Hydrogen-Assisted Subcritical Crack Growth Rate in Cr-Mo Steels

2014 ◽  
Author(s):  
Shinji Konosu ◽  
Ryuji Uemori ◽  
Masao Yuga ◽  
Hiroshi Yamamoto
Keyword(s):  
Growth Rate ◽  
Crack Growth Rate ◽  
Crack Growth ◽  
Subcritical Crack Growth ◽  
Temper Embrittlement ◽  
Potential Drop ◽  
Constant Load ◽  
Life Assessment ◽  
Experimental Database ◽  
Ambient Temperatures

Hydrogen substantially reduces fracture properties such as threshold stress intensity factor KIH and tearing resistance dJ/da in conventional Cr-Mo steels. In order to enable the life assessment of a reactor with a hydrogen-induced crack using a failure assessment diagram (FAD), an experimental database of hydrogen-assisted subcritical crack growth rates da/dt is requisite. However, there are very few studies concerning the effects of hydrogen- and temper-embrittlement on da/dt at ambient temperatures in 2.25Cr-1Mo steels with high and low impurity levels. In this paper, vacuum melted lab heats of 2.25Cr-1Mo steel were supplied with compositional controls. Some specimens were embrittled by step cooling heat treatment (SCHT). Subcritical crack growth rate at a constant load was obtained by means of the potential drop method for 2.25Cr-1Mo steel with initial internal hydrogen (3.2 mass ppm).

Failure Analysis and Life Assessment of Thermal Fatigue Crack Growth in a Nickel-Base Superalloy Based on EBSD Method

2015 ◽  
Author(s):  
Daisuke Kobayashi ◽  
Masamichi Miyabe ◽  
Masahiro Achiwa
Keyword(s):  
Growth Rate ◽  
Fatigue Crack ◽  
Crack Growth Rate ◽  
Fatigue Crack Growth ◽  
Crack Growth ◽  
Fatigue Crack Growth Rate ◽  
Fracture Mode ◽  
Fracture Morphology ◽  
Life Assessment ◽  
The Individual

In the case of failure incidents involving important components, it is necessary to clarify the fracture mechanism by failure analysis. In the case of conventional steel materials, according to the individual fracture mode the fracture surfaces have unique fracture morphology corresponding to tensile, impact, creep and fatigue conditions. We can identify the mechanism of a fracture by observing its fracture surface, and this is known as the fractography. However regarding nickel-base superalloys, any differences in fracture morphology are unfortunately barely distinguishable, which makes it difficult to conduct fractography. In this paper, in order to characterize the damage behavior of IN738LC, the misorientation analysis within grains by using the electron backscattered diffraction (EBSD) method across almost all the whole range of specimens has been carried out. As a result, it was found that the cross section of fracture samples have unique distinguishable morphology corresponding to the individual fracture mode. Furthermore, the striations corresponding to the fatigue crack growth rate was found in the crack cross-sectional sample. It was considered that the EBSD striation observed on the cross section reveals the fatigue crack growth rate, as with striations found in the fatigue fracture surface such as conventional steel materials. On the case study of the actual (service and damaged) gas turbine blade, the EBSD analysis as the fractography revealed the mechanism of cracking and the fatigue crack growth rate. Thus, it is concluded that the misorientation analysis of damage materials allows the qualitative estimation of the fracture mode and the quantitative life assessment of the fatigue crack growth.

Novel Concepts on the Growth of Corrosion Fatigue Small and Short Cracks

2015 ◽  
Vol 227 ◽  
pp. 3-6
Author(s):  
Mikolaj Lukaszewicz ◽  
Shen Gi Zhou ◽  
Alan Turnbull
Keyword(s):  
Growth Rate ◽  
Crack Growth Rate ◽  
Crack Growth ◽  
Corrosion Fatigue ◽  
Potential Drop ◽  
Crack Size ◽  
Short Crack ◽  
Additional Contribution ◽  
Small Cracks ◽  
Turbine Steel

Corrosion fatigue small, short and long crack growth rates have been determined for a 12Cr steam turbine steel in aerated 300 ppb Cl- + 300 ppb SO42- solution and in air at 90 °C. The crack growth rate for short and long cracks was monitored by direct current potential drop (DCPD) and for the small cracks by combining high resolution optical microscopy and DCPD. Comparison of the fatigue growth rate demonstrated that in solution the short crack growth rate was remarkably enhanced in comparison to long cracks, when the crack size is smaller than 250 μm. This enhancement was attributed to the electrochemical crack size effect associated with greater anodic polarisation of the short crack in such low conductivity solution. However, such enhanced growth was not observed for small cracks, which was rationalised on the basis of additional contribution of current from the pit limiting crack-tip polarisation.

scholarly journals An Approach to Estimate SCC Growing Rate in Slow Strain Rate Tensile Test Based on EPFEM

2019 ◽  
Vol 2019 ◽  
pp. 1-7 ◽  
Author(s):  
Shuai Wang ◽  
He Xue ◽  
Yinghao Cui ◽  
Fuqiang Yang ◽  
Rui Guo
Keyword(s):  
Growth Rate ◽  
Plastic Deformation ◽  
Crack Growth Rate ◽  
Tensile Test ◽  
Real Time ◽  
Crack Growth ◽  
Potential Drop ◽  
Light Water ◽  
The Relationship ◽  
Current Potential

The slow strain rate tensile test (SSRT) is a common means to investigate stress corrosion cracking (SCC) in key engineering structural materials of light-water reactors, and it is an important task to real-time monitor the crack growing length and rate of the specimen during the test. Because the specimen is placed in an autoclave with high-pressure and high-temperature water environment-simulated light-water reactor, the current potential drop method, which includes current potential drop (DCPD) and alternating current potential drop (ACPD), is the main means to real-time monitor crack growth rate in the SCC test. As a supplementary means to obtain the crack growth rate during the test, the SSRT process of nickel-based Alloy 600 CT specimens is investigated by using the elastic-plastic finite element method (EPFEM) in this paper. Based on the consideration that both the elastic-plastic deformation and crack length of the specimen would affect the relationship between the load and the displacement of the loading point during the SSRT test, the relationship between the loading point displacement caused by crack propagation ΔLc and plastic deformation ΔLp is separated by EPFEM. Then, the SCC crack growth rate and the real-time crack length are obtained. This proposed approach could be used to improve the test results in the SSRT test.

Life Assessment of an HP-IP Rotor Under Creep Service Conditions

1990 ◽  
Vol 112 (2) ◽  
pp. 237-242
Author(s):  
V. P. Swaminathan ◽  
N. S. Cheruvu ◽  
A. Saxena
Keyword(s):  
Growth Rate ◽  
Crack Growth Rate ◽  
Crack Initiation ◽  
Crack Propagation ◽  
Crack Growth ◽  
Creep Crack Growth ◽  
Life Assessment ◽  
Propagation Time ◽  
Operating Life ◽  
Prediction Approach

A CrMoV high and intermediate-pressure (HP-IP) rotor exhibited in-service cracking in the blade groove walls. Cracks initiated from a notch and propagated under creep conditions to a depth of 0.255 in. (6.5 mm). Since the unit was base loaded, the contribution of cyclic loading (startups and shutdowns) was not significant. The total life of the groove walls is predicted by applying a life prediction approach that accounts for both crack initiation and propagation. Crack initiation life is based on short-term creep tests extrapolated to long times using a time-temperature parameter. Crack propagation time is estimated using a newly developed time-dependent fracture mechanics concept. A parameter Ct is used to express the crack growth rate under creep loading conditions. Pertinent material properties such as creep deformation and creep crack growth rate were obtained by testing the material from the subject rotor. The results obtained by this analysis show that about 50 percent of the estimated life was spent in crack propagation. Predicted service life time compares very favorably with the actual operating life of the rotor.

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