24 questions and answers about mold heat treatment​

  1. How to heat treat H13 mold steel to achieve a hardness of 58°C?
  
  Heating and quenching at 1050~1100℃ and oil quenching can meet the requirements, but generally hot work molds do not require such high hardness. Such high hardness will have poor performance and be difficult to use. Generally, HRC46~50 has good performance. durable.
  
  2. What is used to clean the surface of the mold after heat treatment?
  
  Supplementary question: Generally, molds are polished with a whetstone and then nitrided. After nitriding, the black layer needs to be wiped white with a whetstone. It is very troublesome to polish it again. If it is not whitened, it will not be able to produce a mirror surface. The material is H13. Yes, there are many kinds of imported ones. If there is a potion that can whiten them, you can polish them directly.
  
  (1) It can be cleaned with stainless steel pickling solution or hydrochloric acid. Sand blasting is also available. Grinding with a grinder is expensive and requires a large amount of processing, which may result in substandard dimensions. If it cannot be washed away with hydrochloric acid, I guess you are using high-chromium mold steel? Is it D2 or H13? The oxide layer of high chromium mold steel is difficult to wash off. You should be able to use stainless steel pickling liquid, which is sold in abrasive tools stores or stainless steel stores.
  
  (2) Don’t you have stainless steel pickling paste? That kind of works. For mold steels with high chromium content such as H13, the oxide layer is difficult to wash off with hydrochloric acid. There is another way. Since the mold has been ground with a whetstone, the surface is relatively smooth. In fact, you can just use a coarse oil stone to polish it first, or sand it with an abrasive belt, and then heat treat it. When you come back, polish it with a fine oil stone. You can also use a fiber wheel to polish first, which can effectively remove the black skin, and then grind and polish. Or sandblast. Try sandblasting with 800-mesh boron carbide. It should be able to remove the black skin without much effort and re-sanding.
  
  3. How does the heat treatment plant heat treat metal?
  
  There are a lot of equipment in heat treatment plants. Furnaces include box-type furnaces and pit-type furnaces. Box-type furnaces are the most commonly used. Many heat treatments can be processed here, such as the heating processes of annealing, normalizing and quenching, and tempering. heat treatment.
  
  In fact, it is an electrically heated furnace. The furnace is first heated to a predetermined temperature, and then the workpiece is thrown in. It waits for a period of time to reach the predetermined temperature, then keeps it warm for a period of time, and then takes it out, or cools it together in the furnace. Pit-type furnaces are generally As carburizing treatment equipment, it is a furnace buried underground. After the workpiece is put in, it is sealed, and then some carbon-rich liquid, such as kerosene or methanol, is dripped into the furnace. Then at high temperature, these liquids decompose into carbon atoms and penetrate into the workpiece. surface.
  
  The quenching pool is a place for quenching. It is a pool with aqueous solution or oil in it. It is the place where the workpieces coming out of the box furnace are quenched and cooled. Generally, they are thrown in directly and then fished out after a period of time. There are also other devices, such as high-frequency machines, which are devices that can convert 50 Hz power frequency electricity into a super-powerful 200K Hz current. For example, a common higher power of 200 kilowatts is used, and then an internal circuit is used. The coil made of copper pipes for cooling water is placed outside the workpiece. Generally, for workpieces of tens of millimeters, you will see the surface of the workpiece turn red in a few seconds to more than ten seconds. When the surface temperature reaches a predetermined value, then there is a The water jacket rises and sprays quenching fluid onto the surface of the workpiece to complete the quenching process. These are the common ones.
  
  4. Our Cr12 or Cr12MoV materials have been heat treated and cracked several times recently. Why?
  
  It is better for the hardware mold maker to inform you of the size, shape and heat treatment requirements of the parts, as well as the heat treatment process curve you adopt, otherwise it will be difficult to tell. These two types of steel are of the same type, which are high-carbon, high-chromium ledeburite steels that have a tendency to cold crack. The heat treatment process is also more complex.
  
  Let me talk about my experience without the above information: 950-1000C quenching, oil cooling, HRC>58. In order to obtain hot hardness and high wear resistance, the quenching temperature is increased to 1115-1130C, oil cooling. Thin ones can be air-cooled. In order to reduce deformation, they can also be cooled with salt liquid at 400-450C. Do not temper at 300-375C, as it will reduce the toughness of the tool and cause temper brittleness. In addition, temper immediately after quenching. For quenching above 1100C, temper 2-3 times at 520C. Please note that too high quenching temperature will tend to decarburize. For this reason, pre-heat treatment - spheroidizing annealing - can be performed before quenching.
  
  5. How to distinguish between heat-treated parts and workpieces without heat treatment?
  
  Supplementary question: A worker accidentally mixed a raw part without heat treatment with a batch of debugged and heat-treated workpieces. How to distinguish them now? Do not cut the workpieces to look at the metallography. That will damage the product. Be urgent. Delivery? Heat treatment process 30Cr is normalized, then quenched, and then tempered. The raw part is a casting without heat treatment. Both have been shot blasted and cannot be distinguished by discoloration, and the hardness is between 35-45, so it is impossible to tell the difference based on hardness.
  
  If it cannot be judged by hardness and heat treatment oxidation color. It is recommended to identify by tapping the sound. The metallographic structure of castings and quenched + tempered workpieces are different, and the internal friction is different. It may be possible to distinguish by gently tapping.
  
  6. What does overburning in heat treatment mean?
  
  Exceeding the specified heating temperature will cause the grains to grow and various mechanical properties to deteriorate, such as increased brittleness, decreased toughness, easy deformation and cracking, etc. Controlling the heating temperature can avoid overburning. It means that when the steel is heated above a certain temperature within the solid-liquidus temperature range, the chemical composition of the austenite grain boundaries changes, and melting occurs locally or throughout the grain boundaries. At this time, S, P and other compounds will be enriched on the grain boundaries, resulting in a reduction in the grain boundary bonding force and a serious deterioration in mechanical properties. Overfired steel cannot be remedied by heat treatment or machining methods.
  
  7. What are the causes and preventive measures of mold quenching cracks?
  
  Causes:
  
  1) The mold material has severe network carbide segregation.
  
  2) There is mechanical processing or cold plastic deformation stress in the mold.
  
  3) Improper mold heat treatment operation (heating or cooling too fast, improper selection of quenching cooling medium, cooling temperature too low, cooling time too long, etc.).
  
  4) The mold has complex shape, uneven thickness, sharp corners and threaded holes, which causes excessive thermal stress and structural stress.
  
  5) The mold quenching heating temperature is too high, resulting in overheating or overburning.
  
  6) The mold is not tempered in time after quenching or the tempering and heat preservation time is insufficient.
  
  7) When the mold is reworked, quenched and heated, it is heated and quenched again without intermediate annealing.
  
  8) The mold is heat treated and the grinding process is improper.
  
  9) When the mold is processed by EDM after heat treatment, there will be high tensile stress and microcracks in the hardened layer.
  
  Preventive measures:
  
  1) Strictly control the inherent quality of mold raw materials
  
  2) Improve the forging and spheroidizing annealing process to eliminate network, ribbon, and chain carbides and improve the uniformity of the spheroidized structure.
  
  3) After mechanical processing or cold plastic deformation, the mold should be stress-relieved annealed (>600℃) and then heated and quenched.
  
  4) For molds with complex shapes, asbestos should be used to plug the threaded holes, wrap dangerous sections and thin-walled areas, and use graded quenching or isothermal quenching.
  
  5) Annealing or high-temperature tempering is required when repairing or refurbishing the mold.
  
  6) The mold should be preheated during quenching and heating, and precooled during cooling, and a suitable quenching medium should be selected.
  
  7) The quenching heating temperature and time should be strictly controlled to prevent the mold from overheating and overburning.
  
  8) After quenching, the mold should be tempered in time, and the heat preservation time should be sufficient. High-alloy complex molds should be tempered 2-3 times.
  
  9) Choose the correct grinding process and suitable grinding wheel.
  
  10) Improve the mold EDM process and perform stress relief and tempering.
  
  8. How to carry out heat treatment of large stamping molds? In particular, trimming molds are often produced with burrs and cannot operate normally.
  
  (1) For mold heat treatment, vacuum heat treatment should be used as much as possible to obtain the minimum deformation.
  
  (2) The mold can adopt a spliced ​​structure and be divided into small pieces for heat treatment. It is better to use slow wire cutting, which has high precision, high smoothness and small deformation. The gap is guaranteed and the burrs will be small. See if your device has poor accuracy.
  
  (3) Large trimming burrs. In addition to what the above mentioned, I think there is a high possibility that the punch is stressed on one side and is not strong enough. Is the punch too thin? Does design depend on the knife? There are also residual stresses in the sheet after heat treatment, and deformation will occur after wire cutting. You can consider milling larger wire cutting holes in advance and then heat treatment, leaving 3~4mm for wire cutting.
  
  9. I use H13 steel to make a hot extrusion die. The forged workpiece is brass and the heat treatment is 45~48°. The die diameter is 120mm and the height is 70mm. Will the die crack after working for several hours?
  
  (1) The forging temperature is approximately 900~1000℃? Is the temperature too high? Molds that are not fully preheated before use may be prone to cracking. Improper mold design may also cause cracking. Increase the tempering temperature of the mold to narrow the gap with the actual forging temperature, and the tempering will actually be longer.
  
  (2) This should be considered comprehensively. If necessary, a metallographic examination should be done to basically judge the cause.
  
  10. What are the causes and prevention of soft spots on the mold surface?
  
  Causes:
  
  1) The surface of the mold has oxide scale, rust spots and partial decarburization before heat treatment.
  
  2) After the mold is quenched and heated, the cooling and quenching medium is improperly selected, and there are too many impurities or aging in the quenching medium.
  
  Preventive measures:
  
  1) Oxide scale and rust spots should be removed before mold heat treatment, and the mold surface should be properly protected during quenching and heating. Vacuum electric furnaces, salt bath furnaces and protective atmosphere furnaces should be used as much as possible for heating.
  
  2) When cooling the mold after quenching and heating, a suitable cooling medium should be selected, and the cooling medium used for a long time should be filtered frequently or replaced regularly.
  
  11. Is the mold structure poor before heat treatment?
  
  Causes:
  
  1) There is severe carbide segregation in the original structure of the mold steel material.
  
  2) Poor forging process, such as too high forging heating temperature, small deformation, high forging stop temperature, slow cooling rate after forging, etc., makes the forging structure coarse and contains network, band and chain carbides, making the ball It is difficult to eliminate during chemical annealing.
  
  3) Poor spheroidizing annealing process, such as too high or too low annealing temperature, short isothermal annealing time, etc., can cause uneven spheroidizing annealing structure or poor spheroidization.
  
  Preventive measures:
  
  1) Generally, good quality mold steel materials should be selected according to the working conditions of the mold, production batch size and the toughening properties of the material itself.
  
  2) Improve the forging process or use normalizing preparatory heat treatment to eliminate the unevenness of network and chain carbides and carbides in the raw materials.
  
  3) Solid solution refinement heat treatment can be performed on high-carbon mold steels with severe carbide segregation that cannot be forged.
  
  4) Formulate correct spheroidizing annealing process specifications for the forged mold blank. Quenching and tempering heat treatment and rapid uniform spheroidizing annealing can be used.
  
  5) Install the furnace reasonably to ensure the uniformity of the temperature of the mold base in the furnace.
  
  12. After the mold is quenched, the structure becomes coarse, which will cause the mold to break during use, seriously affecting the service life of the mold?
  
  Reasons:
  
  1) Mold steel materials are confused, and the actual steel quenching temperature is much lower than the required quenching temperature of the mold material (for example, GCr15 steel is regarded as 3Cr2W8V steel).
  
  2) The correct spheroidization process was not carried out before the mold steel was quenched, resulting in poor spheroidization structure.
  
  3) The mold quenching heating temperature is too high or the holding time is too long.
  
  4) The mold is placed improperly in the furnace, and overheating is likely to occur near the electrode or heating element.
  
  5) For molds with large cross-section changes, improper selection of quenching heating process parameters will cause overheating in thin sections and sharp corners.
  
  Preventive measures:
  
  1) Steel materials should be strictly inspected before entering the warehouse to prevent confusion and random placement of steel materials.
  
  2) Correct forging and spheroidizing annealing should be performed before mold quenching to ensure a good spheroidizing structure.
  
  3) Correctly formulate mold quenching and heating process specifications and strictly control quenching heating temperature and holding time.
  
  4) Regularly check and calibrate temperature measuring instruments to ensure normal operation of the instruments.
  
  5) When the mold is heated in the furnace, an appropriate distance should be maintained from the electrode or heating element.
  
  13. How should the heat treatment be carried out when making cold molds made of Cr12MoV steel?
  
  High hardness, high wear resistance and high toughness. Optimized treatment: heating and quenching at 980~1200℃, oil quenching (machine oil), tempering once at 400℃, tempering once at 240℃, HRC57~61, super durable and no chipping.
  
  14. What is the reason why H13 mold steel cracks after heat treatment? The quenching temperature is 1100 ℃ and it is cooled in oil?
  
  Metallographic analysis can be performed to see if there is decarburization of the material on the surface. If it cracks, it is usually caused by decarburization. H13 is generally used for extrusion dies, and the hardness requirements of the material are not very high. Are you using a vacuum? It is recommended to test the furnace at 1030~1050℃.
  
  15. What material is usually used for the guide posts and bushings of the mold? What kind of heat treatment is used to achieve what performance requirements?
  
  (1) In mainland China, 45# carbon structural steel or carbon tool steel is used. The heat treatment and quenching hardness is about HRC45, which cannot reach HRC58~62. It is so high that it is easy to break.
  
  (2) A plate with high requirements is to use SKD61 or SKD11 and H13 and other heat treatment quenching hardness around HRC51.
  
  16. What is the purpose of heat treatment in mold manufacturing? How to apply?
  
  Supplementary question: Is heat treatment performed after the template is processed? Which is the main step?
  
  The role of heat treatment in mold manufacturing: improves hardness and wear resistance, thereby extending its life; strengthens strength, reduces deformation, and ensures the accuracy and precision stability of the mold.
  
  17. Analysis of the failure reasons of the mold?
  
  Failures are mostly caused by fracture, wear and deformation, the main reasons being improper heat treatment and poor mold processing. Therefore, rational selection of materials, correct formulation of heat treatment processes, and improvement of heat treatment quality play a key role in extending the service life of molds. Mold heat treatment includes pre-heat treatment and final heat treatment. The ultimate goal of heat treatment is to make the mold have good surface quality and a reasonable combination of strength, plasticity and toughness.
  
  18. Why does Cr12mov mold steel fall off piece by piece after heat treatment?
  
  (1) You may have exceeded the temperature too much during quenching and overheated, resulting in coarse grains, severe decarburization, coarse martensite, coarse grains at the fracture surface, and low toughness and plasticity.
  
  (2) The heating temperature is too high and the holding time is too long, resulting in serious decarburization of the material surface, coarse grains, poor bonding force, and significant reduction in plasticity.
  
  Suggestions: Calibrate the heating equipment; adjust the quenching and tempering temperature and time; conduct sample heat treatment process tests and conduct necessary performance testing and analysis.
  
  19. What are the advantages of salt bath heat treatment?
  
  Advantages: small amount of deformation when heated uniformly, less oxidation and decarburization, fast heating, can quickly transform the internal structure of the workpiece, good thermal insulation performance and uniform heating performance, can be heated by solid solution treatment, has wide applicability, and can be processed almost without oxidation Shipping processing, etc.
  
  Disadvantages: The working environment is harsh, there is some corrosion to the workpiece, the service life is relatively short, in terms of working space size and power, the power is large but the size is small. Moreover, waste salt has some pollution to the environment. If the customer has higher requirements for non-oxidation, it can be considered. The cost is average.
  
  20. What method is used to test the heat treatment effect?
  
  Simply check surface hardness. Accurate inspection: check the quenching layer depth, core hardness, quenching metallographic structure grade, and quenching and tempering metallographic structure grade. Critical hardness value of quenching layer depth = minimum quenching hardness value X0.8.
  
  21. What are the functions of cryogenic treatment of mold materials?
  
  Cryogenic treatment is a continuation of the cooling process of the workpiece after quenching. Its application in the mold industry is mainly reflected in cold work mold steel, high-speed steel, bearing steel, cold work molds and mold accessories. There are cases of cryogenic technology application. Cryogenic cooling will change some related mechanical properties. The main functions are as follows: Improve the hardness and strength of the workpiece, ensure the dimensional accuracy of the workpiece, improve the wear resistance of the workpiece, improve the impact toughness of the workpiece, improve the internal stress distribution of the workpiece, and improve the fatigue strength , improve the corrosion resistance of the workpiece.
  
  22. Is the wire cutting cracking after heat treatment a problem with the template or the heat treatment?
  
  (1) Some people think it is a problem of heat treatment. Because steel without heat treatment is difficult to crack during wire cutting. Wire cutting cracking is due to stress concentration during cutting. Therefore, during heat treatment, not only must the hardness required for heat treatment be achieved, but internal stress must also be eliminated. The key to eliminating internal stress is the tempering temperature and tempering time. If the tempering temperature is too low, the internal stress cannot be eliminated no matter how long the tempering time is; if the tempering temperature is too high, although the stress is eliminated, it cannot meet the requirements; if the tempering temperature is appropriate, but the tempering time is too short, the stress It cannot be completely eliminated. Therefore, the key to wire cutting cracking after heat treatment is the control of tempering temperature and tempering time.
  
  (2) The situation of wire cutting cracking after heat treatment is more complicated. There is the possibility of insufficient tempering after quenching of the mold, and there is also the possibility of defects during mold forging. If it is a general carbon steel mold, insufficient tempering is the main reason and needs to be given priority. For high-alloy molds, there may also be defects in the material itself, such as impurities, carbide segregation, etc., which require high-power metallographic inspection. judge.
  
  23. What are the causes of Cr12MoV bursting during heat treatment?
  
  (1) Whether the cooling medium cools too fast (cannot use salt water, aqueous agent, etc.)
  
  (2) Before quenching, it may not be annealed properly, resulting in excessive internal stress
  
  (3) The material metallurgy is not good (non-metallic inclusions, band-shaped Structure, eutectic carbide)
  
  (4) During quenching, the furnace temperature rises too fast
  
  (5) Failure to temper in time
  
  24. What is non-heat treatment strengthening?
  
  Surface treatment: such as hard chromium plating to increase the wear resistance of parts;
  
  shot peening: used for parts working under alternating stress, which can greatly improve fatigue strength;
  
  rolling: using rolling tools to treat the surface of parts at room temperature Apply pressure to cause plastic deformation on the metal surface, modify the micro-geometry of the metal surface, improve surface finish, and improve the fatigue strength, wear resistance and hardness of the parts.