Why Certified SAG Mill Discharge Grates Still Wore Out in Six Months

A copper concentrator was replacing its SAG mill discharge grates every six months. The specification called for chromium white iron to ASTM A532 Class III Type A. The parts arrived certified, and they kept wearing out.

The gap was not in the grade. It was in the hardness distribution through the section.

What was happening inside the casting

The foundry was pouring to the low end of its process window. Brinell hardness at the wear face measured 600 to 650 HB, inside the specification. At 25 mm into the section it measured 480 HB, and work-hardening under load was inconsistent. Once the surface layer wore through, the material underneath failed much faster than a properly through-hardened casting would.

The certificate did not lie. But one Brinell test per heat, taken from a coupon poured with the batch, does not tell you what is happening inside a 200 kg grate section.

Four checks for high-chrome wear castings from China

Brinell hardness at three points per grate: wear face, 25 mm depth and back face. Reject if the range exceeds 100 HB.
Metallographic examination of the first article in each production lot, submitted to the buyer for approval before the balance of the order is released. It catches carbide morphology problems that hardness alone will not.
Pouring log per shift, showing melt temperature, mould temperature and cooling rate, included in the release package rather than supplied on request.
Third-party pre-shipment inspection with sectioning of one random part in 20. It destroys one part. That is cheaper than a set of grates that lasts four months.

The six-month replacement cycle was not a specification failure. It was a verification failure. For mining procurement teams sourcing wear parts from China, the standard tells you what to buy and the hold points tell you what to inspect.

Meritus Technology Shanghai
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