The words get used interchangeably and they should not be. Build-up restores dimensions: a worn shaft, a corroded flange face, a sealing land that has been eaten away. Overlay puts a different material on a surface: a corrosion-resistant skin on a cheap substrate, a hard face on a soft part.
Both are welded deposits and both need the same discipline about heat and dilution. The difference is what you are buying. Build-up buys back the shape the part used to have; overlay buys a surface property the part never had. Confusing the two leads to a deposit that is dimensionally correct and metallurgically wrong.
Replacement is often impossible. A cast housing may have no drawing and no supplier, a shaft may be part of a matched assembly, and a seal face may be machined to a fit that a new part would not reproduce without rework. Welding restores the worn area and keeps everything that is still good.
It is also quick when the wear is local. A worn band around a shaft can be built up and turned back to size in less time than a replacement part takes to arrive, which matters when equipment is down. The economics only work, though, if the deposit is sound enough to machine without opening up porosity.
A worn surface is contaminated by definition. It has been in contact with whatever caused the wear, it has usually oxidised, and it often carries a smear of material from the part that wore against it. Grinding back to clean, bright metal across the whole deposit area is the starting point, not a refinement.
The shape of the preparation matters too. A worn groove should be opened out to a shallow, wide form so that each bead can be laid against clean metal rather than into a crevice. Trying to fill a narrow, deep wear scar produces lack of fusion at the walls, and the deposit will lift when the part is machined.
Dilution is the proportion of parent metal that melts into the deposit. It is the variable that quietly determines whether an overlay behaves like the material you specified or like a thin version of the substrate. High dilution on a corrosion-resistant overlay means the first pass is not really the alloy you wanted; it is a mixture.
The usual answer is a buffer layer: a first pass of a compatible, ductile material that bonds to the substrate and provides a clean base for the functional layer. The buffer takes the dilution so the overlay does not have to. It costs a pass and it is the difference between a coating that lasts and one that spalls.
Deposits are wide, and wide deposits are made of many beads. If those beads are laid side by side in one run, the middle of the patch gets hot and stays hot, and the result is a hard, stressed region that tends to crack at its edges once the part cools.
Staggering the sequence, jumping across the patch rather than working along it, keeps the average temperature down. On a circular part, working on opposite sides and letting each bead cool before returning is standard. On a long flat patch, backstep sequencing does the same job.
Every deposit will be machined afterwards, so the deposit has to be bigger than the finished size and consistent in that oversize. A patch that is generous in the middle and thin at the edges will finish undersize at the edges, which means the whole repair has to be redone.
The allowance also has to account for distortion. A built-up shaft will not stay perfectly round, and a flange face will not stay flat, so the allowance has to cover both the machining cut and the movement. Underestimating it is one of the commonest reasons a repair fails at the lathe rather than at the torch.
Build-up on a thick or stiff part creates a lot of restraint. The deposit wants to contract and the surrounding metal will not let it, so the stress has to go somewhere. On hardenable materials the risk is greater, because the deposit and the heat-affected zone can both harden as they cool.
Pre-heat and controlled cooling are the usual controls, along with a deposit material chosen for toughness rather than for maximum hardness. Where the part is large, keeping it warm between passes and covering it afterwards is often enough. Where it is small and highly restrained, the sequence and the buffer layer do more work than any amount of pre-heat.
A repair is only verifiable against a measurement. The worn area should be measured before any welding, so the amount to be restored is known rather than estimated, and the finished part should be measured against the original nominal dimensions.
Recording the before and after also builds a picture over time. A part that comes back for the same repair repeatedly is telling you something about the design or the operating conditions, and the numbers make that visible. Without them, the third repair looks like bad luck rather than a pattern.
Overlay and build-up work is usually urgent, which is exactly when the specification gets skipped. A short list of questions prevents most of the failures, and none of them takes long to answer.
Most of our overlay work is on exhaust components and fixtures that have worn at a joint or a sealing face. The pipe work behind it is described under exhaust pipe sections and exhaust pipe work, the welding procedures on our TIG stainless steel welding pages, and complete titanium assemblies under titanium exhaust systems. Fabricated parts are listed under fabricated components and finished stainless systems under stainless steel exhaust systems.
