Topic:   FUNDAMENTALS OF MECHANICS AND MECHANISMS OF STRESS CORROSION CRACKING -CRACK TIP MECHANICS AND STRESS ENHANCED OXIDATION

Abstract

Stress corrosion cracking (SCC) of structural materials in nuclear power plants fabricated withstainless steels, nickel based alloys and carbon or low alloy steels hasbeen receiving a great concerns for elucidation of the cracking mechanism and for development of life time prediction technology. The generalized crack growth rate (CGR) formulation developed by Fracture Research Institute shown below (hereafter dented as FRI model)is based on a deformation/oxidation mechanism and a theoretical crack tip strain rate equation derived by the author.

The effects of crack tipoxidation, crack tip mechanics and their interactions on crack growth are taken into consideration in developing the above formulation forquantitative prediction of SCC CGRlaboratory data and actual plant data of CGR for those materialsboth in BWR and in PWR primary water have beenevaluated from a view point of this CGR formulation, emphasizing the effects of temperature, K, yield strength and K variations with time. The generalized formulation provides a unique basis for interpreting CGR as well as a unified prediction procedure for CGR. The procedure by use of this equation can plotthe CGR data within a narrow scattered band even under various testing condition.

Accurate disposition of flaws found in plants depends on qualified databases and prediction scheme. Accelerated CGR tests have been used in laboratories to obtain the relevant CGR data in a shorter time by modifying some of the testing condition such as temperature, loading condition. This is also worthwhile to mention that the use of hardened materials by cold work etc also accelerate the CGR of interest. This acceleration in CGR is necessary to predict the CGR behavior in the plants where a long-term CGR prediction is of importance and have to be predicted from the acceleration CGR test results obtained in laboratories. An accelerated test should be fast, mechanism-consistent and result-extrapolative. Quantitative models, such as the FRI generalized CGR model, provide a useful insight to correlate the accelerated test results to real plant behavior. As suggested in the formulation shown above, the test duration can be shortened by several ways such as by increasing the oxidation rate constant, κa, by increasing the crack tip strain rate, or by decreasing m. For example, change in water chemistry can increase κa for BWR by increase in dissolved oxygen. For nickel base alloys, such as alloy 600 and alloy 182 weldments in PWR primary water chemistry conditions, in temperature ranges between ca. 300℃ (water) and 400℃ (superheated steam), which are relevant to plant operation temperatures, there appears to be a continuous IGSCC mechanism. Also, activation energy has been generally accepted to be ca. 180 kJ/mol for SCC initiation and ca. 130 kJ/mol for crack growth. The oxidation rate constant, κa, significantly increases with increasing temperature. Testing at high temperature is a possible way to get qualified CGR data in shorter test durations. For example, for a SCC initiation test or a SCC propagation test, increasing the temperature from 290℃ to 360℃, CGR can increase by ca. 21 timesfaster. Increasing K or YS in a certain range can also increase the crack tip strain rate for the same material in the same environment without changing the cracking mechanism. For example, an increase in YS from 200MPa to 600MPa can result in an increase in CGR by a factor of ca. 10. So test time can be greatly reduced by using materials with higher YS. This method is especially useful for evaluating materials with relatively low SCC sensitivity. This method might also work for shortening test duration when a rather long term test is required to get a rather long SCC crack that is necessary for precise evaluation of the CGR of weldment specimens based on irregular crack fronts.  Meanwhile, caution should be taken to avoid too severe deformation that may possibly change the cracking mechanism.

Recent analysis on surface oxides and also crack tip oxides by in-situ Micro Raman Spectroscopy, in-situ Contact Electric Resistance technique and direct TEM/AES analysis provide a useful information on the protectiveness of surface oxides which may most likely to control the ka of the system of materials and environments. Extensive work has been done on the in-situ and ex-situ oxide analysis on the surface of smooth SSRT specimens as well as crack surfaces of specimens as well as real crack taken from the NPP. Detailed analysis on oxides reveals various important aspects of oxidation kinetics and equilibrium. Based upon such an evaluation on oxides, it is strongly recommended to put the research emphasis in analyzing the dynamics of oxidation at the interfaces of water/outer oxide/inner oxide/metal. Special emphasis should also be focused on the existence of affected layer at the inner oxide/underlying metal interface since this region is an oxidation front. Some new program focused on this behavior will be also introduced in the lecture.

By combining two approaches of macroscopic and microscopic analysis of the oxidation kinetics, CGR can be predicted in more deterministic way by the FRI generalized crack growth model based upon more accurate determination of fundamental parameters such as oxidation kinetics and stress effects on oxidation. The unified parameter in the FRI generalized CGR formulation, crack tip strain ratecan be used to quantify CGR emphasizing the effects of K, YS, dK/dt, and temperature from a view point of interface oxidation kinetics.

The FRI generalized theoretical CGR model provides a deterministic and unified way to quantify SCC behavior of austenitic alloys such as stainless steels, nickel base alloys and weldments in LWR environments, especially concerning the effects of multiple parameters relating to materials, environments and mechanics.

 

Acknowledgments

This work has been supported by the Grant-in Aid for COE Research (No. 11CE2003) and by PEACE II program, which has been supported by the Ministry of Education, Culture, Sports, Science and Technology, Japan, EPRI(USA), SKI(SWEDEN), EDF(FRANCE), TEPCO, KEPCO, ToEPCO, JAPCO, HITACHI Ltd., MHI and TOSHIBA Co. The author wishes to thank Prof. Z-P Lu and Dr. Y. Takeda of Fracture Reliability Research Institute, Tohoku University for their stimulating discussion.

 

Curriculum Vitae -- Tetsuo Shoji