Knowledge of Physical Properties of Materials for Forging Mold Processing II
The hardness of forging die processing materials is not only related to strength indicators, but also closely related to the wear resistance of the mold. Under the allowable conditions of impact toughness, cross-sectional shrinkage rate, and elongation rate, the hardness (or wear resistance) should be improved as much as possible. The hardness and redness of the forging die are important properties of the forging die material, and the die should be able to maintain its shape and size unchanged when working at high temperatures.
As is well known, there is a contradictory relationship between the tensile strength, yield strength, hardness and cross-sectional shrinkage, elongation, and impact toughness of the same forging die material. If the former is improved, the latter must be sacrificed, which requires selecting the corresponding performance matching based on the actual working environment of the specific die.
It must be pointed out that the hardness of materials can not only reflect the strength level to a certain extent, but also have a corresponding relationship between hardness and wear resistance. Generally, materials with high hardness also have good wear resistance. Therefore, many forging mold materials usually only specify the requirements for hardness.
1. Physical properties of mold processing materials – fatigue performance
The fatigue performance of forging die materials includes two types: mechanical fatigue and cold and hot fatigue. The hot work mold undergoes alternating cycles from high mechanical load to zero mechanical load and between 200 ℃ and 600 ℃ temperatures for a long time. The superposition of stress caused by the loading and unloading of these two types of pulses leads to the emergence of microcracks on the surface of the mold cavity over time; Further development of microcracks will accelerate mold wear, generate fragments, and lead to mold failure. Therefore, it is required that forging mold materials should have good resistance to mechanical fatigue and cold and hot fatigue performance.
2. Physical properties of mold processing materials – tissue stability
Usually, the working temperature of hot work molds ranges from 200 ℃ to 600 ℃ or even higher. Under such a wide temperature range and the long-term effects of pulsed thermal and mechanical loads, it can cause changes in the microstructure and instability of properties (such as hardness) of forging mold materials, leading to accelerated wear, plastic deformation, and cracking, thereby causing premature failure of the mold. Therefore, the stability of forging die materials is of great significance for the service life of forging dies. The good thermal conductivity and heat resistance of forging die materials can 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 is to keep the mold at working temperature for a long time (usually choose a predetermined lifespan), and then measure the corresponding hardness at different temperatures and times. The hardness value can qualitatively reflect the tensile strength and wear resistance level of the material.
3. Physical properties of mold processing materials – cold and hot processing process performance
Forging die materials are used in a state of high hardness, high strength, high wear resistance, and sufficient toughness, and their cold and hot processing is very difficult. Therefore, it is required that forging die materials should have good smelting, casting, forging, cutting, heat treatment, surface treatment, and cold and hot processing performance that have an important impact on the comprehensive technical and economic indicators of the mold.
4. Physical properties of mold processing materials – metallurgical quality
The metallurgical quality of forging mold materials has a significant impact on the reliability, service life, and economy of the mold. The low magnification and high magnification structure and mechanical properties of any part of the mold must meet the requirements of the technical conditions, and there must be no visible non-metallic and metallic inclusions, shrinkage cavities, uneven grain size, severe segregation and porosity, white spots, cracks, naphthalene like fractures and other low magnification structure defects, as well as high magnification structure defects such as overburning, overheating, severe decarburization, and carburization.
After determining the composition of the mold steel (according to national or enterprise standards), the main factors determining the quality of the mold steel module are metallurgical technology and subsequent processing technology. Production practice has proven that the steel smelted by electric slag furnace is relatively good, with high purity, density, uniformity, and small anisotropy. The modules made by forging hot deformation, repeated upsetting and drawing, and corresponding heat treatment of electric slag steel ingots are more suitable for manufacturing forging molds.
5. Physical properties of mold processing materials – hardenability
The higher the hardenability of mold steel, the deeper its hardening layer (from surface martensite structure to semi martensite structure), and the better the wear resistance of the mold. Hardenability is an important indicator of the ability of mold steel to undergo quenching, and is an important basis for selecting mold steel and its heat treatment process.
6. Physical properties of mold processing materials – Physical properties
The forging die material requires good thermal conductivity to ensure that the heat on the surface of the mold chamber is quickly transmitted and dissipated, and to avoid the mechanical properties of the mold being reduced due to the high surface temperature of the working part. Maintaining the required hardness of the mold is beneficial for reducing thermal wear and thermal fatigue damage.
The physical properties of mold processing materials have advantages such as high hardness, high strength, high toughness, good dimensional stability, no need for heat treatment, reduced time for heat treatment and precision machining, good wear resistance and processing performance, and excellent fatigue resistance. They are applied in the field of hot forging molds, such as automotive crankshafts, connecting rods, gas stove heads, axes, and other hot processing and forging, with high toughness and stability. Nitriding can also improve the service life, making them the trusted best choice in the field of mold steel.