The impact of mold wear and failure on castings

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The phenomenon that the contact surface of the mold gradually loses material due to the relative movement of the mold surface is called wear. When the mold is working, it will come into contact with the forming surface of the blank, causing relative motion and causing wear. When this wear changes the mold’s size or the state of the surface of the mold so that it cannot work normally, it is called wear. Failure generally includes normal wear failure and abnormal wear failure.

 

Normal wear failure refers to the failure caused by uniform friction and wear between the working part of the mold and the part material, which changes the shape, size, accuracy, etc. of the working part of the mold; abnormal wear failure refers to the failure caused by local external force or environment Under the action of factors, the working part of the mold and the part material are engaged, causing the shape, size, accuracy, etc. of the working part of the mold to suddenly change and lead to failure.

 

There are many types of wear. According to different wear mechanisms, they can be divided into abrasive wear, adhesive wear, fatigue wear, cavitation and erosion wear, corrosive wear, etc.

 

  1. Abrasive wear

The hard protrusions and foreign hard particles on the surface of the workpiece scratch the surface of the mold during processing, causing the material on the surface of the mold to fall off, which is called abrasive wear. When the abrasive grains are in contact with the workpiece and the mold surface, the force acting on the abrasive grains can be divided into two components, parallel and perpendicular to the mold surface. The vertical component forces the abrasive grains to press into the metal surface, and the parallel component forces the abrasive grains to press into the metal surface. The relative tangential motion between the abrasive particles and the metal surface constitutes a complete abrasive wear process. When molding is used, the hardness of the mold is usually higher than that of the workpiece, so the abrasive particles are first pressed into the workpiece. When the mold and the workpiece move relative to each other, the mold is scraped, and small fragments are cut off from the surface of the mold, causing abrasive wear. When there are pits, grooves, and other defects on the surface of the mold, abrasive particles may be embedded in them or bonded to the surface of the mold and move with the mold. The abrasive particles will plow or wrinkle the surface of the workpiece, affecting the processing quality of the workpiece.

 

The factors that affect abrasive wear mainly include the shape and size of the abrasive particles, the ratio of the hardness of the abrasive particles to the hardness of the mold material, the surface pressure between the mold and the workpiece, the thickness of the workpiece, etc. The sharper the shape of the abrasive grains, the greater the wear amount; the larger the size of the abrasive grains, the greater the wear amount of the mold. However, when the size of the abrasive grains reaches a certain value, the wear amount will stabilize within a certain range; When the ratio of grain hardness to mold material hardness is less than 1, the amount of wear is small. When the ratio increases above 1, the amount of wear increases sharply, and then gradually remains within a certain range; as the surface pressure of the mold and workpiece increases, the amount of wear increases. will continue to increase. When the pressure reaches a certain value, the increase in wear is slowed down because the sharp corners of the abrasive particles become blunt; the greater the thickness of the workpiece, the deeper the abrasive particles are embedded in the workpiece, and the wear on the mold is reduced.

 

From the above analysis, it can be seen that increasing the hardness of the mold material can improve the ability to resist the embedding of abrasive particles and help reduce the wear of the mold; surface wear-resistant treatment of the mold can improve its resistance while ensuring that the mold has a certain toughness. Wear performance: During the use of the mold, timely cleaning of abrasive particles on the surface of the mold and workpiece can reduce the probability of abrasive particle invasion and effectively reduce mold wear.

 

  1. Adhesive wear

Due to the uneven surface of the mold and the workpiece, the adhesion point breaks during relative motion and the mold material peels off, which is called adhesive wear. Since there is a certain degree of unevenness on the surface of the mold and the workpiece, only a few microscopic convex parts are in contact, and the pressure on the peak is very large (sometimes as high as 5000MPa), which leads to plastic deformation of the local surface of the mold, and due to plastic deformation and Friction generates high heat, which destroys the lubricating film and oxide film on the surface of the mold material, causing the surface of the fresh mold material to be exposed, and generates mutual attraction and interpenetration between atoms with the workpiece material, resulting in local adhesion between the materials. With the relative movement between the mold and the workpiece and the rapid cooling of the contact part, the peak metal is equivalent to a local quenching process. The metal strength and hardness of the adhesive part increase rapidly, forming quenching cracks, and causing tearing during the movement. cracking and eventually peeling, causing adhesive wear.

 

The main factors affecting adhesive wear include material properties, material hardness, surface pressure, etc. According to the strength theory of metals, it can be known that the failure of plastic materials depends on shear stress, and the failure of brittle materials depends on normal stress. In surface contact, the maximum normal stress acts on the surface, and the maximum shear stress occurs at a certain depth from the surface. Therefore, it can be seen that the higher the plasticity of the material, the more serious the adhesive wear will be. When the same metal or metals with high mutual solubility form a friction pair, the adhesion effect is obvious, and adhesive wear is easy to occur. From the perspective of the material’s organizational structure, metal materials with multi-phase structures have higher resistance to adhesive wear than single-phase metal materials due to their strengthening effects. The closer the hardness of the mold material and the workpiece material is, the more serious the wear will be. As the surface pressure increases, the amount of adhesive wear will continue to increase, but will gradually slow down after reaching a certain range.

 

From the above discussion, it can be seen that choosing a mold material with low miscibility with the workpiece material can reduce the solubility between the two materials and reduce the amount of adhesive wear; rationally selecting lubricants to form a lubricating oil film can prevent or reduce direct contact between the two metal surfaces. , effectively improve its ability to resist adhesive wear; use a variety of surface heat treatment methods to change the miscibility and organizational structure of the metal friction surface, and try to avoid the same type of metals from rubbing against each other, which can reduce adhesive wear.

 

  1. Fatigue wear

Under the action of cyclic stress, the phenomenon of fatigue spalling of surface metal when the two contact surfaces move with each other is called fatigue wear. During the relative motion between the mold and the workpiece, a certain force will be borne. There are variable contact pressures and shear stresses on the surface and sub-surface of the mold. After these stresses act repeatedly for a certain period, local plastic deformation will occur on the surface of the mold. and cold work hardening phenomena. In those relatively weak places, crack sources will form due to stress concentration and expand under the action of external forces. When the cracks extend to the metal surface or intersect with longitudinal cracks, wear spalling will occur.

 

The factors that affect fatigue wear mainly include the metallurgical quality of the material, material hardness, surface roughness, etc. The gas content in steel, the type, size, shape, and distribution of non-metallic inclusions are all important factors affecting fatigue wear. In particular, the presence of brittle and angular non-metallic inclusions destroys the continuity of the matrix. , under the action of cyclic stress, stress concentration will be formed at the sharp corners of the inclusions, and fatigue cracks will be formed due to cold work hardening caused by plastic deformation. The impact of material hardness is quite special. Generally, increasing hardness can increase the fatigue resistance of the mold surface. However, when the hardness is too high, it will accelerate the expansion of fatigue cracks and accelerate fatigue wear. The rough surface of the material will cause contact stress to act on a smaller area, forming a large contact stress and accelerating fatigue wear. Therefore, reducing the surface roughness value of the mold can improve the wear resistance of the mold.

 

To improve the fatigue resistance of the mold, appropriate lubricants should be selected to lubricate the surfaces of the mold and the workpiece, avoid or reduce direct contact between the mold and the workpiece material, reduce contact stress, and reduce fatigue wear. In addition, the surface of the mold can be treated with surface techniques such as shot peening and rolling at room temperature, so that the working surface of the mold will produce a certain residual compressive stress due to pressure deformation, which will help improve the fatigue wear resistance of the mold.

 

Other wear and tear

In addition to the above-mentioned main forms of wear, there are also cavitation wear, erosion wear, corrosion wear, etc.

 

(1) Cavitation wear. Due to the bursting of bubbles on the metal surface, instantaneous high temperature, and impact, the phenomenon of forming tiny pits and pits on the surface of the mold is called cavitation wear. When the mold surface comes into contact with the liquid and produces relative motion, the bubbles formed on the surface will flow to the high-pressure area. When the pressure the bubbles bear exceeds its internal pressure, it will burst and generate extremely high temperatures in an instant. and impact force, acting on the local surface of the mold. After repeated effects, fatigue cracks will form on the near-surface of the mold. After expansion, local metal will separate from the mold surface or vaporize, forming a foam sponge-like cavity, that is, Cavitation wear.

 

(2) Erosion and wear. Tiny particles of solid and liquid repeatedly fall on the surface of the mold in the form of high-speed impact, causing local material loss on the surface of the mold to form pits or pits, which is called erosion wear. Liquid particles impacting the mold surface at high speed will generate high stress when they fall, which will generally exceed the yield strength or strength limit of the metal material, causing local plastic deformation or partial fracture of the mold surface material. Those liquid particles with low velocity will also cause fatigue cracks on the surface of the mold after repeated impact, thus forming pits and pits, leading to erosion and wear.

 

(3) Corrosion and wear. During the working process, chemical or electrochemical reactions occur between the mold surface and the surrounding environmental medium, and the friction between the mold and the workpiece causes the material on the mold surface to fall off, which is called corrosive wear. When friction occurs between the mold material and the workpiece material in a certain environment, the metal material will react chemically or electrochemically with the environmental medium and form reactants. It is worn away in the subsequent relative movement between the mold and the workpiece, which forms corrosive wear. Corrosion and wear often occur in humid or high-temperature environments, especially in the presence of acids, alkalis, salts, and other media. Common forms of corrosion wear are generally oxidation corrosion wear and special medium corrosion wear. It can be seen that the properties of the mold material, the working environment medium, temperature, humidity, etc. are all important factors that affect corrosion and wear. The corrosion and wear resistance of the mold can be improved by correctly selecting mold materials and reasonably improving the working environment.

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