H13 (4Cr5MoSiV1) is a hot work die steel widely used internationally. It has high thermal strength and hardness, high wear resistance and toughness, and good heat fatigue resistance. It is widely used in the manufacture of various forging dies, Hot extrusion dies and die-casting dies for aluminum, copper and their alloys. Hot work die steel is subjected to great impact load, strong friction, thermal stress caused by severe cold and heat cycles, and high temperature oxidation during work, and failure modes such as chipping, collapse, wear, and cracking.

The chemical composition (mass fraction, %) of H13 hot work die steel

The chemical composition characteristics:

• Medium carbon, the mass fraction is 0.32～0.45%, to ensure high hardness, high toughness and high thermal fatigue resistance.

• Adding more elements Cr, Mn, Si, Mn to improve hardenability can change the properties and shape of oxides formed during solidification of steel, and prevent sulfur from forming low-melting FeS on the grain boundaries, and have certain plasticity The presence of MnS can eliminate the harmful effects of sulfur and improve the hot workability of H13 steel; Cr and Si can improve tempering stability.

• The addition of elements Mo, V, and Mo that produce secondary hardening can also prevent the second type of temper brittleness and improve temper stability.
  

Failure influencing factors:

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

1. Chemical composition and metallurgical quality

H13 steel belongs to the type of hypereutectoid alloy steel. There are many defects such as non-metallic inclusions, carbide segregation, loose center and white spots in the structure, which greatly reduces the strength, toughness and thermal fatigue resistance of die steel. H13 steel is generally divided into ordinary H13 steel and high-quality H13 steel according to its quality. High-quality H13 steel adopts more advanced production technology, with pure steel quality, uniform structure, slight segregation, and higher toughness and thermal fatigue properties. Ordinary H13 steel must be forged to break up large non-metallic inclusions, eliminate carbide segregation, refine carbide, and uniform structure.

2. Mold design

When designing the mold, the external dimensions of the module should be determined according to the material and geometric dimensions of the formed parts to ensure the strength of the mold. In addition, too small fillet radii, wide-width thin-walled sections with large differences in wall thickness and improper positions of holes and grooves can easily cause excessive stress concentration and cracks during the heat treatment and use of the mold. Therefore, sharp corners should be avoided as much as possible in the mold design, and the positions of holes and slots should be arranged reasonably.

3. Manufacturing process

• Forging process H13 steel contains more alloy elements, has greater resistance to deformation during forging, and has poor thermal conductivity of the material. The eutectic temperature is low, and it will overburn if you are not careful. Therefore, the heating should be preheated in the range of 800-900℃, and then heated to the initial forging temperature of 1065-1175℃. In order to break up large non-metallic inclusions, eliminate carbide segregation, refine carbide, and uniform structure, it is necessary to repeatedly upset and draw length 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, and it is easy to produce transverse cracks in the core. Therefore, H13 steel should be slowly cooled after forging.
  

  
  

• Cutting process The surface roughness of the cutting process has a great influence on the thermal fatigue performance of the mold. The surface of the mold cavity should obtain a low surface roughness without leaving knife marks, scratches and burrs. These defects cause stress concentration and induce thermal fatigue cracks; therefore, when processing the mold, the corner radius transition of the complex part should be prevented from leaving knife marks, and the burrs on the edges of the holes, grooves and roots should be polished.
  

  
  

• Grinding process During the grinding process, local frictional heat generation can easily cause defects such as burns and cracks, and generate residual tensile stress on the ground surface, leading to premature failure of the mold. Burns caused by grinding heat can temper the surface of the H13 mold until tempered martensite is formed. The brittle untempered martensite layer will greatly reduce the thermal fatigue performance of the mold. If the grinding surface is locally heated to over 800°C and the cooling is insufficient, the surface material will be re-austenitized and quenched into martensite, so the surface layer of the mold will generate high structural stress, and the grinding process The extremely rapid temperature rise of the mold surface will cause thermal stress, and the superposition of structural stress and thermal stress can easily cause grinding cracks in the mold.
  

  
  

• EDM EDM is an indispensable finishing method in the modern mold manufacturing process. During spark discharge, the local instantaneous temperature is as high as 1000℃, which melts and vaporizes the metal at the discharge. There is a thin layer of melted and resolidified metal on the surface of the EDM, and there are many microscopic cracks in it. Under the microscope, this thin layer of metal is bright white, that is, bright white layer. Studies have shown that for high-alloyed H13 steel, the microstructure of the white bright layer formed by EDM is primary martensite, retained austenite and eutectic carbides. There is a large amount of untempered primary martensite. Micro cracks. When the H13 steel mold is under load during work, these micro cracks can easily develop into macro cracks, leading to early fracture and early wear of the mold. H13 steel molds should be tempered again after EDM to eliminate internal stress, but the tempering temperature should not exceed the highest tempering temperature before EDM.
  

  
  

• Heat treatment process Reasonable heat treatment process can make the mold obtain the required mechanical properties and improve the service life of the mold. However, if the heat treatment defects are caused by improper heat treatment process design or improper operation, it will seriously harm the carrying capacity of the mold, cause early failure and shorten the working life. Heat treatment defects include overheating, overburning, decarburization, cracking, uneven hardened layer and insufficient hardness. After the H13 steel mold has been in service for a certain period of time, when the accumulated internal stress reaches a dangerous limit, the mold should be subjected to stress relief and tempering, otherwise the mold will be cracked due to the internal stress when it continues to serve.
  
  

4. Use and maintenance of mold

• Preheating of the mold HI3 steel has a high content of alloy elements and poor thermal conductivity, so the mold should be fully preheated before working. If the preheating temperature is too high, the temperature of the mold is too high during use, the strength is reduced, and plastic deformation is likely to occur, which causes the mold surface to collapse; Initiation of cracks. After comprehensive consideration, the preheating temperature of the H13 steel mold is determined to be 250-300°C, which can reduce the temperature difference between the mold and the forging to avoid excessive thermal stress on the mold surface, and effectively reduce the plastic deformation of the mold surface.
  

  
  

• Cooling and lubrication of the mold In order to reduce the thermal load of the mold and avoid the mold temperature from being too high, it is usually forced to cool during the intermittent work of the mold, which will cause periodic heating and chilling of the mold. Thermal fatigue cracks. Therefore, the mold should be cooled slowly after use, otherwise thermal stress will occur, which will cause the mold to crack and fail. H13 steel molds can be lubricated with water-based graphite with a graphite content of 12% when working, reducing the forming force, ensuring the normal flow of metal in the cavity and smooth demolding of the forging; in addition, graphite lubricant also has a heat dissipation effect, which can reduce H13 The working temperature of the mold.
  
  

Failure analysis plan

The manufacture of H13 steel molds has to go through a series of process links including design, material selection, forging, annealing, machining and heat treatment. Improper process design or improper process operation in each process link will cause premature failure of the mold and reduce the service life of the mold. Hot work die steel often has failure modes such as cracking, collapse, wear and cracking. The failure mode, degree and location of hot work die steel are recorded in a series of process links such as design, material selection, forging, annealing, machining and heat treatment. Important information.

Observe and analyze the macroscopic features, microstructures and failure modes at the failure location of the H13 steel mold, and use the theories and methods of metallurgy, material physics and fracture mechanics to prompt the macroscopic features and materials at the failure location of the H13 steel mold The relationship between microstructure and failure mode and mold design, material selection and processing technology, so as to propose scientific and reasonable process improvement measures.

(1) Analysis of chemical composition of raw materials and metallurgical quality

Improving the cleanliness of H13 steel, especially reducing the sulfur content is an effective measure to increase the life of H13 steel molds. The sulfur content of high-quality H13 steel is between 0.005 and 0.008%. H13 steel is a hypereutectoid steel with a relatively high content of alloying elements. Carbide segregation occurs during smelting and casting, and the steel ingot forms a coarse carbide segregation zone after forging and rolling. The carbide segregation zone and the residual dendrites, shrinkage cavity, porosity and inclusions directly affect the structure and performance of the H13 steel mold, which is one of the important reasons for the early failure of the mold. The analysis of the chemical composition and metallurgical quality of raw materials can assess whether the raw materials are qualified, which can be used to guide the formulation of scientific and reasonable forging processes and heat treatment processes.

Test method: take a sample of H13 steel raw materials, analyze its chemical composition, and evaluate whether its chemical composition meets the requirements; cut a sample from the center of the steel, grind and polish it, etch it with a 4% nitric acid alcohol solution, and check it on an optical microscope. Micro-organization, the grade of carbide segregation zone and inclusion grade are evaluated according to relevant national technical standards.

(2) Analysis of mold microstructure

Microstructure analysis can determine whether there are carbide segregation zones, large non-metallic inclusions, network carbides, eutectic carbides and tempered martensite at the failure location of the mold; micro-domain composition analysis can determine the chemical composition of the mold failure location Distribution characteristics; microhardness analysis can determine the mechanical properties of the mold failure location. Comprehensively analyze the microstructure, microhardness and micro-zone composition at the failure location of the mold, reveal the macro morphology and the micro mechanism of the failure mode at the failure location of the mold, and correctly evaluate the current forging, spheroidizing annealing, quenching and tempering Process, and propose scientific and reasonable process improvement measures.

Test method: cut a sample from the failure position of the mold, grind and polish, etch with 4% nitric acid alcohol solution, check the microstructure on an optical microscope or scanning electron microscope, measure the hardness on a microhardness tester, and use Auger Energy Determine the micro-zone composition on the spectrum analyzer.

Process control measures

From the chemical composition and structure characteristics of H13 steel, it can be seen that the hot working process has a great influence on the structure and performance of H13 steel molds. In order to prevent the early failure of H13 steel molds, extend the service life and improve economic benefits, scientific and reasonable Thermal processing technology.

1. Forging process

H13 steel has high content of alloying elements, poor thermal conductivity, and relatively low eutectic temperature, which can easily cause overburning. For the billet with a diameter greater than Ø70mm, it should be preheated in the range of 800～900℃, and then heated at the initial forging temperature of 1065～1175℃. During forging, the length and upsetting is performed multiple times, and the total forging ratio is greater than 4.

2. Spheroidizing annealing process

The purpose of the spheroidizing annealing process is to homogenize the structure, reduce the hardness, improve the cutting performance, and prepare the structure for quenching and tempering. The spheroidizing annealing process is heat preservation (1h+1min)/mm at 845～900℃, then the furnace is cooled to 720～740℃ isothermal (2h+1min)/mm, and finally the furnace is cooled to 500℃ and air cooling. The spheroidizing annealing structure is Granular pearlite, hardness less than 229HBS. The quality of spheroidization can be evaluated according to the first level diagram of GB/T1299-2000 standard.

3. Quenching and tempering process

The best heat treatment process for H13 steel is oil quenching or stage quenching after heating at 1020～1080℃, and then two tempering at 560～600℃. The microstructure is tempered troostite + tempered sorbite + residual carbide , The microhardness is 48～52HRC. For molds (die-casting molds) that require high thermal hardness, the upper limit heating temperature can be quenched. For molds that require toughness (hot forging dies), the lower limit heating temperature can be quenched.