In heat treatment of mold steel, quenching is a common process. However, for various reasons, sometimes quench crack will inevitably occur, leading to loss of previous work. Analyzing causes of cracks, taking corresponding preventive measures have significant technical and economic benefits. Common quench crack have following 10 types.

Cracks are axial, thin and long. When mold is completely hardened, that is, centerlessly quenched, core is transformed into quenched martensite with the largest specific volume, resulting in tangential tensile stress. The higher carbon content of mold steel, the greater tangential tensile stress generated. When tensile stress is greater than strength limit of steel, longitudinal cracking are formed. Following factors have intensified occurrence of longitudinal cracks:
(1) Steel contains many low-melting harmful impurities such as S, P, Sb, Bi, Pb, Sn, As, etc. During rolling of ingot, there is a serious longitudinal segregation distribution along rolling direction, which is prone to stress concentration and longitudinal quench crack, or vertical cracks formed by rapid cooling after raw materials are rolled away and remain in product, which causes final quench crack to expand and form longitudinal cracks. ;
(2) Longitudinal cracking are easily formed when mold size is within quench cracking sensitive size range of steel (carbon steel tool's dangerous cracking size is 8-15mm, medium and low alloy steel's dangerous size is 25-40mm) or quenching cooling medium selected greatly exceeds critical quenching cooling rate of steel.

(1) Strict inspection of raw materials in warehouse, do not put into production for steel materials with excessive levels of harmful impurities;
(2) Try to choose steel for vacuum smelting, refining outside furnace or electroslag remelting;
(3) Improve heat treatment process, use vacuum processing heat, protective atmosphere heating, fully deoxidizing salt bath furnace heating and analysis quenching, isothermal quenching;
(4) Changing from centerless quenching to centered quenching, that is, incomplete hardening, to obtain a lower bainite structure with high toughness, which can greatly reduce tensile stress, can effectively avoid longitudinal cracking and quenching distortion of mold.

Crack feature is perpendicular to axial direction. In unhardened molds, there are large peaks of tensile stress at transition between hardened and unhardened regions. Large molds are likely to form large peaks of tensile stress when they are rapidly cooled. Resulting axial stress is greater than tangential stress, resulting in transverse crack. Lateral segregation of low melting point harmful impurities such as S, P, Sb, Bi, Pb, Sn, As in forged module or module has lateral micro-cracks, which expand to form lateral cracks after quenching.

(1) Module should be reasonably forged. Ratio of length to diameter of raw material, that is, forging ratio, is preferably selected between 2-3. Double cross deformation is forged between forgings. After five to five drawing multiple fire forging, carbides and impurities in steel are fine, small and evenly distributed on steel matrix. Forged fiber structure is distributed non-directionally around cavity, which greatly improves lateral mechanical performance of module, reduces and eliminates stress sources;
(2) Select ideal cooling rate and cooling medium: fast cooling above Ms point of steel, greater than critical quenching cooling rate of steel, stress generated by supercooled austenite in steel is thermal stress, surface layer is compressive stress, inner layer is tensile stresses, which cancel each other out, effectively preventing formation of thermal stress cracks. Slow cooling between Ms-Mf of steel significantly greatly reduce structural stress during formation of quenched martensite. When 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. Making full use of thermal stress, reducing phase transition stress, controlling total stress to be negative, can effectively avoid occurrence of transverse quench crack. CL-1 organic quenching medium is an ideal quenching agent. At the same time, it can reduce and avoid distortion of quenching mold, control reasonable distribution of hardened layer. Adjusting different concentration ratio of CL-1 quenching agent can obtain different cooling speeds, required hardened layer distribution, and meet needs of different mold steels.

 

It often occurs in abrupt shapes such as mold edges, bosses, knife marks, sharp angles, right angles, notches, holes, and die wiring flashing. This is because stress at corners during quenching is 10 times average stress on smooth surface.
(1) The higher carbon (C) content and alloying element content in steel, the lower Ms point of steel and the lower Ms point reduces by 2℃. Quench cracking tendency increases by 1.2 times, Ms point decreases by 8℃, and quench cracking tendency increases 8 times.
(2) Different structural transformations in steel and same structural transformation at different times. Due to specific tolerances of different structures, huge structural stresses are caused, leading to formation of arc-shaped cracks at interface of structures;
(3) Tempering is not timely after quenching, or tempering is inadequate, residual austenite in steel is not sufficiently transformed and remains in use state, which promotes redistribution of stress, or martensite transformation of residual austenite during service of mold generates new internal stresses. When comprehensive stress is greater than strength limit of steel, arc crack are formed;
(4) It has a second type of tempered brittle steel. After quenching, it is tempered at a high temperature and slowly cooled, causing harmful impurity compounds such as P and S in steel to precipitate along grain boundaries, which greatly reduces grain boundary bonding force and toughness, increases brittleness, and form arc crack under external force during service.

(1) Improve design, make shape as symmetrical as possible, reduce sudden changes in shape, increase process holes and stiffeners, or use combined assembly;
(2) Rounded corners are replaced by right angles and sharp corners, blind holes are replaced by through holes, improve machining accuracy and surface finish, reduce source of stress concentration. For unavoidable right angles, sharp corners, blind holes, etc., general hardness is not high. It can be bandaged or stuffed with iron wire, asbestos rope, refractory mud, etc., artificially creating a cooling barrier, making it slowly cool and quench, avoiding stress concentration, and preventing formation of arc crack during quenching;
(3) Quenched steel should be tempered in time to eliminate part of quenching internal stress and prevent quenching stress from expanding;
(4) Tempering for a long time, improve fracture toughness value of mold;
(5) Full tempering to obtain stable tissue performance;
(6) Multiple tempering to fully transform retained austenite and eliminate new stresses;
(7) Reasonable tempering to improve fatigue resistance and comprehensive mechanical properties of steel parts;
(8) For second-type tempered brittle mold steel, it should be quickly cooled (water-cooled or oil-cooled) after high-temperature tempering, which can eliminate second-class tempered brittleness, prevent and avoid arc-shaped cracks during quenching.

When mold is in service, hardened layer is peeled from steel matrix piece by piece under effect of stress. Due to difference in specific volume between surface structure of mold and core structure, surface layer forms axial and tangential quenching stresses during quenching, tensile stresses are generated in radial direction and abruptly change in the interior. Stripping cracks occur in a narrow range of sharp changes in stress, which often occurs during cooling of surface chemical heat treatment mold. Because surface chemical modification and steel matrix phase change are not time-varying, expansion of quenched martensite in inner and outer layers does not proceed at the same time, resulting in large phase transformation stress, which causes chemically treated infiltration layer to peel from matrix structure. Such as flame surface hardened layer, high frequency surface hardened layer, carburizing layer, carbonitriding layer, nitriding layer, boronizing layer, metalizing layer and so on. It is not advisable to quickly temper chemically penetrated layer after quenching, especially low-temperature tempering below 300℃, which will cause surface layer to form tensile stress, while steel substrate core and transition layer form compressive stress. When tensile stress is greater than compressive stress, chemically penetrated layer will be pulled apart.

(1) Concentration and hardness of chemical infiltration layer of mold steel should be gradually reduced from surface to inside to enhance bonding force between infiltration layer and substrate. Diffusion treatment after infiltration can make transition between chemical infiltration layer and substrate uniform;
(2) Diffusion annealing, spheroidizing annealing, quenching and tempering treatments are performed before chemical treatment of mold steel to fully refine original structure, which can effectively prevent and avoid occurrence of peeling crack, ensure product quality.

Crack depth is relatively shallow, generally about 0.01 to 1.5 mm, which is radiant and is also known as cracking. Main reasons are:
(1) Raw material has a deep decarburization layer, which has not been removed during cold cutting process, or finished mold is heated in an oxidizing atmosphere furnace to cause oxidative decarburization;
(2) Metal structure of decarburized surface layer of mold is different from that of steel matrix martensite, specific volume is different. Decarburized surface layer of steel has a large tensile stress during quenching. Therefore, surface metal is often pulled into a network along grain boundary;
(3) Raw material is coarse-grained steel, original structure is coarse, and there are large massive ferrites, which cannot be eliminated by conventional quenching, remains in quenched structure, or temperature control is inaccurate, instrument fails, structure is overheated or even overheated, grain coarsening, loss of grain boundary bonding force, carbides of steel precipitate along austenite grain boundaries during mold quenching and cooling, grain boundary strength is greatly reduced, toughness is poor, brittleness is large, mesh cracks along grain boundaries under tensile stress.

(1) Strictly check chemical composition, metallographic structure and flaw detection of raw materials. Unqualified raw materials and coarse-grained steel should not be used as mold materials;
(2) Use fine grain steel and vacuum electric furnace steel, check depth of decarburized layer of raw materials before putting into production. Cold cutting machining allowance must be greater than depth of decarburized layer;
(3) Formulate advanced and reasonable heat treatment process, choose microcomputer temperature control instrument, control accuracy reaches ±1.5℃, and calibrate instrument on-site on time;
(4) Final treatment of mold products uses vacuum electric furnace, protective atmosphere furnace, fully deoxidized salt bath furnace to heat mold products and other measures to effectively prevent and avoid formation of network cracks.

 

Mold steel is mostly medium and high carbon alloy steel. After quenching, some supercooled austenite has not been converted into martensite, remains in use state as residual austenite, which affects performance. If it is kept below zero, it will promote martensite transformation of retained austenite. Therefore, essence of cold treatment is to continue quenching. Quenching stress at room temperature and zero degrees are superimposed. When superimposed stress exceeds strength limit of material, a cold-treated crack is formed.

(1) Put mold in boiling water and cook for 30-60min before cold treatment after quenching, which can eliminate 15% -25% quenching internal stress and stabilize residual austenite, then perform conventional cold treatment at -60 ℃, or deep cooling treatment at -120 ℃. The lower temperature, the more amount of retained austenite is transformed into martensite, but it is impossible to complete transformation. Experiments show that about 2% to 5% of retained austenite is retained, and a small amount of retained austenite is retained as needed to relax stress and act as a buffer. Because retained austenite is soft and tough, it can partially absorb rapid expansion energy of martensitization and ease transformation stress;
(2) After cold treatment is completed, mold is taken out and heated in hot water, which can eliminate 40% -60% of cold treatment stress. After temperature is raised to room temperature, it should be tempered in time. Cold treatment stress is further eliminated to avoid formation of cold treatment crack, obtain stable tissue performance, ensure no distortion occurs during storage and use of mold products.

It usually occurs in process of quenching and tempering of finished product of mold. Most of microcracks formed are perpendicular to grinding direction, and depth is about 0.05-1.0mm.
(1) Improper pre-treatment of raw materials, failure to fully eliminate block, network, and band-shaped carbides of raw materials and serious decarburization;
(2) Final quenching heating temperature is too high, overheating occurs, grains are coarse, and more retained austenite is formed;
(3) Stress-induced phase transformation occurs during grinding, which transforms retained austenite into martensite, structural stress is large. In addition, due to insufficient tempering, more residual tensile stress is left, which is superimposed on the stress of grinding structure, or due to grinding speed, large amount of feed and improper cooling, grinding heat of metal surface layer sharply rises to quenching heating temperature, and then grinding fluid cools, causing secondary hardening of grinding surface layer. A variety of stresses are combined, exceeding material's strength limit, causing surface metal grinding cracks.

(1) Raw materials are reformed and forged, double cross-shaped deformation is upset forging. After four to four draws, forged fibrous structure is symmetrically distributed around cavity or axis, and last high temperature residual heat is used for quenching. Tempering at high temperature can fully eliminate block, mesh, band and chain carbides, and make carbides finer to grade 2-3;
(2) Formulate advanced heat treatment process to control final quenched residual austenite content not exceeding standard;
(3) Temper in time after quenching to eliminate quenching stress;
(4) Appropriately reducing grinding speed, grinding amount, grinding cooling rate can effectively prevent and avoid formation of grinding cracks.

Crack appeared in process of on-line cutting of quenched and tempered module. This process changed distribution of stress field on metal surface layer, middle layer and core. Residual internal stress of quenching was out of balance and deformation, and a large tensile stress appeared in a certain area. When this tensile stress is greater than strength limit of mold material, crack is caused, and crack is an arc tail-shaped rigid metamorphic layer crack. Experiments show that wire cutting process is a local high-temperature discharge and rapid cooling process, which causes metal surface layer to form a solidified layer of as-cast dendritic structure, generates a tensile stress of 600-900MPa and a high-stress secondary quenched white bright layer with a thickness of about 0.03 mm.

(1) Severe carbide segregation in raw materials;
(2) Instrument fails, quenching heating temperature is too high, grains are coarse, strength and toughness of material are reduced, and brittleness is increased;
(3) Quenched workpiece is not timely and insufficiently tempered, excessive residual internal stress and new internal stress formed during wire cutting process are superimposed to cause wire cutting cracks.

(1) Strict inspection of raw materials before they are put in storage to ensure that composition of raw materials is qualified. For unqualified raw materials, forging must be improved, carbides must be broken, chemical composition and metallographic structure can be put into production after reaching technical conditions. Finished product before module heat treatment needs to leave a certain amount of grinding after quenching, tempering, and wire cutting;

(2) Check instrument before entering furnace, select microcomputer temperature control, temperature control accuracy ±1.5℃, vacuum furnace, protective atmosphere furnace heating to prevent overheating and oxidative decarburization;
(3) Use of graded quenching, isothermal quenching, and tempering in time after quenching, multiple tempering, to fully eliminate internal stress and create conditions for wire cutting;
(4) Formulate scientific and reasonable wire cutting technology.

 

When mold is in service, micro fatigue fatigue cracks formed under repeated effects of stress slowly propagate, leading to sudden fatigue fracture.
(1) Raw materials have hairline, self-spotting, porosity, looseness, non-metallic inclusions, severe segregation of carbides, band structure, and block free ferrite metallurgical structure defects, which destroy continuity of matrix structure and form uneven stress concentration. 112 in steel ingot is not excluded, resulting in formation of white spots during rolling. There are harmful impurities such as Bi, Pb, Sn, As, and S, P in steel. P in steel is liable to cause cold brittleness, s is likely to cause hot brittleness. S, P harmful impurities are more likely to form fatigue sources;
(2) Excessive chemical infiltration layer, excessive concentration, excessively penetrated layer, too shallow hardened layer, and low hardness in transition zone can cause material's fatigue strength to decrease sharply;
(3) When die surface is rough, low precision, poor finish, knife marks, lettering, scratches, bumps, corroded matte are also likely to cause stress concentration and fatigue fracture.

(1) Strict material selection to ensure material quality, control content of low melting point impurities such as Pb, As, Sn and S, P non-metallic impurities not exceed standard;
(2) Material inspection is carried out before production, and unqualified raw materials are not put into production;
(3) Electroslag remelting and refining steel with high purity, few impurities, uniform chemical composition, fine grains, small carbides, good isotropic properties, and high fatigue to strengthen surface of mold surface by shot peening, surface chemical infiltration layer modification and strengthening treatment, so that metal surface layer is pre-stressed, which can offset tensile stress generated when mold is in service, and improve fatigue strength of mold surface;
(4) Improve precision and smoothness of mold surface processing;
(5) Improve chemical properties of chemically infiltrated and hardened layers;
(6) Microcomputer is used to control thickness, concentration and thickness of hardened layer.

This crack often occurs during use. Metal mold is cracked due to chemical reaction or electrochemical reaction process, which causes damage from surface to internal structure and corrosion, which is stress corrosion cracking. Die steel has different corrosion resistance due to different microstructures after heat treatment. The most corrosion-resistant structure is austenite (A), the most corrosion-resistant structure is bainite (T), followed by ferrite (F), martensite (M), pearlite (P), and sorbite (S). Therefore, it is not suitable to obtain T structure in heat treatment of mold steel. Although quenched steel is tempered, due to insufficient tempering, quenching internal stress still exists more or less. When mold is in service, new stress will also be generated under external force. Where stress exists in metal mold, stress corrosion cracking will occur.

(1) Mold steel should be tempered in time after quenching, fully tempered, and tempered multiple times to eliminate internal stress during quenching;
(2) Generally, tempering of mold steel after quenching is not suitable at 350-400℃, because T structure often appears at this temperature, T structure mold should be reprocessed, mold should be antirusted to improve corrosion resistance;
(3) Hot work mold is preheated at low temperature before service, cold work mold is subjected to a low temperature tempering after a period of service to eliminate stress, which can not only prevent and avoid stress corrosion cracks, but also greatly improve service life of mold, kill two birds with one stone, have significant technical and economic benefits.