Analysis of 10 Types of Cracks in Die Steel Quenching and Measures:
1, longitudinal crack
Cracks are axial, thin and long. When the mold is completely hardened, that is, without quenching, the heart is transformed into the hardened martensite with the largest specific volume, resulting in tangential tensile stress. The higher the carbon content of the mold steel, the greater the tangential tensile stress, and the tensile stress. Above the strength limit of the steel causes longitudinal crack formation. The following factors exacerbate the generation of longitudinal cracks: (1) The steel contains more harmful impurities such as S, P, ***, Bi, Pb, Sn, As and other low melting points, and the steel ingot is longitudinally severed along the rolling direction. Segregation distribution, easy to produce stress concentration to form longitudinal quenching cracks, or longitudinal cracks formed by rapid cooling after rolling of raw materials are not processed to remain in the product leading to the final quenching cracks to expand and form longitudinal cracks; (2) mold size is sensitive to quenched and cracked steel The size cracks (carbon steel tool steel quenching danger size 8-15mm, medium and low alloy steel dangerous size 25-40mm) or the selected quenching cooling medium greatly exceeds the critical quenching cooling rate of the steel are easy to form longitudinal cracks.
Preventive measures: (1) Strictly inspect the raw materials for storage, and not put the steel with excessive content of harmful impurities into production; (2) Use vacuum smelting as much as possible, external refining or electroslag remelting mold steel; (3) Improve heat treatment process and adopt vacuum heating , protection atmosphere heating and full de-oxygenation bath heating and graded quenching, isothermal quenching; (4) change the centerless quenching to quench the heart that is not completely quenched, get high strength and toughness of the lower bainite and other measures to significantly reduce the pull Stress can effectively avoid longitudinal cracking and quench distortion of the mold.
2, transverse crack
The crack feature is perpendicular to the axial direction. Unhardened dies have large tensile stress peaks at the transitions between hardened and unhardened regions. Large rapid tensile stress peaks occur when large molds are cooled quickly. The resulting axial stress is greater than the tangential stress, resulting in lateral forces. crack. Transverse segregation of low-melting-point harmful impurities such as S, P.***, Bi, Pb, Sn, and As in the forging module or transverse microcracks in the module exists, and the transverse cracks are formed after quenching.
Preventive measures: (1) The module should be reasonably forged. The ratio of the length of the raw material to the diameter, ie, the forging ratio, is preferably between 2-3. Forging uses a double cross-shaped change to forge, and after forging by five to five draws, the steel is forged. The carbides and impurities are fine and small, uniformly distributed in the steel matrix, the forged fibrous tissue is distributed non-directionally around the cavity, and the transverse mechanical properties of the module are greatly increased, the stress source is reduced and eliminated; (2) The ideal cooling rate and cooling medium are selected : Fast cooling above the Ms point of the steel, greater than the critical quenching cooling rate of the steel, the stress generated by the undercooled austenite in the steel is thermal stress, the surface is compressive stress, the inner layer is tensile stress, cancel each other, effectively prevent thermal stress The formation of cracks slows down between Ms and Mf of the steel, significantly reducing the structural stress when quenched martensite is formed. When the sum of the thermal stress and the corresponding stress in the steel is positive (tensile stress), it is easily quenched and cracked, and when it is negative, it is not easy to quench crack. Make full use of thermal stress, reduce the phase change stress, and control the total sum of stress to be negative, which can effectively avoid transverse quench cracking. The CL-1 organic quenching medium is an ideal quenching agent. At the same time, it can reduce and avoid quenching mold distortion, and can also control the reasonable distribution of the hardened layer. Adjusting the concentration of CL-1 quenching agent in different concentrations, different cooling rates can be obtained to obtain the required hardened layer distribution to meet the needs of different mold steel.
3, arc crack
Occurs in the mold corners, notches, holes, die wiring flash edge shape mutations. This is because the stress generated at the corners during quenching is 10 times the average stress of the smooth surface. In addition, (1) the higher the content of carbon (C) and alloying elements in the steel, the lower the Ms point of the steel, and the lowering of the Ms point by 2°C, the quenching cracking tendency is increased by 1.2 times, the Ms point is lowered by 8°C, and the quenching cracking tendency is Increase by 8 times; (2) different tissue transformations and the same organizational transformations in steel vary in time, due to different tissue specific tolerances, resulting in great tissue stresses, leading to arc cracks at the interface of the tissue; (3) no tempering after quenching, Insufficient tempering or tempering, residual austenite in the steel is not fully transformed, remains in the service state, promotes stress redistribution, or generates new internal stress due to martensitic transformation of retained austenite when the mold is in service, when the comprehensive stress Arc cracks are formed when the strength exceeds the limit of the steel. (4) The second type of temper brittleness steel, quenched and tempered at low temperature, causes harmful compounds such as P and s in the steel to precipitate along the grain boundary, which greatly reduces the grain boundary. The combination of strength and toughness, increased brittleness, the formation of arc cracks under the action of external forces during service.
Preventive measures: (1) Improve design, try to make shape symmetry, reduce shape changes, increase process holes and stiffeners, or use combined assembly; (2) Fillet generation right-angled and sharp-edged sharp edges, through-holes blind holes, improve Processing accuracy and surface finish, reducing the source of stress concentration, can not avoid the normal hardness requirements such as right angle, sharp edges, blind holes, etc., can be used to wrap or stuff iron wire, asbestos rope, refractory mud, etc., causing artificial cooling barrier, Allow it to cool and quench slowly to avoid stress concentration and prevent formation of arc cracks during quenching; (3) Quenched steel should be tempered in time to eliminate some quenching internal stress and prevent quenching stress from expanding; (4) Longer time tempering, improved mold resistance Fracture toughness value; (5) Fully tempered to obtain stable microstructure and properties; Multiple tempering to fully transform and eliminate new austenite; (7) Reasonable tempering, improving fatigue resistance and comprehensive mechanical properties of steel; For the second type of temper brittle die steel after rapid tempering (water-cooled or oil-cooled) at high temperature, the temper brittleness of the second type can be eliminated to prevent and avoid arc crack formation during quenching.
4, peel crack
When the mold is in service, the hardened layer is stripped from the steel substrate under stress. Due to the difference in specific volume between the surface and the core of the mold, the surface layer forms an axial and tangential quenching stress during quenching, generates a tensile stress in the radial direction, and mutates to the inside. Peeling cracks occur at a narrow range of rapidly changing stress and often occur at the surface. During the cooling process of the chemical heat treatment of the surface layer, due to different chemical modification of the surface layer and different phase transformation of the steel substrate, the quenched martensite of the inner layer and the outer layer are not simultaneously performed, and a large phase change stress is generated, resulting in the chemical treatment of the penetration layer from the matrix structure. Peel off. Such as flame surface hardened layer, high frequency surface hardened layer, carburized layer, carbonitriding layer, nitriding layer, boronized layer, infiltration metal layer. After chemical quenching, quenching should not be followed by rapid tempering. In particular, rapid heating at below 300°C and low temperature tempering will promote the formation of tensile stress in the surface layer. The compressive stress is formed in the core and transition layer of the steel matrix. When the tensile stress is greater than the compressive stress, The chemically penetrated layer was pulled apart and peeled.
Preventive measures: (1) The concentration and hardness of the chemically penetrated layer of the die steel should be reduced from the surface to the inside, and the bonding force between the permeation layer and the substrate should be enhanced. After the infiltration, the diffusion treatment can make the chemical penetration layer and the matrix uniform; (2) Moulds Before the chemical treatment of steel, diffusion annealing, spheroidizing annealing, and tempering treatment are carried out to fully refine the original structure, which can effectively prevent and avoid peeling cracks and ensure product quality.
5, mesh crack
Crack depth is shallow, generally about 0.01-1.5mm deep, radial, alias crack. The main reasons are: (1) the raw material has a deep decarburized layer, which is not removed by cold cutting, or the finished mold is heated in an oxidizing atmosphere furnace to cause oxidative decarburization; (2) the mold decarburized surface metal structure and steel matrix martensite Carbon content is different, different specific volume, steel decarburized surface quenching produces large tensile stress, therefore, the surface metal is often cracked along the grain boundary into a network; (3) raw material is coarse grain steel, the original coarse structure, There is massive ferrite, conventional quenching can not be eliminated, remains in the quenched structure, or temperature control is inaccurate, the instrument fails, overheating of the tissue, even overheating, grain coarsening, loss of grain boundary binding force, mold quench cooling When the carbides of the steel are precipitated along the austenite grain boundaries, the strength of the grain boundaries is greatly reduced, the toughness is poor, and the brittleness is large. Under the action of the tensile stress, the carbides break apart along the grain boundaries.
Preventive measures: (1) Strict raw material chemical composition, metallographic organization and inspection of flaws, unqualified raw materials and coarse-grained steel are not suitable for mold materials; (2) selection of fine-grained steel, vacuum furnace steel, review of decarbonization of raw materials before production Layer depth, cold cutting allowance must be greater than decarburized layer depth; (3) formulate advanced and reasonable heat treatment process, select microcomputer temperature control instrument, control accuracy of ± 1.5 °C, regular on-site calibration instrument; (4) mold product final treatment The use of vacuum electric furnaces, protective atmosphere furnaces and heating of mold products through a fully deoxidized salt bath furnace, etc., effectively prevent and avoid the formation of network cracks.
6, cold crack
Most of the die steels are medium and high carbon alloy steels. After quenching, part of the undercooled austenite has not been transformed into martensite, and remains in the used state as residual austenite, which affects the service performance. If the cooling is continued below zero, the retained austenite can promote martensitic transformation. Therefore, the essence of the cold treatment is that the quenching continues. The quenching stress at room temperature is superimposed with the quenching stress at zero, and a cold crack is formed when the stress at the overlap exceeds the strength limit of the material.
Preventive measures: (1) Prior to cold treatment after quenching, the mold is placed in boiling water for 30-60 minutes to eliminate 15%-25% quenching internal stress and stabilize the retained austenite, and then perform a normal cold treatment at -60°C, or carry out -120 °C cryogenic treatment, the lower the temperature, the more the amount of retained austenite into martensite, but it is not possible to complete the transformation, the experiment shows that about 2% -5% retained austenite remains, as needed A small amount of retained austenite can relax the stress and act as a buffer. Because the retained austenite is soft and tough, it can partially absorb the rapid expansion energy of martensite and ease the stress of phase transformation; (2) Take out the heat of the mold after the cold treatment is finished. The temperature rise in the water can eliminate 40%-60% cold treatment stress, and it should be tempered immediately after heating up to room temperature, and the cold treatment stress can be further eliminated to avoid the formation of cold treatment cracks, to obtain stable organization and performance, and to ensure that no distortion occurs during the storage and use of the mold products.
7, grinding crack
Often occurs in the mold quenching, tempering after cold grinding process, the majority of the formation of micro-cracks perpendicular to the grinding direction, depth of about 0.05-1.0mm. (1) Raw materials are improperly pre-treated, which can not fully eliminate the blocky, mesh, banded carbides and severe decarburization of raw materials; (2) The final quenching heating temperature is too high, overheating occurs, crystal grains are coarse, and more residuals are generated. Austenite; (3) stress-induced phase transformation during grinding, transforming retained austenite into martensite, high tissue stress, combined with insufficient tempering, leaving more residual tensile stress, and grinding Superimposed tissue stress superposition, or due to the grinding speed, large amount of feed and improper cooling, resulting in a sharp increase in the metal surface grinding heat to the quenching heating temperature, followed by grinding liquid cooling, resulting in secondary quenching of the surface layer, a variety of stress In general, beyond the material strength limit, surface metal grinding cracks are caused.
Preventive measures: (1) Change the raw materials to forge, and double cross-shaped deformation to forging and forging; after forging, the forged fibrous structures are symmetrically distributed around the cavity or axis, and the last fire heat is used. Quenching, followed by high temperature tempering, can fully eliminate massive, mesh, ribbon and chain carbides, and refine carbides to 2-3 grades; (2) Formulate advanced heat treatment process to control the final quenching Cristobalite content does not exceed the standard; (3) timely quenching after quenching, eliminate the quenching stress; (4) properly reduce the grinding speed, grinding capacity, grinding cooling speed, can effectively prevent and avoid the formation of grinding cracks.
8, wire cutting crack
The crack appeared in the on-line cutting process of the quenched and tempered module. This process changed the stress field distribution state of the metal surface layer, the middle layer and the core, and the quenching residual internal stress lost the equilibrium deformation, resulting in a large tensile stress in a certain area. The tensile stress caused the cracking of the mold material at the limit of strength, and the crack was a cracked arc-shaped rigid metamorphic layer. Experiments show that the wire cutting process is a local high-temperature discharge and rapid cooling process, so that the metal surface dendritic as-cast microstructure solidified layer, resulting in 600-900MPa tensile stress and thickness of about 0.03mm high stress secondary quenching white bright layer. Causes of cracks: (1) raw materials have severe carbide segregation; (2) failure of the instrument, quenching heating temperature is too high, coarse grains, reduce the material toughness, increase brittleness; (3) quenched parts did not timely tempering and back Insufficient fire, excessive residual internal stress, and superposition of new internal stresses formed during wire cutting results in wire-cut cracks.
Preventive measures: (1) Strictly inspect the raw materials before they are put into storage, ensure that the raw materials are organized and qualified, and change the unqualified raw materials to forge and crush the carbides so that the chemical composition and metallographic structure can reach the technical conditions before they can be put into production. Before the module heat treatment, the finished products shall be quenched, tempered and wire cut after a certain amount of grinding; (2) The instrument is calibrated before entering the furnace, and the microcomputer is used to control the temperature, the temperature control accuracy is ±1.5°C, and the vacuum furnace and the protective atmosphere furnace are heated. Prevention of overheating and oxidative decarburization; (3) Adopting graded quenching, isothermal quenching and quenching followed by timely tempering, multiple tempering, fully eliminating internal stress, creating conditions for wire cutting; (4) Formulating scientific and rational wire cutting process.
9, fatigue fracture
The microscopic fatigue cracks formed by the repeated action of the alternating stress during the service of the mold slowly expand, resulting in a sudden fatigue fracture. (1) raw materials have hairline, self-point, porosity, loose, non-metallic inclusions, serious segregation of carbides, banded structure, massive free ferritic metallurgical defects, destroy the continuity of the matrix structure, resulting in uneven stress concentration . 112 in the ingot did not exclude, resulting in the formation of white spots during rolling. The existence of ***, Bi, Pb, Sn, As and S, P and other harmful impurities in the steel, P in the steel easily lead to cold and brittle, and s easily lead to hot brittle, S, P harmful impurities are easy to form a source of fatigue; (2) Excessive thickness of the chemically penetrated layer, excessive concentration, excessive penetration layer, excessively hardened layer, and low hardness in the transition zone all lead to a drastic decrease in the fatigue strength of the material; (3) When the die surface is rough, the precision is low, and the smoothness is poor , And knife marks, lettering, scratches, bumps, corrosion pockmarks, etc. can also easily lead to stress concentration fatigue fracture.
Preventive measures: (1) Strict material selection, material quality, control of low-melting-point impurities such as Pb, As, Sn, and non-metallic impurities of S, P, etc.; (2) Material inspection prior to production, and unqualified raw materials not put into production; (3 ) Electroslag remelted refined steel with high purity, less impurities, uniform chemical composition, fine crystal grains, small carbides, good isotropic performance, and high fatigue strength, shot peening and surface hardening on the mold surface Chemical modification of the osmotic layer is modified so that the surface of the metal is pre-stressed to counteract the tensile stress generated during the service of the mold and improve the fatigue strength of the mold surface; (4) to improve the precision and finish of the mold surface; (5) to improve the chemical seepage The microstructure of the layer and hardened layer; the use of computer-controlled chemical layer thickness, concentration and hardened layer thickness.
10, stress corrosion crack
This crack often occurs during use. Metal mold due to chemical reaction or electrochemical reaction process, causing damage from the table to the internal structure of the corrosion damage and cracking, which is the stress corrosion cracking. Due to the different microstructures of the mold steel after heat treatment, the corrosion resistance is also different. The most corrosion resistant structure is austenite (A), the most easily eroded structure is troostite (T), followed by ferrite (F)-martensite (M)-pearlite (P)-sornite ( S). Therefore, the mold steel heat treatment should not be T-tissue. Although the quenched steel is tempered, due to insufficient tempering, the quenching internal stress still exists more or less. When the mold is in service, it will also generate new stress under the action of external force. When stress exists in the metal mold, there will be stress. Corrosion cracks occur.
Preventive measures: (1) After quenching, mold steel should be tempered in time, fully tempered, and tempered several times to eliminate quenching internal stress; (2) Die steel should generally not be tempered at 350-400°C after quenching, because T steel Frequently at this temperature, T-tissue molds should be re-treated. The molds should be rust-proofed to improve the corrosion resistance. (3) The hot-work molds should be preheated at low temperatures before they are put into service. Tempering and eliminating stress can not only prevent and avoid the occurrence of stress corrosion cracking, but also can greatly increase the service life of the die, serve two purposes, and have significant technical and economic benefits.