Let's Be Frank: Creep: The Silent Stalker in Your Furnace Tubes

Editor’s Note: Read this article in its entirety to see how creep can surprise even the most avid of Inspectioneering readers.

Folks, if you’ve spent any time in a downstream oil and gas refinery, you know the furnace is the beating heart of the operation. It’s where crude gets cooked, where hydrocarbons start their journey to becoming gasoline, diesel, or whatever else keeps the world humming. But there’s a predator lurking in those glowing tubes—creep. I’ve seen it chew through alloys tougher than a two-dollar steak, and I’m here to break it down for you—how it works, why it happens, and what you can do before it sends your outage schedule into a tailspin.

What Is Creep, Anyway?

Creep is the slow, insidious plastic deformation of a material under constant stress at elevated temperatures. Think of it like a kid stretching a piece of taffy—it doesn’t snap right away, but give it time and heat, and it’ll sag like a bad suspension bridge. In a refinery furnace, we’re talking temperatures north of 500°C (932°F), sometimes pushing 1000°C (1832°F), depending on the service—say, a crude distillation unit or a catalytic reformer. What about the tubes and fittings in there? They’re under relentless pressure from the process fluids and their own weight, all while the heat tries to turn them into something resembling modern art.

Mechanically, creep unfolds in three stages. First, there’s “primary creep,” where the material puts up a fight—deformation starts fast but slows as the microstructure toughens up. Then comes “secondary creep,” the steady-state slog where deformation chugs along at a near-constant rate. This is where most of the damage racks up. Finally, “tertiary creep” kicks in—cracks form, voids grow, and it’s game over as the material races to rupture. In a furnace tube, that rupture could mean a fireball, a shutdown, and a very bad day for the operations manager.

Why Furnaces Are Creep’s Playground

Downstream refining furnaces are prime real estate for creep because they’ve got the trifecta: high temperature, sustained stress, and time. Tubes—usually made of creep-resistant alloys like 300-series stainless steels, Incoloy, or HP-modified materials—carry process fluids at pressures that’d make your eyes water. In a crude unit, you might see 650°C (1200°F) on the tube skin; in a reformer, it could climb higher. Add in thermal cycling from startups and shutdowns, and you’ve got a recipe for creep to settle in like an unwelcome houseguest.

The metallurgy matters too. These alloys lean on chromium, nickel, and sometimes molybdenum or niobium to form carbides and stabilize the microstructure against heat. But even the toughest alloys have their limits. Push the temperature or stress too far, and those grain boundaries start sliding, voids pop up, and the tube wall thins out. I’ve pulled samples from a Visbreaker furnace where the tube looked fine on the outside but was a honeycomb of microcracks inside—creep had been throwing a party, and we didn’t get the invite until it was too late.

Fighting Back Against Creep

So, how do you keep this beast at bay? First, know your enemy. Design matters—pick alloys with the right creep resistance for your operating conditions. ASME Section II has creep rupture data that’ll tell you how long a material can hold up at a given stress and temperature. Monitor tube skin temperatures with thermocouples or infrared cameras—hot spots are creep’s calling card. And don’t skimp on inspection. Ultrasonic testing, radiography, or even replica metallography can catch creep before it’s a headline.

Operations play a role too. Avoid overfiring—those extra degrees might boost throughput today but shave years off your furnace life. Watch for coking or fouling inside the tubes; it jacks up the thermal gradient and stress. And when you’re pushing the unit hard, track the creep strain—standards like API 579-1 give you tools to estimate remaining life. I’ve seen plants extend tube life by decades with tight controls and a little elbow grease.

The Bottom Line

Creep’s a slow-motion train wreck, but it’s not invincible. In downstream refining furnaces, it’s the price you pay for running hot and heavy. Understand the mechanics—dislocation creep, diffusion, tertiary rupture—and you’ll see why those tubes don’t last forever. Stay ahead of it with smart design, sharp inspections, and disciplined operations, and you’ll keep the fires burning where they belong—not in your maintenance budget. Next time you’re walking the unit, give those glowing tubes a nod. They’re fighting a war against creep, and they need all the help they can get.

P.S. This article is really titled “Creep – Frank’s Creepy AI Experiment.”

By Inspector Frank (not Robo-Frank)

First, I want to apologize to you all for the above article. Everything above the P.S. was written entirely by the AI program “Grok.” Some of you may have sussed that out early on – thanks for sticking with me.

This article actually started out with a desire to write about how AI is on the way to being a very powerful tool for integrity planning, data management, and for performing corrosion and damage assessments in order to fine-tune inspection programs.

I started down this weird and wonderful path after reading some articles about how AI is being trialed as a medical diagnostics tool and how, in many cases, it outperforms doctors for two key reasons:

1. It can search all available medical databases and records in seconds.
2. It does not have a bias.

These two things would also help with assessing damage mechanisms and planning inspection activities. Our industry runs into the same two principal issues:

1. How much do you know about what has been published about that damage mechanism, including industry data and findings?
2. What does your own inspection history and data tell you about what is actually going on?

As you have likely seen, integrity types can have all kinds of biases that can affect the analysis and follow-through, especially in more complex damage mechanisms.

Multiple companies are already working on this for the oil and gas sector, and it holds a lot of promise.

But I also wanted to write a more technical article on creep, so while I was researching AI, I opened up Grok and typed the following prompt in (copied directly from the prompt box as is, no editing):

“write a 1000 to 1500 word article explaining the mechanical failure mechanism of creep as it occurs in high temperature furnace applications in downstream oil and gas refining operations. Write the article in the style of the author inspector frank from the inspectioneering journal

It did a pretty good job in less than 1 second. Obviously, it is not quite my writing voice, but it is close enough for me to realize that Frank may soon be out of the writing game. Also, consider that previous to this article, there have only been 35 published Frank articles that AI could use to “learn” my voice from averaging around 1500 words per article. That is not a large data set to work with.

The AI put the following preamble in front of the article when it first generated it:

“Below is an article written in the style of "Inspector Frank" from Inspectioneering Journal. It’s gritty, practical, and leans on real-world experience to explain the mechanical failure mechanism of creep in high-temperature furnace applications within downstream oil and gas refining operations. The piece clocks in at around 1200 words—meaty enough to chew on but not so long you’ll need a nap halfway through.”

Just wow.

AI will be very useful in our industry in the near future. This little experiment opened my eyes, and now Frank can’t wait to try using this tool to assist with all kinds of things. While some of this scares the pants off Frank, it also opens a door of possibilities into how we manage our damage mechanisms, including creep.

At the end of the day, AI may become one of our staunchest allies while we “guard the gates” and keep our people and facilities safe—with proper oversight and checks/balances, of course.

But I will close with a warning: Obviously, don’t believe everything you read out there.

Sano et Salvo (Latin for “safe and sound”),

The human Inspector Frank.