II. Depth Comparison: Which is Superior?

1. Surface Integrity and Fatigue Life

• Shot Polishing followed by Grinding (Route 1): The residual compressive stress layer produced by shot blasting is typically 0.2-0.5 mm deep. However, the subsequent delicate grinding process will remove this reinforcing layer. If the grinding amount is controlled within 0.1 mm, some compressive stress may be retained; if the grinding amount is large (e.g., above 0.3 mm), the reinforcing layer will be removed entirely, and even tempering or secondary quenching due to grinding heat may occur, leading to tensile stress on the surface and negating the benefits of shot blasting.

• Grinding followed by Shot Polishing (Route 2): The surface finish after fine grinding is high (Ra 0.4-0.8 μm), but the grinding process easily generates micro-cracks or tensile stress on the surface. The final shot blasting process can effectively cover grinding marks, rebuild or even improve the residual compressive stress on the surface (typically improving contact fatigue life by 20%-40%), while sealing grinding microcracks, significantly improving the gear’s resistance to pitting and bending fatigue.

• Conclusion: In terms of improving fatigue life, grinding followed by polishing is far superior.

2. Dimensional Accuracy and Deformation Control

• Polishing followed by grinding (Route 1): Shot blasting is a severe work-hardening process that creates a work-hardened layer on the gear surface. For high-alloy steels (such as 20CrMnTi and 17CrNiMo6), this hardened layer is very hard and brittle, leading to a decrease in the grinding ratio (G ratio), extremely rapid wear of the grinding wheel, and a high risk of grinding burns. Furthermore, the impact force of shot blasting can cause microscopic warping in thin-walled or slender gears, posing a significant challenge to dimensional control during subsequent precision grinding.

• Grinding followed by polishing (Route 2): Precision grinding uses the cutting action of the grinding wheel to remove excess material, accurately correcting heat treatment deformation and ensuring tooth profile, tooth direction, and cumulative errors. Shot blasting is a non-cutting cold working process; as long as the shot size and strength are well controlled, it causes almost no macroscopic dimensional changes (usually at the micrometre level) and does not compromise the accuracy after precision grinding.

• Conclusion: In terms of ensuring high precision, grinding followed by polishing is the only option.

3. Production Costs and Efficiency

• Polishing before grinding (Route 1): While seemingly cleaning the surface and protecting the grinding wheel, grinding actually removes the strengthening layer, wasting the initial strengthening energy. Furthermore, to remove the surface unevenness (orange peel effect) caused by shot blasting, the grinding allowance must often be increased, thereby increasing wheel consumption and machine tool time.

• Grinding before polishing (Route 2): This eliminates the rough cleaning process before grinding, but increases the need for post-grinding cleaning. However, modern CNC grinding machines have built-in high-pressure cooling and filtration systems, eliminating the need for excessive reliance on shot blasting to remove oxide scale before grinding. Shot blasting, as the final process, has relatively lower equipment investment, faster processing speed (batch processing), and lower overall cost.

• Conclusion: Economically, grinding before polishing is more advantageous, especially for mass production.

4. Special Cases: The Game of Carburised Layer Depth

One viewpoint suggests that if the carburised layer is shallow (e.g., 0.8-1.0 mm), grinding before polishing may penetrate it, resulting in insufficient hardness. • In this regard, Route 1 (shot blasting followed by grinding) is indeed safer because shot blasting does not remove material, and grinding only requires a small feed to ensure the hardness of the entire tooth surface.

• However, Route 2 (grinding followed by shot blasting) can be addressed by “allowing for grinding allowance within the carburised layer depth.” For example, if an effective hardened layer of 0.8mm is required, and the actual carburization is controlled at 1.0-1.1mm, grinding removes 0.2mm, leaving 0.8-0.9mm, which fully meets the requirements. As long as the process calculations are accurate, this is not an obstacle.

 

III. Decision-Making Guidelines: How to Choose?

There is no absolute “right” or “wrong,” only “suitable” or “unsuitable.” The following core factors should be considered when choosing a process route:

Scenario A: “Grinding before polishing” is mandatory (Recommended mainstream approach)

1. High-precision gears (grade 6 and above, such as automotive transmissions and aerospace gears): Grinding is essential to ensure tooth profile; shot blasting can only be used as a final strengthening method.

2. Gears with a large module (M>4) or wide tooth surfaces: These parts have a deep carburised layer (typically >1.2mm), and grinding will not damage the core hardness.

3. Expensive alloy steel materials (such as Ni-Cr-Mo series): These materials have excellent hardenability but high grinding sensitivity. Grinding before polishing can prevent burning and significantly improve load-bearing capacity, extending the life of expensive parts.

4. Gears requiring high contact fatigue strength: Wind power gears, heavy truck transmission gears, must use grinding before polishing to obtain maximum residual compressive stress.

Scenario B: Situations where “polishing before grinding” can be considered

1. Shallow carburised gears (adequate layer depth < 0.8mm): Such as some instrument gears and lightly loaded gears, even a slight grinding amount will expose the iron. First, shot blasting removes the oxide scale and provides slight strengthening, then lightly grinds to the required dimensions.

2. Small factories with limited equipment: Lacking high-precision CNC grinding machines, they can only rely on grinding allowance to “scrape” together precision. First, shot blasting to clean the surface helps improve grinding efficiency.

3. “Rotten teeth” with severe deformation after carburising: Extensive grinding (> 0.5mm) is needed to correct the deformation. In this case, first shot blasting to remove the thick oxide scale and prevent grinding wheel clogging is a practical remedial measure.

IV. Key Details of Process Implementation

Regardless of the chosen route, the following details determine success or failure:

1. Regarding “Grinding Burn”: If grinding followed by polishing is chosen, the concentration, flow rate, and cleanliness of the grinding fluid are crucial. Once grinding burn occurs (secondary quenched martensite or tempered troostite appears on the surface), shot blasting cannot eliminate this brittle layer and may even cause microcracks to propagate. Strict control of the grinding feed rate and the number of finishing passes is essential.

2. Regarding “Shot Peening Intensity”: When grinding followed by polishing, the shot peening intensity (Almen arc height value) should not be too high. Excessive intensity will damage the surface roughness of the ground teeth and may even cause metal spalling at the tooth tip fillets. Fine shot (such as S110-S170 steel shot) is typically used for gentle strengthening.

3. Regarding “Deburring”: After grinding, the sharp edges of the gears will have burrs. If deburring is not performed before shot blasting, the shot will hit the root of the burrs, causing stress concentration and potentially breaking the burrs, causing injury. It is recommended to add a deburring process (such as brushing or tumbling) after grinding before shot blasting.

4. Regarding “clamp marks”: During gear grinding, the clamping areas may have a carbon-free layer or soft bands. During shot blasting, the deformation behaviourbehaviour of these areas differs from that of the hardened layer; careful observation is necessary to avoid stress concentrations that could lead to cracking at the clamping points.

V. Summary

In conclusion, “carburising followed by fine grinding and then shot blasting” is the gold standard for modern high-precision, high-reliability gear manufacturing. It perfectly resolves the contradiction between dimensional accuracy and surface strengthening, maximising material potential and extending gear service life.

“Shot blasting followed by fine grinding” is more of a compromise solution limited by the depth of the carburised layer, equipment precision, or specific material properties, or used in the roughing stage with a large grinding allowance.

As a heat treatment technician or process engineer, when dealing with specific parts, you should prioritise calculating the degree of match between the carburised layer depth and the grinding amount. As long as the grinding amount does not exceed 30%-40% of the carburised layer depth, firmly choose “fine grinding followed by shot blasting”. This is not only an advancement in technology, but also a reflection of responsibility for product quality. In future gear manufacturing, with the development of dry cutting, hard rolling, and high-power grinding technologies, post-grinding shot blasting—a composite process combining cold and hot grinding—will inevitably become a key link in enhancing the core competitiveness of domestically produced gears.