1. Longitudinal crack
The cracks are axial, thin and long in shape.
When the die is completely quenched, that is, centerless quenching, the center is transformed into quenched martensite with the largest specific volume, generating tangential tensile stress.
The higher the carbon content of the die steel, the greater the tangential tensile stress generated.
When the tensile stress is greater than the strength limit of the steel, longitudinal cracks are formed.
The following factors aggravate the generation of longitudinal cracks:
(1) The 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 presents 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 raw material rolling are not processed and retained in the product, causing the final quenching cracks to expand and form longitudinal cracks;
(2) Longitudinal cracks are easy to form when the die size is within the range of quenching crack sensitive size of steel (the dangerous size of quenching crack of carbon tool steel is 8-15mm, and the dangerous size of medium and low alloy steel is 25-40mm) or the selected quenching cooling medium greatly exceeds the critical quenching cooling speed of the steel.
(1) The warehousing inspection of raw materials shall be strictly carried out, and the steel with harmful impurities exceeding the standard shall not be put into production;
(2) Vacuum smelting, furnace refining or electroslag remelting die steel shall 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, graded quenching and isothermal quenching;
(4) Changing from centerless quenching to central quenching, i.e. incomplete quenching, and obtaining lower bainite structure with high strength and toughness can greatly reduce the tensile stress and effectively avoid the longitudinal cracking and quenching distortion of the die.
2. Transverse crack
The crack is characterized by being perpendicular to the axis.
For the unhardened die, there is a large tensile stress peak at the transition part between the hardened zone and the unhardened zone.
Large tensile stress peaks are easily formed when large dies are rapidly cooled.
As the axial stress formed is greater than the tangential stress, transverse cracks are generated.
Transverse segregation of S, P, Bi, Pb, Sn, As and other low melting point harmful impurities in the forging module or transverse microcracks exist in the module, and transverse cracks are formed after quenching.
(1) The module shall be forged reasonably. The ratio of length to the diameter of raw materials, that is, the forging ratio, should preferably be 2-3.
The double cross type variable direction forging shall be adopted for forging.
After five upsetting and five drawing, the multi fire forging shall be carried out to make the carbide and impurities in the steel fine and evenly distributed on the steel matrix.
The forging fiber structure shall be distributed non directionally around the cavity, greatly improving the transverse mechanical properties of the module, reducing and eliminating the stress source;
(2) Select the ideal cooling rate and cooling medium: fast cooling above the Ms point of the steel is 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 layer is compressive stress, and the inner layer is tensile stress, which offset each other, effectively preventing the formation of thermal stress cracks.
Slow cooling between Ms -Mf of the steel can greatly reduce the organizational stress when forming quenched martensite.
When the sum of thermal stress and corresponding stress in steel is positive (tensile stress), it is easy to quench crack, and when it is negative, it is not easy to quench crack.
Related reading: Quenching Crack vs. Forging Crack vs. Grinding Crack
Make full use of thermal stress, reduce phase transformation stress, and control the total stress to be negative, which can effectively avoid transverse quenching cracks.
CL-1 organic quenchant is an ideal quenchant, which can reduce and avoid the distortion of the quenching die and control the reasonable distribution of the hardening layer.
By adjusting the proportion 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.
Related reading: What Materials Are Usually Used for Stamping Dies?
3. Arc cracks
It often occurs at sudden changes in the shape of die corners, notches, cavities, and die connection flash, because the stress generated at corners during quenching is 10 times the average stress of smooth surfaces.
(1) The higher the carbon (C) content and alloying element content in the steel, the lower the Ms point of the steel.
The Ms point decreases by 2 ℃, then 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 in steel are different at the same time.
Due to the different specific tolerances of microstructures, huge structural stress is caused, which leads to the formation of arc-shaped cracks at the interface of microstructures;
(3) If the quenching is not timely tempered or the tempering is not sufficient, the residual austenite in the steel is not fully transformed, which is retained in the service state to promote the redistribution of stress, or when the die is in service, the residual austenite undergoes martensitic transformation to produce new internal stress, and when the comprehensive stress is greater than the strength limit of the steel, arc-shaped cracks will be formed;
(4) The second kind of tempered brittle steel is tempered 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, greatly reducing the grain boundary adhesion and strength toughness, increasing brittleness, and forming arc cracks under the external force during service.
(1) Improve the design, make the shape symmetrical as far as possible, reduce the sudden change of shape, 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, reduce stress concentration sources.
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. can be used for binding or filling, and artificial cooling barriers can be created to slow cooling and quenching, avoid stress concentration, and prevent the formation of arc cracks during quenching;
(3) Quenched steel shall be tempered in time to eliminate part of quenching internal stress and prevent quenching stress from expanding;
(4) Temper for a long time to improve the fracture toughness of the die;
(5) Fully tempered to obtain stable structure and properties;
(6) Repeated tempering can fully transform residual austenite and eliminate new stress;
(7) Reasonable tempering can improve the fatigue resistance and comprehensive mechanical properties of steel parts;
The mold steel with the second type of temper brittleness shall be cooled quickly after high temperature tempering (water cooling or oil cooling) to eliminate the second type of temper brittleness and prevent and avoid the formation of arc cracks during quenching.
4. Peeling cracks
When the die is in service, under the effect of stress, the hardened layer is peeled off from the steel matrix piece by piece.
Due to the different specific volumes of the surface and central structures of the die, 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 sharp changes in stress, which often occur during the cooling process of the die 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 the quenched martensite in the inner and outer layers is different, resulting in large transformation stress.
This causes the chemical treatment layer to peel off 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 temper the chemical layer rapidly after quenching, especially when tempering at a low temperature below 300 ℃ and heating rapidly, which will cause tensile stress to form on 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 stripped.
(1) The concentration and hardness of the chemical infiltration layer of the die steel should be reduced slowly from the surface to the inside, and the bonding force between the infiltration layer and the matrix should be enhanced.
Diffusion treatment after infiltration can make the chemical infiltration layer and the matrix transition uniform;
(2) Before chemical treatment of die steel, diffusion annealing, spheroidizing annealing and quenching and tempering treatment shall be carried out to fully refine the original structure, which can effectively prevent and avoid peeling cracks and ensure product quality.
5. Mesh cracks
The crack depth is relatively shallow, generally about 0.01-1.5mm deep, radiating, nicknamed crack.
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 an oxidizing atmosphere furnace to cause oxidative decarburization;
(2) The structure of the decarburized surface metal of the die is different from the carbon content and specific volume of the martensite in the steel matrix.
The decarburized surface of the steel produces large tensile stress during quenching.
Therefore, the surface metal is often cracked into a network along the grain boundary;
(3) The raw material is coarse grain steel. The original structure is coarse and there is massive ferrite, which cannot be eliminated by conventional quenching.
It remains in the quenching structure, or the temperature control is inaccurate, the instrument fails, the structure overheats, or even overburns, the grain coarsens, the grain boundary bonding force is lost.
When the die is quenched and cooled, the steel carbide 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, there is a network crack along the grain boundary.
(1) The chemical composition, metallographic structure and flaw detection of raw materials shall be strictly checked, and unqualified raw materials and coarse grain steel shall not be used as die materials;
(2) Fine-grain steel and vacuum electric furnace steel shall be selected, and the depth of decarburization layer of raw materials shall be rechecked before production.
The allowance for cold cutting must be greater than the depth of the decarburization layer;
(3) Formulate advanced and reasonable heat treatment process, select microcomputer temperature control instrument with control accuracy of ± 1.5 ℃, and calibrate the instrument on site regularly;
(4) Vacuum electric furnace, protective atmosphere furnace and fully deoxidized salt bath furnace are used for final treatment of mold products to effectively prevent and avoid the formation of network 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 zero and the cooling continues, the retained austenite can undergo martensitic transformation.
Therefore, the essence of cold treatment is quenching.
The quenching stress at room temperature and the quenching stress at zero temperature are superposed.
When the superposed stress exceeds the strength limit of the material, a cold treatment crack will be formed.
(1) Before quenching and cooling treatment, the die shall be boiled in boiling water for 30-60min to eliminate 15% – 25% of the quenching internal stress and stabilize the residual austenite.
Then, the die shall be subjected to normal cooling treatment at – 60 ℃ or cryogenic treatment at – 120 ℃.
The lower the temperature is, the more the residual austenite will be transformed into martensite, but it is impossible to complete the transformation.
The experiment shows that about 2% – 5% of the residual austenite is retained, and a small amount of residual austenite can be retained as required to relax the stress.
It plays a buffering role. Because the residual austenite is soft and tough, it can partially absorb the rapid expansion energy of martensitization and ease the transformation stress;
(2) After cold treatment, take out the mold and put it into hot water to raise the temperature, which can eliminate 40% – 60% of the cold treatment stress.
When the temperature rises to room temperature, it should be tempered in time.
The cold treatment stress should be further eliminated to avoid the formation of cold treatment cracks, obtain stable organizational performance, and ensure that the mold products are not distorted during storage and use.
7. Grinding cracks
It often occurs in the cold grinding process after quenching and tempering of the die products.
Most of the micro cracks formed are perpendicular to the grinding direction, about 0.05-1.0mm deep.
(1) Improper pretreatment of raw materials, failure to fully eliminate massive, reticulated and banded carbides of raw materials and serious 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, stress induced phase transformation occurs, which transforms the residual austenite into martensite.
The structural stress is large. In addition, due to insufficient tempering, there are many residual tensile stresses left, which are superimposed with the grinding structural stress, or due to large grinding speed, feed rate and improper cooling, the grinding heat of the metal surface rises sharply to the quenching 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, grinding cracks will be caused on the surface metal.
(1) The raw materials are modified and forged for many times with double cross shaped variable direction upsetting and drawing.
After four upsetting and four drawing, the forging fiber structure is symmetrically distributed in 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, reticulated, banded and chain carbides and refine the carbides to 2-3 levels;
(2) Formulate advanced heat treatment process to control the content of final quenched residual austenite not exceeding the standard;
(3) Temper and eliminate quenching stress timely after quenching;
(4) Proper reduction of grinding speed, grinding quantity and grinding cooling speed can effectively prevent and avoid the formation of grinding cracks.
8. Wire cutting cracks
This crack occurs in the online cutting process of the quenched and tempered module.
This process changes the stress field distribution state of the metal surface layer, middle layer and center.
The quenching residual internal stress is out of balance and deformed, and a large tensile stress appears in a certain area.
When this tensile stress is large enough to dry the strength limit of the die material, it causes cracking.
The crack is an arc tail shaped rigid metamorphic layer crack.
The experiment shows that the wire cutting process is a partial high temperature discharge and rapid cooling process, which makes the metal surface form a dendritic solidified layer of as cast structure, producing 600-900MPa tensile stress and 0.03mm thick high stress secondary quenching white layer.
Causes of cracks:
(1) Serious carbide segregation exists 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 the brittleness;
(3) Quenched workpieces are not tempered in time and the tempering is not sufficient, and excessive residual internal stress and new internal stress formed during wire cutting lead to wire cutting cracks.
(1) Strictly check the raw materials before warehousing to ensure that the organizational composition of the raw materials is qualified.
Unqualified raw materials must be forged to break the carbides so that the chemical composition and metallographic structure meet the technical conditions before they can be put into production.
Before the heat treatment of modules, the finished products shall be quenched, tempered and wire cut after a certain amount of grinding is reserved;
(2) Calibrate the instrument before entering the furnace, select the microcomputer to control the temperature, with the temperature control accuracy of ± 1.5 ℃, vacuum furnace and protective atmosphere furnace for heating, and strictly prevent overheating and oxidative decarburization;
(3) Grading quenching, isothermal quenching and timely tempering after quenching, multiple tempering, fully eliminating internal stress, creating conditions for wire cutting;
(4) Formulate scientific and reasonable wire cutting process.
9. Fatigue fracture
When the die is in service, the micro fatigue cracks formed under the repeated action of alternating stress slowly expand, leading to sudden fatigue fracture.
(1) There are cracks, self spots, pores, looseness, non-metallic inclusions, severe carbide segregation, banded structure and massive free ferrite metallurgical defects in raw materials, which destroy the continuity of matrix structure and form uneven stress concentration.
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 the steel.
P in the steel is easy to cause cold brittleness, while S is easy to cause hot brittleness.
If S, P harmful impurities exceed the standard, they are easy to form a fatigue source;
(2) Too thick, too high concentration, too thick, too shallow hardened layer and low hardness of transition zone can lead to a sharp reduction of fatigue strength of materials;
(3) When the die surface is rough in processing, low in accuracy, poor in finish, as well as knife lines, lettering, scratches, bruises, corrosion pits, etc., it is also easy to cause stress concentration and fatigue fracture.
(1) Strictly select materials, ensure the materials, and control the content of Pb, As, Sn and other low melting point impurities and S, P non-metallic impurities not exceeding the standard;
(2) Material inspection shall be carried out before production, and unqualified raw materials shall not be put into production;
(3) The electroslag remelting refined steel with high purity, less impurities, uniform chemical composition, fine grains, small carbides, good isotropic properties, and high fatigue strength shall be selected to strengthen the surface of the die surface by shot peening and surface chemical infiltration, so that the metal surface is pre pressed to offset the tensile stress generated when the die is in service, and improve the fatigue strength of the die surface;
(4) Improve the machining accuracy and finish of the die surface;
(5) Improve the structure and properties of the chemical layer and hardened layer, and use microcomputer to control the thickness, concentration and hardened layer thickness of the chemical layer.
10. Stress corrosion cracking
This crack 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 inside.
This is called stress corrosion cracking.
The corrosion resistance of die 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 is in turn ferrite (F) – martensite (M) – pearlite (P) – sorbite (S).
Therefore, T structure is not suitable for heat treatment of die 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 stress will be generated under the action of external force.
Stress corrosion cracks will occur whenever there is stress in the metal mold.
(1) After quenching, the die steel shall be tempered timely, fully and repeatedly to eliminate the internal stress of quenching;
(2) Generally, it is not suitable to temper the die steel at 350-400 ℃ after quenching.
Because T structure often occurs at this temperature, the die with T structure should be reprocessed, and the die should be treated with rust prevention to improve the corrosion resistance;
(3) The hot working die shall be preheated at low temperature before service, and the cold working die shall be tempered at low temperature to eliminate stress after a stage of service, which can not only prevent and avoid the occurrence of stress corrosion cracks, but also greatly improve the service life of the die, kill two birds with one stone, and can achieve significant technical and economic benefits.
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