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Analysis and measures of ten kinds of crack in die steel quenching
Date：2017-04-05 Hits：
1, longitudinal crack
The crack is axial and the shape is thin and long. When the mold is not completely quenched when quenching, the heart into the largest volume of quenched martensite, produce tensile stress of die steel carbon content is higher, the tangential tensile stress is larger, when the tensile stress is larger than the steel strength limit cause of longitudinal crack formation. The following factors exacerbated by longitudinal cracks: (1) S, containing more P, Bi, Pb, * * *, Sn, As and other low melting point impurities in the steel ingot rolling along the rolling direction is the longitudinal distribution of serious segregation, easy to produce stress concentration to form a vertical quenching crack, longitudinal crack or raw material rolling for fast cooling after the formation of unprocessed products lead to the final drop retained in the quenching crack expand to form a longitudinal crack; (2) the mold size in the steel quenching crack sensitive size range (carbon tool steel quenching crack risk size 8-15mm, low alloy steel dangerous size 25-40mm) or choice of quenching medium greatly exceeds the critical the quenching cooling rate of the steel are easy to form longitudinal crack.
Preventive measures: (1) strictly check the storage of raw materials, production of harmful impurity content does not exceed the standard of steel; (2) try to use the vacuum smelting, refining or electroslag remelting steel mold; (3) improvement of heat treatment process, using vacuum heating, heating and sufficient oxygen and protective atmosphere heating and quenching bath furnace isothermal quenching; (4) a variable that is not entirely determined for quenching quenching hardenability, high strength and toughness to obtain lower bainite and other measures to reduce the tensile stress, can effectively avoid mold longitudinal cracking and quenching distortion.
2, transverse crack
The crack feature is perpendicular to the axial direction. No quenching mold, in the presence of hardened zone and Non Hardened Zone transition part of large tensile stress, easy to form large tensile stress large mold rapid cooling, due to the formation of the axial stress is larger than the tangential stress, resulting in cracks. The S module, P.***, Bi, Pb, Sn, As, such as the low melting point of harmful impurities in the lateral segregation or the module has a lateral micro crack, after quenching the formation of transverse cracks.
Preventive measures: (1) the module should be reasonable forging, raw material length and diameter ratio of forging ratio preferably in between 2 - 3, with double cross to forging forging, upsetting after five five more than pulling fire forging, the carbide and impurity in steel is thin and small, evenly distributed in the steel matrix, forging fibrous tissue around the cavity without directional distribution, greatly improve the mechanical properties of the transverse module, to reduce and eliminate the stress source; (2) the cooling rate and the cooling medium to select the ideal: in the steel above Ms, fast cooling, is greater than the critical cooling rate of steel, steel stress produced by thermal cooling austenite the surface stress, compressive stress, inner tension, offset each other, effectively prevent the thermal stress crack formed between steel Ms - Mf slow cooling, greatly reduce the formation of martensite during tissue stress. When the sum of the thermal stress and the corresponding stress in steel is positive (Zhang Yingli), it is easy to crack. Make full use of thermal stress, reduce the phase transformation stress, control the total stress is negative, can effectively avoid the occurrence of transverse quenching crack. CL-1 organic quenching medium is an ideal quenching agent, at the same time, it can reduce and avoid the distortion of quenching mould, and can also control the reasonable distribution of hardened layer. Adjusting the ratio of different concentration of CL-1 quenching agent, different cooling rate can be obtained, and the hardened layer distribution can be obtained to meet the requirements of different die steels.
3, arc crack
Mutations in the die edge angle, gap, die cavity, flash and other shapes often occur in connection. This is because the stress generated at the edges of the quenching is 10 times the average stress of the smooth surface. In addition, (1) carbon steel (C) and the content of alloy elements is higher, the lower point of steel Ms, 2 DEG Ms point is reduced, quenching crack tends to increase 1.2 times, 8 DEG Ms low, quenching crack tend to increase 8 times; (2) the transformation of different tissues in steel and the same organization is not at the same time, due to different organization than tolerance, causing huge stress, resulting in the formation of arc crack at the junction of the organization; (3) after quenching or tempering tempering is not timely, not fully, steel retained austenite is not fully change, keep in service, promote the redistribution of stress, or die service when the residual austenite martensite transformation stress when new, comprehensive stress is greater than the ultimate strength of steel as a result of arc crack; (4) with second types of temper brittleness of steel after quenching, high temperature tempering, resulting in steel P, s and other harmful impurity compounds precipitate along the grain boundary, Greatly reduce the interface bonding strength and toughness increased brittleness, while serving under the action of external force arc crack formation.
Preventive measures: (1) to improve the design, try to make the symmetrical shape, reduce the mutation, increase the process holes and reinforcing ribs, or by the assembly; (2) generation of sharp edges and sharp corners rounded rectangular through holes, blind hole generation, improve the machining accuracy and surface finish, reduce the stress concentration source, who can not avoid the right angle sharp, sharp edges, hole general hardness requirements is not high, use wire, asbestos rope, refractory mud bandage or packing, man-made barrier to slow cooling, cooling and quenching, avoid stress concentration, to prevent quenching arc crack formation; (3) shall promptly eliminate part of tempering hardened steel, quenching the internal stress and prevent quenching stress expansion; (4) a long time tempering, improve the mold resistance value of fracture toughness; (5) fully tempering, stable microstructure and properties; repeated tempering of the remaining austenite and fully eliminate A new stress; (7) reasonable tempering, improve fatigue resistance of steel parts and comprehensive mechanical properties; for there are second types of die steel after high temperature tempering tempering brittleness should be fast cooling (water cooling or oil cooling), can eliminate two kinds of temper brittleness, prevent and avoid arc quenching crack formation.
4, peel crack
Under the stress of the mould, the hardened hardened layer is stripped from the steel matrix. Because of the die surface and core structure of different volume, quenching is formed on the surface of axial and tangential quenching stress, radial tensile stress, and to internal mutations, stress in the narrow crack generated at the peeling dramatically, often occurs in the processing of surface chemical heat mold cooling process, because the surface chemistry change of phase transition and steel matrix and inner layer caused by quenching expansion not at the same time, have a big transformation stress, leading to chemical treatment of infiltration in the matrix layer from peeling. For example, the surface hardening layer, the high frequency surface hardening layer, the carburized layer, the carbonitriding layer, the nitrided layer, the boride layer and the metal layer. The chemical layer after quenching tempering is not fast, especially below 300~C low temperature tempering rapid heating, will cause the surface tensile stress, and the steel core and transition layer formed compressive stress, when the stress is greater than compression stress, resulting in chemical layer is tensile peeling
Preventive measures: (1) the die steel chemical layer from the surface to the inner concentration and hardness decrease, enhanced permeability layer and substrate binding capacity, permeability after treatment can make the chemical diffusion between substrate and layer transition uniform; (2) annealing, spheroidization annealing, quenching and tempering before chemical processing mould steel, fully refine the original structure, can effectively prevent and avoid stripping crack, to ensure product quality.
5, mesh crack
Crack depth is shallow, generally about 0.01-1.5mm deep, radiation, alias cracking. The main reasons are: (1) raw materials have deep decarburization layer, cold cutting is not removed, or finished mold heating in an oxidizing atmosphere in the furnace caused by oxidation and decarburization; (2) die decarburized layer of metal structure and martensitic steel matrix of different carbon content, volume, surface decarburization quenching generated when large the tensile stress, therefore, the surface metal often along the grain boundaries are ripped into mesh; (3) raw material is coarse grain steel, the original coarse microstructure, existing blocky ferrite, conventional quenching can not be eliminated, retained in the quenched temperature control, or allowed, instrument failure, occurrence of overheating, even burning, grain coarse, lose the strength of grain boundary and die quenching of steel carbides precipitated along the austenite grain boundary and grain boundary strength greatly reduced, poor toughness, brittleness, under the tensile stresses in a network along the grain boundary cracking.
Preventive measures: (1) the strict chemical composition of raw material. The microstructure and inspection, unqualified raw materials and coarse grain steel for die material; (2) selection of fine grain steel, vacuum furnace steel, pre production of raw materials were decarburized depth, cold machining allowance must be larger than the depth of decarburization (; 3) for advanced and reasonable heat treatment process, the microcomputer temperature control instrument, the control accuracy of plus or minus 1.5 DEG C, the timing of live instrumentation calibration; (4) the final processing mold products using vacuum furnace, furnace atmosphere and the deoxidation salt bath furnace heating mold products and other measures to effectively prevent and avoid reticular crack formation.
The crack is axial and the shape is thin and long. When the mold is not completely quenched when quenching, the heart into the largest volume of quenched martensite, produce tensile stress of die steel carbon content is higher, the tangential tensile stress is larger, when the tensile stress is larger than the steel strength limit cause of longitudinal crack formation. The following factors exacerbated by longitudinal cracks: (1) S, containing more P, Bi, Pb, * * *, Sn, As and other low melting point impurities in the steel ingot rolling along the rolling direction is the longitudinal distribution of serious segregation, easy to produce stress concentration to form a vertical quenching crack, longitudinal crack or raw material rolling for fast cooling after the formation of unprocessed products lead to the final drop retained in the quenching crack expand to form a longitudinal crack; (2) the mold size in the steel quenching crack sensitive size range (carbon tool steel quenching crack risk size 8-15mm, low alloy steel dangerous size 25-40mm) or choice of quenching medium greatly exceeds the critical the quenching cooling rate of the steel are easy to form longitudinal crack.
Preventive measures: (1) strictly check the storage of raw materials, production of harmful impurity content does not exceed the standard of steel; (2) try to use the vacuum smelting, refining or electroslag remelting steel mold; (3) improvement of heat treatment process, using vacuum heating, heating and sufficient oxygen and protective atmosphere heating and quenching bath furnace isothermal quenching; (4) a variable that is not entirely determined for quenching quenching hardenability, high strength and toughness to obtain lower bainite and other measures to reduce the tensile stress, can effectively avoid mold longitudinal cracking and quenching distortion.
2, transverse crack
The crack feature is perpendicular to the axial direction. No quenching mold, in the presence of hardened zone and Non Hardened Zone transition part of large tensile stress, easy to form large tensile stress large mold rapid cooling, due to the formation of the axial stress is larger than the tangential stress, resulting in cracks. The S module, P.***, Bi, Pb, Sn, As, such as the low melting point of harmful impurities in the lateral segregation or the module has a lateral micro crack, after quenching the formation of transverse cracks.
Preventive measures: (1) the module should be reasonable forging, raw material length and diameter ratio of forging ratio preferably in between 2 - 3, with double cross to forging forging, upsetting after five five more than pulling fire forging, the carbide and impurity in steel is thin and small, evenly distributed in the steel matrix, forging fibrous tissue around the cavity without directional distribution, greatly improve the mechanical properties of the transverse module, to reduce and eliminate the stress source; (2) the cooling rate and the cooling medium to select the ideal: in the steel above Ms, fast cooling, is greater than the critical cooling rate of steel, steel stress produced by thermal cooling austenite the surface stress, compressive stress, inner tension, offset each other, effectively prevent the thermal stress crack formed between steel Ms - Mf slow cooling, greatly reduce the formation of martensite during tissue stress. When the sum of the thermal stress and the corresponding stress in steel is positive (Zhang Yingli), it is easy to crack. Make full use of thermal stress, reduce the phase transformation stress, control the total stress is negative, can effectively avoid the occurrence of transverse quenching crack. CL-1 organic quenching medium is an ideal quenching agent, at the same time, it can reduce and avoid the distortion of quenching mould, and can also control the reasonable distribution of hardened layer. Adjusting the ratio of different concentration of CL-1 quenching agent, different cooling rate can be obtained, and the hardened layer distribution can be obtained to meet the requirements of different die steels.
3, arc crack
Mutations in the die edge angle, gap, die cavity, flash and other shapes often occur in connection. This is because the stress generated at the edges of the quenching is 10 times the average stress of the smooth surface. In addition, (1) carbon steel (C) and the content of alloy elements is higher, the lower point of steel Ms, 2 DEG Ms point is reduced, quenching crack tends to increase 1.2 times, 8 DEG Ms low, quenching crack tend to increase 8 times; (2) the transformation of different tissues in steel and the same organization is not at the same time, due to different organization than tolerance, causing huge stress, resulting in the formation of arc crack at the junction of the organization; (3) after quenching or tempering tempering is not timely, not fully, steel retained austenite is not fully change, keep in service, promote the redistribution of stress, or die service when the residual austenite martensite transformation stress when new, comprehensive stress is greater than the ultimate strength of steel as a result of arc crack; (4) with second types of temper brittleness of steel after quenching, high temperature tempering, resulting in steel P, s and other harmful impurity compounds precipitate along the grain boundary, Greatly reduce the interface bonding strength and toughness increased brittleness, while serving under the action of external force arc crack formation.
Preventive measures: (1) to improve the design, try to make the symmetrical shape, reduce the mutation, increase the process holes and reinforcing ribs, or by the assembly; (2) generation of sharp edges and sharp corners rounded rectangular through holes, blind hole generation, improve the machining accuracy and surface finish, reduce the stress concentration source, who can not avoid the right angle sharp, sharp edges, hole general hardness requirements is not high, use wire, asbestos rope, refractory mud bandage or packing, man-made barrier to slow cooling, cooling and quenching, avoid stress concentration, to prevent quenching arc crack formation; (3) shall promptly eliminate part of tempering hardened steel, quenching the internal stress and prevent quenching stress expansion; (4) a long time tempering, improve the mold resistance value of fracture toughness; (5) fully tempering, stable microstructure and properties; repeated tempering of the remaining austenite and fully eliminate A new stress; (7) reasonable tempering, improve fatigue resistance of steel parts and comprehensive mechanical properties; for there are second types of die steel after high temperature tempering tempering brittleness should be fast cooling (water cooling or oil cooling), can eliminate two kinds of temper brittleness, prevent and avoid arc quenching crack formation.
4, peel crack
Under the stress of the mould, the hardened hardened layer is stripped from the steel matrix. Because of the die surface and core structure of different volume, quenching is formed on the surface of axial and tangential quenching stress, radial tensile stress, and to internal mutations, stress in the narrow crack generated at the peeling dramatically, often occurs in the processing of surface chemical heat mold cooling process, because the surface chemistry change of phase transition and steel matrix and inner layer caused by quenching expansion not at the same time, have a big transformation stress, leading to chemical treatment of infiltration in the matrix layer from peeling. For example, the surface hardening layer, the high frequency surface hardening layer, the carburized layer, the carbonitriding layer, the nitrided layer, the boride layer and the metal layer. The chemical layer after quenching tempering is not fast, especially below 300~C low temperature tempering rapid heating, will cause the surface tensile stress, and the steel core and transition layer formed compressive stress, when the stress is greater than compression stress, resulting in chemical layer is tensile peeling
Preventive measures: (1) the die steel chemical layer from the surface to the inner concentration and hardness decrease, enhanced permeability layer and substrate binding capacity, permeability after treatment can make the chemical diffusion between substrate and layer transition uniform; (2) annealing, spheroidization annealing, quenching and tempering before chemical processing mould steel, fully refine the original structure, can effectively prevent and avoid stripping crack, to ensure product quality.
5, mesh crack
Crack depth is shallow, generally about 0.01-1.5mm deep, radiation, alias cracking. The main reasons are: (1) raw materials have deep decarburization layer, cold cutting is not removed, or finished mold heating in an oxidizing atmosphere in the furnace caused by oxidation and decarburization; (2) die decarburized layer of metal structure and martensitic steel matrix of different carbon content, volume, surface decarburization quenching generated when large the tensile stress, therefore, the surface metal often along the grain boundaries are ripped into mesh; (3) raw material is coarse grain steel, the original coarse microstructure, existing blocky ferrite, conventional quenching can not be eliminated, retained in the quenched temperature control, or allowed, instrument failure, occurrence of overheating, even burning, grain coarse, lose the strength of grain boundary and die quenching of steel carbides precipitated along the austenite grain boundary and grain boundary strength greatly reduced, poor toughness, brittleness, under the tensile stresses in a network along the grain boundary cracking.
Preventive measures: (1) the strict chemical composition of raw material. The microstructure and inspection, unqualified raw materials and coarse grain steel for die material; (2) selection of fine grain steel, vacuum furnace steel, pre production of raw materials were decarburized depth, cold machining allowance must be larger than the depth of decarburization (; 3) for advanced and reasonable heat treatment process, the microcomputer temperature control instrument, the control accuracy of plus or minus 1.5 DEG C, the timing of live instrumentation calibration; (4) the final processing mold products using vacuum furnace, furnace atmosphere and the deoxidation salt bath furnace heating mold products and other measures to effectively prevent and avoid reticular crack formation.
