"Mold steel" knowledge

Die steel

For the use of the mold, the use of the mold is very wide, and the working conditions of the various molds are very different. Therefore, a wide range of materials are used for the mold, and the mold steel is the most widely used mold material. From general carbon structural steels, carbon tool steels, alloy structural steels, alloy tool steels, spring steels, high-speed tool steels, stainless heat-resistant steels, to martensitic-aged steels and powdered high-speed steels required for special molds, Powder high alloy die steel. Die steel can be generally divided into three categories: cold die steel, hot die steel, and plastic die steel

Cold working die steel

Cold work die steel is mainly used to make a die that press-forms a cold state workpiece. Such as: cold blanking dies, cold stamping dies, cold drawing dies, imprinting dies, cold extruding dies, thread pressing dies and powder pressing dies. Cold working die steels range from a wide variety of carbon tool steels, alloy tool steels, high speed tool steels to powdered high speed tool steels and powdered high alloy die steels.

2. Hot work die steel

Hot work die steel is mainly used to make the die that presses the work under high temperature conditions. Such as: hot forging die, hot extrusion die, die-casting die, hot die forging die. Commonly used hot work die steels include alloy steels with medium and high carbon content added with Cr, W, Mo, V and other alloying elements; hot work die steels with special requirements are sometimes made of high-alloy austenitic heat-resistant die steels. .

Plastic mold steel

Due to the variety of plastics, the requirements for plastic products vary greatly, and various performance requirements are also put forward for the materials used to make plastic molds. Therefore, many industrially developed countries have formed a wide range of plastic mold steel series. Including carbon structural steel, carburizing plastic mold steel, pre-hardened plastic mold steel, age hardening plastic mold steel, corrosion-resistant plastic mold steel, easy-cut plastic mold steel, overall hardened plastic mold steel, martensite Aging steel and plastic mold steel for mirror polishing.

Performance requirements:

Use performance

A strength properties

(1) Hardness Hardness is the main technical index of die steel. The die must maintain its shape and size under the action of high stress and must have a sufficiently high hardness. The hardness of cold die steel is kept at about HRC60 at room temperature, and the hot die steel is generally maintained at HRC40~55 according to its working conditions. For the same steel grade, the hardness is proportional to the deformation resistance within a certain range of hardness values, but there may be obvious differences in the plastic deformation resistance between steels with the same hardness value and different composition and organization.

(2) Red Hardness Hot working molds that operate at high temperatures require the stability of their structure and properties to maintain a sufficiently high hardness. This property is called red hardness. Carbon tool steels and low-alloy tool steels can usually maintain this performance in the temperature range of 180-250°C. Chrome-molybdenum hot work die steels generally maintain this performance in the temperature range of 550-600°C. The red hardness of steel mainly depends on the chemical composition of the steel and the heat treatment process.

(3) Compressive yield strength and compressive bending strength The mold is often subjected to high-strength pressure and bending during use. Therefore, the mold material should have a certain compressive strength and bending strength. In many cases, the conditions for compressive and flexural tests are close to the actual operating conditions of the mold (for example, the compressive yield strength of the measured mold steel is consistent with the deformation resistance exhibited by the punch during operation). . Another advantage of the flexural test is that the absolute value of the strain amount is large, which can more sensitively reflect the difference in deformation resistance between different steel grades and under different heat treatment and tissue conditions.

B toughness

During the working process, the mold is subjected to impact loads. In order to reduce the damage in the form of breakage, chipping, etc. during use, the mold steel is required to have certain toughness.

The chemical composition, grain size, purity, carbides, inclusions, etc., number, morphology, size, and distribution of the die steel, and the heat treatment system of the die steel and the metallurgical structure obtained after the heat treatment are all of the steel. The toughness has a big impact. In particular, the degree of cleanliness of the steel and the distortion of the hot working have a more pronounced effect on its transverse toughness. The toughness, strength and wear resistance of steel are often conflicting. Therefore, we must reasonably choose the chemical composition of the steel and use reasonable refining, thermal processing and heat treatment processes so that the wear resistance, strength and toughness of the mold material can be optimally matched.

Impact toughness is the total amount of energy absorbed by the sample during the entire fracture process during a single impact. However, many tools are fatigue fractured under different working conditions. Therefore, conventional impact toughness cannot fully reflect the fracture properties of die steel. Test techniques such as multiple energy impact fracture work or multiple fracture life and fatigue life are being used.

C Wear resistance

The most important factor in determining the service life of a mold is often the wear resistance of the mold material. The die is subjected to considerable compressive stress and friction during operation, requiring the die to maintain its dimensional accuracy under strong friction. The wear of the mold is mainly mechanical wear, oxidation wear and melt wear. In order to improve the wear resistance of the die steel, it is necessary to maintain the die steel with high hardness, but also to ensure that the composition, morphology and distribution of the carbides or other hardened phases in the steel are reasonable. For heavy-duty, high-speed wear under the conditions of service molds, the surface of the mold steel can be required to form a thin and dense oxide film with good adhesion, to maintain the lubrication effect, to reduce the melting and wear between the mold and the workpiece, such as sticking, welding and so on. Can reduce the mold surface oxidation oxidation wear. Therefore, the working conditions of the mold have a great influence on the wear of the steel.

Abrasion resistance can be measured using a simulated test to determine the relative wear resistance index, as a parameter to characterize the level of wear resistance in different chemical compositions and tissue states. The lifetime before the specified burr height is presented, reflecting the wear resistance of various steel grades; the test is based on Cr12MoV steel (Ñ”=1) for comparison. Figure 1-2-3 shows the results of the wear resistance test of a standard abrasive tool, which is a good reflection of the wear resistance of tool steel under abrasive wear conditions.

D Thermal fatigue resistance

In addition to the cyclical changes in the load under the service conditions of hot die steel, it is also subjected to high temperature and periodic quenching and rapid heating. Therefore, the evaluation of fracture resistance of hot die steel should pay attention to the thermal mechanical fatigue fracture properties of materials. . Thermo-mechanical fatigue is an index of comprehensive performance. It includes thermal fatigue performance, mechanical fatigue crack growth rate, and fracture toughness.

The thermal fatigue performance reflects the working life of the material before the thermal fatigue crack initiation, the material with high thermal fatigue resistance, and the number of thermal cycles of the induced thermal fatigue crack; the mechanical fatigue crack propagation rate reflects the material in the forging after the thermal fatigue crack initiation. Under the effect of pressure, when the crack expands to the inside, the amount of expansion of each stress cycle; the fracture toughness reflects the resistance of the material to the instability of existing cracks. For materials with high fracture toughness, cracks must develop at the crack tip with a sufficiently high stress intensity factor for crack propagation. This means that cracks must have large crack lengths. Under the premise of constant stress, a fatigue crack already exists in a mold. If the fracture toughness value of the mold material is high, the crack must be expanded deeper to allow the instability to expand.

In other words, the thermal fatigue resistance determines the part of the life before fatigue crack initiation; and the crack growth rate and fracture toughness can determine the part of the life that occurs at the subcritical expansion after crack initiation. Therefore, in order to obtain a high life for hot die, the die material should have high thermal fatigue resistance, low crack growth rate, and high fracture toughness value.

The index of the thermal fatigue resistance can be measured by the number of thermal cycles of the induced thermal fatigue cracks, or the number of fatigue cracks and the average depth or length after a certain thermal cycle.

E bite resistance

Occlusive resistance is actually the resistance to cold welding. This property is more important for mold materials. During the test, usually under the condition of dry friction, the test tool steel sample and the material with the occlusal tendency (such as austenitic steel) are subjected to constant speed dual friction motion, and the load is gradually increased at a certain speed. At this time, the rotation The moment also increases correspondingly. This load is called the "occluding critical load." The higher the critical load, the stronger the occlusal resistance.

Process performance

In the mold production cost, the material cost generally accounts for 10% to 20%, while the mechanical processing, heat treatment, assembly and management costs account for more than 80%, so the process performance of the mold is the main factor affecting the production cost of the mold and the ease of manufacture. one.

A Machinability

- Thermal processing performance, refers to thermoplasticity, processing temperature range, etc.;

- Cold processing performance, refers to cutting, grinding, polishing, cold drawing and other processing performance.

Most cold working die steels belong to hypereutectoid steels and to the lanthanum steels. The hot working and cold working properties are not very good. Therefore, the thermal processing and cold working process parameters must be strictly controlled to avoid defects and waste products. On the other hand, by increasing the purity of the steel, reducing the content of harmful impurities, and improving the microstructure of the steel to improve the hot-working and cold-working properties of the steel, the production cost of the mold is reduced.

In order to improve the cold-workability of die steels, since the 1930s, various elements such as S, Pb, Ca, Te and other free-cutting elements or the graphitization of carbon in die steels have been studied. Die steel is cut to further improve its cutting performance and grinding performance, reduce tool wear and reduce costs.

B Hardenability and Hardenability

Hardenability mainly depends on the chemical composition of the steel and the original microstructure prior to quenching; hardenability depends primarily on the amount of carbon in the steel. For most cold die steels, hardenability is often one of the main considerations. For hot work die steels and plastic die steels, the general die size is large, especially for the manufacture of large-scale die sets, whose hardenability is more important. In addition, for molds with complex shapes that are prone to heat-treatment deformation, in order to reduce quench distortion, quenching media with weaker cooling capacity, such as air cooling, oil cooling, or salt bath cooling, are often used as much as possible in order to obtain the required hardness and hardened layer. Depth, you need to use better hardenability of mold steel.

C Quenching temperature and heat treatment deformation

In order to facilitate production, the quenching temperature range of the die steel is required to be as wide as possible, especially when the die is flame-heated and locally quenched. Because it is difficult to accurately measure and control the temperature, the die steel is required to have a wider quenching temperature range.

In the heat treatment of the mold, especially in the quenching process, volume change, shape warping, distortion, etc. are required. In order to ensure the quality of the mold, the heat treatment and deformation of the mold steel are required to be small, especially for the complex shape precision mold, it is difficult to finish after quenching The requirements for heat treatment deformation are more severe and should be made of micro-deformed die steel.

D Oxidation, Decarbonization Sensitivity

In the heating process of the mold, if oxidation and decarburization occur, its hardness, wear resistance, service performance and service life will be reduced; therefore, the oxidation and decarburization sensitivity of the mold steel is required to be good. For mold steels with high molybdenum content, due to their strong oxidative and decarburization sensitivities, special heat treatments such as vacuum heat treatment, controlled atmosphere heat treatment, and salt bath heat treatment are required.

other factors

In selecting the die steel, in addition to the use of performance and process performance, the versatility of the die steel and the price of the steel must also be taken into account. Mold steel is generally not used in large quantities. In order to facilitate the preparation of materials, the generality of steel should be considered as much as possible, and the mass-produced universal mold steel should be used as much as possible in order to facilitate procurement, stock preparation and material management. In addition, a comprehensive analysis must also be made economically, taking into account the manufacturing costs of the molds, the production batch of the workpieces, and the cost of the molds allocated to each workpiece. Comprehensive analysis from the technical and economic aspects to ultimately select a reasonable mold material.