Influence of the Applied Stress Rate on the Stress Corrosion Cracking of 4340 and 3.5NiCrMoV Steels

Sridhar Ramamurthy (2011). Influence of the Applied Stress Rate on the Stress Corrosion Cracking of 4340 and 3.5NiCrMoV Steels PhD Thesis, School of Mechanical and Mining Engineering, The University of Queensland.

       
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Author Sridhar Ramamurthy
Thesis Title Influence of the Applied Stress Rate on the Stress Corrosion Cracking of 4340 and 3.5NiCrMoV Steels
School, Centre or Institute School of Mechanical and Mining Engineering
Institution The University of Queensland
Publication date 2011-06
Thesis type PhD Thesis
Supervisor Professor Andrej Atrens
Professor Leo Lau
Professor David Shoesmith
Total pages 348
Total colour pages 14
Total black and white pages 334
Subjects 09 Engineering
Abstract/Summary Stress Corrosion Cracking (SCC) can occur in engineering structures where stressed metallic parts are in contact with an aggressive environment, and can lead to catastrophic failures. High strength steels are particularly susceptible; increasing yield strength significantly decreases the threshold stress intensity factor, KISCC. As a consequence, SCC of high strength steels has been widely studied and is an on-going effort. In this doctoral dissertation, an effort was made to understand the SCC behaviour of two high strength steels, namely 4340 and 3.5NiCrMoV steel under the following conditions: (1) SCC behaviour in aerated 90 ºC distilled water. (2) SCC behaviour in aerated 30 ºC distilled water. (3) SCC behaviour under hydrogen charging conditions. Stress corrosion cracking experiments were conducted using the Linearly Increasing Stress Test (LIST), which was developed as part of this dissertation. LIST is an improved form of the Constant Extension Rate Test (CERT) and was used to study both crack initiation and crack propagation under the above experimental conditions. In addition, a potential drop method was developed to determine the threshold stress at which crack initiation occurred. The fracture surface morphologies were examined using scanning electron microscopy (SEM). Because the cracking is intergranular in the system of interest, localized electrochemical measurements were performed to determine the grain boundary electrochemical properties using micro-reference electrodes, and secondary ion mass spectrometry (SIMS) measurements were carried out to determine the grain boundary composition and the presence of grain boundary precipitates that could aid intergranular crack growth. LIST experiments conducted in 90 and 30 ºC distilled water showed that SCC occurred at all the applied stress rates for 4340 steel, while for 3.5NiCrMoV steel SCC did not occur at applied stress rates greater than 0.034 MPa s-1 in 90 ºC water and 0.002 MPa s-1 in 30 ºC water. For the 4340 steel in 90 ºC water, the threshold stress at which cracks initiated was controlled by the development of corrosion pits at lower applied stress rates. At both temperatures, it was shown that the applied stress rate controlled the crack tip strain rate. The crack velocity increased with the increase in the applied stress rate and the crack tip strain rate; the maximum crack velocity at both temperatures corresponded to the plateau crack velocity, vII, determined from fracture mechanics tests at the same temperatures. Moreover, no effect of the steel composition on the crack velocity could be identified for the steels under investigation in the present work, indicating a similar rate limiting step that was independent of steel composition for both steels. Finally the fracture surface morphology was mostly intergranular in both steels, and the applied stress rate did not influence the intergranular fracture surface morphology. Under hydrogen charging conditions, LISTs performed on as-quenched 4340 and 3.5NiCrMoV steels indicated that cracking occurred at all applied stress rates for both steels in contrast to the trends for experiments in 90 ºC and 30 ºC distilled water at open circuit potential where cracking did not occur at higher applied stress rates. The dynamic charging decreased the fracture stress and the threshold stress much more dramatically than that in the prior tests, however both stresses were still dependent on the applied stress rate. The measured crack velocity increased with increasing applied stress rate and the crack tip strain rate, and no effect of the steel composition on the crack velocity could be identified for the steels under investigation in the present work, indicating a similar rate limiting step that was independent of steel composition for both steels. Moreover, the exponents of the least square fit data were similar to those from 90 ºC and 30 ºC distilled water experiments, indicating that the same rate limiting step could be operating in all three sets of experiments. Finally, the fracture surface morphology was mostly intergranular fracture, with a significant amount of plasticity at higher applied stress rates. The results from the grain boundary potential measurements using Ag/AgCl micro-reference electrodes, on the as quenched 3.5NiCrMoV steel at room temperature indicated that some grain boundary potentials were more negative than their grain interiors, and the potential difference between the filmed and the film free surface was of the order of 1 V. These results suggest that, thermodynamically, the grain boundaries can dissolve preferentially and there was sufficient driving force for this dissolution, thus supporting the possibility of the anodic dissolution mechanism in this steel. SIMS imaging of as-quenched 4340 steel specimens before and after cathodic charging indicated the presence of carbides in the steel. Hydrogen appeared to be trapped at the carbide particles. Sulphides were present as discrete particles; however they did not appear to play a significant role in trapping hydrogen. The presence of carbides at the grain boundaries and the hydrogen trapped by the carbides could contribute to the anodic dissolution and hydrogen embrittlement mechanisms of stress corrosion cracking. In summary the results presented in this dissertation indicate that the applied stress rate controlled the crack-tip strain rate and that the crack velocity was dependent on both the crack tip strain rate and the applied stress rate. The maximum crack velocities increased with the increase in the applied stress rates, to a maximum of vII measured with fracture mechanics tests. Because no effect of the steel composition on the crack velocity could be identified for the steels under investigation in the present work and was dependent only on the crack-tip strain rate, the rate-limiting step for the crack propagation was controlled by the crack-tip strain rate. The grain boundary potential measurements and SIMS analysis of the grain boundaries indicate that the grain boundaries can thermodynamically provide favourable crack propagation path, thus explaining the intergranular nature of the cracking observed in this study.
Keyword Stress corrosion cracking, High Strength Steel, 4340 steel, 3.5NiCrMoV Steel
Linearly Increasing stress test (LIST), Potential drop measurements, Scanning electron microscopy (SEM), secondary ion mass spectrometry (SIMS)
Threshold stress, Fracture Stress, Crack tip strain rate, Crack velocity, Rate limiting step
Additional Notes Colour Pages: 5-10, 5-11, 5-26, 6-10, 6-11, 6-28, 6-30, 6-31, 8-7, 8-8, 8-10, 8-11, 8-13, 8-14

 
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Created: Tue, 04 Oct 2011, 14:49:56 EST by Mr Sridhar Ramamurthy on behalf of Library - Information Access Service