The Number After “Mn” Decides How Your Hammers Fail
Most wear part quotations say “high manganese steel” and stop there. That phrase covers grades whose service behaviour differs enough to change your hammer life by weeks — and buying on it is like ordering “steel bar” without a specification.
The designations that matter are Mn13Cr2, Mn18Cr2, Mn20Cr2 and Mn24Cr2, along with the molybdenum-bearing grades MnMo0.5 and MnMo1. This guide explains what separates them, and how to work out which one your machine actually wants.
What the Numbers Mean
The convention is straightforward once you know it. “Mn” is manganese and the number following it is the nominal manganese percentage. “Cr2” means roughly two percent chromium. So Mn18Cr2 is an austenitic manganese steel with about eighteen percent manganese and two percent chromium.
The classic material behind all of these is Hadfield steel, developed in the 1880s at around thirteen percent manganese. What made it remarkable then is what still makes it useful now: it arrives soft and tough, and hardens dramatically at the surface under impact while the core stays ductile.
Work Hardening Is the Whole Point
An austenitic manganese casting leaves the foundry at a hardness far below what a wear part appears to need. Buyers new to the material sometimes read the delivered hardness figure and conclude the parts are too soft. They are not measuring the thing that matters.
Under repeated impact the struck surface transforms and hardens substantially, while the material a few millimetres below stays tough. The part effectively builds its own wear-resistant skin in service, and keeps rebuilding it as the surface is worn away. That is why manganese steel survives shock loads that shatter a harder, uniformly hard casting.
Manganese steel needs impact to work.
On light, low-shock duty the surface never fully hardens, and the part wears faster than a plain hard casting would. It is the wrong choice for gentle abrasion — and the right one wherever things arrive in the chamber that should not be there.
Grade by Grade
Raising manganese raises the capacity to work harden and generally improves toughness, at some cost in machinability and in how quickly the surface reaches full hardness. In practical terms:
- Mn13Cr2
- The general-purpose grade and the most widely specified. Well proven across scrap shredding and crusher wear parts, readily cast in most sections, and the sensible default where nothing about the duty is extreme. If you do not know what you need, this is where the conversation starts.
- Mn18Cr2
- A step up in manganese for heavier auto shredding and larger rotors. It tolerates higher impact energy before cracking, which is why it appears so often on hammers for large car shredders. In genuinely light duty it will not out-perform Mn13Cr2 — the extra capacity only pays where the machine works it.
- Mn20Cr2
- For sustained heavy shock, where Mn18Cr2 is still coming out cracked rather than worn. This is a specification driven by evidence from your own worn parts, not something to jump to speculatively.
- Mn24Cr2
- The highest manganese in normal wear part use, for the most severe impact duty. Casting and heat treatment become more demanding at this level, so it is specified where the failure mode clearly justifies it.
- MnMo0.5 / MnMo1
- Molybdenum-bearing manganese grades. Molybdenum raises strength and helps in heavier sections where a plain manganese grade can be sluggish to develop properties through thickness. Useful on large castings and where a part must resist deformation as well as wear.
Section Thickness Changes the Answer
A point rarely discussed in sales conversations: manganese steel does not behave identically at every thickness. In heavy sections it is harder to achieve uniform properties through the full cross section, because the quench that fixes the austenitic structure has to reach the centre of the casting.
This is where the molybdenum grades earn their place. On a large hammer or a thick liner, MnMo0.5 or MnMo1 can deliver more consistent properties through thickness than a plain high manganese grade at the same nominal manganese level.
It is also why the weight and dimensions of your part matter when asking for a grade recommendation. The same designation on a small hammer and on a very large one is not the same product in service.
Reading Your Own Parts to Choose
The evidence you need is sitting in your scrap bin. Look at how the last set came out of the machine:
Impact energy exceeded what the grade could absorb. Move up in manganese — Mn13Cr2 to Mn18Cr2, or Mn18Cr2 to Mn20Cr2. Do not respond by going harder; that makes it worse.
Abrasion is the limit, not impact. There is headroom to move toward a harder specification, or toward a dual hardness construction with a hard face on a tough body.
The part is yielding rather than wearing. A molybdenum-bearing grade, which raises strength, is usually the right direction here.
Chromium’s Job in These Grades
The “Cr2” suffix is easy to overlook. At around two percent, chromium is not there to make the steel stainless or to dominate wear resistance the way it does in a high chrome white iron. It raises yield strength and helps the material resist the initial deformation before work hardening takes over.
The practical effect is a part that holds its shape better early in life. On a hammer that must maintain its profile to keep delivering energy efficiently, that is worth having.
Why Two Suppliers’ Mn18Cr2 Can Differ
A designation is a chemistry range, not a guarantee of identical performance. Two castings that both meet Mn18Cr2 can behave differently in service, and the reasons are usually process rather than chemistry.
Heat treatment is the largest factor. Austenitic manganese steel must be solution treated and quenched properly to fix the structure; an incomplete cycle leaves carbides at the grain boundaries and a part that cracks early despite a conforming analysis. Casting soundness matters too — porosity or inclusions concentrated in a highly stressed section will initiate a crack whatever the grade says.
This is why batch certification is worth asking for, and why a foundry that can discuss its heat treatment cycle is telling you something a chemistry certificate alone does not.
What to Send When Asking for a Grade
A useful recommendation needs more than a part name:
- The grade currently in use, if it is known
- Tonnes processed by the last set, rather than calendar weeks
- How the last set ended — cracked, worn round, deformed, or elongated at the eye
- What goes into the machine, including the things that are not supposed to
- The weight and rough dimensions of a single part
The last point matters more than it appears. Section thickness changes which grade will develop good properties, so a recommendation made without it is only half informed.
Send photographs of your last set of worn parts along with the tonnes they ran and what you are processing. We will tell you which grade the evidence points to — from Mn13Cr2 through Mn24Cr2 and the molybdenum grades — and say plainly if the answer is that your current specification is already right.
Related Reading
For the failure modes behind these choices, see why shredder hammers fail early. For the chrome irons used in blow bars, see how chrome content changes blow bar performance. Products: shredder hammers, ELV shredder hammers and shredder wear parts.