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FAQs for Mold Heat Treatment

How can the hardness of H13 die steel reach 58HRC through heat treatment?

It can be reached by 1050 ~ 1100 ℃ heating quenching and oil quenching.

But the general hot work die steel does not be required with such high hardness because the performance will be very poor under high hardness.

Generally, the hardness in the range of HRC46-50 will perform not only well but also with good durability.

After the heat treatment of the mold, what is the surface whitened with?

  • It can be washed with stainless steel acid or hydrochloric acid. Sandblasting treatment also can be adopted.

If the grinding machine is used, the cost is high, the amount of processing is large, and the size may not be up to the standard.

If hydrochloric acid can’t clean the surface, it is estimated that you’re using die steel with high chromium. It’s D2 or H13 die steel?

The oxide layer of high chromium die steel is difficult to wash off.

Stainless steel acid lotion should wash it off. They are available in the mold or stainless steel shop.

  • Stainless steel pickling cream works

The oxide layer of high chromium mold steels is hard to remove with hydrochloric acid.

There’s another way.

Now that the mold has been polished with oilstone, the surface is smooth.

In fact, you can grind with the coarse whetstone, or abrasive belt, and then do the heat treatment.

After that, grinding with fine oilstone again.

And some people do it this way:

Grinding with the fiber wheel first which can effectively remove the blank skin, and then grinding and polishing, or sandblasting.

How does the heat treatment plant treat metal?

There’s a lot of equipment in the thermal treatment plant.

The stove has a batch-type furnace and pit furnace.

The batch-type furnace is used the most. Many heat treatments can be handled in this furnace, such as annealing, normalizing, quenching and tempering.

In fact, it is a furnace heated by electricity. Heat the furnace to a predetermined temperature and then throw the workpiece in it. Wait for a while to reach the predetermined temperature, keep warm for a while, then take it out. Or cooling inside together with the furnace.

The pit furnace is usually used as carburizing equipment which is buried underground.

When the workpiece is placed in, it is sealed and then dripped into the furnace with some carbon-rich liquids, such as kerosene or methanol. At high temperature, the liquid dissolves into carbon atoms which penetrate into the surface of the workpiece.

A quenching pond is a place for quenching. It has an aqueous solution or oil in it, which used to cool hardened workpieces from the batch-type furnace.

Usually just throw it in and wait a while for getting it out.

There are other devices, such as high-frequency machines, which is an overpowered device that converts a 50Hz power current into a 200KHz current.

For example, the common one has a maximum power of 200Kw. A coil made of a copper tube with internal cooling water is placed on the outside of the workpiece (usually a few tens of millimeters of the workpiece), and within a few seconds, you can see the surface of the workpiece turning red. When the surface temperature reaches the predetermined value, a water jacket is raised to spray quenching solution to the surface of the workpiece to complete the quenching process.

The commonly seen equipment is above mentioned types.

Why are Cr12 or Cr12MoV materials cracked after heat treatment?

It’s very difficult to find the causes without the size and shape of parts, heat treatment requirements, and adopted heat treatment techniques.

These two types of steel belong to the same category which is high-chromium lepidolite steel. The steel itself has a cold cracking tendency.

The heat treatment technique is also complex.

Here are some of my experiences without the above information:

950-1000℃ quenching, oil cooling, HRC>58. In order to obtain thermal hardness and high wear resistance, the quenching temperature is increased to 1115-1130℃, oil cooling.

For fine and thin mold, air cooling method can be adopted. In order to reduce deformation, cooling with the salt solution under the temperature of 400-450℃.

Don’t tempering under 300-375℃, which will reduce the toughness of the tool and occurring temper brittleness. In addition, immediately tempering after quenching.

If quenching under the temperature higher than 1100℃, 2-3 times tempering should be adopted under 520℃.

Please note that high quenching temperature will tend to decarbonize. For this reason, pre-heat treatment can be carried out before quenching (spheroidizing annealing).

How to distinguish heat treated parts from non-heat treated parts?

Supplementary:

“ The workman accidentally mixed an unheated raw part with a batch of well heat-treated parts. Both of them have been undershot blasting treatment, so it’s not able to tell by the color. Besides, both the hardness are in the range of HRC35-45, so it’s also not able to be distinguished by hardness. Now how do you tell them apart? Don’t tell me to cut the workpiece to see the metallographic structure, it will destroy the product.

Answers:

If you can’t tell by hardness or oxidizing colors of heat treatment, I suggest you identify them by tapping sound.

The metallographic structure between casting and workpiece under quenching and tempering are different. There is a difference in internal consumption, so you can tell by tapping sound.

What does overheat mean in heat treatment?

If exceed the specified heating temperature, the grain will grow up and various mechanical properties will deteriorate, such as increased brittleness, decreased toughness, easy deformation and cracking, etc.

The overheat can be avoided by controlling the heating temperature.

The steel cannot be remedied by heat treatment or machining after overburning.

Causes and preventive measures of quenching cracks in molds?

Causes:

  • Severe reticular carbide segregation exists in mold materials.
  • There is mechanical machining or cold-plastic deformation stress in the mold.
  • Improper heat treatment of mold (Heating or cooling too fast, improper selection of quenching cooling medium, the cooling temperature is too low, the cooling time is too long)
  • The mold has a complex shape, uneven thickness, sharp angle and threaded hole, which make the thermal stress and structural stress become too large.
  • Too high mold quenching heating temperature produce overheating or overburning.
  • After quenching, the tempering of the mold is not timely or the tempering time is insufficient.
  • When the mold is sent back and heated by quenching, it is heated again without intermediate annealing.
  • The improper grinding process of mold heat treatment
  • There are high tensile stress and microcracks in the hardened layer during electrosparking machining after heat treatment.

Preventive measures:

  • Strictly control the inner quality of mold material
  • Improveforging and spheroidizing annealing technology to eliminate reticular, banded, chain carbides and improve the uniformity of spheroidized tissue.
  • After machining or cold plastic deformation, the mold should do destressing annealing (>600℃) before quenching.
  • For the complicated mold, asbestos should be used to plug the threaded hole, bind the dangerous section and thin wall, and adopt graded quenching or isothermal quenching.
  • Annealing or high-temperature tempering is required when the mold is repaired or renovated.
  • Mold should be preheated when quenching, precooled when cooling, and a suitable quenching medium should be selected.
  • The quenching heating temperature and time should be strictly controlled to prevent the mold from overheating and overburning.
  • The mold should be tempered timely after quenching, and the heat preservation time should be sufficient. High alloy complex mold should be tempered 2-3 times.
  • Choose the right grinding technique and the right grinding wheel.
  • Improve the electric spark machining technique and do the destressing tempering.

How to heat up large stamping die, especially for edge cutting mold?

  • Choose vacuum heat treatment as far as possible for mold heat treatment to obtain the minimum deformation.
  • Molds can be spliced into small pieces for heat treatment. It is best to use slow cutting which featured high precision, high finish and small deformation. If the clearance is guaranteed, the burrs will be small. Check if your device has poor accuracy.
  • In addition, I think it is quite possible that the strength of the convex mold is not strong enough. There is also residual stress after the heat treatment of sheet metal, and deformation will occur after wire cutting. Large wire cutting holes can be considered for pre-milling and reheat treatment, leave 3-4mm.

Causes and prevention of soft spot on mold surface?

Causes:

  • The surface of the mold has an oxidized layer, rust spot and partial decarbonization before heat treatment.
  • After the mold is quenched and heated, the quenching medium is not selected properly. Excessive or aging of impurities is in the quenching medium.

Preventive measures:

  • The oxide skin and rust stain should be removed before heat treatment. Protect the die surface properly when quenching and heating. Vacuum electric furnace, salt bath furnace and protective atmosphere furnace should be used for heating.
  • When the mold is quenched and heated, the proper cooling medium should be chosen. The cooling medium used for a long time should be filtered regularly or replaced regularly.

Poor structure before heat treatment?

Causes:

  • Severe carbide segregation exists in the original structure of die steel materials.
  • Poor forging technology such as high heating temperature, small deformation, high stop forging temperature and slow cooling speed after forging which is difficult to eliminate spheroidizing annealing because the forging structure is large and there are reticular, banded and chain carbides.
  • Poor spheroidized annealing technology such as too high or too low annealing temperature, short isothermal annealing time, etc which may cause the spheroidized annealing unevenness or poor spheroidization.

Preventive measures:

  • In general, according to the working conditions of the mold, the production quantity and the strength and toughness of the material itself, the mold steel material with good quality should be chosen.
  • To improve the forging process or adopt the preparation heat treatment of normalizing to eliminate the non-uniformity of reticular and chain carbides and carbides in raw materials.
  • High carbon die steel with severe carbide segregation that cannot be forged can be treated with solid solution refinement.
  • Correct spheroidized annealing process specifications for the forged billets can be formulated based on the quenched-tempered heat treatment and rapid uniform spheroidizing annealing technique.
  • Reasonable installation of the furnace to ensure the temperature uniformity of the billet in the furnace.

After quenching, the structure of the mold is large, which will cause the fracture of the mold when it is used, which seriously affect the service life of the mold

Causes:

  • Mix up the mold steel. The actual quenching temperature of steel is far lower than that of required mold material (For example, GCr15 steel is treated as 3Cr2W8V steel)
  • Mold steel has not been properly spheroidized before quenching which leads to poor spheroidization.
  • Mold quenching heating temperature is too high or the heat preservation time is too long.
  • The mold is improperly placed in the furnace and may overheat near the electrode or heating unit.
  • For molds with a large change in cross-section, improper selection of quenching and heating process parameters leads to overheating at the thin section and sharp angle.

Preventive measures:

  • The steel should be inspected strictly before entering the warehouse to prevent the steel from being mixed and disorderly.
  • Correct forging and spheroidization annealing should be carried out before quenching to ensure good spheroidization.
  • Make the mold quenching and heating process specification correctly, strictly control the quenching heating temperature and heat preservation time.
  • Check and correct the thermometer regularly to ensure the instrument works normally.
  • The mold should be kept at an appropriate distance from the electrode or heating unit when heated in the furnace.

How to heat treat Cr12MoV steel to make the cold mold?

High hardness, high wear resistance and high toughness optimization treatment:

  • 980~1200℃ quenching;
  • Oil quenching (machine oil)
  • Tempering at 400℃ for one time
  • Tempering at 240℃ for one time
  • HRC57~61

What is the reason for H13 die steel cracking after heat treatment (quenching temperature 1100℃,oil cooling)?

Metallographic analysis can be carried out to check whether there is decarbonization on the surface of materials.

Cracking is usually caused by decarbonization.

H13 is generally used to make extrusion dies, and the hardness requirement for the materials is not very high.

I suggest you try the heat treatment under the temperature in the range of 1030-1050℃.

What material is usually used for guide column and guide sleeve of mold? What kind of heat treatment should be adopted and what kind of performance should be achieved?

  • Use # 45 carbon structural steel or carbon tool steel. The hardness of heat treatment quenching is around HRC45, less than HRC58~62. It breaks easily even at that high hardness.
  • Generally speaking, SKD61 or SKD11 will be used for the high requirement. The quenching hardness of heat-treated H13 die steel is around HRC51.

What are the uses of heat treatment in mold making?

Role of heat treatment in mold manufacturing:

  • Increase hardness and wear resistance, thus improving its life;
  • The strength is strengthened and the deformation is reduced to ensure the accuracy and precision stability of the mold.

Analysis of mold failure causes

Most failures are caused by fracture, wear and deformation and the main reasons are improper heat treatment and bad mold machining

Therefore, a reasonable selection of materials, correct formulation of the heat treatment process and improvement of heat treatment quality play a key role in extending the service life of the mold.

Mold heat treatment includes preheating treatment and final heat treatment.

The ultimate purpose of the heat treatment is to make the mold have good surface quality, strength, plasticity and toughness.

The reasons for the dropping of Cr12mov die steel pieces by pieces after heat treatment

Possible Causes:

  • When quenching, the temperature exceeds too much, overheating, resulting in coarse grain, serious decarbonization, large martensite, coarse grain fracture, low toughness and plasticity.
  • The high heating temperature and long heat preservation time lead to serious decarbonization of material surface, coarse grain structure, poor binding force and significant reduction of plasticity.

Suggestions:

  • Check the heating equipment;
  • Adjust the quenching and tempering temperature and time;
  • Conduct heat treatment process test for sample and conduct necessary performance test analysis.

Advantages of salt bath heat treatment?

Advantages:

  • Uniform heating, small deformation
  • A small amount of decarbonization without oxidation
  • Heating fast can quickly transform the internal structure of the workpiece
  • Good thermal insulation and heating uniformity
  • It can be heated by solid solution treatment
  • Extensive applicability
  • Able to conduct approximately non-oxidation delivery handling

Disadvantages:

  • The working environment is bad and the workpiece is corroded to some extent
  • The service life is relatively short
  • The working space is small and the power is large
  • Waste salt has pollution to the environment

What method is used to check the heat treatment effect?

  • Simply check the surface hardness
  • Check the depth of the hardened layer, the hardness of the center, the structure grade of the hardened metallography and the structure grade of the tempered metallography.

The critical hardness value of hardened depth=Minimum hardening hardness×0.8。

What is the role of deep cooling treatment of mold material?

Deep cooling treatment is the continuation of workpiece cooling process after quenching.

The application in mold industry is mainly in cold mold steel, high-speed steel, bearing steel, cold mold and mold fittings

Deep cooling will change some related mechanical properties. The main functions are as follows:

  • Improve the hardness and strength of workpiece
  • Ensure the dimension precision of workpiece to improve workpiece wear resistance
  • Improve the impact toughness of workpiece
  • Improve the internal stress distribution of workpiece
  • Increase fatigue strength
  • Improve corrosion resistance of workpiece

What are the reasons for Cr12MoV heat treatment explosion?

  • Is the cooling medium cooling too fast (can’t use salt water, pure water, etc)?
  • Before quenching, it may not be annealed properly, which results in excessive internal stress
  • Poor material metallurgy (non-metallic inclusion, banded structure, eutectic carbide)
  • When quenching, the furnace temperature rises too fast
  • Not timely tempering

What is non-heat treatment strengthening?

Surface treatment:

Such as hard chromium plating to increase the wear resistance of parts.

Shot peening strengthening:

The fatigue strength of the parts can be improved greatly under alternating stress.

Rolling:

Using the rolling tool to apply pressure to the surface of parts at room temperature, to make the metal surface produce plastic deformation, modify the micro-geometric shape of the metal surface, improve the surface smoothness, the fatigue strength, wear resistance and hardness of the parts.