Thermal fatigue is a specific type of fatigue failure mechanism that is induced by cyclic stresses generated due to repetitive fluctuations in the temperature. These temperature change in the material induces thermal expansion (or contraction) and if surrounding material or external constraints hinder this expansion, thermal stresses arise. The degree of damage is affected by the magnitude and frequency of the temperature swings and typically appears in the form of one or more surface cracks on the component.

This paper presents two case histories of thermal fatigue failure along with detailed metallurgical analysis of stainless steel pressure components in a downstream Aromatics plant.

The first case study is on failure of a UNS 31803 duplex stainless steel heat exchanger tubing that occurred after four and half years in service. The damage was in the form of multiple parallel circumferential surface cracks with few cracks propagating through the wall thickness of tube. The equipment was subjected to a temperature differential of about 100 °C every 4 hours of operation during alternate heating & cooling cycle.

The second failure discussed is on 6 inch diameter UNS(1) 32100 process piping after around ten years of service. This is primarily a thermal mix point wherein demineralized water is injected in a hot gas stream resulting in a temperature gradient of more than 100 °C. The failure mode was also characterized by many circumferential cracks at location of injection as well as about 2-3 diameters downstream injection point.

Subject
External surfaces, Water, Materials, Piping, Microscopy, Tubes, Corrosion cracking, Stress cracking, Mechanical failure, Internal surfaces, Metals, Thermal fatigue, Cracks
Keywords:
Thermal Fatigue, Stainless Steel, Cyclic stresses, Temperature differential
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2019
Association for Materials Protection and Performance (AMPP)
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