Yes we Weld [not just for fabrication - but on Cast iron as well] predominately to build up edges and corners, often on manifolds – rarely in casings / castings with any considerable mass. We Metalock these as it is safe, predictive - doesn’t cause inbuilt stresses or misalignment associated with welding – and can be performed everywhere. Metalock requires little or no dismantling, has no HAZ Heat Affected Zone and the embrittlement  or dangers that carries.

Cast iron is hard to weld, risky and not cheap if carried out correctly.  Following are some things to be aware of.

Grades and Environment

1]   There are many different grades of cast iron, some more weldable than others. And it is difficult to tell them apart. A component from one country [say a Cummins block from Germany] repairs beautifully, the same casting from another country [say Brazil] – very average.

2]    The environment the cast iron has been in makes a difference. Exhaust manifolds are impregnated with carbon. Blocks, sumps and casings are contaminated with oils / lubricants.

Composition

Mild steel has less than 0.2 percent carbon.
Medium carbon steel has more than 0.2% and will harden if heated red hot and quenched.
Cast iron has anywhere from 3 to 8 full percent of carbon. And will get very hard and brittle if you don’t control the heat - that’s preheat level and duration, Heat applied during welding and controlled post weld cool down period.

Insights

Following are a series of insights on welding castings.  A lot of our large jobs come from small beginnings where people have welded them.

A hasty nickel or braze weld will further damage the casting and can prevent Metalocking the crack later. You may be looking at a replacement of that casting or cylinder block.

Do not arc/nickel weld a cracked engine block. Welding an assembled engine is fraught with danger. Successful cast iron welding in these applications requires preheat to around 600°C.

In fact any electric welding on cast iron without really knowing what to do and carrying out all steps required is doomed to failure. Cast iron cannot withstand the contraction and hardening caused by welding with insufficient preheating. The brand of rod is unimportant as it is the heat that changes the cast iron itself - precipitating carbides in the structure adjacent to weld causing such hardness that it cannot be machined. Cast iron welding should be carried out by experienced welders used to high temperature oven welding. Cast iron requires preheat to around 500°C for brazing and 750°C for fusion welding.

The Rules

High temp preheat when welding in the center of castings. Limit low temp Arc / Nickel welding and brazing to corners and edges. Preheat and maintain appropriate temperature during the entire welding process. Cool casting very,very slowly to allow the weld affected area to move with the contraction of the weld, rather than crack.

Expansion and contraction

When metals are heated, they expand - When metals cool they contract. There is no consequence when these actions are unrestricted. Restricted expansion / contraction cause welded cast iron to crack.  Temperature increases in the HAZ does not allow the cast iron unrestricted expansion as it is constrained by the colder iron around it.  Forcing the iron to “grow” outwards or thicker. A permanent change.  When it cools down / contracts this draws the surrounding material in – the stresses invoked causing fractures, sometimes not discernable until in service [even worse then].

Note: If the weld or parent material adjacent to the weld cracks - the part was too cold prior to welding. If the parent material adjacent to the weld hardens, the part cooled down too fast.

The Temperatures

Cast irons critical temperature at which it can be hardened or annealed is 700°C+ [most grades]
Cast iron can be hardened to Rc 65, practically unmachinable.

Rapid cooling of iron through water or oil quenching is obviously bad news. What people don’t realise is that iron can be air quenched in these circumstances as the surrounding material quickly draws the heat away – causing localised hardening. It is important the casting is maintained at 700°C for 30 minutes or more and cooled to 640°C for another 30 minutes, etc. Now the iron is annealed and hardening was avoided.  In fact a 24 hour controlled cooling period for castings is a good rule of thumb.

The casting is an inert object –if it hardens or cracks it’s really the person controlling the temperature who is at fault.

Different Cast Welding Techniques

Nickel welding
If using nickel rods on cast iron confine it to corners and edges where heat can dissipate without causing stresses.   Preheat area to dull red, arc weld and post heat straightway – don’t let it air quench. Less amps is required when the casting is preheated. 

Powder welding / Puddle torching
Preheat the casting to 490°C, apply a thin coat to protect surface from ferrite oxide deposits and continue preheat to 700°C.  As the area builds up temperatures can reach 980°C in the area where it bonds.  If no preheat - restrict use to corners and edges.

Brazing
Preheat casting to 490-500°C before increasing the temperature in the target area.  Bronze rod melts at 940°C. Bonding temp is 980°C. Braze on an incline of 30° +.  Don’t flat braze as this can causes pin holes or “burnt” edges.  Working uphill enables the vee to be filled from bottom to top in one motion.  Use thin shunts /move fast, keep area ahead of puddle tinned.  Post heat to 480°C.  End with a 24 hour controlled slow cooling period

Fusion welding
Fusion welding requires melting and puddling of base metal as filler is added.  Typical applications are dense castings such as cylinder heads, which we prefer to Metalock. A 750°C preheat is required. General consensus is to use natural gas, not propane as the heating source, with oxy-acetylene gas and bare cast iron rod for the actual welding process.  Heat and maintain casting at bright red during the entire procedure.  Cast iron melts at around 1200°C.  A post heat of 750°C for approx. 20 minutes is needed. Follow with 24 hour controlled slow cooling period

The Metalock process avoids these problems.