Work Hardening in Manganese Jaw Crusher Plates

Work hardening jaw plate behaviour is the reason a manganese casting can arrive soft from the foundry and end up among the hardest steel surfaces in a crushing plant.

A quarry replaced a set of plates that had run 90,000 tonnes and found the worn face reading far harder than the same point had read when the plate was fitted.

Nothing had been added to the steel; the surface had been hammered into a harder state by the rock passing over it.

Manganese steel work hardening explains that change, and a jaw plate hardness change measured on a returned part is the evidence a plant can use to judge whether the duty suits the grade.

This article covers how the transformation happens, what it looks like in the readings, and how to record it so that one set of plates can be compared with the next.

1. Work Hardening in Manganese Jaw Crusher Plates

A jaw plate cast in manganese steel leaves the foundry in a soft condition, and that softness is a design choice rather than a compromise.

The austenitic structure of the grade is tough and ductile, so the plate can deform under impact instead of cracking.

Work hardening in manganese jaw crusher plates takes place when the surface is loaded past its yield point again and again, and the crystal structure transforms in that surface layer.

The transformed layer is shallow, extending perhaps 5 to 15 millimetres below the worn face on a typical plate.

Underneath it the steel stays soft and tough, which is what allows the plate to absorb a shock that a through hardened casting would fracture under.

Three conditions decide how much hardening a plate develops.

The load per unit area at the contact point, which rises as the feed gets coarser.

The number of loading cycles, which is set by the stroke rate and the hours the plate spends in the machine.

And the temperature at the surface, because the transformation slows noticeably once the contact area runs warm.

2. Manganese Steel Work Hardening: How the Surface Changes

Manganese steel work hardening is a surface event, and the depth of the affected layer is what separates a plate that is working from one that has been worn past its useful state.

A fresh plate commonly reads between 180 and 220 HB when it is measured at the face.

After a period in a hard rock chamber the same face may read 350 to 500 HB, while the core of the section stays close to its original figure.

That gradient is the point of the grade, because a hard shell over a tough core resists abrasion without losing the capacity to absorb impact.

A plant that measures the face and ignores the core will misjudge the casting, and it may reject a plate that is behaving exactly as the grade intends.

Three features of a finished face show that the transformation has taken place.

A polished, slightly dished surface rather than a deeply gouged one.

A face that rings differently under a light hammer, though a hardness test remains the reliable check.

And a worn profile that has held its shape instead of rounding off at the edges within the first weeks.

3. What a Jaw Plate Hardness Change Looks Like in Service

A jaw plate hardness change is the difference between the reading taken on a new plate and the reading taken at the same point after service.

Recording both readings is what turns an impression into a figure that can be compared across sets and across machines.

A plate that has gained 150 HB at the face is doing what the grade allows, while a plate that has gained 40 HB has not been loaded hard enough to transform.

The second case points at a soft or a fine feed rather than at a casting fault.

Three readings make up a useful hardness record.

The reading at the feed opening, where the load on the plate is heaviest.

The reading at the discharge end, where the material is fine and the contact pressure is lower.

And the reading taken on the core of the section after the plate has been cut, which shows whether the plate hardened through or at the service face alone.

A jaw plate hardness change of 150 to 250 HB between those extremes is normal on a hard rock duty.

A change of less than about 50 HB over a full plate life is a signal that the chamber is being fed too gently for the grade to do its work.

4. Why a Work Hardening Jaw Plate Differs from a Pre-Hardened Part

A work hardening jaw plate relies on the duty to reach its hardness, while a pre-hardened plate arrives at the site with the hardness already in place.

Each approach suits a different plant, and the choice is decided by the feed rather than by the price of the casting.

A pre-hardened plate is hard from the first hour, which helps where the feed is fine and abrasive but carries little impact.

Manganese is tough from the first hour, which helps where the feed carries large lumps that would crack a brittle plate before it wore.

A work hardening jaw plate is the right answer where the feed supplies the load the transformation needs, and a plant blending two rock types will often run manganese in the primary and a harder grade further down the plant.

The table below sets out how the two behave at the machine.

A plant that fits manganese into a fine feed duty will pay for toughness it never uses, and it will see a short plate life as a result.

5. Manganese Steel Work Hardening and the Feed It Needs

Manganese steel work hardening depends on the feed delivering enough load to the surface, so a plant feeding a soft rock should not expect the grade to transform at all.

Limestone, soft sandstone and some oxidized ores do not generate the contact pressure the change requires.

A plant in that position is usually better served by a harder plate that does not depend on the duty to reach its working hardness.

The same logic applies at the other end, where a blast design producing very large lumps will overload the plate before the face has had a chance to harden.

Four feed conditions support the transformation well.

A charge with a substantial share of lumps above half the width of the opening.

A choke fed chamber, which keeps the load distributed across the plate face.

A setting that holds material in the chamber long enough to be loaded more than once.

And a rock with a moderate to high silica content, which supplies the abrasive contact that drives the change.

A feed that is too fine passes through quickly, and the plate then wears at its soft hardness for its whole life.

Delonshine Technology reviews the feed size distribution before recommending a manganese grade, because the same casting behaves differently at two plants running the same tonnage.

6. Measuring a Jaw Plate Hardness Change on a Returned Part

Measuring a jaw plate hardness change properly takes a portable tester and a marked point on the plate.

The face of a manganese plate is not flat after service, so a reading taken on a ridge will not compare with one taken in a hollow.

Marking the measuring points on the new plate before it is fitted is what makes the later comparison valid.

Four measurements at marked points along the centre line give a profile that can be plotted against tonnage, while a single reading in the middle of the plate misses the difference between the feed end and the discharge end.

Three practical points keep the readings usable.

Cleaning the surface to bare metal, since a layer of dust or scale will read low.

Using the same tester and the same indenter for every reading in a series.

And taking the reading with the plate well supported, because a plate resting on an uneven surface can read differently.

7. Work Hardening Jaw Plate Grades and Their Limits

A work hardening jaw plate is normally cast in a manganese grade with a defined carbon and manganese content, and the balance between them decides how the plate behaves.

A standard grade carries roughly 1.1 to 1.4 per cent carbon and 11 to 14 per cent manganese.

A modified grade raises the manganese and may add chromium or molybdenum, which increases the depth of hardening without making the casting brittle.

The choice between them turns on the feed rather than on a general preference, since a harder modified grade will crack in a chamber fed with very large lumps.

Three limits apply to any manganese plate.

The transformation depth is finite, so a plate worn past that layer loses the benefit and then wears at its soft rate.

The surface can be over-hardened by extreme impact, which produces spalling at the face rather than useful hardness.

And the grade loses toughness if it is heated above about 250 degrees Celsius in service, which is why a chamber that runs hot is a different problem from a chamber that runs hard.

8. Manganese Steel Work Hardening in Maintenance Planning

Manganese steel work hardening becomes a planning tool once a plant holds hardness readings tied to tonnage.

A plate whose face reaches a working hardness within the first few thousand tonnes will then wear at a rate that can be forecast.

A plate that takes much longer to reach that hardness is telling the plant that the chamber is being fed too gently.

Four items belong in the record for each set of plates.

The hardness readings at the marked points, taken before fitting and again after removal.

The tonnage passed and the hours the machine ran to pass it.

The feed size distribution and the setting in use.

And a photograph of the worn face, which shows whether the wear was even or concentrated in one zone.

Shenyang Delonshine Technology keeps the grade and the heat treatment of each casting on record, so a plant comparing two sets is comparing like with like.

Delonshine Technology supplies manganese plates to a standard or a modified specification, and it states which one suits a given feed rather than leaving the choice to the buyer.

Shenyang Delonshine Technology Co Ltd treats the hardness record as part of the wear part specification, because a grade that is right for one chamber is often wrong for the next.

9. Frequently Asked Questions about Manganese Steel Work Hardening

9.1 What is work hardening in a jaw plate?

Work hardening in a jaw plate is the local increase in hardness that a manganese steel develops when its surface is loaded past its yield point repeatedly. A work hardening jaw plate starts soft and tough, which lets it absorb impact without cracking, and the loaded face transforms into a much harder layer as rock passes over it. The change is shallow, usually a few millimetres to about fifteen millimetres deep, and the core of the section stays tough beneath it. That combination of a hard shell over a ductile core is the reason the grade is used in primary crushing.

9.2 How much does manganese steel work hardening change the surface?

Manganese steel work hardening commonly raises the face from around 180 to 220 HB when new to somewhere between 350 and 500 HB after service in a hard rock chamber. The size of the gain depends on the contact pressure and the number of loading cycles rather than on the grade alone. A plate that gains a hundred and fifty to two hundred and fifty HB at the feed end is working normally. A plate that gains very little has not been loaded hard enough for the transformation to take place.

9.3 What does a jaw plate hardness change of less than 50 HB mean?

A jaw plate hardness change of less than 50 HB over a full life usually means the feed is too fine or too soft to load the surface hard enough. The plate then wears at close to its as-cast hardness and gives a shorter life than the grade could deliver. The same figure can also point at a chamber that is running wide, so material passes through with little compression. Checking the feed size distribution and the setting is the first step, and a harder plate that does not depend on work hardening is often the better answer.

9.4 How should a plant measure a work hardening jaw plate?

A work hardening jaw plate should be measured at points marked on the plate before it is fitted, so the later reading is taken at the same place. Four points along the centre line give a profile from the feed opening to the discharge end, and the same tester and indenter should be used for every reading in the series. The surface is cleaned to bare metal first, since dust or scale will read low. The core reading is taken after the plate has been cut, and it shows how deep the transformation reached.