Kazunori Fukada

Fatigue Strength Of A Radical Nitrided Ni-baseSuper Alloy

Radical nitriding was applied to Ni-base super alloy, Alloy 718, which is difficult to nitride and the fatigue strength of the alloy was also investigated in comparison with the aged alloy under push-pull loading at room temperature and 500°C. By radical nitriding, compound layer of about 5 μm was formed and specimen surface was hardened over HV1300. Consequently, fatigue strength was increased in wide life region, especially beyond 10 cycles at room temperature. This was mainly caused by the suppression of crack initiation due to formation of hard compound layer. There was no or little difference in fatigue strengths between specimen removed nitrided layer by electro-polishing and aged specimen. That is, radical nitriding method is effective to improve the fatigue strength at room temperature without the damage of matrix.

Ultrasonic Fatigue of Radical Nitrided Ni-Base Superalloy

Ultrasonic and rotating bending fatigue tests were carried out for aged and nitrided Ni-base super alloys to investigate the effects of loading frequency and nitriding on fatigue strength. Loading frequencies were 19.5 kHz under ultrasonic and 50 Hz under rotating bending, respectively. Fatigue strength under ultrasonic was higher than that under rotating bending in both alloys. Moreover, in both tests, fatigue strength was improved by nitriding. The increase in fatigue strength by nitriding was large in ultrasonic fatigue. These results were discussed through the successive observation of fatigue process at specimen surface and fracture surface observation.

Effect of Radical Nitriding on Fatigue Strength of Ni-Base Superalloy

In order to investigate the effect of nitriding on the crack initiation and propagation behavior of Ni-base super alloy, Alloy 718, rotating bending fatigue tests were carried out until 108 cycles at room temperature. By nitriding at 500°C for 12h, compound layer of about 5μm in thickness was formed and the initiation of a fatigue crack was strongly suppressed causing the increase in fatigue strength. A crack initiated in brittle manner at the compound layer in all of fractures. However the crack propagated in ductile manner controlled by the property of the base alloy. That is, there is no or little influence of nitriding on the crack growth rate of the alloy.

Internal fracture process in fatigue of radical nitrided bearing steel

Rotating bending fatigue tests were carried out for a radical nitrided bearing steel in order to investigate the initiation and propagation behavior of an internal crack. Surface fracture occurred under high stress levels and fish-eye fracture happened under low stress levels so that the S-N curve appeared in the shape of two-step lines. The depth of the origin of internal fracture was confined in a relatively narrow region near the boundary between the hardened and nonhardened matrix. By two-step loading fatigue tests in the internal fracture region, a circular trace corresponding to the stress change was observed w i t h the fisheye. In the internal fracture region, a crack was initiated at the early stage of the fatigue process, that is, most of the fatigue life of internal fracture was occupied by the growth life of an internal crack.

On Fish-eye Fracture Of Radical Nitrided Die Steel.

In order to mhestlgate the effect of radical mtndmg on fatlgue sbength of die steel, rotatmg bending fatigue tests \\ere carned out at room tempeiahre and 150°C Fatigue st~ength u a s increased by mtridmg at both temperatures due to surface hardenmg and compressne resldual stress generated in the surface region Comparing a i th the non-iutnded steel in uhich the fiactuie o q i n \\as at i~uface slip bands 01 surface defects the mtrided steel kactured m-ith the ongin at Inner mclusions at both temperahues In tlie mtrided steel, cracks mtlated from mclusions in the non-hardened lajer or near the boundarl, behleen hardened and non-ha~dened layers The difference In fatigue strength betneen the h\o temperatures \\as discussed bdsed on the feature of fish-eye