Hardfacing alloys and how to choose

Hardfacing alloys come in many families, each balancing hardness, toughness, corrosion resistance, heat resistance and cost differently. The catalogue of a single filler maker may list dozens of products, and the names give little away. Choosing well means knowing the families, what each is good at and what it is not, and matching them to the wear the part suffers, the base it is laid on and the cost the job can bear.

This chapter surveys the main families of hardfacing alloys, recalls the railway specification of 1920 that included a manganese steel electrode for hard-wearing surfaces, and shows how the workshop uses a simple weighted comparison to choose a deposit for a part.

A railway specification of 1920

Hornor’s Spot and Arc Welding of 1920 reproduced a railway’s specification for its arc welding electrodes, numbered by type. Among the mild steel and medium-carbon electrodes for general work, it listed a manganese steel electrode, with about 10 to 14% manganese and about 1.0 to 1.25% carbon, for all hard-wearing surfaces where extreme toughness and medium hardness were needed, naming track parts, steam shovel dippers and dipper teeth among its uses. It also listed a medium-carbon steel electrode for axles, forgings and piston rods, or wherever enough carbon was wanted to limit abrasive wear. Even then, the railway distinguished between wear that needed toughness, for which it chose manganese steel, and wear that needed hardness, for which it chose more carbon. The modern families extend the same distinction.

Martensitic steels

Martensitic hardfacing alloys are low- or medium-alloy steels that harden as they cool from welding, forming a hard but reasonably tough structure. They are used for building up and for moderate abrasion and metal-to-metal wear, such as tractor rollers, idlers, shafts and sprockets, and they can often be laid in several layers and, at the softer grades, machined. They are the nearest modern descendants of the railway’s medium-carbon electrode.

Austenitic manganese steels

Austenitic manganese steels, the alloys of the railway’s manganese electrode, are soft when deposited but work-harden dramatically under impact: the surface struck repeatedly becomes very hard, while the metal beneath stays tough. They are the standard choice for building up and surfacing manganese steel castings, such as crusher hammers, jaws and excavator teeth, and for parts that suffer heavy impact with moderate abrasion. They need care in welding, because manganese steel embrittles if it is overheated, as Chapter 5 explains.

Chromium carbide irons

High-chromium, high-carbon alloys form large, hard chromium carbides in an iron matrix as they solidify. The carbides, harder than quartz, give excellent resistance to low-stress abrasion by sand, soil, gravel and coal, and the alloys are among the cheapest hardfacing for abrasion. They are brittle, so they relieve their shrinkage stresses by forming fine cracks across the bead, check cracks, which are normal and usually harmless in abrasion service (Chapter 6), and they are limited to two or three layers and moderate impact.

Complex carbide alloys

Complex carbide alloys add elements such as niobium, vanadium, molybdenum and tungsten to form carbides even harder and finer than chromium carbides, giving better resistance to high-stress abrasion and, in some grades, keeping their hardness at elevated temperatures. They cost more than plain chromium carbide alloys and are used where those wear too fast, such as in sinter plants, high-stress grinding and severe mineral abrasion.

Tungsten carbide composites

Tungsten carbide deposits contain particles of very hard tungsten carbide held in a tougher matrix of iron, nickel or another alloy. They give the highest abrasion resistance of the common hardfacing families, used for drill bits, earthmoving tool edges, augers and mixer blades in severe abrasion, but the tungsten carbide is expensive, it dissolves if overheated during welding, which is why low-dilution, low-heat processes suit it (Chapter 3), and it is brittle under heavy impact.

Cobalt- and nickel-based alloys

Cobalt-based alloys, containing chromium, tungsten and carbon, keep their hardness at high temperatures and resist corrosion, erosion and galling; they are used on valve seats, hot-working tools and parts in chemical and power plants. Nickel-based alloys with chromium, boron and silicon resist corrosion and moderate wear and serve as the matrix for many tungsten carbide deposits. Both families are expensive and are used where their particular properties are needed, often in thin, low-dilution layers.

Other alloys

Bronzes and other copper alloys are used for bearing surfaces and metal-to-metal sliding wear where a dissimilar, low-friction surface is wanted. Stainless steels serve as buffer layers and for corrosion with mild wear. Tool steel compositions are used on cutting and forming tools.

Figure 1The main hardfacing alloy families

	Best against	Toughness	Relative cost
Martensitic steels	build-up; moderate abrasion; metal-to-metal	good	low
Austenitic manganese	heavy impact; work-hardens in service	very high	low to moderate
Chromium carbide irons	low-stress abrasion by sand, soil, rock	low; check-cracks	low
Complex carbides	high-stress abrasion; some at temperature	low to moderate	moderate
Tungsten carbide composites	severe abrasion	low under impact	high
Cobalt-based	heat, corrosion, galling, erosion	moderate	very high
Nickel-based	corrosion with wear; matrix for tungsten carbide	moderate	high

Choosing with a weighted comparison

The workshop chooses deposits with a simple weighted comparison. For each part, it lists the properties that matter, gives each a weight according to its importance for that part, scores each candidate alloy family on each property from experience and filler makers’ data, and adds up the weighted scores. The method does not replace judgement or trials, but it makes the reasoning visible and stops one property, usually hardness, crowding out the others.

Figure 2Weighted score for a hardfacing choice on the quarry's bucket edges

score = Σ (weight × rating)

Weights

abrasion 0.5, impact 0.3, cost 0.2

Chromium carbide

abrasion 4, impact 2, cost 5

Manganese steel

abrasion 2, impact 5, cost 4

Tungsten carbide composite

abrasion 5, impact 2, cost 1

1. chromium carbide: 0.5 × 4 + 0.3 × 2 + 0.2 × 5 = 3.6
2. manganese steel: 0.5 × 2 + 0.3 × 5 + 0.2 × 4 = 3.3
3. tungsten carbide: 0.5 × 5 + 0.3 × 2 + 0.2 × 1 = 3.3

On these weights, chromium carbide comes out ahead for the bucket edges, which is what the workshop chose. Had the quarry’s rock been much larger, with more impact, the impact weight would have risen and manganese steel might have won; had the material been extremely abrasive and the downtime cost very high, tungsten carbide might have justified its price. The comparison makes those trade-offs explicit.

The base metal decides too

The alloy must also suit the base it is laid on. Chromium carbide deposits suit carbon and low-alloy steels but can crack badly on manganese steel without a buffer; manganese deposits suit manganese steel castings but can form brittle zones on carbon steel if diluted; tungsten carbide composites need a base and process that will not dissolve their carbides. Filler makers’ data state which bases each product suits and whether a buffer layer is needed. A deposit chosen for the wear but unsuited to the base will fail at the fusion line.

Combining alloys on one part

Many parts suffer different wear in different places, and the best treatment uses different alloys where each is needed. A crusher hammer’s body, which takes the impact, is built up with manganese steel, while the narrow wearing faces that rock slides across may get a thin pattern of a carbide alloy. A loader tooth’s tip, which digs into rock, may get a tougher deposit than its sides, which are scoured by gravel. An auger flight’s leading edge, which meets stones, may get a tougher deposit than its face, which meets only grain. Combining alloys needs care where they meet, because some combinations form brittle mixtures, and the filler makers’ compatibility guidance applies, but the result can outlast any single alloy on the whole part.

Price per kilogram is not cost

Alloy families differ widely in price per kilogram, and the cheapest filler is rarely the cheapest result. The cost that matters is the cost per tonne of material handled or per hour of service, which includes the filler, the labour to apply it, the downtime to change parts and the parts themselves. An expensive alloy that triples a part’s life can be far cheaper overall than a cheap one that adds a third; a cheap alloy that does the job well is better than an expensive one whose extra resistance outlasts the part beneath it. Chapter 10 builds the cost per tonne for the quarry’s hammers and edges.

Consumable forms and processes

Each alloy family comes in forms for different processes: covered electrodes for manual metal arc, solid and tubular wires for gas-shielded and open-arc welding, strip and wire with flux for submerged arc, powders for plasma transferred arc, laser and thermal spray, and rods for oxy-fuel surfacing. The process changes dilution, deposition rate and heat input (Chapters 3 and 7), so the choice of alloy and process go together. Open-arc, self-shielded tubular wires, which need no gas, are widely used for hardfacing in the field, where wind would blow shielding gas away.

Trials before commitment

A weighted comparison narrows the choice; a trial settles it. The workshop runs field trials whenever a choice is close or a part is expensive: a few parts surfaced with each candidate, fitted together in the same machine so that they see the same conditions, and measured at fixed intervals. On a crusher, that might be alternate hammers on the same rotor; on a loader, the two halves of a bucket edge surfaced differently; on a farm, alternate tillage points across one implement. A trial over a few weeks or a season gives a wear rate for each deposit under the customer’s real conditions, which no catalogue can.

Keeping the range small

A workshop that stocks every alloy its suppliers offer ends up with shelves of partly used consumables, some of which absorb moisture and spoil, and welders who are unsure which to use. The workshop keeps a short list: a martensitic build-up wire, a manganese steel electrode and wire, a chromium carbide open-arc wire and electrode, a buffer-layer stainless electrode, and tungsten carbide rods for occasional severe cases, each with a written procedure. New products are added only after a trial shows they do something the list cannot. The short list makes procedures repeatable and stock manageable.

Reading the data sheet

Each filler’s data sheet gives the information needed to use it: the alloy family and nominal composition, the deposit’s hardness and the conditions under which it was measured, the recommended number of layers, the bases it suits and whether a buffer is needed, the preheat and interpass temperatures, the welding parameters, the forms and sizes, and the safety data. The workshop reads every data sheet before first use and copies the key items onto its procedure card. A filler used outside its data sheet’s conditions, too many layers, the wrong base, no preheat, is the commonest cause of hardfacing that fails early.

Hazards in the alloys

Many hardfacing alloys contain elements whose fume is hazardous: chromium, particularly in forms produced by welding high-chromium alloys; manganese, in large amounts in manganese steel deposits; nickel and cobalt. Their safety data sheets set out the hazards and controls, which typically include local exhaust ventilation and, often, respiratory protection, because hardfacing produces large amounts of fume over long welding times. Setting Up a Welding Shop by the Numbers sizes fume extraction in its chapter on fume extraction and ventilation.

A failure: the hardest was not the best

This is an illustration. A grain terminal had chosen a tungsten carbide composite for its chute liners because it was the most abrasion-resistant deposit its supplier offered. The liners resisted the grain’s abrasion superbly, but the cost was several times that of a chromium carbide deposit, and the liners were being replaced not for wear but because the plates beneath them corroded and cracked in the terminal’s damp conditions long before the deposit wore out.

The workshop’s comparison for the next set weighted corrosion and cost as well as abrasion. A chromium carbide deposit, with its higher chromium also resisting the damp, on a stainless backing plate, lasted as long as the plates, at a fraction of the cost. The hardest deposit had outlived everything else, which meant it had been too expensive.

Exercise: re-weighting for impact

For the quarry’s crusher hammers, the workshop weights impact 0.6, abrasion 0.3 and cost 0.1, with the same ratings as in the bucket edge example. Which family comes out ahead?

Chromium carbide scores 0.3 × 4 + 0.6 × 2 + 0.1 × 5 = 1.2 + 1.2 + 0.5 = 2.9. Manganese steel scores 0.3 × 2 + 0.6 × 5 + 0.1 × 4 = 0.6 + 3.0 + 0.4 = 4.0. Tungsten carbide scores 0.3 × 5 + 0.6 × 2 + 0.1 × 1 = 1.5 + 1.2 + 0.1 = 2.8.

Manganese steel comes out clearly ahead for the hammers, which suffer heavy impact, matching both the railway’s choice of 1920 for dipper teeth and modern practice for manganese castings. In practice, the workshop builds up hammers with manganese steel and may add a thin pattern of a carbide deposit on the faces that see most abrasion, a combination Chapter 5 discusses.