Cast iron welding repair

Gear cases, pump bodies, housings and brackets are often cast iron, and they fail by cracking. Welding cast iron is a skill of its own because the metal behaves nothing like steel when it is heated. This page is about that repair. For the joint on steel parts, see the general welding page.

At a glance

• Cracks found first with Magnaflux
• Preheat and slow cooling to protect the casting
• Filler chosen for cast iron, often nickel based
• Honest advice on whether to repair or replace
• Machining after the weld, in the same shop

Why cast iron cracks when it is welded

Cast iron holds a lot of carbon, much of it as flakes of graphite. That makes it good for vibration damping and wear, and it also makes it brittle. A weld heats a small spot to melting while the metal around it stays cool, and cast iron cannot stretch to absorb that difference. The result is a new crack beside the weld.

Fast cooling makes it worse. Carbon in the heated zone can form hard, brittle structures that crack as they cool, which is why the heat-affected zone matters so much on castings.

Find the whole crack before you weld

Cast iron is magnetic, so Magnaflux crack inspection works well on it. Magnetic particles gather along a crack and show where it ends. A weld that stops short of the crack tip leaves the crack free to grow.

Once the crack is mapped, it is opened up with a groove so the weld can reach the bottom. Drilling a small hole at each crack tip is a common general practice to stop it running further.

Preheat, slow cooling and filler

The usual approach on a casting is to warm the whole part evenly before welding, keep it warm during the work and let it cool slowly, often under insulation. This is the idea behind preheat and interpass temperature. Short weld passes, peened lightly, also relieve stress.

Filler for cast iron is often nickel based, because it stays soft enough to take the stress. Which approach fits depends on the casting, the crack and the job the part does.

Repair or replace: we look at it first

Some castings are better replaced, such as a part with many cracks, oil-soaked metal that will not accept a weld or a crack in a place that carries heavy load. We would rather tell you that after a look than weld a part that will fail again. The guide on repair or replace sets out how to decide.

Send photos of the crack from two sides, the machine the part came from and what the part does. If it is a housing that must be bored or faced after welding, mention that too, because it changes how much stock is left on.