Studies on the Fatigue Fracture of Tufftrided Steel

Observation of Notch Root of Crack by Means of Micro Beam X-Rays

Abstract

As previously reported the present authors carried out experimental investigations on the fatigue fracture of tufftrided steel, with emphasis on observation of changes in residual stress due to cyclic stressing, by rotary bending using the cylindrical type specimen, and by reversed bending using the plate type specimen, and pointed out that no change was observed either in residual stress or in micro structure during the fatigue process. In the process of fatigue fracture there are three stages, the crack initiation, the crack propagation and the fracture, and the process appears only in the locally limited area of the tufftrided steel specimens.
In order to clarify the mechanism of fatigue in tufftride treatment it is required that suitable experimental method will be devised for observation of the fatigue process within the limited area, and to meet this requirement the specimen for this experiment must be notched, so that the initiation of fatigue crack and its propagation may be easily observed. In view of the fact that the X-ray micro beam diffraction technique has many advantages for enacting observation of the changes in the microstructure within the limited area under cyclic stressing, the investigation on the tufftrided low carbon steel specimen was performed through observing the changes in the microstructure during the fatigue process, using the optical microscope by the X-ray micro beam diffraction method.
The results obtained in this way are summarized as follows.
(1) Under the same number of cyclic stressing to fracture crack occurs in the tufftrided specimen at its notch root in the earlier period of total fatigue life than in the non-tufftrided specimen, while crack propagation rate is lower in the tufftrided specimen than in the non-tufftrided specimen.
(2) The area of plastic deformation in the vicinity of fatigue crack of the tufftrided steel is very small, and narrow straight slip lines in parallel are observed the area.
(3) When the carbon steel is nitrified by tufftriding, the incomplete substructure are formed in the nitrified layer.
(4) The increases of total misorientation, micro lattice strain and excess dislocation density and the decrease of subgrain size at the notch root are observed in the earlier period of total fatigue life. These variables are maintained, however, to stand almost at constant value even after the crack initiation.