Hardfacing and Surfacing by the Numbers
Chapter 5 of 10 · 10 min read
Manganese steel and impact wear
Rebuild austenitic manganese steel parts that work-harden under impact, keep them cool by counting the heat each bead adds, remove the work-hardened skin, and join them to carbon steel.
Austenitic manganese steel is the material of crusher hammers, jaws, cones, excavator teeth, railway crossings and many other parts that take heavy impact. It is soft and tough when new, and the surface that is struck becomes harder and harder in service, while the metal beneath stays tough enough to absorb the blows. That combination makes it nearly ideal for heavy impact wear, and rebuilding manganese parts with weld deposits is one of the commonest hardfacing jobs. It is also one of the easiest to get wrong, because manganese steel loses its toughness if it is overheated, and a careless rebuild can turn a tough casting brittle.
This chapter explains how manganese steel work-hardens and why heat embrittles it, calculates how quickly welding heats a crusher hammer, sets out the workshop’s procedure for rebuilding hammers, and covers joining manganese steel to carbon steel.
What manganese steel is
The classic austenitic manganese steel, often named after its inventor, Robert Hadfield, contains roughly 12 to 14% manganese and a little over 1% carbon. Hornor’s 1920 railway specification gave its manganese electrode about 10 to 14% manganese and 1.0 to 1.25% carbon, close to the same composition. The high manganese keeps the steel in its austenitic form at room temperature, after the castings are heat treated by heating and rapid quenching in water, and in that form the steel is tough, ductile and not very hard. It is also non-magnetic, which gives a simple and useful workshop test: a magnet does not stick to austenitic manganese steel, though it may stick weakly to a heavily work-hardened surface.
Work hardening
When austenitic manganese steel is struck or deformed, its surface hardens greatly, becoming several times harder than when new, to a depth that grows with the severity of the impact. The hardened skin resists abrasion while the tough core beneath absorbs the blows without cracking. Under heavy impact, the skin is continually worn away and renewed. Under light impact or pure abrasion, the surface never hardens much, and manganese steel wears quickly, which is why it is chosen for impact, not for sliding abrasion by sand.
Why heat embrittles it
The toughness of manganese steel depends on its carbon staying dissolved in the austenite, which the quench after casting achieves. If the steel is held at elevated temperatures for long enough, the carbon comes out of solution as carbides along the grain boundaries, and the steel becomes brittle. Welding heats the metal next to the weld, and if a part is allowed to get too hot overall, or a region is held hot for too long, the heat-affected zone and then larger regions embrittle. The filler makers’ guidance is therefore to keep manganese steel cool during welding: no preheat, a low maximum interpass temperature that the filler maker specifies, short beads, skip welding, and pauses to let the part cool.
How fast a hammer heats
The heat added by welding can be estimated, and the arithmetic shows why rebuilding a hammer takes patience.
- Bead
- 150 mm at 1.8 kJ/mm
- Efficiency
- about 0.8
- Hammer
- 60 kg, specific heat about 500 J/(kg·°C)
- heat into the hammer ≈ 0.8 × 1.8 × 150 ≈ 216 kJ per bead
- average rise ≈ 216,000 ÷ (60 × 500) ≈ 7.2 °C per bead
- about 14 beads raise the whole hammer by about 100 °C, with the weld region much hotter
A rebuild may need dozens of beads, so a hammer welded continuously would soon be far hotter than any filler maker allows. The near-weld region heats much faster than the average suggests, which is why temperature is measured close to the weld, not on the far side of the hammer.
Working on several parts in rotation
The workshop’s answer is to rebuild hammers in sets, in rotation. A welder lays a few beads on the first hammer, moves to the second, then the third and fourth, and by the time the welder returns to the first, it has cooled. The rotation keeps every hammer below the interpass limit and keeps the welder working continuously, so the set of four takes little longer than one hammer welded with waits. With four hammers and about a dozen beads per visit, each hammer gets a long cooling interval between visits, and the welder’s arc time per hour stays high, which is the economic case for sending hammers in sets as well as the metallurgical one. Some workshops cool the parts with water sprays or by standing them in water up to just below the weld area, as some filler makers suggest for manganese steel, which speeds the cooling further.
- Inspect and weighcheck for cracks; record mass for wear and balance
- Remove the work-hardened skingrind back to softer, sound metal; grind out cracks
- Build up in rotationmanganese steel electrode or wire; short stringer beads; peen each bead; four hammers in turn
- Check temperaturetemperature crayon close to the weld before each new bead; wait if over the limit
- Hardface wearing faces if specifiedthin pattern of a compatible carbide deposit
- Weigh and matchbring each set within the crusher maker's mass tolerance
Removing the work-hardened skin
A worn manganese part has a work-hardened skin that is hard and has lost much of its ductility, and it often carries fine cracks from the impacts. Welding over it is a mistake: the hard skin cracks under the weld’s shrinkage, and the cracks run into the new deposit. The workshop grinds the worn face back until the hardness falls to near that of the unworked steel, checked with a file or a portable hardness tester, and grinds out any cracks completely, confirming with dye penetrant, before welding.
Peening
Peening each manganese bead while it is still hot, with a round-nosed hammer or a needle peener, stretches the deposit and counteracts its shrinkage, reducing the stress that would otherwise crack it or the base. It also starts the work hardening of the deposit’s surface. Peening is a standard part of most manganese steel welding procedures; the filler maker’s guidance says whether and how to peen each product.
Identifying the casting
Before rebuilding, the workshop confirms that a part really is austenitic manganese steel, because the procedure is very different from that for carbon or low-alloy steel, and the wrong one damages either. The magnet test is the first check: a new or lightly worn manganese casting does not attract a magnet. The part’s maker, its part number and its markings confirm it. Some wear parts that look similar are made of martensitic or chromium-molybdenum steels, which need preheat rather than cooling, and treating one of those as manganese steel, welding it cold, can crack it. A part that cannot be identified is treated as unknown and tested before any welding.
Cracks in manganese castings
Manganese castings often arrive with cracks from impact, especially at corners and around bolt holes. Small cracks can be ground out and filled with manganese deposit, following the same cool-welding rules; cracks that run deep into the casting, or that have grown from internal casting defects, may mean the part is near the end of its life. The workshop grinds each crack out until dye penetrant shows it gone, and if the excavation would go deeper than the build-up limits allow or weaken a highly loaded section, it recommends a new part rather than a repair.
Building up in layers
Manganese build-up deposits can be laid in many layers, because they are tough and work-harden rather than crack, which is why manganese steel is the normal build-up for manganese castings. Each layer is laid in short beads, peened, and allowed to cool before the next. The workshop restores the hammer’s profile with templates and the mass with the scales, building slightly more where the hammer wears fastest so that the rebuilt shape matches the new one’s balance. The final layer is left as welded on faces that will work-harden in service, or dressed where the profile must be accurate.
Work-hardening deposits in service
A freshly rebuilt manganese surface is soft and wears relatively quickly at first, until impact has hardened it. The first hours in service therefore wear the rebuilt hammer a little faster than later hours, and the workshop’s wear measurements are taken over long enough periods to average this out. Some operators deliberately run rebuilt parts on softer material first, where practical, to start the work hardening gently; most simply accept the slightly faster initial wear.
Hardfacing manganese parts
Manganese build-up deposits work-harden in service like the casting, so a hammer rebuilt with manganese steel behaves much like a new one. Where the wear also includes significant abrasion, a thin pattern of a harder deposit on the faces that see most sliding can extend the life, but the harder deposit must be compatible with manganese steel and is laid over a manganese build-up, not directly on a work-hardened surface. Chromium carbide deposits laid directly on manganese steel without a suitable build-up or buffer can crack and spall under the impact. The filler makers’ guidance lists compatible combinations.
Joining manganese steel to carbon steel
Manganese steel parts are sometimes welded to carbon steel, such as manganese teeth or wear plates on a carbon steel bucket. Welding them directly with a manganese filler risks a brittle zone where the filler mixes with the carbon steel, and welding them with a carbon steel filler risks a brittle zone in the manganese side. The usual choice is an austenitic stainless steel filler of a type the filler makers recommend for dissimilar joints, which tolerates dilution from both sides and stays tough. The manganese side is kept cool, as always, and the carbon steel side preheated if its own composition requires it.
Fume from manganese deposits
Manganese steel deposits contain about as much manganese as the castings, and welding them produces fume rich in manganese compounds, which can harm the nervous system with prolonged exposure. Rebuilding a set of hammers means hours of arc time in one place, so the workshop treats manganese welding as high-fume work: local exhaust at the arc, respiratory protection as the risk assessment requires, and work rotation so that no welder spends whole days rebuilding manganese parts. The filler’s safety data sheet sets out the hazards and the controls, and Welding Health and Safety by the Numbers deals with fume control in general.
The running case: crusher hammers
This is an illustration. The quarry’s impact crusher uses a set of four manganese steel hammers, each about 60 kg new, which wear to about 45 kg before they are turned or rebuilt. The workshop rebuilds a set at each scheduled shutdown, following the procedure above, and returns them weighed and matched within the crusher maker’s tolerance. Rebuilding adds about 12 kg of manganese deposit per hammer, and the rebuilt hammers last about 85% as long as new ones at a little under half the price of a new set. Chapter 10 turns that into a cost per tonne crushed.
Measuring temperature properly
Temperature-indicating crayons, which melt at a stated temperature, are the simplest way to check a manganese part’s temperature: a mark of a crayon rated at the interpass limit, made close to the weld, melts if the part is too hot. Contact thermometers and infrared thermometers are also used, though infrared readings on shiny or scaled surfaces need care. The workshop’s welders check before each new bead on a hammer, and the rotation means they rarely have to wait.
A failure: the hammer that broke
This is an illustration. A workshop elsewhere had rebuilt a single manganese crusher hammer in one continuous session to meet an urgent request, laying bead after bead without pauses, and the hammer grew too hot to touch for a long time afterwards. Back in the crusher, it broke across its body within days, the fracture showing a coarse, brittle surface.
The prolonged heating had embrittled a large region of the casting. A hammer breaking in an impact crusher can damage the rotor, the liners and anything in the crusher’s path, so the cost of the failure was many times the cost of the hammer. The quarry now sends only sets of hammers for rebuilding, so that rotation is always possible, and keeps a rebuilt spare set so that no rebuild is ever rushed.
Exercise: heat in a rebuild
Rebuilding a 40 kg manganese steel jaw needs about 30 beads, each about 120 mm long at 1.6 kJ/mm, with about 80% of the heat entering the part. Estimate the average temperature rise per bead and for the whole rebuild if done continuously, taking a specific heat of 500 J/(kg·°C). What does this suggest?
Each bead puts about 0.8 × 1.6 × 120 ≈ 154 kJ into the jaw, raising its average temperature by about 154,000 ÷ (40 × 500) ≈ 7.7 °C. Thirty beads, welded continuously, would raise it by about 230 °C on average, with the region near the weld far hotter.
That would certainly exceed a manganese steel filler maker’s interpass limit and risk embrittling the jaw. The rebuild should be done in rotation with other parts, with pauses and temperature checks, or with the jaw partly immersed in water as the filler maker allows, so that it never gets near the limit.
