Causes of mould failure and preventive measures?
In the process of production and application, the mould often fails in various situations and wastes a lot of manpower and material resources, thus affecting the production schedule.
(1) plastic deformation
The plastic deformation is the deformation caused by the load greater than the yield strength.For example, the cavity collapse, the enlargement of the type hole, the collapse of the corner, and the upsetting and longitudinal bending of the convex mold.In particular, the working surface of the mold is in contact with high temperature materials, so that the surface temperature of the cavity tends to exceed the tempering temperature of the hot die steel, and the inner wall of the groove is collapsed or pressed under pressure due to softening.When the hardened steel is used as the cold upsetting die, the mold is heated by the quenching, and the inner hole is sprayed with water to produce a hardening layer.When the die is in use, such as the cold heading force is too large, the base compressive yield strength under the hardened layer is not high, and the cavity of the die cavity is collapsed.Under the same hardness, the steel with different chemical compositions has different compressive strength. When the hardness of steel is 63HRC, the yield strength of the following four types of steel is from high to low: W18Cr4V> Crl2> Crl2.
(2) wear failure
Wear failure refers to the cutting edge of the cutting edge, the Angle of the circle, the surface depression, the surface groove mark, the peeling of the mucous membrane (the surface of the molds is stuck to the blank metal in the friction).In addition, in the work, the convex mould is converted into high pressure gas after the lubricant is burned, and the surface of the convexity is scoured to form cavitation.
Cold washed, if the load is not big, mainly oxidation wear types, wear also can to some degree of occlusal abrasion, when parts stamping part dull or load is bigger, the occlusal abrasion situation will become serious, which makes wear, the wear resistance of steel depends not only on the hardness, also depends on the nature of the carbide, size, distribution and quantity, in die steel, high speed steel and high chromium steel wear resistance is higher at present.However, in the presence of serious carbide segregation or large particles of carbide in steel, these carbides are easy to flake off, causing abrasive wear and accelerating wear.Lighter cold made of steel (sheet blanking, stretching, bending, etc.) impact, the load is not large, mainly for static wear.Under the condition of static wear, the carbon content of die steel is much, and the wear resistance is large.In the condition of impact wear (such as cold heading, cold extrusion, hot forging, etc.), too much carbide in die steel does not help to improve wear resistance, but it reduces wear resistance due to impact abrasive wear.
Studies have shown that under the condition of impact abrasive wear, die steel carbon content to 0.6% as the ceiling, cold heading die work under impact load conditions, such as too many carbides in the steel, easily by impact wear surface spalling.These peeling hard particles will become abrasive particles and speed up wear.The surface of the cavity of the hot mold is reduced by the softening of high temperature. In addition, the oxide iron oxide also ACTS as abrasive, and also has high temperature oxidation corrosion.
(3) fatigue failure
Fatigue failure characteristics: after a certain of mold parts, the initiation of the tiny crack, and gradually expand to the deep, expanded to a certain size, the bearing capacity of the severely weakened the mold caused by the rupture.Fatigue crack initiation in stress larger area, especially the stress concentration area (size transition, gap, mark, wear away the place such as crack), the fatigue fracture when the fracture is divided into two parts, part of the formation of fatigue crack development by fatigue rupture section, presents the shells, fatigue source is located in the shell.The other part is abrupt fracture, presenting uneven and rough section.
Mold steel has high yield strength and low fracture toughness when working in high hardness.High yield strength helps delay the generation of fatigue crack, but the low fracture toughness of fatigue crack propagation rate sped up and the critical length decreases, and the fatigue crack propagation greatly reduces cycle number, therefore, cold work die fatigue life depends mainly on the fatigue crack initiation time.
Hot-work die general serving in medium or low hardness condition, die fracture toughness than cold work die is much higher, therefore, in hot mould, fatigue crack propagation velocity is lower than the cold work mould, critical length is greater than the cold work mould, hot-work die fatigue crack of subcritical expansion cycle cooler for much longer, but hot-work die surface quenching, thermal shock is very easy to initiation of cold and hot fatigue crack, hot-work die of fatigue crack initiation time is much shorter than cold work mould, as a result, many fatigue fracture of hot-work die life depends mainly on fatigue crack growth when asked.
(4) fracture failure
The common forms of fracture failure are collapse, splitting, breaking, splitting and so on.The main part of cold mold is mechanical force (impact pressure).Hot-work die are in addition to the mechanical force, stress and thermal stress and organization, there are a lot of hot-work die working temperature is higher, and adopts the forced cooling, the internal stress can be far more than mechanical stress, therefore, many hot-work die fracture is mainly related to the internal stress is too large.
There are two kinds of mold fracture process: one-time fracture and fatigue fracture.One - time fracture is a sudden fracture of the die during stamping. Once the crack is formed, it is unstable and expanded.Its main reasons are severe overloading or severe embrittlement of mold materials (such as overheating, insufficient tempering, severe stress concentration and serious metallurgical defects).
The causes of die failure and preventive measures are as follows.
(1) unreasonable structure design causes failure.
The stress concentration is caused by the sharp Angle of rotation (the stress concentration is more than ten times higher than the average stress) and the large section change, which is often the source of many early failure of the molds.And in the process of heat treatment and quenching, the sharp Angle can cause the residual tensile stress and shorten the die life.
Preventive measures: the transition of each part of the convex mold should be smooth and smooth. Any small knife mark will cause intense stress concentration, and its diameter and length should meet certain requirements.
(2) failure caused by poor quality of mold materials.
Internal defects of mold materials such as loose, shrinkage cavity, inclusion, segregation of components, uneven distribution of carbides, original surface defects (such as oxidation, decarbonization, folding, scarring, etc.) affect steel performance.
Excessive inclusion causes failure.There are inclusions in steel is the root of mold internal cracks, especially brittle oxide and silicate, plastic deformation does not occur in the hot pressure processing, will only cause the brittle fracture and form micro cracks.In the subsequent heat treatment and use of the crack further expansion, and cause the mold cracking.In addition, in the grinding process, the mountain is caused by the peeling of large particles, resulting in surface holes.
The surface decarburization causes failure.Die steel in hot pressure processing and annealing, often due to the heating temperature is exorbitant, heat preservation time is too long, steel surface decarburization, caused serious decarburization steel after machining, sometimes still residue has decarburized layer, so that when quenching, due to the different layer inside and outside the organization (surface decarburization layer for ferrite, internal pearlite) makes organization transformation, and the crack generation.
The unevenness of carbon distribution causes failure.Crl2, C, r12MoV die steel, such as carbon and alloy element content is higher, the formation of a lot of eutectic carbides, the carbides in the forging is small, easy to strip and reticular segregation, leading to frequent Zuo fire crack along the banded carbide distribution. Mould in use cracks further expand, die cracking caused by failure.
Preventive measures: when steel is forged, the mould should be forged in many directions, so that the eutectic carbide in steel can be crushed to be even finer and even, and ensure that the steel carbide is not uniform.
(3) improper machining of molds.
(1) of the cutting knife mold cavity parts or punch round parts in machining, often due to feed too deep to leave a mark, local severe stress concentration, when quenching treatment, the stress concentration areas susceptible to micro cracks.
Preventive measures: in the final cutting of parts, the amount of feed should be reduced to improve the surface finish.
(2) electric processing causes the loss of efficiency in machining of die, because a lot of heat generated in the discharge, heated to very high temperature will make the mold by processing parts, make the organization change, form the so-called abnormal electric processing layer, the abnormal surface due to the high temperature melting, and then quickly solidified, the layer with a white under the microscope, there are many subtle internal crack, white areas of quenching under layer, called hardening layer, then into the hot effect is abate, the temperature is not high, only produce temper, said tempered layer.Measurement of hardness distribution: melting and solidification layer, hardness is very high, up to 610 ~ 740 HRC, 30 microns thickness of hardening layer hardness 400 ~ 500 HRC, thickness of 20 ^ ^ «.Tempering is a high temperature-tempering, with a soft tissue, with a hardness of 380 ~ 400HRC and a thickness of m.
Preventive measures: remove the resolidification layer in the abnormal layer by mechanical method, especially the microcrack;After electroprocessing, a low temperature tempering is carried out to stabilize the abnormal layer in case of micro-crack propagation.
(3) caused by the failure of grinding Mold cavity surface during grinding, due to the large grinding speed and grinding wheel fairly granular or poor cooling conditions, all can lead to overheating or cause surface grinding surface to soften, reduce the hardness, make the mould in use due to wear or thermal stress and grinding cracks, leading to early failure.
Preventive measures: grinding wheel with strong cutting force or poor bonding;Reduce the supply of workpiece;Choose the appropriate coolant;Grinding with 250 ~ 350 ℃ after tempering, to grinding stress.
(4) the mold heat treatment process is not suitable.
There are more carbon and alloy elements in the heating speed die steel, and the thermal conductivity is poor. Therefore, the heating speed cannot be too fast and should be carried out slowly.In order to prevent oxidation and decarburization, and to prevent oxidation and decarbonization, the heating rate should not be too fast, and the heat penetration should be slow.In this way, there will be no great thermal stress, which is safer.If the mold is heated quickly, the heat stress will be generated inside and outside the mold.If it is not properly controlled, it is easy to produce deformation or crack and must be preheated or slowed down to prevent it.
Oxidation and decarburization die quenching is carried out at high temperature, if not strictly controlled, the surface is easy to oxidize and decarbonize.In addition, after the surface decarburization, due to the difference in inner and outer layers, large tissue stress occurred in the cooling, resulting in the quenching crack.
Precautionary measures: packing materials can be packed and packed with anti-oxidation and decarburizing materials.