Hardfacing and Surfacing by the Numbers
Chapter 2 of 10 · 10 min read
Building up and hardfacing
Separate building up a worn part with a tough deposit from hardfacing it against wear, lay padding beads that merge, calculate the metal and time a build-up needs, and know when to replace instead.
A worn part usually needs two things: metal put back where it has worn away, and a surface that will resist the wear better next time. They are different jobs and often need different deposits, and confusing them is behind many failed repairs. Building up restores a part’s shape and size, usually with a deposit similar to the base metal, tough, crack-resistant and often machinable. Hardfacing applies a thinner layer of a wear-resistant alloy to the working surface. A badly worn part is usually built up first, then hardfaced, because most hardfacing alloys cannot be laid in thick layers without cracking or spalling.
This chapter explains the difference between the two, describes how padding beads are laid so that they merge into a sound deposit, calculates the weld metal and time needed to rebuild a loader bucket edge, and sets out when replacing a part is better than rebuilding it.
The railway shops’ build-up
Railway workshops in the early twentieth century rebuilt worn parts on a large scale, and their practice shows the logic. Cravens’s 1921 treatise listed driving wheels built up on their treads instead of being turned down, and axle journals built up to their original size. Turning a worn wheel’s tread back to its correct profile on a lathe removes metal all round its circumference, shortening its life; building up the worn places restores the profile with little metal removed. Wanamaker and Pennington described building up wheel flanges, journal boxes and shafts with the metallic arc, laying the build-up layers parallel to a shaft’s axis. And Hornor quoted a railway’s annual comparison in which filling worn spots by arc welding cost about $330 against about $2,680 by the older methods, a saving of nearly 90%, among savings across its whole list of repairs that made arc welding standard practice.
Preparing the worn surface
A build-up is only as sound as the surface beneath it. Worn parts arrive dirty, oily, rusty and often with fatigue cracks, old deposits and work-hardened layers. Before welding, the workshop cleans the worn area back to bright metal by grinding or gouging, removes any old hardfacing that is cracked, spalled or of an unknown alloy, and checks the area for cracks with dye penetrant or magnetic particle inspection, grinding out any it finds. On manganese steel, the work-hardened surface layer is ground off, because it is brittle and cracks under welding (Chapter 5). Skipping this preparation is the commonest cause of build-ups and hardfacing that crack or flake off early.
Restoring shape with templates
Many wearing parts have shapes that matter: a tillage point’s profile decides how it penetrates soil, a crusher hammer’s face decides how it strikes rock, a pump impeller’s vanes decide its flow. The workshop keeps profile templates, cut from sheet steel to the shape of new parts, for the parts it rebuilds often, and checks the build-up against them as it proceeds, so that the rebuilt part works like the original. A build-up that restores the metal but not the shape may wear unevenly, perform poorly or unbalance the machine.
Balance and mass
Rotating parts such as crusher hammers and fan blades must be balanced. A rebuilt hammer that is heavier or lighter than its partners on the rotor unbalances the crusher, causing vibration that damages bearings and the frame. The workshop weighs crusher hammers before and after rebuilding, rebuilds them in matched sets, and adjusts the deposit to bring each set within the crusher maker’s mass tolerance. Weighing also measures the wear: the difference between a new hammer’s mass and a worn one is the metal lost, and over a season it gives the wear rate per tonne crushed.
Building up
A build-up deposit’s job is to restore the part’s dimensions with metal that is sound, tough and compatible with the base. For carbon and low-alloy steel parts, the build-up is usually a low-hydrogen deposit of strength similar to or a little below the base metal, which can be laid in many layers without cracking and machined if needed. For manganese steel parts, it is a manganese or stainless steel deposit compatible with manganese steel (Chapter 5). For cast iron, it is a nickel-based deposit. The build-up must be thick enough to restore the shape, leaving room for the hardfacing layers on top, and its surface must be sound, because a hardfacing laid over porosity or cracks in the build-up will fail with it. The workshop inspects every build-up visually before hardfacing it.
Hardfacing
A hardfacing deposit’s job is to resist wear, and its properties are chosen for the wear the part suffers (Chapter 1). Most hardfacing alloys are hard and, to a greater or lesser degree, brittle, so they are applied in one to three layers, as the filler maker specifies, over a build-up or the sound base metal. Thick multi-layer hardfacing of the harder alloys tends to crack through and spall, especially under impact, because each layer adds shrinkage stress and the deposit cannot stretch to relieve it. The common rule, set out in filler makers’ data, is to build up to within a few millimetres of the final surface with a tough alloy and finish with the specified number of hardfacing layers.
| Purpose | Typical deposit | Layers | |
|---|---|---|---|
| Building up | restore shape and size | tough, crack-resistant, compatible with the base | as many as needed |
| Buffer layer | separate base from hardfacing | tough, ductile; often stainless or manganese steel | one or two |
| Hardfacing | resist wear | hard alloy chosen for the wear | usually one to three |
A buffer layer, a tough intermediate deposit between the base and the hardfacing, is used where the base and the hardfacing are not compatible, where the base is crack-sensitive, or where the hardfacing must be protected from cracks in the base. Chapter 8 discusses buffer layers with preheat.
Padding: beads that merge
Hornor’s 1920 manual taught arc welders to build up a surface by padding: laying a succession of beads side by side. Hornor stressed that each bead must be laid with the arc melting into the edge of the previous bead and the plate beneath it together, so that the beads merge into a continuous deposit. If the arc is held on the parent metal only, each bead fuses to the plate but not to its neighbour, and when the build-up is machined, the gaps between beads show as slag spots and voids. The manual also noted that the beads could be run lengthwise or crosswise of the work.
The rule is the same today. Each bead overlaps the previous one, typically by about a third to a half of its width, so that the surface is continuous and the beads are fused to each other. The overlap also sets how many beads cover a given width: beads 12 mm wide laid at a pitch of 8 mm cover 120 mm in 15 beads. Too little overlap leaves valleys and lack of fusion between beads; too much piles up the deposit and wastes metal.
Rebuilding a bucket edge
This is an illustration. One of the quarry’s loader bucket edges has worn thin along a 2.4 m length: across a wear zone about 120 mm wide, the edge has lost an average of about 8 mm of thickness. Rebuilding it means replacing that metal.
- Worn zone
- 120 mm wide × 8 mm deep × 2,400 mm long
- Density of steel
- about 7,850 kg/m³
- volume = 120 × 8 × 2,400 ≈ 2.3 million mm³
- mass ≈ 2.3 × 10⁶ × 7.85 × 10⁻⁶ ≈ 18 kg of weld metal
- arc time ≈ 18 ÷ 5 ≈ 3.6 hours; paid time ≈ 3.6 ÷ 0.35 ≈ 10 hours
Eighteen kilograms of weld metal and about ten hours of welder’s time is a substantial job, and on top of it come preheating, the hardfacing layers and inspection. At the workshop’s rates, the rebuild costs about as much as a new bolt-on edge. The calculation shows why heavily worn edges are often better replaced than rebuilt, and why the cheapest time to protect an edge is before it wears, with a thin hardfacing pattern on the new edge. It also shows how quickly weld metal adds up over a large area: a few millimetres of wear across a wide surface is many kilograms of deposit, and the time to lay it is measured in hours, not minutes. Estimating the volume before quoting, as here, prevents the most common underquote in rebuilding work.
When to replace instead
Rebuilding makes sense when the part is expensive or slow to obtain, when the worn metal is modest compared with the part, when the base metal is sound and weldable, and when the rebuilt part will last as long as a new one or longer. Replacement makes sense when the wear is so heavy that rebuilding costs as much as a new part, when the base metal is cracked or work-hardened beyond repair, when the part is cheap and readily available, or when it is designed to be replaced, such as bolt-on edges and teeth. The decision of Machine Part Repairs by the Numbers, in its chapter on repair or replace, applies, with downtime, risk and cost weighed together.
Build-up limits
Even tough build-up alloys have limits. Each layer adds heat and shrinkage, and a very thick build-up on a thin part can distort it, crack it or change its properties. Some parts lose strength when heated, such as heat-treated steels, and limit how much build-up they can take. Manganese steel castings become brittle if overheated (Chapter 5). The filler maker and the part maker give the limits; where a part has worn beyond them, it is replaced.
Machining the build-up
Some rebuilt parts must be machined to size, such as shafts, bearing seats and precision surfaces, and others are used as welded, such as bucket edges and tillage points. Machined build-ups need a deposit that can be machined, which rules out the harder hardfacing alloys, and enough allowance to clean up to sound metal, as Machine Part Repairs by the Numbers explains for shaft journals. As-welded surfaces need a smooth enough finish not to interfere with the material flowing over them, and in some parts, such as chutes, the bead direction matters (Chapter 9).
The running case: bucket edges
This is an illustration. The workshop compared rebuilding the quarry’s worn bucket edges with replacing them. Rebuilding the badly worn edges cost about as much as new ones and took longer, so the quarry changed to replacing edges once worn below a set thickness, and the workshop hardfaced each new edge before fitting with a thin pattern of a chromium carbide deposit along its wearing face (Chapters 6 and 9). The new edges lasted about twice as long as plain ones, and no edges were rebuilt. The workshop’s time went into preparing and hardfacing new edges in batches, a quicker and more predictable job than rebuilding worn ones, and the quarry kept two hardfaced spare edges in stock so that a change could be made at the next shutdown.
A failure: hardfacing as build-up
This is an illustration. A welder elsewhere had rebuilt a badly worn crusher hammer entirely with a hard chromium carbide alloy, laying six or seven layers to restore its shape, because that was the alloy on hand. The hammer went back into the crusher, and within a shift large pieces of the deposit had spalled off, the remaining deposit was criss-crossed with cracks, and the hammer was scrapped.
The hard alloy could not be laid in so many layers, and the crusher’s impact finished what the shrinkage stresses had started. The workshop’s procedure for hammers now builds up with a manganese steel deposit compatible with the casting, then finishes with the hardfacing layers the filler maker specifies.
Exercise: beads and layers
A chute liner area 300 mm wide has worn about 12 mm below its original surface. It is to be built up and then finished with two hardfacing layers, each about 3 mm thick. The build-up beads are 14 mm wide, deposit about 3 mm thick per layer and are laid at a pitch of 9 mm. How many beads per layer and how many build-up layers are needed to restore the original surface?
The two hardfacing layers will add about 2 × 3 = 6 mm, so the build-up must restore the other 12 − 6 = 6 mm. At about 3 mm per layer, that is 2 build-up layers. Each layer needs about 300 ÷ 9 ≈ 34 beads across the width, about 68 build-up beads in all, followed by about the same number for each hardfacing layer.
Planning the build-up and hardfacing thicknesses together, before welding, avoids finishing proud of the original surface, which would interfere with the flow of material down the chute, or short of it, which would leave a step where wear concentrates.
