How to choose grinding wheel for grinding stainless steel?

Stainless steel is an alloy steel with high corrosion resistance and high strength. It is named because when the chromium content reaches a certain proportion, a dense chromium oxide film will form on the surface, which has the properties of rust prevention and corrosion resistance.

Types of stainless steel products

In the classification of metal materials, people often divide stainless steel into five categories according to their metallographic characteristics, namely: martensitic stainless steel, ferritic stainless steel, austenitic stainless steel, austenitic-ferritic duplex stainless steel, and precipitation-hardened stainless steel. Among them, austenitic stainless steel is the most common, accounting for about 70% of the total stainless steel.

At present, stainless steel with Cr and Ni elements added at the same time accounts for more than 60% of the total output of stainless steel. Compared with structural steel, stainless steel has poor thermal conductivity, low elastic modulus, high elongation, and large cross-sectional shrinkage, which brings many difficulties to processing, especially grinding.

 

Grinding characteristics of stainless steel

1. High grinding force and high grinding temperature

Stainless steel materials have the characteristics of high temperature, high strength, and high toughness. During grinding, the lattice distortion is large and the plastic deformation is severe. The cutting force per unit area of ​​stainless steel can reach 90GPa, and the grinding force is 1.5-2.2 times that of 45# steel. The grinding resistance is large and the temperature of the grinding zone is high. The thermal conductivity of stainless steel is low, about 1/3 of 45# steel, which makes the surface of the workpiece instantly generate a high temperature of 1000-1500℃, the surface of the workpiece is easily burned, the workpiece produces cracks, and sometimes the surface of the workpiece is annealed, and the annealing depth can reach 0.01-0.02mm.

 

2. Severe work hardening tendency

When grinding stainless steel, the workpiece surface is severely deformed, stress and strain increase, and work hardening occurs. In the work hardening layer, the grains are distorted, the sand grains are worn and passivated, and vibration marks and scratches are produced.

 

3. Grinding dust adheres to the grinding wheel

Due to the great toughness of stainless steel, under certain grinding temperature, contact pressure and relative speed conditions, grinding dust easily reacts with sand particles, so grinding dust adheres to the grinding wheel, fills the gaps between the abrasive particles, and causes the grinding wheel to be rapidly blocked. At this time, the grinding conditions deteriorate rapidly, the heat is serious, and the surface roughness decreases significantly. The adhesion of grinding dust varies with different stainless steels. The grinding adhesion of concentrated nitric acid-resistant stainless steel and heat-resistant stainless steel is relatively serious, followed by 1Cr18Ni9Ti, 1Cr13, 2Cr13, etc.

 

4. Grinding dust is not easy to cut off, and abrasive particles are easy to blunt

When grinding stainless steel materials, the grinding force is large, and the abrasive particles are easy to blunt, causing the grinding wheel to wear. When grinding stainless steel, the grinding ratio is G=6-12, while the grinding ratio is G=40-80 when grinding ordinary carbon steel and low alloy steel. Therefore, the grinding ratio of stainless steel is only about 1/7 of that of ordinary steel.

 

5. Burning of workpiece surface

Burning of workpiece surface is one of the common defects in grinding, especially austenitic stainless steel, which is more sensitive to burning.

 

6. Scratches on workpiece surface

When grinding stainless steel materials, the surface of the workpiece is easily scratched, so the roughness of the processed surface is required to be high. Causes of scratches: There are some protruding abrasive particles on the working surface of the grinding wheel, which scratch the surface of the workpiece before falling off. The grinding chips that fall off the grinding wheel fall between the grinding wheel and the workpiece, causing scratches on the surface of the workpiece. The grinding fluid is not clean, mixed with grinding chips, and the supply of grinding fluid is insufficient, resulting in scratches during the grinding process. 7. Deformation of workpiece Due to the large linear expansion coefficient of stainless steel, the grinding temperature has a greater impact on the size of the workpiece, which is easy to cause errors in dimensional measurement. In addition, the hardening trend of stainless steel is serious, and residual stress is easy to generate on the surface of the workpiece. Especially when grinding slender shaft workpieces, thin-walled workpieces and other workpieces with poor rigidity, the workpiece is easily deformed under the action of grinding force.

 

Solution

1. Selection of abrasive types

When grinding stainless steel, white corundum (WA) is generally used. White corundum has good cutting performance and self-sharpening properties.

 

The effect of grinding stainless steel with cubic boron nitride (CBN) grinding wheel is better. Due to the high hardness of CBN, the abrasive grains are not easy to wear, the chemical stability is good, and there is no chemical affinity with iron elements, it is not easy to clog the CBN grinding wheel during grinding. The CBN grinding wheel has a small grinding force, low grinding heat, and does not burn the workpiece. However, the price of CBN grinding wheel for stainless steel is higher than that of ordinary grinding wheel, and it needs to be trimmed according to the actual situation.

 

The grinding effect of single crystal corundum grinding wheel is second only to CBN grinding wheel. Because each abrasive grain is a spherical polyhedral single crystal, there is no crack and residual stress caused by mechanical crushing. If the sudden crushing of a single abrasive grain is less, and the hardness and toughness are well matched, the single crystal corundum has good grinding performance. When grinding acid-resistant stainless steel, grinding scratches can be reduced, especially single crystal corundum with large aperture has good application effect in production.

(single crystal corundum grinding wheel)

 

When grinding Cr17Ni7A1 precipitation hardened stainless steel, use microcrystalline corundum (MA) grinding wheel. Microcrystalline corundum has high toughness, good cutting performance, long service life, self-sharpening cracking along the microcrystalline gap, will not cause large particles or the entire abrasive particles to fall off, which is conducive to reducing grinding heat and grinding wheel blockage. When grinding austenitic stainless steel, use a grinding wheel made of microcrystalline corundum and green silicon carbide, which can greatly reduce the roughness of the workpiece and reduce or eliminate burns.

2. Selection of abrasive grain size

Abrasive grain size has a direct impact on surface roughness. Stainless steel has high toughness and is easily clogged by grinding chips. If a fine-grained grinding wheel is used, the abrasive loses its cutting effect and the surface finish of the workpiece is not high. The results show that F36 and F46 grains are used for rough grinding, and F60 grains are used for fine grinding. F46 and F60 grain grinding wheels are used for both rough and fine grinding.

 

3. Selection of binder

Stainless steel has the characteristics of high toughness, high temperature resistance, high strength, and high grinding force. Therefore, the grinding wheel is required to have high strength and be able to withstand large impact loads. When grinding stainless steel, a ceramic bond grinding wheel can be used. It has good waterproof, heat resistance, and corrosion resistance. It can maintain good cutting performance and high production efficiency. The disadvantage is that it is brittle and cannot withstand large impacts and bending. Resin bond grinding wheels are used for cutting, grooving, and centerless grinding of stainless steel. They have high strength, good elasticity, impact resistance, and many pores. They can be used for higher circumferential speeds (35-75m/s). However, the hardness of the resin bond grinding wheel is low. Putting the resin grinding wheel in paraffin can prevent the influence of alkaline solution. In addition, when the temperature is higher than 150℃, the resin bond will soften, reduce strength, and even burn, so it should be cooled during grinding.

 

4. Selection of grinding wheel hardness

When grinding stainless steel, a grinding wheel with lower hardness should be selected, and the grinding wheel has better self-sharpening. If the hardness is too low, the abrasive grains are not firmly bonded, and they fall off before being blunted, which will greatly reduce the service life of the stainless steel grinding wheel.

 

In short, when grinding stainless steel, CBN grinding wheels and ordinary grinding wheels should be selected according to the economic situation and the actual situation of the grinding material. In addition, ordinary abrasive belts and CBN abrasive belts are also used for grinding in practical applications.