Kazuhiro Morino

Fatigue Crack Growth Resistance in Ni-Base Super Alloy

Rotating bending fatigue tests were carried out to investigate the influence of grain size on the resistance to crack growth of Ni-base super alloy, Inconel 718, using the materials with grain sizes of about 20, 50,100 and 200 たm, at room temperature. The larger grain size, the smaller crack growth rate, though the static strength was decreased with increase in grain size. The growth rate of a small crack was uniquely determined by the term ja na (Small crack growth law), where ja and a are the stress amplitude and the crack length, respectively, and n is a constant. The resistance to crack growth among materials was evaluated based on the law and showed a good correlation with tensile strength jB, meaning that fatigue life can be predicted by using only j B BB.

Fatigue Properties of Ni-Base Superalloy in Long Life Region

Fatigue properties of Inconel 718 at 500°C were investigated under rotating bending. Fatigue strength was higher at 500°C than at room temperature. Fracture occurred by the initiation and propagation of a surface crack in short life region. The early propagation of a surface crack smaller than 20-30 μm was suppressed by oxidation at 500°C. This is a main reason for the higher fatigue strength at 500°C. However, the crack growth after the retardation of a small crack growth at 500°C was accelerated reflecting the lower static strength. On the other hand, in long life region, though a surface crack initiated at the early stage of fatigue life similar to the case in short life region, the crack did not propagate over 20-30 μm, and an internal fracture occurred. Consequently S-N curve at 500°C showed a duplex property and the fatigue limit was not recognized. The subsurface damage was initiated at the early stage of fatigue life.

Fatigue Strength of Nitrided 18Ni 300 Grade Maraging Steel

In the present paper, the influences of nitriding on the fatigue strength and the fracture mechanism of maraging steel were investigated under rotating bending. Nitridings were carried out at 753K and 843K for 1h. The fatigue strength was increased by nitriding due to hardening and formation of compressive residual stress in the surface layer. However, the increase in fatigue strength and the fracture mechanism were markedly influenced by nitriding temperature. That is, the fatigue strength was markedly increased in the specimen nitrided at 753K, and the increase in fatigue strength was very small in the specimen nitrided at 843K. Moreover, fracturing of the specimens nitrided at 753K occurred from internal inclusions in the long life region, though the surface fracture occurred in the short life region of the specimens nitrided at 753K and in the whole life region of the specimens nitrided at 843K. Consequently, S-N curve in the specimen nitrided at 753K showed a two-step shape. These results were discussed from viewpoints of hardening and brittleness due to nitriding. Introduction Recently, applications of high strength metals have been increasing, because service conditions of machines and structures have become sever and sever. Maraging steel is an ultra-high strength steel which has both high strength and high ductility [1,2]. However, fatigue strength is relatively low in comparison with the high static strength. This is mainly caused by its high notch sensitivity [3]. Therefore, many studies on improvement of fatigue strength of this steel have been carried out from the points of view of controlling the microstructure and surface modification [4-8]. For example, nitriding is an effective treatment for wear, corrosion and fatigue, however, the effect of nitriding on the fatigue strength is different depending on the nitriding condition, material, stress level and so on [9-11]. On the other hand, in high strength steels and surface modified steels, fracture mechanism changes from surface fracture to internal fracture with the decrease in stress level in super long fatigue region and the presence of fatigue limit for internal fracture is not clarified [12-14]. In the present study, rotating bending fatigue tests were carried out for maraging steels nitrided at different temperatures in order to investigate the influence of nitriding condition on the fatigue strength. Material and Experimental Procedures Material used was a 300 grade maraging steel whose chemical composition in mass percent was 0.005C, 0.03Si, 0.04Mn, 0.002S, 18.69Ni, 4.89Mo, 8.92Co, 0.10Al, 0.91Ti and remainder Fe. Specimens were machined after solution treatment of 1093K for 1h. Figure 1 shows the shape and dimensions of specimen. Prior to aging or nitriding, all the specimens were electro-polished about 20 m from the surface layer to remove the scratch and work-hardened layer. Aging was carried out for 2h at 753K or 843K to achieve the maximum strengths in both of tensile and fatigue or form the reversion austenite [6], Key Engineering Materials Online: 2003-10-15 ISSN: 1662-9795, Vols. 251-252, pp 33-40 doi:10.4028/www.scientific.net/KEM.251-252.33

Fatigue Strength of Radical Nitrided Spheroidal Graphite Cast Iron

In order to investigate the effect of radical nitriding on fatigue strength of spheroidal graphite cast iron FCD400, rotating bending fatigue tests were carried out at room temperature. Fatigue strength was increased by nitriding. Fracture started from microshrinkages or abnormal graphites in the same manner as that of as cast iron. However, fracture mode was different according to the stress level. That is, fracture occurred from specimen surface at high stress levels and it occurred from defects in subsurface at low stress levels. The cause of these results was the hardened layer and the compressive residual stress, which suppressed the initiation of the surface crack.

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

Study of Crack Propagation Behaviors on Low Cycle Fatigue in Spheroidal Graphite Cast Iron based on Observation of Surface and Fracture Section.

In order to make clear the effect of the crack coalescence for crack growth rate, low cycle fatigue tests were carried out using two kinds of spheroidal graphite cast iron (SGI). The crack propagation behaviors from the initiation crack to the fracture were investigated to observe surface and fracture section. When a main crack was connected with preexisting cracks starting from microshrinkages on neighboring surface, crack growth rate was accelerated. After connection, the rate was suppressed. So, the crack growth curve in SGI showed the state of steps. As a whole, its crack growth rate was the same as average rate in SGI. That is, the crack coalescence was not the main factor for accelerating crack growth rate. In addition, it was clear that the large variation of the crack growth curves in SGI was caused by the crack coalescence and the difference of the crack initiation length.