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What exactly is metal heat treatment?

Metal heat treatment is one of the important processes in mechanical manufacturing. Compared with other processing technologies, heat treatment generally does not change the shape and overall chemical composition of the workpiece, but by changing the internal microstructure of the workpiece, or changing the chemical composition on the surface of the workpiece, endowing or improving the performance of the workpiece. Its characteristic is to improve the intrinsic quality of the workpiece, which is generally not visible to the naked eye. As some people say, mechanical processing is surgery, heat treatment is medicine, representing the core competitiveness of a country’s manufacturing industry.

meatl heat treatment

The technological process

Heat treatment process generally includes heating, heat preservation, cooling three processes, sometimes only heating and cooling two processes. These processes are interconnected and uninterruptible.

heating

(heating)

When the metal is heated, the workpiece is exposed to the air, which often occurs oxidation and decarburization (that is, the carbon content on the surface of steel parts decreases), which has a very adverse effect on the surface performance of the parts after heat treatment. As a result, the metal should normally be heated in a controlled atmosphere or protective atmosphere, molten salt and vacuum, and can also be protected by coating or packaging methods.

The heating temperature is one of the important technological parameters in the heat treatment process. The selection and control of the heating temperature is the main problem to ensure the quality of the heat treatment.The heating temperature varies with the metal material being treated and the purpose of the heat treatment, but it is generally heated above the phase transition temperature to obtain high-temperature tissue.In addition, the transformation takes a certain time, so when the surface of the metal workpiece reaches the required heating temperature, it must be kept at this temperature for a certain time, so that the internal and external temperatures are consistent and the microstructure changes completely. This period of time is called the heat preservation time.

heat preservation (2)

(heat preservation)

When using high-energy density heating and surface heat treatment, the heating speed is very fast, and generally, there is no heat preservation time, while the heat preservation time of chemical heat treatment is often longer.

cooling

(cooling)

Cooling is also an indispensable step in the process of heat treatment. The cooling method varies from process to process, mainly controlling the cooling rate.

Process classification

The heat treatment technology of metal can be divided into three types: integral heat treatment, surface heat treatment, and chemical heat treatment. According to the different heating medium, heating temperature, and cooling method, each category can be divided into several different heat treatment processes. The different microstructure can be obtained by different heat treatment process for the same metal, so it has different properties. Iron and steel are the most widely used metal in industry, and the microstructure of iron and steel is the most complex, so there are many kinds of heat treatment technology for iron and steel.

Integral heat treatment is a metal heat treatment process that heats the workpiece as a whole and then cools it at an appropriate rate to obtain the required metallographic structure and change its overall mechanical properties. Overall heat treatment of iron and steel has roughly annealing, normalizing, quenching, and tempering four basic processes, namely the “four fires” of heat treatment.

Quenching

Quenching process

The hardening of steel is a heat treatment process in which the steel is heated to the critical temperature above Ac3 (subeutectoid steel) or Ac1 (hypereutectoid steel), held for a period of time, all or part of the steel is austenitized, and then the steel is cooled faster than the critical cooling speed to below Ms (or isothermal near Ms) for martensite (or bainite) transformation.

Process: heating, heat preservation, and cooling.

The essence of quenching is the transformation of martensite or Bainite by supercooled austenite to obtain martensite or Bainite structure.

The purpose of quenching :(1) to greatly improve the rigidity, hardness, wear resistance, fatigue strength, and toughness of steel, so as to meet the different requirements of various mechanical parts and tools;(2) Through quenching to meet the ferromagnetism, corrosion resistance and other special physical and chemical properties of some special steels.

Application: Quenching process is most widely used, such as tools, measuring tools, molds, bearings, springs, and automobiles, tractors, diesel engines, cutting machine tools, pneumatic tools, drilling machinery, agricultural machinery and tools, petroleum machinery, chemical machinery, textile machinery, aircraft, and other parts are used in the quenching process.

The quenching medium

meatl heat treatment 1

The medium used for quenching cooling the workpiece is called quenching cooling medium (or quenching medium).The ideal quenching medium should have the condition that the workpiece can be quenched into martensite without causing too much quenching stress.

The commonly used quenching media are water, aqueous solution, mineral oil, molten salt, molten alkali, and so on.

Low water

Water is a quenching medium with a strong cooling capacity.

Advantages: wide source, low price, the stable composition is not easy to deteriorate.

Faults: unstable cooling capacity, easy to make the workpiece deformation or cracking. In the “nose” area of the C curve (around 500 ~ 600℃), the water is in the stage of vapor film, and the cooling is not fast enough, which will form a “soft point”.However, in the martensite transition temperature region (300 ~ 100℃), the water is in the boiling stage and the cooling is too fast, which tends to make the martensite transition speed too fast and generate great internal stress, leading to deformation and even cracking of the workpiece. When the water temperature rises, the water contains more gas or water mixed with insoluble impurities (such as oil, soap, mud, etc.), which will significantly reduce its cooling capacity.

Application: Suitable for quenching and cooling of carbon steel workpiece with small section size and simple shape.

● Brine and lye

Add an appropriate amount of salt and alkali into the water, make the high-temperature workpiece immersed in the cooling medium, in the steam film phase precipitated salt and alkali crystal and immediately burst, the steam film will be destroyed, the surface of the workpiece oxide is also blasted, so as to improve the cooling ability of the medium in the high temperature area, its defect is the corrosive medium.

Application: Under normal circumstances, the concentration of saltwater is 10%, the concentration of caustic soda aqueous solution is 10% ~ 15%.Can be used as a quenching medium for carbon steel and low alloy structural steel workpieces, the use temperature should not exceed 60℃, after quenching should be cleaned in a timely manner and anti-rust treatment.

Low oil

The cooling medium is usually mineral oil (mineral oil). Such as oil, transformer oil, and diesel oil. The oil is generally 10, 20, 30 oil, the larger the oil, the greater the viscosity, the higher the flashpoint, the lower the cooling capacity, the corresponding increase in the use temperature.

Quenching way

● Single liquid quenching

It is a quenching operation in which austenite chemical parts are immersed in a quenching medium and cooled to room temperature. The quenching medium of a single liquid includes water, brine, alkali water, oil, and a specially prepared quenching agent.

Advantages: simple operation, conducive to the realization of mechanization and automation.

Disadvantages: The cooling rate is limited by the cooling characteristics of the medium and affects the quenching quality.

Application: Single – liquid quenching is only suitable for a carbon steel workpiece with a simple shape.

● Double liquid quenching

The austenite chemical component is immersed in a medium with a strong cooling capacity first. Before the steel component reaches the temperature of the quenching medium, it is immediately taken out and then cooled in another medium with weak cooling capacity, such as water before oil, water before air, etc..Double-liquid quenching reduces the tendency of deformation and cracking, which is difficult to master in operation and has certain limitations in application.

● Martensite graded quenching

It is to immerse the austenite chemical parts in the liquid medium (salt bath or alkali bath) at the martensite point of steel with a slightly higher or lower temperature and keep the appropriate time. After the inner and outer layers of the steel parts reach the medium temperature, they are taken out for air cooling, so as to obtain the martensite structure quenching process, also known as graded quenching.

Advantages: The graded quenching can effectively reduce the phase transition stress and thermal stress and reduce the quenching deformation and cracking tendency due to air cooling after the graded temperature stays to the same temperature inside and outside the workpiece.

Application: suitable for alloy steel and high alloy steel workpiece with high deformation requirement, and also for carbon steel workpiece with small cross-section size and complex shape.

● Bainite isothermal quenching

It is a quenching process, sometimes called isothermal quenching, in which steel parts are austenitized and quickly cooled to isothermal bainite conversion temperature range (260 ~ 400℃) to convert austenite into bainite, and the general heat preservation time is 30 ~ 60min.

● Compound quenching

The workpiece was cooled to below Ms to obtain 10% ~ 20% Martensite, and then isothermal in the lower bainite temperature region. This cooling method can obtain the M+B structure of the workpiece with a large cross section. The martensite formed during pre-quenching can promote bainite transformation and temper martensite at isothermal. Compound quenching for alloy tool steel workpiece can avoid the first type of temper brittleness and reduce the residual Austenite volume, that is, the tendency of deformation and cracking.

Tempering

Tempering process

Tempering is a heat treatment process in which the quenched workpiece is reheated to an appropriate temperature below the lower critical temperature and cooled to room temperature in air, water, oil and other media after holding for a period of time.

The purpose of tempering :(1) eliminate the residual stress of the workpiece during quenching to prevent deformation and cracking;(2) Adjust the hardness, strength, plasticity and toughness of the workpiece to meet the performance requirements;(3) Stabilize the structure and size to ensure the accuracy;(4) Improve and improve machining performance.

Classification of tempering

● Low temperature tempering

Refers to the tempering of the workpiece at 150~250℃.

Objective: To maintain high hardness and wear resistance of quenched workpiece and reduce quenching residual stress and brittleness.

Tempered martensite is the tissue obtained by tempering martensite at low temperatures.

Application: cutting tools, measuring tools, molds, rolling bearings, carburizing and surface quenching parts, etc.

● Moderate heat

Refers to the tempering of the workpiece between 350 ~ 500℃.

Objective: To obtain high elasticity and yield point, appropriate toughness. The tempering trochtite is obtained after tempering, which means that the ferrite matrix formed by tempering martensite is distributed in the complex phase structure of extremely fine spherical carbide (or cementite).

Application: spring, forging die, impact tool, etc.

● High temperature tempering

Refers to the workpiece tempering above 500℃.

Objective: To obtain better comprehensive mechanical properties of strength, plasticity, and toughness.

After tempering, the tempered Soxhlet is obtained, which means that the ferrite matrix formed by tempering Martensite is distributed in the complex phase structure of fine spherical carbide (including cementite).

Is the fire

Normalizing

Normalizing process

Normalizing is a metal heat treatment process in which the steel is heated to 30-50℃ above the critical temperature (the temperature of complete austenitizing), and then taken out from the furnace to be cooled in air or by a water spray, spray, or air blow after holding the steel for an appropriate time.

Objective :(1) to make grain refinement and carbide distribution uniform;(2) Remove the internal stress of the material;(3) Increase the hardness of the material.

Advantages :(1) normalizing cooling rate is slightly faster than annealing cooling rate, so the pearlite lamellar space obtained is smaller, the normalizing structure is finer than annealed structure, so its hardness and strength are higher;(2) External cooling of a normalizing furnace does not occupy equipment and has high productivity.

Application: suitable only for carbon steel and low and medium alloy steel, not for high alloy steel. Because the austenite of high alloy steel is very stable, air cooling will also result in martensite tissue.

The specific purpose

(1) For low carbon steel and low alloy steel, normalizing can improve its hardness to improve its machinability;

(2) For medium carbon steel, normalizing can replace tempering treatment to prepare for high frequency quenching, and reduce deformation of steel parts and processing costs;

(3) For high carbon steel, normalizing can eliminate the network cementite structure and facilitate spheroidizing annealing;

(4) Normalizing can be used instead of quenching for large steel forgings or steel castings with sharp changes in section to reduce the tendency of deformation and cracking or to prepare for quenching;

(5) For the hardened counter repair parts of steel, the influence of overheating can be eliminated by normalizing so that the steel can be re-quenched;

(6) It is used for cast iron to increase the pearlite body and improve the strength and wear resistance of the casting.

Annealing

Annealing process

The process of heat treatment in which a metal or alloy is heated to an appropriate temperature, held for a certain period of time, and then slowly cooled (usually as the furnace cools) is called annealing.

The essence of annealing is to heat steel to austenitizing for pearlite transformation, and the annealed tissue is the nearly balanced one.

Purpose of annealing:

(1) Reduce the hardness of steel, improve the plasticity, and facilitate machining and cold deformation processing;

(2) Uniform steel chemical composition and structure, refine grain, improve steel performance or prepare for quenching structure;

(3) Eliminate internal stress and work hardening to prevent deformation and cracking.

Annealing method

1. Complete annealing

Process: Heat the steel to Ac3 above 20~30℃, after holding for a period of time, cool it slowly (along with the furnace) to obtain a heat treatment process (complete austenitizing) with a nearly balanced structure. In actual production, in order to improve productivity, annealing cooling to about 500℃ will be taken out of the oven for air cooling.

Objective: To refine grain, uniform structure, eliminate internal stress, reduce hardness, and improve the machinability of steel. The microstructure of subeutectoid steel after complete annealing is F+P.

Application: Complete annealing is mainly used for subeutectoid steel (WC =0.3~0.6%), generally medium carbon steel and low – and medium-carbon alloy steel castings, forgings, and hot-rolled profiles, and sometimes used for their welds.

Incomplete annealing

Process: Heat the steel to Ac1~Ac3(subeutectoid steel) or Ac1~Accm(hypereutectoid steel) after heat preservation and slow cooling to obtain a heat treatment process that is close to the equilibrium structure.

Application: it is mainly used to obtain spherical pearlite structure of hypereutectoid steel to eliminate internal stress, reduce hardness, and improve machinability.

3. Isothermal annealing

Process: Heat the steel to a temperature higher than Ac3(or Ac1). After holding the steel for an appropriate period of time, it is quickly cooled to a certain temperature in the pearlite region, and isothermal maintenance is made to convert austenite into pearlite, and then air-cooled to room temperature.

Objective: Similar to full annealing, the transformation is easy to control.

Application: suitable for more stable steels: high carbon steel (wc> 0.6%), alloy tool steel, high alloy steel (total amount of alloy elements > 10%). Isothermal annealing is also beneficial to obtain uniform structure and properties. However, it is not suitable for large section steel parts and large quantities of charge, because isothermal annealing is not easy to make the inside of the workpiece or the batch workpiece reach isothermal temperature.

4. Spheroidizing annealing

Process: A heat treatment process for spherification of carbides in steel to obtain granular pearlite. When heating to a temperature above Ac1 of 20~30℃, the holding time should not be too long, generally, 2~4h is appropriate. The cooling method is usually furnace cooling or about 20℃ below Ar1 for a long time isothermal.

Objective: To reduce hardness, uniform structure, and improve machinability in preparation for quenching.

Application: Mainly used in eutectoid steel and hypereutectoid steel, such as carbon tool steel, alloy tool steel, bearing steel, etc.Spheroidal pearlite is obtained by spheroidal annealing. In spheroidal pearlite, the cementite is spherical with fine particles dispersed on the ferrite matrix. Compared with lamellae, spherical pearlite has a lower hardness and is easy to be machined, and austenite grains are not easy to be coarse and less prone to deformation and cracking during quenching and heating.

5. Diffusion annealing (uniform annealing)

Process: A heat treatment process in which the ingot, casting, or forging is heated to a temperature slightly below that of the solid phase line for a long period of time and then cooled slowly to eliminate chemical inhomogeneity.

Objective: To eliminate the dendrite segregation and regional segregation during solidification and homogenize the composition and structure.

Application: Used in some high quality alloy steel and serious segregation alloy steel castings and ingot. The heating temperature of diffusion annealing is very high, usually 100~200℃ above Ac3 or Accm. The specific temperature depends on the segregation degree and steel type. The holding time is generally 10~15 hours. After diffusion annealing, complete annealing and normalizing treatment are required to refine the structure.

6. Stress relief annealing

Process: Heat the steel to a certain temperature below Ac1 (generally 500~650℃), hold the heat, and then cool with the furnace.

The stress annealing temperature is lower than A1, so the stress annealing does not cause tissue change.

Objective: To eliminate residual internal stress.

Application: Mainly used to eliminate the residual stress of castings, forgings, welding parts, hot-rolled parts, cold drawn parts, etc.If these stresses are not eliminated, they may cause deformation or cracks in the steel after a certain period of time or during subsequent machining.

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