Research On Fatigue Properties Of X80 Large Strain Pipeline Steel

Tensile test, microhardness test, metallographic microscope, scanning electron microscope(SEM) and electron back-scattered diffraction(EBSD) are employed to study microstructure and mechanic properties of X80 large strain pipeline steel. The experimental results showed that yield strength is 636 MPa, tensile strength is 723 MPa,elongation rate is 16.2% and vickers hardness is 225 HV for X80 large strain pipeline steel.It’s microstructure is made up of polygonal ferrite bainite structure and a small amount of M-A. In the whole section, ferrite and bainite ratio is about 50~60%.Fatigue test are carried out on a MTS Landmark 370 type servo hydraulic fatigue testing machine to research fatigue behavior of X80 large strain pipeline steel and fitting the data to draw the fatigue limit. The experimental results showed that Paris linear regression equation is S=2973N-0.14 and the fatigue limit is about 480 MPa of X80 large strain pipeline steel with the R=0.1. The fatigue crack propagation rate test was performed using an INSTRON8874 uniwersal testing machine. The experimental results showed that △k can divide thecurve of da/dn-△k into AB and BC two parts and △k=30MPa*m1/2at that time. Finally,the fatigue fracture surface and crack growth rate and propagation path were analyzed. It showed that the fracture surfaces of the large stress level and small stress level are both ductile and brittle mixed fracture,fatigue crack produced at the surface of the rolling surface of the steel plate; In fatigue crack propagation process, high-angle grain boundaries of ferrite, high strength bainite, tiny M/A constituent in the bainite and the second crack which exist in ferrite and bainite interface also can restrain the crack propagation.