Heat Treatment Technology of Die Casting Mold

Metal die-casting has the characteristics of high production efficiency, saving raw materials, reducing production cost, good product performance, and high precision, and it is widely used in production.

The working surface of the die casting mold, directly in contact with liquid metal, bears high pressure, high-speed flow of liquid metal erosion, and heating in the workpiece after mold release and is subject to rapid cooling. Therefore, thermal fatigue cracking, thermal wear, and hot melt corrosion are common forms of failure of die casting mold, so it requires to die casting mold with hot and cold fatigue resistance, high-temperature strength and toughness, resistance to liquid metal erosion performance, high heat resistance and high thermal conductivity, good oxidation resistance and high hardenability and wear resistance, etc. and high thermal conductivity, good oxidation resistance and high hardenability and wears resistance, etc.

Heat treatment is an important part of improving the service life of the die-casting mold. Investigation shows that: heat treatment or improper operation led to mold fracture failure, which accounted for about 60% of the total failure. Therefore, in the die-casting mold production, we must carry out the correct operation of heat treatment.

Table of Contents

Die casting mold manufacturing process route

1. General die casting die
Forging – spheroidal annealing – mechanical rough machining – stabilization – finishing forming – quenching and tempering – clamp assembly.

2. Die casting die with complex shape and high precision requirement
Forging – spheroidizing annealing (or tempering treatment) – rough machining – tempering – electric machining or finishing forming – clamping and grinding – nitriding (or nitrogen-carbonitriding) -Grinding and polishing.

Die casting mold conventional heat treatment process

The heat treatment process is widely used in die-casting mold manufacturing. It can improve the performance of mold parts and extend the service life of the mold. In addition, heat treatment can also improve the processing performance of die casting mold, improve the quality of processing, and reduce tool wear. Therefore, mold manufacturing occupies a very important position.

Die-casting mold is mainly made of steel, and its manufacturing process of conventional heat treatment for spheroidal annealing, stabilization treatment, tempering and quenching, and tempering. The steel organization structure will change through these heat treatment processes so that the die-casting mold will obtain the required organization and performance.

1. Pre-treatment
After forging die casting die billet, spheroidal annealing or tempering heat treatment must be used on the one hand to eliminate stress to reduce hardness, easy cutting, and at the same time for the final heat treatment to prepare the organization. After annealing, a uniform organization can obtain and diffuse the carbide distribution to improve the die steel’s toughness. Because the effect of tempering treatment is better than spheroid annealing, high toughness requirements of the mold often lead to tempering instead of spheroid annealing.

2. Stabilization treatment
Die-casting mold is generally a more complex cavity, in rough machining will produce large internal stress, in quenching will produce deformation. To eliminate the stress, generally, after rough machining should be stress relief annealing, that is, stabilization treatment.

The process is heating temperature 650 ℃ -680 ℃, insulation 2-4h after the furnace air cooling. The shape of the more complex die-casting mold needs furnace cooling to 400 ℃ or less out of the furnace air-cooling. Die quenching and tempering for EDM processing. The processing surface will produce a metamorphic layer, easy to cause wire-cutting cracks, and should also be a lower temperature stress relief annealing.

3. Quenching and preheating
Die-casting die steel is mostly high-alloy steel. Because of its poor thermal conductivity, the quenching and heating must be carried out slowly, often taking preheating measures. For anti-deformation requirements of the mold is not high. In the case of no cracking, the number of preheating can be less, but anti-deformation requirements of high mold must be preheated several times. Lower temperature (400 ℃ -650 ℃) preheating, generally in the air furnace; higher temperature preheating, salt bath furnace should be used, preheating time is still according to 1 min/mm.

4. Quenching heating
For typical die-casting die steel, high quenching heating temperature is conducive to improving thermal stability and resistance to softening, reducing the tendency of thermal fatigue. Still, it will cause grain growth and boundary formation carbide, so toughness and plasticity decline, leading to serious cracking. Therefore, die-casting die requires a high toughness, often using low-temperature quenching, and requires a high-temperature strength, the use of higher temperature quenching.

To obtain good high-temperature performance, to ensure that the carbide can be fully dissolved, to get the composition of uniform austenite, die-casting die quenching and holding time are relatively long, generally in the salt bath furnace heating insulation factor of 0.8-1.0 min/mm.

5. Quenching and cooling
For the shape of simple, anti-deformation requirements of the die-casting die using oil cooling; and complex shape, anti-deformation requirements of the die-casting die using graded quenching. To prevent deformation and cracking, no matter what cooling method is used, it is not allowed to be cooled to room temperature; generally, it should cool to 150℃-180℃, tempered immediately after a certain time of uniform heat. The uniform heat time can be calculated by 0.6 min/mm.

6. Tempering
Die casting mold must be fully tempered, generally tempered three times. The first tempering temperature is chosen in the temperature range of the second hardening; the second tempering temperature is chosen to make the mold reach the required hardness; the third tempering should be lower than the second l0℃-20℃. After tempering is used, oil cooling or air cooling, tempering time is not less than 2 h.

Die casting mold surface strengthening process

Conventional overall quenching has been difficult to meet the high surface wear resistance of die-casting die and matrix toughness requirements.

Surface strengthening treatment can not only improve the die casting die surface wear resistance and other properties and can make the matrix maintain sufficient toughness while preventing molten metal adhesion and leaching, which is very effective in improving the overall performance of the die casting die, saving alloying elements, significantly reduce costs, give full play to the potential of the material, as well as better use of new materials.

Production practice shows that the surface strengthening treatment improves the quality of die-casting die and extends the service life of the important mold measure. Die-casting molds often used in the surface strengthening process are carburizing, nitriding, nitrogen-carbon infiltration, boron infiltration, chromium infiltration, and aluminizing.

1. Carburizing
Carburization is currently the most widely used in the mechanical industry as a chemical heat treatment method. Its process is characterized by: low and high carbon low alloy die steel and high carbon high alloy steel dies in the active medium (carburizing agent), heated to 900 ℃ -930 ℃, so that carbon atoms into the surface layer of the mold, followed by quenching and low-temperature tempering so that the surface layer and the heart of the mold has a different composition, organization, and performance.

Carburizing is also divided into solid carburizing, liquid carburizing, and gas carburizing. Recently, it has been developed to control atmosphere carburizing, vacuum carburizing, benzene ion carburizing, etc.

2. Nitriding
The process of nitrogen infiltration into the steel surface is called steel nitriding. Nitriding can make mold parts to obtain higher surface hardness, wear resistance, fatigue performance, red hardness, and corrosion resistance than carburizing. Because the nitriding temperature is lower (500-570 ℃), nitriding mold parts deformation is smaller.

Nitriding methods are solid nitriding, liquid nitriding, and gas nitriding. At present, ion nitriding, vacuum nitriding, electrolytic nitriding, high-frequency nitriding, and other new technologies are being widely used to shorten the nitriding time and obtain a high-quality nitriding layer.

3. Nitrocarburizing
Nitrogen-carbonitriding is a low-temperature nitriding process (530℃-580℃) in which nitrogen and carbon are simultaneously infiltrated in a medium containing active carbon and nitrogen atoms, and nitriding is the main process. The brittleness of the nitriding layer is small, and the co-infiltration time is much shorter than the nitriding time. Die casting die by nitrogen-carbonitriding can significantly improve its thermal fatigue performance.

Harsh working conditions, die-casting mold with good high-temperature mechanical properties, resistance to hot and cold fatigue performance, resistance to liquid metal corrosion performance, oxidation resistance and high hardenability and wear resistance, etc.; heat treatment is the main manufacturing process to determine these properties.

Die casting mold heat treatment is through the organization of steel to change the structure so that the mold surface obtains high hardness and wear resistance. At the same time, the heart still has sufficient strength and toughness while effectively preventing molten metal sticky mold from leaching. Selection of the appropriate heat treatment process can reduce scrap and significantly improve mold life.

How to prevent heat treatment of die casting molds deformation and cracking

In the production and processing of die-casting mold parts, due to their complex shape and structure, there are more obvious differences in the cross-sectional dimensions of each part so that when heat treatment is carried out, the heating and cooling rates of each part are not the same. This situation can lead to the formation of very different thermal stresses, tissue stresses, and phase change volumes in the various parts of the part. This can cause abnormal expansion or contraction of the part volume, resulting in large deviations in size and shape or even cracking.

The causes of heat treatment deformation and cracking of die-casting molds are many, including the chemical composition of the steel and the original organization, the structural shape and cross-sectional size of the part, the heat treatment process, and other factors that will be involved. In actual production, deformation is often impossible to eliminate and can only minimize the degree of its occurrence, but as long as the appropriate means, cracking can be completely avoided.

 

Preparatory heat treatment

The so-called initial heat treatment is relative to the final heat treatment; that is, before the final heat treatment is added to play a primary role in the heat treatment step; this step can provide the final heat treatment to provide a good machined property or tissue form. Common preparatory heat treatment processes include annealing, normalizing, and tempering.

For the initial heat treatment of eutectic steel stamping dies, the focus is on eliminating reticulated secondary carburization within the forging, grain refinement, and internal stresses. The specific process is first normalized and then spheroids and annealed. Stamping concave die parts should first be stabilized using low-temperature tempering. For those dies with more complex shapes and high accuracy requirements, the possibility of deformation and cracking during heat treatment is higher, so after their rough machining is completed. Before finishing begins, a suitable tempering treatment is carried out to prepare the tissue for the final heat treatment to avoid cracking as much as possible.

Quench heating methods and protection of parts

Quenching and tempering are the processes most likely to cause distortion and cracking of parts. Some small die-casting molds, slender cylindrical parts, high alloy steel mold parts, etc., should avoid the direct heating quenching method. First, preheat it to 520 to 580 degrees Celsius and then put it into the medium temperature salt bath furnace, heating it to quenching temperature. The practice has shown that deformation is significantly smaller with this heating method of parts then directly in the electric furnace or reflection furnace heating quenching, and cracking can also be largely avoided.

 

If the heating temperature is too high, quenching austenitic parts will make the grain coarse and easy to cause oxidation, decarburization, and other phenomena, resulting in parts deformation and cracking. The temperature of the words low will cause shrinkage of the parts inside the hole, the size of the hole diameter becomes smaller. Therefore, the heating temperature should be allowed within the range; try to choose the upper limit of temperature quenching. For alloy steel, the heating temperature is high, which will cause the bore to expand, and the size of the bore becomes larger, and the best choice is to allow a lower temperature limit.

 

In addition, in the quenching and tempering treatment, it is necessary for parts prone to deformation and cracking, measures to effectively protect the shape and cross-sectional symmetry, and the internal stress to maintain balance. This is particularly true for parts that are complex in shape. Common methods of protection include bundling, filling, and blocking.

Optimization of the cooling method and choice of coolant

When the die-casting mold parts are heated, they should not be put directly into the coolant after being removed from the furnace. This can easily cause excessive local temperature differences and deformation and cracking. The correct method is to place the part in the air for pre-cooling before putting it into the coolant for quenching. To ensure that the cooling rate is uniform in all parts, the part should be rotated properly after being placed in the coolant, preferably in a variable direction.

 

The choice of coolant is equally important. For alloy steels, isothermal or graded quenching using potassium nitrate and sodium nitrite heat baths is an effective method of reducing distortion, especially for handling die-casting molds with complex shapes and precise dimensional requirements. Some porous mold parts shrink when cooled in oil and expand when cooled in nitrate. The rational use of two different media can also reduce the distortion of parts due to quenching.

Tempering treatment control

After quenching, die-casting mold parts in the coolant should not stay in the air for too long. Still, they should be promptly put into the tempering furnace for tempering treatment to eliminate the internal stress of the parts and reduce deformation and crack tendency. Especially for some die-casting mold parts that need to be wire-cut, using graded quenching and multiple tempering heat treatments before the wire-cut process can effectively improve the hardenability of the parts so that the internal stress is evenly distributed and less likely to deformation and cracking. In the tempering process, it is important to avoid low-temperature tempering brittleness and high-temperature tempering brittleness.

Heat Treatment Frequently Asked Questions and Answers

Quenching Common Problems and Tips for Solving Them

*Ms point decreases with the increase of C%

When quenching, the temperature at which the supercooled Wirthian body begins to metamorphose into the Mada bulk is called the Ms point, and the temperature at which metamorphosis is completed is called the Mf point. The higher the %C content, the lower the Ms point temperature. 0.4% C carbon steel has a Ms temperature of about 350°C, while 0.8% C carbon steel is reduced to about 200°C.

The quenching solution can be added with the appropriate additives

(1) Salt can be added to the water to double the cooling rate: the cooling rate of brine quenching is fast, and there will be no quenching and cracking and uneven quenching phenomenon; it can be called the ideal quenching and hardening with coolant. The proportion of salt added to the weight percentage of 10% is appropriate.

(2) impurities in the water than pure water are more suitable for quenching fluid: the addition of solid particles in the water will help clean the workpiece’s surface and destroy the vapor film effect, which increases the cooling rate and prevents the occurrence of quenching spots. Therefore quenching treatment, not pure water but a mixture of water quenching technology, is a very important concept.

(3) polymer can be blended with water into a water-soluble quenching fluid: polymer quenching fluid can be blended according to the degree of water from water to oil cooling rate of the quenching fluid; very convenient, and no fire, pollution, and other public hazards, quite forward-looking.

(4) Dry ice plus ethanol can be used for deep cooling treatment capacitive fluid: dry ice can be added to ethanol to produce a uniform temperature of -76 ℃, a very suitable low-temperature coolant.

The correlation between hardness and quenching speed

By changing the quenching and cooling rate of the steel, different hardness values will be obtained, mainly due to the different organizations produced within the steel. When the cooling rate is slow and the steel passes through the Ps curve, the temperature of the metamorphosis of the Worsted body is higher and the Worsted body will produce a poloid, the point where the metamorphosis begins is the Ps point and the point where the metamorphosis ends is the Pdf point, the hardness of the poloid is smaller. If the cooling rate is accelerated and the cooling curve does not cut through the Ps curve, the Wirthtian body will metamorphose into the harder Jatan bulk. The hardness of Asada bulk is related to the carbon content of the solid solution, so the hardness of Asada bulk will increase with the %C content. Still, after 0.77%C, there is no significant increase in the amount of solid carbon solution in Asada bulk, and the hardness changes tend to moderate.

The difference between quenching and tempering cooling methods

There are three common cooling methods for quenching, namely: 

(1) continuous cooling

 (2) constant temperature cooling

(3) stage cooling. 

To reduce the occurrence of quenching and cracks during the quenching process above the critical zone temperature, rapid cooling above the critical cooling rate is appropriate; when entering the danger zone, the use of slow cooling is an extremely important key technique. Therefore, stage cooling or constant temperature cooling (hemp tempering) is most appropriate when this type of cooling is applied.

Common cooling methods for tempering include rapid and slow cooling, with alloy steels generally using rapid and tool steels preferably using slow cooling. When cooled sharply at tempering temperature, tool steels are susceptible to cracking due to residual austenite metamorphosis, called tempering cracks.

The role of residual austenite after quenching

The presence of austenite and residual austenite in quenched workpieces can cause cracks when left at room temperature for a long period. In addition, another major disadvantage of the residual Wasteland body is its low hardness, which deteriorates the cutting properties of the tool. Deep cooling can induce the metamorphosis of jasper bodies so that they cannot be metamorphosed even if they are cooled further. External machining can be used to metamorphose unstable residual bodies into jasper bodies to reduce residual bodies’ effect on the steel’s properties.

Reasons for lack of hardness after quenching

The purpose of quenching is to achieve a satisfactory hardness on the surface of the steel. If the hardness value is unsatisfactory, it may be caused by the following factors: (1) the quenching temperature or the temperature of the Wasteland body is not sufficient; (2) it may be due to insufficient cooling rate; (3) if decarburization occurs on the surface of the workpiece before the heat treatment, the effect of surface hardening of the workpiece will be greatly reduced; (4) if there is rust or black skin on the surface of the workpiece, the hardness of the area will be significantly insufficient. (4) If there is rust or black skin on the workpiece’s surface, the area’s hardness will be significantly insufficient, so it is advisable to use the bead strike method to remove the surface of the workpiece before applying quenching treatment.

Causes of quenching and cracking

The main causes of quench cracking are the size and shape of the workpiece, the carbon content, the cooling method, and the pre-treatment method. Hardening cracking occurs in steel heat treatment because the quenching process generates metamorphic stresses, which are related to the process of nephrite metamorphosis, and usually, the steel does not break as soon as the nephrite metamorphosis begins, but when the nephrite metamorphosis is about 50% complete (at a temperature of about 150°C), i.e., before the end of the quenching process. This is why the quenching process requires rapid cooling at high temperatures and slow cooling at low temperatures. If the key to quenching can be mastered, “fast and then slow,” quenching cracks can be minimized.

 

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