Retained Austenite in Mill Liners: The 30% Rule
A medium-manganese white iron liner leaves heat treatment with nearly half its matrix still austenite. It shrugs off impacts — and wears away like cheese. Run the same casting through deep cold treatment and hardness climbs from 43.5 to 57.5 HRC, but impact toughness falls from 7.1 to 3.8 J/cm². The liner you actually want lives between those two extremes, and one measured experiment pinned down where.
Eight grades of austenite, one variable
Researchers in Wuhan and Beijing took a medium-manganese white cast iron (C 2.08%, Mn 4.95%, Cu 1.02%, ~0.6% Mo) — the workhorse chemistry for small and mid-size ball mill liners — and changed nothing except the retained austenite content. Sub-zero treatment at temperatures from 0 °C down to −196 °C moved the austenite level from 48.78% to 23.16% in eight measured steps, while carbide volume (about 20%) and carbide shape stayed constant. That last point is what makes the data worth trusting: the only moving part was austenite.
| Retained austenite | Bulk hardness | Impact toughness |
|---|---|---|
| 48.78% (as heat-treated) | 43.5 HRC | 7.1 J/cm² |
| 23.16% (−196 °C, 2 h) | 57.5 HRC | 3.8 J/cm² |
| 30–35% (the spec window) | 51–53 HRC | ≥ 4.5 J/cm² |
On a simulated liner wear rig — quartz sand at 0.212–0.425 mm, 190 g/min feed, 80 N load, 0.5 m/s sliding — wear resistance peaked at roughly 30% austenite and dropped off on both sides. The authors’ engineering verdict: for liners in this material class, hold retained austenite between 30% and 35%.
Why both extremes lose
Austenite is a two-faced phase. Fresh, it measures only HV 245–365 against martensite’s 600–900. But under repeated grinding and impact it work-hardens to HV 600–900 in the worn skin, and being tough and plastic, it blunts crack tips instead of launching them through the carbide network.
Too much of it, around 48%, and the bulk matrix simply stays soft. Quartz grit ploughs deep gouges and scoring pits into the face; the wear test pieces came out scarred with heavy ploughing.
Too little, near 23%, and the opposite failure shows up: the matrix is hard but unforgiving, and the eutectic carbides lose the cushioning support around them. Carbide particles crack, then spall off in chunks — you lose the hardest phase in the material by making the matrix too hard. Impact data agrees, with toughness nearly halved at the deep-frozen end.
The 30–35% window is where the surface work-hardens under duty while the core stays forgiving.
Each liner family has its own number
Retained austenite is not a universal setting — it depends on the matrix the austenite lives in:
| Liner material | Retained austenite | Reason it works |
|---|---|---|
| Hadfield steel (ZGMn13) | ~100% by design | the whole point is impact-driven hardening — see our work-hardening guide |
| Alloyed Hadfield, 350 °C aged | austenitic, precipitation-hardened | carbide dispersion adds 45% wear life without losing toughness (the aging argument) |
| Air-hardening bainitic cast steel | ~11% (measured by magnetic analysis) | thin austenite films between bainite ferrite laths stop cracks |
| High-chromium steel (ZGCr13SiMo type) | ~10% | enough to keep a 57 HRC steel from cracking; the Japanese CIX-2L benchmark ran 8.5% |
| Medium-Mn white iron | 30–35% target | the balance point in the curve above |
Note the direction of travel: as matrices get harder, the austenite fraction drops. A bainitic steel at 11% and a white iron at 33% can both be right — they are different load-bearing systems, and comparing them on austenite percentage alone is meaningless.
What this means on a purchase order
Retained austenite is a number a competent foundry can quote, measure and hold. X-ray diffraction gives it in half an hour. When you evaluate liner suppliers — especially for white iron and alloy cast iron liners — ask three things:
- What is your target retained austenite window for this grade and duty?
- Show an XRD phase report for a production casting, not a lab button.
- What heat-treatment route holds that window in a 100 mm section, not just a test bar?
A supplier who cannot answer has no specification — only outcomes. Those foundries are the reason two “identical” liner sets can differ 2× in service life, a gap we covered in our materials guide and the ZGMn13 failure analysis.
One honest caveat before anyone copies numbers blindly: the 30% figure comes from one material system under one set of test conditions. Treat it as the shape of the answer, not a law of nature. In your ore, with your ball charge and your pH, the optimum can shift a few points either way — but it will still be an interior point, never an extreme.
QM Mill Liners quotes retained austenite windows and supplies XRD phase reports with alloy liner orders — because a hardness number without a microstructure behind it is half a promise. Send us your worn liner samples and we will read the metallurgy back to you — contact here.
FAQ
What is retained austenite in a mill liner? Untransformed austenite left in the matrix after heat treatment. It is softer than martensite but toughens the structure and work-hardens under grinding, so a controlled amount improves wear life.
How much retained austenite should a liner have? For medium-manganese white iron, lab and field data support 30–35%. Hadfield steel runs nearly 100% by design; bainitic and high-chromium steels run near 10%. The right number depends on the matrix.
Is retained austenite bad? Excess austenite softens the matrix and speeds abrasive wear; too little embrittles it and causes carbide spalling. Controlled austenite — not zero — is what makes liners last.