Pumps crack, corrode, and wear out — but a cracked pump doesn't always mean a new pump. At Rumix, we use a proven four-step welding process to fix it, bring it back to spec, and put it right back to work. Here's how it's done.
Step one
Preheat the metal
Before striking a single arc, we heat the entire pump body — or at least the area around the repair — using a torch or an oven. This brings the metal to a specific temperature called the preheat temperature, which depends on the type of metal the pump is made from.
Why? Because cold metal next to a hot weld is like pouring boiling water into a cold glass — it can crack. Preheating slows the cooling rate so the weld and the base metal shrink together at the same pace. For cast iron pumps, we typically preheat to 300–600°F (150–315°C). For carbon steel, 200–400°F is the usual range.
Wild fact
Cast iron has almost zero flexibility — it can't bend even slightly before it snaps. That's why skipping preheat on cast iron is basically asking for a brand-new crack. The weld metal wants to shrink as it cools; if the surrounding iron is cold and rigid, it pulls apart like a tug-of-war — and the iron loses every time.
Step two
Weld the repair
Now we lay down the weld — filling the crack, building up worn surfaces, or joining a broken section back together. Our welders use short passes called stringer beads and select filler rods matched exactly to the pump's metal. For cast iron, nickel-based electrodes are the go-to because pure nickel is stretchy enough to handle the stress without cracking.
The pump stays warm the entire time. Letting it cool mid-weld is a classic mistake. On large pump housings, we wrap the part in insulating blankets between passes to hold the heat in.
Wild fact
A weld pool is hotter than the surface of the sun — hitting around 7,000°F (3,900°C) at the arc. The metal goes from room temperature to liquid and back to solid in a fraction of a second. That extreme temperature swing in a tiny spot is exactly why all the heating and cooling steps around it matter so much.
Step three
Stress relieve (or slow-cool)
After welding, the metal is full of internal tension — invisible forces locked inside from the heating and cooling. Left alone, those stresses can cause warping, cracking later, or premature failure under pressure. The fix is post-weld heat treatment (PWHT), also called stress relieving.
The pump goes back into the oven — or we use ceramic insulating pads with electric heating elements — brought up to around 1,100–1,250°F (595–675°C) for carbon steel, held there for an hour or so, then cooled very slowly. On simpler jobs, wrapping the pump in insulating blankets overnight can be enough.
Wild fact
You can't see residual stress. A pump can look perfect — shiny, smooth, no cracks — and be absolutely loaded with internal stress ready to split the metal the moment it goes back under pressure. PWHT is essentially invisible repair work, fixing a problem you can't see with a process you can barely detect. That's why some shops skip it and deeply regret it later.
Step four
Machine it back to spec
Welding adds metal and distorts dimensions — the pump is no longer the right size or shape. This is where Rumix Machining and Fabrication earns its name. The pump goes to our lathe, mill, or boring machine and the weld is cut, ground, or bored back to the original engineering drawings. Bearing seats, seal faces, impeller fits, and flange surfaces are all brought back to exact measurements.
This final step turns a rough weld repair into a pump that works as well as — or better than — the original. Machining removes any surface porosity and leaves a clean, precise surface for seals and bearings to run on.
Wild fact
Pump seal faces need to be flat to within 0.0003 inches — thinner than a human hair, which is about 0.003 inches. After welding something at 7,000°F, our machinists bring it back to a tolerance you couldn't feel with your fingertip even if you tried. That contrast — brutal heat followed by microscopic precision — is what makes pump repair genuinely impressive engineering.
Quick Reference
Large industrial pump housings can cost $50,000–$500,000 new. A weld repair might cost $2,000–$10,000 and be done in days, not months.
A full repair — preheat, weld, stress relieve, machine — typically takes 3 to 10 days depending on pump size and damage.
Most steel and iron pumps can. Some aluminum alloys are tricky. Hardened steels need special filler rods and very controlled heat.
Non-destructive testing (NDT) — dye penetrant, magnetic particle, or ultrasound — finds hidden cracks before the pump goes back into service.