Analysis and preventive measures of quenching crac

Analysis and preventive measures of quenching cracks in die steel

quenching is a common process in the heat treatment of die steel. However, for various reasons, sometimes quenching cracks will inevitably occur, resulting in the waste of previous efforts. It has remarkable technical and economic benefits to analyze the causes of cracks and take corresponding preventive measures. Common quenching cracks have the following 10 types

1. Longitudinal crack

the crack is axial, and the shape is thin and long. When the die is completely quenched, i.e. centerless quenching, the center is transformed into quenched martensite with the largest specific volume, resulting in tangential tensile stress. The higher the carbon content of the die steel, the greater the tangential tensile stress. When the tensile stress is greater than the strength limit of the steel, longitudinal cracks are formed. The following factors have aggravated the generation of longitudinal cracks: (1) the steel contains a lot of harmful impurities with low melting points such as s, P, Sb, Bi, Pb, Sn, as, etc., and the ingot is seriously segregated longitudinally along the rolling direction during rolling, which is easy to produce stress concentration and form longitudinal quenching cracks, or the longitudinal cracks formed by rapid cooling of raw materials after rolling are not processed and retained in the products, resulting in the expansion of the final quenching cracks and the formation of longitudinal cracks; (2) Longitudinal cracks are easy to form when the die size is within the quenching crack sensitive size range of steel (the dangerous size of quenching crack of carbon tool steel is mm, and the dangerous size of medium and low alloy steel is mm) or when the selected quenching cooling medium greatly exceeds the critical quenching cooling rate of the steel

preventive measures: (1) strictly inspect the warehousing of raw materials, and do not put into production steel products with excessive harmful impurities; (2) Try to use vacuum smelting, refining outside the furnace or electroslag remelting die steel; (3) Improve the heat treatment process, adopt vacuum heating, protective atmosphere heating, fully deoxidized salt bath furnace heating, step quenching and isothermal quenching; (4) Measures such as changing centerless quenching to centerless quenching, i.e. incomplete quenching, obtaining lower bainite structure with high strength and toughness, greatly reducing tensile stress, and effectively avoiding longitudinal cracking and quenching distortion of the die

2 transverse crack

the crack feature is perpendicular to the axial direction. There is a large peak value of tensile stress in the transition part between the hardened area and the unhardened area of the unhardened mold. When the large mold is rapidly cooled, it is easy to form a large peak value of tensile stress, because the formed axial stress is greater than the tangential stress, resulting in transverse cracks. Transverse segregation of harmful impurities with low melting points such as s, Bi, Pb, Sn, as in the forged module or transverse microcracks in the module, and transverse cracks are formed after quenching

preventive measures: (1) the module should be forged reasonably, and the ratio of raw material length to diameter, i.e. the forging ratio, is best selected between 23. The forging adopts double cross directional forging, and through five upsetting and five drawing multi fire forging, the carbides and impurities in the steel are fine and small, evenly distributed in the steel matrix, and the forged fiber structure has no directional distribution around the cavity, which greatly improves the transverse mechanical properties of the module and reduces and eliminates the stress source; (2) Choose the ideal cooling rate and cooling medium: fast cooling above the MS point of the steel, which is greater than the critical quenching cooling rate of the steel. The stress produced by supercooled austenite in the steel is thermal stress, the surface layer is compressive stress, and the inner layer is tensile stress, which counteracts each other, effectively preventing the formation of thermal stress cracks, slow cooling between MS and MF of the steel, and greatly reducing the structural stress when quenching martensite is formed. When the sum of thermal stress and corresponding stress in steel is positive (tensile stress), it is easy to crack, and when it is negative and has become a new type of fire hazard, it is not easy to crack. Making full use of thermal stress, reducing phase transformation stress and controlling the total stress to be negative can effectively avoid transverse quenching cracks. CL-1 organic quenching medium is an ideal quenching agent, which can reduce and avoid the distortion of quenching die, and control the reasonable distribution of hardening layer. By adjusting the ratio of CL-1 quenchant with different concentrations, different cooling rates can be obtained, and the required hardened layer distribution can be obtained to meet the needs of different die steels

3 arc cracks

often occur at the sharp changes in the shape of mold corners, notches, holes, female mold wiring flash and so on. This is because the stress generated at the edges and corners during quenching is 10 times the average stress of the smooth surface. In addition, (1) only when carbon (c) is contained in the steel, can better development be achieved. The higher the content of alloying elements and the lower the MS point of the steel, the lower the MS point is. If the MS point is reduced by 2 ℃, the quenching cracking tendency increases by 1.2 times, and if the MS point is reduced by 8 ℃, the quenching cracking tendency increases by 8 times; (2) The dissimilarity of different structure transformation and the same structure transformation in steel, due to different structure ratio tolerance, causes huge structure stress, and leads to the formation of arc cracks at the structure interface; (3) Tempering is not timely after quenching, or tempering is not sufficient, residual austenite in the steel is not fully transformed, and remains in the service state to promote stress redistribution, or martensitic transformation of residual austenite occurs during mold service to produce new internal stress, and arc cracks will be formed when the comprehensive stress is greater than the strength limit of the steel; (4) It has the second kind of tempered brittle steel. After quenching, it is tempered at high temperature and cooled slowly, resulting in the precipitation of harmful impurity compounds such as P and s along the grain boundary, which greatly reduces the binding force and strength toughness of the grain boundary, increases brittleness, and forms arc cracks under the action of external forces during service

preventive measures: (1) improve the design, try to make the shape symmetrical, reduce the sudden change of shape, increase the process holes and stiffeners, or adopt combined assembly; (2) Round corners replace right angles and sharp edges, and through holes replace blind holes, so as to improve machining accuracy and surface finish, and reduce the source of stress concentration. For places where right angles, sharp edges, blind holes, etc. cannot be avoided, the general hardness requirements are not high. Iron wire, asbestos rope, refractory mud, etc. can be used to wrap or plug, artificially create a cooling barrier, make it cool and quench slowly, avoid stress concentration, and prevent the formation of arc cracks during quenching; (3) Quenched steel should be tempered in time to eliminate some quenched internal stress and prevent the expansion of quenched stress; (4) Tempering for a long time can improve the fracture toughness of the die; (5) Fully tempered to obtain stable microstructure and properties; (6) Multiple tempering can fully transform retained austenite and eliminate new stress; (7) Reasonable tempering can improve the fatigue resistance and comprehensive mechanical properties of steel parts; (8) For the die steel with the second kind of temper brittleness, it should be cooled quickly (water cooling or oil cooling) after high temperature tempering, which can eliminate the second kind of temper brittleness and prevent and avoid the formation of arc cracks during quenching

4 peeling cracks

when the die is in service, the quenching hardening layer is peeled off from the steel matrix one by one under the action of stress. Due to the difference in specific volume between the surface layer structure and the core structure of the mold, the surface layer forms axial and tangential quenching stress during quenching, generates tensile stress in the radial direction, and mutates inward, and produces peel cracks in the narrow range of sharp changes in stress, which often occurs in the cooling process of the mold after surface chemical heat treatment. Due to the difference between the chemical modification of the surface layer and the transformation of the steel matrix, the quenching martensite expansion of the inner and outer layers is not carried out at the same time, resulting in large transformation stress, As a result, the chemical treatment layer is stripped from the matrix structure. Such as flame surface hardening layer, high-frequency surface hardening layer, carburizing layer, carbonitriding layer, nitriding layer, boriding layer, metalizing layer, etc. The chemical infiltration layer should not be tempered quickly after quenching, especially the low-temperature tempering and rapid heating below 300~c will promote the formation of tensile stress on the surface layer, while the compressive stress will be formed on the center of the steel matrix and the transition layer. When the tensile stress is greater than the compressive stress, the chemical infiltration layer will be pulled apart

preventive measures: (1) the concentration and hardness of chemical infiltration layer of die steel should be reduced gently from the surface to the inside, and the adhesion between infiltration layer and matrix should be enhanced. Diffusion treatment after infiltration can make the transition between chemical infiltration layer and matrix uniform; (2) Diffusion annealing, spheroidizing annealing and quenching and tempering treatment are carried out before chemical treatment of die steel to fully refine the original structure, which can effectively prevent and avoid peeling cracks and ensure product quality

5-shaped crack

the crack depth is shallow, generally about 0 5mm, radial, alias cracking. The main reasons are: (1) the raw materials have a deep decarburization layer, which is not removed by cold cutting, or the finished mold is heated in an oxidizing atmosphere furnace, resulting in oxidative decarburization; (2) The metal structure of the decarburized surface layer of the die is different from that of the martensite of the steel matrix in terms of carbon content and specific volume. When the decarburized surface layer of the steel is quenched, it produces large tensile stress. Therefore, the surface metal is often pulled and cracked along the grain boundary; (3) The raw material is coarse-grained steel, with coarse original structure and massive ferrite, which cannot be eliminated by conventional quenching and remains in the quenched structure, or the temperature control is incorrect, the instrument fails, the structure overheats or even burns, the grain coarsens, and the grain boundary adhesion is lost. When the die is quenched and cooled, the carbide of the steel precipitates along the austenite grain boundary, the grain boundary strength is greatly reduced, the toughness is poor, and the brittleness is large, Under the action of tensile stress, it cracks along the grain boundary in shape

preventive measures: (1) strictly control the chemical composition of raw materials Metallographic structure and flaw detection, unqualified raw materials and coarse-grained steel should not be used as mold materials; (2) Select fine grain steel and vacuum electric furnace steel, recheck the decarburization layer depth of raw materials before production, and the cold cutting allowance must be greater than the decarburization layer depth; (3) Formulate advanced and reasonable heat treatment process, select microcomputer temperature control instrument, and the control accuracy reaches 1.5 ℃, and calibrate the instrument on site regularly; (4) The final treatment of mold products adopts measures such as vacuum electric furnace, protective atmosphere furnace and fully deoxidized salt bath furnace to heat mold products, so as to effectively prevent and avoid the formation of current cracks

6 cold treatment cracks

die steels are mostly medium and high carbon alloy steels. After quenching, some supercooled austenite has not been transformed into martensite, which remains in the service state and becomes residual austenite, affecting the service performance. If it is cooled below zero, martensitic transformation of retained austenite can be promoted. Therefore, the essence of cold treatment is to continue quenching. The quenching stress at room temperature and the quenching stress at zero temperature are superimposed. When the superimposed stress exceeds the strength limit of the material, the cold treatment crack will be formed

preventive measures: (1) after quenching, boil the mold in boiling water for 30 60min before cooling treatment, which can eliminate 15%-25% of the quenching internal stress and stabilize the residual austenite. Then carry out -60 ℃ conventional cooling treatment or -120 ℃ deep cooling treatment. The lower the temperature is, the more the residual austenite is transformed into martensite, but it is impossible to complete the transformation. The experiment shows that about 2%-5% of the residual austenite is retained, Retaining a small amount of retained austenite as required can relax the stress and play a buffer role. Because the retained austenite is soft and tough, it can partially absorb the rapid expansion energy of martensitization and alleviate the transformation stress; (2) After the cold treatment, take out the mold and put it into hot water for heating, which can eliminate 40% - 60% of the cold treatment stress. After heating to room temperature, it should be tempered in time, and the cold treatment stress should be further eliminated to avoid the formation of cold treatment cracks, obtain stable microstructure and properties, and ensure that the mold products will not be distorted during storage and use

7 grinding cracks

often occur in the cold grinding process after quenching and tempering of die products. Most of the micro cracks formed are perpendicular to the grinding direction, with a depth of about 0.05 1.0mm. (1) Improper pretreatment of raw materials, failure to fully eliminate blocky, like, banded carbides and serious decarburization of raw materials; (2) The final quenching heating temperature is too high, overheating occurs, the grain is coarse, and more residual austenite is formed; (3) During grinding, stress-induced phase transformation occurs, so that residual austenite is transformed into martensite, and the structural stress is large. In addition, due to insufficient tempering, more residual tensile stress is left, which is superimposed with the grinding structural stress, or due to the grinding speed, large feed amount and improper cooling, the grinding heat of the metal surface is sharply raised to the quenching heating temperature, followed by the cooling of the grinding fluid, resulting in the secondary quenching of the grinding surface, and the synthesis of various stresses, If the strength limit of the material is exceeded, the surface metal grinding crack will be caused

preventive measures: (1) change the forging of raw materials and double cross several times