Quantification of Damage to Nitrided Layer by Polishing Repair after Nitriding of Molds | H13 Mold Nitriding Secondary Repair Specification
After nitriding, it was directly polished and repaired, and 56% of the nitride on the surface of the mold was removed, resulting in a significant decrease in wear resistance and anti stick aluminum properties.
If the removal amount of a single polishing is greater than 0.006mm, the compound layer will be completely ground off, leaving only the diffusion layer, and the resistance to aluminum infiltration will decrease by 52%.
Grinding wheel particle size # 240 or below, high cutting force, local overheating, secondary tempering softening of nitride layer, hardness drop HRC3-5.
The polishing blade direction is crossed and disorderly, forming intermittent scratches on the surface. After production, the scratch location will preferentially stick aluminum and accumulate slag.
After repair, low-temperature stress relief was not carried out, and surface tensile stress was introduced by grinding, resulting in a 50% reduction in microcrack initiation period under cold and hot cycling.
Repeated nitriding of the same cavity for more than 2 times leads to continuous accumulation of matrix brittleness and significantly increases the risk of thermal fatigue cracking.
Low pressure die-casting molds (LPDC), reverse pressure casting molds (CPC), differential pressure casting molds for wheel hubs, low-pressure casting molds for wheel hubs, gravity casting molds, motorcycle wheel hub casting molds, structural component casting molds, aluminum alloy wheel hub molds, aluminum casting forming molds, low-pressure casting tooling molds, many outsourcing factories refine and polish after nitriding, which is a typical process sequence error. Zhejiang Xinfeng Machinery standard process: cavity precision machining+polishing → cleaning and degreasing → nitriding treatment; After nitriding, only light throwing of oil stones is allowed to remove small black spots, with a removal amount controlled within 0.002mm. If there are significant cavity defects after nitriding and a large amount of polishing allowance is required, the nitriding must be removed, re polished, and nitrided again. It is not recommended to directly perform thick grinding. Overseas customers often receive nitriding reports, but they do not know that the thickness of the compound layer on the report is in the original nitriding state, and the thickness will shrink after polishing. The technical agreement may add a clause stating that after nitriding is completed, large-scale machining and polishing will no longer be carried out, and only a small amount of oilstone trimming is allowed.
FAQ
1. Question: What is the maximum allowable amount of polishing removal after nitriding?
Answer: Try to control it to ≤ 0.002mm and only lightly repair the oilstone.
2. Question: What are the consequences of polishing over 0.006mm?
Answer: The removal of the compound layer significantly reduces the anti stick and wear-resistant properties of the aluminum.
3. Question: Why is coarse-grained grinding wheel prohibited?
Answer: Local overheating, tempering softening of nitride layer, and hardness degradation.
4. Question: What treatment is required after nitriding repair and polishing?
Answer: For large polishing amounts, it is recommended to remove nitrogen and diazotize; Micro adjustment for low-temperature stress relief.
5. Question: What is the standard process sequence for the mold cavity?
Answer: Precision machining and polishing → degreasing and cleaning → nitriding.
6. Question: What are the hidden dangers of repeated nitriding?
Answer: The accumulation of matrix brittleness makes it more prone to thermal fatigue cracking.
