I can’t tell you how many times I’ve been told that you can’t weld cast iron. To an extent, a lot of the concerns are true, it’s pretty miserable stuff to work with. But there are ways to make it work.
In this post I’m going to share what some of these options are, along with the pros and cons of each.
Why Welding Cast Iron is Such a Challenge
This is probably the best place to start. What’s the big deal with cast iron? It melts fairly easily, so why is it so hard to weld?
Welding cast iron is difficult because it has a high carbon content (about 10x that of most steels), and it’s brittle and prone to cracking. Thermal stresses are challenging to remove from the Heat Affected Zone, and cracks will likely propagate over time.
This can be a nightmare if not done right – you’ll get a part that just keeps breaking into pieces or instantly snaps under stress. Although, to be fair, cast iron will crack even on a good day if it’s under tensile load.
That said, there are a few ways to reduce this risk.
Prepping the Weld Area
This is really critical with iron. There will likely be a considerable amount of rust on the part that you’re working on, so make sure you do a good job of getting down to bare, shiny metal.
Generally, welding two pieces of iron together is a bad idea, you’ll be better off with brazing. More on brazing later. Welding cast iron works much better with repair jobs, like cracked parts or when you drilled a hole in the wrong place and you need to fill it in.
If there’s a crack that you’re trying to repair, you need to properly dress it. It should be pretty obvious once you’re down to bare metal, the crack will be visible as a dark line. Sometimes using a finer disc to smooth out the metal surface will make it easier to clearly see the crack.
On either end of the crack, drill a hole. This will prevent the crack from propagating. I like to actually drill slightly past the end of the visible crack. That way, the crack will extend to the hole and then stop. It makes it less likely that there’s a fracture on the other side of the hole that will just let the crack pick up where it left off.
If you don’t drill the ends of the crack before you weld it, it’s really unlikely that the part will stay together once it’s welded back up. It could very well just keep cracking as you weld it, or it’ll just break over time.
Grind down until you’ve reached the bottom of the crack. For most jobs, this will mean grinding right through the metal.
This is my preferred welding approach whenever it’s possible. It’s not terribly difficult, but it helps if you’re a reasonably experienced welder. Knowing how to properly preheat metal and control your heat on the fly are good skills to have under your belt before trying this.
- Most stable way to actually weld cast iron, but it’s still not perfect.
- Ideal for parts that have to endure thermal cycling.
- Sometimes it’s just not feasible to get a part hot enough to properly preheat.
- Setup is more involved – you’ll need a rosebud torch and a way of slowing the cool, like an insulating blanket or dry sand.
- The weld area is rarely stronger than the unaffected area.
- There is a risk that the area around the weld will crack, even when the process is done properly.
- Can be done with MIG, TIG, or arc welding machines.
- Filler rod/wire suitable for iron. I like to use 308L but there are a lot of options out there that work just fine.
By preheating the iron to 500-1200 degrees F, you’ll substantially reduce the internal stresses in the iron. This will help prevent the part from cracking around the weld in the heat-affected zone.
If you have an infrared thermometer, then by all means use it. If you don’t, though, here’s a trick to know what the temperature is:
Iron starts to glow dull red at about 900 degrees F. So if you get your rosebud torch on it and let it just start to glow, then you know you’re at a good temperature for welding. It’s best to do this inside in a darker area so that you’ll notice when it starts to glow.
Take your time heating it up, make sure it’s slow and even. Also, don’t let the iron get hotter than 1400 F (bright red). That’s the critical temperature range of iron. Welding that hot = bad.
Ideally, heat up the entire part. If the part is too big and you can’t heat the whole thing up, then heat the largest area possible. You’ll want the area around the weld to be properly preheated for several inches (I like to shoot for about 12″ at least) to properly disperse the internal stresses.
Don’t let the part cool down too much while you’re working on it. Stop and reheat as many times as you need to in order to keep the part above 500 F. Bad things happen when you’re trying to do preheated welds below that – the internal stress just doesn’t dissipate and you’ll end up with a cracked part.
Honestly there are dozens of options for filler materials. Feel free to experiment. A lot of people recommend nickel-based fillers, and they do work really well, especially if you need to machine the part after welding. They’re just really expensive.
If you’re doing something that doesn’t need to be machined, like an exhaust manifold, then try using a 308L wire. It still isn’t cheap, but it’s not as bad as nickel. It will hold together well and handle the heat.
Weld using a low current. Too high and there will be extra stresses that are guaranteed to crack later on, if not while you’re welding.
Pro Tip: Don’t start and stop your welds at the same point when laying down additional beads. You’ll get a buildup of stresses and a potential cracking point. Instead, try using a L or J shape of sweeping motion on to your weld line, so each weld starts and ends in a different place.
The nice thing about the preheated welding is that you don’t need to stitch your welds as long as you don’t overheat it. You can just give’r and git’r done. Much more so than the non-preheated approach, anyway.
After Welding Instructions
After every bead, make sure that you thoroughly peen the weld. This will help to work out extra stresses and prevent cracking later on.
One thing that’s really important is a slow cooldown. Wrap the part in an insulating blanket or bury it in dry sand to let the part cool slowly and allow the stresses to relax. Fast air cooling = cracking.
Overall, I find the preheating method works best for anything that will be experiencing thermal cycling (like exhaust manifolds or old engine blocks). This will be the most effective way of having a strong (ish) weld that won’t crack immediately.
For more decorative pieces, though, you might be able to get away without preheating.
For starters, you don’t want to be doing this welding dead cold. Ideally, the workpiece will be heated up to about 100 degrees F, otherwise it’ll crack for sure.
A rule of thumb for this is to have the workpiece at a temperature where you can hold it with your bare skin without it burning you. Some guys just toss the part in a BBQ for 15 minutes and bake it up to temperature. Don’t let it cool off too much while you’re working on it, but once you get going then likely the heat from welding will keep it warm enough.
- Easier to set up, don’t need a torch to preheat or slow cooling medium.
- For lots of applications it works good enough.
- More prone to cracking than preheating method.
- Much more internal stresses.
- Can be done with MIG, TIG, or arc welding machines.
- Filler rod/wire suitable for iron. I like to use 308L but there are a lot of other options out there that work just fine.
- Insulating blanket or dry sand that can handle high temperatures. This is used for letting the part cool slowly.
This is essentially the same as what’s required for the preheating approach.
- Grind off all the rust in the weld area and make sure that you have shiny bare metal.
- Drill holes on either end of the crack to prevent it from propagating.
- Grind down to the bottom of the crack.
- Grind a bevel for the weld.
Again, you’re going to want to keep your voltage low and control your heat. Stitch weld a maximum of 1″ at a time, and allow it to cool off before continuing. It’s really important that the work doesn’t glow while you’re working on it.
If you start at either end and “hopscotch” your stiches, you won’t really have to wait for it to cool since it will cool off while you’re welding another section.
After Welding Instructions
Peening is absolutely critical if you’re not preheating. Ideally use a pneumatic hammer or an equivalent tool so you can very thoroughly remove the stresses.
Inspect it very closely for cracking after it’s been peened. Go to town on in with a wire brush and see if there are any visible areas where the cracking has started. Try to touch up these areas as best you can, they’ll only get worse with time.
This is actually a really good option whenever the application will let you do it. Brazing will restore the part to very nearly its original strength.
Iron takes brazing extremely well because it’s so porous. It will usually take a little bit of extra prep to make sure that the surface is clean enough, but it’s normally worth it.
Prep the metal like you would for welding – bevel or notch the area to be brazed. You’ll also want to clamp the part as best you can – iron will really tend to warp as it cools down, and clamping will help to counteract this.
Many handbooks will tell you that brazing isn’t ideal for parts that operate at over 400 degrees F, since the braze will often lose considerable strength at that temperature. Honestly, though, there are a lot of guys that braze repair exhaust manifolds and they last just fine. The braze softens, but it’s good enough. Just maybe avoid using this if it’s a part that’s under load at that temperature.
Common brazing rods for iron are silver, brass or nickel. I usually use brass but do whatever makes you happy.
- Really solid, reliable way of repairing cracks and joining metal together.
- Can also work for joining dissimilar materials. For example, steel to iron.
- Extremely visible – any repairs will be noticeable since the braze will be a different color. This may not be practical for certain cosmetic applications.
- Brazing torch, or oxyacetylene welding torch. Larger pieces will need larger torches.
- Brazing rods – brass, silver and nickel are common choices.
- Insulating blanket for slow cooling.
Cleaning the Iron
There are a few extra challenges that are more noticeable with brazing. One of these challenges is dealing with graphite.
Since iron has such a high carbon content (roughly 2-4%), this carbon isn’t all completely absorbed in the metal. Instead of being nicely and consistently distributed throughout the iron, it clumps up and crystallizes into graphite flakes.
This can be a real pain when you’re brazing, it makes wetting very difficult. Even if the surface is freshly ground and clean, iron that’s on the higher carbon end of the spectrum can give you problems.
I like to go to town on it with a really clean brass wire brush. This just helps to knock out some of these graphite flakes. After that, wipe the surface down with alcohol until no more grey graphite comes off as you’re wiping it.
If you have the equipment to blast the piece with steel or nickel shot, then this is the time to use it. It’ll do a really good job of clearing out the excess graphite.
After that, use an oxidizing flame (one with too much oxygen so it makes a loud hissing noise) to help burn off any remaining graphite. You might see this blow off as sparks when the surface gets hot.
Once that’s all done, you should be good to go.
This is really important. If you don’t clamp the piece down firmly, there’s no way you’re getting it straight.
If you can, clamp it down to a solid, flat welding table with C-clamps. If you’re working on a housing of some kind, you might want to consider even machining a little fixture for yourself that will be able to really firmly hold everything in place.
You need your clamping to be dead solid so nothing can move on you while you’re brazing. Otherwise the part is going to warp like crazy and you’ll have a miserable time realigning everything. This is a step worth spending time on, don’t rush it.
Once you’re sure that the surfaces are clean and everything is firmly clamped, it’s time to heat up the part.
Add a decent amount of flux to the area that you’ll be brazing, and try to be as thorough as possible. Heat the workpiece up nice and steadily, not too fast, especially if it’s a smaller piece that you’re working on.
You need to get the iron up to a nice cherry red for this to work. The flux will melt totally clear at this point. Don’t get so hot that the iron starts to melt.
Once the iron is nice and bright, scrape the brazing rod against the surface. If it’s hot enough and the flux is doing its job, it should immediately wet the surface of the metal and stick solidly to it.
Take your time and gradually build up the brazing filler until it’s a bit higher than the beveled area. That way you’ll be able to grind it back down so it’s half respectable.
Really, this takes a bit of skill. Be patient with yourself and don’t expect the first one to come out looking pristine. Ideally try a few practice pieces before you braze some iron that’s actually important for something.
It’s absolutely critical that you don’t unclamp the part until it is cool to the touch. There’s so much internal stress in iron that’s been heated up that you’ll likely never get it straight again.
Slow cooling is really important, too. Once you’re done brazing, cover the part with an insulating blanket to slow it down. You’ll want it to take several hours for it to be cool enough to touch.
You’re probably going to be best off just insulating it and letting it cool overnight. That way you’ll really be minimizing the risk of cracking and warping.
Ultimately, I’ve found brazing to give the best results, mechanically speaking. It might not look the prettiest once it’s done, since the color of the braze will highlight the repair, but it should be very durable.
Really, though, there is no one approach that will work for every application. If you find yourself often working with iron, it’d be good to learn all of these methods of repair.
Do you have any tricks or sure-fire ways of working with iron? Or any questions? Share them in the comments.