Improvement on the Service Life of Aluminum Magnesium Alloy Dies (Part Two)
2. Selection of steel for aluminum magnesium alloy die casting dies
Comprehensive factors such as the type, chemical composition, metallographic structure, hardness, toughness and hypoploid structure of mold materials are important reasons for the failure of aluminum magnesium alloy die casting molds. Poor working conditions require aluminum magnesium alloy die casting molds to have high performance of anti tempering stability and resistance to cold and heat fatigue, good resistance to high temperature, high pressure and high speed liquid aluminum magnesium alloy erosion ability, high strength and good toughness.
2.1 3Cr2W8V (H21) steel
3Cr2W8V (H21) steel contains more tungsten, chromium and vanadium elements, which has good hardenability, tempering stability and high thermal strength, and is suitable for die casting molds with high bearing capacity, good thermal strength and good tempering stability.
2.2 4Cr5MoSiV1 (H13) steel
4Cr5MoSiV1 (H13) steel has good toughness and resistance to cold and heat fatigue. It is not easy to produce thermal fatigue cracks. Even if thermal fatigue cracks appear, they are thin and short and not easy to expand. There is no need to preheat before use and it can be cooled by tap water, which has good thermal strength.
2.3 4Cr5Mo2MnSiV1 (Y10) steel
Molybdenum with a mass fraction of about 2% is added, supplemented by elements such as vanadium and mud to improve thermal stability, and appropriate amounts of silicon and manganese are added to increase strength of the matrix, obtaining good thermal fatigue performance and resistance to molten metal corrosion.
2.4 4Cr5MoSiV (H11) steel
4Cr5MoSiV (H11) steel belongs to the tungsten hot work die steel, which has good toughness, good thermal strength, thermal fatigue performance and certain wear resistance under medium temperature conditions. 4Cr5MoSiV (H11) steel is quenched and heat treated under lower austenitizing temperature conditions, which has small heat treatment deformation and small tendency to produce oxide skin, and can resist the erosion effect of molten aluminum.
2.5 3Cr3Mo3VNb (HM3) steel
3Cr3Mo3VNb (HM3) steel is a new type of hot forging die steel with high strength and good toughness. Add element Nb under the condition of low carbon content to improve tempering resistance and thermal strength, which has obvious tempering secondary hardening effect, and can effectively overcome early failure of the mold due to thermal wear, thermal fatigue, thermal cracking, etc.
2.6 4Cr3Mo3SiV (H10) steel
4Cr3Mo3SiV (H10) steel has better hardness, heat resistance and wear resistance, very good hardenability and good toughness, better tempering resistance and thermal stability than that of H13 steel, better impact and fracture toughness than that of 3Cr2W8V steel at working temperature of 500 to 600 ℃. When the tempering temperature exceeds 260℃, the hardness of the steel is higher than that of H13 steel. Using mold materials with high strength and good toughness is a very important measure to improve performance of aluminum magnesium alloy die casting molds and extend the service life of the mold. For example, a certain mold is made from 3Cr2W8V steel with a dimension of φ180x85mm, hardness 42 to 46HRC, and can only die-cast 900 pieces when it is put into use. Later, the mold is made from 4Cr3Mo3SiV, and the service life was increased to 1000.000 pieces.
3. Selection of heat treatment process for aluminum magnesium alloy die casting molds
The heat treatment of the aluminum magnesium alloy die casting mold is to change the structure of the mold steel, so that the mold can obtain the required structure and performance, and can extend the service life of the mold. The heat treatment process specification should be determined according to the mold material, shape, size and complexity.
3.1 Pre-heat treatment
The pre-heat treatment of the die casting mold can adopt three processes, namely continuous annealing, isothermal annealing and quenching and tempering heat treatment. The purpose is to obtain a uniform structure and dispersed carbide before the final heat treatment, improving the strength and toughness of the steel. The continuous annealing process is relatively simple, and a better granular pearlite structure can also be obtained. For die casting molds with complex shapes and high requirements, isothermal annealing can be used to obtain a more ideal granular pearlite structure.
3.2 Quenching and preheating
Steel used for die casting molds are high alloy steel with poor thermal conductivity. Preheating measures are often taken during quenching and heating. The times of preheating and temperature depend on the composition of the mold steel and the requirements for mold deformation. For molds with low quenching temperature, simple shape and low deformation requirements, a preheating with a temperature of 800℃ to 850℃ should be adopted during quenching heating on condition that cracking doesn’t occur. For molds with higher temperature quenching, complex shapes and high deformation requirements, secondary preheating with temperatures of 600 to 650°C and 800 to 850°C is necessary. The purpose is to reduce the stress generated during the heating process and make the overall structure of the mold uniform.
3.3 Quenching heating
The quenching heating temperature of the die casting mold can be implemented according to the quenching heating specification of each steel grade. For example, the quenching temperature of 3Cr2W8V steel is from 1050 to 1150℃, and H13 steel from 1020 to 1100℃. In order to ensure the full dissolution of carbides and obtain uniform austenite and good high temperature performance, the quenching and heating holding time of die casting molds should be appropriately extended. Generally, the heating holding coefficient in the salt bath furnace is from 0.8 to 1.0min/mm.
3.4 Quenching cooling
The oil quenching speed is fast and good performance can be obtained, but the tendency of deformation and cracking is great. Generally, oil cooling is used for die casting molds with simple shapes and low deformation requirements; for die casting molds with complex shapes and high deformation requirements, grading quenching should be used to prevent mold deformation and cracking. Quench cooling should be as slow as possible to reduce quenching deformation. Heating and quenching in a vacuum resistance furnace, gas quenching can be adopted for cooling. Heating and quenching in a salt bath, grading quenching can be adopted for cooling. When the mold is quenched and cooled, it should be tempered immediately after being cooled to 150 to 200℃, and it is not allowed to cool to room temperatures.
3.5 Tempering
The hardness of the die casting mold is achieved by tempering, and the hardness of the die casting mold cavity directly affects the hot and cold fatigue service life of the mold. When materials and quenching temperatures are different, tempering temperatures are also different. For example, hardness of the 3Cr2W8V steel aluminum magnesium alloy die casting mold is generally from 42 to 48HRC, and its tempering temperature is generally selected between 560 and 620 ℃. However, if high temperature quenching is used, the tempering temperature is as high as 670℃. If the quenching temperature is 1150℃, the tempering temperature is 650℃ and the hardness will be 45HRC. The hardness is 35HRC after quenching at a temperature of 1050℃ and tempering at a temperature of 650℃ .
3.6 Surface strengthening treatment
After the die casting mold is quenched and tempered, the surface hardness is not very high. Obtain high hardness and wear resistance on the surface of the die casting mold, while the core part still maintains sufficient strength and toughness. Meanwhile, the anti sticking performance of the aluminum magnesium alloy die casting mold can be improved. Surface nitriding or nitrocarburizing treatment can be performed on the die casting mold. Using toughening treatment and surface strengthening treatment is an important way to improve the performance and service life of the mold. For example, the nitrocarburizing heat treatment medium of H13 die casting mold is ammonia gas plus ethanol, and the process is 580℃x4.5h. After quenching at a temperature of 1030°C and tempering at a temperature of 600°C and 580 gas nitrocarburizing heat treatment, the surface hardness of the mold is above 900HV and the matrix hardness is from 46 to 48HRC, which greatly improve wear resistance, fatigue resistance and corrosion resistance of the mold.
4. Conclusion
In the production of aluminum magnesium alloy die casting molds, it is necessary to analyze and study failure causes according to mold working conditions, correctly selecting mold materials, reasonably formulating heat treatment processes to ensure hardness, wear resistance, core strength and toughness of mold surfaces, preventing corrosion of metal liquid and mold sticking, effectively reducing the rejection rate and significantly increasing the service life of the mold. Production practice has proved that preheating the aluminum magnesium alloy die casting mold to an effective and economical temperature can reduce the temperature difference between the mold and the workpiece, reduce the generation of mold cracks, extend the service life of the mold and increase productivity. Of course, during the use of aluminum magnesium alloy die casting molds, correct use, reasonable management and careful maintenance are also effective measures to reduce the early failure of the mold and increase the service life of the mold.
2.1 3Cr2W8V (H21) steel
3Cr2W8V (H21) steel contains more tungsten, chromium and vanadium elements, which has good hardenability, tempering stability and high thermal strength, and is suitable for die casting molds with high bearing capacity, good thermal strength and good tempering stability.
2.2 4Cr5MoSiV1 (H13) steel
4Cr5MoSiV1 (H13) steel has good toughness and resistance to cold and heat fatigue. It is not easy to produce thermal fatigue cracks. Even if thermal fatigue cracks appear, they are thin and short and not easy to expand. There is no need to preheat before use and it can be cooled by tap water, which has good thermal strength.
2.3 4Cr5Mo2MnSiV1 (Y10) steel
Molybdenum with a mass fraction of about 2% is added, supplemented by elements such as vanadium and mud to improve thermal stability, and appropriate amounts of silicon and manganese are added to increase strength of the matrix, obtaining good thermal fatigue performance and resistance to molten metal corrosion.
2.4 4Cr5MoSiV (H11) steel
4Cr5MoSiV (H11) steel belongs to the tungsten hot work die steel, which has good toughness, good thermal strength, thermal fatigue performance and certain wear resistance under medium temperature conditions. 4Cr5MoSiV (H11) steel is quenched and heat treated under lower austenitizing temperature conditions, which has small heat treatment deformation and small tendency to produce oxide skin, and can resist the erosion effect of molten aluminum.
2.5 3Cr3Mo3VNb (HM3) steel
3Cr3Mo3VNb (HM3) steel is a new type of hot forging die steel with high strength and good toughness. Add element Nb under the condition of low carbon content to improve tempering resistance and thermal strength, which has obvious tempering secondary hardening effect, and can effectively overcome early failure of the mold due to thermal wear, thermal fatigue, thermal cracking, etc.
2.6 4Cr3Mo3SiV (H10) steel
4Cr3Mo3SiV (H10) steel has better hardness, heat resistance and wear resistance, very good hardenability and good toughness, better tempering resistance and thermal stability than that of H13 steel, better impact and fracture toughness than that of 3Cr2W8V steel at working temperature of 500 to 600 ℃. When the tempering temperature exceeds 260℃, the hardness of the steel is higher than that of H13 steel. Using mold materials with high strength and good toughness is a very important measure to improve performance of aluminum magnesium alloy die casting molds and extend the service life of the mold. For example, a certain mold is made from 3Cr2W8V steel with a dimension of φ180x85mm, hardness 42 to 46HRC, and can only die-cast 900 pieces when it is put into use. Later, the mold is made from 4Cr3Mo3SiV, and the service life was increased to 1000.000 pieces.
3. Selection of heat treatment process for aluminum magnesium alloy die casting molds
The heat treatment of the aluminum magnesium alloy die casting mold is to change the structure of the mold steel, so that the mold can obtain the required structure and performance, and can extend the service life of the mold. The heat treatment process specification should be determined according to the mold material, shape, size and complexity.
3.1 Pre-heat treatment
The pre-heat treatment of the die casting mold can adopt three processes, namely continuous annealing, isothermal annealing and quenching and tempering heat treatment. The purpose is to obtain a uniform structure and dispersed carbide before the final heat treatment, improving the strength and toughness of the steel. The continuous annealing process is relatively simple, and a better granular pearlite structure can also be obtained. For die casting molds with complex shapes and high requirements, isothermal annealing can be used to obtain a more ideal granular pearlite structure.
3.2 Quenching and preheating
Steel used for die casting molds are high alloy steel with poor thermal conductivity. Preheating measures are often taken during quenching and heating. The times of preheating and temperature depend on the composition of the mold steel and the requirements for mold deformation. For molds with low quenching temperature, simple shape and low deformation requirements, a preheating with a temperature of 800℃ to 850℃ should be adopted during quenching heating on condition that cracking doesn’t occur. For molds with higher temperature quenching, complex shapes and high deformation requirements, secondary preheating with temperatures of 600 to 650°C and 800 to 850°C is necessary. The purpose is to reduce the stress generated during the heating process and make the overall structure of the mold uniform.
3.3 Quenching heating
The quenching heating temperature of the die casting mold can be implemented according to the quenching heating specification of each steel grade. For example, the quenching temperature of 3Cr2W8V steel is from 1050 to 1150℃, and H13 steel from 1020 to 1100℃. In order to ensure the full dissolution of carbides and obtain uniform austenite and good high temperature performance, the quenching and heating holding time of die casting molds should be appropriately extended. Generally, the heating holding coefficient in the salt bath furnace is from 0.8 to 1.0min/mm.
3.4 Quenching cooling
The oil quenching speed is fast and good performance can be obtained, but the tendency of deformation and cracking is great. Generally, oil cooling is used for die casting molds with simple shapes and low deformation requirements; for die casting molds with complex shapes and high deformation requirements, grading quenching should be used to prevent mold deformation and cracking. Quench cooling should be as slow as possible to reduce quenching deformation. Heating and quenching in a vacuum resistance furnace, gas quenching can be adopted for cooling. Heating and quenching in a salt bath, grading quenching can be adopted for cooling. When the mold is quenched and cooled, it should be tempered immediately after being cooled to 150 to 200℃, and it is not allowed to cool to room temperatures.
3.5 Tempering
The hardness of the die casting mold is achieved by tempering, and the hardness of the die casting mold cavity directly affects the hot and cold fatigue service life of the mold. When materials and quenching temperatures are different, tempering temperatures are also different. For example, hardness of the 3Cr2W8V steel aluminum magnesium alloy die casting mold is generally from 42 to 48HRC, and its tempering temperature is generally selected between 560 and 620 ℃. However, if high temperature quenching is used, the tempering temperature is as high as 670℃. If the quenching temperature is 1150℃, the tempering temperature is 650℃ and the hardness will be 45HRC. The hardness is 35HRC after quenching at a temperature of 1050℃ and tempering at a temperature of 650℃ .
3.6 Surface strengthening treatment
After the die casting mold is quenched and tempered, the surface hardness is not very high. Obtain high hardness and wear resistance on the surface of the die casting mold, while the core part still maintains sufficient strength and toughness. Meanwhile, the anti sticking performance of the aluminum magnesium alloy die casting mold can be improved. Surface nitriding or nitrocarburizing treatment can be performed on the die casting mold. Using toughening treatment and surface strengthening treatment is an important way to improve the performance and service life of the mold. For example, the nitrocarburizing heat treatment medium of H13 die casting mold is ammonia gas plus ethanol, and the process is 580℃x4.5h. After quenching at a temperature of 1030°C and tempering at a temperature of 600°C and 580 gas nitrocarburizing heat treatment, the surface hardness of the mold is above 900HV and the matrix hardness is from 46 to 48HRC, which greatly improve wear resistance, fatigue resistance and corrosion resistance of the mold.
4. Conclusion
In the production of aluminum magnesium alloy die casting molds, it is necessary to analyze and study failure causes according to mold working conditions, correctly selecting mold materials, reasonably formulating heat treatment processes to ensure hardness, wear resistance, core strength and toughness of mold surfaces, preventing corrosion of metal liquid and mold sticking, effectively reducing the rejection rate and significantly increasing the service life of the mold. Production practice has proved that preheating the aluminum magnesium alloy die casting mold to an effective and economical temperature can reduce the temperature difference between the mold and the workpiece, reduce the generation of mold cracks, extend the service life of the mold and increase productivity. Of course, during the use of aluminum magnesium alloy die casting molds, correct use, reasonable management and careful maintenance are also effective measures to reduce the early failure of the mold and increase the service life of the mold.
