Thermal Fatigue Cracking

Thermal fatigue cracking is the cracking that happens after a metal is repeatedly heated and cooled rapidly. Heat resistant alloys all have high coefficients of thermal expansion; most of which expand at a rate close to 1/5” per foot when heated from room temperature up to 1800°F. Since metal expands when heated and then contracts the same amount when cooled again, the metal is continually strained in alternating directions. After a number of cycles, the metal eventually yields to the constant strain leading to cracks. There are 3 important factors to consider regarding equipment that will be thermally cycled:

  • Design – the design of the material to be used in high temperature service should be loose as the material will expand and need space to grow.
  • Use Lighter Sections – Thinner sections allow for metal to cool more quickly and uniformly.
  • Grain Size – Alloys with finer grains will have better thermal cycling properties than those with larger grains. Ideally a grain size of ASTM 4.0 or finer is desirable.

Tyler Reno