Creep Resistance: What It Is & How the Right Steel Prevents It

Metals are among the most durable materials on earth, but when subjected to constant stress and elevated temperatures, even the strongest materials will deform. For metal components, this phenomenon is called “creep.”

Left unaddressed, the consequences of creep include loss of efficiency, unpredictable operation, and in extreme cases, catastrophic failure. While eliminating creep entirely is not feasible, engineers mitigate its impact by estimating creep rates and selecting materials suitable to the application. Sullivan Steel offers valuable technical expertise to support these efforts.

We asked Craig Darragh, a consulting metallurgist with over 40 years of industry experience, to discuss creep and creep-resistant alloys.

Defining Creep in Steel

What is creep in steel? Creep is the change in length or dimension of a component under constant load. It is similar to metal fatigue, except that fatigue occurs under a cyclic load.

At room temperature, metals generally experience dimensional changes only when the load is changed, but creep occurs and is accelerated at higher heat levels. Creep also doesn’t happen right away. A part can creep for months or even years before breaking or showing signs that it will break.

There are three stages of creep: the primary, secondary and tertiary. The most pertinent is the second stage, the steady state, because it reveals the rate of creep over time before it reaches the third and final stage, failure.

Key Takeaways:

• All metals experience creep under a constant load.
• Creep is faster at elevated temperatures.
• The secondary stage of creep is constant and more predictable.

How Creep Impacts Components

A fastener, such as a bolt and nut in a jet engine, is a perfect example of an application with significant creep prevalence.

When a nut is tightened onto a bolt, it is then under a constant load. The tightening will cause the initial creep because of the strain associated with loading the nut and bolt, then, in the secondary stage, they will experience a fairly constant change in length, or a constant rate of creep. Finding this rate helps an engineer determine how the length changes under a consistent load and temperature over extended periods.

Further, the engineer’s concern then becomes how the impact of creep causing the engine bolt to change impacts the entire mechanism – what happens to the engine if the bolt keeps creeping? That is a primary design consideration when selecting materials for applications where creep is an issue.

Key Takeaways:

• The secondary stage of creep (where the creep rate is constant), helps with estimating how a component's length will change over time at a specific temperature.
• Engineers must consider how the change in length of a small component affects the larger system over time.

How is Creep Prevented?

Creep is typically prevented by using a creep-resistant alloy. Creep-rupture and stress-rupture data for a particular steel can help design engineers select the ideal material, considering the size and shape of the part, and what portion of that part will be subject to the effects of creep.

However, creep is just one part of a design. What creep-related data is most important to the application depends on the application itself.

Key Takeaways:

• Some alloys are more creep-resistant than others.
• Creep-rupture and stress-rupture data will help with steel component design.
• Designers should prioritize material data specific to their application.

What Makes an Alloy Creep Resistant

Creep occurs when, at the microstructural level, planes of the atoms move relative to each other. That movement can be influenced by including alloying elements with atoms of different sizes that will interrupt the slip, that is, not allow it to occur as easily.

That said, creep-resistant steels often sacrifice other properties like hardening characteristics or corrosion resistance. It is, therefore, important to which performance characteristics are most important for the steel in a given application to achieve. Sometimes that means talking to someone who really knows a lot about the alloy you want to use.

Key Takeaways:

• Creep can be mitigated by selecting alloys with the appropriate microstructure and performance characteristics for its intended application.
• Using an alloy with strong creep resistance may mean sacrificing other material properties.

Sullivan Steel Stocks Creep-Resistant Grades

Sullivan deals in specialty steels. We stock in-demand grades that deliver superior performance across a range of characteristics that are difficult to achieve with conventional steels.

For example, Greek Ascoloy (also known as 418, Alloy 418, AISI 418, and AMS 5616) possesses several advantageous performance attributes, including creep resistance up to 1,000°F (538°C). An iron-based, martensitic grade, Greek Ascoloy’s chemistry includes nickel, tungsten, and chromium. Other creep resistant grades in Sullivan Steel’s lineup include stainless bearing steels Jethete M152 and CX13VDW, both of which meet AMS 5719 Type 2. 

For components in high-temperature service where creep is a concern, talk to the experts at Sullivan. We support customers with technical and metallurgical expertise. We don’t just recommend a grade and leave it at that—we can work with you to find a steel solution that fits.

If you’re facing a complex challenge in your application, we’d be delighted to be part of the solution.

Learn more about Greek Ascoloy, Jethete M152, CX13VDW, start a Live Chat, or request a quote to get started.