Heating and quenching at 1050~1100℃ and oil quenching can meet 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 and durability.

Additional question: I run a mold polishing shop. Generally, molds are polished with a whetstone before being nitrided. After nitriding, black layer needs to be wiped white with a whetstone. It is very troublesome to polish it again. Without rubbing it white, you won't be able to get a mirror surface. There are many kinds of materials, including H13 and imported. If there is a potion that can whiten it, it can be polished 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? 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, oxide layer is difficult to wash off with hydrochloric acid. There is another method that I use myself. Since your mold has been polished with a whetstone, 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, then heat treat it. When it come back, polish it with a fine oil stone. Method I use is to use a fiber wheel to polish it first, which can effectively remove black skin, then grind and polish it. Or sandblast, try sandblasting with 800-mesh boron carbide. It should be able to remove black skin without much effort and re-sanding.

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, and many heat treatments can be processed here, such as heating processes of annealing, normalizing and quenching, and tempering, which are common heat treatments.
In fact, it is an electrically heated furnace. First heat furnace to a predetermined temperature, throw workpiece in, wait for a period of time to reach predetermined temperature, keep it warm for a period of time, then take it out or cool it together in the furnace. Pit furnace is generally used as carburizing treatment equipment. It is a furnace buried underground. After workpiece is put in, it is sealed, then some carbon-rich liquid, such as kerosene or methanol, is dripped into furnace. Then at high temperature, these liquids decompose into carbon atoms and penetrate into surface of workpiece.
Quenching pool is a place for quenching. It is a pool with aqueous solution or oil in it. It is the place where workpieces coming out of 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, common maximum power is 200 kilowatts, and then use a coil made of copper pipes with cooling water inside to be placed outside workpiece. Generally, workpiece is tens of millimeters. In a few seconds to more than ten seconds, you will see surface of workpiece turn red. When surface temperature reaches a predetermined value, a water jacket will rise up and spray quenching liquid onto surface of workpiece to complete quenching process. These are common ones.

It is best to inform hardware mold maker of size, shape and heat treatment requirements of parts, as well as heat treatment process curve you adopt, otherwise it will be difficult to tell. These two types of steel are of same type, which are high-carbon, high-chromium ledeburite steels that have a tendency to cold crack. Heat treatment process is also more complex.
Let me talk about my experience without above information: 950-1000C quenching, oil cooling, HRC>58. In order to obtain hot hardness and high wear resistance, quenching temperature is increased to 1115-1130C, oil cooling. Thin ones can be air-cooled. In order to reduce deformation, they are also cooled with salt liquid at 400-450C. Do not temper at 300-375C, as it will reduce toughness of 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.

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 workpieces to look at metallographic phase. This will damage product and we need to rush shipment? Heat treatment process 30Cr is normalized, then quenched, and tempered. Raw part is a casting without heat treatment. Both have been shot blasted and cannot be distinguished from each other due to discoloration, and hardness is between 35-45, making it indistinguishable based on hardness. If it cannot be judged by hardness and heat treatment oxidation color. I suggest you identify it by tapping sound. Metallographic structure of castings and quenched + tempered workpieces are different, and internal friction is different. It may be possible to distinguish by gently tapping.

 

Exceeding specified heating temperature will cause grains to grow and various mechanical properties to deteriorate, such as increased brittleness, decreased toughness, easy deformation and cracking, etc. Controlling heating temperature can avoid overburning. It means that when steel is heated above a certain temperature within solid-liquidus temperature range, chemical composition of austenite grain boundaries changes, and melting occurs locally or throughout grain boundaries. At this time, S, P and other compounds will be enriched on grain boundaries, resulting in a reduction in grain boundary binding force and a serious deterioration in mechanical properties. Overfired steel cannot be remedied by heat treatment or machining methods.

Cause:
1) Mold material has severe network carbide segregation.
2) There is mechanical processing or cold plastic deformation stress in 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) Mold has complex shape, uneven thickness, sharp corners and threaded holes, which causes excessive thermal stress and structural stress.
5) Mold quenching heating temperature is too high, resulting in overheating or overburning.
6) Mold is not tempered in time after quenching or tempering and heat preservation time is insufficient.
7) When mold is reworked, quenched and heated, it is heated and quenched again without intermediate annealing.
8) Mold is heat treated and grinding process is improper.
9) When mold is processed by EDM after heat treatment, there will be high tensile stress and microcracks in hardened layer.
Precaution:
1) Strictly control inherent quality of mold raw materials
2) Improve forging and spheroidizing annealing process to eliminate network, ribbon, chain carbides and improve uniformity of spheroidized structure.
3) After mechanical processing or cold plastic deformation, mold should be stress-relieved annealed (>600℃), then heated and quenched.
4) For molds with complex shapes, asbestos should be used to plug threaded holes, wrap dangerous sections and thin-walled areas, use graded quenching or isothermal quenching.
5) Annealing or high-temperature tempering is required when repairing or refurbishing mold.
6) Mold should be preheated during quenching and heating, precooled during cooling, and a suitable quenching medium should be selected.
7) Quenching heating temperature and time should be strictly controlled to prevent mold from overheating and overburning.
8) Mold should be tempered in time after quenching, and heat preservation time should be sufficient. High-alloy complex molds should be tempered 2-3 times.
9) Choose correct grinding process and suitable grinding wheel.
10) Improve mold EDM process, perform stress relief and tempering.

 

(1) For mold heat treatment, vacuum heat treatment should be used as much as possible to obtain minimum deformation.
(2) Mold can adopt a spliced structure and be divided into small pieces for heat treatment. It is best to use slow wire cutting, which has high precision, high smoothness and small deformation. Gap is guaranteed and burrs will be small. See if your device has poor accuracy.
(3) Large trimming burrs. In addition to what above mentioned, I think there is a high possibility that punch is not strong enough due to stress on one side. Is punch too thin? Does design depend on knife? There are also residual stresses in 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.

(1) Forging temperature is approximately 900~1000℃? Is temperature too high? Molds that are not fully preheated before use may be prone to cracking. Improper mold design may also cause cracking. Increase tempering temperature of mold to narrow gap with actual forging temperature, and tempering will actually be longer.
(2) This should be considered comprehensively. If necessary, a metallographic examination should be done to basically judge cause.

Cause:
1) Surface of mold has oxide scale, rust spots and partial decarburization before heat treatment.
2) After mold is quenched and heated, cooling and quenching medium is improperly selected, and there are too many impurities or aging in quenching medium.
Precaution:
1) Oxide scale and rust spots should be removed before mold heat treatment, 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 mold after quenching and heating, a suitable cooling medium should be selected. Cooling medium used for a long time should be filtered frequently or replaced regularly.

Cause:
1) There is severe carbide segregation in original structure of 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 forging structure coarse and has network, band and chain carbides, which are difficult to eliminate during spheroidizing 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.
Precaution:
1) Generally, good quality mold steel materials should be selected according to working conditions of mold, production batch size and toughening properties of material itself.
2) Improve forging process or use normalizing preparatory heat treatment to eliminate unevenness of network, chain carbides and carbides in 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 forged mold blank. Quenching and tempering heat treatment, rapid and uniform spheroidizing annealing can be used.
5) Install furnace reasonably to ensure uniformity of temperature of mold base in furnace.
12. After mold is quenched, structure becomes coarse, which will cause mold to break during use, seriously affecting service life of mold?
Causes:
1) Mold steel materials are confused, and actual steel quenching temperature is much lower than required quenching temperature of mold material (such as treating GCr15 steel as 3Cr2W8V steel).
2) Correct spheroidization process was not carried out before mold steel was quenched, resulting in poor spheroidization structure.
3) Mold quenching heating temperature is too high or holding time is too long.
4) Mold is placed improperly in furnace, and overheating is likely to occur near 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.
Precaution:
1) Steel materials should be strictly inspected before entering 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, strictly control quenching heating temperature and holding time.
4) Regularly check and calibrate temperature measuring instruments to ensure normal operation of instruments.
5) When mold is heated in furnace, an appropriate distance should be maintained from electrode or heating element.

 

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.

Metallographic analysis can be performed to see if there is decarburization of material on the surface. If it cracks, it is usually caused by decarburization. H13 is generally used for extrusion dies, and hardness requirements of material are not very high. Do you use a vacuum furnace? It is recommended to try it at 1030~1050℃.

(1) In inland areas, 45# carbon structural steel or carbon tool steel is used. Heat treatment and quenching hardness is about HRC45, which cannot reach HRC58~62. It is so high that it is easy to break.
(2) Generally, those with high requirements are to use heat treatment quenching hardness of about HRC51 such as SKD61 or SKD11 and H13.

Supplementary question: Is heat treatment performed after mold plate is processed? Which is main step?
Role of heat treatment in mold manufacturing: improves hardness and wear resistance, thereby extending its life; strengthens strength, reduces deformation, ensures accuracy and precision stability of mold.

Failures are mostly caused by fracture, wear and deformation, 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 service life of molds. Mold heat treatment includes pre-heat treatment and final heat treatment. Ultimate goal of heat treatment is to make mold have good surface quality and a reasonable combination of strength, plasticity and toughness.

(1) You may have exceeded temperature too much during quenching and overheated, resulting in coarse grains, severe decarburization, coarse martensite, coarse grains at fracture surface, low toughness and plasticity.
(2) Heating temperature is too high and holding time is too long, resulting in serious decarburization of material surface, coarse grains, poor bonding force, and significant reduction in plasticity.
Suggestions: Calibrate heating equipment; adjust quenching and tempering temperature and time; conduct sample heat treatment process tests, conduct necessary performance testing and analysis.

Advantages: Small deformation when heated uniformly, less oxidation and decarburization, fast heating, can quickly transform internal structure of workpiece, good thermal insulation performance and heating uniformity, can be heated by solid solution treatment, has wide applicability, and can be shipped with almost no oxidation. Disadvantages: Working environment is harsh, there is some corrosion to workpiece, service life is relatively short, in terms of working space size and power, power is larger but size is smaller. Moreover, waste salt has some pollution to environment. If customer has higher requirements for non-oxidation, it can be considered. Cost is average.

Simply check surface hardness. Accurate inspection: check quenching layer depth, core hardness, quenching metallographic structure grade, quenching and tempering metallographic structure grade. Critical hardness value of quenching layer depth = minimum value of quenching hardness X0.8.

Cryogenic treatment is a continuation of cooling process of workpiece after quenching. Its application in 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. Main functions are as follows: Improve hardness and strength of workpiece, ensure dimensional accuracy of workpiece, improve wear resistance of workpiece, improve impact toughness of workpiece, improve internal stress distribution of workpiece, and improve fatigue strength, improve corrosion resistance of workpiece.

 

(1) I am engaged in heat treatment, and I personally 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 hardness required for heat treatment be achieved, but internal stress must also be eliminated. Key to eliminating internal stress is tempering temperature and tempering time. If tempering temperature is too low, internal stress cannot be eliminated no matter how long tempering time is; if tempering temperature is too high, although stress is eliminated, it cannot meet requirements; if tempering temperature is appropriate but tempering time is too short, stress cannot be completely eliminated. Therefore, key to wire cutting cracking after heat treatment is control of tempering temperature and tempering time.
(2) Situation of wire cutting cracking after heat treatment is more complicated. There is possibility of insufficient tempering after quenching of mold, there is also possibility of defects during mold forging. If it is a general carbon steel mold, insufficient tempering is main reason and needs to be given priority. For high-alloy molds, there may also be defects in material itself, such as impurities, carbide segregation, etc., which which require high-power metallographic inspection to judge.

(1) Whether 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) Poor material metallurgy (non-metallic inclusions, banded structures, eutectic carbides).
(4) During quenching, furnace temperature rises too fast.
(5) Failure to temper in time

Surface treatment: such as hard chromium plating, to increase wear resistance of parts; shot peening: used for parts working under alternating stress, which can greatly improve fatigue strength; rolling: using rolling tools to apply pressure to surface of part at room temperature to cause plastic deformation on metal surface, modify micro-geometry of metal surface, improve surface finish, and improve fatigue strength, wear resistance and hardness of part.