High temperature temper brittleness Many alloy steels are tempered between 500 and 550 ° C after quenching, or embrittled at a slow cooling rate of 500 to 550 ° C after tempering at temperatures above 600 ° C. If it is reheated to a temperature above 600 ° C and then rapidly cooled, the toughness can be restored, so it is also called reversible temper brittleness. Tempering definition The heat treatment process of quenching steel, which is heated under AC1 and kept warm after cooling.
Tempering and tempering of steel is a heat treatment process in which the workpiece is hardened and heated to a temperature below AC1, kept for a certain period of time, and then cooled to room temperature.
Tempering is generally followed by quenching, the purpose of which is:
(a) Eliminating residual stress generated during quenching of the workpiece to prevent deformation and cracking;
(b) Adjust the hardness, strength, ductility and toughness of the workpiece to meet the performance requirements;
(c) Stabilize the organization and size to ensure accuracy;
(d) Improve and improve processability. Therefore, tempering is the last important step in obtaining the required performance of the workpiece.
According to the tempering temperature range, tempering can be divided into low temperature tempering, medium temperature tempering and high temperature tempering.
(1) Tempering of low temperature tempered workpieces below 250 °C.
The purpose is to maintain the high hardness and wear resistance of the quenched workpiece and reduce the quench residual stress and brittleness.
After tempering, tempered martensite is obtained, which refers to the structure obtained when the quenched martensite is tempered at a low temperature.
Mechanical properties: 58 ~ 64HRC, high hardness and wear resistance.
Applications: cutting tools, measuring tools, molds, rolling bearings, carburized and surface hardened parts.
(2) Tempering of the medium temperature tempered workpiece between 250 and 500 °C.
The aim is to obtain a higher elasticity and yield point with appropriate toughness. Preheat treatment
After tempering, the tempered torsite is obtained, which means that the ferrite matrix formed during the tempering of the martensite is distributed with a fine phase structure of extremely fine spherical carbides (or cementite).
Mechanical properties: 35 ~ 50HRC, high elastic limit, yield point and certain toughness.
Applications: Springs, forging dies, impact tools, etc.
(3) Tempering of high temperature tempered workpieces above 500 °C.
The aim is to obtain comprehensive mechanical properties with good strength, ductility and toughness.
After tempering, tempered sorbite is obtained, which refers to a multiphase structure in which fine spheroidal carbides (including cementite) are distributed in the ferrite matrix formed during martensite tempering. Mechanical properties: 200 ~ 350HBS, better comprehensive mechanical properties.
Applications: Widely used in a variety of more important structural members such as connecting rods, bolts, gears and shaft parts.
The composite heat treatment process in which the workpiece is quenched and tempered at high temperature is called quenching and tempering. Quenching and tempering is not only the final heat treatment, but also can be used for pre-heat treatment of some precision parts or induction hardened parts.
Precautions
A heat treatment process in which the steel quenched into martensite is heated to a temperature below the critical point A1, held for a suitable period of time, and then cooled to room temperature. The purpose of tempering is to eliminate the quenching stress and transform the steel structure into a relatively stable state. The plasticity and toughness of the steel are improved without reducing or appropriately reducing the hardness and strength of the steel to achieve the desired properties. Medium carbon and high carbon steels usually have a high hardness after quenching, but they are very brittle and generally require tempering before they can be used. The quenched martensite in steel is a supersaturated solid solution of carbon in α-Fe, which has a body-centered square structure, and its squareness c/a increases with the increase of carbon content (c/a=1+0.045wt). %C). Martensite structure is thermodynamically unstable and has a tendency to transition to stable tissue. Many steels have a certain amount of retained austenite after quenching, which is also unstable and will change during tempering. Therefore, the tempering process is essentially a complex transformation process in which the hot steel is heated in a certain temperature range to make the thermodynamically unstable microstructure in the steel transition to a stable state. The content and form of the transformation depend on the chemical composition and structure of the hardened steel, as well as the heating temperature (see martensitic transformation) for secondary preheating.
Carbon steel tempering process
The structural transformation during the tempering of quenched carbon steel is representative of all kinds of steel. The tempering process includes martensite decomposition, carbide precipitation, transformation, aggregation and growth, ferrite recovery and recrystallization, and residual austenite decomposition. The transitions in the tempering of low and medium carbon steels are schematically summarized in Figure 1. According to their reaction temperatures, they can be described as four stages overlapping each other. The first stage of tempering (below 250 ° C) Martensite is unstable at room temperature, the interstitial carbon atoms can move slowly in the Martens, producing some degree of carbon segregation. As the tempering temperature increases, the martensite begins to decompose, and ε-carbide precipitates in the medium and high carbon steel (Fig. 2), and the squareness of the martensite decreases. The increase in hardness observed after tempering of high carbon steel at 50 to 100 ° C is due to the precipitation hardening of ε-carbide in martensite (see desolvation). The ε-carbide has a close-packed hexagonal structure, which is in the form of a strip or a thin rod, and has a certain orientation relationship with the matrix. The nascent ε-carbide is likely to remain coherent with the matrix. After tempering at 250 ° C, the martensite still retains about 0.25% carbon. Martensite containing less than 0.2% carbon does not undergo ε-carbide precipitation when tempered below 200 °C, only carbon segregation, and tempering at higher temperatures directly decomposes cementite. The second stage of tempering (200 ~ 300 ° C) residual austenite transformation. It is tempered to a temperature range of 200 to 300 ° C. The retained austenite is not completely transformed in the quenched steel, and decomposition will occur to form a bainite structure. This change is more pronounced in medium carbon and high carbon steels. For carbon steels and low alloy steels containing less than 0.4% carbon, this transformation is essentially negligible due to the small amount of retained austenite. The third stage of tempering (200 ~ 350 ° C) martensite decomposition is completed, the squareness disappears. The ε-carbide is converted to cementite (Fe3C). This conversion is carried out by the dissolution of ε-carbide and the recrystallization of cementite. The cementite initially formed and the matrix maintain a strict orientation relationship. Cementite tends to nucleate at the interface between the ε-carbide and the matrix, at the martensite interface, at the twin boundary in the high-carbon martensite sheet, and at the original austenite grain boundary (Fig. 3). The formed cementite is initially in the form of a film and then gradually spheroidized into granular Fe3C. The fourth stage of tempering tempering (350 ~ 700 ° C) cementite spheroidization and growth, ferrite recovery and recrystallization. The cementite starts to spheroidize at 400 ° C, and aggregates grow after 600 ° C. During the process, smaller cementite particles dissolve in the matrix and carbon is transported to the larger particles that are selected for growth. Carbide particles located on the boundaries between the martensite grain boundaries and the original austenite grain boundaries are the fastest to spheroidize and grow because they are much easier to diffuse in these regions. The ferrite undergoes a recovery process at 350 to 600 °C. At this time, in the low-carbon and medium-carbon steels, the dislocations in the lath martensite lath and the lath boundary are merged and rearranged, so that the dislocation density is significantly reduced, and the original Markov slab is formed. The bundles are closely related to the long strip of ferrite grains. The original martensite lath boundary can be kept stable to 600 ° C; in high carbon steel, the ferrite formed by the disappearance of twin crystals in the needle-shaped martensite still maintains its needle-like morphology. Significant recrystallization occurs in the ferrite between 600 and 700 °C, forming equiaxed ferrite grains. Thereafter, the Fe3C particles are continuously coarsened, and the ferrite grains are gradually grown.
The influence of alloying elements on the general tempering process The alloying element silicon can delay the nucleation and growth of carbides and strongly block the transformation of ε-carbide into cementite; adding about 2% silicon in steel can make ε - The carbide is kept to 400 °C. In carbon steel, the squareness of martensite disappears at 300 ° C, and steel containing elements such as Cr, Mo, W, V, Ti and Si can maintain a certain square after tempering at 450 ° C or even 500 ° C. degree. Explain that these elements can delay the decomposition of iron-carbon supersaturated solid solutions. Conversely, Mn and Ni promote this decomposition process (see alloy steel).
The alloying elements also have a large effect on the amount of retained austenite after quenching. The retained austenite forms a fine network around the martensite lath; these austenites decompose after tempering at 300 ° C, and a cementite film is produced at the lath boundary. When the residual austenite content is high, such a continuous film is likely to be one of the causes of tempered martensite brittleness (300 to 350 ° C). Alloying elements, especially Cr, Si, W, Mo, etc., enter the cementite structure, and the coarsening temperature of the cementite particles is increased from 350 to 400 ° C to 500 to 550 ° C, thereby suppressing the temper softening process. It hinders the grain growth of ferrite.
Special Carbides and Secondary Hardening When strong carbide forming elements are present in the steel at a sufficiently high concentration, they can be substituted for cementite to form their own special carbides at temperatures ranging from 450 to 650 °C. The diffusion and redistribution of alloying elements are required for the formation of special carbides, and the diffusion coefficients of these elements in iron are several orders of magnitude lower than those of elements such as C and N. Therefore, a certain temperature tempering condition is required before nucleation grows. For the same reason, the growth rate of these special carbides is very low. The highly dispersed special carbide formed at 450 to 650 ° C maintains its dispersion even after long-term tempering. Figure 4 shows that the formation of alloy carbide between 450 and 650 ° C strengthens the matrix, causing the hardness of the steel to rise again and peak. This phenomenon is called secondary hardening.
Performance of steel after tempering The properties of hardened steel after tempering depend on its internal microstructure; the microstructure of the steel varies with its chemical composition, quenching process and tempering process. Carbon steel can obtain better mechanical properties after tempering between 100 and 250 °C. A typical change in the mechanical properties of alloy structural steel after tempering between 200 and 700 ° C is shown in Figure 5. It can be seen from Fig. 5 that as the tempering temperature increases, the tensile strength σb of the steel decreases monotonously; the yield strength σ0.3 increases slightly and then decreases; the area shrinkage ψ and the elongation δ continuously improve; Using the fracture toughness K1c as an indicator) the overall trend is rising, but there are two minimum values ​​between 300-400 ° C and 500-550 ° C, which are correspondingly called low temperature temper brittleness and high temperature temper brittleness. . Therefore, in order to obtain good comprehensive mechanical properties, alloy structural steels tend to temper in three different temperature ranges: ultra high strength steel is about 200 ~ 300 ° C; spring steel is around 460 ° C; quenched and tempered steel is back at 550 ~ 650 ° C fire. Carbon and alloy tool steels are required to have high hardness and high strength, and the tempering temperature generally does not exceed 200 °C. Alloy structural steel, die steel and high speed steel with secondary hardening during tempering are tempered in the range of 500 to 650 °C.
Tempering brittle low temperature temper brittleness The embrittlement of many alloy steels after quenching into martensite at 250-400 °C tempering. The embrittlement that has occurred cannot be eliminated by reheating, and is therefore also called irreversible temper brittleness. A lot of research has been done on the cause of temper softening which causes low temperature temper brittleness. It is generally believed that when the quenched steel is tempered in the range of 250-400 ° C, the cementite precipitates at the prior austenite grain boundary or at the martensite interface to form a thin shell, which is the main cause of low temperature temper brittleness. A certain amount of silicon is added to the steel to delay the formation of cementite during tempering, which can increase the temperature at which low temperature temper brittleness occurs. Therefore, the ultrahigh strength steel containing silicon can be tempered at 300-320 ° C without embrittlement. Conducive to improving the overall mechanical properties.
High temperature temper brittleness Many alloy steels are tempered between 500 and 550 ° C after quenching, or embrittled at a slow cooling rate of 500 to 550 ° C after tempering at temperatures above 600 ° C. If it is reheated to a temperature above 600 ° C and then rapidly cooled, the toughness can be restored, so it is also called reversible temper brittleness. It has been proved that impurity elements such as P, Sn, Sb and As in steel are segregated to the prior austenite grain boundary at a temperature of 500-550 °C, resulting in high-temperature temper brittleness; elements such as Ni and Mn can be formed with impurity elements such as P and Sb. The grain boundary synergistic segregation (Cosegregation), Cr element promotes this synergistic segregation, so these elements all aggravate the high temperature temper brittleness of steel. On the contrary, the interaction of molybdenum and phosphorus hinders the segregation of phosphorus at the grain boundary and can reduce the high temperature temper brittleness. Rare earth elements also have a similar effect. The rapid cooling of steel after tempering at temperatures above 600 °C can inhibit the segregation of phosphorus, which is often used in heat treatment to avoid high temperature temper brittleness.
When the car exhaust pipe is tempered to watch the racing game, when the car is decelerating in the curve, sometimes it will make a very loud humming sound, just like shooting a gun. This is the sound of the exhaust pipe tempering. Its English major The term is called BACKFIRE. The car needs rapid acceleration, so compared with the engine of the ordinary civilian car, the racing engine is more often set in the state of fuel enrichment, the mixing ratio is very thick, so that more fuel can participate. Combustion releases energy. At the moment of sudden oil withdrawal, there will always be some unburned mixed gas entering the exhaust system, and the hot exhaust pipe will be ignited again to cause deflagration (the loud noise emitted at this time is the sound of the gun you hear). In addition, in order to reduce the exhaust back pressure and improve the power, the exhaust pipe of the car is mostly in-line type, there is no three-way catalytic package and silencer for the civilian car, so there will be such undisguised detonation sound, which is powerful. It can even be seen that the flame is ejected from the end of the exhaust pipe, which makes people feel very irritating. Compared to naturally aspirated engines, those turbocharged racing engines are more prone to tempering because they are mostly equipped with so-called partial ignition systems.
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