Influence of heat treatment on fatigue behaviour of 4130 AISI steel

The 4130 steel is widely used in petroleum and gas industry. During the use, it can be exposed to high temperatures for long duration as well as to severe cyclic loading as a consequence of start and shut down procedure. Both have an effect on microstructure and on mechanical strength, especially fatigue resistance. The goal of this study is to characterize the influence of a thermal treatment on high temperature fatigue behaviour of this steel.The material has been investigated in its as-received condition and after a high temperature heat treatment (around 1000 °C and slowly cooled down in calm air). The microstructure changes from a bainitic to a ferrito-pearlitic one. The fatigue tests are conducted at 450 °C under total strain control ranging from to 1.5%. It is shown that the ferrito pearlitic steel exhibits a primary and secondary hardening while the bainitic one is ore stable after the initial hardening. The fatigue resistance is better for the ferrito pearlitic steel than for the bainitic steel when plastic strain variation is considered but the conclusion inverses with the cyclic stress amplitude. Crack nucleation as well as crack growth were found to be promoted in the bainitic steel.