Resistance of a material against repeated temperature change in continuous operation.
Temperature changes exert very high load on the material. They usually result in a limited service life of the material; the material may become useless due to cracks or deformations. The challenge for engineers is that thermal fatigue resistance or thermal shock resistance is not a basic material parameter which can be calculated such as hardness or tensile strength; it rather depicts sophisticated characteristics which very much depend on the operational conditions. Basically, we differentiate between four main types:
1. So-called firing cracks which occur on the hot side of the work piece in the form of a coherent network of cracks. In this connection, larger cracks throughout the whole wall thickness may occur.
2. Deformations which impair the function of the component or prevent its reconstruction after assembly. Deformations especially occur in components made of nodular graphite cast iron.
3. Large continuous cracks which destroy the component with the first temperature changes. These cracks must be considered as defects because they are based on incorrectly composed materials, wrong structure, casting defects, excessive internal tensions or on a construction which is not adapted to loads due to temperature changes.
4. High level of internal and external oxidation: Although this failure is primarily caused by high operating temperatures, temperature changes considerably accelerate scaling and internal oxidation.
The reason for all these different types of material damage and destruction are tensions which are caused by constraints in thermal expansion or contraction when heating and cooling the component. They are proportional to the temperature difference, the elasticity module and the thermal expansion coefficient of the material. They may occur if a cyclically heated component is rigidly clamped, or due to temperature differences in one part during heating and cooling. The simple relation applies to the tensions σth occurring under this conditions (eq 1):
E = elasticity module
Î± = thermal expansion coefficient
Î”T = temperature difference
S = mold factor
Î³ = Poisson’s ratio
However, this equation can neither be used for the quantitative estimation of tension nor for the calculation of a characteristic material value for thermal fatigue resistance. Therefore, it cannot be used as a basis for construction. The reason is that the material parameters used for this equation depend on temperature and time.
Thermal stresses are compressive stresses for the warmer part of the casting and tensile stresses for the colder part. In some cases, thermal stresses may almost reach or even exceed tensile strength which easily causes cracks. They usually occur after a certain number of temperature changes when the high fatigue strength is exceeded.
Almost all cracks occur during the cooling phase unless the compressive stresses duirng the heating of the warm part are so high that the tensile strength in the cold part of the component is exceeded.