Understanding Creep in Furnace Components

Modern engineering demands materials which can withstand immense heat, high pressure and significant load. Under these relentless forces components undergo a time dependent and life-limiting change known as ‘Creep’.

Creep is the slow, permanent deformation of a material that operates at high temperatures, while under constant stress over time. In furnace components this includes reformer tubes, furnace tubes, headers, and supports. Remember, creep also applies to turbine blades in power units for instance, or any other components which experience high temperatures and stress over time.

For metals, creep damage typically occurs when temperatures are above 35% of the absolute melting temperature of the material. Creep can occur well below the Yield¹ limit of the material and across a wide range of temperatures, for instance Lead (Pb) can experience creep at room temperatures while Iron (Fe) will begin to creep around 343°C.

The creep process is understood and categorised in 3 stages, which are discussed below:

The 3 Stages of Creep

1. Primary Creep (Transient Creep)

Primary creep, or transient creep, begins rapidly when components are first exposed to high temperatures and load. The primary creep rate will gradually decrease with time because the material undergoes strain hardening², which increases its resistance to further deformation.

• Material undergoes an initial (primary) high creep rate.
• The rate of primary creep slows down as the material internally strengthens.
• Primary creep accounts for a relatively small proportion of total creep life.

Primary Creep in Furnace Components

• Primary creep is frequently seen after furnace start-up or following significant temperature increases.
• Generally, it is not a major concern unless operating conditions exceed the design limits.

¹ Yield limit refers to the stress levels required for a material to be permanently deformed. Typically measured via Tensile testing. For materials without a distinct yield point, the ‘0.2% offset method’ is used to define it.

² Strain hardening (also known as work-hardening) occurs when a material becomes stronger and harder as it undergoes plastic deformation._

2. Secondary Creep (Steady-State Creep)

During secondary creep, the material reaches a balance between strain hardening and recovery mechanisms. Deformation will continue at a nearly constant rate.

• Secondary creep is the longest lasting stage of creep rate.
• Secondary creep is stable and predictable.
• Most design calculations for furnace tubes will be based on this phase.

Secondary Creep in Furnace Components

• Tubes may operate for thousands of hours in secondary creep.
• The components wall thickness, furnace operating temperatures and stress determine how long this stage may last.
• Regular inspection is required to detect when components are nearing the end of this stage.

3. Tertiary Creep (Accelerating Creep)

The deterioration of the material accelerates during Tertiary creep due to the following:

• Grain boundary damage.
• The formation of voids within the material.
• Micro-cracking.
• Wall thinning.
• Metallurgical degradation.

During this stage, the tertiary creep rate accelerates quickly until rupture occurs. Bulging or swelling of the affected tubes may become visible upon inspection. The final outcome of tertiary creep is that components will fail if the material is not replaced. This stage is critical because failure can occur suddenly once tertiary creep begins.

Tertiary Creep in Furnace Components

• Tube diameter will expand (creep bulging).
• Egg shaped tubes may be visible on inspection.
• Surface cracking will occur.
• Tubes will undergo a reduction in wall thickness.
• Components experience elevated surface temperatures.

Identifying and Managing Creep in Furnace Components

Design engineers and inspectors are most concerned with identifying the transition from secondary creep to tertiary creep, as this will indicate if components are approaching the end of their safe operating life and will need replacing. Furnace design matters in the fight against creep and of course alloy selection for your specific operation is critical.

For furnace design purposes, a key reference is ASME Section II Part D, which focuses on material properties, with data on tensile strength, yield strength, design stresses, external pressure charts, and other relevant data to determine the allowable stresses of the material. Operations should avoid overfiring; the extra temperatures may increase output but could shave years off the components operating life.

Inspection should monitor tube skin temperatures with thermocouples or infrared cameras because hot spots will identify creep. Ultrasonic testing, radiography, or even replica metallography can catch creep before it’s a headline causing failure.