A Study On The Multiaxial Fatigue And Pre-corroded Fatigue Behavior Of Nuclear Power Steels

In this paper,we studied on the multiaxial fatigue behavior of 316 LN austenitic stainless steel and pre-corroded fatigue behavior of 15MND5 bainitic steel used in pressurized water reactor(PWR)nuclear power plant.The wrought 316 LN austenitic stainless steels were widely used in primary circuit in PWR due to its good mechanical properties and corrosion resistance.Uniaxial and multiaxial fatigue(circular path,rhombus path and square path),multiaxial ratcheting fatigue tests were conducted at room temperature.The conclusions can be listed as follows:As for multiaxial fatigue tests,316 LN materials exhibited obvious non-proportional additional hardening.The circular loading paths showed superlative additional hardening.The axial and torsional effective stress response exhibited rapid initial hardening,gradual softening and stress rapidly falling to failure at high effective strain amplitude.At low strain amplitudes,the cycle stability was shown at the second stage.As for higher torsional strain,the 316 LN under circular path loading showed hardening of axial and torsional direction.Only torsional stress response showed a little higher under rhombus and square loading path.The FS model can predict the multiaxial nonproportional fatigue life better than SWT model.On the basis of both models,a new modified model was proposed and the predicted fatigue lives were well.As for multiaxial ratcheting fatigue tests,the axial-torsional stress-controlled results showed no obvious difference on axial ratcheting strain accumulation under circular path and square path.The ratcheting strain accumulation rate was higher on first stage and axial strain accumulated slowly then increased rapidly again until failure.The effects of low cycle fatigue behavior of 16MND5 bainitic steel pre-corroded in simulated PWR were also studied.The porous corrosion surface of pre-corroded materials tended to generate pits as a result of promoting contact area to the fresh metal,which promotes crack initiation.For original materials,the fatigue cracks initiated at inclusions in metal matrix.Moreover,the simulated PWR environments degrade the mechanical properties and low cycle fatigue behavior of 16MND5 steels remarkably.Pre-corrosion of 16MND5 mainly affected plastic term of Coffin-Manson equation.