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Xinghua Tongbu Casting Co., Ltd.

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About various quenching cracks of heat-resistant steel after heat treatment

2019-11-16
second

Common heat treatment quenching cracks of heat-resistant steel are as follows:

1. Longitudinal cracks

The crack is axial thin long. When the mold is completely hardened, that is, coreless quenching, the core is transformed into a quenched martensite with a large specific volume, which produces tangential tensile stress. The higher the carbon content of the mold steel, the greater the tangential tensile stress. Above the strength limit of this steel, longitudinal cracks are formed. The following factors have exacerbated the occurrence of longitudinal cracks:

(1) Steel contains more harmful impurities with low melting point such as S, P, Sb, Bi, Pb, Sn, As, etc. When the steel ingot is rolled, it is severely segregated in the longitudinal direction along the rolling direction, it is easy to produce stress concentration to form longitudinal quenching cracks Longitudinal cracks formed by rapid cooling after rolling of raw materials are processed remain in the product, causing quenching cracks to expand to form longitudinal cracks;

(2) The size of the mold is within the sensitive size range of steel quenching cracking (carbon tool steel quenching dangerous size is 8-15mm, medium low alloy steel dangerous size 25-40mm) the selected quenching cooling medium greatly exceeds the critical quenching of the steel It is easy to form longitudinal cracks at the cooling rate.

Large tooling design

The crack is axial thin long. When the die is completely hardened, that is, coreless quenching, the core is transformed into specific volume quenched martensite, which produces tangential tensile stress. At the extreme strength of steel, longitudinal cracks are formed. The following factors have exacerbated the occurrence of longitudinal cracks:

Precaution:

(1) Strictly check the raw materials in storage, do put into production steel products with harmful impurities exceeding the standard;

(2) Try to use vacuum smelting, refining outside the furnace electroslag remelting mold steel;

(3) Improve the heat treatment process, use vacuum processing heat, protective atmosphere heating full deoxygenation salt bath furnace heating analysis quenching, isothermal quenching;

(4) Turning centerless quenching into centering quenching means incomplete hardening, obtaining lower bainite structure with high toughness other measures, greatly reducing tensile stress, effectively avoiding longitudinal cracking quenching distortion of the mold.

2. Transverse cracks

The crack characteristics are perpendicular to the axial direction. Unhardened molds have large tensile stress peaks in the transition between the hardened zone the unhardened zone. Large molds tend to form large tensile stress peaks during rapid cooling. The resulting axial stress is greater than the tangential stress, resulting in lateral crack. The lateral segregation of low-melting point harmful impurities such as S, P, Sb, Bi, Pb, Sn, As in the forging module the lateral micro-cracks exist in the module. After quenching, they expand to form lateral cracks.

Precaution:

(1) The module should be forged reasonably, the ratio of the length of the raw material to the diameter, that is, the forging ratio, is selected between 2-3, the double cross-shaped forging between the forgings is forged by five headings five draws multiple fires, so that the carbides impurities in the steel It is thin, small evenly distributed on the steel substrate, the forged fiber structure is directionally distributed around the cavity, which greatly improves the lateral mechanical properties of the module reduces eliminates the source of stress;

(2) Choose the ideal cooling rate cooling medium: rapid cooling above the Ms point of the steel, greater than the critical quenching cooling rate of the steel, the stress generated by the supercooled austenite in the steel is thermal stress, the surface layer is compressive stress, the inner layer For tensile stress, they cancel each other, effectively prevent the formation of thermal stress cracks, slow cooling between the Ms-Mf of the steel, greatly reduce the structural stress when forming quenched martensite. When the sum of the thermal stress the corresponding stress in the steel is positive (tensile stress), it is easy to quench cracking, when it is negative, it is easy to quench cracking. Make full use of thermal stress, reduce phase transformation stress, control the sum of stress to be negative, which can effectively avoid the occurrence of transverse quenching cracks. CL-1 organic quenching medium is an ideal quenching agent, at the same time, it can reduce avoid the distortion of quenching die, also control the reasonable distribution of hardened layer. By adjusting the ratio of CL-1 quenching agent to different concentrations, different cooling rates can be obtained, the required hardened layer distribution can be obtained to meet the needs of different mold steels.

3. Arc crack

It often occurs at the corners of the mold, bosses, knife lines, sharp corners, right angles, notches, holes, female mold wiring flash other sudden changes in shape. This is because the stress generated at the corners during quenching is 10 times the average stress on the smooth surface.

(1) The higher the carbon (C) content alloying element content in the steel, the lower the Ms point of the steel. If the Ms point is lowered by 2 ° C, the tendency of quenching cracks increases by 1.2 times, the Ms point decreases by 8 ° C, the tendency of quenching cracks increases by 8 Times

(2) Different structural transformations in the steel the same structural transformations have different timings. Due to the specific tolerance of different structures, huge tissue stress is caused, which leads to the formation of arc cracks at the junction of the tissues;

(3) tempered in time after quenching, insufficient tempering, the residual austenite in the steel is fully transformed, remains in use to promote the redistribution of stress, the martensite phase occurs in the residual austenite when the die is in service A new internal stress is generated by the transformation, an arc crack is formed when the integrated stress is greater than the strength limit of the steel;

(4) It has the second type of tempered brittle steel. After quenching, the high temperature tempering is slowly cooled, resulting in the precipitation of harmful impurity compounds such as P S in the steel along the grain boundary, greatly reducing the bonding strength toughness of the grain boundary, increasing brittleness, when in service Under the action of external force, arc-shaped cracks are formed.

Precaution:

(1) Improve the design, try to make the shape symmetrical, reduce the shape mutation, increase the process holes ribs, use combination assembly;

(2) Rounded corners replace right angles sharp edges, through holes replace blind holes, improve processing accuracy surface finish, reduce stress concentration sources, generally have low hardness requirements for corners, sharp edges, blind holes, etc. It can be bandaged stuffed with iron wire, asbestos rope, refractory mud, etc., artificially creating a cooling barrier, which is slowly cooled quenched to avoid stress concentration prevent the formation of arc-shaped cracks during quenching;

(3) The quenched steel should be tempered in time to eliminate part of the internal stress of quenching prevent the expansion of quenching stress;

(4) Tempering for a longer time to improve the fracture toughness value of the mold:

(5) Fully tempered to obtain stable organizational performance;

(6) Tempering multiple times to fully transform residual austenite eliminate new stress;

(7) Reasonable tempering to improve the fatigue resistance of steel parts comprehensive mechanical mechanical properties;

(8) For steels with the second type of tempered brittleness, the steel should be quickly cooled (water-cooled oil-cooled) after high-temperature tempering, which can eliminate the second-type tempered brittleness prevent avoid arc-shaped crack shapes during quenching.

4. Peeling crack

When the mold is in service, the hardened layer is peeled off the steel matrix under stress. Due to the different specific volume of the surface structure the core structure of the mold, the surface layer forms axial tangential quenching stress during quenching, which produces tensile stress in the radial direction abrupt changes to the inside. Peeling cracks occur at a narrow range of stress changes, which often occurs in During the cooling process of the surface chemical heat treatment mold, the internal external quenching martensite expansion is carried out at the same time due to the difference in the surface chemical modification the steel matrix phase transformation, resulting in large phase transformation stress, resulting in the chemical treatment of the infiltration layer the matrix structure Peel off. Such as flame surface hardening layer, high frequency surface hardening layer, carburizing layer, carbonitriding layer, nitriding layer, boronizing layer, metalizing layer, etc. It is suitable for rapid tempering after quenching of chemical infiltration layer, especially rapid heating at low temperature below 300 ℃, will cause the surface layer to form tensile stress, the compressive stress will be formed in the core transition layer of the steel matrix. The infiltrated layer was pulled apart.

Precaution:

(1) The concentration hardness of the chemical infiltration layer of the mold steel should be gradually reduced the surface to the inside, the bonding force between the infiltration layer the substrate should be enhanced. After diffusion, the diffusion treatment can make the transition between the chemical infiltration layer the substrate uniform;

(2) Diffusion annealing, spheroidizing annealing, tempering treatment are carried out before the chemical treatment of the die steel to fully refine the original structure, which can effectively prevent avoid peeling cracks ensure product quality.

5. Mesh crack

The depth of the crack is relatively shallow, generally about 0.01 to 1.5mm deep, it is radial, it is also known as crack. The main reasons are:

(1) The raw material has a deep decarburization layer, which is removed by cooling cutting, the finished mold is heated in an oxidizing atmosphere furnace to cause oxidative decarburization;

(2) The metal structure of the decarburized surface layer of the mold is different that of the steel matrix martensite, the specific volume is different. When the decarburized surface layer of the steel is quenched, a large tensile stress is generated. Therefore, the surface metal is often pulled into a network along the grain boundary. ;

(3) The raw material is coarse-grained steel, the original structure is coarse, there is large block ferrite, which can be eliminated by conventional quenching, it remains in the quenched structure, the temperature control is inaccurate, the instrument fails, the organization overheats even burns. The grains are coarsened the bonding strength of the grain boundaries is lost. When the mold is quenched cooled, the carbides of the steel are precipitated along the austenite grain boundaries. The grain boundary strength is greatly reduced, the toughness is poor, the brittleness is large. split.

Precaution:

(1) Strictly check the chemical composition, metallographic structure flaw detection of raw materials. Unqualified raw materials coarse grain steel should be used as mold materials;

(2) fine-grained steel vacuum electric furnace steel, check the depth of the raw material decarburization layer before commissioning. The cold machining allowance must be greater than the depth of the decarburization layer;

(3) Formulate advanced reasonable heat treatment process, choose microcomputer temperature control instrument, the control accuracy reaches ± 1.5 ℃, regularly check the instrument on site;

(4) The treatment of mold products adopts measures such as vacuum electric furnace, protective atmosphere furnace heating of mold products through a fully deoxidized salt bath furnace to effectively prevent avoid the formation of network cracks.


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