Know about Metal Materials & Heat Treatment Terms?

Heat Treatment
In production, the operation of heating, holding at a certain temperature, and then cooling steel to cause solid - state phase transformation, thereby changing its internal structure and improving its mechanical properties, is called heat treatment.

Normalizing
The metal heat treatment process in which the workpiece is heated to 30℃ - 50℃ above Ac3 (Ac refers to the final temperature at which free ferrite is completely transformed into austenite during heating, generally between 727℃ and 912℃) or Acm (Acm is the critical temperature line for the complete austenitization of hypereutectoid steel during actual heating), held for a period of time, and then taken out of the furnace and cooled in air, or by water spraying, mist spraying, or air blowing.

Quenching
The heat treatment process in which steel is heated to a certain temperature above Ac3 or Ac1, held for a certain time, and then taken out and cooled in water or oil to obtain martensite.

Austempering
The treatment method in which an austenitized workpiece is quenched into a molten salt with a temperature slightly higher than Ms, held isothermally for a sufficient time to allow the supercooled austenite to undergo bainite transformation at a constant temperature, and then taken out and cooled in air after the transformation is completed is called austempering.

Step Quenching
The treatment method in which an austenitized workpiece is quenched into a molten salt with a temperature slightly higher or slightly lower than Ms. After the temperature inside and outside the workpiece becomes uniform, it is taken out of the molten salt and cooled in air to room temperature to obtain a martensite structure. This treatment method is called step quenching.

Single - Liquid Quenching
An austenitized workpiece is immersed in a quenching medium until the transformation is completed.

Double - Liquid Quenching
The heat treatment process in which an austenitized workpiece is first placed in a cooling medium with strong cooling capacity for a certain time. When it is cooled to slightly above Ms, the workpiece is immediately taken out and placed in another cooling medium with a slower cooling capacity to be transformed into martensite.

Tempering
A heat treatment process in which quenched steel is heated to a certain temperature below the critical point A1, held for a certain time, and then cooled to room temperature.

Tempered Sorbite Quenching
When carbon steel is tempered at 500℃ - 650℃, a multiphase structure composed of coarse - grained cementite and polygonal ferrite is obtained.

Tempered Troostite Quenching
When carbon steel is tempered at 350℃ - 500℃, a multiphase structure composed of fine - grained cementite and acicular ferrite is obtained.

Tempered Martensite Quenching
When carbon steel is tempered below 250℃, a multiphase structure composed of a supersaturated α solid solution and dispersedly distributed carbides is obtained.

Annealing
It is a heat treatment process in which steel is heated to a certain temperature above or below the critical point, held for a certain time, and then cooled with the furnace. It is the most widely used and has the most types of heat treatment processes. The purposes of different types of annealing also vary.

Isothermal Annealing
A heat treatment process in which a hypoeutectoid steel workpiece is heated to 20℃ - 30°C above A3, held for a certain time, and then isothermally treated at a certain temperature in the pearlite transformation range below Arl. After it is transformed into pearlite, it is taken out of the furnace and air - cooled. It can effectively shorten the annealing time, improve production efficiency, and obtain a uniform structure and properties.

Full Annealing
The heat treatment process in which castings, forgings, weldments, and hot - rolled profiles of hypoeutectoid steel are heated to 20℃ - 30°C above A3, held for a certain time, and then cooled with the furnace to 500℃ - 600°C and taken out of the furnace for air cooling. Its purpose is to refine grains, reduce hardness, improve machinability, and eliminate internal stress.

Spheroidizing Annealing
An annealing process in which a workpiece of hypereutectoid steel or alloy tool steel is heated to 20℃ - 30°C above Ad, held for a certain time, and then cooled with the furnace to about 500°C and taken out of the furnace for air cooling (ordinary spheroidizing annealing), or cooled to 20°C below Arl, held isothermally for a certain time, and then cooled to about 500°C and taken out of the furnace for air cooling (isothermal spheroidizing annealing) to obtain granular pearlite. Its purpose is to reduce hardness, homogenize the structure, improve machinability, and prepare the structure for quenching.

Diffusion Annealing
For important or alloy steel ingots or castings with non - uniform chemical compositions such as dendritic segregation, in order to achieve homogenization of chemical compositions, they can be heated to 150℃ - 300°C, held for a long time, and then slowly cooled with the furnace. Since diffusion annealing requires long - term heating at high temperatures, the austenite grains are very coarse. Therefore, a full annealing or normalizing must be carried out again to refine the grains and eliminate overheating defects.

Recrystallization Annealing
The heat treatment process in which a cold - deformed metal is heated above the recrystallization temperature and held for an appropriate time to transform the deformed grains into uniform equiaxed grains is called recrystallization annealing.

Stress - Relief Annealing
An annealing process in which the workpiece is slowly heated with the furnace to 500℃ - 600°C to eliminate the residual internal stress caused by deformation processing, casting, and welding processes, improve the dimensional stability of the workpiece, and prevent deformation and cracking. After being held for a period of time, it is slowly cooled with the furnace to below 200℃ - 300°C and taken out of the furnace.

Surface Heat Treatment of Steel
A type of heat treatment method that enables the surface of a part to obtain high hardness and wear resistance while the core still retains good toughness and plasticity.

Carburizing
Carburizing is the process of allowing carbon atoms to penetrate into the surface layer of a workpiece to increase the carbon content of the surface layer, generally Wc = 8% - 1.05%. After carburizing, the workpiece is quenched and then tempered at a low temperature to make the surface achieve high hardness and high wear resistance, while the core has sufficient strength and toughness, achieving the purpose of a hard outer layer and a tough inner layer.

Nitriding
It is a process of infiltrating nitrogen into the surface of a steel part. The purpose of nitriding is to further improve the hardness and wear resistance of the steel part surface, as well as the fatigue strength and corrosion resistance.

Thermal Spraying
It is a process technology that uses special equipment to heat and melt or soften a certain solid material and accelerate it to be sprayed onto the surface of a workpiece to form a special thin layer, so as to improve the corrosion resistance, wear resistance, high - temperature resistance, etc. of the machine part.

Physical Vapor Deposition (PVD Method)
It is a vapor deposition method that uses physical methods to generate deposited atoms or ions without chemical reactions occurring in the chamber.

Chemical Vapor Deposition (CVD Method)
It is a method in which thermal energy or radiant energy is input into a gas - phase reaction chamber filled with a gas at any pressure to cause a certain chemical reaction in the gas phase. As a result, a solid - state thin film is deposited on a specific surface of the workpiece.

Metal Ion Implantation
It is a treatment process in which high - energy ion beams are injected into the surface of a metal material to form an extremely thin near - surface alloy, thereby changing the physical, chemical, and mechanical properties of the substrate surface.

Electroless Plating
The process method in which a part is placed in a plating tank filled with chemical agents of special compositions. After a certain period of time, an electrochemical reaction occurs between the chemical agents, and a coating of a certain thickness is obtained on the surface of the workpiece, is called electroless plating.

Critical Quenching Diameter
It refers to the maximum hardenability diameter that a round bar specimen can obtain when quenched in a certain medium (that is, the maximum diameter at which the core is quenched into half - martensite), denoted by D0.

Isothermal Transformation
Isothermal transformation refers to rapidly cooling austenitized steel to a certain temperature below A1, allowing the supercooled austenite to undergo structural transformation during the holding process, and then cooling it to room temperature after the transformation is completed.

Continuous Cooling Transformation
When an austenitized steel part is continuously cooled from a high temperature to room temperature at a certain cooling rate, the structural transformation completed during the continuous cooling process is called continuous cooling transformation.

Martensite
It is the name of a structure in ferrous metal materials, which is a supersaturated solid solution of carbon in α - Fe.

Plate - like Martensite (Needle - like Martensite)
It is a typical martensite structure formed in medium - and high - carbon steels and high - nickel iron - nickel alloys. The spatial shape of plate - like martensite is lenticular. Since the specimen grinding surface intersects with it, it appears needle - like or bamboo - leaf - like under an optical microscope. Therefore, plate - like martensite is also called needle - like or bamboo - leaf - like martensite.

Lath Martensite
It is the martensite formed from austenite with low carbon content and is a typical martensite structure in low - carbon steel, medium - carbon steel, and stainless steel. Since the microstructure is composed of groups of laths, it is called lath martensite.

Stabilization of Austenite
Within the martensite transformation temperature range, if the cooling is stopped at a certain temperature and the workpiece is held for a period of time and then the cooling is continued, the martensite transformation does not start immediately. Instead, it restarts after a period of time, resulting in a corresponding increase in the amount of retained austenite. This phenomenon is called the stabilization of austenite. Since it is caused by isothermal holding, it is called thermal stabilization.

Supercooled Austenite
When austenite is cooled below the critical temperature, it is in a thermodynamically unstable state and will decompose and transform during cooling. This unstable austenite that exists below the critical transformation temperature and is about to undergo transformation is called supercooled austenite.

Bainite
When austenite is supercooled to a temperature range between the pearlite transformation temperature and the martensite transformation temperature, a transformation that combines shear transformation and short - range diffusion will occur, and the transformation product is called bainite or bainitic ferrite.

Hardenability of Steel
The hardenability of steel refers to the ability of austenitized steel to obtain martensite during quenching. Its magnitude can be expressed by the depth of the hardened layer obtained by quenching the steel under certain conditions. The deeper the hardened layer, the better the hardenability of the steel.

Hardening Capacity of Steel
Hardening capacity refers to the maximum hardness that the martensite structure formed at a cooling rate exceeding the critical cooling rate can reach under ideal quenching conditions, also known as hardenability.

Actual Grain Size
The size of austenite grains obtained under a specific heating condition is called the actual grain size. The actual grain size is different from the initial grain size. The initial grain size is the grain size when austenite has just formed (that is, when its grain boundaries have just contacted), while the austenite referred to by the actual grain size has been held for a certain period of time. The diameter of the actual grains is larger than that of the initial grains.

Temper Brittleness
Temper brittleness refers to the phenomenon that the toughness of quenched steel decreases after tempering. When quenched steel is tempered, as the tempering temperature increases, the hardness decreases and the toughness increases. However, there are two troughs in the relationship curve between the tempering temperature and the impact toughness of many steels, one between 200℃ - 400℃ and the other between 450℃ - 650℃. The phenomenon that the impact toughness decreases as the tempering temperature increases. Temper brittleness can be divided into the first - type temper brittleness and the second - type temper brittleness.

High - Temperature Temper Brittleness
The brittleness that occurs when quenched steel is tempered in the temperature range of 500℃ - 650℃ is called high - temperature temper brittleness, also known as the second - type temper brittleness. This type of temper brittleness mainly occurs in steels containing alloying elements such as Cr, Ni, Mn, and Si.

Low - Temperature Temper Brittleness
The brittleness that occurs when quenched steel is tempered in the temperature range of 250℃ - 400℃ is called low - temperature temper brittleness, also known as the first - type temper brittleness. Almost all quenched steels will show this brittleness when tempered at about 300℃.