Formation Of Reversed Austenite Research Articles

Improvement of Fatigue Strength of Maraging Steel by Shot Peening

Rotating bending fatigue tests up to 108 cycles were carried out to investigate the effects of shot peening on the fatigue strength and the fracture mechanism in wide range of fatigue life of an 18% Ni maraging steel by using shot particles of various sizes or hardness. Fatigue strength was increased markedly by shot peening in the wide region of fatigue life. The S-N curves showed duplex S-N properties because of the transition of fracture origin from the specimen surface in the short life region to the subsurface in the long life one. Double shot peening by using superhard fine particles was effective to improve the fatigue strength for surface fracture, though the fatigue strength for an internal fracture was only slightly influenced. Fatigue strength for the internal fracture was increased by formation of reversion austenite, suppressing intergranular cracking.

Effects of Microstructure and Humidity on Fatigue Strength of Maraging Steel

Rotating bending fatigue tests were carried out for maraging steels with different grain size in moist air in order to investigate the effects of humidity, grain size and reversion austenite on fatigue strength of the steel. Fatigue strength was decreased by humidity, and the decrease in fatigue strength was large in larger grain sized steel. Both of the initiation and propagation of a crack were promoted by humidity. Many cracks initiated at the specimen surface and intergranular cracks were observed at specimen surface and on fracture surface when humidity was high. The decrease in fatigue strength by humidity was suppressed by formation of reversion austenite.

Influence of Reversion Austenite on Initiation and Propagation of Fatigue Crack of Maraging Steel.

Rotating bending fatigue tests were carried out for 18%Ni maraging steel in order to investigate the influence of reversion austenite on crack initiation and its propagation of maraging steel. Fatigue strength was increased by formation of reversion austenite. The increase in the fatigue strength was large near the fatigue limit. This was caused from that the crack initiation and its early propagation were suppressed by the formation of reversion austenite and the suppression was marked with decrease in stress level. Although the fatigue strength decreased by humidity, the formation of reversion austenite relieved the sensitivity of fatigue strength for humidity.

715 マルエージング鋼の疲労き裂の発生及び伝ぱに及ぼす逆変態オーステナイトの影響

Rotating bending fatigue tests were carried out for 18%Ni maraging steel in order to investigate the influence of reversion austenite on crack initiation and its propagation of maraging steel. Fatigue strength was increased by formation of reversion austenite. The increase in the fatigue strength was large near the fatigue limit. This was caused from that the crack initiation and its early propagation were suppressed by the formation of reversion austenite and the suppression was marked with decrease in stress level. Although the fatigue strength decreased by humidity, the formation of reversion austenite relieved the sensitivity of fatigue strength for humidity.

716 マルエージング鋼の切欠感度に及ぼす逆変態オーステナイトの影響

Rotating bending fatigue tests were carried out for 18% Ni maraging steel in order to investigate the influence of reversion austenite on notch sensitivity of maraging steel. Fatigue strength was increased by formation of reversion austenite. The increase in the fatigue strength was large near the fatigue limit. In the range of notch radius ρ larger than 0.05 mm, fatigue limit was determined by the limit for crack initiation in the material without reversion austenite, whereas a non-propagating crack was observed in only the notched specimen of ρ=0.05 mm in the material formed reversion austenite. These were caused by suppressions of the crack initiation and its early propagation due to formation of reversion austenite. However, the influence of reversion austenite on notch sensitivity was very small.

Influence of Reversion Austenite on Notch Sensitivity of Maraging Steel.

Rotating bending fatigue tests were carried out for 18% Ni maraging steel in order to investigate the influence of reversion austenite on notch sensitivity of maraging steel. Fatigue strength was increased by formation of reversion austenite. The increase in the fatigue strength was large near the fatigue limit. In the range of notch radius ρ larger than 0.05 mm, fatigue limit was determined by the limit for crack initiation in the material without reversion austenite, whereas a non-propagating crack was observed in only the notched specimen of ρ=0.05 mm in the material formed reversion austenite. These were caused by suppressions of the crack initiation and its early propagation due to formation of reversion austenite. However, the influence of reversion austenite on notch sensitivity was very small.

ショットピーニングしたマルエージング鋼の疲労強度に及ぼす逆変態オーステナイトの影響

A maraging steel has low fatigue strength for its high static strength. In the present study, the influences of shot peening and reversion austenite on fatigue strength of a maraging steel were investigated to achieve the improvement of fatigue strength of the steel. By shot peening, fatigue strength was markedly increased and fracture mode changed in order of surface, combined type of surface and interior and interior fractures with decrease of the stress level. Fatigue strength in the surface fracture region was still increased by double shot peening which improved the surface integrity i.e. surface roughness, hardness and compressive residual stress. On the other hand, fatigue strength in the interior fracture region was increased by the formation of reversion austenite where as it was not affected by the shot peening condition.

３００級１８Ｎｉマルエージング鋼の疲労特性に及ぼす時効条件および逆変態オーステナイトの影響

The influence of aging condition on fatigue strength of the 300 grade 18Ni maraging steel has been investigated in relation to the behavior of age hardening and the formation of reversion austenite. In this study, rotating bending fatigue tests were performed for three series of specimens with different aging condition; as solution-treated without aging, aged for various time at 753K which is the temperature applied for the industrial aging treatment, and overaged to form a small amount of reversion austenite. Effect of reversion austenite on fatigue strength was examined using specimens with the same static strength which had been controlled by varying aging temperature and time, namely under-aging or over-aging. The main results obtained are as follows.(1) In the case of 753K aging, the fatigue limits of specimens aged for 11ks to 48ks were nearly the same value, although an under-aged (2.8ks) specimen has as much lower value as a solution-treated specimen without aging treatment.(2) A small amount of reversion austenite is effective for increasing fatigue resistance. For instance, 2vol% of austenite was enough for improving fatigue limit of the maraging steel used, from 580MPa to 640MPa at the same hardness level of Hv 610.

１８％Ｎｉマルエージング鋼の疲労特性に及ぼす逆変態オーステナイトの影響

The influence of reversion austenite on the fatigue behavior of a maraging steel has been investigated by using 18%Ni-8%Co-5%Mo steel. The behaviors of age hardening and reversion from martensite to austenite were examined first under various aging conditions in the temperature range of 750-850K. These aging conditions were chosen so that a selected volume fraction of austenite (γvol%=1, 2, 3 or 24%) was formed in the age hardened maraging steel. The specimens were subjected to fatigue test, crack growth test and fractography. The results obtained are as follows:(1) The formation of reversion austenite results in a great improvement of fatigue strength. Especially, a small amount of austenite (2-3vol%) markedly increases the fatigue limit of a maraging steel without reducing the hardness so much. Due to the existence of reversion austenite, crack growth tends to be suppressed and the maximum length of non-propagating crack becomes longer.(2) The existence of reversion austenite improves the fatigue fracture toughness.

１８％Ｎｉマルエージング鋼の時効硬化挙動と疲労特性

The influence of aging treatment on the fatigue strength of 18 Ni-maraging steels has been investigated in relation to the behavior of age hardening and the formation of reversion austenite in an 18Ni-8Co-5Mo steel. In the present study, fatigue test was performed on two series of specimens in order to make the effect of age hardening phases on fatigue property clear; in the first series, the strength of test pieces was controlled to have the same proof strength by varying aging temperature and time, and in the second series, it was changed by varying aging time at 750K, which is usually applied as the aging temperature in the commercial 18Ni-maraging steel. Fractography was also examined to study the fracture mode. The main results obtained are as follows.(1) The fatigue limit is almost independent of the aging condition of specimens as far as they have the same proof strength. The computed fatigue strength, however, decreases when specimens have been hardened through the precipitation of low temperature phases.(2) In the case of 750K aging, specimens strengthened to around the maximum hardness (Hv 450) have the almost same value in the fatigue limit independent of small difference in the aging time, although the fatigue limit of under-aged specimens was much lower.(3) A small amount of reversion austenite, which is formed in an over-aging condition, is very effective in increasing fatigue resistance. An amount of 3 vol% austenite was enough for improving the fatigue limit of a maraging steel used.