Performance Requirements of Mold Steel

Now we place focus on the mold steel. This is a material that helps in fabricating the mold. As a result, it must be top-notch. There are expectations for the mold steel, which marks it as quality, and we will analyze them in the following sections.

  • Working condition of the die casting mold
  • Performance requirements of die-casting mold materials
  • Common grades of die casting molds
  • Die-casting die steel material quality requirements and inspection standards
  • Production process and judgment of advantages and disadvantages of die-casting die steel

1.Die-casting Mold Condition

In the process of die casting, the molten metal enters the mold at high pressure, high speed, and high temperature, forming a violent erosion on the surface of the mold cavity, causing corrosion and wear on the surface of the mold. And the die-casting mold is produced at a higher temperature, the surface of the mold is cooled and heated, and the long-term reciprocating cycle is performed.

Research and analysis show that the die-casting mold is mainly affected by three kinds of stress

  1. Thermal stress caused by heat exchange in each die casting cycle
  2. Chemical-physical effects of molten metal on die-casting mold materials during die-casting operations
  3. Local mechanical stress generated during demolding.

The long-term effect of these stresses will lead to the failure of the die casting mold.

2.Die-casting Mold Materials

The die-casting mold materials should have specific properties;

  • High-temperature resistance: It has good resistance to high temperature and tempering.
  • High pressure resistance: There must be sufficient high-temperature impact toughness.
  • High temperature wear resistance: good wear resistance, erosion resistance.
  • Thermal fatigue: Because the mold is continuously subjected to cold and heat cycles during the die casting process, the material must have excellent thermal fatigue crack resistance
  • Good hardenability and small heat treatment deformation: It is necessary to obtain uniform hardness during the heat treatment of the mold, so good hardenability is required. It is especially important to use materials with small heat treatment deformation, especially for large-sized molds.

3.Common Grades of Die-casting Mold Materials

There are many kinds of die-casting mold steels on the market, and there are many patterns. Sometimes I don’t know how to choose the right steel. And we may not know much about mold steel, so now let’s understand the knowledge of mold steel.

Common hot tooling steel grades are as follows:

Common die-casting mold material standards include the ASTM standard in the United States, the JIS standard in Japan, the DIN standard in Germany, and the GB standard in China. In addition, many steel companies also have their own internal standards. Often the internal standards of these companies are higher than the industry standards of the country where the company is located.

There are also industry standards set by some industry organizations, the most common is NADAC’s #207-2016 Special quality die steel & heat treatment acceptance criteria for die casting dies

4.Die-casting Die Steel Material Quality Requirements and Inspection Standards

Taking NADCA #207-2016 as an example, let’s look at the 8 quality requirements that need to be met to meet the professional use of die-casting die steel.

A. Chemical composition

B. Annealing hardness

C. Purity

D. Ultrasonic flaw detection

E. Resilience

F. Grain size

G. Annealing the tissue

H. Band segregation

A.Die Casting Die Steel

The percentage of main alloying elements and impurities

NADCA specifies the category and composition comparison of special steel for die casting, which determines the basic properties of the material;

Grade A corresponds to general-purpose die-casting die steel, which is mainly used in fields with lower requirements. This steel grade can not undergo electro slag remelting (ESR), but other composition standards must meet the regulations. The approximate grades corresponding to A-grade are refined 1.2344, 8407 2M, SKD61

Grade B corresponds to general-purpose die-casting die steel, which requires secondary refining. The approximate grades are electro slag 1.2344, 8407, DAC, etc.

Grade C has high toughness, high hardenability, and higher tempering resistance, suitable for large-scale long-life die-casting molds; the corresponding similar grades are 8418 (dievar) DAC-magic DHA31

Class D corresponds to 1.2343, which requires electro slag remelting. The main highlight is the material’s toughness, which is suitable for large die-casting molds. However, the wear resistance and tempering resistance are not as good as A and B.

E-class performance is between B and C, and the approximate grades are EX1, EX2, DHA21. But the market uses very little.

The content of harmful impurities such as S and P in the die-casting mold material is specified

S and P are harmful elements that will affect the performance of die casting molds. Therefore, its content must be controlled not to exceed the standard. Some large steel mills will further specify the content of As Sn Pb Sb Bi on this standard, such as As+ Sn+ Pb+ Sb+ Bi≤0.04%

And the content of HO N, such as H≤1.5PPM O≤20PPM N≤100PPM

B.Annealed Hardness

When the mold material is delivered, the hardness in the annealed state is less than or equal to 235HB to ensure that the mold material has good machining performance and microstructure.


Magnify 100 times, observe the type and quantity of non-metallic inclusions in the steel under a microscope, and grade them and must be less than or equal to the values ​​specified in the following table A Sulfide B Aluminide C Silicate D Spherical oxide Specified in ASTM E45

D.Ultrasonic flaw detection

NADAC stipulates that ultrasonic inspection must be performed to avoid cracks, voids, and severe segregation but does not specify that the acceptance level of acceptance is to be negotiated between the buyer and the seller. The specific qualification level can refer to SEP1921:E/e or GB/T4162AA, and dense (such as loose, white spots, etc.) and continuous defects (such as crack shrinkage) are not allowed.

E.Impact toughness

Toughness is the ability of the material to absorb energy, and the die-casting mold material requires high impact toughness.

Toughness measurements can be determined by Charpy impact testing.

The test’s principle is that the pendulum’s potential energy is converted into kinetic energy, the sample is impacted, and the test sample absorbs the kinetic energy. The pendulum motion then decays until it stops. The energy absorbed by the sample is measured, and then the impact toughness of the sample is calculated. as the picture shows

F.Grain size

Because the grain size greatly influences the material properties, NADCA stipulates that the ASTM E112 comparison method should be used under a 100 times microscope to measure the grain size of die-casting die steel, requiring ≥ grade 7 or higher.

Annealed Microstructure: As-received steel shall consist essentially of a ferritic matrix with a homogeneous distribution of spheroidized carbides.

G.Annealed Microstructure

Take the annealed and polished sample, corrode it with 5% nitric acid alcohol solution, and observe it under a microscope with a magnification of 500 times. NADCA provides an annealed microstructure rating chart to evaluate whether the sample is qualified or not. Eligible levels are AS1-AS5

H. Banded Segregation

Take the annealed and polished sample, corrode it with 5% nitric acid alcohol solution, and observe it under a microscope with a magnification of 50 times. Use NADCA’s Banding Segregation Rating Chart to compare microscopic banding or microscopic chemical composition segregation. The annealed microstructure should be free of severe banding.

We can ask the steel factory to provide qualification documents to ensure reliable after-sales service when necessary.

5.Production Process and Judgment of the Quality of Die-casting Die Steel

The production of die steel requires the following conditions:

  • The smelting process of electric arc furnace + out-of-furnace refining + vacuum degassing + electro slag can effectively remove the five harmful elements such as S, P, and gas content in the material
  • Use homogenization treatment to reduce band segregation effectively
  • Forging and blanking process with large forging ratio (6-8). Guarantee the density and flaw detection quality of flat steel modules.
  • The preheating process of ultra-refinement + spheroidizing annealing is adopted to ensure that the die-casting die steel has a good microstructure and grain size.
  • Adopt advanced equipment and complete technological process to ensure the final impact toughness of the material.

Bad Process

  • Among them, the induction furnace cannot remove the five harmful gases of S and P, and can only adjust the element composition of the material.
  • Due to the blind pursuit of speed, the electroslag furnace used for inferior steel results in extremely poor crystalline structure and purification effect, which affects the purity and original structure of the steel. Moreover, there is no gas protection in the secondary smelting process, and there is no vacuum remelting technology and equipment, which leads to a further increase in the gas content in the steel.
  • The forging process is rough, and the forging ratio is seriously insufficient, affecting the steel’s density. Often flaw detection reveals inherent defects, resulting in poor performance.

Therefore, when choosing the die-casting mold material, choose large-scale factories as much as possible and steel with brand quality assurance.

The advanced production equipment and reasonable and necessary production process in the steel factory are the fundamental basis to ensure the inherent quality of die-casting die steel.

Some daily tips for judging the quality of die steel:

  • Judging from whether the machining is smooth or not and the roughness after polishing. Good materials tend to be brighter.
  • Observe whether there are visible impurities and cracks on the surface of the polished mold
  • Observe the hardness difference of the dies from the uniform quenching and tempering temperature
  • Test whether the hardness is uniform in different parts of the material and whether there are particularly hard areas.
  • Ultrasonic flaw detection
  • Use a portable metallograph to check the annealing structure and segregation of the material
  • Measure the composition of materials with a portable spectrometer to check whether the content of key elements is sufficient? Are there high levels of other unrelated elements? Is there a big difference in the composition of the same batch of materials?