Short answer: nitrided elements suit clean, mildly abrasive duty; powder-metallurgy grades suit abrasive fillers; HIP-clad and bimetal grades suit the most aggressive wear plus corrosion. All three are typically supplied at 58–62 HRC, so hardness alone does not tell you which will last.
Almost every supplier quotes 58–62 HRC, and buyers reasonably assume the parts are equivalent. They are not. Two elements at the same surface hardness can differ by a factor of five in service life, because hardness describes the surface and wear depends on the microstructure beneath it, the depth of the hardened case, and how the material behaves when it is hot and loaded.
The useful questions are different: how deep is the hardened layer, what happens when it is breached, and does the material resist the specific wear mechanism in your process.
| Grade | How it works | Strengths | Weaknesses |
|---|---|---|---|
| Nitrided | Hard case diffused into the surface of a tough core | Good wear resistance, excellent toughness, moderate cost | Case is thin; once breached, wear accelerates sharply |
| Through-hardened tool steel | Hardness throughout the section | Uniform wear resistance, can be re-machined | Less tough; can crack under shock loading |
| Powder metallurgy (PM) | Alloyed powder consolidated with a fine, uniform carbide distribution | High wear resistance with good toughness; consistent | Higher cost, longer lead time |
| HIP-clad / bimetal | Wear-resistant alloy metallurgically bonded onto a tough base | Best abrasion plus corrosion resistance; deep wear layer | Highest cost; specialist repair |
| Duty | Recommended grade | Reason |
|---|---|---|
| Unfilled or lightly filled polymer | Nitrided | Wear is mild; toughness matters more than hardness |
| Colour masterbatch with TiO2 | Nitrided, or PM in the dispersion zone | TiO2 is abrasive but the load is moderate |
| Carbon black masterbatch | PM | High structure carbon black is aggressive and the duty is continuous |
| Glass fibre above 30 percent | PM or HIP-clad | Continuous abrasion at high stress |
| Mineral filler above 45 percent | PM or HIP-clad | Loading drives wear rate directly |
| Corrosive plus abrasive (flame-retardant, filled) | HIP-clad, corrosion-resistant alloy | Hardness alone does not resist chemical attack |
| Food and feed contact | Nitrided or hardened stainless | Corrosion resistance and food-contact compliance come first |
The comparison that matters is cost per tonne of product, not cost per piece. A nitrided element at a third of the price of a PM element is the cheaper choice only if it lasts more than a third as long. In glass-filled duty it often lasts a fifth as long, which makes the expensive part the cheap one.
There is a second effect that is easy to miss. Once clearance opens, quality falls before the part fails visibly, so a cheap element that wears quickly can cost more in rejected product than in replacement parts. That is why we usually recommend spending on the dispersion zone first — it wears fastest and its wear affects quality most directly.
PVD coatings such as TiN, CrN and DLC add surface hardness and reduce friction. They help in specific duties, particularly where sticking or galling is the problem, and they are not a substitute for the right base material. A coated nitrided element still has a thin nitrided case underneath, and once both layers are breached the wear rate is the same as uncoated.
For food contact, verify that any coating is approved for the application before specifying it.
Grade and finish work together: surface finish of screw elements covers the grinding and polishing side. For hygiene-driven material choices, see food and feed extrusion parts. For highly abrasive mineral filler duty, mineral-filled compounding shows how the grade choice plays out.
No. The number describes surface hardness only. Case depth, core toughness, carbide distribution and heat treatment quality all differ, and they determine service life far more than the headline figure.
Typically a few tenths of a millimetre, which is why nitrided elements degrade quickly once the case is breached. Ask for the specified case depth rather than accepting the hardness figure alone.
In abrasive duty, usually yes. The comparison is cost per tonne of product: if a PM element lasts more than three times as long as a nitrided one at three times the price, it is the cheaper option and it also protects quality.
Yes, and it is often the right answer — nitrided in the conveying zones, PM in the dispersion zone. Avoid mixing grades within a single functional zone, because the harder part takes less wear and shifts load onto its neighbour.
No. A coating improves surface behaviour but sits on top of the base material. Once it and any nitrided case are worn through, performance reverts to whatever the base material can do.