Discussion of Failure Analysis in H13 Steel Die

H13 steel is the most widely used of the hot work mold steel in the world. It has great heat intensity properties, hardness and wear resistance. The toughness and anti-fatigue performance are good. It is widely used to make all kinds of hammer dies, hot-extrusion dies and etc. Hot die steel needs to sustain huge impulse load, friction and high-temperature oxidation. But something always happens during work time. Such as the crack, dent, abrasion, formation of cracking and etc. The chemical composition chart of H13 die steel as below.

Its characteristics of chemical composition
Its carbon content can make sure the high hardness, toughness and other properties. Adding the elements Cr, Mn, Si, Mn, which can make the hardenability get better to change the nature and shape of the oxide in the time of steel become solidification. That move can avoid the formation of low melting point Fes on the grain boundary. Instead, existence as Mns with certain plasticity. Then it will clean out the bad influence of sulfur to make the hot workability of H13 mold steel. Adding the elements of Mo, V to make it harden secondary. And it can enhance temper resistance.

The factors of failure
It is a complicated question. But we can analysis it on four ways of material, design, manufacture and use.

Chemical composition and the quality of material
H13 steel belongs to the type of graphitic alloy steel. There is so many nonmetallic inclusion in it. And has the defects of the center be loosen and etc. That will make the strength, toughness and etc get down. And the quality of H13 steel has two levels for common and high-quality. High quality H13 steel is made from high technology and material. It has better toughness and thermal fatigue property. However, the common one needs the processing of forging. Which will destroy the big non-metallic inclusion and make the carbide get small.

The design of the die
The design of mold depends on the shapes and specification of products to make sure the size of the die. That was good for the strength of die. Furthermore, too small radius of rounded corner, ill-suited groove position and etc will make the die gets big pressure during the heat treatment. That is easy to cause crackle. So must make the taper angle, hole and etc at the right position in the mold.

Production technique
Forging Process There are so many alloy elements in H13 steel. The resistance of deformation is big at the time of forging. And the thermal conductivity of the material is bad. So the eutectic temperature is low. That is easy to burn. So preheated temperature range must be at 800-900℃. Then heated to the initial forging temperature of 1065-1175℃. In order to crush large non-metallic inclusions and make the carbide get small, the forging ratio needs to be bigger than four. At the cooling process after forging, it is possible to get quenching crack. And the core is easy to get transverse cracks. So H13 steel must be slowly cooled after the forging.

The machining of cutting The roughness of the surface after the machining of cutting, which has a large impact on the thermal fatigue property of steel mold. The roughness of the steel mold surface must be lower. The knife mark and cullet cut are not supposed at here. Those shortages will cause pressure over control to make the thermal fatigue crack. So, must pay attention to handle these things in the mold at the time of processing.

The machining of grinding At the process of grinding, the generation of heat by friction at the parts which is easy to cause the problems of crack, empyrosis and etc. And also make the mold surface gets residual tensile stress. That will make the mold get failure early. If the temperature of some parts of grinding surface up to over 800℃, and not make it cool fully. Then the material of the surface layer will be re-austenize. And quenching into martensite. So the surface layer of the die will get high stress. At the same time, that will make the possibility of getting grinding crack be higher.

The technology of heat treatment Taking heat treatment in the right way will make the mold get what it needs about the properties and raise the time of working. But if do it in the wrong way, it will cause the defects of heat treatment. It will harm hugely to its load capacity. The defects of heat treatment are decarburization, superheating, dehiscence, strength is not enough and etc. After a period of time of use, the internal stress accumulates to a damage level. It must do a work of strain relief tempering. If not, it will dehiscence during work time.

The use and care of H13 steel die
The preheat of steel mold H13 steel has high content alloy elements and its thermal conductivity is bad. So it must fully preheat before the use. The temperature of preheating too high, then the temperature of the mold will be higher during the work. And its strength will get down, it will easy to get plastic deformation then make the surface subsides. However, if the temperature of the preheating is too low, the temperature of the surface has a big change. With high heat stress, get the crackle is more easily. The temperature of preheating for H13 steel mold is at the range of 250-300℃. That can reduce the temperature difference between the mold and the forgeable piece to avoid excessive thermal stress on the surface of the mold, and effectively avoid the plastic deformation of the mold surface.

The lubrication and cooling of steel mold In order to decrease the heat load of mould, people always will make it cooling directly in the interval of work. However, it will cause a bad impact on the mold. It should be cool slowly to avoid thermal stress and cracking. Need to use the graphitic lubricant to make sure the H13 steel die work in the right way. Its power of radiating is great. That will decrease the temperature of H13 mold at the time of work.

How to fix it?
Analysis of chemical composition and quality of material
Raise the purity of H13 steel. Decrease the content of sulfur which is the most efficient way to increase the working life of H13 steel mold. The sulfur content is at the range of 0.005-0.008% of high-quality H13 steel. According to the analysis of chemical composition and quality of material can get the result of if it is eligibility. That will help us to make the right decision on the technology of heat treatment and forging.

Test Method: Sampled the H13 steel stuff to analyze its chemical composition and if it complies to the requirements. And take the sample at the steel center. Then grinding, polishing, etching with 4% nitric acid alcohol solution, examining the microstructure on an optical microscope, and assessing the grade of carbide segregation and the grade of inclusion according to relevant national technical standards.

The way of technology control
Forging Technology The ingot that diameter bigger than 70mm must preheat at the temperature range of 800-900℃. Then, it is heated at an initial forging temperature of 1065 to 1175 ° C. And the total forging ratio must bigger than 4.

Spheroidizing Annealing It can make the strength get lower and improve the cutting machining properties. Do a good start for quenching and tempering. The spheroidizing annealing process is maintained at 845℃ to 900℃(1 h +1 min)/mm. And then cooled to 720℃ ~ 740℃ isothermal (2h + 1 min)/mm at the furnace. Cooling to 500 ° C at the furnace, and finally out of the furnace to air-cooled.

Quenching and Tempering Technology There is the best way of H13 steel heat treatment technology. After heating at 1020°C~1080°C, then oil hardening or martempering. Tempered twice at 560-600 °C. The microstructure is tempered toughness + tempered sorbite + residual carbide, and the microhardness is 48-52HRC.

The manufacture of H13 steel die must do a good job at design, the material chosen, forging, annealing, heat treatment and etc to make sure the quality at a high level. Every step is important and has a big influence on the mold product.