Hardfacing and Surfacing by the Numbers
Chapter 6 of 10 · 10 min read
Carbide deposits, abrasion and check cracking
See how hard carbides in a softer matrix resist abrasion, why carbide deposits crack across their beads and when that is harmless, how thick to lay them, and when overlay wear plate is the better buy.
For abrasion by sand, soil, gravel, coal and ore, the workhorse of hardfacing is a carbide deposit: an alloy that, as it solidifies, forms hard carbide particles in a softer metal matrix. The carbides do the work of resisting the abrasive, and the matrix holds them in place. Carbide deposits are cheap, easily applied and outstandingly effective against low-stress abrasion. They also crack, in a regular pattern across every bead, which alarms people who have not seen it before and, in the wrong service, leads to spalling.
This chapter explains how carbide deposits resist abrasion, why they form check cracks and when those cracks matter, how thick they may be laid, and when buying ready-made chromium carbide overlay plate is better than hardfacing a part.
Carbides and matrix
In a chromium carbide deposit, the high chromium and carbon combine as the weld metal solidifies to form long, hard chromium carbide crystals, set in a matrix of iron alloy. The carbides are harder than quartz, the hardest common abrasive, so sand and rock particles cannot scratch them easily; instead, they wear away the softer matrix between the carbides until the carbides stand proud, and then the particles slide over the carbide tops. The more carbides there are, and the closer together, the less matrix is exposed and the slower the wear. In complex carbide and tungsten carbide deposits, the carbides are harder still.
Carbide size and the abrasive
The size of the carbides relative to the abrasive particles matters. When the abrasive particles are much larger than the spacing between carbides, they ride over the carbide tops and cannot reach the matrix, so wear is slow. When the particles are fine enough to fit between the carbides, they scour out the matrix around the carbides, undermining them until they fall out, so wear is faster. Fine abrasives, such as fine sand and coal dust, therefore favour deposits with fine, closely spaced carbides, such as many complex carbide alloys; coarser abrasives are well resisted by the larger carbides of ordinary chromium carbide deposits.
Check cracks
Chromium carbide deposits are brittle and cannot stretch to relieve the shrinkage stress as they cool. Instead, they crack, forming fine cracks across the bead at intervals, usually running from the bead’s surface down to, but not into, the base. These are check cracks or relief cracks, and in most carbide deposits they are expected and harmless: they relieve the stress that would otherwise build up across the deposit and pull it off the base, and the narrow cracks do not let the abrasive in far enough to matter.
| What it looks like | What it means | |
|---|---|---|
| Check cracks | fine, regular, across the bead, stopping at the base | normal stress relief; harmless in abrasion service |
| Cracks into the base | running from the deposit into the part | defect: may grow by fatigue and break the part |
| Cracks along the fusion line | between the deposit and the base | defect: the deposit will spall off in pieces |
| Spalling | flakes of deposit lost | wrong deposit for impact, too thick, or poor bond |
The important distinction is between cracks that stay in the deposit and cracks that run into the base or along the fusion line. A check crack that continues into the base can grow by fatigue under service loads and eventually break the part; a crack along the fusion line separates the deposit from the base, so that the deposit spalls off in pieces. Both are defects, avoided by a suitable build-up or buffer layer, the right number of layers, and, where the base is crack-sensitive, preheat (Chapter 8).
Crack-free deposits
Some applications cannot accept check cracks: parts that suffer significant impact, where the cracks can grow and the deposit spall; parts in corrosive service, where the cracks let corrosive material reach the base; and parts that must seal or carry pressure. For these, filler makers offer crack-free carbide alloys, usually with lower carbide content and somewhat less abrasion resistance, or the deposit is laid with a preheat and controlled cooling that reduces cracking. The choice is a trade between abrasion resistance and crack-free integrity, made for each part.
How thick
Carbide deposits are generally limited to two or three layers, because each layer adds shrinkage stress and thicker deposits crack more severely and are more likely to spall. Each layer is typically a few millimetres thick, depending on the process and parameters, so the permitted layers add up to a modest total thickness. Where more thickness is needed, the part is built up with a tough alloy and the carbide layers laid on top (Chapter 2). The filler maker’s data state the maximum number of layers for each product, and exceeding it is one of the commonest causes of spalling.
Abrasion resistance and carbide fraction
The abrasion resistance of a carbide deposit rises with the fraction of its volume that is carbide, because more of the surface is protected by carbide and less is exposed matrix.
The trend is why carbide content, controlled by the filler’s composition and by dilution (Chapter 3), matters so much: a first layer diluted to a lower carbide fraction wears much faster than the filler’s data suggest.
Bead direction
The direction in which carbide beads are laid relative to the flow of material affects wear. Beads laid across the flow present a series of ridges and valleys: the ridges take the wear, and the valleys can fill with packed fines that protect the deposit, which helps in some materials. Beads laid along the flow let material run smoothly down the valleys, which can scour the valleys and the edges of the beads, but give a smoother path for sticky material that would otherwise build up. Check cracks, which run across each bead, lie along the flow in cross-flow beads and across it in along-flow beads, and in very abrasive service the cracks across the flow can be worn into grooves. Chapter 9 deals with bead patterns in more detail.
Welding over old carbide deposits
Worn carbide deposits are often renewed rather than removed. A deposit worn evenly, with sound remaining metal, can usually take a fresh layer, provided the total does not exceed the filler maker’s limit and the old deposit is cleaned and free of loose fragments. A deposit that has cracked into the base, spalled, or reached its layer limit must be removed, by gouging and grinding, down to sound build-up or base, before it is rebuilt. Laying new carbide over an old deposit that already has the maximum number of layers is a common cause of spalling, because the total thickness is then far beyond the limit.
Corrosion and carbide deposits
Many carbide deposits contain enough chromium to resist mild corrosion, which helps in damp abrasive service such as wet coal, ore slurries and grain terminals. Their check cracks, however, can let corrosive moisture reach the base steel, which corrodes beneath the deposit and can undermine it. In wet service, the workshop chooses higher-chromium or crack-free deposits, lays them on stainless or corrosion-resistant build-ups where the base would otherwise corrode, and keeps the base metal’s exposed edges protected.
Cutting and fitting overlay plate
Overlay plate brings its own fabrication practices. It is cut by plasma or abrasive methods from the carbide side, because the carbide face resists oxy-fuel cutting; it is rolled with the carbide on the outside of the curve, within the maker’s minimum radius, so that the deposit is not compressed and crushed; and it is attached by bolts countersunk into the carbide face, by plug welds from the back, or by welding the backing plate’s edges, never by welding on the carbide face. The workshop learned these practices from the plate maker’s fabrication guide before cutting its first sheet, and its first panels fitted without damage.
Overlay wear plate
For large, flat or gently curved wearing surfaces, such as chute liners, hopper linings and truck tray liners, an alternative to hardfacing in place is chromium carbide overlay plate: a mild steel backing plate with a carbide deposit laid on one face by an automatic process in a factory, sold in sheets. The plate is cut, rolled and bolted or welded in place like ordinary plate. Its deposit is laid under controlled conditions, with consistent thickness and dilution, and it is often cheaper per square metre than hardfacing in the workshop, especially for large areas. Its limits are that it cannot follow complex shapes, the carbide face cannot be bent sharply, and cutting it needs plasma or abrasive methods, because the carbide face resists oxy-fuel cutting.
Inspecting carbide deposits
Inspection of carbide deposits looks for the defects that matter, not the check cracks that are expected. The workshop checks each finished deposit visually for spalled or lifted areas, for cracks that run beyond the deposit into the base, especially at the edges where the deposit ends, for porosity and for even coverage and thickness. Dye penetrant on the base metal beside the deposit shows whether any crack has run out of the deposit into the part. A tap with a small hammer can reveal a deposit that has separated from its base: a sound deposit rings, a lifted one sounds dull. Thickness is checked against the plan with a gauge or by measuring the part’s dimensions before and after hardfacing.
Edges and ends
The edges and ends of a carbide deposit are its weak points. The base metal just beyond the deposit wears faster than the deposit and can be undercut, leaving the deposit’s edge unsupported, so that it breaks away. The workshop carries the deposit slightly beyond the area of heaviest wear, finishes it with a tapered edge rather than a step, and, on edges that face the flow, wraps the deposit round the edge so that the material meets deposit rather than base. Checking how a previous deposit failed, often by undercutting at its edges, guides where the next one should end.
The running case: chute liners
This is an illustration. The grain terminal’s chute liners, about 18 m² in all, had been hardfaced in place by the workshop every year or so with two layers of chromium carbide deposit, taking about 40 hours of welding each time. The workshop compared buying overlay plate, cutting and fitting it, with hardfacing in place. The plate, fitted in bolted panels, cost a little more in materials but much less in labour and downtime, and its consistent deposit lasted longer than the hand-laid one. The terminal changed to overlay plate for the flat liners and kept workshop hardfacing for the curved transition pieces that the plate could not follow.
Grinding and machining carbide deposits
Carbide deposits are too hard to machine with ordinary tools and are usually used as welded. Where a smooth surface or accurate dimension is needed, they are ground with suitable abrasive wheels. Where a hardfaced part must fit accurately, such as a wear plate that must sit flush with its neighbours, the deposit’s thickness is controlled during welding rather than corrected afterwards. The workshop plans hardfacing to finish at the right height, using templates and thickness checks, so that grinding is limited to blending edges.
A failure: the spalled liner
This is an illustration. A transfer chute, struck by large lumps of ore falling from a conveyor head, had been hardfaced with four layers of a high-carbide chromium deposit to give it a thick wear layer. Within weeks, the deposit had spalled away in plate-sized pieces where the lumps struck, leaving bare steel that wore through quickly.
The deposit was too thick for its alloy, and the impact of the falling lumps was too great for a brittle carbide deposit. The chute was rebuilt with an impact plate of a tougher deposit, or a manganese steel liner, at the point of impact, and a two-layer carbide deposit where the ore slid rather than struck. Matching the deposit to the wear at each point of the chute, rather than one deposit for all, solved the problem.
Exercise: layers and thickness
A chromium carbide filler’s data sheet allows a maximum of two layers, each about 3.5 mm thick. A chute liner has worn 12 mm. Plan the rebuild, and say what would happen if the welder laid four layers of the carbide deposit instead.
The carbide deposit may be at most about 2 × 3.5 = 7 mm thick. To restore 12 mm, the liner is first built up about 12 − 7 = 5 mm with a tough, compatible build-up alloy, in perhaps two layers, then finished with the two carbide layers.
Four layers of carbide deposit, about 14 mm, would exceed the filler maker’s limit. The deposit would build up high shrinkage stresses, crack more severely, possibly through to the base or along the fusion line, and be prone to spalling, especially if the chute sees any impact. The rebuilt liner would likely fail sooner than one built up correctly, despite having more of the expensive alloy on it.
