According to the literature [3~8], the typical treatment process and mechanical performance of some cold work die steels are listed in Table 1, which can be used for reference when selecting the die materials and formulating the processing technology.
2 Some Problems in Failure Analysis of Cold Work Dies
Through the investigation and analysis of the failure conditions of domestic cold heading, cold punching and cold extrusion tools, the statistical results are shown in Table 2.
From the statistical results shown in Table 2, the main failure types of cold work molds are overload failure and wear failure, accounting for approximately 80% to 90% of the total failures. Cold heading molds are mainly cracked or abnormally worn (partial shedding), and cold extrusion molds are mainly based on brittle fracture or wear failure, while cold punching molds are mainly based on wear failure. The proportions of brittle cracking failure in cold extrusion and cold heading with high working stress and large fluctuating stress are obviously higher than those with low working stress.
The fatigue fracture failure rate collected in Table 2 is relatively small. This is due to the fact that cold working mold materials mostly use high-carbon, high-alloy steels. The hardness is as high as 60HRC. The fatigue fracture morphology is difficult to distinguish and count.
There are many reasons for the failure of cold working molds. Besides the factors of the mold materials, there are many factors related to the precision and condition of working equipment, product materials and surface quality, mold structure and processing precision, operator quality, etc. Some of these random factors give failure analysis. Increase difficulty.
2.1 The relationship between working stress, hardness and life of cold working die steel
The measurement statistics show that for products made of steel, the average working stress of the cold extrusion die material is about 2500 MPa, the hardness is 62-64HRC, the cold-heading die is about 1500MPa, the hardness is 58-62HRC, the cold-punching die is about 500MPa, and the hardness is used 60-62HRC, where the stress of the cold extrusion die is the largest. In fact, it is also subjected to a random load of 10% to 20%, and the local stress of the mold must exceed the above stress.
The effect of working hardness of cold working die steel on the service life is the result of comprehensive action. Figure 1 shows the working hardness, failure type, and service life chart of cold-extruded W6Mo5Cr4V2 steel punch (extrusion 20Cr steel).
The relationship between hardness and life of W6Mo5Cr4V2 steel shown in Fig. 1 shows that there are two low life zone hardness zones in the life curve (ie, C and B zones), when less than 63HRC (based on plastic deformation failure) or greater than 64HRC (brittle fracture failure is The main) low life area. For the early failure of the cold mold should carefully analyze the impact of material factors and other factors.
2.2 Major Failure Modes of Cold Work Dies
The main failure modes of cold working molds are overload failure, wear failure, and fatigue failure. The typical failure fracture shape is shown in Fig. 2, discussed separately below.
2.2.1 Overload failure
Overload failure refers to the failure of the material itself to withstand the failure caused by the action of the working load (including random fluctuating loads), including failure due to insufficient toughness and insufficient strength. Among them, attention should be paid to failure of brittle fracture due to insufficient toughness.
(1) Failure of insufficient material toughness
Due to the absence of macroscopic signs and sudden bursts of failure before such failures, it is the most dangerous accident in the failure of cold work tools. In the past, such failures have also led to personal accidents, which have caused great losses in production safety and economic construction. This kind of instability under the failure of fracture in the cold extrusion and cold heading mold easily appear, such as the punch fracture, cracking, and even burst, characterized by no obvious plastic deformation before the failure, the macro fracture without shear lip, And relatively flat, resulting in permanent failure of the mold can not be repaired.
This failure is associated with insufficient mold material toughness and high stress. The analysis and calculation of the actual bearing capacity of the cold extrusion die shows that the ability to withstand the work strain before the punch failure is thousands of times higher than the energy consumption of the fracture, which shows that the punch bears high potential kinetic energy and low fracture resistance during work. According to the energy conservation principle, almost all the energy of the punch fracture potential becomes extended kinetic energy, and its extended limit speed can reach 103m/s. When there is stress concentration in the die structure, such as r≤1mm in the transition zone of the hexagonal cold heading punch, the stress concentration factor Kt=2, when the cold extrusion punch step is r=3mm, Kt=1.3, even the machining tool marks Grinding, rough traces, etc., can become weak links and produce unstable fractures.
High-carbon, high-alloyed cold work die steels are tempered martensite and secondary precipitated phases, containing more residual carbides, high material hardness, and matrix energy absorption and relaxation stress. The ability to strain is low and the distribution of primary carbide inhomogeneities severely reduces the toughness of the material. Therefore, such failure fractures cannot see macroscopic deformation, and the micro-deformation size is roughly equivalent to the carbide spacing.
(2) Insufficient strength failure
In cold heading and cold extrusion punches, the compression resistance and bending resistance of the material are not enough, and it is easy for the bow head to buckle and the bending deformation to fail. Such failures are likely to occur in the development of new products because of the excessive work load and the low hardness of the mold. The actual experience shows that the hardness of the cold head punch is less than 56HRC, and the hardness of the cold head punch is less than 62HRC. This type of failure is easy to occur; at the same time, it shows that the material strength is insufficient, there is more plasticity, and the toughness potential can be exerted.
The empirical methods for resolving such early failures are: brittle fracture failure reduction (enhancement); deformation failure increase hardness (enhancement).
2.2.2 Wear failure
Wear failure refers to the friction loss between the working part of the mold and the material being processed, and the failure caused by changes in the shape and size of the working part (edge, punch). It also includes normal wear failure and abnormal wear failure.
(1) Normal wear failure
For cold stamping and cold extrusion dies, which require strict surface dimensions, the die life depends on the wear resistance of the surface under the premise of ensuring the material. Uniform frictional wear between the working part of the mold and the material being processed causes failures caused by changes in the shape and dimensions of the working part (blade, punch). Usually the die has a long service life. For example, the punching die and extrusion die with high surface quality are easy to produce such failure (Figure 2c).
(2) Abnormal wear failure
Under the action of local high pressure, the working part of the mold and the material to be processed are bitten together - the material to be processed is "cold-welded" to the surface of the mold (or the material of the mold is "cold-welded" to the surface of the material), causing the product to be processed (or mold material) The failure of sudden changes in the shape and size of the surface causes scratches in the surface quality of the processed product. This kind of failure occurs easily in drawing, bending and cold extrusion dies (Fig. 2d).
2.2.3 Fatigue failure (multi-failure fatigue failure)
Cold working mold loads are periodically applied at a certain impact velocity and with a certain amount of energy. This state is similar to the small energy multi-shot fatigue experiments (periodic loading and unloading with certain energy). Because the fatigue life of the die material is mostly 1000-5000 times, there is no obvious boundary between crack fatigue source and crack diffusion zone.
Die steel fatigue and structural steel fatigue are very different. Since the initiation period of fatigue cracks of brittle materials accounts for most of the life, crack initiation and propagation are difficult to distinguish in most cases. Careful analysis of the fatigue microscopic pattern shows that the crack initiation is mostly at the weak links of the material surface, such as grain boundaries, carbides, and stress concentration sites. Experiments show that when the impact fatigue crack initiation is about 0.1mm microcracks, the life expectancy accounts for more than 90% of the total life span. From the fracture, it is difficult to observe the stable expansion zone and fatigue zone of the structural steel. Once the crack is generated, the instability will expand rapidly. After the shot peening treatment of high-speed steel, due to the surface residual compressive stress, the crack source position is transferred to the subsurface about 0.2mm (Fig. 2e), and improving the surface stress state of the material is an effective way to improve the multi-flood fatigue resistance. Multi-shot fatigue failures are common in heavy-duty molds such as cold-extrusion, cold-headed punch dies.
Domestic mold steel
2.3 Surface Cracking Sensitivity of Crl2 Steel
The surface cracking sensitivity of high hardness materials is an indicator reflecting the failure resistance performance of the surface of the material. Surface cracking is a common problem in molds, and traditional methods are difficult to evaluate. It is an effective and simple method to detect the cracking susceptibility of the surface of cold work tool steel by Vickers hardness test method.
Cracking tests were performed on a Bloile hardness tester. The specimens were first ground mechanically polished and etched with a 4% nitric acid alcohol solution, using a diamond quadrangular pyramid indenter with an angle of 136o between opposite faces under a certain load. Press into the metal surface under test (Figure 3a). Observe the changes in the boundary shape of the indented squares under a microscope. Measure 3-5 points for each sample. Force analysis shows that cracks may occur only when the stress acting on the boundary is greater than the threshold value of cracking in the local area of ​​the material, so that the surface cracking sensitivity of the material can be judged. Figures 3b and 3c show the cracking susceptibility test results and morphology of Crl2 steel.
2.4 Notch Sensitivity of High Speed ​​Steel W6Mo5Cr4V2 Steel
High-speed steel mold structure inevitably exists macro and micro stress concentration, stress concentration of high hardness materials has important research and application significance for fracture failure analysis.
W6Mo5Cr4V2 steel notch sensitivity studies use axisymmetric notched tensile specimens. Notched stress concentration factors were 4.5, 3.8, 3.2, 2.4 and 1, respectively, and W6Mo5Cr4V2 steels were tempered at 1170°C, 1190°C, 1210°C and 1230°C for quenching at 560°C. Figure 5 shows the relationship between austenite temperature, notch radius and notch fracture toughness K1(r) of W6Mo5Cr4V2 steel.
Experiments show that the notch fracture toughness of W6Mo5Cr4V2 steel increases linearly with the radius r1/2 of the notch: K1(r) = K1(ro) + Yr1/2; the quenching temperature rises and the fracture toughness K1(ro) decreases, K1(r) The slope of the -r1/2 curve also dropped from Y=491 to 173. Different slope changes indicate that the low temperature slope Y value is higher than the high temperature state, indicating that low temperature quenching is more conducive to improving the ability of the material to resist stress concentration.
2.5 F6Mo5Cr4V2 steel fracture fractal dimension and treatment process
At the same time as the observation of the fracture electron scanning electron microscope, the electron beam was scanned along a certain direction along the fracture surface using the SEM to obtain a secondary electron line graph. This curve reflects the secondary electron line intensity change along the fracture in this direction. The irregularity of the fracture surface of the material can be statistically described quantitatively by the fractal dimension D (dimensionless). Fig. 5 is the corresponding diagram of the scanning fracture and the secondary electron wire in different treatment processes of W6Mo5Cr4V2 steel.
The relationship between the fractal dimension and the mechanical properties of fractures in different processes of W6Mo5Cr4V2 steel was measured experimentally. The results are shown in Table 3.
From Table 3, it can be seen that as the quenching temperature increases, the fractal dimension D of W6Mo5Cr4V2 steel decreases, that is, the D value decreases from 1.165 at 1170 hours to 1.375 at 1230. It can be seen that with the fracture toughness K1c and impact toughness The increasing value of the material Ak increases the D value. The value of D is a measure of the degree of fracture irregularity. A large value of D indicates that the surface of the fracture surface is uneven and the energy consumption at break is large. This correspondence can be illustrated by comparing the fracture photograph with the secondary electron line.
Analyzing the relationship between the fractal dimension D of W6M85Cr4V2 steel and HRC, grain size d, and bending strength, we can see that with the increase of steel bending strength δb, the value of D increases, with the increase of hardness HRC, or the increase of grain size d, The decrease of D value and the increase of quenching temperature, W6M85Cr4V2 steel, and δb grain size d increase, hardness HRC increase, and toughness K1c and Ak decrease, so the energy consumption and D value at the time of fracture are reduced.
3 Cold Die Material's Resistance to Fracture
Indicators for evaluating failure resistance or load carrying capacity: σs, σb, σf, Ak, etc. are generally used for the overall material. For cracked body (notched) JIC, KIC, and notch strength. For ductile fractured materials, the experimental data measured by these indicators are relatively stable and have good reproducibility, which can better reflect the failure resistance index of the material.
For brittle fractured materials, including tough-brittle mixed fractures, the evaluation of failure resistance is far from perfect. The main problem is that there is no obvious boundary between the yield and fracture of such materials; the dispersibility of material performance test data is large and the determination is difficult, so the evaluation of such materials at home and abroad is different. In summary, the author thinks:
(1) Brittleness is a material strength plasticity index defined opposite to toughness. The brittleness of materials is evaluated. It is advisable to use a strength-plasticity comprehensive index. The properties of brittle materials cannot be fully expressed by a single strength or plasticity index.
(2) The brittle fracture process of the material is often from the absence of macroscopic cracks - formation of cracked bodies - crack propagation fracture. Therefore, to evaluate the brittleness of materials, it is necessary to consider the properties of brittleness of material without cracking macro-cracking and brittleness of brittle material.
(3) Fracture evaluation of brittle materials without macroscopic cracks. Evaluated by energy consumption of fractures, such as areas of stress-strain such as stretching, compression, and bending (or areas under the force-displacement curve), and energy consumed by impact work. Surface roughness and defect size have a great influence on this performance measurement.
(4) Materials with cracked bodies are evaluated using fracture toughness indicators: such as KIC JIC GIC, which consumes energy from extended cracked bodies, but the fracture toughness values ​​of brittle materials are very low, and the uniformity of crack tip shape and performance affects the performance measurement. Very large, common material fracture toughness distribution diagram 6. After analysis and calculation of the actual bearing capacity of the W6Mo5Cr4V2 steel cold extrusion punch die, it can be known that the brittle failure material withstands the working strain capacity thousands of times higher than the energy consumption of the fracture, and almost all of the energy is converted into expanding kinetic energy, which rapidly bursts the punch.
Although material strength and toughness are not always contradictory to each other, this contradictory relationship has been universal. The question of evaluating the performance of brittle fracture resistance of cold work mold materials remains to be further explored.
Based on practical experience, in the selection of mold materials and the development of a reasonable treatment process, some ways to reduce the early failure of cold molds are proposed.
(1) Refine the grain or grain size of the carbide of the cold work die steel to improve the material strength and fracture toughness and improve the material's resistance to brittle fracture.
(2) Improve the surface quality of the mold. Because brittle materials are particularly sensitive to surface defects (notched stress concentration), the ability to resist brittle fracture can be improved by using methods such as polishing, polishing, and surface hardening.
(3) Toughening and strengthening of the multi-phase structure, adding one or more toughening-enhanced dispersions to the material, which can absorb energy and hinder crack propagation. For example, toughened ZrO2, residual austenite, transformation hardening steel, etc. can play a toughening effect.
(4) Fiber-reinforced composite materials, the use of metal, non-metallic fiber material toughness prepared composites, improve the fracture resistance.
(5) Gradient materials, according to different performance requirements using composite multilayer functionally graded materials, such as surface coating materials with good resistance to brittle to improve the surface treatment toughness.
(6) Different processing techniques, heat treatment techniques, etc., such as brittle fracture failure hardness (enhancement); deformation failure increase hardness (enhancement) and so on. The resistance to brittle fracture can be increased within a certain range.
4 Conclusion
(1) According to the statistical analysis results of the service life and failure of the die, some cold-work die steels and performance selection parameters are proposed;
(2) Analysis of several major failure modes of cold work die steels: overload failure, wear failure and fatigue failure characteristics and morphology, etc.; failure law of typical cold work die; discussion of the relationship between die failure, die life and material process ;
(3) Analyze the surface cracking sensitivity, notch fracture property and brittle fracture mechanism of high hardness die steel.
(4) Emphasizing the importance of proper selection of mold materials and the development of a reasonable treatment process, some methods for reducing brittle fracture are proposed.
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