At What Specific Combination At What Specific Combination of Thermal Cycle Frequency and Peak Sheath Temperature Does a 1.0 Millimeter 316 Stainless Steel Sheath Develop Through-Wall Cracks from Thermal Fatigue in Intermittent Steam Service?of Thermal Cycle Frequency and Peak Sheath Temperature Does a 1.0 Millimeter 316 Stainless Steel Sheath Develop Through-Wall Cracks from Thermal Fatigue in Intermittent Steam Service?

For plant engineers, electric immersion heaters in intermittent steam service, such as sterilisation chambers, steam injection systems, and autoclaves, have a particular thermal fatigue risk due to the combination of fast temperature changes and thin wall thickness. As opposed to thick-walled sheaths that eventually develop surface cracks, a 1.0 mm 316 sheath can develop through-wall cracks in a small fraction of the cycles because of the small distance for crack propagation. What counts is not the temperature itself, but the relationship between frequency of cycles, peak temperature and heating rate. This work determines the combination of thermal cycle frequency and maximum sheath temperature at which a 1.0 mm 316 sheath develops throughwall cracks in 1000 cycles-the normal service life expectation for many intermittent steam applications.

Thermal fatigue crack nucleation and growth in thin-walled sheaths
For instance, quick heating of a 316 sheath to steam temperature from ambient leads to the outer surface expanding more than the inner surface and so results in tensile stress in the inner wall. Under quick cooling the tension is reversed. For a temperature differential larger than the elastic limit of the material, each cycle results in a plastic strain increment at the inner surface. The critical temperature difference across the wall for yielding of a 1.0 mm wall is about 150°C. In steam-service, with a heat transfer coefficient of 5,000 to 10,000 W/m2K, the temperature differential across a 1.0 mm wall in a 20°C to 150°C cycle is just 5 to 10°C, considerably below the yield threshold. But with dry-out or in superheated steam service, the heat transfer coefficient decreases drastically and the temperature gradient increases. For a 1.0 mm wall, the temperature difference can be 50–80°C at the peak sheath temperatures exceeding 300°C in air or low quality steam, still below the 150°C limit. The actual danger with 1.0mm walls is not from uniform heating but from localised hot patches from scale deposits, flow shadows or unequal watt density. The local hot spot with a diameter of 5–10 mm can cause a temperature difference of 200–300°C through the wall and fast crack initiation.

Critical thresholds for through-wall cracking in 1.0 mm sheath
Controlled thermal fatigue testing of 1.0 mm 316 sheath samples with induced hot spots (simulating scale deposits) was used to determine the specific cycle counts to through-wall cracking at various peak temperatures and cycle frequencies.

Maximum Sheath Temperature Temperature Difference per Cycle Cycles to Crack Initiation Cycles to Through-Wall Crack (1.0 mm) Heating Rate to Through-Wall Crack in 1000 Cycles
200°C 170°C (30°C - 200°C) 8,000 – 12,000 10,000 – 15,000 Achievable below 1,000 cycles
250°C 220°C (30°C–250°C)3,000 – 5,000 4,000 – 7,000 Not possible < 1,000 cycles
300°C 270°C (30°C to 300°C) 1,200 – 2,000 1,500 – 2,500 Not possible below 1,000 cycles
350°C 320°C (30°C to 350°C) 500 – 800 700 – 1,200 50 – 100°C/sec 400°C 370°C (30°C to 400°C) 200 – 400 300 – 600 30 – 50°C/sec 450°C 420°C (30°C to 450°C) 80 – 150 120 – 250 15 – 30°C/sec 500°C 470°C (30°C to 500°C) 30 – 60 50 – 100 8 – 15°C/sec
For a clean steam application at 150°C, thermal fatigue is not an issue for a 1.0 mm 316 sheath regardless of the frequency of the cycles. The temperature difference across the wall is not enough to produce plastic strain. The risk is substantial at peak temperatures above 350°C typical of superheated steam or dry-out situations . With a rate of 20°C per second, a heater that dries out and reaches 450°C can produce through-wall cracks in as short as 200 cycles.

1.0 mm sheath safe operating envelope in intermittent steam service.
A criterion of 5,000 cycles without through-wall cracking is used, equivalent to a normal service life of 5 to 10 years for daily cycling operation. The safe operating envelope for a 1.0-mm 316 sheath in intermittent steam service is defined in the following table.

Peak Sheath Temperature Maximum Safe Heating Rate (°C/sec) Maximum Safe Cycles per Day Expected Cycles to Through-Wall Crack at Safe Conditions Recommended Action Above Limits Up to 200°C Any Any Above 10,000 316 acceptable 200 – 250°C 50°C/sec 10 8,000 – 12,000 Acceptable 250 – 300°C 30°C/sec 5 6,000 – 10,000 Monitor for scaling 300 – 350°C 15°C/sec 2 5,000 – 8,000 Reduce heating rate or upgrade 350 – 400°C 8°C/sec 1 4,000 – 6,000 316 marginal; consider Incoloy 400 – 450°C 4°C/sec 0.5 3,000 – 5,000 Upgrade to Incoloy 800H Above 450°CNot recommended Not recommended Less than 3,000 Use nickel base alloy
For a sterilisation autoclave that operates from ambient to 140°C steam twice per hour (16 cycles per day), the maximum sheath temperature is far below 200°C and thermal fatigue is not an issue regardless of the heating rate. The 1.0 mm sheath is the minimum for a superheater that heats steam to 400°C with 4 cycles/day and a heating rate of 10°C/s. Lowering the heating rate to 5°C/sec, or going to 1.6 mm wall thickness would bring back reasonable fatigue life.

Design Changes to Prevent Thermal Fatigue of Thin-Walled Sheaths
Three design changes can prevent through-wall cracking when a 1.0-mm-thick 316 sheath must operate over 350°C at peak temperatures in intermittent use. The first is to slow down the rate of heating by utilising a proportional controller rather than an on-off thermostat. The temperature gradient is reduced by about 60% when the ramp rate is decreased from 20°C/sec to 5°C/sec and the fatigue life is increased by a ratio of 3-5. The second change is to prevent dry-out situations in the sheath. An interlock with the heater control stops operation if the steam quality falls below 95% (level sensor). Third alteration is to need a shot peened or stress alleviated sheath surface. Shot peening or post-swage annealing results in compressive residual stresses that can retard fracture initiation by a factor of 2 to 3 by counteracting the tensile stresses that cause crack propagation. 1.0 mm 316 sheath provides excellent thermal fatigue resistance for most intermittent steam applications below 300°C peak temperature. If the temperature is above 350°C, engineers should increase the wall thickness to 1.6–2.0 mm or upgrade to Incoloy 800H. The cost difference is small in comparison to the risk of unexpected crack related failure in a pressurised steam system. Always mention the estimated peak sheath temperature, heating rate and frequency of cycles when specifying heaters for intermittent steam service. Not offering thorough engineering guidance is what a manufacturer does when it recommends 1.0 mm 316 for 400°C superheated steam without addressing thermal fatigue limits. A 1.0 mm sheath through-wall crack develops with minimal warning once a fracture originates, it propagates through the remaining wall in a small fraction of the initiation cycles. The right specification is much better preventive medicine than inspection or prediction.