Investigations On Uniaxial Tension And Fatigue Behaviors Of Cr17 High-Purity Ferritic Stainless Steel

Because of their low cost and excellent properties,the high-purity ferritic stainless steels have been widely used in the fields of construction,automotive manufacturing,etc.Under service conditions,high-purity ferritic stainless steels are usually subjected to different kinds of loads such as unidirectional,cyclic and other complex loads.Therefore,it is of significantly importance to study the mechanical behaviors of high-purity ferritic stainless steels under unidirectional and cyclic loading conditions.However,investigations concerning these aspects have been rarely carried out so far.In light of this,the Cr17 high-purity ferritic stainless steel is selected as the target material.The uniaxial tensile behaviors at different strain rates and the fatigue behaviors at different constant stress amplitudes as well as the effects of specimen thickness on the uniaxial tensile and fatigue behaviors are systematically studied,in order for providing a reference for their applications in practical engineering.The results of the uniaxial tensile tests show that,with increasing strain rate,the yield strength ?YS and the ultimate tensile strength ?UTS along with the uniform elongation? increase,stemming from the activation of more slip systems at a high strain rate in Cr17 high-purity ferritic stainless steel,which enhances the interaction between dislocations as well as the uniform deformation capacity.More slip systems are activated and deformation becomes stronger on the surface near the fracture,and the number and size of dimples increase on the fracture surface with increasing strain rate.The dislocation structures change from dislocation tangles to cell-wall structure as the strain rate rises from 10-4s-1 to 10-2s-1.The tension-tension fatigue tests of specimens with 0.4 mm and 0.8 mm thickness show that the fatigue life Nf reduces with increasing stress amplitude ??/2 and the Basquin linear relationship between Nf and ??/2 is behaved;The intergranular and sliding cracks on the surface near the fracture of the specimen with 0.4 mm thickness are observed and the intergranular cracks become more obvious on the surface far from the fracture at high stress amplitudes,and cracks along slip bands exist on the surface near the fracture of specimen with 0.8 mm thickness and the intergranular cracks are only observed on the surface far from the fracture at high stress amplitudes.On the fracture surfaces of the specimen with 0.4 mm thickness,only instantaneous fracture zone is found and the number and size of dimples increase with increasing ??/2,but the fatigue resource,cracks propagation zone and rapid fracture zone are all found on the fracture surface of the specimen with 0.8 mm thickness and the crack propagation exhibits an obvious cleavage feature;also,the number and size of dimples increase with increasing ??/2.The corresponding dislocation structures change from the dislocation tangles and loose cells at low ??/2 to the cell-wall structures at high??/2.The uniaxial tension and fatigue properties for Cr17 high-purity ferritic stainless steel exhibit an obvious size effect.As the thickness t of specimen reduces from 0.80 to 0.55 mm,the ? is nearly unchanged,and the ?linearly decreases with the t reducing continuously;the?YS decreases as the thickness t of specimen reduces from 0.80 to 0.35 mm and then remains basically constant;the ?UTS slowly reduces with decreasing t and then rapidly decreases at t<0.55 mm.The differently oriented slip bands are gradually operated and surface bulges are enhanced as the thickness t increases;meanwhile,the size and number of dimples on the fracture surface rise obviously,and the corresponding dislocation structures evolve from separate single dislocations to dislocation tangles and cells as well as cell-wall structures.At the same stress amplitude,the fatigue life Nf reduces with decreasing thickness t.The deformation on the surface becomes weaker lightly and the number and size of dimples on the fracture surface decrease as well as the internal dislocation structures change from dislocation cells and cell-wall to dislocation tangles as the t reduces.There are few dislocation resources inside the thin specimen and they are easy to slide out of the surface,leading to a low capacity of dislocations to coordinate the plastic deformation,and the fatigue life of the thin specimen thus becomes lowered.