If there’s one constant in my 18 years as a supplier of steels for boilers and pressure vessels, it’s that the material only matters as much as the way it’s joined. Every power plant, refinery, or industrial heating unit I’ve worked with over the decades has come back to the same question: How will my chosen welding technique change the steel I bought from you? I don’t just sell steel— I sell the reliability that keeps that steel from failing when it’s holding 1,500 psi of steam at 550°C, or resisting the cyclic stress of daily start-stop cycles. Welding isn’t just melting two pieces together; it’s a transformation that can either make or break a pressure vessel, and I’ve seen firsthand how even small choices in technique turn out to make all the difference. Steels for Boilers and Pressure Vessels
Let’s start with the basics. The steels we supply—mostly carbon-manganese, low-alloy chrome-molybdenum (Cr-Mo), and a small subset of high-strength low-alloy (HSLA) steels, all tailored for boiler and pressure vessel service—are designed to balance strength, creep resistance, and toughness at extreme temperatures. But when you weld them, you introduce a heat-affected zone (HAZ), that strip of metal adjacent to the weld that never melts but still gets heated to temperatures high enough to alter its microstructure. That’s where most failures start: not at the weld itself, but in the HAZ, where grain growth, phase transformations, and residual stresses can leave the metal weaker, more brittle, or more prone to cracking. The welding technique dictates how severe that HAZ damage is, which is why we work so closely with our fabricator customers to match the right process to their steel and their application.
Take shielded metal arc welding (SMAW), commonly called stick welding, which is still the workhorse for on-site boiler repairs and large pressure vessel fabrication. It’s cheap, flexible, and doesn’t require fancy equipment, which makes it go-to for maintenance teams working in tight, remote spaces. But SMAW’s downside is that it applies high, localized heat for a relatively long time. For carbon-manganese steels, that long heat soak causes grain growth in the HAZ—bigger grains mean lower toughness, especially at low temperatures, which is a big risk for vessels that operate in cold climates. For our low-alloy Cr-Mo steels, which are used for high-temperature steam lines, the slow cooling rate after SMAW welding can lead to the formation of hard, brittle martensite in the HAZ. Martensite is great for knives, but terrible for pressure vessels; it’s prone to hydrogen-induced cracking, especially if there’s any moisture in the welding environment. I had a customer 10 years ago who cut corners on preheating a Cr-Mo steam drum repair with SMAW, skipped the post-weld heat treatment (PWHT), and ended up with a 2mm crack in the HAZ that would have become a catastrophic failure during a pressure test. We had to replace the entire drum, and they lost two weeks of production—all because they thought stick welding was a “set it and forget it” process.
Now compare that to gas tungsten arc welding (GTAW), or TIG welding, which is more precise, uses a non-consumable tungsten electrode, and delivers a concentrated heat arc. GTAW is slower and requires more skill, so it’s not ideal for large-scale fabrication, but it’s perfect for root passes—those critical first layers of a weld that form the interior of the pressure vessel, where corrosion and fatigue are most likely to start. The concentrated heat input means a smaller HAZ, less grain growth, and lower residual stress, so GTAW preserves more of the steel’s original toughness and strength. I recommend GTAW for our thinner-gauge boiler tubes, because the smaller HAZ reduces the chance of tube failure during cyclic operation. Last year, a small refinery in Texas called us to replace a set of water-wall tubes that had been failing every six months; they were using SMAW for all welds, and the HAZ in their tubes was developing fatigue cracks within a year. We suggested switching to GTAW for the root passes, combined with our standard preheat and PWHT, and those tubes have been running strong for 18 months now. The customer said the small extra cost for GTAW was nothing compared to the downtime they used to deal with.
Then there’s submerged arc welding (SAW), the workhorse of large, automated pressure vessel manufacturing. SAW uses a granular flux that covers the weld area, shielding it from atmospheric contamination and allowing very high travel speeds with deep, uniform penetration. For thick-shell pressure vessels, like those used in oil refinery hydrocrackers, SAW is the only practical way to get consistent, high-quality welds across 100mm thick steel plates. But SAW’s high heat input means a much larger HAZ than GTAW or even SMAW for thin material. For our HSLA steels, which are designed to have a fine, uniform grain structure for high strength, the slow cooling rate after SAW can cause the HAZ to soften significantly—like, 20-30% lower yield strength than the base steel. That’s a problem because the HAZ becomes the weakest point of the weld joint, even if the weld metal is strong. So for SAW welds, we always adjust the flux composition and the post-weld cooling rate to minimize that softening. I worked with a pressure vessel fabricator in Ohio a couple years back that was using SAW on 80mm thick Cr-Mo plates for a hydrocracker reactor; their initial SAW welds had HAZ softening that failed their pressure tests. We worked with them to switch to a low-carbon flux that promoted faster cooling, and adjusted the preheat temperature to 250°C instead of 150°C, and that fixed the softening issue. The reactor passed its 1.5x overpressure test with zero leaks, and it’s been in operation for three years without any HAZ issues.
Don’t let’s forget about flux-cored arc welding (FCAW), a middle ground between SMAW and SAW that’s popular for medium-sized fabrication, especially when speed is important but automation isn’t feasible. FCAW uses a tubular wire filled with flux, which provides shielding and alloying elements during welding. It’s faster than SMAW, produces fewer defects, and can be used on both thin and thick steels. But FCAW’s flux composition is critical for boiler and pressure vessel steels; if the flux has too much sulfur, it can cause hot cracking in the weld metal, especially when welding our higher-strength carbon steels. I had a customer in Alberta that used a cheap, off-brand FCAW wire on a batch of boiler drums, and 10% of the welds were rejected due to hot cracking. We switched them to a specialty FCAW wire we stock for pressure vessel applications, which has controlled sulfur levels and added manganese to improve crack resistance, and their rejection rate dropped to less than 1%. It’s a small detail, but when you’re fabricating 50 drums a month, that’s $50,000 a month in savings just from choosing the right FCAW consumable.
The biggest new trend in welding for pressure vessels is laser beam welding (LBW), which is gaining traction for thin-walled components like heat exchangers and superheater tubes. LBW uses a high-energy laser beam that delivers extremely focused heat, resulting in a HAZ that’s almost negligible—sometimes just a few millimeters wide, compared to 10-15mm for SMAW on thick material. That means minimal alteration of the steel’s microstructure, higher strength and toughness in the joint, and lower residual stress. For our advanced ultra-supercritical (A-USC) steels, which are designed to operate at 760°C and higher (way beyond the 550°C limit of standard Cr-Mo steels), LBW is the only welding process that doesn’t degrade the steel’s creep resistance. I’ve been testing LBW with a university lab in Japan for the last two years, and the welds we’ve produced have HAZ creep rates that are 40% lower than GTAW welds on the same A-USC steel. The problem is that LBW is expensive, and it’s still mostly limited to thin materials, so it won’t replace SAW or SMAW for large, thick pressure vessels anytime soon. But for the next generation of power plants that are looking to improve efficiency and lower emissions, LBW is going to be a game-changer.
No matter what welding technique you use, there are two non-negotiables that we always emphasize to our customers: preheating and post-weld heat treatment (PWHT). Preheating slows the cooling rate of the HAZ, which prevents martensite formation and reduces residual stress. For our Cr-Mo steels, we require preheat temperatures of 200-300°C, depending on thickness, to avoid cracking. PWHT, on the other hand, relieves residual stress, improves toughness, and homogenizes the microstructure of the weld and HAZ. I can’t tell you how many times fabricators skip PWHT to save time, only to end up with a failed vessel months later. Last year, a customer in Australia cut corners on PWHT for a 10mm thick carbon steel boiler tube assembly, and during a routine inspection, we found a 3mm crack in the HAZ. They had to shut down the entire plant for three days to repair it, which cost them over $2 million in lost production. That’s a huge sum, and all it would have taken was a few hours of PWHT to prevent.
As a supplier of steels for boilers and pressure vessels, my job isn’t just to send out high-quality material—it’s to help my customers understand how their welding choices will interact with that material. I’ve seen too many pressure vessel failures that weren’t caused by bad steel, but by bad welding practices: incorrect preheat, wrong consumables, skipping PWHT, choosing the wrong technique for the application. Every weld is a compromise between heat input, speed, and quality, and the best fabricators don’t just go for the fastest or cheapest process—they go for the one that preserves the steel’s performance.
If you’re fabricating a new boiler, repairing an existing pressure vessel, or just looking to reduce downtime from weld-related failures, I’m here to help. We work with fabricators, engineering firms, and plant operators across the world, providing not just steels, but also guidance on welding techniques, preheating schedules, and PWHT requirements tailored to your specific application. Don’t take chances with a component that operates under extreme pressure and temperature—reach out to us, and we’ll help you match the right steel and welding process to keep your equipment running safely and reliably for decades.
Pre-Engineered Buildings Industry References:
- ASME Boiler and Pressure Vessel Code, Section IX: Welding, Brazing, and Fusing Qualifications, 2023 Edition.
- Lippold, J.C., Kotecki, D.J., Welding Processes for Pressure Vessels and Boilers, Woodhead Publishing, 2005.
- Bhadeshia, H.K.D.H., “Welding of Advanced High-Strength Steels,” Science and Technology of Welding and Joining, Vol. 10, No. 1, 2005, pp. 1-8.
- Panda, S.K., Mukherjee, S., “Effect of Welding Techniques on Microstructure and Mechanical Properties of Cr-Mo Steels for High-Temperature Applications,” Journal of Materials Engineering and Performance, Vol. 26, No. 9, 2017, pp. 4212-4223.
- Kim, Y.J., et al., “Laser Beam Welding of Advanced Ultra-Supercritical Steels for Power Plant Applications,” International Journal of Pressure Vessels and Piping, Vol. 175, 2019, Article 103937.
Raysteel Resources
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