2018 July 10    Metal & Shop, Tools 


With cast iron, welding tends to create brittle areas around the heat-affected zone.  This may be less likely to occur with brazing, since it doesn't melt the base metal.

Question is, can you braze a cast iron pulley? 

Should you?  Let's find out.  (Also please read the Disclaimer.)

And in the process, maybe we can learn a couple things about torch brazing.

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In This Article

1. Safety

2. Gearing Up

3. The Pre-Heat

4. The Braze

5. Safety, Again.

6. The Sand Bucket


A miscalculation with simple machines, and the next thing you know, you're figuring out if it's a good idea to braze a cast iron pulley....

1. Safety - READ THIS!!

Is it a good idea to braze a cast iron pulley that's chipped? 

Short Answer:  Depends on the rotational speed, the mass of the chip, and how good the repair is. 

Offhand-- and I'm not guaranteeing this is safe-- I know of a 4" or 5" brazed pulley that rotates at probably 1,000 rpm. 

Mower deck pulleys can rotate fast:  3,000 rpm or more.  And some of them are greater than 5" diameter.  In general, fast-rotating objects become dangerous quickly.  When there's a failure, fragments can move at hundreds of feet per second.

In the 19th and early 20th Centuries, industrial flywheel explosions were known to take out entire buildings.  A seven-foot cast iron flywheel doesn't even have to rotate that fast to become scary.

Up into the 1970's there were many terrible accidents involving clutch plates and flywheels that exploded.  Usually this happened at drag races.  Often it happened elsewhere, such as down the main drag of town.  Automotive clutch disasters still happen sometimes, even today. 

When a spinning flywheel gives way at 6,000 to 8,000 rpm, it can look like a bazooka round hit the car.  Some of these flywheel explosions literally cut the cars in half. 

Those flywheels are larger than a mower spindle pulley, which makes them a lot more dangerous.  That said, any fast-rotating wheel or pulley can explode under the wrong set of conditions. 

"Warning:  Never Operate with Mower Deck Cover Removed"........... this is also why you don't ship grinding wheels with no cushioning, as a certain online retailer stubbornly insists on doing.

So, we're only going to use this pulley as an example of brazing principles.  That, and maybe I'll save it for some low-rev contraption powered by a bicycle or hand crank.

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2. Gearing Up

Brazing, like machining or welding, is not something you should just dive into on a lark;  you need to learn safe handling of OA tanks, and that sort of thing.

However, if you have some basic experience already, you'll probably want your own set of tanks, regulators, etc.

It's a good idea to invest in at least a semi-decent kit from Victor, Miller / Smith, or Harris. I'm not saying generic brands are always bad, because I've repaired a lot of stuff with an unlabeled Chinese set that has worked for many years.  But if you're just starting out, get something of a known brand where there's some customer support.

Welding goggles are crucial.  For oxyacetylene work, you'll need goggles that are shade #5. Old-style round-lens welding goggles are very popular for oxyacetylene work.  There's also this style which I've used extensively, but I don't really like the way they fit. 

Best thing I've found so far is this type of goggle. I haven't tried that exact brand, but Hobart offers one that looks identical, and it fits comfortably.  And it will fit over prescription glasses.

For the filler metal, I used regular low-fuming bronze.  However, capillary brazing might work better with Harris Safety Silv 45


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3. The Pre-Heat

The area to be brazed will have to be about 1650 Fahrenheit.  If you try to use a torch to heat the whole thing up, you'll waste all your welding gas. 

Even after heating this pulley for twenty minutes with the brazing torch tip, it was nowhere near hot enough.  In fact, it wasn't even dull red. 

Just to heat it as much as I did, it wasted huge amounts of welding gas. 

An electric oven might get this to 500 F, which could help save welding gas.  It should be hotter, though, which is going to require a propane burner and some firebrick.  Either that, or a charcoal forge.  (Not wood this time;  find out why.)

I had the pulley clamped in a bench vise.  I wasn't concerned if the vise itself got to 500 degrees or more;  this is the ugly vise that I hack-repaired.  If it breaks, I'll just weld it back together again.  Then again, you don't want something like this to fall apart when you've got a chunk of 500-degree cast iron clamped in it.  Where would it roll?  Just not all that safe.  (As I've said:  don't be too quick to try some of this stuff...)

So, why does it matter what temperature you preheat cast iron?  Let's find out.

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4. White Cast Iron

Deliberate formation of white cast iron involves heating gray cast iron to the melting temperature, somewhere about 2200 degrees F. 

Arc welding can get hot enough for that, easily.  However, brazing gets the immediate area hot enough that some transition might occur.  I don't know for sure how much would form, if at all.  Really we can't know by looking at it.  It takes some fancy equipment you would probably find at a university metallurgy lab.  (If they even have anything related to iron and steel production anymore.)

Pulleys are usually made of gray cast iron or ductile iron.  I think they do this for a reason.  Some types of pulley might be made of white cast iron, but probably not "high-speed" pulleys.  White cast iron is highly resistant to wear, but it's brittle. 

With insufficient preheat, it could be possible to form small amounts of white cast iron around the braze joint. 

So you could get an area that would be brittle, and it could develop stress cracking. 

For a high-speed pulley, that would be bad.

Here's what I'd do next time, if I were going to braze a high-speed pulley (which might not be a good idea, as I've said).  I would make sure to preheat the entire thing to at least a dull red heat, which is about 1000 Fahrenheit.  Heat the brazing area to the required temp (1630 to 1650 F), complete the braze, then keep it post-heated with the torch at just below the melting point of the filler metal.  Cool as slowly as possible... couple days.

If you're just going to use the pulley for a hand-crank machine (something where it's rotating at 50 or 100 rpm) then it's not going to matter nearly as much.  A homemade ice cream maker, at fifty or sixty rpm, is another alternative.

Incidentally, if you had some way to keep the brazed pulley at 1650 Fahrenheit for seven straight days, packed in rust powder, you could actually transition any white cast iron into malleable iron.  Just sayin'. 

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5. The Brazing

It is possible to braze things together with big fat braze joints that look like welds, but this is not the ideal way.  Try to have good fitup, where the molten filler metal flows in by capillary action. 

Contrary to popular belief, low-fuming bronze (LFB) can flow into narrow joints by capillary action.  Silver braze obviously works better for this, though. 

Either type requires clean surfaces, enough heat, and enough flux.

A reassembled cast iron break might be no more than 0.005" gap.  Silver braze works great where there is very close fitup, as narrow as 0.001".  Bronze braze joints might gain some strength by having thick metal buildup across the two pieces (like welds), but it will fill narrow gaps if done right.

If you've soaked enough heat into the workpiece, the end of a flux-coated brazing rod should liquefy without much help.  A little bit of heat applied to the end of the brazing rod is OK, as long as the workpiece is already hot enough to liquefy the flux coating to water-clear by itself.

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5. Safety, Again.

A good braze joint is considerably stronger than cast iron itself.  This is why, "in theory", brazing would be OK for this.

That said, it's only as good as the adhesion to the base metal.  That can be decreased by surface dirt, soot, insufficient heat, etc.  There's also the possible formation of white cast iron, as we've seen. 

This brazing attempt didn't fill the gap perfectly.  The cast iron simply wasn't hot enough.  There's one section where I can definitely see it didn't take.  I haven't done a hammer test on this yet, because I might actually save this for a low-rpm machine that I might build.  Or a piece of art.  But we'll see.

Whether it had looked perfect or not, I'm not going to use it on a lawnmower.  This link has a lot of brand-new pulleys for different mowers.  This is one area not to be a cheapskate.  If your high-speed pulley gets broken and they still manufacture one in that size, just buy the new pulley.

But what if it was the only one of its kind? 

Again it depends on the rotational speed, or more accurately, the surface speed.  If a high-speed pulley has a chunk out of it from unsuccessful use of a gear puller, we've seen why it's kind of risky to try metal-bonding the chunk back in place.  It's probably safer to grind off the sharp edges and just run it with the chunk missing.  Grind out another equal-sized area on the opposite side of the pulley to balance it.  Then there's no fragment that can fly off.  Get the pulley re-balanced at a machine shop, if necessary.  (You're still going to want to run that inside a protective cover, though... never run a lawnmower deck with the safety covers removed.)

6. The Sand Bucket

We looked at the problem of white cast iron, and how it might form when the metal cools too fast.  The most durable cast iron repairs are the ones that were cooled slowly when the cast iron was at a uniformly high heat. 

If the air is cold, you might even braze the piece while it's on the hot sand and then just shovel sand over it when it's done.

Play sand appears to work OK for slow-cooling the cast iron, from what I've tried.  I can't guarantee that you wouldn't form any white cast iron at all, though.  That's why I keep saying that if you're going to braze a pulley, make sure it's for something low-speed. 

Pre-evaporate the moisture out of the sand, using direct sun on a dry day, turning it over a few times with a shovel.  Keep the sand in a purpose-built metal box, or the empty bottom of an old BBQ grill.  Use something with support legs, nothing flimsy that will collapse with your nice 1600-degree piece of brazed cast iron....

A full day later, the workpiece should be cool enough to handle. 

Machine off the excess bronze, or use a file. (Here's something that should be ideal for such a purpose, though I don't have one.)

Low-speed pulley, now. 

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What did we learn from this?  First, we saw that low-fuming bronze can indeed fill narrow gaps.  It would have been even more effective, had I preheated the workpiece to a higher temperature. 

Brazing pulleys is certainly possible, but is it a good idea?  If you're trying to repair lawnmower parts that spin fast, I would highly recommend just buying a new pulley. 

Use little exercises like these to learn your brazing technique, but don't mess around with safety when you're dealing with fast-rotating objects.  Be smart, be safe, and stand far away from the dragstrip when they're shifting gears at 7,000 rpm.

I hope you enjoyed this article.  If it helped you in any way, please help me out by using any of these links to purchase your gear.  It's the only way I can keep this website online and bringing you helpful articles like this one.   Thanks!!


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