Heat treatment process generally includes three heating, heat preservation, cooling process, sometimes only two heating and cooling process. The process to connect to each other, completely.

Heating is one of the important process of heat treatment. Heating of metal heat treatment method are many, the first is to use charcoal and coal as a heat source, and application of liquid and gas fuel. The application of electric heating is easy to control, and no environmental pollution. The heat source can be used to direct heating, but can be by molten salt or metal, and floating particles with indirect heating.

Metal heating workpiece when it is exposed to the air, often occur oxidation and decarburization (that is, the surface carbon content lower) steel parts, the performance on the surface of the parts after heat treatment have a bad effect. Thus metals usually should be in a controlled atmosphere or protective atmosphere, molten salt and a vacuum heating, coating or packing method to protect the heating is also available.

The heating temperature is one of the important process parameters of heat treatment process, selection and control of heating temperature, is a major problem to ensure the quality of heat treatment. Heating temperature is processed and varies the purpose of the metal material and heat treatment, but is usually heated to above phase transition temperature, in order to obtain high temperature. Another change will take time, so when the heating temperature of metal surface is achieved, still must in the temperature keeping time, make internal and external temperature, the microstructure change completely, this paragraph of time is called the heat preservation time. Heating with high energy density and surface heat treatment and heating speed, generally there is no heat preservation time, and the chemical heat treatment of heat preservation time tend to be long.

Cooling is indispensable in the process of heat treatment process steps, cooling method for craft different and different, main is to control the cooling rate. General annealing cooling speed of the slowest, normalizing the cooling speed faster, quenching cooling speed is faster. But also with different steel grade have different requirements, such as empty hard steel can be hardened by normalizing the same cooling rate.

The classification of the heat treatment process

Metal heat treatment process can be divided into the overall heat treatment, surface heat treatment and chemical heat treatment three categories. According to the heating medium, heating temperature and cooling method is different, each categories and can be divided into a number of different heat treatment process. The same kind of metal in different heat treatment process, can obtain different organizations, which have different performance. Steel is one of the most widely used industrial metals, and the microstructure of iron and steel is the most complex, therefore there are many different kinds of steel heat treatment process.

Integral heat treatment is heated for artifacts as a whole, and then cooling at the appropriate speed, obtain microstructure, to change its overall mechanical properties of metal heat treatment process. Steel overall roughly annealing heat treatment, the four basic normalizing, quenching and tempering process.

The overall heat treatment process

Annealing is the workpiece of heating to proper temperature, according to the material and workpiece size in different heat preservation time, and then slowly cooling, the purpose is to make the metal internal organization at or near equilibrium state and obtain good process performance and use of performance, or for further quenching organization.

Normalizing is after the workpiece heated to proper temperature cooling in the air, the effect of normalizing and annealing are similar, just get the organization of a finer, commonly used to improve the cutting performance of material, also sometimes used for low requirements for some parts as the final heat treatment.

After quenching is the workpiece heating temperature, water, oil or other inorganic salts, organic aqueous solution such as rapid cooling in the quenching cooling medium. Hardened steel after quenching, but at the same time become fragile.

In order to reduce the brittleness of steel after quenching of steel is higher than the room temperature and under 650 ℃ at some stage in the proper temperature for a long time of heat preservation, again for cooling, this process is called tempering.

Annealing, normalizing, quenching and tempering is the overall heat treatment of the "four" fire, closely related to the quenching and tempering, often, be short of one cannot.

The "four" fire with the different heating temperature and cooling methods, and developed different heat treatment process. In order to obtain a certain amount of strength and toughness, combine quenching and high temperature tempering process, known as conditioning. Some alloy after quenching formation of supersaturated solid solution, put it at room temperature or slightly higher under proper temperature to keep a long time, in order to improve the hardness and strength of the alloy or electrical magnetic, etc. The heat treatment process called aging treatment.

The pressure deformation and heat treatment processing effective and combined closely, obtain good strength and toughness of components with method called thermomechanical treatment; In negative pressure or vacuum atmosphere heat treatment is called vacuum heat treatment, it can not only make the workpiece no oxidation, no decarburization, keeping the surface is bright and clean after processing, improve the performance of the workpiece can also lead to infiltration agent for chemical heat treatment.

Surface heat treatment is the only heating workpiece surface to change the surface mechanical properties of metal heat treatment process. For heating the workpiece surface and not only make excessive internal heat into the workpiece, using the heat source must have a high energy density, the larger on unit area of the workpiece and the heat, or local can short the surface layers of components or transient at high temperature. The main method of surface heat treatment has the flame hardening and induction heating heat treatment, the common source of heat oxygen acetylene and oxygen propane flame, induced current, laser and electron beam, etc.

Chemical heat treatment is by changing the workpiece surface chemical composition, microstructure and mechanical properties of metal heat treatment process. The chemical heat treatment and surface heat treatment difference is that the latter has changed the chemical composition of workpiece surface. Chemical heat treatment is artifacts in a medium containing carbon, nitrogen and other alloying elements (gas, liquid and solid) in heating, heat preservation for a long time, so that the workpiece surface infiltration of carbon, nitrogen, boron and chromium element. After the infiltration of elements, sometimes even on other heat treatment such as quenching and tempering. Chemical heat treatment of the main methods of carburizing, nitriding, metallic cementation.

Heat treatment is the mechanical parts and one of the important working procedure in process of mould manufacturing. In general, it can ensure and improve the properties of the workpiece, such as wear resistance, corrosion resistance, etc. Can also improve the organization and the stress state of blank, for all kinds of cold and hot working.

Such as white cast iron after annealing treatment for a long time can get the malleable iron, improve plasticity; The service life of the gear are the correct heat treatment process, can be better than without heat treatment of gear exponentially or dozens of times to improve; In addition, the low price of carbon steel through the infiltration of some alloying elements will have a certain price, performance of alloy steel, can replace some heat resistant steel, stainless steel; Workers die is almost all need after heat treatment before use.

The means of whole heat treatment process

A, the kinds of annealing

Will deviate from the equilibrium state of the steel is heated to proper temperature, heat preservation at a certain time, and then slowly cooling (along with the furnace cooling), get close to equilibrium heat treatment process of the organization.

Steel has a variety of annealing process, according to the heating temperature can be divided into two categories: one is above the critical temperature (Ac1 or Ac3) annealing, also known as phase change of recrystallization annealing, including full annealing, incomplete annealing, spheroidizing annealing and diffusion annealing, etc.; Another kind is below the critical temperature of annealing, including recrystallization annealing, the stress relieving, etc.

1. Completely annealing and isothermal annealing

Full annealing and weighing annealing crystallization, commonly referred to as "for annealing, is mainly used for the annealing of hypoeutectoid composition of carbon steel and alloy steel casting, forging and hot rolled profiles, sometimes used for welding structure. Generally used as a final heat treatment, not heavy piece or as a preliminary heat treatment in some artifacts.

2. The spheroidizing annealing

Spheroidizing annealing is mainly used for hypereutectoid steel and alloy tool steel, such as, measuring tool and die steel used in the manufacture cutters). The main purpose is to reduce the hardness and improve cutting workability, and to prepare for after quenching.

3. The stress relieving

The stress relieving is also called low temperature annealing (or high temperature tempering), the annealing is mainly used to eliminate the casting, forging, welding parts, rolling, residual stress of cold-drawn pieces, etc. If these stress will not be eliminated, will cause the steel after a certain period of time, or during subsequent machining deformation or cracks.

Second, quenching, the most commonly used cooling medium is salt water, oil and water. Brine quenching workpiece, easy to get high hardness and smooth surface, it is not easy to produce soft point of quenching is not hard, but it is easy to make the workpiece deformation is serious, even cracking. Oil as quenching medium is only applicable to the stability of the supercooled austenite bigger some alloy or carbon steel workpiece quenching of small sizes.

The purpose of the three, steel tempering

1. Reduce the brittleness, eliminate or reduce internal stress, there is a big internal stress after the steel quenching and brittleness, if not timely tempering will often result in deformation and cracking of steel.

2. To obtain the required mechanical properties of workpiece, workpiece after quenching, high hardness and brittleness is big, in order to meet the requirements of different performance of various artifacts, can be tempered with appropriate to adjust the hardness, decrease the brittleness, get the necessary toughness, plastic.

3. Stable workpiece size

4. For some annealing softening hard alloy steel, after quenching (or normalizing) high temperature tempering, is often used to make carbides in steel appropriate gathered themselves together, and the hardness is reduced, and machining.

A supplement to the heat treatment means

(1) annealing: refers to the metal material of heating to proper temperature, keep a certain amount of time, and then slowly cooling heat treatment process. Common annealing process include: recrystallization annealing, the stress relieving, spheroidizing annealing and full annealing, etc. Annealing: the purpose of the main is to reduce the hardness of metal materials, improve the plasticity, processing and machining and pressure, and reduce the residual stress, improving group woven and composition of the homogenization, or for heat treatment after ready for the organization.

(2) the normalizing refers to heat the steel or steel to or more than on the critical point temperature (steel), 30 ~ 50 ℃ at a time, in the stillness of the air cooling heat treatment process. : the purpose of normalizing the main is to improve the mechanical properties of low carbon steel, improve the cutting processability, refine the grain size, remove the tissue defects, such as heat treatment after ready for the organization.

(3) hardening: refers to heat the steel to the Ac3 or Ac1 point temperature (steel) above a certain temperature, keep a set time, and then in the appropriate cooling speed, to obtain martensitic (or bainite) heat treatment process of the organization. Common salt bath quenching, quenching process of fire are martensite interrupted quenching, bainite isothermal quenching, surface hardening and selective hardening, etc. Steel quenching of purpose: to get the required martensite structure, improve the hardness, strength and wear resistance, such as heat treatment after ready for the groups.

(4) tempering: refers to the steel after hardening, and then heated to a temperature below, heat preservation time, and then cooling to room temperature of heat treatment process. Common tempering process are: low temperature tempering, the tempering temperature, high temperature tempering, and multiple tempering, etc.

Tempering purpose: mainly to eliminate steel produced during quenching stress, the steel with high hardness and wear resistance, and with desired plasticity and toughness, etc.

(5) conditioning: refers to the steel or steel quenching and high temperature tempering composite heat treatment process. The conditioning treatment used in steel quenched and tempered steel. It usually refers to the carbon structural steel and medium carbon alloy steel.

(6) carburizing, carburizing is directed into the steel surface layer of carbon atoms. Is made of low carbon steel workpiece surface layer with high carbon steel, after quenching and low temperature tempering, the workpiece surface layer with high hardness and wear resistance, and the central part of the workpiece remain toughness of low carbon steel and plastic.

Some of the common heat treatment concept

Normalizing: steel or steel heated to a critical point AC3 or ACM more appropriate temperature keeping time, cooled in the air after get pearlite heat treatment process of the organization.

2. Annealing annealing: hypoeutectoid steel workpiece heated to above AC3 20 to 40 degrees, heat preservation after a period of time, with the slow cooling furnace (or buried in the sand or lime cooling) to 500 degrees Celsius in air cooling heat treatment process

3. The solution heat treatment: keep single-phase zone temperature alloy is heated to high temperature, the excess phase dissolves into solid solution, and then rapidly cooling, to get the supersaturated solid solution heat treatment process

4. Aging: alloy after solution heat treatment or cold plastic deformation at room temperature or slightly above room temperature to keep, the phenomenon of its performance change with time.

5. Solid solution treatment: make various phase in the alloy dissolves, solid solution and improve the toughness and corrosion resistance performance, eliminate stress and softening, in order to continue processing molding

6. Aging treatment: in the precipitation strengthening phase temperature heating and heat preservation, make the strengthening phase precipitates out, hardening, increase the intensity

7. Quenching: the steel austenitizing with proper cooling, cooling speed after all or certain parts within the cross section within the scope of martensite and unstable organization structure change of heat treatment process

8. Tempering: after hardening of the workpiece heated to a critical point AC1 under proper temperature keeping time, method of cooling, then used to meet the requirements to obtain required by the organization and performance of heat treatment process

9. Steel carbonitriding: carbonitriding is to steel surface infiltration process of carbon and nitrogen at the same time. Traditionally carbonitriding, also known as cyanide, is currently in medium temperature gas carbonitriding and low temperature gas carbonitriding (i.e., gas soft nitriding) are more widely application. Medium temperature gas carbonitriding's main purpose is to improve the steel hardness, wear resistance and fatigue strength. Low temperature gas carbonitriding with nitriding is given priority to, its main purpose is to improve the wear resistance of steel and resistance to bite.

Conditioning treatment 10. The quenching and tempering: general practice combining high temperature quenching and tempering heat treatment known as conditioning treatment. Conditioning treatment is widely used in various important structural parts, especially those who work under alternating load of connecting rod, bolts, gears and shaft, etc. Conditioning treatment after get tempered sorbite organization, its mechanical performance than the same hardness of normalizing sorbite for optimal organization. It depends on the high temperature tempering temperature and hardness and tempering stability of steel and the workpiece with the size of section, generally between HB200-350.

11. Brazing: the two kinds of artifacts together with solder heat treatment process