Hangbo Alloy Group
Shanghai · Est. 2012
Thermal fatigue cracks nickel alloys under repeated heating and cooling. Learn the mechanism and which grades resist cyclic thermal stress in furnaces and turbines.
Thermal fatigue is cracking caused by repeated cycles of heating and cooling, where uneven expansion and contraction build up strain at notches, welds, and changes in section. Unlike creep, which acts under steady high-temperature load, thermal fatigue is driven by the temperature swing itself - every startup, shutdown, and load change adds a strain cycle. Furnace skids, reformer outlets, turbine casings, and heat-treat fixtures all live in this regime, and the wrong alloy cracks in surprisingly few cycles. This guide explains the mechanism, the metallurgy that resists it, and the grades engineers specify for cyclic heat.
When one part of a component heats faster than another, the hot region wants to expand but is held back by the cooler mass around it; on cooling the reverse happens. These constrained movements produce alternating tensile and compressive strain at stress concentrators - corners, weld toes, bolt holes, and abrupt section changes. Above a certain strain range, the material accumulates fatigue damage cycle by cycle until a crack appears.
The decisive parameter is the thermal strain range, set by the temperature swing and the alloy's coefficient of thermal expansion. A high-expansion alloy swinging through a wide temperature band sees large strain every cycle, so it cracks sooner. Low-cycle fatigue (LCF) life - the number of cycles to failure - drops sharply as the strain range rises.
Two metallurgical traits extend thermal-fatigue life. First, a low coefficient of thermal expansion keeps the strain range small for a given temperature swing; alloys such as Inconel 617 and the low-expansion superalloys are favored for exactly this. Second, high elevated-temperature ductility and crack-growth resistance let the material absorb strain without initiating cracks, and a stable, oxidation-resistant surface prevents oxide notches from acting as crack starters.
For furnace and turbine hardware the usual choices balance expansion, strength, and oxidation resistance:
Design-wise, generous radii, uniform wall sections, and strain-tolerant joints do as much for life as alloy choice. Pair this with the creep and stress-rupture guide when the component also sees steady load.
Creep is slow, time-dependent deformation under steady load at high temperature; thermal fatigue is cracking from repeated temperature swings regardless of a sustained load. A component can fail by thermal fatigue in a few hundred cycles even when the steady stress is far below the creep limit - see our creep and stress-rupture guide for the steady-load side.
Low-expansion, high-ductility grades such as Inconel 617 and Hastelloy X are staples for cyclic heat, with Alloy 602CA for the hottest, most oxidizing cycles. Furnace fixtures follow our fixture selection guide.
Use generous fillet radii, avoid abrupt section changes, keep wall thickness uniform, and specify strain-tolerant joints (bellows, flexible hangers). Controlling the temperature swing where possible - slower heat-up and cool-down - directly cuts the strain range each cycle.
As a dedicated nickel alloy manufacturer serving power generation, aerospace, chemical processing, oil & gas sectors, Hangbo Alloy Group supplies the grades discussed here across all standard product forms with full ASTM/ASME documentation and EN 10204 3.1 certification. Our material is specified for applications where corrosion resistance, high-temperature strength, and traceable quality are non-negotiable. See our production & testing facilities for more on how every heat is verified.