Hardfacing and Surfacing by the Numbers
Chapter 7 of 10 · 10 min read
Processes for surfacing
Compare manual metal arc, flux-cored, submerged arc, oxy-fuel, plasma, laser and thermal spray for surfacing by deposition, dilution, portability and cost, and time the covering of a large area.
The same hardfacing alloy can be laid by many processes, and the process changes the deposit’s dilution, its quality, the heat put into the part, the time the job takes and what it costs. A covered electrode and a stinger can hardface a bucket tooth in a paddock; a submerged arc head on a manipulator can cover a mill roll in a fraction of the time; a laser can lay a thin, almost undiluted layer of tungsten carbide on a valuable tool. Choosing the process is as important as choosing the alloy, and the two are usually chosen together, and the right choice depends on the part, the area, the alloy, the location and the number of parts.
This chapter compares the main surfacing processes, calculates how long each would take to cover a large area, and sets out how the workshop chooses among them.
Manual metal arc
Covered electrodes are the most versatile surfacing process: simple, portable equipment works in the workshop or the field, in any position, on almost any part, with a wide choice of hardfacing electrodes. Their drawbacks are a low deposition rate, typically a kilogram or two an hour of arc time, frequent electrode changes, slag removal between layers, and moderate to high dilution depending on technique. They suit small parts, repairs, awkward positions and field work, and almost every repair welder already has the equipment.
Flux-cored and open-arc wires
Tubular wires, fed continuously by a wire feeder, deposit hardfacing alloys several times faster than electrodes, with fewer stops. Gas-shielded wires give clean deposits in the workshop; self-shielded, open-arc wires need no gas and work outdoors in wind, which makes them the commonest choice for hardfacing earthmoving and mining equipment in the field. Dilution is moderate and controllable with parameters. They suit medium to large areas and repeated work.
Submerged arc
Submerged arc welding, with the arc buried under a granular flux, deposits metal at high rates, often many kilograms an hour, with excellent quality and little fume or arc light. It needs mechanised travel, usually with the part rotating under a fixed head or a head moving over the part on a manipulator, and works only in the flat position. Its dilution can be high at high currents; strip cladding, using a wide strip electrode instead of a wire, spreads the heat and gives lower dilution over large areas. It suits rolls, rollers, idlers, large flat plates and repeated parts of any size.
Oxy-fuel surfacing
Oxy-fuel surfacing, the method of the early manuals, melts the filler rod onto a surface that is heated until it just sweats, with very low dilution (Chapter 3). It is slow, it needs a skilled hand, and the long heating can distort thin parts, but it suits small, precise deposits and expensive alloys such as tungsten carbide rods, where low dilution keeps the carbides intact.
Plasma transferred arc
Plasma transferred arc surfacing uses a constricted plasma arc to melt a metal powder fed into it onto the surface, with precise control of heat, low dilution and smooth, even deposits. It is mechanised, and the equipment is costly, but it suits valuable parts and production surfacing of items such as valve seats, tools and mining picks, and it handles alloys and carbide-rich powders that arc processes would dilute or degrade.
Laser cladding
Laser cladding melts a powder or wire onto the surface with a laser beam, giving very low dilution, a narrow heat-affected zone and little distortion. It is the most precise and most expensive of the fusion processes, used for high-value parts, thin layers of costly alloys, and parts that must not be heated much.
Thermal spray
Thermal spray processes, including flame spray, arc spray and high-velocity oxy-fuel spraying, propel molten or softened particles onto a prepared surface, building up a coating without melting the base. The bond is mainly mechanical, so there is no dilution and little heat into the part, but the coating is thinner and generally less able to take impact than a fused deposit. Thermal spray suits worn shafts and bearing seats, corrosion protection and thin wear-resistant coatings, and some sprayed coatings are fused afterwards to improve their bond.
| Deposition rate | Dilution | Where it suits | |
|---|---|---|---|
| Manual metal arc | low | moderate to high | small parts, repairs, field, any position |
| Open-arc and flux-cored wire | moderate to high | moderate | medium to large areas; field and workshop |
| Submerged arc | high | moderate to high; lower with strip | rolls, plates, repeated parts, flat position |
| Oxy-fuel | low | very low | small deposits; tungsten carbide rods |
| Plasma transferred arc | moderate | low | valuable parts; production; carbide powders |
| Laser cladding | low to moderate | very low | high-value parts; thin layers; low distortion |
| Thermal spray | moderate | none | coatings; shafts; low heat input |
Time to cover an area
The deposition rate, together with the operator factor, sets how long a large surfacing job takes, and the differences between processes are large.
- Deposit
- 2 m² × 5 mm, about 78.5 kg
- Manual metal arc
- about 1.5 kg/h, operator factor 30%
- Open-arc wire
- about 5 kg/h, operator factor 35%
- Submerged arc
- about 12 kg/h, operator factor 50%
- manual metal arc: 78.5 ÷ 1.5 ÷ 0.30 ≈ 174 paid hours
- open-arc wire: 78.5 ÷ 5 ÷ 0.35 ≈ 45 paid hours
- submerged arc: 78.5 ÷ 12 ÷ 0.50 ≈ 13 paid hours
At the workshop’s illustrative rate of A$130 an hour, the labour alone would be about A$22,700 by manual metal arc, about A$5,800 by open-arc wire and about A$1,700 by submerged arc, before the alloy, which is the same weight in each case. For large areas, the faster processes, or ready-made overlay plate (Chapter 6), are far cheaper; for small parts, the set-up time of mechanised processes outweighs their speed. The comparison also shows why quotes for large surfacing jobs vary so widely between workshops: a workshop quoting manual metal arc for a job another would do by submerged arc is not overcharging, it is simply using a slower process, and the customer is entitled to ask which process each quote assumes.
Heat input and the part
The process also decides how much heat goes into the part, which matters for parts that distort, embrittle or soften. Manganese steel parts need low heat input and cool working (Chapter 5); hardened parts such as boron steel tillage points lose hardness if heated too much; thin parts distort. High-deposition processes put more heat in per unit time, which can be an advantage on thick parts that need preheat anyway and a disadvantage on thin or heat-sensitive ones. Laser and plasma processes, with their concentrated, controlled heat, suit heat-sensitive parts best.
Positions and part handling
Surfacing goes fastest and best in the flat position, where the pool is supported and high currents can be used. Turning parts so that their wearing faces are horizontal, with positioners, turntables and simple fixtures, raises deposition rates and lowers defects. Large field equipment cannot be turned, so field surfacing is often done in vertical and overhead positions, at lower currents and rates, with electrodes and wires designed for those positions. When the workshop estimates a surfacing job, it notes the positions as well as the area, because a square metre of vertical surfacing takes far longer than a square metre in the flat position.
Consumable cost and recovery
The alloy is the same weight whatever the process deposits it, but the amount bought differs, because processes waste different amounts. Covered electrodes lose their coatings as slag and leave stubs; flux-cored wires lose their flux; spatter and grinding waste some of everything. The ratio of metal deposited to consumable bought, the deposition efficiency, varies widely between processes and products, and the filler makers’ data give it. For an expensive alloy, a process with a higher deposition efficiency can save a significant share of the consumable cost, which is another reason powder processes such as plasma transferred arc suit costly carbides.
Preparation for each process
Each process has its own preparation needs. Arc processes need the surface clean of rust, oil and old loose deposit for sound fusion; submerged arc needs flat, accessible surfaces and careful earthing through rotating parts; plasma and laser processes need clean surfaces and accurate set-up of the head’s distance and path; thermal spray needs the surface roughened, usually by grit blasting, so the coating can key to it, and kept clean until spraying. Poor preparation spoils any process, but the precise processes are the least forgiving.
Fume and light by process
The processes differ in the fume and arc light they produce, which affects the controls needed. Open-arc and flux-cored wires used for hardfacing produce large amounts of fume, especially at high currents with high-alloy wires; manual metal arc produces moderate amounts per kilogram but over long times; submerged arc produces very little fume and no visible arc, because the arc is buried in the flux; plasma, laser and thermal spray need enclosures or local extraction for their own fume and, for lasers, protection from the beam. The process choice therefore affects the workshop’s safety arrangements as well as its productivity.
Field and workshop
Much hardfacing is done in the field, on equipment that cannot easily be brought to a workshop: excavator buckets, crusher liners, dragline parts. Field work favours portable processes, manual metal arc and self-shielded open-arc wire, powered by engine-driven welders, and it adds the challenges of weather, access and safety. Workshop work allows mechanised processes, fixtures, positioners and controlled conditions, and gives more consistent results. The workshop does field hardfacing for the quarry’s large fixed equipment and brings everything movable back to the workshop.
Matching process to quantity
The number of parts changes the best process as much as their size. For a single worn part, the process the workshop already has set up, usually manual metal arc or flux-cored wire, is almost always the right choice, because any set-up of a mechanised process would cost more than it saves. For a batch of a few dozen identical parts, a simple fixture and a carriage begin to pay. For hundreds of parts a year, a dedicated set-up with a positioner, a mechanised head and a written procedure gives consistent deposits at the lowest cost per part. The workshop reviews its repeated jobs each year to see which have grown enough to justify mechanising.
Mechanisation
Mechanising surfacing, with a carriage, a manipulator or a rotating positioner, brings consistency as well as speed. A mechanised head lays beads of constant size, overlap and dilution, while manual surfacing varies with the welder’s skill and fatigue over long jobs. For repeated parts, such as idlers, rollers and tillage points in quantity, a simple fixture and a carriage pay for themselves quickly. Welding Automation by the Numbers covers mechanised welding more generally.
The running case: choosing processes
This is an illustration. The workshop’s processes, after a review, were: manual metal arc for small repairs and field work on awkward parts; self-shielded open-arc wire for bucket edges, teeth and field hardfacing of the quarry’s liners; gas-shielded flux-cored wire for crusher hammer build-up in the workshop, using the rotation of Chapter 5; a small submerged arc set-up with a rotating positioner for the terminal’s conveyor idlers; and oxy-fuel with tungsten carbide rods for the few auger parts that needed it. Overlay plate replaced in-place hardfacing for flat liners. Each process had a written procedure, a set of parameters and a person responsible for its equipment, so that the choice of process was part of the job plan rather than whatever machine was free that day.
A failure: the hand-surfaced roll
This is an illustration. A local mill had asked a welding contractor to rebuild a worn roller, about 600 mm in diameter and 1.5 m long, by hand with electrodes, because the contractor had no mechanised equipment. The job took nearly two weeks, the deposit was uneven, and the roller had to be ground heavily to make it round, removing much of the expensive hardfacing.
The workshop’s quote for the next roller used its submerged arc set-up with the roller rotating under the head, with an even, overlapping helical deposit that needed only light finishing. The work took two days. For large, regular parts, the mechanised process was faster, more consistent and cheaper, even after its set-up.
Exercise: tillage points by two processes
The workshop hardfaces 400 tillage points a year, each needing about 60 g of deposit. Compare the arc time by manual metal arc at 1.5 kg/h and by gas-shielded flux-cored wire at 4 kg/h. If set-up and handling add 3 minutes per point either way, what is the total time for each?
The total deposit is 400 × 0.06 = 24 kg. By manual metal arc, the arc time is 24 ÷ 1.5 = 16 hours; by flux-cored wire, 24 ÷ 4 = 6 hours. Handling adds 400 × 3 = 1,200 minutes, 20 hours, either way.
The totals are about 36 hours by manual metal arc and 26 hours by flux-cored wire, a saving of about 10 hours. Because handling is so large a share, a fixture that holds several points and speeds handling could save as much again, which suggests that, for small repeated parts, handling deserves as much attention as the process.
