1. Longitudinal crack
The cracks are axial, thin and long in shape.
When the matrix is completely quenched, i.e., centerless quenching, the center is transformed into quenched martensite with a higher specific volume, generating tangential tensile stress.
The higher the carbon content of the matrix steel, the greater the tangential tensile stress generated.
When the tensile stress is greater than the strength limit of the steel, longitudinal cracks form.
The following factors aggravate the generation of longitudinal cracks:
(1) Steel contains a lot of S, P, Bi, Pb, Sn, As and other harmful impurities with low melting point.
When the steel ingot is rolled, it has a serious longitudinal segregation distribution along the rolling direction, which is easy to produce stress concentration and form longitudinal quenching cracks, or the longitudinal cracks formed by rapid cooling after rolling of the raw material are not processed and retained in the product, causing the final tempering cracks to expand and form longitudinal cracks;
(2) Longitudinal cracks are easy to form when the die size is within the size range sensitive to quenching crack of steel (the dangerous quenching crack size of carbon tool steel is 8-15mm, and the dangerous size of medium and low alloy steel is 25-40 mm) or the selected quenching cooling medium far exceeds the critical quenching cooling speed of the steel.
Preventive measures:
(1) Raw material storage inspection must be strictly carried out, and steel with harmful impurities exceeding the standard must not be put into production;
(2) Vacuum smelting, furnace refining or electroslag remelting should be selected as far as possible;
(3) The heat treatment process is improved by adopting vacuum heating, protective atmosphere heating, fully deoxidized salt bath furnace heating, graduated quenching and isothermal quenching;
(4) Changing from centerless quenching to center quenching, that is, incomplete quenching, and obtaining lower bainite structure with high strength and toughness can greatly reduce the tensile stress and effectively prevent longitudinal cracking and quenching distortion of the die.
2. Transverse crack
The crack is characterized by being perpendicular to the axis.
For the unhardened matrix, there is a large tensile stress peak in the transition part between the hardened zone and the unhardened zone.
Large tensile stress peaks are easily formed when large dies are cooled rapidly.
As the axial tension formed is greater than the tangential tension, transverse cracks are generated.
Transverse segregation of S, P, Bi, Pb, Sn, As and other harmful low-melting impurities in the forging module or transverse microcracks exist in the module, and transverse cracks are formed after quenching.
Preventive measures:
(1) The module must be forged reasonably. The ratio of length to diameter of raw materials, that is, the forging ratio, should preferably be 2-3.
Double cross type variable direction forging should be adopted for forging.
After five upsetting and five drawings, multi-fire forging should be carried out to make the carbide and steel impurities fine and evenly distributed in the steel matrix.
The forged fiber structure will be distributed non-directionally around the cavity, significantly improving the transverse mechanical properties of the module, reducing and eliminating the stress source;
(2) Select the optimal cooling rate and cooling medium: rapid cooling above the Ms point of the steel is greater than the critical tempering cooling rate of the steel.
The stress generated by subcooled austenite in the steel is thermal stress, the surface layer is compressive stress, and the inner layer is tensile stress, which compensate each other, effectively preventing the formation of thermal stress cracks.
The slow cooling between Ms -Mf of steel can greatly reduce the organizational stress during the formation of quenched martensite.
When the sum of the thermal stress and the corresponding stress in the steel is positive (tensile stress), it is easy to extinguish the crack, and when it is negative, it is not easy to extinguish the crack.
Related Reading: Crack Tempering vs. Crack Forging vs. Crack Forging Crack rectification
Make full use of thermal stress, reduce phase transformation stress, and control the total stress to be negative, which can effectively prevent transverse quenching cracks.
CL-1 organic quenching is an ideal quenching, which can reduce and avoid the distortion of the quenching matrix and control the reasonable distribution of the hardening layer.
By adjusting the quenching ratio CL-1 with different concentrations, different cooling rates can be obtained and the required distribution of the hardened layer can be obtained to meet the needs of different matrix steels.
Related Reading: What materials are typically used for stamping dies?
3. Arc Cracks
It often occurs in sudden changes in the shape of die corners, notches, cavities and die connecting burrs, because the stress generated in the corners during quenching is 10 times the average stress of smooth surfaces.
Furthermore,
(1) The higher the carbon content (C) and alloy element content in the steel, the lower the Ms point of the steel.
The Ms point decreases by 2℃, so the quenching cracking tendency increases by 1.2 times, the Ms point decreases by 8℃, and the quenching cracking tendency increases by 8 times;
(2) The transformation of different microstructures and the transformation of the same microstructure into steel are different at the same time.
Due to the different specific tolerances of the microstructures, enormous structural stresses are caused, which leads to the formation of arc-shaped cracks at the interface of the microstructures;
(3) If quenching is not tempered in a timely manner or tempering is not sufficient, the residual austenite in the steel is not fully transformed, which is retained in the service state to promote stress redistribution, or when the die is in service , the residual austenite undergoes martensitic transformation to produce new internal stresses, and when the comprehensive stress is greater than the strength limit of the steel, arc-shaped cracks will be formed;
(4) The second type of tempered brittle steel is quenched slowly at high temperature after quenching, which leads to the precipitation of P, S and other harmful impurities in the steel along the grain boundary, significantly reducing the grain boundary adhesion and toughness, increasing brittleness and forming arc cracks under external force during service.
Preventive measures:
(1) Improve the design, make the shape as symmetrical as possible, reduce the sudden shape change, increase the process hole and reinforcing rib, or adopt combined assembly;
(2) Round corners replace right angles and sharp corners and sharp edges, and through holes replace blind holes to improve processing accuracy and surface finish while reducing sources of stress concentration.
For places where it is impossible to avoid right angles, sharp corners and sharp edges, blind holes, etc., the general hardness requirements are not high, iron wire, asbestos rope, fire-resistant mud, etc. filler and artificial cooling barriers can be created to delay cooling and quenching, prevent stress concentration, and prevent arc cracks from forming during quenching;
(3) Quenching steel must be quenched in time to eliminate part of the internal quenching stress and prevent the expansion of quenching stress;
(4) Tempering for a long time to improve the fracture toughness of the matrix;
(5) Fully tempered to obtain stable structure and properties;
(6) Repeated tempering can completely transform residual austenite and eliminate new stresses;
(7) Reasonable tempering can improve the fatigue resistance and comprehensive mechanical properties of steel parts;
Mold steel with the second type of quenching brittleness must be cooled quickly after high-temperature tempering (water quenching or oil quenching) to eliminate the second type of quenching brittleness and prevent and prevent the formation of arc cracks during tempering.
4. Peeling cracks
When the die is in service, under the effect of stress, the hardened layer is removed from the steel die piece by piece.
Due to the different specific volumes of the surface and core structures of the matrix, axial and tangential quenching stresses are formed in the surface layer during quenching, tensile stresses are generated in the radial direction, and sudden changes occur internally.
Peeling cracks are generated in the narrow range of sudden stress changes, which generally occur during the matrix cooling process after chemical heat treatment of the surface layer.
Because the chemical modification of the surface layer is different from the transformation of the steel matrix, the expansion of quenched martensite in the inner and outer layers is different, resulting in large transformation stress.
This causes the chemical treatment layer to detach from the matrix.
Such as flame surface hardening layer, high frequency surface hardening layer, carburizing layer, carbonitriding layer, nitriding layer, boronizing layer, metallizing layer, etc.
It is not suitable to quench the chemical layer quickly after quenching, especially when tempering at a low temperature below 300 ℃ and heating quickly, which will cause tensile stress to form in the surface layer and compressive stress to form in the center of the steel matrix and the transition layer.
When the tensile stress is greater than the compressive stress, the chemical layer will be pulled and removed.
Preventive measures:
(1) The concentration and hardness of the chemical infiltration layer of the matrix steel should be slowly reduced from the surface to the inside, and the bonding strength between the infiltration layer and the matrix should be increased.
Diffusion treatment after infiltration can uniform the chemical infiltration layer and matrix transition;
(2) Before chemical treatment of die steel, diffusion annealing, spheroidizing annealing and quenching and tempering treatment should be carried out to fully refine the original structure, which can effectively prevent and prevent cracking and ensure the quality of the product.
5. Cracks in the mesh
The depth of the crack is relatively shallow, generally about 0.01-1.5 mm deep, radiating, called fissure.
The main reasons are:
(1) The raw material has a deep decarburization layer, which is not removed during cold cutting, or the finished mold is heated in a furnace with an oxidizing atmosphere to cause oxidative decarburization;
(2) The structure of the decarburized surface metal of the matrix is different from the carbon content and specific volume of martensite in the steel matrix.
The decarburized surface of the steel produces high tensile stress during quenching.
Therefore, the metal surface is often broken into a network along the grain boundary;
(3) The raw material is coarse-grained steel. The original structure is coarse and contains massive ferrite, which cannot be eliminated by conventional quenching.
It remains in the tempering structure, or the temperature control is inaccurate, the instrument fails, the structure overheats, or even overburns, the grain becomes coarser, the grain boundary bonding strength is lost.
When the matrix is quenched and cooled, steel carbide precipitates along the grain boundary of austenite, the grain boundary strength is greatly reduced, the toughness is low, and the brittleness is large.
Under the action of tensile stress, a network crack occurs along the grain boundary.
Preventive measures:
(1) The chemical composition, metallographic structure and flaw detection of raw materials must be strictly checked, and unqualified raw materials and coarse-grained steel must not be used as matrix materials;
(2) Fine-grain steel and electric vacuum furnace steel should be selected, and the depth of the decarburization layer of raw materials should be checked again before production.
The tolerance for cold cutting must be greater than the depth of the decarburization layer;
(3) Formulate an advanced and reasonable heat treatment process, select a microcomputer temperature control instrument with a control accuracy of ±1.5℃, and calibrate the instrument on site regularly;
(4) Electric vacuum furnace, protective atmosphere furnace and fully deoxidized salt bath furnace are used for the final treatment of mold products to effectively prevent and prevent the formation of web cracks.
6. Cold treatment cracks
Most die steels are medium and high carbon alloy steels.
After quenching, some undercooled austenite is not transformed into martensite and remains as residual austenite in service, which affects service performance.
If the temperature is below freezing and cooling continues, the retained austenite may undergo martensitic transformation.
Therefore, the essence of cold treatment is cooling.
The room temperature quenching stress and the zero temperature quenching stress are superimposed.
When the superimposed stress exceeds the strength limit of the material, a cold treatment crack will be formed.
Preventive measures:
(1) Before quenching and cooling treatment, the die should be boiled in boiling water for 30-60 minutes to eliminate 15% – 25% of the internal quenching stress and stabilize the residual austenite.
Then the die must undergo normal cooling treatment at –60℃ or cryogenic treatment at –120℃.
The lower the temperature, the more residual austenite will be transformed into martensite, but it is impossible to complete the transformation.
The experiment shows that about 2% to 5% of residual austenite is retained, and a small amount of residual austenite can be retained as needed to relax the stress.
It plays a buffer role. Because residual austenite is soft and tough, it can partially absorb the rapid expansion energy of martensitization and alleviate the transformation stress;
(2) After cold treatment, take out the mold and put it in hot water to increase the temperature, which can eliminate 40% to 60% of the stress of cold treatment.
When the temperature rises to room temperature, it must be tempered in time.
The stress of cold treatment must be further eliminated to prevent the formation of cold treatment cracks, achieve stable organizational performance, and ensure that mold products are not distorted during storage and use.
7. Grind cracks
It often occurs in the cold grinding process after quenching and tempering of die products.
Most of the microcracks formed are perpendicular to the grinding direction, about 0.05-1.0 mm deep.
(1) Inadequate pretreatment of raw materials, failure to fully eliminate solid, cross-linked and banded carbides from raw materials and severe decarburization;
(2) The final quenching heating temperature is too high, overheating occurs, the grain is coarse, and more residual austenite is generated;
(3) During grinding, a stress-induced phase transformation occurs, which transforms the residual austenite into martensite.
The structural stress is great. Furthermore, due to insufficient tempering, many residual tensile stresses remain, which are superimposed on the structural grinding stress, or due to the large grinding speed, feed rate and inadequate cooling, the grinding heat of the metal surface increases sharply to tempering. heating temperature, and then the grinding fluid cools, resulting in secondary quenching of the grinding surface, which is a combination of multiple stresses.
If the strength limit of the material is exceeded, cracks will be caused on the metal surface.
Preventive measures:
(1) Raw materials are modified and forged many times with variable direction upsetting and stretching in the shape of a double cross.
After four upsetting and four stretching, the forged fiber structure is distributed symmetrically in a wavy shape around the cavity or axis.
The final high-temperature waste heat is used for quenching, followed by high-temperature tempering, which can fully eliminate blocky, cross-linked, banded and chain carbides and refine carbides by 2-3 levels;
(2) Formulate advanced heat treatment process to control the final quenched residual austenite content that does not exceed the standard;
(3) Temper and eliminate quenching stress in a timely manner after quenching;
(4) Properly reducing the grinding speed, grinding amount and grinding cooling speed can effectively prevent and prevent the formation of grinding cracks.
8. Cracks when cutting the wire
This crack occurs in the online cutting process of the quenched and tempered module.
This process changes the state of stress field distribution of the metal surface layer, middle layer and center.
The internal residual stress of quenching is unbalanced and deformed, and a large tensile stress appears in a certain area.
When this tensile stress is great enough to dry out the ultimate strength of the matrix material, it causes cracking.
The fissure is an arc-tail-shaped rigid metamorphic layer fissure.
The experiment shows that the wire cutting process is a high temperature partial discharge and rapid cooling process, which causes the metal surface to form a solidified dendritic layer like molten structure, producing tensile stress of 600-900MPa and 0.03 mm thick secondary high voltage quenching white layer.
Causes of cracks:
(1) There is serious segregation of carbide in raw materials;
(2) The instrument fails, the quenching heating temperature is too high, and the grain is coarse, reducing the strength and toughness of the material and increasing brittleness;
(3) The quenched parts are not tempered in time and the tempering is not sufficient, and the excessive residual internal stress and new internal stress formed during wire cutting lead to wire cutting cracks.
Preventive measures:
(1) Strictly check the raw materials before storage to ensure that the organizational composition of the raw materials is qualified.
Unqualified raw materials must be forged to break carbides so that the chemical composition and metallographic structure meet the technical conditions before being put into production.
Before heat treatment of modules, the finished products must be quenched, tempered and wire cut after reserving a certain amount of grinding;
(2) Calibrate the instrument before entering the furnace, select the microcomputer to control the temperature, with temperature control accuracy of ± 1.5 ℃, vacuum furnace and protective atmosphere furnace for heating, and strictly prevent overheating and oxidative decarburization;
(3) Classification quenching, isothermal quenching and timely quenching after quenching, multiple quenching, completely eliminating internal stress, creating conditions for wire cutting;
(4) Formulate a scientific and reasonable thread cutting process.
9. Fatigue fracture
When the die is in service, fatigue microcracks formed under the repeated action of alternating stresses slowly expand, leading to sudden fatigue fracture.
(1) There are cracks, self-stains, pores, looseness, non-metallic inclusions, severe carbide segregation, banded structure and massive metallurgical defects of free ferrite in the raw materials, which destroy the continuity of the matrix structure and form uneven concentration of tensions.
112 in the steel ingot was not eliminated, resulting in the formation of white spots during rolling.
There are Bi, Pb, Sn, As, S, P and other harmful impurities in steel.
P in steel is easy to cause cold brittleness, while S is easy to cause hot brittleness.
If harmful impurities S, P exceed the standard, they will be easy to form a source of fatigue;
(2) Too thick hardened layer, too high concentration, too thick and too shallow hardened layer, and low transition zone hardness can lead to a sharp reduction in the fatigue strength of materials;
(3) When the surface of the die is rough in processing, poor precision, poor workmanship, as well as knife lines, letters, scratches, bruises, corrosion pits, etc., it is also easy to cause stress concentration and fatigue fracture.
Preventive measures:
(1) Strictly select materials, guarantee materials, and control the content of Pb, As, Sn and other low-melting impurities and S, P non-metallic impurities that do not exceed the standard;
(2) Material inspection must be carried out before production, and unqualified raw materials must not be put into production;
(3) Refined electroslag remelting steel with high purity, less impurities, uniform chemical composition, fine grains, small carbides, good isotropic properties and high fatigue resistance should be selected to strengthen the die surface surface by shot peening and surface chemicals infiltration, so that the metal surface is pre-pressed to compensate for the tensile stress generated when the die is in service and improve the fatigue resistance of the die surface;
(4) Improve machining accuracy and die surface finish;
(5) Improve the structure and properties of the chemical layer and hardened layer, and use a microcomputer to control the thickness, concentration and hardened layer thickness of the chemical layer.
10. Stress corrosion cracking
This cracking often occurs during use.
The metal mold cracks due to the chemical reaction or electrochemical reaction process, which causes damage and corrosion of the structure from the surface to the interior.
This is called stress corrosion cracking.
The corrosion resistance of matrix steel is different due to different structures after heat treatment.
The most corrosion-resistant structure is austenite (A), and the most easily corroded structure is troostite (T), which in turn is ferrite (F) – martensite (M) – pearlite (P) – sorbite (S) .
Therefore, the T frame is not suitable for heat treatment of shaped steel.
Although the quenched steel has been tempered, due to insufficient tempering, the internal stress in quenching still exists more or less.
When the mold is in service, new stresses will be generated under the action of external forces.
Stress corrosion cracking will occur whenever there is stress on the metal mold.
Preventive measures:
(1) After quenching, the die steel must be quenched timely, completely and repeatedly to eliminate the internal stress of quenching;
(2) Generally, it is not suitable to quench die steel at 350-400 ℃ after quenching.
Because T-structure often occurs at this temperature, the T-structure die must be reprocessed, and the die must be treated with rust prevention to improve corrosion resistance;
(3) The hot working die must be preheated at a low temperature before service, and the cold working die must be quenched at a low temperature to eliminate stress after a service phase, which can not only prevent and prevent the occurrence of stress corrosion cracking, but also greatly improves the service life of the die, kills two birds with one stone, and can achieve significant technical and economic benefits.
