1. Quenching
1. What is quenching?
The quenching of steel is a heat treatment process in which the steel is heated to a critical temperature above Ac3 (sub-eutectic steel) or Ac1 (trans-eutectic steel), insulation for a period of time, so that it is fully or partially austenitic, and then a rapid cooling rate greater than the critical cooling rate to less than Ms (or isothermal near Ms) for martensitic (or bainite) transformation. The solid solution treatment of aluminum alloys, copper alloys, titanium alloys, tempered glass and other materials or the heat treatment process with a rapid cooling process is usually called quenching.
2. The purpose of quenching:
1) Improve the mechanical properties of metal finished materials or parts. For example, improve the hardness and wear resistance of tools, bearings, etc., improve the elastic limit of springs, and improve the comprehensive mechanical properties of shaft parts.
2) Improve the material properties or chemical properties of some special steels. Such as improving the corrosion resistance of stainless steel, increasing the permanent magnetism of magnetic steel, etc.
When quenching and cooling, in addition to the need to reasonably select the quenching medium, there must also be a correct quenching method, the commonly used quenching methods, mainly single liquid quenching, double liquid quenching, graded quenching, isothermal quenching, local quenching and so on.
3. Steel workpieces have the following characteristics after quenching:
(1) Unbalanced (i.e., unstable) tissues such as martensitic, bainite, and residual austenite are obtained.
(2) There is a large internal stress.
(3) Mechanical properties can not meet the requirements. Therefore, steel workpieces are generally tempered after quenching
Second, tempering
1. What is tempering?
Tempering is the quenched metal or parts heated to a certain temperature, after a certain period of heat preservation, in a certain way to cool the heat treatment process, tempering is quenched immediately after the operation, usually is also the last process of heat treatment of the workpiece, so the joint process of quenching and tempering is called final treatment.
2. The main purposes of quenching and tempering are:
1) Reduce internal stress and reduce brittleness, quenched parts have great stress and brittleness, such as no timely tempering will often produce deformation or even cracking.
2) Adjust the mechanical properties of the workpiece, after the workpiece is quenched, the hardness is high, the brittleness is large, in order to meet the different performance requirements of various workpieces, it can be adjusted by tempering, hardness, strength, plasticity and toughness.
3) Stabilize the workpiece size. By tempering, the metallographic structure can be stabilized to ensure that no deformation occurs during future use.
4) Improve the cutting performance of some alloy steels.
3. The role of tempering is:
(1) Improve the stability of the organization, so that the workpiece no longer undergoes tissue transformation during use, so that the workpiece geometry and performance remain stable.
(2) Eliminate internal stress in order to improve the performance of the workpiece and stabilize the geometric dimensions of the workpiece.
(3) Adjust the mechanical properties of steel to meet the requirements of use.
The reason why tempering has these effects is because when the temperature rises, the atomic activity ability is enhanced, and the atoms of iron, carbon and other alloying elements in steel can be diffused relatively quickly, realizing the rearrangement and combination of atoms, so that the unstable unbalanced structure is gradually transformed into a stable equilibrium organization. The elimination of internal stress is also related to the decrease in metal strength at elevated temperatures. Generally, when steel is tempered, the hardness and strength decrease and the plasticity increases. The higher the tempering temperature, the greater the change in these mechanical properties. Some alloy steels with high alloying element content will precipitate some fine metal compounds when tempered in a certain temperature range, which will increase the strength and hardness. This phenomenon is called secondary hardening.
Tempering requirements: Workpieces with different uses should be tempered at different temperatures to meet the requirements in use.
(1) Tools, bearings, carburized hardening parts, surface quenching parts are usually tempered at low temperatures below 250 °C. After low temperature tempering, the hardness does not change much, the internal stress is reduced, and the toughness is slightly improved.
(2) The spring is tempered at a medium temperature of 350~ 500 °C, which can obtain high elasticity and necessary toughness.
(3) Parts made of medium carbon structural steel are usually tempered at high temperature at 500~ 600 °C to obtain a good fit of suitable strength and toughness.
When steel is tempered at about 300 ° C, it often increases its brittleness, which is called the first type of tempering brittleness. Tempering should generally not be made during this temperature range. Some medium carbon alloy structural steels are also prone to brittleness if they are slowly cooled to room temperature after tempering at high temperatures. This phenomenon is called the second type of tempering brittleness. The addition of molybdenum to steel, or cooling in oil or water when tempered, can prevent the second type of tempering brittleness. This brittleness can be eliminated by reheating the second type of tempered brittle steel to the original tempering temperature.
In production, the performance requirements of the workpiece are often based on. According to the different heating temperatures, the tempering is divided into low temperature tempering, medium temperature tempering, and high temperature tempering. The heat treatment process combining quenching and subsequent high-temperature tempering is called quenching and tempering, that is, it has a high degree of strength and good plastic toughness.
1) Low temperature tempering: 150-250 °C, M back, reduce internal stress and brittleness, improve plastic toughness, have high hardness and wear resistance. Used in the manufacture of measuring tools, tools and rolling bearings, etc.
2) Medium temperature tempering: 350-500 °C, T back, with high elasticity, a certain degree of plasticity and hardness. Used to make springs, forging dies, etc.
3) High temperature tempering: 500-650 °C, S return, with good comprehensive mechanical properties. Used to make gears, crankshafts, etc.
Third, the fire is in full swing
1. What is a normalizing fire?
Normalizing is a heat treatment that improves the toughness of steel. After heating the steel member to 30~50 °C above the Ac3 temperature, it is kept warm for a period of time and is air-cooled. The main feature is that the cooling rate is faster than annealing and lower than quenching, and the crystalline grain of the steel can be refined in a slightly faster cooling when normalizing, which can not only obtain satisfactory strength, but also significantly improve the toughness (AKV value) and reduce the cracking tendency of the components. After some low-alloy hot-rolled steel plates, low-alloy steel forgings and castings are normalized, the comprehensive mechanical properties of the material can be greatly improved, and the cutting performance is also improved.
2. Normalizing fire has the following purposes and uses:
(1) For sub-eutectic steel, normalizing is used to eliminate the overheated coarse crystal structure and Wei's structure of casting, forging and weldment, and the strip-like structure in the rolled product; refine the grains; and can be used as a pre-heat treatment before quenching.
(2) For over-eutectic steel, normalizing can eliminate the mesh secondary carburizing and refine the pearlite, which not only improves the mechanical properties, but also facilitates the future spheroid annealing.
(3) For low-carbon deep-drawn thin steel plates, normalizing can eliminate the free carburized body of the grain boundary to improve its deep-drawing performance.
(4) For low-carbon steel and low-carbon low-alloy steel, the use of normalizing can obtain more fine sheet pearlite structure, so that the hardness is increased to HB140-190, avoiding the phenomenon of "sticking knife" during cutting and improving machinability. For medium carbon steel, it is more economical and convenient to use normalizing in the case of both normalizing and annealing.
(5) For ordinary medium carbon structural steel, in the case of low mechanical properties, normalizing can be used instead of quenching and high temperature tempering, which is not only easy to operate, but also makes the structure and size of the steel stable.
(6) High temperature normalizing (150 ~ 200 °C above Ac3) Due to the high diffusion rate at high temperature, the composition segregation of castings and forgings can be reduced. The coarse grains after high temperature normalizing can be refined by a second lower temperature normalizing.
(7) For some low and medium carbon alloy steels used in steam turbines and boilers, normalizing is often used to obtain bainite structure, and then tempered at high temperature, which has good creep resistance at 400 ~ 550 °C.
(8) In addition to steel parts and steel, normalizing is also widely used in ductile iron heat treatment, so that it obtains a pearlite matrix and improves the strength of ductile iron.
Since the normalizing fire is characterized by air cooling, the ambient temperature, stacking method, air flow and workpiece size have an impact on the microstructure and performance after the normalizing fire. Normalizing can also be used as a classification method for alloy steels. Usually according to the diameter of 25 mm specimen heated to 900 ° C, the air cooled the resulting structure, the alloy steel is divided into pearlite steel, bainite steel, martensitic steel and austenitic steel.
Fourth, annealing
1. What is annealing?
Annealing is a metal heat treatment process that slowly heats a metal to a certain temperature, keeps it for a sufficient amount of time, and then cools it at a suitable rate. Annealing heat treatment is divided into complete annealing, incomplete annealing and stress relief annealing. The mechanical properties of annealed materials can be detected by tensile tests or by hardness tests. Many steels are supplied in annealing heat treatment, steel hardness testing can use Rockwell hardness tester, test HRB hardness, for thinner steel plates, steel strips and thin-walled steel pipes, surface Rockwell hardness tester can be used to detect HRT hardness.
2. The purpose of annealing is to:
(1) Improve or eliminate various tissue defects and residual stresses caused by steel in the casting, forging, rolling and welding processes, and prevent deformation and cracking of workpieces.
(2) Soft chemical parts for cutting processing.
(3) Refine the grain, improve the structure to improve the mechanical properties of the workpiece.
(4) Prepare for the final heat treatment (quenching, tempering).
3. Common annealing processes are:
(1) Complete annealing. It is used to refine the coarse superheating structure with poor mechanical properties of medium and low carbon steel after casting, forging and welding. The workpiece is heated to a temperature above 30 to 50 ° C above the temperature of ferritic conversion into austenite, insulation for a period of time, and then slowly cooled with the furnace, and the austenite is transformed again during the cooling process, which can make the structure of the steel thinner.
(2) Spheroidal annealing. It is used to reduce the high hardness of tool steels and bearing steels after forging. The workpiece is heated to 20 to 40 ° C above the temperature of the steel that begins to form austenite, and after heat preservation, it is slowly cooled, and the sheet-like carburized body in the pearlite becomes spherical during the cooling process, thereby reducing the hardness.
(3) Isothermal annealing. It is used to reduce the high hardness of some alloy structural steels with high nickel and chromium content for cutting processing. Generally, it is cooled to the most unstable temperature of austenite at a faster speed, and the austenite is converted to tonite or sonite at a relatively fast speed, and the hardness can be reduced.
(4) Recrystallization annealing. It is used to eliminate the hardening phenomenon (increased hardness and decreased plasticity) of metal wire rods and sheets in the process of cold drawing and cold rolling. The heating temperature is generally 50 to 150 ° C below the temperature at which the steel begins to form austenite, and only in this way can the process hardening effect be eliminated to soften the metal.
(5) Graphitized annealing. Cast iron containing a large amount of carburized body is turned into malleable cast iron with good plasticity. The process operation is to heat the casting to about 950 ° C, cool it properly after a certain period of insulation, and decompose the carburized body to form a flocculent graphite.
(6) Diffusion annealing. It is used to homogenize the chemical composition of the alloy casting and improve its usability. The method is to heat the casting to the highest possible temperature without melting, and keep warm for a long time, and then cool down after the diffusion of various elements in the alloy tends to be evenly distributed.
(7) Stress-relieving annealing. It is used to eliminate internal stresses in steel castings and weldments. For steel products after heating, the temperature of austenite begins to form below 100 ~ 200 ° C, and it is cooled in the air after heat preservation, and the internal stress can be eliminated.