Reasons for mold failure and its’ preventive measures
Time：2019-04-08 11:19:32 / Popularity： / Source：
- Mold fails
1. Plastic deformation.
Plastic deformation is deformation caused by load being greater than yield strength. For example, cavity is collapsed, hole is enlarged, edge is collapsed, punch is thickened and longitudinally bent. Especially for hot working molds, working surface is in contact with high temperature material, so that surface temperature of cavity often exceeds tempering temperature of hot working die steel, and inner wall of groove is collapsed or piled up due to softening. When low-hardenability steel is used as a cold boring die, after mold is heated by quenching, inner hole is sprayed and cooled to produce a hardened layer. When mold is in use, if cold heading force is too large, base compressive yield strength under hardened layer is not high, mold cavity is collapsed. Yield strength of die steel generally increases with increase of carbon content from some alloying elements. Under same hardness, steel with different chemical compositions has different compressive strength. When hardness of steel is 63HRC, order of yield strength of following four steels from high to low is: W18Cr4V>Cr12>Cr6WV>5CrNiW.
2. Wear and tear.
Wear failure refers to passivation of blade, rounding of corners, depression of plane, groove of surface, and peeling of mucous membrane (blank metal is stuck on the working surface of mold during friction). In addition, during operation of punch, lubricant is converted into a high-pressure gas after combustion, surface of punch is vigorously washed to form cavitation.
In the case of cold punching, if load is not large, type of wear is mainly oxidation, and wear may also be a certain degree of bite wear. When edge portion becomes dull or punching load is large, wear of bite becomes serious, wear is accelerated, and wear resistance of die steel depends not only on its hardness, but also on nature, size, distribution and quantity of carbide. In die steel, wear resistance of high speed steel and high chromium steel is high. However, in the case of severe carbide segregation or large particle carbides in steel, these carbides are easily peeled off, causing abrasive wear and accelerated wear. Lighter cold work die steel (sheet punching, stretching, bending, etc.) impact, load is not large, mainly static wear. Under static wear conditions, mold steel has a large carbon content and a large wear resistance. Under impact wear conditions (such as cold heading, cold extrusion, hot forging, etc.), excessive carbides in die steel do not contribute to improvement of wear resistance, but rather wear resistance due to impact abrasive wear.
Research shows that under condition of impact abrasive wear, carbon content of die steel is limited to 0.6%, and cold die works under impact load condition. For example if there is too much carbide in die steel, it is prone to impact wear and surface peeling. These exfoliated hard particles will become abrasive particles, accelerating rate of wear. Cavity surface of hot work die is reduced in wear resistance due to high temperature softening. In addition, iron oxide scale acts as an abrasive, as well as high temperature oxidation corrosion.
3. Fatigue failure.
Root cause of fatigue damage of molding is special ring load, and all factors that can promote increase of surface tensile stress can accelerate initiation of fatigue crack.
When cold working die is operated under high hardness, die steel has high yield strength and low fracture toughness. High yield strength is beneficial to delay generation of fatigue cracks, but low fracture toughness accelerates expansion rate of fatigue cracks and decreases critical length, which greatly shortens number of fatigue crack growth cycles. Therefore, fatigue life of cold work die mainly depends on fatigue crack initiation time.
Hot work die is generally used in medium or low hardness conditions, and mold fracture toughness is much higher than that of cold work dies. Therefore, in hot work dies, fatigue crack growth rate is lower than that of cold work dies, and critical length is greater than cold work. Subcritical expansion period of fatigue crack of hot mold is much longer than that of cold mold. However, surface of hot work die is easily affected by rapid cooling and rapid heat. Fatigue crack initiation time of hot work die is much shorter than that of cold work die. Therefore, fatigue fracture life of many hot molds depends mainly on the time of fatigue crack propagation.
4. Fracture is invalid.
Common forms of fracture failure are: chipping, caries, splitting, breaking, cracking, etc. Driving force of different die fractures is different. Cold working mold is mainly subjected to mechanical force (rushing pressure). In addition to mechanical forces, thermal molds also have thermal stresses and microstructure stresses. Many hot work mold have higher operating temperatures and forced cooling. Internal stress can far exceed mechanical stress. Therefore, fracture of many hot molds is mainly related to excessive internal stress.
There are two types of mold fracture processes: one-time fracture and fatigue fracture. One-time fracture is a mold that sometimes breaks suddenly during stamping, and once crack is initiated, it is unstable and expands. Main reason for this is severe overload or severe embrittlement of mold material (such as overheating, insufficient tempering, severe stress set and severe metallurgical defects).
- Cause of mold failure and preventive measures
A. Unreasonable structural design causes failure.
Sharp corners (where stress concentrations are more than ten times higher than average stress) and excessive cross-section changes cause stress concentrations, often becoming sources of early failure of many molds. And in heat treatment quenching process, sharp corner causes residual tensile stress and shortens life of molding.
Precautionary measures: Transition of each part of punch should be smooth. Any small tool marks will cause strong stress concentration, and its diameter and length should meet requirements.
B. Failure caused by poor quality of moulding materials.
Internal defects of moulding materials, such as looseness, shrinkage, segregation of inclusions, uneven distribution of carbides, and defects of original surface (such as oxidation, decarburization, folding, scarring, etc.) affect properties of steel.
1 Excessive inclusions cause failure.
There is a source of cracks in steel inside mold, especially brittle oxides and silicates, which do not undergo plastic deformation during hot pressing, and only cause brittle fracture to form microcracks. In subsequent heat treatment and use, crack is further expanded to cause cracking of mold. In addition, in grinding, surface holes are caused by peeling of large particle inclusions.
2 Surface decarburization causes failure.
When hot-pressing and annealing, mold steel is often too hot, holding time is too long, and surface of steel is decarburized. After mechanical processing of severely decarburized steel, sometimes decarburized layer remains. Thus during quenching, tissue transition is inconsistent due to difference in inner and outer layers (surface decarburization layer is ferrite body and interior is pearlite), and cracks are generated.
3 carbides are unevenly distributed, causing failure.
Crl2, Cr112MoV and other mold steels have higher carbon content and alloying elements, forming many eutectic carbides. These carbides tend to exhibit banded and network segregation when forging is relatively small, resulting in cracks along banded carbides during quenching. Cracks in mold are further expanded during use, causing mold cracking failure.
Precautionary measures: When steel is satin rolled, mold should be forged in multiple directions repeatedly, so that eutectic carbide in the steel is crushed to be finer and more uniform, to ensure steel carbide non-uniformity level requirements.
C. Improper machining of molding.
1 Tool marks in the cutting: In machining process, cavity part of molding or rounded part of punch is often left with localized knife mark because feed is too deep, which causes serious stress concentration. When quenching, microcracks are likely to be generated in concentrated portion.
Precautions: In the last cut of roughing of part, amount of feed should be minimized to improve surface finish of mold.
2 Electrical processing causes failure. When mold is electrically processed, a large amount of heat is generated due to discharge, which will heat mold to a very high temperature, causing structure to change, forming a so-called electromachining abnormal layer, melting on the surface of abnormal layer due to high temperature, and then solidifying quickly. The layer is white under microscope, there are many fine cracks inside, the area under white layer is quenched, called quenching layer, and then heat is weakened, temperature is not high, only tempering occurs, it is called tempering layer. Hardness distribution of section is measured: molten resolidified layer with a high hardness of 610 to 740 HRC, a thickness of 30 μm, quenching layer with a hardness of 400 to 500 HRC, and a thickness of 20 μm. Tempering is high temperature tempering, structure is soft, hardness is 380-400HRC, and thickness is 10μm.
Precautionary measures: 1 Mechanically remove re-solidified layer in open layer, especially micro-cracks; 2 Perform a low-temperature tempering after electrical machining to stabilize anomalous layer to prevent microcrack propagation.
3 Grinding process caused failure. When mold cavity surface is ground, grinding speed is too large, grinding wheel is too fine or cooling condition is poor, which will cause surface of grinding to overheat or cause surface to soften and hardness to decrease, so that mold is scratched due to severe wear or thermal stress during use, leading to early failure.
Precautionary measures: 1 Use coarse grinding wheel with strong cutting force or grinding wheel with poor cohesiveness; 2 Reduce feed amount of workpiece; 3 Select appropriate coolant; 4 Use tempering at 250~350 °C after grinding to remove grinding stress .
D. Mold heat treatment process is not suitable.
Selection of heat treatment process parameters such as heating temperature, length of holding time, and cooling rate are all mold failure factors.
1 Heating rate: Mold steel contains more carbon and life-changing elements, thermal conductivity is poor. Therefore, heating speed should not be too fast, and should be carried out slowly to prevent mold from being deformed and cracked. When heating and quenching in an air furnace, in order to prevent oxidation and decarburization, packing protection heating is adopted, heating rate should not be too fast, heat transfer should be slow. In this way, no large thermal stress is generated and it is safer. If molding is heated at a high speed, heat is quickly transferred, and a large thermal stress is generated inside and outside mold. If it is not properly controlled, it is easy to produce deformation or cracks, and it must be prevented by preheating or slowing down acceleration.
2 Effects of oxidation and decarburization. Mold quenching is carried out at high temperatures. If not strictly controlled, surface is easily oxidized and decarburized. In addition, after surface of mold is decarburized, due to difference in inner and outer layers, large structural stress occurs during cooling, and quenching cracks are caused.
Precautions: Packing protection can be used, and tank is filled with anti-oxidation and decarburization filling moulding materials.
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