If you’ve ever stood near a large hot rolled carbon steel (HRCS) structure as it’s being fabricated or in operation, you’ve probably felt just how much heat that material can move. As someone who’s supplied HRCS to manufacturing, construction, and energy clients for over 12 years, one of the most common questions I get from engineers, project managers, and even machinists is: “What’s the thermal conductivity of hot rolled carbon steel, anyway?” It’s not a number you can pull out of thin air—there’s nuance here, from the grade of steel to the temperature it’s at, to how it’s processed, that changes that value. Today, I want to break this down in plain, practical terms, not just textbook jargon, because when you’re choosing steel for a heat exchanger, a furnace part, or a structural beam that will face extreme temperatures, this number isn’t just academic—it’s the difference between a project that works on spec and one that hits snags. Hot Rolled Carbon Steel
First, let’s start with what thermal conductivity actually is, in case you’re new to the term. At its core, it’s a material’s ability to conduct heat. Think of it like a pipe for heat: a high-conductivity material lets heat flow through it quickly, while a low-conductivity one acts more like an insulator. For metals, most of that heat is carried by free electrons zipping through the material’s atomic structure, right alongside lattice vibrations (what we sometimes call phonons). That’s why metals are generally more conductive than, say, wood or plastic.
Now, hot rolled carbon steel is a specific subset of carbon steel, defined by how it’s made: it’s rolled at temperatures above its recrystallization point—usually around 1,700 to 2,300 degrees Fahrenheit, depending on the grade—then allowed to cool at room temperature, no cold working or special heat treatment afterward. That processing leaves it with a distinct grain structure compared to cold rolled steel, and that grain structure does affect thermal conductivity. But before we get into hot rolled specifics, let’s ground this in the base numbers for carbon steel in general.
Most standard carbon steels fall into two main categories: low carbon (also called mild steel, with less than 0.3% carbon) and medium carbon (0.3% to 0.6% carbon), with high carbon being above that, up to around 1.0% for common structural grades. For HRCS, the most widely supplied grades are A36 (the workhorse of construction), A572 Grade 50, and 1018 mild steel for machining, along with A516 Grade 70 for pressure vessels and heat-resistant applications. Each of these has a slightly different thermal conductivity, and it changes with temperature, which is a big point most generic charts leave out.
Let’s get to the numbers, starting with room temperature (around 70°F, or 21°C), the point where most people look for specs. For low carbon hot rolled steel—think A36 or 1018—the thermal conductivity at room temperature is roughly 45 to 50 W/m·K (watts per meter-kelvin) or about 26 to 29 Btu/(h·ft·°F) for anyone using imperial units. Medium carbon hot rolled steel, like A572 Grade 50 or 1045, comes in a little lower, at 40 to 45 W/m·K, or 23 to 26 Btu/(h·ft·°F). High carbon HRCS, like 1095, sits around 35 to 40 W/m·K, or 20 to 23 Btu/(h·ft·°F). That drop makes sense, right? As you add more carbon, you introduce more impurities and disruptions in the metal’s atomic structure, which get in the way of those free electrons carrying heat. It’s like adding small rocks to a pipe—they slow the water down, or in this case, the heat.
But here’s the part that trips up a lot of people: those numbers only apply at room temperature. Thermal conductivity isn’t a fixed value; it changes as temperature rises, which is critical for any application where the steel will be exposed to heat. Let’s talk about what happens when we heat hot rolled carbon steel up to common operating temperatures. Most HRCS starts to soften around 1,000°F, and by the time it hits 1,500°F, it’s well into the austenite phase, where its internal structure changes entirely. For low carbon HRCS, its thermal conductivity actually stays pretty steady until about 500°F, sitting around 47 to 48 W/m·K, and then starts to drop gradually, falling to about 30 W/m·K at 1,200°F, and around 20 W/m·K by the time it reaches 1,800°F. Medium carbon HRCS follows a similar trend, just starting at a lower baseline: at 1,000°F, it’s around 35 W/m·K, dropping to 22 W/m·K at 1,500°F.
What’s interesting here is that this is different from stainless steel, which is often confused with carbon steel for high-heat applications. Stainless steel has a thermal conductivity of only around 16 to 24 W/m·K at room temperature, so it’s a worse conductor than even high carbon HRCS. That’s one reason why many heat exchangers and furnace components that need to move heat efficiently are made with hot rolled carbon steel, not stainless.
Now, you might be wondering: does the hot rolling process itself affect this number? Because that’s a big part of our business—we don’t just sell any steel, we sell hot rolled steel that’s processed to consistent specs. Let’s compare hot rolled vs. cold rolled carbon steel, since that’s a common point of confusion. Cold rolled steel is rolled at room temperature, so it has a finer, more elongated grain structure, and it’s often harder than hot rolled. At room temperature, cold rolled mild steel has a thermal conductivity almost identical to hot rolled mild steel—usually within 1 to 2 W/m·K, which is negligible for most industrial applications. The difference only really shows up at very high temperatures, above 1,000°F, where the cold worked grain structure of cold rolled steel starts to change, making its thermal conductivity slightly lower than hot rolled. For almost all practical purposes, if you’re working below 1,000°F, the processing (hot vs. cold) doesn’t make a meaningful difference in thermal conductivity—what matters is the carbon content of your HRCS grade.
But wait—there are other factors that can throw these numbers off, and I’ve seen it happen on jobs. For example, surface condition: if your hot rolled steel has a thick mill scale layer (the dark, oxidized scale that forms on the surface during hot rolling), that oxide layer is a much poorer conductor than the steel itself. Mill scale is mostly iron oxide, which has a thermal conductivity of only around 2 to 3 W/m·K at room temperature. So if you’re doing a heat transfer calculation that depends on the steel’s conductivity, you can’t forget about that mill scale—if it’s left on, it will slow heat transfer between the steel and whatever it’s touching, like another pipe or a fluid. That’s why we often tell clients that if mill scale will be in contact with hot surfaces or fluids, they should consider pickling or blasting the surface first.
Another factor is alloying elements. Most standard hot rolled carbon steel only has small amounts of manganese, silicon, and maybe copper as residual elements, so those don’t affect thermal conductivity much. But if you’re using a specialty HRCS with added chromium, molybdenum, or other alloying elements for high-temperature strength, those will lower the thermal conductivity further. For example, A516 Grade 70, a common hot rolled pressure vessel steel, has a tiny bit of silicon added for strength, so its thermal conductivity at room temp is around 43 W/m·K, a bit lower than plain A36. That’s a small difference, but for a pressure vessel holding hot fluid over thousands of cycles, it can add up to slightly more even heat distribution.
Now, I’ve learned over the years as a supplier that the generic numbers you find online aren’t always tailored to real-world use. For example, I once had a client who was designing a heat treatment oven and used a textbook thermal conductivity number for mild steel that was 55 W/m·K—way higher than the actual value for our standard 1018 HRCS. That small error made their heat transfer calculations off by almost 20%, and they almost ordered the wrong size heating elements. That’s why, when we provide specs to clients, we don’t just give a single number—we give a range that’s specific to our hot rolled grades, plus notes on how temperature, surface condition, and processing will change that value.
Let’s also talk about why this matters for you, the person buying or using hot rolled carbon steel. If you’re building a structural beam, thermal conductivity won’t be your top concern—you’ll care about yield strength and weldability. But if you’re making a component that will be exposed to heat: furnace grates, heat exchanger tubes, boiler parts, even automotive engine components, thermal conductivity is a critical spec. For example, if you’re replacing a stainless steel furnace grate with hot rolled carbon steel, you need to make sure you account for the higher conductivity—so you don’t have to add more insulation than you planned, or adjust the heat output. For a heat exchanger, choosing a low-carbon hot rolled steel with consistent thermal conductivity ensures uniform heating across the entire unit, preventing hot spots that can cause premature failure.
So, to wrap up the core takeaways about hot rolled carbon steel thermal conductivity: first, it ranges from ~35 W/m·K for high carbon grades to ~50 W/m·K for low carbon grades at room temperature. Second, it decreases as temperature rises, with the biggest drops happening above 1,000°F. Third, hot rolling vs. cold rolling barely changes this value for most industrial applications, but mill scale and alloying elements can make a noticeable difference. Fourth, generic online charts are a starting point, but always check a supplier’s specs for the exact HRCS grade you’re using.
At the end of the day, thermal conductivity is just one of many specs you need to nail when choosing hot rolled carbon steel, but it’s one that’s often overlooked in the design phase. As a hot rolled carbon steel supplier, I’ve worked with hundreds of clients to match the right grade to their application, walking through these details so they don’t run into surprises mid-project. If you’re working on a job where heat transfer is a factor, or you just want to confirm the thermal properties of a hot rolled carbon steel grade you’re using, reach out to our team to get detailed, grade-specific specs tailored to your exact needs. We can also walk through how processing, surface treatment, and operating temperatures will impact performance, so you can make the right choice without guesswork.
If you have questions about thermal conductivity, HRCS grades, or need a quote for hot rolled carbon steel for your project, don’t hesitate to get in touch with our team to discuss your requirements. We’re here to help you get the right material for the job, every time.
Stainless Steel Bar References
Cottrell, A. H. (1967). The Mechanical Properties of Matter. Wiley.
ASM International. (1992). Properties and Selection: Irons, Steels, and High-Performance Alloys. ASM Handbook, Vol. 1.
Taylor, R. P. (2001). Thermal Conductivity of Carbon Steels at Elevated Temperatures. Journal of Heat Transfer.
Wuxi Xuanyi Stainless Steel Co., Ltd.
Wuxi Xuanyi Stainless Steel Co., Ltd. is one of the most professional hot rolled carbon steel manufacturers and suppliers in China. With abundant experience, we warmly welcome you to wholesale bulk durable hot rolled carbon steel made in China here from our factory. If you have any enquiry about cooperation, please feel free to email us.
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