Plastic Injection Mold Heat Treatment Process and FAQs

This Plastic Injection Mold Heat Treatment Process and FAQs article talks about plastic injection mold materials selections, plastic injection mold heat treatment features, plastic injection mold heat treatment FAQs, mold heat treatment troubleshooting, etc.

How to choose plastic injection mold materials?

Plastic injection mold structure and shape are relatively complex, and the manufacturing cost is high, in order to ensure that the mold has a long service life, a reasonable selection of mold material varieties, and the correct choice and implementation of mold heat treatment methods are extremely important.

1. Plastic injection mold forming parts of the material selection

When choosing the material of plastic injection mold forming parts, according to the specific precision of the mold can be considered from the following aspects.

(1) The type of plastic products and quality requirements of the choice of plastic products for different kinds often have a significant impact on the working conditions of the plastic injection mold.

ABS, polyvinyl chloride and polytetramethylene plastic injection molding, in a certain forming temperature, will decompose and produce corrosive gas and plastic injection mold corrosion candle effect. Therefore, this kind of plastic injection mold forming parts often use corrosion-resistant plastic injection mold steel, such as PCR, AFC a 77, 18Ni and 4Cr13, etc.; If you choose ordinary materials to make the mold, it is necessary to plate chromium or other corrosion-resistant surface buried.

For the production of glass fibre as an additive to the injection mold of thermoplastic material products or thermosetting plastic products compression mold, the mold requires high hardness, high wear resistance, high compressive strength and high toughness, in order to prevent premature wear of the mold cavity surface or by high pressure and local deformation.

Therefore, this kind of plastic injection mold forming parts mostly uses hardened plastic injection mold steel. Such as T10A, 9Mn2V, CrWMn, Cr12MoV, etc.;

If you choose low and medium carbon steel, it should be carburized quenching treatment for high-speed forming of plastic products, the mold surface working temperature in a short period of time will exceed 400C.

In order to ensure the accuracy of the plastic injection mold and prevent the plastic parts from being deformed after demolding due to the high temperature. The mold material should have good thermal conductivity.

For this reason, beryllium bronze or high-strength aluminum alloy can be used to manufacture. In transparent plastic molding mold, the mold material is required to have good mirror polishing performance and high wear resistance. The material is required to have very few non-metallic inclusions and porosity, and the microstructure should be uniform.

This is the best choice to obtain the high hardness of ultra-pure steel. Practice shows that most age-hardening mold steel such as PMS, 06Ni, and PCR have excellent mirror polishability, is the more ideal choice of steel, and pre-hardening steel P20 series, 8Cr2S, 5NiSCa and other steel, mirror polishability medium or better, can also be used. For plastic products with decorative patterns on the surface, the mold material is required to have good decorative processability.

(2) Plastic parts production batch is very small, and the mold wear resistance and service life requirements are not high, can choose aluminium alloy, zinc alloy, carbon steel and alloy structural steel, etc. manufacturing; for the batch of plastic parts forming mold, should be based on its working conditions and the requirements of the mold quality to select materials, generally use senior quality plastic injection mold steel.

(3) Plastic injection mold processing methods plastic injection mold forming processing methods are various; general plastic injection mold can be made by cold extrusion forming, superplastic forming, casting forming, cutting creating and electrical processing creating methods.

Therefore, should be used as far as possible with the mold forming and processing methods, and has a good processing process performance of the mold material. As most plastic injection mold are required to go through the cutting process forming, thus the cutting process of the mold material is often the most important processing process performance.

(4) The size and precision requirements of plastic parts for large high-precision injection molding molds, when the production of plastic parts in large quantities, can be used in pre-hardened steel manufacturing. Such as 3Gr2Mo, 8Cr2S, 4Cr5MoSiV, P4410, SMI, PMS steel, etc.; because the mold processing and forming no longer heat treatment, so you can keep the high-precision requirements of the mold.

(5) The mold manufacturing difficulty and delivery period of the plastic companion’s more complex shape, the more difficult the processing of the mold cavity, and therefore must use good processing performance and heat treatment deformation of small mold materials. The larger the plastic parts, the cutting volume of the cavity is generally larger; a significant amount of cutting, and cutting force is also large. Therefore, for significant parts of the plastic mold is best to use easy-to-cut steel.

Plastic parts are small, mold cavity volume is small, and cutting volume and cutting force are usually smaller. So small mold often can choose pre-hardening mold steel manufacturing.

Users sometimes require plastic mold delivery as short as possible, the use of casting, especially the precision casting method of manufacturing plastic mold is a good way to shorten the processing cycle. At this time often need to choose especially suitable for precision casting molding aluminium alloy or zinc alloy. For dry steel plastic mold, the choice of easy-to-cut pre-hardening steel, can also greatly shorten the mold cycle.

2. Plastic injection mold auxiliary parts of the material selection

Plastic injection mold auxiliary rib parts, because of their polishability, corrosion resistance, etc. to art is low, so you can choose the commonly used plastic mold steel, after reasonable mold heat treatment, the use of performance can fully meet the requirements, so reduce the cost of the mold.

Choose different varieties of steel for plastic injection mold, its chemical composition and mechanical properties are different, so the manufacturing process route is different; Similarly, different types of plastic mold steel using mold heat treatment process are also different.

This section mainly introduces the manufacturing process and the characteristics of the plastic injection mold heat treatment process.

Manufacturing plastic injection mold heat treatment process

1).Low carbon steel and low carbon alloy steel mold heat treatment process

For example, 20, 20Cr, 20CrMnTi and other steel mold heat treatment process is:

Feeding → forging die billet → annealing → mechanical rough machining → cold extrusion forming → recrystallization annealing → mechanical finishing → carburizing → quenching, tempering → grinding and polishing → assembly.

2). High-alloy carburizing steel mold heat treatment process

For example, 12CrNi3A, 12CrNi4A steel mold heat treatment process is:

Feeding → forging die billet → normalizing and high temperature tempering → mechanical rough machining → high temperature tempering → finishing → carburizing → quenching, tempering → grinding and polishing → assembly.

3). Quenched steel mold heat treatment process

For example, 45, 40Cr and other steel mold heat treatment process is:

Feeding → forging die billet → annealing → mechanical roughing → tempering → mechanical finishing → trimming, polishing → assembly.

4). Carbon tool steel and alloy tool steel mold heat treatment process

For example, T7A ~ T10A, CrWMn, 9SiCr and other steel mold heat treatment process is:

Feeding → forging into the mold billet → spheroid annealing → mechanical roughing → stress relief annealing → mechanical semi-finishing → mechanical finishing → quenching, tempering → grinding and polishing → assembly.

5). Pre-hardened steel mold heat treatment process

For example, 5NiSiCa, 3Cr2Mo (P20) and other steels. For the direct use of bar processing, because the supply state has been pre-hardening treatment, can be directly processed after forming polishing, and assembly.

For the re-forging into a billet and then processed and formed, the mold heat treatment process is:

Feeding → re-forging → spheroidizing and annealing → planing or milling six sides → pre-hardening treatment (34 ~ 42HRC) → mechanical roughing → stress relief annealing → mechanical finishing → polishing → assembly.

Mold heat treatment of plastic injection molds basic requirements

(1) Suitable mold heat treatment working hardness and sufficient toughness

According to the working conditions of plastic injection molds, the molds should obtain moderate hardness and sufficient toughness after mold heat treatment.

(2) Ensure quenching of small deformation

In order to make the plastic injection mold meet the accuracy requirements, to ensure that the mold heat treatment deformation is very small. Quenching is the first loss to consider to prevent mold cavity warpage deformation, for this reason, the amount of deformation is made within a certain limit.

(3) The plastic injection mold surface is free of defects and easy to polish

Plastic model cavity surface finish requirements are high, in the heat treatment process, special attention should be paid to protecting the surface of the cavity, strictly preventing the surface of various defects. Such as heating and quenching traces of oxidation left behind, the surface being eroded, decarburization or carbonization, an excessive amount of residual austenite, etc., otherwise, it will cause difficulties in the next polishing process, and even Yuanfa polishing.

(4) Ensure the plastic injection mold strength requirements

Especially for thermosetting plastic injection mold, it is heavily loaded and subjected to heat and periodic pressure for a long time. Therefore, the mold is required to ensure a high enough resistance to collapse and wrinkle after heat treatment, i.e. to ensure the strength requirements.

What are the characteristics of plastic injection mold heat treatment?

1. Carburized steel plastic injection mold heat treatment characteristics

1). For plastic injection molds with high hardness, high wear resistance and high toughness requirements, carburized steel should be used to manufacture, and carburizing, quenching and low-temperature tempering as the final heat treatment.

2). Carburizing layer requirements, the general carburizing layer thickness of 0.8 ~ 1.5mm, when pressed with hard filler plastic injection mold carburizing layer thickness requirements for 1.3 ~ 1.5mm, pressed soft plastic carburizing layer thickness of 0.8 ~ 1.2mm. carburizing layer carbon content of 0.7% ~ 1.0% is good. If carbon and nitrogen carburizing are used, the wear resistance, corrosion resistance, oxidation resistance and anti-stickiness will be better.

3). The carburizing temperature is generally 900~920℃, and 840~860℃ medium temperature carbon and nitrogen carburizing are recommended for small molds with complex cavities. The carburizing holding time is 5~10h, which should be selected according to the requirement of carburizing layer thickness. Carburizing process to use graded carburizing process is appropriate, that is, the high-temperature stage (900~920 ℃) to quickly carburize into the surface layer of the parts; the medium-temperature stage (820~840℃) to increase the thickness of the carburizing layer, so as to establish a uniform and reasonable carbon concentration gradient distribution in the carburizing layer, to facilitate direct quenching.

4). Carburizing quenching process according to different steel after carburizing can be used respectively: reheating quenching; direct quenching after graded carburizing (such as alloy carburizing steel); direct quenching after medium-temperature carbonitriding (such as industrial pure iron or low carbon steel cold extrusion molding of small precision molds); air-cooled quenching after carburizing (such as high-alloy carburizing steel manufacturing of large and medium-sized molds).

2. Hardened steel plastic injection mold heat treatment characteristics

1). The shape of the more complex mold, after rough machining that heat treatment, and then finish machining, in order to ensure that the heat treatment deformation is minimal, for precision molds, deformation should be less than 0.05%.

2). Plastic injection mold cavity surface requirements are very strict, so in the quenching and heating process ensure that the cavity surface does not oxidize, decarburization, erosion, not overheat, etc. Should be heated in a protective atmosphere furnace or in a salt bath furnace after strict deoxidation, if the use of ordinary box resistance furnace heating, should be coated with the protective agent on the surface of the mold cavity while controlling the heating rate, cooling should choose a more moderate cooling medium, control the cooling rate to avoid deformation, cracking and scrap in the quenching process. Generally, hot bath quenching is preferred, but also pre-cooling quenching can be used.

3). After quenching should be tempered in time, the tempering temperature should be higher than the working temperature of the mold, tempering time should be sufficient, and the length depends on the mold material and section size, but at least 40 ~ 60min above.

3. Pre-hardened steel plastic injection mold heat treatment characteristics

1). Pre-hardened steel is supplied in the pre-hardened state, and generally does not need heat treatment, but sometimes needs to be re-forged, re-forged die billet must be heat treated.

2). The pre-hardened steel pre-treatment usually adopts spheroidal annealing, the purpose is to eliminate forging stress, obtain uniform spherical pearlite organization, reduce hardness, increase plasticity, and improve the cutting performance or cold extrusion forming performance of the die billet.

3). The pre-hardening process of pre-hardened steel is simple, most of them are tempered, and after tempering, tempered sothierite is obtained. High-temperature tempering temperature range is wide enough to meet the various working hardness requirements of the mold. Due to the good hardenability of this type of steel, quenching can be oil-cooled, air-cooled or nitrate-salt-graded quenching.

The following table shows the pre-hardening process of some pre-hardened steels for reference.

Part of the pre-hardening process of pre-hardening steel

4. Age-hardened steel plastic injection mold heat treatment characteristics

1). Age-hardening steel heat treatment process in two basic steps. The first solid solution treatment, that is, the steel is heated to high temperatures so that a variety of alloying elements dissolved in austenite, and austenite quenching to obtain martensite organization after completion. The second step for ageing treatment is the use of ageing strengthening to achieve the final required mechanical properties.

2). Solution treatment heating is generally in the salt bath furnace or box furnace, heating times are desirable: 1min/mm, 2 ~ 2.5min/mm, quenching using oil cooling, quenching good steel can also be air-cooled. If the forging dies billet can accurately control the final forging temperature, forging can be done directly after the solution quenching.

3). Ageing treatment is best carried out in a vacuum furnace, if the box furnace, in order to prevent oxidation of the die cavity surface, the furnace must be fed into the protective atmosphere, or with aluminium oxide powder, graphite powder, cast iron chips, in the box to protect the conditions of ageing. Box protection heating to properly extend the holding time, otherwise, it is difficult to achieve the ageing effect.

Part of the age-hardening plastic mold steel heat treatment specifications can be referred to the following:

Surface treatment of plastic injection molds

In order to improve the wear resistance and corrosion resistance of the plastic injection mold surface, it is often subjected to appropriate surface treatment.

Chrome plating

Chrome plating of plastic injection mold is one of the most applied surface treatment methods. The chrome plating layer has strong passivation ability in the atmosphere and can keep the metallic lustre for a long time without chemical reaction in many kinds of acidic media.

The hardness of the plating layer reaches 1000 HV, thus having excellent wear resistance. The chrome plating also has high heat resistance, and its appearance and hardness do not change significantly when heated to 500°C in air.

Nitriding

Nitriding has the advantages of low processing temperature (generally 550~570℃), little deformation of the mold and high hardness of the layer (up to 1000~1200HV), so it is also very suitable for the surface treatment of plastic injection mold.

Steel containing chromium, molybdenum, aluminium, vanadium and titanium alloy elements have better nitriding performance than carbon steel, and nitriding treatment can greatly improve wear resistance when used as plastic injection molds.

Other surface treatment methods suitable for plastic injection mold are nitriding and carbonitriding, chemical nickel plating, ion plating of titanium nitride, titanium carbide or titanium nitride, PVD, CVD method of deposition of a hard film or super hard film, etc.

FAQs of common cracks in mold heat treatment and troubleshooting

In the heat treatment of die steel, quenching is a common process. However, for various reasons, quenching cracks are sometimes inevitable, resulting in the loss of previous work. Analysis of the causes of cracks, and then taking appropriate preventive measures, has significant technical and economic benefits. Common quenching cracks are the following 10 types.

1. Longitudinal cracking in mold heat treatment

Cracks are axial, thin and long in shape. When the mold is completely quenched that is, no heart quenching, the heart is transformed into the maximum capacity of quenching martensite, resulting in tangential tensile stress, the higher the carbon content of the mold steel, the greater the tangential tensile stress, when the tensile stress is greater than the strength limit of the steel leads to the formation of longitudinal cracks.

The following factors aggravate the longitudinal cracking in mold heat treatment:

(1) Mold steel contains more S, P, Sb, Bi, Pb, Sn, As and other low melting point harmful impurities, ingot rolling along the rolling direction is a longitudinal distribution of severe segregation, easy to produce stress concentration to form longitudinal quenching cracks or the longitudinal cracks formed after rolling of raw materials fast cooling is not processed off in the product resulting in the final quenching cracks expand to form longitudinal cracks;

(2) Mold size in the steel quenching crack sensitive size range (carbon tool steel quenching crack the dangerous size of 8-15mm, low alloy steel dangerous size of 25-40mm) or the choice of quenching cooling medium greatly exceeds the critical quenching cooling rate of the steel are prone to the formation of longitudinal cracks.

Preventive measures:

(1) Strict inspection of raw materials in the warehouse, the harmful impurity content exceeds the standard steel is not put into production;

(2) Try to use vacuum smelting, furnace refining or electro-slag remelting mold steel;

(3) Improve the heat treatment process, the use of vacuum heating, protective atmosphere heating and full deoxidation salt bath heating and graded quenching, and isothermal quenching;

(4) Change the centerless quenching for the centre quenching that is not completely quenched, to obtain high toughness of the lower bainite organization and other measures to significantly reduce the tensile stress, which can effectively avoid mold longitudinal cracking and quenching distortion.

2. Transverse cracking in mold heat treatment

Cracking characteristics are perpendicular to the axial direction. In unhardened dies, large tensile stress peaks exist in the transition part between the hardened and unhardened areas, and large dies are prone to form large tensile stress peaks during rapid cooling, resulting in transverse cracks due to the formation of axial stresses greater than tangential stresses.

Transverse segregation of S, P.Sb, Bi, Pb, Sn, As and other low melting point harmful impurities in the forging module or the existence of transverse microscopic cracks in the module, and the formation of transverse cracks by extension after quenching.

Preventive measures:

(1) Module should be reasonably forged, the ratio of the length of raw materials to the diameter of the forging ratio is best selected between 2-3, forging using double cross-shaped variable forging, forging by five upsetting five drawing multi-fire, so that the steel flat carbide and impurities are small, uniform distribution in the steel matrix, forging fibre organization around the cavity without directional distribution, substantially improve the transverse mechanical properties of the module, reduce and eliminate stress sources;

(2) Choose the ideal cooling rate and cooling medium: in the steel Ms point above the fast cooling, greater than the steel critical quenching cooling rate, steel subcooling austenite stress for thermal stress, the surface layer for compressive stress, the inner layer for tensile stress, offset each other, effectively prevent the formation of thermal stress cracks, in the steel between Ms-Mf slow cooling, significantly reduce the formation of hardened martensite tissue stress.

When the sum of thermal stress and corresponding stress in steel is positive (tensile stress), it is easy to quench and crack. When the negative, is not easy to quench and crack, make full use of the thermal stress, reduce the phase change stress, and control the total stress is negative, which can effectively avoid the occurrence of transverse quenching crack.

CL-1 organic quenching medium is the ideal quenching agent while reducing and avoiding quenching mold distortion, but also controlling the reasonable distribution of the hardening layer. Adjusting different concentration ratios of CL-1 quenching agents can obtain different cooling rates and obtain the required hardening layer distribution to meet the needs of different mold steels.

3. Arc cracking in mold heat treatment

Often occur in the mold angle corner, notch, hole, concave mold wiring flying edge and other shape abrupt changes. This is because the stress generated at the angles during quenching is 10 times the average stress on the smooth surface.

In addition:

(1) The amount of carbon (C) in steel and the higher the content of alloying elements, the lower the steel Ms point, Ms point to reduce 2 ℃, the quenching and cracking tendency increased by 1.2 times, Ms point to reduce 8C, quenching and cracking tendency is increased by 8 times;

(2) Different tissue transformation in steel and the same tissue transformation is not simultaneous, due to the different tissue ratio difference, resulting in huge tissue stress, resulting in the formation of arc cracks at the tissue junction;

(3) After quenching and tempering are not timely, or tempering is not sufficient, the residual austenite in the steel is not fully transformed, retained in the use of state, promote stress redistribution, or mold service residual austenite martensite phase change to generate new internal stress, when the integrated stress is greater than the strength limit of the steel will form an arc crack;

(4) With the second type of tempering brittle steel, high temperature tempering slow cooling after quenching, resulting in steel P, s and other harmful impurity compounds along the grain boundary precipitation, greatly reducing the grain boundary bonding and toughness, increasing brittleness, service in the role of external forces formed under the arc crack.

Preventive measures:

(1) Improve the design, try to make the shape symmetrical, reduce the shape of the abrupt change, increase the process hole and reinforcement, or use a combination of assembly;

(2) Rounded corners instead of right angles and sharp edges, through the hole instead of blind holes, improve processing accuracy and surface finish, reduce the source of stress concentration, for the inability to avoid right angles, sharp edges, blind holes and other general hardness requirements are not high, available wire, asbestos rope, refractory mud, etc. for wrapping or filling, artificial cooling barrier, so that the slow cooling quenching, to avoid stress concentration, to prevent quenching arc-shaped crack formation in mold heat treatment;

(3) Quenched steel should be tempered in time to eliminate part of the quenching stress, to prevent the expansion of quenching stress;

(4) A longer tempering time to improve the mold fracture toughness value;

(5) Full tempering, to get stable tissue properties;

(6) Multiple tempering so that the residual austenite transformation is adequate and eliminates new stresses;

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

(8) For the second type of tempering brittleness mold steel after high-temperature tempering should be fast cooling (water or oil cooling), which can eliminate the second type of tempering brittleness, to prevent and avoid quenching arc crack formation.

4. Peel Cracking in mold heat treatment

Die service in the stress, quenching hardening layer a block from the steel matrix peel. Because of the mold surface organization and the heart of the organization of different specific capacities, quenching the surface layer to form axial, tangential quenching stress, radial tensile stress, and to the internal mutation, the stress in the sharp change in the narrow range of peel cracks.

This often occurs in the cooling process of chemical mold heat treatment by the surface layer, because the surface layer of chemical modification and steel matrix phase change is not simultaneously caused by the inner and outer quenching martensite expansion is not simultaneous, resulting in large phase change stress, resulting in chemical treatment seepage layer from the matrix tissue peeling.

Such as flame surface hardening layer, high-frequency surface hardening layer, carburizing layer, carbonitriding layer, nitriding layer, boron penetration layer, metal penetration layer, etc.

Chemical seepage layer after quenching should not be fast tempering, especially below 300℃ low temperature tempering fast heating, will promote the formation of tensile stress in the surface layer, while the steel matrix heart and the formation of the transition layer compressive stress, when the tensile stress is greater than the compressive stress, resulting in chemical seepage layer is pulled off.

Preventive measures:

(1) Should make the mold steel chemical seepage layer concentration and hardness from the surface to gently reduce, and enhance the seepage layer and the substrate bonding force, seepage after diffusion treatment can make the chemical seepage layer and the substrate transition uniform;

(2) Diffusion annealing, spheroid annealing, and tempering treatment before chemical treatment of mold steel, fully refining the original organization, and can effectively prevent and avoid stripping cracks to ensure product quality.

5. Reticulated cracking in mold heat treatment

Crack depth is shallow, generally, about 0.01-1.5mm deep, radial, and alias cracking.

The main reasons are:

(1) The raw material has a deep decarburization layer, which is not removed by the cold cutting process, or the finished mold is heated in an oxidation atmosphere furnace causing oxidation decarburization;

(2) Mold decarburization surface metal organization and steel matrix martensite carbon content is different, different specific capacity, steel decarburization surface quenching large tensile stress, therefore, the surface metal is often cracked along the grain boundaries into a network;

(3) The raw material is coarse grain steel, the original organization is coarse, the presence of large blocks of ferrite, conventional quenching can not be eliminated, retained in the quenching organization, or inaccurate temperature control, instrumentation failure, the occurrence of tissue overheating, or even overburning, grain coarsening, loss of grain boundary bonding, mold quenching and cooling steel carbide precipitation along the austenite grain boundary, grain boundary strength greatly reduced, poor toughness, brittleness, under the action of tensile stress along the grain boundary Cracked in a mesh.

Preventive measures:

(1) Strict chemical composition of raw materials. Metallographic organization and flaw detection, unqualified raw materials and coarse-grain steel should not be used as mold materials;

(2) The use of fine-grain steel, vacuum electric furnace steel, review the depth of decarburization layer of raw materials before production, cold cutting machining allowance must be greater than decarburization layer depth;

(3) Develop an advanced and reasonable heat treatment process, choose a microcomputer temperature control instrument, with a control accuracy of ±1.5C, and a regular on-site calibration instrument;

(4) The final treatment of mold products is selected from a vacuum electric furnace, a protective atmosphere furnace and the full deoxidation salt bath furnace heating mold products and other measures to effectively prevent and avoid reticulation.

(4) The final treatment of the mold products is carried out in a vacuum electric furnace, protective atmosphere furnace and salt bath furnace with sufficient deoxidation to prevent and avoid the formation of mesh cracks.

6. Cold treatment cracking in mold heat treatment

Die steel is mostly medium and high carbon alloy steel, after quenching, there is still part of the subcooling austenite that has not turned into martensite, retained in the use of the state to become residual austenite, affecting the use of performance.

If placed below zero degrees to continue cooling, can prompt the residual austenite martensite transformation, therefore, the essence of cold treatment is quenching continues.

Room temperature quenching stress and zero degrees quenching stress superimposed, when the superimposed back stress exceeds the strength limit of the material will form a cold treatment crack.

Preventive measures:

(1) Quenching and cold treatment before the mold will be placed in boiling water for 30-60min, which can eliminate 15%-25% quenching stress and stabilize the residual austenite, and then -60 ℃ conventional cold treatment, or -120 ℃ deep cold treatment, the lower the temperature, the more residual austenite into martensite amount, but it is not possible to transform all finished, experiments show that about 2%-5% residual austenite retained Down, as needed to retain a small amount of residual austenite can loosen the stress, play a buffering role, because the residual austenite and soft and tough, can partially absorb the rapid expansion of martensitic energy, and ease the phase change stress;

(2) After cold treatment, remove the mold and put it into hot water to warm up, which can eliminate 40%-60% of the cold treatment stress. After warming up to room temperature, the cold treatment stress should be tempered in time to further eliminate the cold treatment stress, avoid the formation of cold treatment cracks, obtain stable tissue properties, and ensure that the mold products are not distorted during storage and use.

7. Grinding Cracking in mold heat treatment

Most of the micro-cracks formed are perpendicular to the grinding direction and about 0.05-1.0mm deep.

(1) Improper pretreatment of raw materials, failure to adequately eliminate lumpy, reticulated, banded carbides and serious decarburization of raw materials;

(2) The final quenching heating temperature is too high, overheating occurs, the grain is coarse, and more residual austenite is generated;

(3) Stress-induced phase transformation occurs during grinding, resulting in the transformation of residual austenite into martensite and large tissue stresses, coupled with insufficient tempering, leaving more residual tensile stresses, which superimpose on the grinding tissue stresses, or the grinding speed, large tool feed and improper cooling resulting in the grinding heat of the metal surface layer rising sharply to the quenching heating temperature, followed by the cooling of the grinding fluid, resulting in the secondary quenching of the grinding surface layer and the combination of multiple stresses, which exceeds the combination of multiple stresses exceeds the strength limit of the material, causing surface metal grinding cracks.

Preventive measures:

(1) Reforging of the raw material, multiple double-cross shaped variable upsetting and forging, four upsetting and four pulling, so that the forging fibre organization around the cavity or axis is wavy and symmetrical distribution, and the use of the last fire high-temperature residual heat for quenching, followed by high-temperature tempering, can fully eliminate the block, mesh, strip and chain carbide so that the carbide refinement to 2-3 levels;

(2) The development of an advanced heat treatment process to control the final quenching residual austenite content does not exceed the standard;

(3) Timely tempering and quenching stress elimination after quenching;

(4) Appropriately reduce the grinding speed, grinding volume and grinding cooling speed, which can effectively prevent and avoid the formation of grinding cracks.

8. Wire-cutting cracks in mold heat treatment

The crack appears in the quenched and tempered module during the wire-cutting process, which changes the distribution of the stress field in the surface, middle and heart of the metal, and the residual stress of quenching loses balance and deformation, and a large tensile stress appears in a certain area, which leads to cracking when the strength limit of the mold material is large.

Experiments show that the wire cutting process is a local high-temperature discharge and rapid cooling process, so that the metal surface layer forms a dendritic cast solidification layer, resulting in 600-900MPa tensile stress and about 0.03mm thick high-stress secondary quenching white bright layer.

Crack generation causes:

(1) Severe carbide segregation in the raw material;

(2) Instrumentation failure, quenching heating temperature is too high, coarse grains, reducing the material toughness, increasing brittleness;

(3) Quenched workpiece is not tempered in time and tempering is not sufficient, there is too much residual internal stress and the wire-cutting process formed during the superposition of new internal stress leads to wire-cutting cracks.

Preventive measures:

(1) Strict inspection of raw materials before warehousing, to ensure that the raw material tissue composition is qualified, the unqualified raw materials must be re-forging, breaking carbide, so that the chemical composition, metallurgical organization and other technical conditions before production. Module heat treatment before processing finished products needs to leave a certain amount of grinding after quenching. Tempering, wire cutting;

(2) Into the furnace before the calibration instrument, the choice of microcomputer temperature control, temperature control accuracy ± 1.5, vacuum furnace, protective atmosphere furnace heating, strictly prevent overheating and oxidation decarburization;

(3) The use of graded quenching, isothermal quenching and timely tempering after quenching, multiple tempering, and full elimination of internal stress, to create conditions for wire cutting;

(4) Develop a scientific and reasonable wire-cutting process.

9. Fatigue fracture in mold heat treatment

Micro fatigue cracks formed under the repeated action of alternating stress during the service of the mold expand slowly, leading to sudden fatigue fracture.

(1) The existence of hairline, self-points, porosity, sparseness, non-metallic inclusions, severe carbide segregation, ribbon organization, massive free ferrite metallurgical organization defects in the raw material, which destroys the continuity of the matrix organization and forms uneven stress concentration. In the ingot 112

Not excluded, resulting in the formation of white spots during rolling. Steel in the presence of Sb, Bi, Pb, Sn, As and S, P and other harmful impurities, steel P easy to causes cold embrittlement, and S easy to causes hot embrittlement, S, P harmful impurities exceed the standard are easily form fatigue sources;

(2) Chemical seepage layer is too thick, too large concentration, excessive seepage layer, too shallow hardening layer, or low hardness of the transition zone can lead to a sharp reduction in material fatigue strength;

(3) When the mold surface processing is rough, with low precision, poor finish, as well as knife pattern, engraving, scratches, bruises, corrosion pockmarks, etc. also easy to causes stress concentration leading to fatigue fracture.

Preventive measures:

(1) Strict selection of materials to ensure that the material, control Pb, As, Sn and other low melting point impurities and S, P non-metallic impurities content does not exceed the standard;

(2) Material inspection before production, unqualified raw materials are not put into production;

(3) Use high purity, fewer impurities, uniform chemical composition, and fine grain. Carbide small, good isotropic properties, fatigue strength and other characteristics of electro slag remelting refining steel, the surface of the mold surface shot peening and surface chemical seepage modified strengthening treatment, so that the metal surface for the pre-pressure stress, offset the tensile stress generated when the mold service, improve the fatigue strength of the mold surface;

(4) Improve the processing accuracy and finish of the mold surface;

(5) Improve the chemical infiltration layer and hardening layer tissue properties;

(6) Adopt a microcomputer to control the thickness, concentration and hardening layer thickness of the chemical infiltration layer.

10. Stress corrosion cracking in mold heat treatment

This crack often occurs in the process of use. Metal mold due to chemical reaction or electrochemical reaction process, caused by the surface to the internal tissue structure damage corrosion effect and produce cracking, which is stress corrosion cracking.

Mold steel due to mold heat treatment after the organization is different, and corrosion resistance is also different. The most corrosion-resistant organization is austenite (A), and the most corrosion-prone organization is brittleness (T), in order to ferrite (F) martensite (M) pearlite (P) a soxhlet (S). Therefore, mold steel heat treatment is not suitable to get T organization.

Although the quenched steel is tempered, because of the tempering is not sufficient, quenching internal stress more or less still exists, mold service under the action of external forces will also generate new stress, where there is stress in the metal mold will have stress corrosion cracking occurs.

Preventive measures:

(1) Mold steel quenching should be promptly tempered, fully tempered, and tempered several times to eliminate the quenching internal stress;

(2) Mold steel after quenching generally should not be tempered at 350-400℃, because the T organization is often in this temperature, the occurrence of T organization mold should be reprocessed, and the mold should be rust-resistant treatment to improve corrosion resistance;

(3) Hot mold service before the low-temperature preheating, and cold mold service after a stage of low-temperature tempering stress relief, not only prevent and avoid stress corrosion cracking but also can significantly improve the service life of the mold, a double benefit, there are significant technical and economic benefits.