Heat treatment of mold includes preliminary heat treatment, final heat treatment, and surface strengthening treatment. Generally, heat treatment defect refers to various defects that occur in final heat treatment process or in subsequent process, such as quenching, deformation, poor hardness, electric machining cracking, grinding crack, early damage of mold and so on. Following is a more detailed analysis.

 

Causes and precautions for quenching are as follows:
1, shape effect, mainly caused by design factors, such as rounded corner R is too small, hole position is improperly set, section transition is not good.
2, overheating (overheating), mainly caused by inaccurate temperature control, process setting temperature is too high, uneven furnace temperature and other factors, preventive measures include maintenance, proofreading temperature control system, correction process temperature, adding horns between workpiece and furnace floor.
3, decarburization, mainly caused by overheating (or over-burning), unheated heating of air furnace, small machine allowance, forging or pre-heat treatment residual decarburization layer and other factors, preventive measures are controlled atmosphere heating, salt bath heating, vacuum furnace, box furnace using box protection or use anti-oxidation coating; machining allowance is increased by 2~3mm.
4, improper cooling, mainly due to improper choice of coolant or too cold, should master quenching medium cooling characteristics or tempering treatment.
5, poor mold steel structure, such as serious carbide segregation, poor forging quality, improper preparation of heat treatment, etc., preventive measures are to use correct forging process and a reasonable preparation of heat treatment system.

Reasons for lack of hardness and preventive measures are as follows:
1. Furnace or method of entering cooling tank is improper, process temperature should be corrected, temperature control system should be checked and repaired. When loading furnace, workpiece spacing should be placed evenly, dispersed into tank, and it is forbidden to accumulate or bundle into tank to cool.
2. Quenching temperature is too high. This is caused by improper process setting temperature or temperature control system error. Process temperature should be corrected and temperature control system should be overhauled.
3. Over-tempering, this is caused by tempering temperature setting is too high, temperature control system fault error or furnace temperature is too high, process temperature should be corrected and temperature control system should be overhauled. Loaded at no higher than set furnace temperature.
4. Improper cooling, reason is that pre-cooling time is too long, cooling medium is not properly selected, quenching medium temperature is getting higher and cooling performance is degraded, mixing is poor or trough temperature is too high, etc. Measures: quick release and sinking; master cooling characteristics of quenching medium; oil temperature 60 ~ 80℃, water temperature below 30℃. When quenching amount is large and cooling medium is heated, cooling quenching medium should be added or other cooling tanks should be used for cooling; stirring of coolant is strengthened; and it is taken out at Ms+50℃.
5. Decarburization, which is caused by residual decarburization layer of raw materials or quenching heating, preventive measures are controlled atmosphere heating, salt bath heating, vacuum furnace, box furnace adopting box protection or using anti-oxidation coating; machining allowance is increased by 2~3mm.

In mechanical manufacturing, quenching deformation of heat treatment is absolute, and deformation is not relative. In other words, it's just a matter of deformed size. This is mainly due to surface embossing effect of martensitic transformation during heat treatment. Preventing heat treatment deformation (dimensional changes and shape changes) is a very difficult task. In many cases. it has to rely on experience to solve it. This is because not only steel type and shape of mold have an effect on heat treatment deformation, improper carbide distribution state, forging and heat treatment methods also caused or aggravate heat treatment deformation. In many conditions of heat treatment, as long as a certain condition changes, deformation degree of steel member changes greatly. Although heat treatment deformation problem is mainly solved by experience and heuristic method for a long time, correctly grasping relationship between mold steel forging, module orientation, mold shape, heat treatment method and heat treatment deformation, grasping heat treatment deformation law from accumulated actual data, and establishing archives data on heat treatment deformation is a very meaningful work.

Decarburization is a phenomenon and reaction in which all or part of carbon in surface layer is lost due to action of surrounding atmosphere when steel is heated or kept warm. Decarburization of steel parts not only causes insufficient hardness, quenching, heat treatment deformation, chemical heat treatment defects, but also has a great influence on fatigue strength, wear resistance and mold performance.

In mold manufacturing, use of electrical discharge machining (electric pulse and wire cutting) is an increasingly common processing method. With widespread application of electrical discharge machining, defects caused by it are correspondingly increased. Since electric discharge machining is a processing method for melting surface of mold by high temperature generated by discharge, a white EDM layer is formed on machined surface, and a tensile stress of about 800 MPa is generated, so that defects such as deformation or cracks often occur during electromachining of mold. Therefore, use of EDM molds must fully understand impact of EDM on mold steel, and take appropriate precautions in advance. Prevent overheating and decarburization during heat treatment, and perform sufficient tempering to reduce or eliminate residual stress. In order to fully eliminate internal stress generated during quenching, high temperature tempering is required, so steels capable of withstanding high temperature tempering should be used (eg DC53 type, ASP-23, high speed steel, etc.) for processing under stable discharge conditions; after discharge machining, it is stabilized and relaxed; reasonable process holes and grooves are set;  re-solidified layer is fully eliminated for use in a sound state; using vector translation principle, a portion of internal stress-cutting drainage that has been concentrated before cutting is dispersed and released.

 

Reason for insufficient toughness may be that quenching temperature is too high, and holding time is too long to cause coarsening of grains, or due to absence of tempering in temper brittle zone.

When there is a large amount of retained austenite in workpiece, tempering transformation occurs under action of grinding heat, thereby generating tissue stress and causing cracking of workpiece. Precautionary measures are: cryogenic treatment after quenching or repeated tempering (mold tempering is usually 2 to 3 times, even for low-alloy tool steel for cold working), and amount of retained austenite is minimized.

Heating and quenching at 1050~1100℃, oil quenching, can meet requirements, but general hot work mold does not require such high hardness. For such high hardness, performance will be very poor, not easy to use, generally good performance in HRC46~50, durable.

Problem: general mold is polished with oil stone and then nitriding. After nitriding, it is necessary to use oil stone to rub black layer of white. It is very troublesome to polish, and material is H13. There are many kinds of imported ones. If there is a potion that can be washed white, it can be polished directly.
(1) It can be washed with stainless steel pickling solution or hydrochloric acid. Sandblasting is also possible. If grinding of grinding machine is high, processing volume is large, which may make size not meet standard. If you can't wash hydrochloric acid, is it estimated that you are using high chromium mold steel? Is it D2 or H13? Oxide layer of high chromium die steel is more difficult to wash off. It should be possible to use stainless steel pickling, which is available in abrasive shops or stainless steel stores.
(2) Do you have stainless steel pickling cream? H13 is a mold steel with a relatively high chromium content, and oxide layer is difficult to wash off with hydrochloric acid. There is also a way, since mold has been worn by oil stone, surface is relatively smooth. In fact, it can be polished only with coarse oilstone or with a sanding belt, and then heat treated. Then polish with fine stone. It can also be polished with a fiber wheel to effectively remove black skin and then grind it. Or sand blasting, try to use a 800-mesh boron carbide for sand blasting, you should be able to remove black skin, and do not need to spend too much effort to re-grind.

There are many equipments in heat treatment plant. There are probably box furnaces and pit furnaces. Box furnaces are used most. Many heat treatments can be treated here, such as annealing, normalizing, quenching and tempering.
In fact, it is an electric heating furnace, first heat furnace to a predetermined temperature, then throw workpiece into it, wait for a period of time to a predetermined temperature, keep it for a while, then take it out, or cool it together in furnace. Pit furnace is generally used as a carburizing treatment equipment, it is a furnace buried in the ground. After workpiece is placed, it is sealed, some carbon-rich liquid, such as kerosene or methanol, is dropped into furnace, then liquid is decomposed into carbon atoms to penetrate surface of workpiece at a high temperature.
Quenching tank is a quenching place, which is a pool with an aqueous solution or oil. It is the place where quenching of workpiece from box furnace is cooled. Usually it is directly thrown in, and then wait for a while to take out. There are other devices, such as high-frequency machines, which are ultra-high-power devices that can convert 50 Hz power frequency into a 200K Hz current, such as a common 200 kW maximum power, then a coil made of a copper tube with internal cooling water is placed on the outside of workpiece. Generally, a workpiece of several tens of millimeters, you see that surface of workpiece turns red after a few seconds. When surface temperature reaches a predetermined value, a water jacket is lifted up to spray quenching liquid onto surface of workpiece to complete quenching process.

Problem: worker accidentally mixes a non-heat treated living piece with a batch of debugged heat-treated workpieces. How to distinguish them, do not cut workpiece? Heat treatment process 30Cr is normalized, re-quenched, and then tempered. Green part is not heat treated. Both of them have been polished and color change can not be separated. Hardness is between 35 and 45, which cannot be distinguished by hardness.
If it is not possible to judge by hardness and heat treatment oxidation color. It is recommended to distinguish by tapping sound. Metallurgical structure of casting and quenching + tempering workpieces are different, internal friction is different. It may be distinguished by light tapping.

Exceeding specified heating temperature, grain grows, mechanical properties deteriorate, such as brittleness, toughness decreases, deformation and cracking are easy. Controlling heating temperature can avoid overheating. When steel is heated above a certain temperature in temperature range of solid liquidus, chemical composition changes in austenite grain boundary, and local or entire grain boundary appears to be melted. At this time, S, P and other compounds are enriched on grain boundary, resulting in a decrease in the grain boundary bonding force and a serious deterioration in mechanical properties. After overburning, steel cannot be remedied by heat treatment or processing.

Cause:
1) There is severe network carbide segregation in mold material.
2) There are machining or cold plastic deformation stresses in mold.
3) Improper heat treatment of mold (heating or cooling too fast, improper selection of quenching cooling medium, low cooling temperature, too long cooling time, etc.).
4) Shape of mold is complicated, thickness is uneven, with sharp corners and threaded holes, so that thermal stress and structural stress are excessive.
5) Mold quenching heating temperature is too high to produce overheating.
6) Tempering of mold after quenching is not timely or tempering holding time is insufficient.
7) When mold is repaired and quenched, it is heated and quenched without intermediate annealing.
8) If mold is heat treated, grinding process is improper.
9) During electrical discharge machining of mold after heat treatment, there are high tensile stresses and microcracks in hardened layer.
Precaution:
1) Strictly control intrinsic quality of mold raw materials.
2) Improve forging and spheroidizing annealing process to eliminate network, ribbon and chain carbides, improve uniformity of spheroidized structure.
3) Mold after mechanical processing or after cold plastic deformation shall be subjected to stress relief annealing (>600℃) , then heated and quenched.
4) For complex shape of the mold, asbestos should be used to block threaded hole, dangerous section and thin wall are wrapped, quenching or austempering is adopted.
5) Annealing or high temperature tempering is required for rework or refurbishment of mold.
6) Mold should be preheated during quenching and heating, pre-cooling measures should be taken during cooling, and suitable quenching medium should be selected.
7) Quenching heating temperature and time should be strictly controlled to prevent overheating and over-burning of mold.
8) After quenching, mold should be tempered in time, holding time should be sufficient, and high alloy complex mold should be tempered 2-3 times.
9) Choose right grinding process and right grinding wheel.
10) Improve mold EDM process, perform stress relief and tempering.

(1) Vacuum heat treatment should be used as much as possible to obtain minimum amount of deformation.
(2) Mold can be spliced and divided into small pieces to heat treatment. It is best to cut with slow wire, high precision, high finish and small deformation. Gap is guaranteed and burrs will be small. See if your device is very poorly accurate.

(1) Forging temperature is about 900~1000℃? Is temperature too high? Mold may be easily cracked without sufficient preheating before use. Unreasonable mold design may also be prone to cracking. Tempering temperature of mold is increased somewhat, difference between actual and forging temperature is reduced, tempering is actually longer.
(2) This should be considered comprehensively. If necessary, it is necessary to make a metallographic phase in order to basically determine cause.

 

Cause:
1) Mold has scale, rust and partial decarburization on the surface before heat treatment.
2) After mold is quenched and heated, cooling and quenching medium is improperly selected, impurities in quenching medium are excessive or aging.
Precaution:
1) Before heat treatment of mold, scale and rust should be removed. When surface of mold is properly protected during quenching, vacuum furnace, salt bath furnace and protective atmosphere furnace should be used for heating.
2) When mold is quenched, heated and cooled, appropriate cooling medium should be selected. Cooling medium used for long-term use should be filtered frequently or periodically.

cause:
1) There is serious carbide segregation in original structure of die steel material.
2) Forging process is not good, such as forging heating temperature is too high, deformation amount is small, stop forging temperature is high, cooling speed after forging is slow, etc., so that forged structure is coarse and has mesh, band and chain carbides, which is difficult to eliminate during spheroidizing annealing.
3) Poor spheroidizing annealing process, such as too high or too low annealing temperature, short annealing time, etc., can cause uneven spheroidizing annealing or poor spheroidization.
Precaution:
1) Generally, according to working conditions, production batch of mold and toughening performance of material itself, try to select a good quality mold steel material.
2) Improve forging process or use normalizing heat treatment to eliminate non-uniformity of network, chain carbides and carbides in raw materials.
3) For high-carbon die steels with severe segregation of carbides that cannot be forged, solid solution heat treatment can be performed.
4) Develop a correct spheroidizing annealing process specification for forged blank, which can be tempered heat treatment and fast uniform spheroidizing annealing.
5) Reasonably install furnace to ensure uniformity of temperature of mold base in furnace.

Causes:
1) Mold steel is confused. Actual steel quenching temperature is much lower than quenching temperature of required mold material (for example, GCr15 steel is regarded as 3Cr2W8V steel).
2) Correct spheroidization process is not performed before mold steel is quenched, and spheroidized structure is poor.
3) Mold quenching heating temperature is too high or holding time is too long.
4) Mold is placed in furnace improperly, and it is easy to generate heat in the vicinity of electrode or heating element.
5) For molds with large cross-section changes, quenching heating process parameters are not properly selected, overheating occurs at thin sections and sharp corners.
Precaution:
1) Steel should be strictly inspected before storage, steel should be prevented from being confused.
2) Correct forging and spheroidizing annealing should be performed before mold is quenched to ensure good spheroidized structure.
3) Properly formulate mold quenching heating process specification, strictly control quenching heating temperature and holding time.
4) Regularly check and calibrate temperature measuring instrument to ensure normal operation of instrument.
5) Mold should be kept at an appropriate distance from electrode or heating element when heated in furnace.

High hardness, high wear resistance and high toughness. Optimized treatment: 980~1200℃heating and quenching, oil quenching (oil) 400℃ tempering once, 240℃ tempering once, HRC57~61, super durable and not chipping.

(1) 45# carbon structural steel or carbon tool steel in China, heat treatment quenching hardness around HRC45, can not reach HRC58~62. If it is too high, very easy to break.
(2) Generally, it is required to use heat treatment quenching such as SKD61 or SKD11 and H13, hardness is about HRC51.

Role of heat treatment in mold making: Improve hardness and wear resistance, thereby improving its life; strengthening strength, reducing deformation, ensuring precision stability of mold.

Most of failure is caused by fracture, wear and deformation, which are mainly caused by improper heat treatment and poor mold making. Therefore, rational selection of materials, proper formulation of heat treatment processes, and improved heat treatment quality play a key role in extending mold life. Heat treatment of mold includes pre-heat treatment and final heat treatment, final purpose of heat treatment is to make mold have a good surface quality and a reasonable fit of strength, plasticity and toughness.

(1) It may be that when quenching, it exceeds too much temperature, overheating, causing coarse grains, severe decarburization, coarse martensite, coarse grain of fracture, low toughness and plasticity.
(2) If heating temperature is too high and holding time is too long, surface of material is decarburized seriously, grain size of structure is coarse, bonding force is poor, and plasticity is significantly reduced.
Recommendation: proofread heating equipment; adjust quenching, tempering temperature and time; do sample heat treatment process test, and carry out necessary performance test analysis.

Advantages: uniform heat, small deformation, less oxidative decarburization, fast heating, can quickly change internal structure of workpiece, good thermal insulation performance and uniform heating performance, can be treated by solution treatment heating, wide applicability, near-non-oxidation shipment processing can be performed.
Disadvantages: Working environment is harsh, workpiece has certain corrosion, service life is relatively short, power is larger, and size is smaller. Moreover, waste salt is somewhat polluted to environment. If customer has high requirements for no oxidation, cost can be considered.

Simply check surface hardness. Accurate inspection: check hardness of hardened layer, hardness of core, microstructure of quenched metallurgy, tempering metallographic organization grade. Hardened layer deep critical hardness value = quenching hardness minimum *0.8.

Cryogenic treatment is a continuation of workpiece cooling process after quenching. Application in mold industry is mainly reflected in application of cryogenic technology in cold work die steel, high speed steel, bearing steel, cold work die and mold parts. Deep 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 stress distribution inside workpiece, improve fatigue strength, and improve corrosion resistance of workpiece.

(1) Some people think that it is a problem of heat treatment. Since wire cut without heat treatment is difficult to crack, wire cut cracking is due to stress concentration during cutting. Therefore, it is necessary to not only achieve hardness required for heat treatment but also internal stress during heat treatment. The key to eliminating internal stress is temperature of tempering and time of tempering. If tempering temperature is too low, internal stress can not be eliminated after tempering time; if tempering temperature is too high, although stress is eliminated, requirement cannot be met; if tempering temperature is appropriate, but tempering time is too short, stress It 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 complicated, and there is a possibility that tempering is insufficient after quenching of mold, and there is also possibility of defects in forging of mold. If it is a general carbon steel mold, tempering is not main reason, priority should be given. For high-alloy molds, there may be defects in material itself, such as impurities, carbide segregation, etc., which require high-magnification metallographic detection to judge.

(1) Whether cooling medium is cooled too fast (no salt water, water, etc.)
(2) Before quenching, annealing may not be good, resulting in excessive internal stress.
(3) Poor metallurgy of materials (non-metallic inclusions, banded structures, eutectic carbides)
(4) When quenching, furnace temperature rises too fast.
(5) Did not temper in time

Surface treatment: such as hard chrome plating, increase wear resistance of parts;
Shot peening: used to withstand workpiece under alternating stress, can greatly improve fatigue strength;
Rolling: Using a rolling tool to apply pressure to surface of part under normal temperature conditions, plastically deform metal surface, correct microscopic geometry of metal surface, improve surface finish, improve fatigue strength, wear resistance and hardness of part.