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How To Reasonably Select Die Steel

How To Reasonably Select Die Steel

How to reasonably select die steel is a complex system engineering, but it is also a key link of die manufacturing. It is an important work related to die life and cost. The following is an analysis from the basic performance, process performance, metallurgical quality, universality and use requirements of die steel.

Basic properties of die steel

When selecting die steel, we must first consider some basic properties of die steel, which must be able to meet the needs of the manufactured die. In general, three properties are the main, namely the wear resistance, toughness, hardness and red hardness of steel. These three properties can comprehensively reflect the comprehensive properties of die steel, and their application scope should be determined to a certain extent. Of course, for the requirements of a mold, one or two of them may be primary, while the other one or two are secondary.

1. Wear resistance of die steel

When the die is working, the surface often has to produce strong friction with the workpiece for many times. In this case, the die must still maintain its dimensional accuracy and surface roughness without early failure. It is required that the die steel can not only bear mechanical wear, but also form a thin and dense attached oxidation die on the friction surface of the die when bearing heavy load and high-speed friction, maintain lubrication, prevent adhesion and welding between the die and the surface of the processed workpiece, resulting in scratch on the surface of the workpiece, and reduce the damage caused by further oxidation on the die surface. In order to improve the wear resistance of die steel, it is necessary to adopt reasonable production and treatment processes to make the die steel not only have high hardness, but also make the composition, morphology and distribution of carbide and other hardening phases in the die steel reasonable. Of course, the lubrication in the working process of the die and the surface treatment of the die steel also have a good impact on improving the wear resistance of the die.

2. Toughness of die steel

For dies subjected to strong impact load, such as punch of cold working die, hot forging die for hammer, cold upsetting die, hot upsetting die, etc., the toughness of die steel is a very important consideration. For dies working at high temperature, the high temperature toughness at working temperature must also be considered. For the die subjected to multi-directional impact load, its isotropy must also be considered. The chemical composition, grain size, carbide, composition, quantity, morphology, size and distribution of inclusions of die steel: metallographic structure and micro segregation will affect the toughness of die steel. The purity of steel and the direction of forging and rolling deformation will have a great impact on the transverse properties. The toughness of die materials often contradicts with wear resistance and hardness. It is an important way to reasonably select the heat treatment and toughness of steel to meet the needs of mold processing, so as to obtain the best mold performance and toughness.

Die Steel

3. Hardness and red hardness

Hardness is the main technical performance index of die steel. The die must have high hardness and strength in order to maintain its original shape and size. Generally, the quenching and tempering hardness of cold working die steel is required to be about HRC60, while that of hot working die steel is about hrc45 ~ 50, and the hot working die steel is required to maintain a certain hardness at its working temperature. Red hardness refers to the ability of die steel to maintain its hardness and structural stability and resist softening at a certain temperature. It is an important performance index for hot work die steel and some heavy-duty cold work die steel. In addition, according to the actual working conditions of different molds, the actual required properties should be considered respectively. For example, the cold and hot fatigue resistance of hot working die steel should be considered, and the erosion resistance of molten metal should be considered for die-casting die; Attention should be paid to the isotropy of heavy-duty cavity die; For hot working dies working at high temperature, its oxidation resistance at working temperature should be considered; For molds working in corrosive media, attention should be paid to their corrosion resistance; For the mold working under high load, its compressive strength, tensile strength, bending strength, fatigue strength and fracture toughness should be considered.

Technological properties of die steel

In the total manufacturing cost of molds, especially for small precision and complex molds, the cost of mold materials often accounts for only 10% ~ 20% of the total cost, sometimes even less than 10%; The expenses of machining, heat treatment, surface treatment, assembly and management account for more than 80% of the cost. Therefore, the process performance of mold materials has become an important factor affecting the mold cost. Improving the process performance of mold can not only make the mold production process simple and easy to manufacture, but also effectively reduce the mold manufacturing cost. The process performance of die materials often needs to be considered as follows.

Machinability of die steel

The machinability of die materials includes cold working properties, such as cutting, grinding, polishing, cold extrusion and cold drawing. Hot working properties include thermoplastic and hot heating temperature range. Die steel mainly belongs to hypereutectoid steel and ledeburite steel, and its cold working and hot working properties are generally not very good. In the production process, the process parameters of hot working and cold working must be strictly controlled to avoid defects and waste products. On the other hand, it is also necessary to improve the purity of steel, reduce harmful impurities, improve the microstructure of steel, and take some measures, In order to improve the process performance of steel and reduce the manufacturing cost of die. In order to improve the machinability and grindability of die steel, since the 1930s, various free cutting die steels have been developed by adding an appropriate amount of sulfur, lead, calcium, rare earth metals and other elements to the steel or the elements that lead to the graphitization of carbon in die steel. Later, it was found that after some free cutting elements were added, some harmful inclusions (such as iron sulfide) would be produced in the die steel, which would reduce the mechanical properties of the steel, especially the transverse plasticity and toughness. Therefore, the molten steel was denatured in the later stage of refining by adding denaturants (such as Sica, rare earth elements, etc.), The formation of calcium rich sulfide or rare earth sulfide makes the sulfide spheroidize, inhibits the adverse effect of sulfur on the mechanical properties of steel, retains and gives play to its favorable role in the machinability and grindability of steel, and further develops the free cutting die steel. Some die materials, such as high vanadium high speed steel and high vanadium high alloy die steel, have poor grindability and low grinding ratio, which is not convenient for grinding. In recent years, the use of powder metallurgy can make the carbides in the steel fine and uniform, completely eliminate the large particle carbides in the high vanadium die steel produced by ordinary process, and not only greatly improve the grindability of this kind of steel, Moreover, the plasticity and toughness of the steel are improved, so that it can be popularized and applied in die manufacturing. Some molds have low requirements for surface roughness, such as plastic molds requiring mirror polishing and some cold working molds. It is necessary to use mold materials with good polishing performance. This kind of steel is often refined by electroslag remelting or vacuum arc remelting to obtain high-purity steel to meet the requirements of mirror polishing.

Properties of die steel

(1) leather pattern processability: some plastic products require the manufacture of surfaces with leather patterns, decorative patterns or text patterns. In order to produce these products, it is required to process corresponding clear patterns and patterns on the surface of the mold pressing these products. The chemical etching process is generally used to process these patterns and leather patterns. The mold material is required to adapt to this chemical etching process. After etching, the leather patterns and patterns with clear patterns and textures can be obtained on the mold surface.
(2) casting process performance: in order to simplify the production process, domestic and foreign countries have been committed to developing the casting process to directly produce the casting blank close to the shape of the finished mold in recent years. For example, China has studied the use of casting technology to produce some cold working molds, hot working molds and glass forming molds. Accordingly, some casting die steels have been developed, which are required to have good casting process properties, such as fluidity, shrinkage and so on.
(3) weldability: some molds are required to weld special wear-resistant or corrosion-resistant materials in the parts with the most severe working conditions. Some molds hope to be repaired by surfacing process during use and then reused. For such molds, it is required to select mold materials with good weldability to simplify the welding process, avoid or simplify the pre welding preheating and post welding treatment process, better meet the needs of the welding process, and develop a number of mold materials with good weldability.
(4) cold deformability: in order to simplify the process and improve the manufacturing efficiency of the mold, some cavity molds produced in batch adopt the cold extrusion process to press the cavity, and the cavity of the mold is directly pressed out with the hardened punch. The mold material is required to have good cold deformability, such as the low-carbon and low silicon steel in the plastic mold steel.

Quenching temperature and quenching deformation

In order to facilitate production, it is hoped that the quenching temperature range of die materials should be wider, especially when some dies require flame heating and local quenching, it is difficult to accurately measure and control the temperature, so die steel is required to adapt to a wide quenching temperature range. During heat treatment, the deformation degree of dies is required to be small, especially some precision dies with complex shapes, It is difficult to repair after hardening, so the requirements for the deformation degree of quenching and tempering are more strict. It should be made of micro deformation die steel.

Hardenability and hardenability

The hardenability mainly depends on the carbon content of the steel, and the hardenability mainly depends on the chemical composition, alloy element content and microstructure before quenching. For most of the cold working dies requiring high hardness, the requirement for hardenability is high; For most hot working molds and plastic molds, the requirements for hardness are not too high, and their hardenability is often considered more; Especially for some large section deep cavity molds, in order to make the core of the mold get good structure and uniform hardness, it is required to select die steel with good hardenability. In addition, for the mold with complex shape, high precision and easy to produce heat treatment deformation, in order to reduce its heat treatment deformation, the quenching medium with weak cooling capacity (such as oil cooling, air cooling, pressure quenching or salt bath quenching) is often used as far as possible. It is necessary to use the mold material with good hardenability to obtain satisfactory quenching hardness and hardening layer depth.

Oxidative decarburization sensitivity

During the heating process of the die, if oxidation and decarburization occur, the shape and performance of the die will be changed, the hardness, wear resistance and service life of the die will be affected, and the early failure of the die will be caused. Some die steels with high molybdenum content are easy to oxidize and decarburize, which has limited their popularization and application for a period of time. After the development of heat treatment process and equipment, special heat treatment processes (such as vacuum heat treatment, controllable atmosphere heat treatment, salt bath heat treatment, etc.) can avoid oxidation and decarburization, and these die steels can be successfully popularized and applied. Although molybdenum based alloys have excellent high temperature properties, their application range is seriously limited due to their easy oxidation at high temperature.

Metallurgical quality of die steel

Metallurgical quality also has a great impact on the performance of die steel. Only with excellent metallurgical quality can we give full play to various properties of die steel. Therefore, suitable materials with excellent metallurgical quality must be selected for the production of precision, complex and long-life molds. Major metallurgical production departments at home and abroad are also doing everything possible to improve the metallurgical quality of die steel. It mainly includes the following aspects:

Smelting quality and forging and rolling process

For high-quality die steel, processes such as out of furnace refining, vacuum treatment, vacuum smelting, powder spraying and electroslag remelting are widely used at home and abroad to reduce the content of harmful elements, oxygen, hydrogen and inclusions in the steel, and fine tune the chemical composition and pouring temperature. Electroslag remelting can also effectively improve the macrostructure and density of steel and improve the isotropy of die steel. The tests of some domestic manufacturers show that the transverse impact toughness value of 4Cr5MoSiV1 steel produced by electric arc furnace is only equivalent to 31% of the longitudinal direction. After electroslag remelting, the transverse impact toughness value of 4Cr5MoSiV1 steel can be equivalent to 70% of the longitudinal direction, which can be increased by more than one time. For die steel with special requirements, powder high speed steel and powder high alloy die steel produced by powder metallurgy process can better improve the microstructure and properties of steel. On the basis of ensuring a certain forging ratio, upsetting drawing forging and cross rolling processes should be adopted as far as possible to improve the isotropy of die materials. In order to reduce the machining allowance and improve the utilization rate of materials, precision forging machine, fast forging hydraulic press and high-precision continuous rolling machine are widely used to provide high-precision steel to meet the needs of die manufacturing.

Heat treatment and finishing of die steel

For the heat treatment of forged and rolled materials, controllable atmosphere or vacuum heat treatment shall be adopted to avoid oxidative decarburization. Some plastic die steels and hot work die steels shall be pre hardened by the Metallurgical Department. For some hot work die materials with high requirements, the Metallurgical Department should refine the structure in advance to eliminate the coarse carbides and chain distributed carbides in the steel and obtain the fine and evenly distributed carbide structure, so as to further improve the various properties of the steel, especially the isotropy. According to some foreign reports, some hot work die steels produce high-quality and high directivity die steels through electroslag remelting multidirectional forging (rolling) – microstructure refinement. The transverse impact toughness value can be equivalent to more than 90% of the longitudinal impact toughness value. Many steel mills have named commodity brands for the steel produced by this process, such as isodisc of Bole steel plant in Austria; Many metallurgical production departments in China are also committed to this work. In addition, considering the metallurgical quality of each part of the steel, when manufacturing the mold, attention should be paid to making the main working surface of the mold (such as cavity or edge) close to the surface of the steel; In general, the surface of the steel is a relatively clean part of the steel, while the center of the steel is an area where the low magnification defects of the steel are relatively concentrated; Especially in the large section ledeburite steel, the uniformity of eutectic carbide in the central part of the steel will be 2 ~ 3 levels higher than that in the surface part. In addition, the direction of the main load of the die should be consistent with the deformation direction of the steel, so as to reduce the adverse impact of the anisotropy of the steel on the die.

Other factors that should be considered in reasonable selection of die steel

When selecting die steel, according to the service conditions and requirements of the die, in addition to the above factors, especially the main performance of the die steel must adapt to the service conditions and requirements of the die, it is also necessary to consider the price and universality of the selected die steel. Generally, when the batch of workpieces produced is large and the size of the die is small, the share of die steel in the die manufacturing cost is very small. The price of die steel can not be taken as the main consideration index, and the more advanced and applicable die steel can be selected as far as possible. For large or super large molds with simple shape, since the cost of mold steel will account for a large share of the total cost of the mold, the mold steel with lower price can be selected according to the batch of production workpieces, or the mold steel with lower price can be selected for the mold body, and in the key working parts of the mold, such as cavity or cutting edge, Using the method of insert or surfacing to insert or overlay the high-grade die steel can not only improve the service life of the die, but also reduce the material cost.
The universality of die steel is also a factor that must be considered when selecting die steel. Generally, the amount of die steel is small, and there are many varieties and specifications. In order to facilitate procurement and material preparation in the market, the universality of materials should be considered. In addition to special requirements, the general die steel produced in large quantities shall be used as far as possible. Due to the relatively mature technology of general die steel, the accumulated production process and use experience are more, and the performance data are relatively complete, which is convenient for reference in the design and manufacturing process. In addition, the selection of general die steel can facilitate the procurement, material preparation and material management.