Mold and Mold Parts, as the basic process equipment in industrial production, the quality and life of molds directly affect the precision, production efficiency and manufacturing cost of products. Among the various factors influencing mold quality, the selection of steel is particularly critical. Rational material selection can not only extend the service life of molds and reduce production costs but also improve the stability of product quality. This article will systematically explain the selection method of steel for mold parts from three dimensions: mold service conditions, steel performance, and economy.
I. Selection Based on Mold Service Conditions
Molds undergo complex mechanical loads, thermal loads, and friction during operation. Therefore, steel must be selected to match the specific service conditions.
Regarding mechanical load, mold parts endure different types of stress during operation. For molds primarily subjected to intense impact loads, such as cold heading dies and punching dies, the toughness of the steel should be prioritized. Under such working conditions, medium-carbon alloy tool steels like 5CrNiMo and 5CrMnMo, or matrix steels with excellent impact toughness, can be selected. For molds bearing high unit pressure, such as cold extrusion dies and stamping dies, high strength and hardness are required to resist plastic deformation. High-carbon high-chromium steels like Cr12MoV or high-speed steels like W6Mo5Cr4V2 can be chosen. For molds subjected to cyclic loads, such as hot forging dies, fatigue strength becomes a key indicator. Steels with high purity and uniform carbide distribution should be selected.
The operating temperature range directly affects the performance of the steel. For cold work molds operating below 200°C, room temperature strength, hardness, and wear resistance are the main considerations; for warm work molds operating in the range of 200-500°C, the steel needs to possess certain tempering stability and hot strength; for hot work molds operating above 600°C, high-temperature strength, oxidation resistance, and thermal fatigue resistance must be prioritized, such as selecting hot work mold steels like H13 (4Cr5MoSiV1).
Friction and wear conditions are also crucial basis for material selection. For working conditions dominated by abrasive wear, such as plastics containing hard particles or powder metallurgy molds, high-carbon high-chromium steels like Cr12 and Cr12MoV, or high-speed steels should be selected. These steels contain a large number of hard carbides, effectively resisting abrasive wear. For conditions mainly involving adhesive wear, such as deep drawing dies and drawing dies, in addition to certain hardness, steels with good surface lubricity should be selected, or surface treatment should be considered.
Corrosive environments also influence steel selection. For molds forming corrosive media, such as certain plastics or chemicals (e.g., PVC plastic molds), martensitic stainless steels with excellent corrosion resistance, like 420 (4Cr13), or precipitation-hardening stainless steels, like 17-4PH, should be selected. These steels contain high chromium content, forming a dense passive film on the surface.
II. Steel for Mold Parts: Selection Based on Steel Performance Characteristics
Different steels have different performance characteristics. Understanding these characteristics is the foundation for rational material selection.
Hardness and strength are the basic performance indicators of mold steel. Cold work molds typically require high hardness of 58-64 HRC to ensure wear resistance and compressive strength; hot work molds focus more on strength and hardness retention at high temperatures, generally using hardness in the range of 40-52 HRC. It should be noted that high strength and high hardness are often accompanied by reduced toughness, so a balance between the two must be sought.
Toughness determines the mold’s ability to resist cracking and chipping. For molds with impact loads, high-toughness steel must be selected. From a materials science perspective, refining the grain size, improving purity, and improving carbide distribution can all enhance steel toughness. For example, steel produced by the electroslag remelting process has significantly better toughness than steel produced by ordinary smelting.
Wear resistance is an important guarantee of mold life. The wear resistance of steel depends on hardness, carbide type, and quantity. High-carbon high-chromium steels contain a large amount of M₇C₃ type carbides with hardness up to 1800 HV; high-speed steels contain MC and M₆C type carbides with hardness exceeding 2500 HV. These hard phases greatly improve the wear resistance of the steel.
Heat treatment performance includes hardenability, heat treatment deformation tendency, and tempering stability. Large molds require high hardenability to ensure uniform overall performance; precision molds require small heat treatment deformation to ensure dimensional accuracy; hot work molds require high tempering stability to ensure performance does not degrade during long-term high-temperature operation.
Machinability includes cuttability, grindability, and polishability. For molds with complex cavities, good cuttability can significantly reduce manufacturing costs; molds requiring high surface quality need excellent polishability, which requires steel with high purity, uniform structure, and moderate hardness.
III. Steel for Mold Parts: Selection Based on Economic Principles
Under the condition of meeting technical requirements, economic efficiency is an important factor that must be considered in material selection.
Material cost is an important part of mold cost. The prices of different steel grades vary greatly, from ordinary carbon tool steels to high-end high-speed steels and powder metallurgy steels, the price may differ by several times or even dozens of times. Material selection should avoid the “only high-grade theory”. Under the premise of meeting the use requirements, steel grades with lower prices should be selected as much as possible.
Processing cost is equally important. Steel with poor machinability will increase cutting time, tool wear, and grinding difficulties, leading to an increase in processing costs. In some cases, choosing slightly more expensive but better machinable steel may result in lower total cost.
Mold life and maintenance costs are the key to long-term economy. Long-life molds can reduce replacement times, improve equipment utilization, and reduce cost per piece. For mass-produced molds, even if the initial investment is high, selecting high-performance steel is often economically reasonable.
Resource utilization efficiency should also be considered. The characteristics of China’s mineral resources determine that the cost of some alloy elements is high. When performance is similar, steel grades rich in alloy elements stored domestically should be preferred. For example, molybdenum-containing steel grades are usually more economical than tungsten-containing steel grades.
IV. Steel Selection Examples for Typical Mold Parts
The selection of plastic mold parts needs to comprehensively consider product appearance requirements, production batch, and plastic type. For ordinary plastics and small production batches, pre-hardened steel such as P20 (3Cr2Mo) can be selected, with a hardness of about 30-36 HRC, which can be used directly after processing; for high-transparency plastics or products with high surface quality requirements, steel with excellent polishability such as NAK80 should be selected; for engineering plastic molds containing reinforcing fillers, high-wear-resistant steel such as S136 (4Cr13) or cemented carbide should be used; for large plastic molds, to ensure uniform overall performance, steel with high hardenability should be selected.
The selection of stamping die parts mainly depends on sheet thickness, strength, and production batch. Thin sheet blanking dies can use high-carbon high-chromium steel Cr12MoV; thick sheet blanking or high-strength steel plate stamping requires tougher steel grades such as DC53; precision high-speed stamping dies should use powder high-speed steel to ensure high wear resistance and dimensional stability.
Die-casting mold parts face harsh working conditions and need to have high-temperature strength, thermal fatigue resistance, and resistance to molten metal erosion at the same time. Zinc alloy die-casting molds can use medium-alloy hot work mold steel such as H13; aluminum alloy die-casting molds should use hot work mold steel with high hardenability such as H13, H11; copper alloy die-casting molds require hot work mold steel with higher tungsten content, such as H21, due to higher operating temperatures.
V. Systematic Material Selection Method
The selection of mold steel is a systematic project that requires comprehensive consideration of technical feasibility and economic rationality. The following systematic approach is recommended:
First, analyze the service conditions of the mold in detail, determine the main failure forms and use requirements, and establish clear technical indicators. Second, preliminarily select several steel grades that may meet the requirements based on the technical indicators. Then, conduct a comprehensive performance comparison of the initially selected steel grades, including mechanical properties, process performance, and service performance. Finally, determine the final selected steel based on cost analysis and resource situation.
It is worth noting that with the progress of materials science, new mold steel grades are emerging constantly. Materials engineers should pay close attention to the dynamics of material development and update their knowledge reserves in a timely manner. At the same time, they should also pay attention to the rational use of traditional mature steel grades and avoid blindly pursuing new materials.
The selection of mold steel is both a science and an art. Only on the basis of an in-depth understanding of material properties and mold requirements can the most reasonable material selection decision be made, thereby manufacturing high-quality, long-life, and low-cost mold products.
