced mechanical characteristics with a yield strength of up to 450MPa.
10. Brazing Treatment Process
Brazing employs filler metals with melting temperatures above 450°C but lower than the base metals’ melting temperatures, and they include silver, copper, and aluminum. Usually, it’s carried out in a controlled condition of oxygen or under vacuum to avoid oxidation of the substance. The strength of the joint can be up to 200 Mpa to 300 Mpa depending on the material used in the process. In the case of surface roughness after brazing, the values were found to be in the range of Ra 0.4 to 0.8 μm, which forms a uniform and strong interface. Brazing is viable for components of HVAC, plumbing, and aerospace applications.
11. Induction Heat Treatment Process
Induction heating heats the metals to their required temperature by the application of varying magnetic fields. Normally, it’s applicable at frequencies ranging from 10 kHz to 500 kHz and raises the surface layer temperature to 800°C-1000°C, followed by rapid cooling. Induction hardening is capable of achieving surface hardness of up to 58 – 65 HRC and case depth of between 0.5μm to 3μm. The surface roughness gets as low as Ra 0.4-0.8 μm, which is known to have a combination of high wear resistance and high fatigue strength in gears, shafts, and other precision parts.
Summary
Our primary goal for writing this article is to let the reader know about different heat treatment types and their technical aspects. The primary purpose of heat treatments is to modify and improve mechanical properties alloys, and metals. Each process is adopted for specific applications and surface properties like strength and durability. These treatments found their extensive use in industries like aerospace, automotive, and manufacturing, for optimizing material performance for various demanding environments. Contact us to get personalized heat-treating services from our expert metallurgists!
