Hey everyone, it’s Jake here from the carriage bolt supply team – and before you ask, yeah, I’m the same guy who posts dumb bolt fail reels on our company Instagram (don’t @ me, my boss still doesn’t get the joke). Today we’re tackling a question I get at least twice a week on calls: “Are 0 carriage bolts suitable for high-temp environments?” 0Carriage Bolts
First off, let’s cut the crap – when people say “high temp,” they mean different things. For a backyard deck builder it’s 150°F and blistering, for a refinery tech it’s 800°F and explosive fumes, and for a foundry worker? Let’s just say that bolt’s gonna glow orange before the day’s out. Most of the time I get this question, folks are mixing up “high temp” with “my specific job’s high temp,” so let’s break this down like I explain it to my cousin who thought 316 stainless was a coffee mug material (true story).
First, let’s clarify what “0 carriage bolts” actually are – yeah, that’s the head size, right? Wait, no, hold up, for anyone new: when we say “0 carriage bolts,” we’re talking about the size designation for the head’s width, not the shaft. Carriage bolts have that round, domed head with a square shank under it, right? The square part stops it from spinning when you tighten the nut, which is why they’re used for things like fence rails, trailer frames, heavy equipment brackets, and sometimes even HVAC ductwork that’s near furnaces. The “0” is just the ANSI (American National Standards Institute) size that translates to a 1/2-inch nominal shaft diameter, for anyone who wants the nerd version. Good, now we’re on the same page.
Now, the big question: high temps. Let’s start with what 0 carriage bolts are typically made of, because that’s 90% of the answer. Most standard 0 carriage bolts – the ones that go to your local hardware store or big box – are made of plain carbon steel, maybe a cheap zinc-plated coating for rust resistance. Here’s the thing about plain carbon steel: it melts at around 2750°F, but it starts to lose its structural strength way before that. At 400°F, carbon steel is about 90% as strong as it is at room temp. At 600°F? We’re down to 75%. By 800°F? It’s half its original strength. And if you throw constant thermal cycling – like a part that goes from room temp to 500°F every hour, expanding and contracting – you get metal fatigue way faster. So standard plain steel 0 carriage bolts? Definitely not for anything over 500°F, and even then, only if the cycle is super mild. I’ve seen guys install these on a small water heater manifold that runs at 450°F, and they lasted 3 years. Install the same ones on a stack that hits 700°F every day for a refinery pump? They snapped in 3 months. Not fun.
But wait – we don’t just sell plain steel, right? That’s why I get the second call: “What if I get the upgraded ones?” Yeah, we stock coated and alloyed 0 carriage bolts for exactly this reason. Let’s talk about common upgrades. First, hot-dip galvanized (HDG) carbon steel. The zinc coating helps with rust, but what does it do for temp? Zinc melts at 787°F, so the coating is gone before you hit 800°F. Under that, though, the carbon steel core is still there – but same strength curve applies. HDG 0 carriage bolts are fine up to 550°F, but not beyond. They’re good for outdoor stuff near grills or patio heaters, but not for industrial high-temp.
Next, stainless steel. This is where things get tricky, because not all stainless is equal for high temps. The most common stainless for bolts is 304, right? What’s its temp rating? Room temp strength is a given, at 800°F it’s around 85% as strong as 304 at room temp, and it’s got good corrosion resistance. But if you go above 1000°F? 304 starts to “sensitize” – that means the chromium in the steel reacts with carbon to form chromium carbides at the grain boundaries, which makes it super prone to corrosion (like rust holes) and weakens it. So 304 0 carriage bolts are good up to 1000°F, but only if it’s a constant temp, not cycling. If it cycles, the fatigue hits hard at 900°F.
Then there’s 316 stainless, which people think is better. It is – a little. It has molybdenum, so better corrosion resistance, but its high-temp strength is almost identical to 304 up to 1200°F. The big upgrade for high temps is 304H or 316H stainless steel – the “H” stands for “high carbon.” Wait, why does carbon matter here? Because at high temps, you want the steel to maintain its structure, and the higher carbon content makes it more resistant to that sensitization we talked about. 304H 0 carriage bolts can handle constant temps up to 1500°F, and even 1700°F if the cycle is slow (like once a day, not every hour). I had a customer last year who used these for a kiln’s access door – runs at 1450°F nonstop, and his bolts have been going strong for 18 months, no issues. Compare that to when he tried 304, which lasted 2 months.
But wait, there’s more. What if you’re going even higher? Like, 2000°F? Stainless won’t cut it – it starts to soften big time above 1500°F. For that, we use alloy steel bolts, specifically ones made with materials like Inconel or high-temperature carbon alloys. We carry Inconel 625 0 carriage bolts, which are basically heat-resistant superalloys. They can handle constant temps up to 2000°F, and even up to 2200°F for short periods. The downside? They’re way more expensive than standard bolts, and they’re harder to work with because they don’t thread as easily. But for applications like furnace flanges, foundry conveyors, or boiler supports where temps are consistently over 1500°F, they’re the only game in town with 0 carriage bolt size.
Now, let’s talk about common mistakes we see customers make all the time. First, guessing. I had a guy last month who bought standard 0 carriage bolts for a smoke stack that hits 900°F, because he “heard stainless works.” It was 304, lasted 3 weeks. He tried to blame us, but when we looked at his specs, he never told us it was a smoke stack, not a fence. Second, ignoring thermal expansion. Different metals expand at different rates. If you bolt a steel carriage bolt to a cast iron part that’s heating up, they expand at different speeds – that can cause the bolt to seize or snap, even if the temp is within the bolt’s rating. Third, forgetting about atmosphere. If your high-temp environment has moisture, sulfur, or other corrosive gases, that changes everything. A bolt that’s fine in a dry 1500°F kiln might corrode in 1000°F flue gas with sulfur in it.
Wait, let’s also address a question I get a lot: “Can I just use thread sealant or high-temp grease to make standard bolts work?” No, and here’s why. Grease breaks down at 500°F max – above that, it turns to carbon, which is just as bad as no lubricant when you’re tightening a bolt. Thread sealant? High-temp silicone sealant might work up to 500°F, but anything higher and it burns away, leaving the threads dry. And dry threads when tightening a bolt at high temp? You’ll strip the shank or break the head before you get it to the right torque. Don’t waste your time or money on hacks – get the right bolt first.
Now, let’s wrap this up with a quick cheat sheet, ‘cause I know you’re scrolling and don’t want to read all the nerd stuff again. For 0 carriage bolts in high temp:
- Under 500°F: Standard carbon steel (zinc or plain) works. Cheap, easy to replace if it fails.
- 500°F to 1000°F: 304 stainless, good for constant temp, avoid cycling if possible.
- 1000°F to 1700°F: 304H or 316H stainless, perfect for most industrial kilns, doors, etc.
- Over 1700°F: Inconel or other high-temp alloys, only for extreme environments where you can’t replace bolts often.
And one last note: if you’re not sure what temp your application hits, measure it first. Buy a cheap infrared thermometer, take readings at the hottest point the bolt will touch, not just the average temp. That’s where most mistakes happen.
Now, if you’re someone who’s working on a project – whether it’s a small backyard patio heater mounting, an industrial boiler bracket, a foundry conveyor, whatever – and you’re not sure if your 0 carriage bolts will hold up to the high temps, hit us up for a chat. We don’t do pushy sales stuff, we just send recommendations based on what actually works, not what makes us the most money. If you need test samples, we can send those too, no hoops to jump through. Just reach out whenever works for you.
AS1252 Bolts Series References
- ASME B18.5: Standard for Fasteners, Carriage Bolts (2020)
- Metallurgy of Stainless Steels: High-Temperature Strength and Corrosion Resistance, ASM International (2019)
- Material Properties for High-Temperature Fastener Applications, Industrial Fasteners Institute (2021)
- NIST: Thermal Expansion Coefficients for Common Bolt Materials (2022)
Ningbo Taida Fastener Manufacture Co., Ltd.
Ningbo Taida Fastener Manufacture Co., Ltd. is well-known as one of the leading 0carriage bolts manufacturers and suppliers in China. Please feel free to buy high quality 0carriage bolts at competitive price from our factory. Good service and punctual delivery are available.
Address: No.286 Galaxy Road, Chengdong Industrial Park, Xiangshan Economic Development Zone, Ningbo China
E-mail: suki@tdfasteners.com
WebSite: https://www.td-fasteners.com/
