Patterns, coverage and thickness

Hardfacing need not cover a whole surface. Beads laid in a pattern, with bare base metal between them, can protect a part almost as well as full coverage in many materials, at a fraction of the deposit, heat and cost. In some materials, a pattern protects better, because the spaces fill with packed fines that themselves resist the abrasive. Choosing the pattern, the bead spacing, the bead direction and the thickness, and deciding exactly where to put the deposit, are as important as choosing the alloy.

This chapter describes the common hardfacing patterns and when each suits, calculates the deposit a pattern saves compared with full coverage, sizes the deposit’s thickness from the wear rate and the service interval, and sets out how to protect edges and ends.

Patterns and the material

The best pattern depends on the material flowing over the part and how it moves.

Figure 1Common hardfacing patterns

	How laid	Suits	Why
Stringer beads across the flow	parallel beads at right angles to the material's movement	sand, fine gravel, soil with fines	fines pack between beads and protect the base
Stringer beads along the flow	parallel beads in the direction of movement	coarse rock, large lumps	rock rides on the beads; sticky material slides off
Crosshatch or waffle	beads in two directions forming a grid	mixed materials; bucket bottoms and sides	traps fines in each cell; protects in all directions
Dot pattern	spots of deposit at intervals	clay, sticky soils; thin parts; impact areas	little heat and distortion; clay sticks in the gaps
Full coverage	overlapping beads over the whole area	very fine, highly abrasive material; slurries	no exposed base for fines to scour

The logic is mostly about what fills the spaces between the beads. In material with plenty of fines, such as sand or crushed rock with dust, beads laid across the flow trap the fines in the valleys between them, where they pack hard and form a protective layer of the material itself, so the base between the beads is protected by the fines and the beads take the wear. In coarse rock without fines, there is nothing to pack, and beads along the flow let the rock ride on the deposit. In sticky clay, a dot pattern lets the clay stick in the gaps and protect the base, while the dots resist the scouring. In fine slurries, the abrasive can scour any exposed base, so full coverage is usually needed.

Spacing

The spacing between beads depends on the size of the material. Beads spaced so that the typical particle cannot reach the base between them, or so that the fines can bridge and pack the gap, protect the base; beads spaced too widely let the material scour the base between them, undercutting the beads. A common rule of practice is to space beads more closely for fine materials and more widely for coarse ones, and to confirm the spacing by trial. The workshop starts from the filler maker’s pattern guidance for the material and adjusts the spacing after inspecting the first parts in service.

How much a pattern saves

A pattern uses deposit in proportion to the fraction of the area it covers.

Figure 2Deposit saved by a stringer pattern compared with full coverage

coverage fraction = bead width ÷ bead pitch

Bead width and pitch

12 mm at 40 mm

Area

1.5 m²

Deposit thickness

4 mm

1. coverage fraction = 12 ÷ 40 = 30%
2. full coverage: 1.5 × 0.004 × 7,850 ≈ 47 kg; pattern: about 30% of that, about 14 kg
3. the pattern saves about 33 kg of deposit, and the time and heat to lay it

Saving about 33 kg of deposit on one bucket bottom saves roughly a day of welding by an open-arc wire process and much of the heat that would distort the bucket. If the pattern protects nearly as well as full coverage in the quarry’s material, as it did in the workshop’s trial, the saving is almost pure gain.

Thickness from the service interval

How thick should the deposit be? Thick enough to last until the part can next be inspected and renewed, and no thicker than the alloy allows. The wear rate measured in service, in millimetres per thousand tonnes or per hundred hours, and the planned interval between services give the thickness needed.

Figure 3Deposit thickness needed to last a service interval at different wear rates

0.2 mm per 1,000 t4 mm of hardfacing, for an interval of 20,000 tonnes

0.3 mm per 1,000 t6 mm of hardfacing, for an interval of 20,000 tonnes

0.4 mm per 1,000 t8 mm of hardfacing, for an interval of 20,000 tonnes

0.5 mm per 1,000 t10 mm of hardfacing, for an interval of 20,000 tonnes

If the alloy is limited to two layers totalling, say, 7 mm, and the wear rate is 0.4 mm per thousand tonnes, a 20,000 tonne interval needs about 8 mm, more than the alloy allows. The workshop then either shortens the interval, renewing the deposit at 15,000 tonnes, chooses a more resistant alloy with a lower wear rate, or adds sacrificial wear bars or buttons that can be replaced. The calculation turns the service plan and the deposit into one decision. It also shows the value of measuring wear rates honestly: a rate guessed too low leads to a deposit that wears through before the service, exposing the base, while a rate guessed too high leads to thick deposits that crack, waste alloy, and may outlast the part they protect.

Where wear concentrates

Wear is rarely even across a part. Bucket edges wear fastest at the corners and the leading edge; crusher hammers at their striking faces and tips; chutes where material lands and where it turns; augers at the outer edge of the flights; pump impellers at the vane tips. Hardfacing placed where wear concentrates, and lighter treatment or none elsewhere, gives the most life for the least deposit. The workshop maps wear on each kind of part from the worn parts that come in, marking the areas that wear fastest, and places its deposits accordingly.

Edges and ends

The edges of a hardfacing deposit, and the edges of the part itself, need particular attention. Base metal just beyond the end of a deposit wears faster than the deposit and is undercut, leaving the deposit’s edge unsupported, so that it breaks away; the workshop carries deposits slightly beyond the area of heavy wear and tapers their ends. Part edges that meet the material head-on, such as a bucket’s cutting edge or a chute’s leading lip, get the deposit wrapped round the edge, so the material strikes deposit rather than bare steel. Corners, where wear comes from two directions at once, get extra deposit or a heavier pattern.

Wear buttons and bars

Where hardfacing cannot be laid thick enough, or the part needs protection that can be renewed quickly, wear buttons and wear bars are an alternative: small pieces of very hard material, such as chromium white iron or tungsten carbide inserts in a steel holder, welded onto the part at intervals. They are welded by their steel holders or backing, so the hard material itself is not welded, and they can be replaced individually as they wear. They work much like a dot or stringer pattern, protecting the surface between them by trapping material. The workshop fits wear buttons on the bucket’s heel and wear bars on its bottom, and hardfaces the edges and corners.

Patterns and distortion

Patterns put much less heat into a part than full coverage, which reduces distortion, important on thin parts such as auger flights and bucket sides. Full coverage of a thin plate on one side can bow it like the flange tilt of Welding Distortion by the Numbers, while a pattern spread over the same area bows it much less. Where full coverage is needed on a thin part, the workshop lays it in a balanced sequence and may preset the part to allow for the bow.

Patterns on round parts

Round parts such as auger flights, rollers, shafts and drill pipes need patterns that suit their rotation and the way material moves over them. On an auger flight, the outer edge does most of the work and gets a continuous deposit wrapped round it, while the face may get a stringer pattern following the spiral or a dot pattern to limit distortion of the thin flight. On rollers and idlers, full coverage laid as a continuous helix by a rotating positioner gives an even surface that runs true. On drill pipes and stabilisers, bands of tungsten carbide deposit at intervals protect against abrasion by the hole wall while leaving most of the pipe free. The workshop sketches the pattern for each round part on its job card, because the obvious pattern on a flat drawing is often wrong once the part rotates.

Pattern and material build-up

Some materials stick and build up on surfaces, such as damp clay, wet coal and sticky grain, and the hardfacing pattern affects whether they do. Ridged patterns can help material release by reducing the contact area, or can hold it in their valleys, depending on the material. A build-up of sticky material can protect a surface from wear, but it also reduces a bucket’s capacity or blocks a chute. The workshop asks customers about build-up as well as wear, and chooses smoother deposits or patterns along the flow where release matters more than wear.

Combining patterns on one part

A single part often needs different patterns in different places. The quarry’s bucket gets continuous deposit on its cutting edge and corners, a crosshatch on its bottom, stringer beads along its sides where material slides out, and wear buttons on its heel, where it drags on the ground. The combination puts the heaviest protection where wear is worst and lighter, cheaper patterns elsewhere. The workshop keeps a marked-up photograph of each treated part type, showing the pattern in each area, so that the next set is treated the same way.

Renewing patterns

Patterns are easy to renew. When the beads of a stringer or crosshatch pattern have worn down but the base between them is still protected, a fresh set of beads laid between or over the old ones restores the protection quickly, with little preparation, provided the layer limits are respected. The workshop schedules pattern renewal at each service interval, measuring the beads’ remaining height and renewing before they wear flush with the base, which is cheaper than rebuilding a base that has started to wear.

The running case: bucket bottoms and edges

This is an illustration. The workshop trialled three treatments on the quarry’s bucket bottoms over a season: full coverage, a stringer pattern across the flow at 40 mm pitch, and a crosshatch at 50 mm. In the quarry’s crushed limestone with plenty of fines, the stringer pattern and the crosshatch both protected the bottoms nearly as well as full coverage, with the base between beads protected by packed fines, at about a third of the deposit and welding time. The workshop adopted the crosshatch for bucket bottoms, with wear bars added, and wrapped the leading edges and corners in a continuous deposit.

Measuring the pattern in service

A pattern’s success is judged by inspection in service. At each service, the workshop measures the height of the remaining beads at fixed points, looks for undercutting of the base between and beside the beads, and checks whether fines are packing between the beads as intended. Beads wearing evenly with the base still protected show a good pattern; beads undercut and breaking away show beads too far apart or running the wrong way for the material. The measurements feed the wear rate used to size the next deposit and the service interval, so that the pattern and the plan improve together.

Thin parts and heat

Thin parts, such as auger flights, light chutes and tillage discs, can be overheated, distorted or even burned through by hardfacing. The workshop uses smaller beads, lower currents, dot or stringer patterns rather than full coverage, and skip sequences that let each area cool, and supports thin parts against a heavier backing plate where distortion would matter. Where a thin part needs heavy protection, a bolted wear plate or replaceable wear strip is often better than hardfacing the part itself.

A failure: beads along the flow in fines

This is an illustration. A sand plant’s chute had been hardfaced with stringer beads running along the flow, widely spaced, by a welder used to rock chutes. The fine, fast-moving sand ran down the channels between the beads, scouring the bare steel there and undercutting the beads, which then broke away. The chute wore through between the beads within weeks.

In fine sand with no coarse material, the beads along the flow simply channelled the abrasive onto the base. Rebuilt with full coverage, or with close-spaced beads across the flow so that the sand packed between them, the chute lasted many times longer. The material decided the pattern, not the welder’s habit, and the plant now specifies the pattern on its work orders.

Exercise: pattern or full coverage

A chute area of 2 m² can be hardfaced either with full coverage 5 mm thick or with beads 10 mm wide at 25 mm pitch, also 5 mm high. Calculate the deposit for each. If both protect equally in the material, what is the saving in weld metal and, at an open-arc deposition of 5 kg/h and an operator factor of 35%, in paid hours?

Full coverage uses about 2 × 0.005 × 7,850 ≈ 78.5 kg. The pattern covers 10 ÷ 25 = 40% of the area, about 0.4 × 78.5 ≈ 31.4 kg. The saving is about 47 kg of weld metal.

At 5 kg/h and 35% operator factor, each kilogram takes about 1 ÷ (5 × 0.35) ≈ 0.57 paid hours, so the saving is about 47 × 0.57 ≈ 27 paid hours, more than three days of welding, provided the pattern really protects as well in the chute’s material, which a trial should confirm.