Creep‐fatigue Crack Growth and Fractography of a Type 304 Stainless Steel at Elevated Temperature

A study into microstructural effects and crack growth behaviour of AISI type 316 stainless steel under creep-fatigue conditions at 550°C within the high strain ranges of 0.9-2.5%, including a 60 min hold time, was undertaken on a high-temperature reverse-bending rig. Throughout the tests, surface cracks on both the tensile-hold and the compressive-hold sides were monitored by means of a plastic-strip replication technique. Additional investigations were conducted on failed specimens to examine the crack morphology in the depth direction, and to examine the function of oxidation; also to study changes of fracture surface morphology, changes in dislocation structures and precipitate configurations corresponding to the different strain ranges. These detailed analyses revealed that the predominantly intergranular long cracks on the tensile-hold side and transgranular short cracks on the compressivehold side are dominant aspects of the investigation. The dislocation structures under creep-fatigue conditions are strain-range dependent, with a clearly defined cell structure at the higher strain ranges and dense dislocation tangles at lower strain ranges. The large reduction in creep-fatigue endurance can be attributed to early crack growth and grain boundary cracking caused by stress relaxation, oxidation, precipitation and, most importantly, the coalescence of the many minor surface short cracks.

Section: Mechanisms Of Creep-fatigue Failurementioning

confidence: 99%

Crack Growth Morphology and Microstructural Changes in 316 Stainless Steel Under Creep‐fatigue Cycling

Gao¹,

Brown²,

Miller³

1995

“…2(d)] is of a mixed type involving intergranular creep and transgranular fatigue in spite of the longer hold time in which more creep rather than fatigue is expected. The difference can be attributed to recovery of grain boundary sliding during the compression hold period in C-C loading as discussed in the previous paper [ 1 11. In C-P loading, grain boundary sliding during the tension hold period would be accumulated by repeated application of the tension hold because there is no compression hold period, resulting in intergranular creep fracture.…”

Section: An Additional Remark On the Fracture Morphology In C-p And Cmentioning

confidence: 94%

Creep‐fatigue Crack Growth and Fractography of Metallic Materials in Air and Vacuum at Elevated Temperatures

Koterazawa

1993

Self Cite

Creep-fatigue crack growth behaviour of a Type 304 stainless steel, a quenched 2$r-1 Mo steel, Hastelloy X, a Ti-6242 alloy, and a low carbon steel under different reversed loading patterns (P-P, C-P and C-C) were investigated in air and a vacuum environment. The results are discussed in the light of fracture mechanics and fractography. Crack growth rates for all of the materials tested were successfully correlated in terms of the cyclic J integral range (AJ) irrespective of the loading patterns. In the low growth rate region, where fatigue fracture was predominant, crack growth rates of all the materials were about the same for the same value of AJ. On the other hand, growth rates were somewhat different, depending on the creep ductility of the material in the region of high growth rate, where creep fracture was predominant. Materials with lower ductility exhibited higher growth rates for the same AJ values. Differences were insignificant between the crack growth rates in air and vacuum and were consistent with the small differences observed in the fracture surface morphology in the two environments. NOMENCLATURE a, a, = semi-crack length, initial semi-crack length da/dN =crack growth rate per cycle A{ = cyclic J integral range AK = stress intensity range E = Young's modulus J = modified J integral (= C * ) P = load S = area of P-6 hysteresis loop 6 = displacement over the gauge length of specimen or at the centre of the crack o = nominal stress in static test W, B, b =width, thickness and ligament length of specimen omax = maximum nominal stress in cyclic test

“…The specimens were heated by electric furnace and maintained at 650 k 1°C during the test. An intermittent overstress with a very short hold time t h (5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20) s in most of the tests) was applied at intervals oft, (0.75-24 h), i.e. long enough when compared with the hold time of overstress.…”

Section: Test Proceduresmentioning

confidence: 99%

Acceleration of Crack Growth Under Intermittent Overloading at Elevated Temperature

Koterazawa

Nosho

1992

Self Cite

Ahtract-Acceleration of crack growth by intermittent overloading was investigated at 650°C by using specimens of different thickness made of Type 304 stainless steel. When the hold time of overload was very short (-20 s), the crack growth rate was significantly accelerated (20-50 times) and the fracture surface morphology showed extremely ductile transgranular fracture by glide plane decohesion or microvoid coalescence, suggesting significant recovery of the material. In the thinner plate specimens, the crack growth rate under intermittent overloading was correlated well with the modified J-integral, i.e. f (C*) and agreed with the growth rate of static creep cracks in a f versus da/dt diagram. In the thicker plate specimens, however, this is not the case and the growth rate was about 20% of that in the thinner plate specimens in the f diagram. Transgranular fatigue type crack growth appeared in the low growth rate region.