Analysis on strength performance, thermal fatigue resistance and bite resistance of mold steel

1. Strength performance
(1) Hardness
Hardness is the main technical index of mold steel. In order to keep its shape and size unchanged under the action of high stress, the die must have high enough hardness. The hardness of cold working mold steel is generally maintained at about HRC60 at room temperature. According to its working conditions, hot working mold steel is generally required to be maintained in the range of hrc40 ~ 55.
For the same steel, within a certain range of hardness value, the hardness is directly proportional to the deformation resistance; However, there may be obvious differences in plastic deformation resistance between steel grades with the same hardness value and different composition and structure.

(2) Red hardness
The hot working mold working at high temperature is required to maintain the stability of its structure and performance, so as to maintain high enough hardness. This performance is called red hardness.
Carbon tool steel and low alloy tool steel can usually maintain this property in the temperature range of 180 ~ 250 ℃, and chromium molybdenum hot working die steel can generally maintain this property in the temperature range of 550 ~ 600 ℃. The red hardness of steel mainly depends on the chemical composition and heat treatment process of steel.
(3) Compressive yield strength and compressive bending strength
The mold is often subjected to high strength pressure and bending in the process of use, so the mold material is required to have certain compressive strength and flexural strength.
In many cases, the conditions of compression test and bending test are close to the actual working conditions of the mold (for example, the measured compressive yield strength of die steel is consistent with the deformation resistance of the punch during operation).
Another advantage of bending test is that the  value of strain is large, which can sensitively reflect the difference of deformation resistance between different steel grades and under different heat treatment and microstructure.

2. Tenacity
In the working process, the die bears the impact load. In order to reduce the damage in the form of breaking and edge collapse in the process of use, the die steel is required to have a certain toughness.
The chemical composition, grain size, purity, quantity, morphology, size and distribution of carbides and inclusions, as well as the heat treatment system and metallographic structure of die steel have a great impact on the toughness of steel. In particular, the purity and hot working deformation of steel have more obvious effects on its transverse toughness.
The toughness, strength and wear resistance of steel are often contradictory. Therefore, it is necessary to reasonably select the chemical composition of steel and adopt reasonable refining, hot working and heat treatment processes, so as to make the wear resistance, strength and toughness of die materials achieve a good combination.
Impact toughness is the total energy absorbed by the specimen in the whole fracture process during one impact. However, many tools are fatigue fracture under different working conditions. Therefore, the conventional impact toughness can not fully reflect the fracture performance of mold steel. Small energy multiple impact fracture work or multiple fracture life and fatigue life are being used.
3. Wear resistance
The wear resistance of mold materials is often an important factor in determining the service life of dies. The die bears considerable compressive stress and friction in work, which requires the die to maintain its dimensional accuracy under strong friction. The wear of die is mainly mechanical wear, oxidation wear and melting wear.
In order to improve the wear resistance of die steel, it is necessary to not only maintain the high hardness of mold steel, but also ensure that the composition, morphology and distribution of carbides or other hardened phases in the steel are reasonable.

For the mold in service under the condition of heavy load and high-speed wear, it is required that the die steel surface can form a thin and dense oxide film with good adhesion, maintain the lubrication, reduce the molten wear such as bonding and welding between the die and the workpiece, and reduce the oxidation wear caused by oxidation on the die surface. Therefore, the working conditions of the die have a great influence on the wear of steel.
The wear resistance can be measured by the simulation test method, and the relative wear resistance index can be used as the parameter to characterize the wear resistance level under different chemical composition and organizational state. To present the service life before the specified burr height and reflect the wear resistance level of various steel grades; The test is based on Cr12MoV steel.
4. Thermal fatigue resistance
Under the service condition, hot work mold steel is not only subjected to the periodic change of load, but also subjected to the action of high temperature and periodic quenching and heating. Therefore, the evaluation of the fracture resistance of hot work die steel should pay attention to the thermomechanical fatigue fracture performance of materials. Thermomechanical fatigue is an index of comprehensive performance, which includes thermal fatigue performance, mechanical fatigue crack growth rate and fracture toughness.
Thermal fatigue performance reflects the working life of materials before thermal fatigue crack initiation. Materials with high thermal fatigue resistance have more thermal cycles for thermal fatigue crack initiation; The mechanical fatigue crack growth rate reflects the growth amount of each stress cycle when the crack propagates internally under the action of forging pressure after the thermal fatigue crack initiation; Fracture toughness reflects the resistance of materials to the unstable propagation of existing cracks.
For the material with high fracture toughness, if the crack wants to propagate unsteadily, it must have a sufficiently high stress intensity factor at the crack tip, that is, it must have a large crack length. On the premise of constant stress, there is already a fatigue crack in a die. If the fracture toughness of the die material is high, the crack must expand deeper before instability propagation can occur.
In other words, the thermal fatigue resistance determines the part of life before fatigue crack initiation; The crack growth rate and fracture toughness can determine the part of life where subcritical propagation occurs after crack initiation. Therefore, in order to obtain high service life of hot working die, the die material should have high thermal fatigue resistance, low crack growth rate and high fracture toughness.
The index of thermal fatigue resistance can be measured by the number of thermal cycles of thermal fatigue crack initiation, or by the number of fatigue cracks and average depth or length after a certain thermal cycle.
5. Occlusal resistance
Occlusal resistance is actually the resistance when “cold welding” occurs. This property is important for die materials.
During the test, under the condition of dry friction, the tested tool steel sample and the material with bite tendency (such as austenitic steel) are subjected to constant speed dual friction movement, and the load is gradually increased at a certain speed. At this time, the torque is also increased accordingly. This load is called “bite critical load”. The higher the critical load is, the stronger the bite resistance is.



About Huizhou Dj Molding Co., Ltd

Dj Mold is a China mold maker of plastic molds- injection mold, die casting moulds, plastic blow molding, rotational molding, medical plastic injection molding, two shot plastic injection molding, insert molding, overmolding, metal injection molding, micro injection molding, powder injection molding, ceramic injection molding, liquid injection molding, husky injection molding, household mold, casting mold, die mold tool, custom molds, china moulds, rapid prototyping tooling, plastic prototyping tooling, punch press tooling, die and tooling for mobile/ cell phone parts, automotive parts, vacuum cleaners, rechargeable tools, telephones, copiers, computers, multimedia speakers, and many other electronic products and household appliances. And also a plastic product manufacturer, mold manufacturer China– plastic parts, plastic water tank, plastic balls, plastic containers, plastic buckle, plastic anchor, plastic hanger, plastic spoon, plastic pipe fitting, plastic tumble, plastic tableware, plastic cups, plastic bottles, plastic tray, plastic cosmetic container, plastic case, plastic food container, plastic chairs, plastic caps, plastic cap closure, plastic tubes, plastic water pipes, plastic knobs, plastic tubing, plastic utility boxes, plastic racks and so on.

Person: James Yuan
Company: Huizhou Dj Molding Company Limited
Add:  LongGang Village,LongXi Town,BoLuo County,HuiZhou City,GuangDong Province, China
Tel: 86-752-6682869
Email: [email protected]


Share this post