24 Questions on Mold Heat Treatment

source：宏森    data：2024-05-10

1. How can H13 mold steel achieve a hardness of 58 ℃ through heat treatment?

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 performance will be poor and difficult to use. Generally, HRC46-50 has good performance and durability.

2. What is used to whiten the surface of the mold after heat treatment?

Question supplement: Generally, molds are first polished with an oilstone before being nitrided. After nitriding, the black layer needs to be wiped white with an oilstone, which is very troublesome to polish. Without wiping white, the mirror surface cannot be produced. There are various materials available, including H13 and imported ones. If there is a solution that can wash white, it can be directly polished.

(1) Stainless steel pickling solution or hydrochloric acid can be used for cleaning. Sandblasting treatment is also acceptable. The cost of grinding with a grinder is high, and the processing volume is large, which may result in substandard dimensions. If hydrochloric acid cannot wash it off, it is estimated that 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. It should be possible to use stainless steel pickling solution, which can be sold in both grinding tool stores and stainless steel stores.

(2) Do you not have stainless steel pickling paste? That kind of thing is possible. Mold steels with high chromium content, such as H13, have an oxide layer that is difficult to remove with hydrochloric acid. There is another way, since the mold has been polished with oil stones, the surface is relatively smooth. In fact, you can first use a coarse oilstone to polish, or use a sandbelt to polish, and then proceed with heat treatment. After returning, use a fine oilstone to polish it. You can also use a fiber wheel to polish first, which can effectively remove the black skin and then grind and polish. Or sandblasting, try sandblasting with 800 mesh boron carbide once, it should be able to remove the black skin without requiring too much effort to regrind.

3. How does a heat treatment plant heat treat metals?

There are a lot of equipment in heat treatment plants, and there are probably box furnaces, well furnaces, and box furnaces that are most commonly used. Many heat treatments can be carried out here, such as annealing, normalizing, quenching heating processes, tempering, and other common heat treatments.

It is actually a furnace heated by electricity. First, the furnace is heated to the predetermined temperature, then the workpiece is thrown in, waited for a period of time to reach the predetermined temperature, kept warm for a period of time, and then taken out, or cooled together in the furnace. A well type furnace is usually used as a carburizing treatment device, which is a buried underground furnace. After the workpiece is placed in the furnace, it is sealed, and then some carbon rich liquids, such as kerosene or methanol, are dropped into the furnace. At high temperature, these liquids decompose into carbon atoms and infiltrate the surface of the workpiece.

A quenching pool is a place for quenching, which is a pool containing aqueous solution or oil. It is the cooling place for quenching workpieces from a box furnace. Generally, it is directly thrown in and then taken out after a period of time. There are also some other devices, such as high-frequency machines, which are devices that can convert 50 Hz power frequency electricity into a high-power current of 200 kHz. For example, a common maximum power of 200 kW is achieved, and a coil made of copper tube with internal cooling water is placed on the outside of the workpiece. Generally, for workpieces of several tens of millimeters, you can see the surface of the workpiece turn red after a few seconds to more than ten seconds. When the surface temperature reaches the predetermined value, a water jacket rises up and sprays quenching liquid onto the surface of the workpiece to complete the quenching process. That's all for the common ones.

4. How many times have our recent Cr12 or Cr12MoV materials undergone heat treatment and cracking? Why?

It is best to inform the size, shape, and heat treatment requirements of the parts on the hardware mold, as well as the heat treatment process curve you have adopted, otherwise it is difficult to explain. These two types of steel belong to the same category, belonging to high carbon and high chromium martensitic steel, which itself has a tendency towards cold cracking. The heat treatment process is also relatively complex.

Below is my experience without the above information: quenching at 950-1000C, oil cooling, HRC>58 To achieve thermal hardening and high wear resistance, the quenching temperature is increased to 1115-1130C and oil cooled. Thin and air cooled, also cooled in salt solution at 400-450C to reduce deformation. Do not temper at 300-375C, as it will reduce the toughness of the tool and cause tempering brittleness. In addition, temper immediately after quenching. Temper 2-3 times at 520C for quenching above 1100C. Please note that excessively high quenching temperatures may have a tendency for decarburization. Therefore, pre heat treatment - spheroidizing annealing can be performed before quenching.

5. How to distinguish between heat-treated parts and workpieces without heat treatment?

Question supplement: The worker accidentally mixed an untreated workpiece with a batch of pre adjusted heat-treated workpieces. How can they be distinguished now? Do not cut the workpiece to see the metallographic structure, as it will damage the product. Urgent shipment is necessary? The heat treatment process for 30Cr involves normalizing, re quenching, and then tempering. The raw material is a casting that has not undergone heat treatment. Both have undergone shot blasting treatment and the color change cannot be distinguished, and the hardness is between 35-45, which cannot be distinguished by hardness.

If it cannot be distinguished by hardness and heat treatment oxidation color. Suggest distinguishing by tapping sound. The metallographic structure of castings and quenched and tempered workpieces is different, and there are differences in internal friction, which can be distinguished by gentle tapping.

6. What does overburning in heat treatment mean?

Exceeding the specified heating temperature can lead to grain growth, deterioration of various mechanical properties such as increased brittleness, decreased toughness, and susceptibility to deformation and cracking. Controlling the heating temperature can avoid overburning. When steel is heated above a certain temperature range in the solid-liquid phase, there is a chemical composition change in the austenite grain boundaries, and local or entire grain boundaries undergo melting. At this point, compounds such as S and P will accumulate on the grain boundaries, leading to a decrease in the bonding strength of the grain boundaries and a serious deterioration of mechanical properties. Overburned steel cannot be remedied through heat treatment or processing methods.

7. The causes and preventive measures of quenching cracks in molds?

Cause of occurrence:

1) The mold material exhibits severe network carbide segregation.

2) There is mechanical processing or cold plastic deformation stress in the mold.

3) Improper heat treatment operation of the mold (heating or cooling too fast, improper selection of quenching and cooling medium, low cooling temperature, long cooling time, etc.).

4) The complex shape, uneven thickness, sharp corners, and threaded holes of the mold result in excessive thermal and structural stress.

5) The mold quenching heating temperature is too high, resulting in overheating or overburning.

6) Delayed tempering or insufficient tempering insulation time after mold quenching.

7) When the mold is repaired and quenched, it is reheated and quenched without undergoing intermediate annealing.

8) Improper grinding process during mold heat treatment.

9) During electrical discharge machining after heat treatment of the mold, there are high tensile stresses and microcracks in the hardened layer.

Preventive measures:

1) Strictly control the intrinsic quality of mold raw materials

2) Improve forging and spheroidizing annealing processes, eliminate network, strip, and chain carbides, and improve the uniformity of spheroidized microstructure.

3) After mechanical processing or cold plastic deformation, the mold should undergo stress relief annealing (>600 ℃) before heating and quenching.

4) For molds with complex shapes, asbestos should be used to block 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 molds.

6) The mold should be preheated during quenching and heated, and pre cooled during cooling, and suitable quenching media should be selected.

7) The quenching heating temperature and time should be strictly controlled to prevent mold overheating and overburning.

8) After quenching, the mold should be tempered in a timely manner, and the insulation 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 electrical discharge machining process and perform stress relief tempering.

8. How to perform heat treatment on large stamping molds? Especially for molds like edge cutting, there are often burrs in production that cannot operate normally.

(1) Vacuum heat treatment should be chosen as much as possible for mold heat treatment to achieve the minimum deformation.

(2) The mold can adopt a splicing structure and be divided into small pieces for heat treatment. It is best to use slow wire cutting, which has high accuracy, high smoothness, and small deformation. The gap is guaranteed, and the burrs will be small. Check if your device's accuracy is very poor.

(3) Apart from the ones mentioned above, I believe that if the convex mold is subjected to force on one side, the possibility of insufficient strength is high. Is the convex mold too thin? Is it designed to rely on a knife? After heat treatment, there is residual stress in the sheet metal, and deformation may occur after wire cutting. It is possible to consider pre milling larger wire cutting holes for reheat treatment, leaving 3-4 mm wire cutting.

9. I used H13 steel to make hot extrusion molds for forging workpieces, which were made of brass with a heat treatment of 45-48 °. The mold diameter was 120mm and the height was 70mm. After working for several hours, the mold cracked?

(1) The forging temperature is approximately 900~1000 ℃? Is the temperature too high? Without sufficient preheating before use, the mold may also be prone to cracking. Unreasonable mold design may also lead to cracking. Raise the tempering temperature of the mold to narrow the gap with the actual forging temperature and achieve a longer tempering time.

(2) This needs to be comprehensively considered, and if necessary, a metallographic examination should be conducted to determine the underlying cause.

10. What are the causes and prevention measures for soft spots on the surface of the mold?

Cause of occurrence:

1) The surface of the mold has oxide skin, rust spots, and localized decarburization before heat treatment.

2) After quenching and heating the mold, improper selection of cooling and quenching medium may result in excessive impurities or aging in the quenching medium.

Preventive measures:

1) Before heat treatment of the mold, oxide skin and rust spots should be removed, and the surface of the mold should be appropriately 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, suitable cooling media should be selected, and long-term cooling media should be regularly filtered or replaced.

11. Poor organization before mold heat treatment?

Cause of occurrence:

1) The original structure of the mold steel material exhibits severe carbide segregation.

2) The forging process is poor, such as high heating temperature, small deformation, high stopping temperature, slow cooling speed after forging, etc., which results in coarse forging structure and the presence of network, strip, and chain shaped carbides, making it difficult to eliminate during spheroidization annealing.

3) Poor spheroidization annealing process, such as high or low annealing temperature, short isothermal annealing time, etc., can cause uneven or poor spheroidization microstructure during spheroidization annealing.

Preventive measures:

1) Generally, high-quality mold steel materials should be selected as much as possible based on the working conditions of the mold, production batch size, and the strength and toughness of the material itself.

2) Improve forging process or adopt normalizing preparation heat treatment to eliminate the unevenness of network and chain shaped carbides and carbides in raw materials.

3) High carbon mold steel with severe carbide segregation that cannot be forged can be subjected to solid solution refinement heat treatment.

4) Developing the correct spheroidization annealing process specifications for the forged billet can be achieved by using quenching and tempering heat treatment and rapid and uniform spheroidization annealing technology.

5) Reasonably charge the furnace to ensure the uniformity of the temperature of the mold billet inside the furnace.

12. After quenching, the structure of the mold is coarse, which will cause the mold to fracture during use, seriously affecting the service life of the mold?

Reason for occurrence:

1) The mold steel is confused, and the actual quenching temperature of the steel is much lower than the required quenching temperature of the mold material (such as treating GCr15 steel as 3Cr2W8V steel).

2) The mold steel was not subjected to the correct spheroidization treatment process before quenching, resulting in poor spheroidization structure.

3) The heating temperature for mold quenching is too high or the insulation time is too long.

4) Improper placement of molds in the furnace can easily cause overheating in areas close to electrodes or heating elements.

5) Improper selection of quenching and heating process parameters for molds with significant cross-sectional changes can result in overheating at thin sections and sharp corners.

Preventive measures:

1) Steel should be strictly inspected before storage to prevent confusion and disorderly placement of steel.

2) Before quenching the mold, correct forging and spheroidizing annealing should be carried out to ensure a good spheroidized structure.

3) Correctly formulate the quenching and heating process specifications for molds, strictly control the quenching and heating temperature and insulation time.

4) Regularly inspect and calibrate temperature measuring instruments to ensure their normal operation.

5) The mold should be kept at an appropriate distance from the electrode or heating element when heated in the furnace.

13. How to perform heat treatment when manufacturing cold molds with Cr12MoV steel?

High hardness, high wear resistance, and high toughness optimization treatment: Heating and quenching at 980-1200 ℃, oil quenching (oil) tempering at 400 ℃ once, tempering at 240 ℃ once, HRC57-61, super durable and non chipping.

14. What is the reason for the cracking of H13 mold steel after heat treatment? The quenching temperature is 1100 ℃ and it is cooled in oil?

Metallographic analysis can be conducted to see if there is any decarburization phenomenon on the surface of the material. If cracking occurs, it is generally caused by decarburization. H13 is usually used for extrusion molds, and the hardness requirements for the material are not very high. Is it recommended to use a vacuum furnace and test at 1030~1050 ℃

15. What material is usually used for the guide pillar and sleeve of the mold? What kind of heat treatment is used and what performance requirements are achieved?

(1) In mainland China, using 45 # carbon structural steel or carbon tool steel, the heat treatment quenching hardness is around HRC45, which cannot reach HRC58-62, but it is so high that it is easy to break.

(2) A high requirement is to use heat treatments such as SKD61, SKD11, and H13 to quench the hardness HRC51.

16. What are the uses of heat treatment in mold manufacturing? How to apply it?

Question supplement: Is it necessary to perform heat treatment after the template is processed, and what is the main process?

The role of heat treatment in mold manufacturing is to improve hardness and wear resistance, thereby increasing its lifespan; Strengthen strength, reduce deformation, and ensure the accuracy and stability of the mold.

17. Analysis of the causes of mold failure?

The majority of failures are caused by fracture, wear, and deformation, mainly due to improper heat treatment and poor mold processing. Therefore, the rational selection of materials, the correct formulation of heat treatment processes, and the improvement of heat treatment quality play a crucial 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 ensure that the mold has good surface quality and a reasonable combination of strength, plasticity, and toughness.

18. Why does Cr12mov mold steel fall off one by one after heat treatment?

(1) You may have exceeded the temperature too much during quenching, overheating, resulting in coarse grains, severe decarburization, coarse martensite, coarse fracture surface, low toughness and plasticity.

(2) Excessive heating temperature and insulation time result in severe decarburization of the material surface, coarse grain size, poor bonding strength, and significantly reduced plasticity.

Suggestion: Verify heating equipment; Adjust the quenching and tempering temperature and time; Conduct heat treatment process tests on the sample and conduct necessary performance testing and analysis.

19. What are the advantages of salt bath heat treatment?

Advantages: Uniform heating with small deformation, less oxidation and decarburization, fast heating, can quickly transform the internal structure of the workpiece, good insulation and heating uniformity, can be used for solid solution treatment heating, wide applicability, and can be used for almost oxidation free shipping treatment.

Disadvantages: The working environment is harsh, causing some corrosion to the workpiece, resulting in a relatively short service life. In terms of workspace size and power, the power is relatively large and the size is relatively small. Moreover, waste salt causes some environmental pollution. If customers have high requirements for non oxidation, it can be considered, and the cost is generally the case.

20. What method is used to test the effectiveness of heat treatment?

Simple inspection of surface hardness. Accurate inspection: Check the depth of the hardened layer, hardness of the core, quenching metallographic structure grade, and quenching and tempering metallographic structure grade. The critical hardness value of the hardened layer depth is the minimum quenching hardness value X0.8.

21. What are the effects of deep cooling treatment on mold materials?

Deep cooling treatment is a continuation of the cooling process of quenched workpieces, and its application in the mold industry is mainly reflected in cold work mold steel, high-speed steel, and bearing steel. There are cases of deep cooling technology application in cold work molds and mold accessories. Deep cooling will change some related mechanical properties, mainly in the following aspects: improving the hardness and strength of the workpiece, ensuring the dimensional accuracy of the workpiece, improving the wear resistance of the workpiece, enhancing the impact toughness of the workpiece, improving the internal stress distribution of the workpiece, enhancing fatigue strength, and enhancing the corrosion resistance of the workpiece.

22. Is the cracking of wire cutting after heat treatment a problem with the template or a problem with heat treatment?

(1) Some people believe it is a problem with heat treatment. Because it is difficult to crack steel without heat treatment during wire cutting, wire cutting cracking is caused by stress concentration during cutting. Therefore, in order to achieve the required hardness during heat treatment, 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, no matter how long the tempering time is, the internal stress cannot be eliminated; If the tempering temperature is too high, although stress is eliminated, it cannot meet the requirements; If the tempering temperature is appropriate, but the tempering time is too short, the stress 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 quite complex, with the possibility of insufficient tempering after mold quenching, as well as 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 consideration. For high alloy molds, there may also be defects in the material itself, such as impurities and carbide segregation, which require high magnification metallographic testing to determine.

23. What are the reasons for the cracking of Cr12MoV during heat treatment?

(1) Is the cooling medium cooling too fast (salt water, water agents, etc. cannot be used)

(2) Before quenching, it may not have been properly quenched, resulting in excessive internal stress

(3) Poor material metallurgy (non-metallic inclusions, banded structure, eutectic carbides)

(4) During quenching, the furnace temperature rises too quickly

(5) Failure to temper in a timely manner

24. What is non heat treatment strengthening?

Surface treatment: such as plating hard chromium to increase the wear resistance of the parts;

Shot peening strengthening: used for parts that work under alternating stress, which can greatly improve fatigue strength;

Rolling: Using rolling tools to apply pressure to the surface of parts at room temperature, causing plastic deformation on the metal surface, correcting the micro geometric shape of the metal surface, improving surface smoothness, enhancing the fatigue strength, wear resistance, and hardness of the parts.