An Investigation of Elevated Temperature Fatigue Crack Initiation in 2 and 1/4 CR-1 MO Low Alloy Steel (original) (raw)
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Procedia Engineering, 2010
The effects of temperature as well as the initial hardness in the range 42 to 50HRC on the fatigue behaviour and life of a 5%Cr tool steel are investigated. Total strain controlled isothermal fatigue experiments between 200°C and 600°C are carried out under 1Hz frequency. The significant softening induced by tempering and/or cyclic loading is related to a strong reduction of the dislocation density clearly observed by transmission electron microscopy and x-ray diffraction. The coalescence of secondary carbides is also involved in the yield strength decrease above 550°C and during cyclic loading. Increasing the initial hardness reduces the cyclic softening rate and increases the fatigue life. Moreover, several isothermal fatigue experiments with regular interruptions at different fractions of life are performed in order to investigate the crack initiation mechanisms. Three main crack initiation sites were observed depending on testing temperatures: non-metallic inclusions, prior austenitic grain boundary and lath boundary.
Mechanisms of fatigue crack growth in low alloy steel
Acta Metallurgica, 1973
A study has been made of fatigue crack propagation in a low alloy steel which is subject to temper embrittlement. Effects of mean stress on the growth rate have been examined and comparisons between temper embrittled and unembrittled conditions have been made. Whereas ...
Fatigue Crack Growth in High Temperature Alloys
Superalloys 1984 (Fifth International Symposium), 1984
Fatigue crack growth rates of a variety of high temperature alloys (nickel-base, iron-base) were investigated at various temperatures in air and in vacuum. Growth rates at intermediate cyclic stress intensity ranges and also in the threshold regime were studied. At room temperature the materials show a rather similar crack growth behaviour in the Paris regime. Differences in the threshold regime can be explained by microstructural and closure effects. Even in vacuum the temperature dependence of the Paris regime cannot be explained only by the temperature dependence of Young's modulus. Oxidation predominantly influences the crack growth rates at elevated temperatures. Under these conditions crack growth in the threshold regime is governed by mechanisms similar to those at room temperature but additionally by oxidation induced mechanisms (crack branching, oxide induced closure) occur.
Analysis of high temperature fatigue crack growth behavior
International Journal of Fatigue, 1997
High temperature fatigue crack growth has been examined in the light of the new concepts developed by the authors. We observe that the high temperature crack growth behavior can be explained using the two intrinsic parameters ⌬K and K max , without invoking crack closure concepts. The two-parameter requirement implies that two driving forces are required simultaneously to cause fatigue cracks to grow. This results in two thresholds that must be exceeded to initiate the growth. Of the two, the cyclic threshold part ⌬K * th is related to the cyclic plasticity, while the static threshold K * max is related to the breaking of the crack tip bonds. It is experimentally observed that the latter is relatively more sensitive to temperature, crack tip environment and slip mode. With increasing test temperature, the cycledependent damage process becomes more time-dependent, with the effect that crack growth is dominated by K max. Thus, in all such fracture processes, whether it is an overload fracture or subcritical crack growth involving stress corrosion, sustained load, creep, fatigue or combinations thereof, K max (or an equivalent non-linear parameter such as J max) remains as one essential driving force contributing to the final material separation. Under fatigue conditions, cyclic amplitude ⌬K (or an equivalent non-linear parameter like ⌬J) becomes the second necessary driving force needed to induce the characteristic cyclic damage for crack growth. Cyclic damage then reduces the role of K max required for crack growth at the expense of ⌬K.
Influence of temperature and exploitation period on fatigue crack growth param.PDF
The influence of exploitation period and temperature on the fatigue crack growth parameters in different regions of a welded joint is analysed for new and exploited low-alloyed Cr-Mo steel A-387 Gr. B. The parent metal is a part of a reactor mantle which was exploited for over 40 years, and recently replaced with new material. Fatigue crack growth parameters, threshold value K th , coefficient C and exponent m, have been determined, both at room and exploitation temperature. Based on testing results, fatigue crack growth resistance in different regions of welded joint is analysed in order to justify the selected welding procedure specification.
The near-threshold fatigue crack propagation behaviour of a high-strength low-alloy steel has been investigated in ambient air and in vacuum so as to determine the role of the environment precisely. The analysis of the results is conducted by taking crack closure effects into account. It is concluded that fatigue crack growth rates measured in ambient air depend upon three processes: intrinsic fatigue crack propagation as observed in vacuum, adsorption of water vapour molecules on freshly created rupture surfaces, which enhances crack propagation, and a subsequent step of hydrogen-assisted cracking. The appearance of intergranular ruptures and oxide layers on rupture surfaces in ambient air is also discussed.
Materials Science and Engineering: A, 1991
Cylindrical specimens of 12% CrMoV steel were tested under high temperature, low cycle fatigue (HTLCF) conditions to investigate the growth of microcracks which nucleate at the surface. Results are presented which indicate that the microcracks maintain an equilibrium shape under HTLCF conditions, making it possible to estimate crack depth from a measure of the crack length along the surface. The individual growth of these microstructurally small cracks was monitored using a surface replication technique during periodic interruptions of the experiments. Crack growth rate data for different microcracks are compared on the basis of the cyclic J-integral AJ to examine the use of fracture mechanics for predicting HTLCF microcrack growth. A determination of AJ is shown to be successful in characterizing the growth of fatigue microcracks in 12% Cr steel at elevated temperatures despite differences in overall orientation at the surface. Good agreement with fatigue crack growth data for through-cracks in CrMoV steels at high temperatures further demonstrates the usefulness of AJ for predicting HTLCF microcrack growth. The results also suggest that large extrapolations of the observed correlation between AJ and crack growth rate could result in unacceptable errors owing to the relative contribution of oxidation damage at the crack tip. 0921-5093/91/$3.50
Materials Science and Engineering: A, 1994
Based on the S-N (stress cycling) data of Ti-6A1-4V alloys and high Cr-Ni steels (JN1 and YUS170) at 4, 77 and 293 K, the fatigue strength and crack initiation behavior in the long-life range were investigated in this study. Ti-6A1-4V rolled alloys and YUS 170 steel exhibited a good fatigue strength in the long-life range at cryogenic temperatures. In Ti-6A1-4V forged alloys and JN1 steel, however, there was a rather sharp drop in the fatigue strength at 4 and 77 K between 106 and 10 7 cycles. Subsurface crack initiation occurred in the long-life range for each material, and the location of its site was determined. The size of the subsurface crack initiation site obviously increased for Ti-6AI-4V forged alloys and JNI steel as the number of cycles increased. We then proposed that the fatigue strength in the long-life range for the present alloys is related to the microcrack growth behavior in their microstructure.
Modelling short fatigue crack growth in a heat-treated low-alloy steel
International Journal of Fatigue, 1995
Fatigue tests have been conducted both in laboratory air and using an intermittent air/corrosion fatigue sequential cycling technique. All tests were performed on smooth specimens made from a quenched and tempered steel and subjected to fully reversed torsional loading. Crack initiation and growth behaviour was recorded using a surface replication technique. Debonding at the metal matrix/ inclusion interface appears to be the main cause of the formation of short cracks. The growth of these short cracks dominates the fatigue lifetime, being significantly influenced by the material microstructure. Empirical models for short and long crack growth were developed by employing elastic plastic fracture mechanics parameters to predict the total fatigue life of a specimen. Later, these crack growth equations derived from air fatigue tests conducted at stresses above the fatigue limit were applied to the crack growth results obtained from intermittent air fatigue/corrosion fatigue tests carried out at sub-fatigue limit stress levels. A reasonable agreement was observed between predicted and experimental crack growth rates (
Journal of Mechanical Science and Technology, 2017
The fatigue crack growth behavior of Mod.9Cr-1Mo steel was investigated as a function of temperature, loading frequency and R ratios in the Paris regime. The relationship between fatigue crack growth rate and stress intensity factor range was acquired for each test condition. The results revealed that crack growth rate was accelerated with increasing temperature and decreasing loading frequency. The influence of the R ratio on crack growth rate was only pronounced at the low loading frequency condition. In order to understand the crack growth mechanism, activation energy analysis and normalized K D analysis were performed. This study suggests that oxidation and the degradation of mechanical properties promote crack growth behavior.