Annealing covers a broad range of thermal processes, including bright annealing, full annealing, homogenizing, hydrogen de-embrittlement, malleablizing, normalizing, recrystallization / crystal growth, solution treating, spheroidizing, and stress relieving.
Austempering quenches cast iron or carbon steel above the austenite transformation ranges in a medium with a high enough rate of heat transfer to prevent the transformation of high temperature formation products. The process is completed at a temperature that is too low for pearlite to form and too high for martensite to form. Austempering steel involves quenching austenite and holding it at a constant temperature (usually between 450° F and 800° F) above the martensite start temperature (Ms) until transformation to bainite is complete. Bainite has a very fine acicular or feathery structure and a dispersion of cementite (iron carbide) and ferrite (alpha iron). Bainite is tougher than quenched and tempered structures in certain steels at specific hardness levels. The martensite transformation is a diffusionless reaction that proceeds rapidly (at the speed of sound) as a shear wave through the material. The martensite transformation often results in distortion and cracking in conventionally quench and tempered parts. Austempering cast iron results in ausferrite.
Carbonitriding is a case hardening process in which the cases are formed by adding carbon and nitrogen through a diffusion process. Case hardening causes a surface layer within a material to form and harden.
Carburizing is a case hardening process in which the cases are formed by adding carbon through a diffusion process. Case hardening causes a surface layer within a material to form and harden. Heat treated parts with a carburized or carbonitride case have a hard, strong surface and a tough, ductile core.
Case hardening consists of the formation and hardening of a compositionally modified surface layer within the material. Cases are formed by adding carbon, nitrogen, chrome, boron or other elements to the steel or metal surface region through diffusion, implantation or other reactions. Case forming or hardening processes are referred to as carburizing, nitriding, ferritic nitriding, carbonitriding, nitrocarburizing, boronizing and carbon restoration.
Quenching and tempering (quenching and drawing) is a conventional process used to harden or strengthen steel or ferrous alloys. In the first step, the ferrous alloy is transformed to austenite and then quenched in water, brine water, air or oil. During the quenching step, the austenite is converted to martensite in a diffusionless shear reaction. Martensite is very hard, but too brittle for practical use in the untempered state. The quenched alloy is then tempered (drawn) or heated at an intermediate temperature to transform the brittle martensite into a very fine dispersion of cementite (iron carbide) phase in an iron ferrite matrix. Tempered martensite or tempered steel is the product of a conventional hardening process. The martensite transformation is a diffusionless reaction that proceeds rapidly (at the speed of sound) as a shear wave through the material. Careful control is required to avoid or reduce any distortion and cracking that may occur due to the rapid martensite transformation. In some cases, a martempering process is advisable to avoid cracking.
Homogenization or homogenized anneal is a thermal treatment that heats an alloy to a temperature at which diffusion reduces compositional segregation. Casting, welding and other melting processes can result in segregation and constitutional liquation and can benefit from a homogenizing anneal in many cases.
Hot isostatic pressing (HIP) use an argon atmosphere or other gas mixtures heated up to 3000º F and pressurized up to 100,000 psi. Evacuated steel or metal cans or parts with a sintered, non-porous surface are used to contain and maintain a seal during HIPing. HIPing is used for densifying high performance ceramics, ferrites and cemented carbides, net-shape forming of nickel-base super alloy and titanium powders, compacting of high-speed tool steel, diffusion bonding of similar and dissimilar materials, and eliminating voids in aerospace castings or creep damaged blades.
Martempering or marquenching is a hardening procedure in which an austenitized ferrous workpiece is quenched into an appropriate medium whose temperature is maintained at the martensite start temperature (Ms) of the workpiece, and held in the medium until its temperature is uniform throughout – but not long enough to permit bainite to form – and then cooled in air. The treatment is usually followed by tempering. When carburized materials are martempered, the controlling martensite start temperature (Ms) is that of the case. Martempering of carburized steels is called marquenching.
Normalizing is a common heat treatment for improving the strength and ductility of ferrous alloys. Normalizing anneal decreases pearlite interlamellar spacing and refines grain size. The normalizing process consists of austenization followed by air cooling. The process consists of heating steels at ~100° F into the fully austenite phase region (area of the iron-carbon phase diagram where alloy is 100% austenite), holding at temperature, and then cooling in air. This heat treat operation is used to erase previous heat treating results in carbon steels or low alloy steels, and to produce a uniform grain structure in forged and cold worked steel parts.
Precipitation hardening consists of two steps: solution treating and then aging. In a solution treating anneal process, an alloy is heated to dissolve alloying elements and cooled rapidly to maintain the alloying elements in solution. After forming or machining operations, the alloy can be heated to an intermediate temperature to age or precipitation-harden the metal. A second phase is precipitated in the aging process, resulting in strain hardening of the lattice.
Shot peening is a mechanical process that work hardens the surface of a material and imparts a residual compressive stress. The compressive residual stress can improve the fatigue performance of shafting and other components under cyclic loads.
Solution treating is the first phase in precipitation hardening, a two-step process that ends with aging. In a solution treating anneal process, an alloy is heated to a point to dissolve alloying elements and cooled rapidly to maintain the alloying elements in solution. After forming or machining operations, the alloy can be heated to an intermediate temperature to age or precipitation-harden the metal. A second phase is precipitated in the aging process resulting in strain hardening of the lattice.
Spheroidizing heats steel to a temperature near the austenite-to-ferrite transformation temperature for a long enough time to coarsen and modify its structure. Generally, spheroidizing is used to improve machining properties and the cold-deformability (cold heading or working) of the steel. The normally lamellar cementite in the pearlite (iron carbide rods in a ferrite matrix) structure is transformed to spheres of cementite in an equiaxed ferrite matrix.
Cryogenic, cold or sub-zero treatments below the martensite finish temperature are used to obtain desired conditions or properties such as dimensional or microstructural stability in steels. The treatment may involve the transformation of retained austenite, which usually requires tempering of the resulting martensite. Depending on the alloy, other stabilization treatments are used. Examples include desensitization treatments of austenitic stainless steels at 879° to 900° C to precipitate out carbides of stabilizing elements (TiC, NbC or TaC). The treatment may also involve the repeated annealing of non-ferrous or ferrous parts during manufacturing to ensure dimensional stability in service.
Stress relieving is a lower temperature annealing process that removes residual stress from forming, cold working or welding operations without causing substantial reductions in strength or hardness.
Other unlisted, specialized, or proprietary heat treating processes.
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Suppliers have presses for hot or cold operations. Hot presses may include isostatic units for HIP processing, or cold bending presses for straightening distorted parts.
Suppliers provide portable equipment that can be used on-site or in the field. For example, resistive heating blankets can be used for stress relieving large structures.
Suppliers have furnaces or equipment that use an inert gas, reducing gas (hydrogen-nitrogen) or other protective atmosphere to prevent oxidation or contamination during thermal processing.
Suppliers have furnaces or equipment with a molten salt bath to prevent oxidation or contamination during thermal processing. Salt baths maintain a very stable and consistent temperature profile, an important part of the aluminum aging or precipitation hardening processes.
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AS 9100 is a set of quality guidelines and requirements published by the Society of Automotive Engineers (SAE) in cooperation with major aerospace manufacturers. AS 9100 is based on ISO 9001 but maintains a particular focus on the aerospace industry.
International Standards Organization (ISO) provides certifications such as ISO 7779, ISO 9000, ISO 9001:1987, ISO 13485, ISO/TS 16949:2002, ISO 14000 and ISO 17025. ISO 7779 is concerned with acoustics, telecommunications equipment, and the measurement of airborne noise emitted by information technology (IT) equipment. ISO 9001:2000 establishes requirements for company quality management systems. ISO 13485 is concerned with medical devices, quality management systems and requirements for regulatory purposes. ISO 14000 establishes requirements for environmental management policies that minimize a company’s harmful effects on the environment. ISO 17025 outlines the general requirements for the competence of testing and calibration laboratories.
ISO 9001:2000 establishes requirements for company quality management systems. Standards range from manufacturing to services such as design, development, production, and installation.
QS 9000 is a quality standard for suppliers of DaimlerChrysler Corporation, Ford Motor Company, and General Motors Corporation. QS-9000 is based on the 1994 edition of ISO 9001, but contains additional requirements that are particular to the automotive industry. Specifically, QS 9000 applies to suppliers of production materials, production and service parts, heat treating, painting and plating, and other finishing services.
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Suppliers can assist with concepts, manufacturing costs, manufacturing techniques and material considerations. Suppliers may also be able to assist in upgrading or redesigning, re-evaluating or modernizing existing products to increase performance and/or reduce manufacturing costs.
Suppliers can provide finishing and coating services such as anodizing, plating, powder coating, spray painting, mass finishing, tumbling, or vibratory finishing.
Suppliers have test and inspection capabilities such as ultrasonic, eddy current, magnetic particle or other nondestructive or destructive inspection methods.
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