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Correct Heat Treatment is Conductive to Long Service Life of Mould
Posted: 09/27/2016 10:55:41  Hits: 40
1. How to conduct heat treatment of H13 die steel can make its hardness reach 58 degrees? 
If the heat quenching or oil quenching temperature is 1050 ~ 1100℃, its hardness can meet the requirement. However, common hot work dies do not require such a high hardness, because high hardness will lead to poor performance. In general, hot work dies are  well performed and durable when the hardness is HRC46 ~ 50.

2. How clean up the surface of heat treated mould? 
Common mould will experience oil stone treatment at first, then, nitriding, oil stone treatment again, finally, polishing. This kind of process is very troublesome, no wipe up, no mirror surface. If there is a lotion can wipe up the surface of H13 mould, polishing can be conducted directly. 

1) Stainless steel acid pickling, hydrochloric acid cleaning and sand blasting are all appropriate. processing can also. Grinding will cost much, and it needs a large amount of machining, which may lead to substandard size. If the hydrochloric acid cannot wash it off, it is estimated that the material is high chromium die casting steel, D2 or H13? The oxide layer of high chromium steel is difficult to wash out. Stainless steel acid pickling liquor can do, which supplied by grinding tool shops or stainless steel stores. 

2) In addition to using stainless steel acid pickling liquor, there is another way to clean it up. Since the mould has been grinded with oil stone, the surface is relatively smooth. In fact, after crude oil stone grinding or sand belt grinding, heat treatment can be done. Then, grind it by fine oil stone. And the better solution is fiber wheel grinding first, which can effectively remove the casting skin, then, grinding and polishing. Or try to do sand blasting in 800 acre boron carbide, in this way, the casting skin may be removed, asking little effort to regrind. 

3. How heat treatment manufacturers conduct heat treatment of metal materials? 
There are much equipment in heat treatment manufacturers, including batch-type furnace, pit furnace. Box-type furnace is used most often. With these equipment, various heat treatment can be done, such as heating process of annealing, normalizing and quenching, as well as tempering these common heat treatment. 

Actually, it is a electricity heating stove. When the stove is heated up to the predetermined temperature, throw the artifacts into it, then, after heat preservation for a period of time, remove them, or cool them in the oven. Usually, pit type furnace is used as a carburizing processing equipment, which is a stove buried to the ground. Put artifacts in, seal, and then drip inside the stove some carbon-rich liquid, such as kerosene or methanol, finally, the liquid resolved into carbon atoms at high temperature permeate into the artifacts surface. 

Quendhing bath is the place for quenching, namely a pool with solution or oil, which is the cooling place of artifacts removed from box-type furnace. Generally, throw artifacts into it directly, and then take out after a period of time. There are some other equipment, such as high frequency machine, which is a device that can turn main frequency furnace of 50 HZ into a super power furnace of 200K HZ. For example, there is a common maximum power of 200 kilowatts. Then, put a coil made from a copper pipe whose interior can flux cooling water on the outside of the artifacts, typically a few millimeters artifacts, people can see the surface of artifacts become red in a few seconds to 10 seconds. When the surface temperature reach the desired value, there is a water jacket spraying quenching liquid to the surface, completing quenching process so far. 

4. Why heat treatment of Cr12 or Cr12MoV materials cracked a few times?
It is better for hardware mold to tell parts size, shape, and heat treatment requirements to those people who will take the heat treatment process of curve, otherwise it's hard to ensure. These two kinds of steel belong to a category, high carbon high chromium ledeburite steel, itself has a tendency to cold cracking. Heat treatment process is more complicated. 

In addition, there are some tips without above date: 950-1000℃, quenching, oil cooling, HRC > 58. In order to obtain the red hardness and high wear resistance, increase quenching temperature to 1115-1130℃, oil cooling. In order to reduce the deformation, thin air quenching can be also conducted in the 400-450℃ salt liquid cooling. Don't temper in 300-375℃, which can reduce the toughness of the tool, leading to temper brittleness. Besides, temper immediately after quenching. If quenching temperature is higher than 1100℃, temper 2-3 times in 520℃. Please note that the high quenching temperature will tend to decarburization, therefore, it is advised to do spheroidizing annealing heat treatment in advance before quenching, . 

5. How to distinguish between heat-treated pieces and artifacts without heat treatment? 
If workers accidentally put a work piece without heat treatment together with a batch of debugging goods that after heat treatment, how to distinguish them now? Don't look cut workpiece, that will damage the product. Heat treatment process of 30CR had through normalizing, quenching and tempering, and raw piece is the die casting piece without heat treatment. Both of them had through the blasting processing, their color cannot be distinguished any more, also, their hardness are between 35 and 45, it is indistinguishable relying on hardness. 

If hardness and oxidation color didn’t work, it is suggested that to distinguish by percussion. Metallographic structures of castingng, quenching and tempering artifacts are different and there are differences among their internal frictions. Through tapping, they are likely to be distinguished. 

6. What’s the meaning of overburning in heat treatment? 
If excess the prescribed heating temperature, the grain will grow, the mechanical properties will become bad, such as higher brittleness, less toughness, easy to deformation and cracking and so on. To control the heating temperature can avoid over burnt. That is, when the steel is heated by the temperature that above  solids-liquid line temperature, chemical composition changes have taken place in the austenite grain boundary and burning-out phenomenons occur in partial or whole grain boundaries. At this time, S, P and other compounds become enrich on grain boundary, resulting in decreased grain boundary binding force and  badly deteriorated  mechanical performance. The steel after overburning cannot be remedied by heat treatment or other processing methods. 

7. What are causes and preventive measures of quenching crack?
Causes: 
1) Serious meshy carbide segregation exist in mould materials. 
2) Machining or cold molding deformation stress. 
3) Improper heat treatment (over-quick heating or cooling, wrong medium, too low cooling temperature, too long cooling time, etc.). 
4) Complex shape, uneven thickness, sharp corner and threaded hole, etc., make the thermal stress and organization stress excessive. 
5) Too high quenching and heating temperature result in overheating or overburning. 
6) Not prompt tempering time after quenching or insufficient tempering heat preservation time. 
7) When quenching for repair, quench again without intermediate annealing. 
8) Improper grinding technology of heat treatment. 
9) During electrosparking after heating, higher tensile stress and micro crack exist in the hardening layer. 

Preventive measures: 
1) Strictly control the immanent quality of raw materials 
2)Improve the forging and spheroidizing annealing process, eliminate the mesh, belt, chain carbide, improve the evenness of spheroidized structure. 
3)The mould that through machining and cold molding deformation should be heated and quenched after stress annealing (> 600 ℃). 
4)For complex shaped mould, plug up threaded hole with asbestos, bind up dangerous section thin wall, and adopt graded quenching or isothermal quenching. 
5)Repairing or refurbishing mould requires annealing or high temperature tempering. 
6)Preheat mould before quenching heating, precool mould before cooling, and select suitable quenching medium. 
7)Strictly control the temperature and time of quenching heating, lest overheating and overburning. 
8)Take Prompt tempering after quenching, ensure sufficient heat preservation time, high alloy complex mould should be tempered 2-3 times. 
9) Select the correct grinding process and the appropriate grinding wheel. 
10) Improve electrosparking process and conduct stress tempering. 

8. How launch heat treatment of large stamping mould? Especially for trimming this kind of mold, because of burr, it cannot run normally. 
1) It is better to choose vacuum heat treatment, in order to obtain the minimum deformation. 
2) Splicing structure can be adopted and small pieces are easier to deal with. It is better to cut with slowly delivered wire, for getting high precision, high brightness, deformation. Guaranteed clearance, smaller burr. See whether your device accuracy is poor or not. 
3)In addition to the above mentioned, cutting burr may be caused by unilateral stress of punch and insufficient strength. Whether the punch is too thin? Whether the design is depended on a knife? And residual stresses after heat treatment of metal, deformation shown after the wire cut. In face of these problems, people can consider to use a larger wire cutting hole in advance for heat treatment, leaving 3 ~ 4 mm wire cut. 

9. There was a H13 steel hot extrusion die forged workpiece, brass, heat treatment in 45 ~ 48 ° diameter of 120 mm, 70 mm high, cracked after it work for hours.
1) Forging temperature is about 900 ~ 1000 ℃. Is that too high? Mould without fully preheated before use may be easy to crack. Unreasonable mold design may also be easy to crack. Increasing the tempering temperature of the mold is in order to narrow the gap and the actual forging temperature, rendering longer tempering time. 
2) This case need overall consideration. Only doing metallographic when necessary can basic judgment be made. 

10. What are causes and preventions of soft spot on mould surface?
Causes: 
1) Oxide skin, rusty spot and partial decarburization exist in the surface before heat treating the mould. 
2) After quenching heating, choosing improper cooling quenching medium results in too much impurities or aging. 
Preventive measures: 
1) Remove oxide skin and rusty spot before heat treatment. Protect mould surface appropriately when quenching heating. Try to adopt the vacuum furnace, salt bath furnace and  special atmosphere furnace for heating. 
2) Choose the appropriate cooling medium when cooling. For long-term used cooling medium, filter frequently or replace periodically. 

11. Poor organization before heat treatment 
Causes: 
1) Serious carbide segregation exist in original organization of raw steel. 
2) Poor forging process, such as forging and heating temperature are too high, small deformation, non-forging temperature is high, cooling speed after forging is slow, etc., make forging organization bulky and produce meshy, band-like and chain carbide, so it is difficult to eliminate soft spot when spheroidizing annealing. 
3) Poor spheroidizing annealing process, such as too high or too low annealing temperature, short isothermal annealing time, etc., may result in uneven spheroidizing annealing organization or bad nodulizing. 

Preventive measures: 
1) According to working conditions, production batch and strengthening and toughening performance of material itself, try to choose the steel material with good quality. 
2) Improve the forging process or use normalizing heat treatment. Eliminate mesh and chain carbide of raw materials and inhomogeneity of carbon compound. 
3)For the carbide that cannot be forged and high carbon die steel of severe segregation,  solution and refine heat treatment can be performed. 
4) Formulate correct specifications of spheroidizing annealing process for forged die blanks. quenching and tempering treatment and rapid spheroidizing annealing can be used. 
5) Charge furnace reasonably. Guarantee the uniformity of mold temperature of billet inside the furnace. 

12. The organization of mold after quenching become bulky and this will make the mold being easy to crack when using, seriously affecting the service life of the die? 
Causes: 
1) Confuse mould steel with actual steel. Quenching temperature of actual steel is far lower than that of the requested mould material (eg. Use GCr15 steel as 3 cr2w8v steel). 
2) No proper spheroidizing process before quenching results in bad spheroidized structure. 
3) Too high quenching heating temperature or too long holding time. 
4) Placing mold improperly in a furnace is prone to overheating when it near the electrode or heating element area. 
5) Choosing improper technical parameters of quenching heating for the mould whose section changes greatly is prone to overheating in thin section and sharp corners. 

Preventive measures: 
1) Test strictly before steel storage. Avoid confuse steel and misplace steel . 
2) In order to ensure good spheroidizing organization, correct forging and spheroidizing annealing should be done before quenching mould. 
3) Make correct specification of quenching heating process, strictly control the quenching heating temperature and holding time. 
4) Detection and correct thermometer table on a regular basis, guarantee the normal work of instrument. 
5) Heating mold in furnace should keep proper distance with electrodes or heating element. 

13. How to conduct heat treatment of cold mold made of Cr12MoV steel ? 
High hardness, high wear resistance and high toughness optimization: heat quenching in 980 ~ 1200 degrees and tempering in 400 degrees after oil quenching, then tempering in 240 degrees, HRC57 ~ 61, super durable blade that won't crack. 

14. Why H13 die steel cracks after heat treatment, quenching temperature is 1100 degrees, oil cooling? 
Metallographic analysis is advisable. See if there is a decarburization phenomenon existing in material surface. In general, cracking is caused by decarburization. H13 is usually used for extrusion die, which dosen’t request high hardness of the material. If use vacuum furnace, 1030 ~ 1050 ℃ is advisable. 

15.What material should be applied to guide pillar and guide sleeve? What kind of heat treatment is appropriate? What level of performance requirements should be reached? 
1) Using 45 # carbon constructional steel or carbon tool steel with quenching hardness of HRC45 degrees or so when heat treating cannot reach requirement of HRC58-62. Even if it can reach, it is very easy to break. 
2) Generally, high requirements are SKD61 or SKD11 and H13 heat treatment, quenching hardness HRC51 degrees or so. 

16. How to apply heat treatment in mould manufacturing?  
Questions added: is heat treatment done after template processing? Which is the main part? 
Heat treatment role in mould manufacturing: improve the hardness, wear resistance, so as to improve their life; Strengthen the intensity, reduce deformation, to guarantee the precision and accuracy stability of the mold.

17. Why the mold failed? 
Failure is mostly caused by fracture, wear and deformation. The main reason is improper heat treatment and mould processing. Therefore, reasonably selecting material, correctly formulating the process of heat treatment and improve the quality of heat treatment play a vital role for extending the service life of mold. Heat treatment include preliminary heat treatment and final heat treatment. The final purpose of heat treatment is to make the mold with overall performance of good surface quality, strength, plasticity and toughness. 

18. Why Cr12mov die steel after heat treatment sheds off piece by piece? 
1) When quenching, maybe over the temperature too much, overheating causes coarse grains, decarburization, bulky martensite, coarse-grain fracture section, low toughness and plasticity. 

2) High heating temperature and too long holding time cause serious decarbonization on material surface, bulky grain structure, poor adhesion strength, significantly reduced plasticity. 

Suggestion: Proofread heating equipment; Adjust quenching, tempering temperature and time; Do heat treatment process tests and necessary performance test and analysis. 

19.What’s the advantage of salt-bath heat treatment? 
Advantages: heated evenly, small deformation, less non-oxidation decarburization, heat fast, can quickly change internal group woven structure of the workpiece, good heat preservation performance and evenness, can undertake heating solid solution treatment, wide application, be able to undertake non-oxidation delivery processing, etc. 

Disadvantage: poor working conditions, has certain corrosion on workpiece, relatively short service life, larger power, smaller size. Besides, discarded salt will produce some pollution to the environment. If customers have higher request for non- oxidation, this method can be taken into consideration and its cost is general. 

20. How inspect the effect of heat treatment? 
Simply check surface hardness. Accurately check the depth of hardening layer, the core hardness, the grade of metallographic structure and the level of quenched and tempered microstructure. The value of deep hardening layer hardness = quenching hardness minimum value, X0.8. 

21. What’s the effect of subzero treatment? 
Cryogenic treatment is the extension of cooling process after quenching workpiece. Application in the mold industry mainly embodied cold-work die steel and high speed steel, bearing steel, cold-work mould and mould parts. Cryogenic treatment will change some mechanical properties and main functions as follows: improve hardness and strength, so as to guarantee the size precision, wear resistance, impact toughness, internal stress distribution, fatigue strength, corrosion resistance. 

22.Is wire cutting cracking after heat treatment due to the problem of templates or heat treatment? 
1)Heat treatment is more likely to result in cracking. It is hard to render the steel before heat treatment cracked when wire cutting. Wire cutting cracking is due to stress concentration when cutting. Therefore, when heat treating, not only need to meet the hardness requirements of heat treatment,but also have to eliminate internal stress. The key to eliminate internal stress is tempering temperature and tempering time. If the tempering temperature is too low, long tempering time also cannot eliminate internal stress; If the tempering temperature is too high, stress can be eliminated while failing to meet the requirements; If the tempering temperature is appropriate, but the tempering time is too short, stress cannot be completely eliminated. So whether wire cutting cracking will occur after the heat treatment depend on the control of tempering temperature and tempering time. 

2) This situation is more complex, inadequate tempering after quenching and defects when forging are likely to be causes. If it is a general carbon steel mold, insufficient tempering might be the main reason and people should give priority to this factor. As for high alloy mold, material itself may be defective, such as impurity, carbide segregation, etc.. These need to be determined by high power metallographic test. 

23. Why doing heat treatment of Cr12MoV will burst and crack? 
1) Cool medium too quickly (can't use brine, water, etc.) 
2) No tempering before quenching may cause excessive internal stress. 
3) Inferior metallurgy material (non-metallic inclusions, banded structure or eutectic carbide). 
4) Temperature rises too fast when quenching.
5) Failed to take prompt tempering.

24. What is non-heat treatment reinforcement? 
Surface treatment: for example, hard chrome plating can increase wear resistance of parts; 
Shot peening: used to withstand working parts under alternating stress, which can greatly improve the fatigue strength; 
Rolling: using rolling tool exert pressure on the surface of parts in a normal temperature, make   plastic deformed metal surface, modify micro geometry shape of metal surface, improve surface finish, improve fatigue strength, wear resistance and hardness of parts. 
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