Knowledge of physical properties of forging die materials

It has good matching of mechanical properties such as tensile strength, yield strength, area shrinkage, elongation, impact toughness and hardness within the range of mold use temperature (room temperature to higher working temperature).

1) Tensile strength and yield strength

Different types of dies focus on the specific indicators of mechanical properties required by forging die materials. At the same time, we should not only consider room temperature mechanical properties and ignore the more important performance indicators of actual working temperature; we should not pursue comprehensive high indicators that are blindly divorced from reality. For example: when cold forging hard-to-deform alloy forgings with extremely high deformation resistance, when the stress on the die exceeds the tensile strength of the forging die material at the service temperature, the die will be cracked and scrapped; if the stress on the die exceeds the forging die material At the yield strength at the operating temperature, permanent deformation such as die collapse will be caused, resulting in out of tolerance forgings. Under such conditions, it is necessary to select forging die materials with high tensile strength and yield strength. Generally, the tensile strength of forging die materials should be greater than 30% of the stress that should be tolerated.

2) Impact toughness, area shrinkage and elongation

Since the selection of the impact toughness index of the forging die material is closely related to the forging load properties, if the impact toughness of the forging die material is low, the die may crack and be scrapped for the dies used for impact load equipment such as forging hammers and screw presses. ; Under such conditions, it is necessary to choose a forging die material with high impact toughness.

For forging die materials with high area shrinkage and elongation, it can continue to be used in the case of tiny cracks, and will not break quickly. Therefore, under the conditions of tensile strength, yield strength and hardness indicators, as far as possible to increase the area shrinkage and elongation.

3) Hardness

In view of the fact that the hardness has a corresponding relationship with the strength index, it is also closely related to the wear resistance of the mold, and the hardness (or wear resistance) should be improved as much as possible under the conditions of impact toughness, section shrinkage and elongation permitting. Forging die hardness and red hardness are important properties of forging die materials, and the die should be able to keep its shape and size unchanged when working at high temperatures.

As we all know, there is a conflicting relationship between the tensile strength, yield strength, hardness, section shrinkage, elongation, and impact toughness of the same forging die material. If the former is improved, the latter must be sacrificed. The actual working environment of the mold is used to select the corresponding performance matching.

It must be pointed out that the hardness of the material can not only reflect the strength level to a certain extent, but also has a corresponding relationship between hardness and wear resistance. Generally, materials with high hardness have good wear resistance. Therefore, many forging die materials are usually only Simply specify the requirements for hardness.

2. Fatigue performance

The fatigue properties of forging die materials include mechanical fatigue and thermal fatigue. The hot work die is cycled alternately between high mechanical load to zero mechanical load and temperature range of 200°C to 600°C for a long time. The stress caused by the loading and unloading of these two pulses is superimposed. Over time, micro-cracks will appear on the surface of the die cavity; micro-cracks Further development will accelerate die wear and produce fragments, which will lead to die failure. Therefore, it is required that the forging die material should have good resistance to mechanical fatigue and thermal fatigue resistance.

3. Organizational stability

Usually, the working temperature of the hot work die is 200℃～600℃ or even higher. Under such a wide temperature range and the long-term action of pulsed thermal and mechanical loads, it will cause changes in the structure and properties of the forging die material (such as hardness). ) instability, leading to accelerated wear, plastic deformation, and cracking, resulting in premature die failure. Therefore, the stability of the forging die material is of great significance to the service life of the forging die. The good thermal conductivity and tempering resistance of the forging die material can make the forging die maintain the hardness and structural stability of the die surface under complex working conditions.

A simple method commonly used to check the structural stability of forging die materials: keep the die at the working temperature of the die for a long time (usually select a predetermined life period), and then measure the corresponding hardness at different temperatures and different times. The hardness value can qualitatively reflect the resistance of the material. level of tensile strength and abrasion resistance.

4. Process performance of cold and hot processing

The forging die material is used in the state of high hardness, high strength, high wear resistance and sufficient toughness, and its cold and hot processing is very difficult. Therefore, it is required that the forging die material should have an important influence on the comprehensive technical and economic indicators of the die. Good smelting, casting, forging, machining, heat treatment and surface treatment and cold and hot processing performance.

5. Metallurgical quality

The metallurgical quality of the forging die material has a very important influence on the reliability, life and economy of the die. The low and high magnification structure and mechanical properties of any part of the mold must meet the requirements of the technical conditions, and non-metallic and metal inclusions, shrinkage holes, uneven grains, severe segregation and porosity, white spots, etc. are not allowed. Low-magnification structural defects such as cracks and naphthalene-like fractures, as well as high-magnification structural defects such as overburning, overheating, severe decarburization, and carburization.

After the composition of the die steel is determined (according to the national or enterprise standards), the main factors that determine the quality of the die steel module are the metallurgical process and subsequent processing technology. Production practice has proved that the steel smelted by electroslag furnace is better, with high purity, compactness, uniformity and small anisotropy. The modules made by electroslag ingots after forging thermal deformation, repeated upsetting and corresponding heat treatment are more suitable for manufacturing. Forging die.

6. Hardenability

The higher the hardenability of the die steel, the deeper the hardened layer (the depth of the martensite structure on the surface to the semi-martensite structure), the better the die wear resistance. Hardenability is an important indicator to measure the ability of die steel to accept quenching, and an important basis for selecting die steel and its heat treatment process.

7. Physical properties

The forging die material requires good thermal conductivity to ensure that the heat on the surface of the die cavity is conducted and dissipated as soon as possible, so as to avoid the reduction of the mechanical properties of the die due to the high surface temperature of the working part. Maintaining the proper hardness of the mold is conducive to reducing thermal wear and thermal fatigue damage.