Three basic principles of mold steel selection

 Three basic principles of mold steel selection

• Meets working conditions

1, Wear resistance

When blank is plastically denatured in mold cavity, it flows and slides along surface of cavity, causing severe friction between surface of cavity and blank, which causes mold to fail due to wear. Therefore, wear resistance of material is one of the most basic and important properties of mold.

Hardness is a major factor affecting wear resistance. In general, the higher hardness of mold parts, the smaller wear amount, and the better wear resistance. In addition, wear resistance is also related to type, quantity, shape, size and distribution of carbides in material.

2, Toughness

Most of working conditions of mold are very bad, and some often suffer from a large impact load, resulting in brittle fracture. In order to prevent mold parts from suddenly breaking during work, mold must have high strength and toughness.

Toughness of mold depends mainly on carbon content, grain size and microstructure of material.

3, Fatigue fracture performance

During working process of mold, under long-term effect of cyclic stress, fatigue fracture is often caused. Form has small energy multiple impact fatigue fracture, tensile fatigue fracture, contact fatigue fracture and bending fatigue fracture.

Fatigue fracture properties of a mold depend mainly on its strength, toughness, hardness, and amount of inclusions in material.

4, High temperature performance

When working temperature of mold is higher, hardness and strength are lowered, resulting in early wear of mold or plastic deformation and failure. Because mold material should have high anti-tempering stability, to ensure that mold has high hardness and strength at working temperature.

5, Resistance to cold and heat fatigue

Some molds are in a state of repeated heating and cooling during working process, causing surface of cavity to be subjected to tensile and pressure-induced stresses, causing surface cracking, spalling, increasing friction, impeding plastic deformation, and reducing dimensional accuracy, thereby resulting in mold failed. Hot and cold fatigue is one of the main forms of hot work die failure. It should have high resistance to cold and heat fatigue.

6, Corrosion resistance

Some molds such as plastic molding, when working, due to presence of chlorine, fluorine and other elements in plastic, strong corrosive gases such as hci, hf is resolved to analyze after heating, eroding surface of mold cavity, increasing its surface roughness, and aggravating wear and tear.

• Mold meets process performance requirements

Mold manufacturing process generally involves several processes such as forging, cutting, and heat treatment. In order to ensure quality of mold and reduce production cost, material should have good forgeability, machinability, hardenability, hardenability and grindability. It should also have small oxidation, decarburization sensitivity, quenching, deformation cracking tendency.

1. Forgeability

It has low hot forging deformation resistance, good plasticity, wide forging temperature range, low tendency for forging cracking and precipitation of reticulated carbides.

2, Annealing processability

Spheroidizing annealing temperature range is wide, annealing hardness is low, fluctuation range is small, and spheroidization rate is high.

3, Cutting processability

Large cutting amount, low tool loss and low surface roughness.

4, Oxidation, decarburization sensitivity

When heated at high temperature, anti-oxidation energy is good, decarburization speed is slow, and it is not sensitive to heating medium, and tendency to produce pitting is small.

5, Hardenability

Uniform and high surface hardness after quenching.

6, Hardenability

After quenching, a deep hardened layer can be obtained, which can be hardened with a mild quenching medium.

7, Quenching deformation cracking tendency

Conventional quenching volume change is small, shape is warped, distortion is slight, and abnormal deformation tendency is low. Conventional quench cracking has low sensitivity and is not sensitive to quenching temperature and workpiece shape.

8, Grindability

Relative loss of grinding wheel is small, maximum amount of grinding without burn is large, and it is not sensitive to quality of grinding wheel and cooling condition, and it is not easy to cause abrasion and grinding crack.

• Mold meets economic requirements

In the selection of mold material, principle of economy must be considered to reduce mold manufacturing process cost as much as possible. Therefore, under premise of satisfying performance of use. Firstly, price is lower, carbon steel can be used without alloy steel, and domestic materials can be used without imported materials. In addition, production and supply of market should also be considered when selecting materials. Selected steel grades should be as small and concentrated as possible, and easy to purchase.

Selection of automotive stamping die materials

Due to different functions of various components in the mold, requirements for materials and principles of selection are also different. Therefore, rational selection of mold materials is also a very important task in die design. It is not only related to cost of parts, but also seriously affects production.

With development of automotive industry, people’s demand for automobiles is growing, and the biggest factor affecting production is how to use die materials.

Although working parts of mold should be made of materials better than those used in other parts. Generally speaking, depending on the conditions and requirements of mold, there are four main types:

1) For mold working parts with simple shape and small stamping size. Common carbon tool steels, such as T8A, T10A, etc.

2) For mold parts with complicated shapes and large stamping parts, alloy tool steel or high speed tool steel, such as Cr12, CrWMn, Cr12MoV, W18Cr4V, W6MoSCr4V2, etc., are often used.

3) For mold working parts with high precision of stamping parts or mold life, commonly used hard alloy or steel-bonded hard alloys, such as YG15, YG20, GW50, etc.

4) For working parts of large molds (such as automobile cover dies), common cast iron or cast carbon steel is used, and some are also welded and strengthened at cutting edge parts, such as gray cast iron HT250, cast steel 2G270-500 and so on.

In selection of mold parts and materials, in addition to the most important selection of working parts, pillars and guide sleeve parts are more prominent. Generally, low carbon steel in high quality carbon steel is used for carburizing and quenching or bearing steel.

It must be pointed out that material selection of mold parts is the most important and rigorous, and requirements for its heat treatment industry must be reasonable.

In addition to above-mentioned parts, it is ok to use medium-carbon steel in carbon structural steel or high-quality carbon steel.

Large drawing parts, deep drawing mold materials for automobile covers

Alloy cast iron or high strength ductile iron can be used. Ductile iron can be immersed in lubricating oil, and graphite in structure has self-lubricating effect, which can effectively reduce friction in drawing, and has low cost and easy processing.

High-strength ductile iron can be austempered by double-medium delayed cooling martensite to obtain high strength and toughness with a hardness of 55~58HRC. Firstly, mold is slowly preheated and then heated to 880~900℃. After heat preservation, air is pre-cooled, then brine is quenched to about 550℃, then it is transferred to oil cooling. When mold temperature drops to about 250℃, it is put into 180~200. Hot oil of °C is kept at moderate temperature for 2~3 hours, then oil temperature is lowered to about 170 °C for 5~7 hours, and finally transferred to air cooling.

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