H13 (4Cr5MoSiV1) and H13R are widely used hot working die steels in the world. They have high thermal strength and hardness, high wear resistance and toughness, and good thermal fatigue resistance. They are widely used in manufacturing various forging dies, hot extrusion dies and die casting dies of Al, Cu and their alloys. Hot-working die steel bears great impact load, strong friction, thermal stress and high temperature oxidation caused by intense cold and hot cycles when working, and often occurs failure forms such as cracking, collapse, wear and tear.

 

The chemical composition (mass fraction,%) of H13 hot working die steel is shown in the attached table.

Chemical composition (mass fraction) (%) of H13 hot working die steel

C

Cr

Mo

V

Si

Mn

P

S

0.32 to 0.45

4.75 to 5.55

1.10 to 1.75

0.80 to 1.20

0.80 to 1.20

0.20 to 0.50

Less than 0.03

Less than 0.03

Its chemical composition characteristics are as follows: 1. Medium carbon with mass fraction of 0.32-0.45% ensures high hardness, high toughness and high thermal fatigue resistance. (2) Adding more elements such as Cr, Mn and Si to improve hardenability. Mn can change the properties and shapes of oxides formed during solidification of steel, avoid the formation of FeS with low melting point at grain boundaries, and exist as MnS with certain plasticity, thus eliminating the harmful effect of sulfur and improving the hot working performance of H13 steel; Cr and Si can improve the tempering stability. (3) Adding the elements Mo and V which produce secondary hardening. Mo and V can also prevent the second kind of tempering brittleness and improve the tempering stability.

I. Influencing factors of failure

The failure of H13 steel die is a very complex technical problem, which can be analyzed from four aspects: material, design, manufacture and use.

1. Chemical Composition and Metallurgical Quality

H13 steel belongs to hypereutectoid alloy steel. There are many defects in the structure, such as non-metallic inclusions, carbide segregation, center porosity and white spots, which greatly reduce the strength, toughness and thermal fatigue resistance of die steel. High quality H13 steel has higher toughness and thermal fatigue properties due to its advanced production process, pure steel, uniform structure and slight segregation. Ordinary H13 steel must be forged after ESR in order to break large non-metallic inclusions, eliminate carbide segregation, refine carbides and uniform structure.

2. Die Design

When designing the die, the shape size of the module should be determined according to the material and geometric size of the forming part to ensure the strength of the die. In addition, too small fillet radius, wide and thin-walled cross section with wide wall thickness difference and inappropriate location of holes and grooves are easy to cause excessive stress concentration and crack initiation in the process of heat treatment and use of dies. Therefore, in the die design, as far as possible to avoid sharp corners, holes, slots should be reasonably arranged.

3. Manufacturing Technology

(1) Forging process

H13 steel contains a lot of alloying elements, and its deformation resistance is high during forging, and its thermal conductivity is poor, eutectic temperature is low, and it will overburn if it is not paid attention to. Therefore, heating should be preheated in the range of 800 - 900 C, and then heated to the initial forging temperature of 1065 - 1175 C. In order to break up large non-metallic inclusions, eliminate carbide segregation, refine carbides and uniform structure, repeated upsetting and elongation are required during forging, and the total forging ratio is greater than 4. In the cooling process after forging, there is a tendency to produce quenching cracks, which are easy to produce transverse cracks in the core. Therefore, H13 steel should be cooled slowly after forging.

(2) Cutting

The machined surface roughness has a great influence on the thermal fatigue performance of the die. The surface roughness of the die cavity should be lower, and no knife marks, scratches and burrs should be left. These defects cause stress concentration and thermal fatigue crack initiation. Therefore, in the process of processing the die, the corner radius transition of complex parts should be prevented from leaving knife marks, and the burrs of the hole, groove edge and root should be polished.

(3) Grinding

In the grinding process, local friction heating easily causes defects such as burns and cracks, and generates residual tensile stress on the grinding surface, which leads to premature failure of the die. The burns caused by grinding heat can temper the surface of H13 die until tempered martensite is formed. The brittle untreated martensite layer will greatly reduce the thermal fatigue performance of the die. If the local temperature of the grinding surface is over 800 C and the cooling is not enough, the surface material will be re-austenitized and quenched into martensite, so the surface layer of the die will produce high structural stress. At the same time, the rapid temperature rise of the surface of the die will cause thermal stress, and the superposition of the structural stress and thermal stress will easily lead to grinding cracks in the die.

(4) Heat treatment process

Reasonable heat treatment process can make the die obtain the required mechanical properties and improve the service life of the die. However, if heat treatment defects occur due to improper design or operation of heat treatment process, it will seriously endanger the bearing capacity of the die, cause early failure and shorten the working life. Heat treatment defects include overheating, overheating, decarbonization, cracking, uneven hardening layer and insufficient hardness. When the accumulated internal stress of H13 steel die reaches the dangerous limit after a certain period of service, the die should be de-stressed and tempered, otherwise the die will crack due to the internal stress when it continues to serve.

4. Use and Maintenance of Dies

(1) Preheating of Die

HI3 steel has high alloy element content and poor thermal conductivity, so the die should be fully preheated before working. When the preheating temperature is too high, the temperature of the die is on the high side, the strength decreases, the plastic deformation is easy to occur and the surface of the die collapses; when the preheating temperature is too low, the instantaneous surface temperature changes greatly, the thermal stress is large, and the cracks are easy to initiate. After comprehensive consideration, the preheating temperature of H13 steel die is determined to be 250-300 C, which can reduce the temperature difference between die and forging to avoid die.