Reasons for mold failure and preventive measures

In the process of production and application of molds, failures of various situations often occur, which wastes a lot of manpower and material resources and affects the production progress. The following mainly describes the basic failure modes of the mold and the causes of failure and preventive measures.

First, the mold failure The basic form of cold and hot mold failure in service can be divided into: plastic deformation; wear; fatigue;
1. Plastic deformation.
Plastic deformation is the deformation caused by the load being greater than the yield strength. For example, the cavity is collapsed, the hole is enlarged, the edge is collapsed, and the punch is thickened and longitudinally bent. Especially for hot working molds, the working surface is in contact with the high temperature material, so that the surface temperature of the cavity often exceeds the tempering temperature of the hot working die steel, and the inner wall of the groove is collapsed or piled up due to softening. When the low-hardenability steel is used as a cold boring die, after the mold is heated by quenching, the inner hole is sprayed and cooled to produce a hardened layer. When the mold is in use, if the cold heading force is too large, the compressive yield strength of the base under the hardened layer is not high, and the cavity of the mold is collapsed. The yield strength of die steel generally increases with the increase of the content of carbon (c) from some alloying elements. Under the same hardness, the steel with different chemical compositions has different compressive strength. When the hardness of steel is 63HRC, The order of the yield strength of the following four steels from high to low is: W18Cr4V>Cr12>Cr6WV>5CrNiW.
2. Wear and tear failure.
Wear failure refers to the passivation of the blade, the rounding of the corners, the depression of the plane, the groove of the surface, and the peeling of the mucous membrane (the billet metal is stuck in the mold worksheet during the rubbing). In addition, during the operation of the punch, the lubricant is converted into a high-pressure gas after combustion, and the surface of the punch is strongly washed to form cavitation.
In the case of cold punching, if the load is not large, the type of wear is mainly oxidation, and the wear may also be a certain degree of bite wear. When the edge portion becomes dull or the punching load is large, the wear of the bite becomes serious, and The wear is accelerated, and the wear resistance of the die steel depends not only on its hardness, but also on the nature, size, distribution and quantity of the carbide. In the die steel, the wear resistance of the high speed steel and the high chromium steel is high. However, in the case of severe carbide segregation or large particle carbides in the steel, these carbides are easily peeled off, causing abrasive wear and accelerated wear. Lighter cold work die steel (thin sheet punching, drawing, bending, etc.) impact, load is not large, mainly static wear. Under static wear conditions, the mold steel has a large carbon content and a large wear resistance. Under impact and wear conditions (such as cold heading, cold extrusion, hot forging, etc.), excessive carbides in the die steel do not contribute to the improvement of wear resistance, but rather wear resistance due to impact abrasive wear.
The research shows that under the condition of impact abrasive wear, the carbon content of the die steel is limited to 0.6%, and the cold die works under the impact load condition. For example, the carbide in the die steel is too much, and it is easy to fix the impact and wear the mountain. The surface is peeled off. These exfoliated hard particles will become abrasive particles, accelerating the rate of wear. The cavity surface of the hot work die is reduced in wear resistance due to high temperature softening. In addition, the iron oxide skin also functions as an abrasive, and also has high temperature oxidation corrosion effect.
3. Fatigue failure.
Characteristics of fatigue failure: Some parts of the mold pass through a certain service period, and crack small cracks, and gradually expand to the depth, when extended to a certain size, severely weaken the bearing capacity of the mold and cause fracture. Fatigue cracks are initiated in areas with large stresses, especially stress concentration parts (size transitions, notches, tool marks, wear cracks, etc.). When fatigue fracture occurs, the broken door is divided into two parts, and part of the fatigue fracture section formed by fatigue crack development. Presented in a shell shape, the source of fatigue is at the apex of the shell. The other part is a sudden break, showing an uneven rough section.
The root cause of the fatigue damage of the mold is the special ring load, and all the factors that can promote the increase of the surface tensile stress can accelerate the initiation of fatigue crack.
When the cold working die is operated under high hardness, the die steel has high yield strength and low fracture toughness. The high yield strength is beneficial to delay the generation of fatigue cracks, but the low fracture toughness accelerates the expansion rate of fatigue cracks and decreases the critical length, which greatly shortens the number of fatigue crack propagation cycles. Therefore, the fatigue life of cold working die mainly depends on fatigue. Crack initiation time.
Hot work dies are generally used in medium or low hardness conditions, and the mold fracture toughness is much higher than that of cold work dies. Therefore, in hot work dies, the fatigue crack growth rate is lower than that of cold work dies, and the critical length is greater than cold work. The subcritical expansion period of the fatigue crack of the hot mold is much longer than that of the cold mold. However, the surface of the hot mold is quenched, and the hot and cold fatigue cracks easily. The fatigue crack initiation time of the hot mold is lower than that of the cold mold. It is much shorter, so the fatigue fracture life of many hot molds depends mainly on the time of fatigue crack propagation.
4. Fracture failure.
The common forms of fracture failure are: chipping, caries, splitting, breaking, cracking, etc. The driving force of different die fractures is different. The 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 molds have higher operating temperatures and forced cooling. The internal stress can far exceed mechanical stress. Therefore, many hot molds The fracture is mainly related to excessive internal stress.
There are two types of mold fracture processes: one-time fracture and fatigue fracture. One-time breakage of the mold sometimes breaks suddenly during stamping, and once the crack is initiated, it is unstable and expands. The main reason for this is severe overload or severe embrittlement of the mold material (such as overheating, insufficient tempering, severe stress set and severe metallurgical defects, etc.).

Second, the cause of mold failure and preventive measures 1. Unreasonable structural design caused failure.
Sharp corners (where stress concentrations are more than ten times higher than the average stress) and excessive cross-section changes cause stress concentrations, often becoming the source of many mold failures early. And in the heat treatment quenching process, the sharp corner causes residual tensile stress and shortens the life of the mold.
Precautionary measures: The transition of each part of the punch should be smooth and smooth. Any small tool marks will cause strong stress concentration, and its diameter and length should meet the requirements.
2. Failure caused by poor quality of the mold material.
Internal defects of the mold material, such as looseness, shrinkage, segregation of inclusions, uneven distribution of carbides, and defects of the original surface (such as oxidation, decarburization, folding, scarring, etc.) affect the properties of the steel.
1 Excessive inclusions cause failure.
There is a source of cracks in the steel inside the mold, especially brittle oxides and silicates, which do not undergo plastic deformation during hot pressing, and only cause brittle fracture to form microcracks. In the subsequent heat treatment and use, the crack is further expanded to cause cracking of the mold. In addition, in the grinding, surface holes are caused by the peeling of large particle inclusions.
2 Surface decarburization causes failure.
When hot-pressing and annealing, the mold steel often has too high heating temperature and too long holding time, which causes decarburization of the steel surface. After the mechanical processing of the severely decarburized steel, sometimes the decarburized layer remains, so that it is quenched. At the time, due to the difference in the inner and outer layers (the surface decarburization layer is a ferrite body and the interior is a pearlite), the structural transformation is inconsistent 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 band and network segregation when forging is relatively small, resulting in the distribution of banded carbides during quenching. The crack, the crack in the mold is further expanded during use, causing the mold to crack and fail.
Precautionary measures: When the steel is satin rolled, the mold should be forged in multiple directions repeatedly, so that the eutectic carbide in the steel is crushed to be finer and more uniform, and the steel carbide unevenness level is required.
3. The mold is not properly attached.
1 Tool marks in the cutting: The cavity part of the mold or the rounded part of the punch is machined, often leaving a knife mark due to the infeed, which causes serious stress concentration. When quenching, it should be Micro-cracks are easily generated in concentrated parts of the mountains.
Precautions: In the last cut of the rough machining of the parts, the amount of feed should be minimized to improve the surface finish of the mold.
2 Electrical processing causes failure. When the mold is electrically processed, a large amount of heat is generated due to the discharge, which will heat the mold to a very high temperature, causing the structure to change, forming a so-called electromachining abnormal layer, melting on the surface of the abnormal layer due to high temperature, and then very Quickly solidified, the layer is white under the microscope, there are many fine cracks inside, and the area under the white layer is quenched, called quenching layer, and then weakened by heat, the temperature is not high, only tempering occurs, weigh back Fire layer. The hardness distribution of the section is measured: the molten resolidified layer has a high hardness of 610 to 740 HRC, a thickness of 30 Î¼m, a hardness of 400 to 500 HRC of the quenched layer, and a thickness of 20 Î¼m. The tempering is high temperature tempering, the structure is soft, the hardness is 380-400HRC, and the thickness is 10Î¼m.
Precautionary measures : 1 Mechanically remove the re-solidified layer in the open layer, especially micro-cracks; 2 Perform a low-temperature tempering after electrical machining to stabilize the anomalous layer to prevent microcrack propagation.
3 grinding process caused failure. When the mold cavity surface is ground, the grinding speed is too large, the grinding wheel is too fine or the cooling condition is poor, which will cause the surface of the grinding table to overheat or cause the surface to soften, and the hardness is lowered, so that the mold is in use. Severe wear, or thermal stress, causes grinding cracks, leading to early failure.
Precautionary measures: 1 Use coarse grinding wheel with strong cutting force or grinding wheel with poor adhesion; 2 Reduce the feed amount of the workpiece; 3 Select suitable coolant; 4 Use tempering at 250~350 Â°C after grinding to remove the grinding stress .
4. The mold heat treatment process is not suitable.
The heat treatment process parameters such as the heating temperature, the holding time and the cooling rate are not suitable, and will become the mold failure factor.
1 Heating rate: The mold steel contains more carbon and life-changing elements, and the thermal conductivity is poor. Therefore, the heating speed should not be too fast, and should be carried out slowly to prevent the mold from being deformed and cracked. In the air furnace heating and quenching, in order to prevent oxidation and decarburization, the boxing protection heating is adopted. At this time, the heating rate should not be too fast, and the heat penetration should be slow. In this way, no large thermal stress is generated and it is safer. If the mold is heated at a high speed, the heat is quickly transferred, and a large thermal stress is generated inside and outside the mold. If it is not properly controlled, it is easy to produce deformation or cracks, and it must be prevented by preheating or slowing the acceleration of temperature.
2 The effects of oxidation and decarburization. Mold quenching is carried out at high temperatures. If not strictly controlled, the surface is easily oxidized and decarburized. In addition, after the surface of the mold is decarburized, due to the difference in the inner and outer layers, large structural stress occurs during cooling, and quenching cracks are caused.
Precautions: Packing protection can be used, and the box is filled with anti-oxidation and decarburization filling materials.
The effect of cooling conditions.
Different mold materials have different tissue states and cooling rates. For high alloy steels, due to the high alloying elements, the hardenability is high, and oil heat treatment, air cooling or even isothermal quenching and grade quenching can be used.

Spotlight Closet

We diviided the led light according to the using eviiroment. Most of our product is the commercial LED Lighting

Commercial lighting is a term used to describe lighting that is used in commercial spaces, including auto dealerships, distribution centers, churches, factories, offices, and warehouses. Unlike residential lighting, commercial lighting is made to withstand more abuse and has a longer lifespan.

While the focus of residential lighting is often on aesthetics, commercial lighting is task orientated. Commercial lighting systems are designed based on what the application is. For example, in an office-type setting, you may see task lighting, which illuminates specific areas where employees need concentrated light to be able to perform their jobs.

Spotlight Closet,Small Led Spotlights,Kitchen Ceiling Spotlights,Spotlight Ceiling Light

Jiangmen Dilin Lighting High-Tech Co., Ltd. , https://www.jmdilinlighting.com