Introduction to the hottest heat treatment process

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Introduction to heat treatment process

surface quenching

surface quenching of steel

when some parts are subjected to alternating loads and impact loads such as torsion and bending, their surface layer bears higher stress than the center. In the case of friction, the surface layer is also constantly worn. Therefore, the surface layer of some parts is required to have high strength, high hardness, high wear resistance and high fatigue limit. Only surface strengthening can meet the above requirements. Table 1 dimensional deviation and appearance surface quenching have the advantages of small deformation and high productivity, so they are widely used in production

according to different heating methods, surface quenching mainly includes induction heating surface quenching, flame heating surface quenching, electric contact heating surface quenching, etc

induction heating surface quenching

induction heating is to use electromagnetic induction to generate eddy current in the workpiece and heat the workpiece. Compared with ordinary quenching, induction heating surface quenching has the following advantages:

1. The heat source is on the surface of the workpiece, with fast heating speed and high thermal efficiency

2. Because the workpiece is not heated as a whole, the deformation is small

3. The heating time of the workpiece is short, and the amount of oxidation and decarburization on the surface is less

4. The surface hardness of the workpiece is high, the notch sensitivity is small, and the impact toughness, fatigue strength and wear resistance are greatly improved. It is conducive to giving full play to the potential of materials, saving material consumption and improving the service life of parts

5. Compact equipment, easy to use, good working conditions

6. It is convenient for mechanization and automation

7. It can be used not only in surface quenching, but also in penetration heating and chemical heat treatment

basic principle of induction heating

place the workpiece in the inductor. When the inductor passes through the alternating current, an alternating magnetic field with the same frequency as the current is generated around the inductor, and an induced electromotive force is generated in the workpiece accordingly, forming an induced current on the surface of the workpiece, that is, eddy current. Under the action of the resistance of the workpiece, this eddy current converts electric energy into heat energy, so that the surface temperature of the workpiece can reach the quenching heating temperature, which can realize surface quenching

properties after induction surface quenching

1. Surface hardness: the surface hardness of workpieces quenched by high and medium frequency induction heating is often 2-3 units (HRC) higher than that of ordinary quenching

2。 Wear resistance: the wear resistance of workpiece after high-frequency quenching is higher than that of ordinary quenching. This is mainly due to the combination of fine martensite grains, high carbide dispersion, high hardness and high compressive stress on the surface of the hardened layer

3。 Fatigue strength: high and medium frequency surface quenching greatly improves the fatigue strength and reduces the notch sensitivity. For the workpiece of the same material, the depth of the hardened layer is within a certain range, and the fatigue strength increases with the increase of the depth of the hardened layer, but when the depth of the hardened layer is too deep, the surface layer is compressive stress, so the fatigue strength decreases with the increase of the depth of the hardened layer, and the brittleness of the workpiece increases. General hardened layer depth δ= (10~20)%D。 More appropriate, where D. Is the effective diameter of the workpiece

annealing process

annealing is a heat treatment process that heats metals and alloys to an appropriate temperature, maintains them for a certain period of time, and then cools them slowly. The hypoeutectoid steel after annealing is ferrite plus lamellar pearlite; Eutectoid steel or hypereutectoid steel is granular pearlite. In a word, the annealed structure is close to equilibrium

purpose of annealing

① reduce the hardness of steel and improve the plasticity, so as to facilitate cutting and cold deformation processing

② refine the grain, eliminate the structural defects caused by casting, forging and welding, homogenize the structure and composition of the steel, improve the performance of the steel or make structural preparations for future heat treatment

③ eliminate the internal stress in the steel to prevent deformation and cracking

types of annealing process

① homogenization annealing (diffusion annealing)

homogenization annealing is an annealing process aimed at reducing the chemical composition segregation and structural heterogeneity of metal ingots, castings or forgings, heating them to high temperature, maintaining them for a long time, and then cooling them slowly, aiming at the homogenization of chemical composition and structure

the heating temperature of homogenization annealing is generally AC3 + (150 ~ 200 ℃), that is, 1050 ~ 1150 ℃. The machine can be operated for 5h only after the holding time is generally 10 ~ 1, so as to ensure the full diffusion and eliminate or reduce the uneven composition or organization. Due to the high heating temperature, long time and coarse grain size of diffusion annealing, complete annealing or normalizing is carried out after diffusion annealing to refine the structure again

② complete annealing

complete annealing, also known as recrystallization annealing, is an annealing process that austenitizes the iron carbon alloy completely and then cools slowly to obtain a near equilibrium structure

complete annealing is mainly used for hypoeutectoid steel, generally medium carbon steel and low and medium carbon alloy structural steel forgings, castings and hot-rolled sections, and sometimes for their welding components. Complete annealing is not applicable to hypereutectoid steel, because it needs to be heated above ACM for complete annealing of hypereutectoid steel. When it is cooled slowly, carburization will precipitate along the austenite grain boundary and distribute in shape, resulting in increased brittleness of the material, leaving hidden dangers for the final heat treatment

the heating temperature of fully annealed carbon steel is generally AC3 + (30 ~ 50 ℃); Alloy steel is AC3 + (500 ~ 70 ℃); The holding time should be determined according to the type of steel, the size of workpiece, the amount of furnace, the type of equipment selected and other factors. In order to ensure the complete pearlite transformation of undercooled austenite, the cooling of complete annealing must be slow, and the furnace shall be cooled to about 500 ℃ with the furnace and then discharged for air cooling

③ incomplete annealing

incomplete annealing is an annealing process that heats the iron carbon alloy to the temperature between AC1 and AC3 to achieve incomplete austenitization, followed by slow cooling

incomplete annealing is mainly applicable to forged and rolled pieces of medium and high carbon steels and low alloy steels. Its purpose is to refine the structure and reduce the hardness. The heating temperature is AC1 + (40 ~ 60) ℃, and it is cooled slowly after heat preservation

④ isothermal annealing

isothermal annealing is an annealing process that heats steel parts or blank parts to a temperature higher than AC3 (or AC1), maintains it for an appropriate time, cools them quickly to a certain temperature in the pearlite temperature range and maintains it isothermal, so that austenite changes into pearlite structure, and then cools them in air

isothermal annealing process is applied to medium carbon alloy steel and low alloy steel to refine structure and reduce hardness. The heating temperature of hypoeutectoid steel is AC3 + (30 ~ 50) ℃, and the heating temperature of hypereutectoid steel is AC3 + (20 ~ 40) ℃. Keep it for a certain time, and conduct isothermal transformation with the furnace cooling to a temperature slightly lower than Ar3, and then exit the furnace for air cooling. The microstructure and hardness of isothermal annealing are more uniform than that of complete annealing

⑤ spheroidizing annealing

spheroidizing annealing is an annealing process for spheroidizing carbides in steel. The steel is heated to 20 ~ 30 ℃ above AC1 for a period of time, and then cooled slowly to obtain the structure of spherical or granular carbides evenly distributed on the ferrite matrix

spheroidizing annealing is mainly applicable to eutectoid steel and hypereutectoid steel, such as carbon tool steel, alloy tool steel, bearing steel, etc. These steels are air cooled after rolling and forging. The obtained structure is lamellar and preserves a surface layer of pearlite and cementite. This structure is hard and brittle, which is not only difficult to cut, but also easy to deform and crack in the later quenching process. After spheroidizing annealing, the spheroidal pearlite structure is obtained, in which the cementite is spherical particles and dispersed on the ferrite matrix. Compared with the flake pearlite, it is not only low hardness and easy to cut, but also austenite grains are not easy to grow during quenching and heating, and the workpiece deformation and cracking tendency are small during cooling. In addition, spheroidizing annealing can sometimes be used for some hypoeutectoid steels that need to improve cold plastic deformation (such as stamping, cold heading, etc.)

the heating temperature of spheroidizing annealing is AC1 + (20 ~ 40) ℃ or ACM - (20 ~ 30) ℃, isothermal cooling or direct slow cooling after holding. Austenization is "incomplete" during spheroidizing annealing, except for the transformation of lamellar pearlite into austenite and the dissolution of a small amount of excess carbides. Therefore, it is impossible to eliminate spheroidal carbides. If there are spheroidal carbides in hypereutectoid steel, normalizing must be carried out before spheroidizing annealing to eliminate them, so as to ensure the normal spheroidizing annealing

There are many spheroidizing annealing processes, and the two most commonly used processes are ordinary spheroidizing annealing and isothermal spheroidizing annealing. Ordinary spheroidizing annealing is to heat the steel to 20 ~ 30 ℃ above AC1 for a proper time, and then slowly cool it with the furnace, cool it to about 500 ℃ and exit the furnace for air cooling. Isothermal spheroidizing annealing is the same as ordinary spheroidizing annealing process. After heating and holding, it is isothermal with the furnace cooling to a temperature slightly lower than AR1. The isothermal time is 1.5 times of its heating and holding time. After isothermal, it is cooled to about 500 ℃ with the furnace and then discharged for air cooling. Compared with ordinary spheroidizing annealing, spheroidizing annealing can not only shorten the cycle, but also make the spheroidizing structure uniform, and can strictly control the hardness after annealing

⑥ recrystallization annealing (intermediate annealing)

recrystallization annealing is a heat treatment process in which the metal after cold deformation is heated to above the recrystallization temperature and maintained for an appropriate time to recrystallize the deformed grains into uniform equiaxed grains, so as to eliminate deformation strengthening and residual stress

⑦ stress relief annealing

stress relief annealing is an annealing process to eliminate the residual stress in castings caused by plastic deformation processing, welding, etc

there is internal stress in the workpiece after forging, casting, welding and cutting. If it is not eliminated in time, the workpiece will be deformed during processing and use, affecting the accuracy of the workpiece. It is very important to use stress relief annealing to eliminate the internal stress produced in the processing process

the heating temperature of stress relief annealing is lower than the phase transformation temperature A1, so there is no structural transformation during the whole heat treatment process. The internal stress is mainly eliminated by the workpiece in the process of heat preservation and slow cooling. In order to eliminate the internal stress of the workpiece more thoroughly, the heating temperature should be controlled during heating. Generally, it enters the furnace at low temperature, and then it is heated to the specified temperature at a heating speed of about 100 ℃/h. The heating temperature of the weldment shall be slightly higher than 600 ℃. The holding time depends on the situation, usually 2 ~ 4H. The holding time for stress relief annealing of castings is set at the upper limit, the cooling rate is controlled at (20 ~ 50) ℃/h, and it can be discharged for air cooling only when it is cooled below 300 ℃

normalizing process

normalizing process is a heat treatment process in which steel parts are heated to 30 ~ 50 ℃ above AC3 (or ACM) and cooled in still air after holding for an appropriate time. Normalizing steel parts heated to 100 ~ 150 ℃ above AC3 is called high temperature normalizing

the main purpose of normalizing medium and low carbon steel castings and forgings is to refine the structure. Compared with annealing, after normalizing, the pearlite lamella is finer and the ferrite grain is smaller, so the strength and hardness are higher

due to the low hardness of low carbon steel after annealing, tool sticking occurs during cutting, and the cutting performance is poor. Improving the hardness by normalizing can improve the cutting performance. Some medium carbon structural steel parts can be normalized instead of quenching and tempering, simplifying the heat treatment process

the normalizing heating knife of hypereutectoid steel is above ACM, so that the original cementite is dissolved into austenite, and then cooled at a faster speed to inhibit the common failure of cementite at austenite grain boundary: precipitation after sample fracture, so as to eliminate the cementite and improve the structure of hypereutectoid steel

parts requiring weld strength of weldments shall be normalized to improve weld group

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