Key Engineering Materials Vols. 297-300

Abstract: Effects of some environmental factors on stress corrosion cracking (SCC) of pipeline steel X-70 both in near-neutral pH environments, including NS4 solution and several solutions containing main types of soil in the eastern part of China, and in high pH solution were studied by means of electrochemical measurement and slow strain rate testing (SSRT). The anodic polarization curves showed different features in near-neutral pH and high pH solutions in terms of active-passive transition behavior. In near-neutral pH solutions, the cracking mode was transgranular with the feature of quasi-cleavage, the susceptibility to SCC increased with decreasing potential, pH and temperature as well as increasing CO2, indicating a dominant mechanism of hydrogen induced cracking (HIC). In high pH solutions, the cracking behavior was similar to that in near-neutral pH solutions when the specimens were polarized at cathodic potentials, but quite different at anodic potentials. A comparison of the electrochemical behavior with the SCC potential region indicated a dominant SCC mechanism associated with anodic dissolution (AD) of X70 in high pH solution at anodic potentials. A preliminary experimental potential (E)-pH-SCC diagram has been established for X70 in near-neutral pH environments.

Abstract: Recently, ceramics was used extensively as structural materials and ceramics components became larger and more complex. Fracture sometimes occurs during firing because of large and complex shape, and this fracture interrupts manufacturing process. The simulation of sintering has been studied to prevent this fracture. However, it was difficult to simulate fracture process because there was little data on strength of green compact. It is necessary to measure strength during sintering in order to perform a useful simulation. In this study, we measured strength of two kind of alumina green compact during sintering. Three point bending test at elevated temperature was performed and strength was estimated at each temperature. A model for strength at relative low temperature was also proposed using the temperature dependence of specific surface area. Furthermore, fracture toughness test was performed and the relationship between strength and fracture toughness was obtained. Strength at relative low temperature increased with temperature. Fracture toughness was proportional to strength at the temperature range where materials demonstrated brittle fracture manner. Strength of each alumina was analyzed using this model.

Abstract: JIS-SPV50Q high strength steel is often employed in construction of liquid petroleum gas (LPG) spherical tanks due to its high strength and good ductility. In general, post weld heat treatment is not performed after welding of SPV50Q high strength steel and welding residual stress will be retained in weldment. Service experience and inspection indicate that higher H2S concentration and welding residual stress result in the environmental failure, such as blistering or hydrogen induced cracking (HIC), sulfide stress corrosion cracking (SSCC) and stress oriented hydrogen induced cracking (SOHIC). In the present paper, the cracking behavior of SPV50Q high strength steel weldment by manual electric arc welding has been investigated in various saturate solutions with different concentrations of H2S. The results of slow strain rate testing, performed at a strain of 1×10-6s-1, reveal the presence of SSCC and HIC in the base metal adjacent to HAZ. The ffects of the different temperatures of post weld heat treatment on cracking are discussed. The suitable post weld heat treatment could increase the resistance of SPV50Q weldment on SSCC or HIC and does not decrease the mechanical properties of SPV50Q weldment.

Abstract: Steam generator began having severe problems 20 years ago. The main causes were wastage, wear, loose parts, denting. These problems were solved by changing the water chemistry treatment method. Therefore these damage types were replaced by stress corrosion cracking on the inner surface of the tubes and most recently by intergranular attack and stress corrosion cracking on the external surface of the tube. Steam generator has experienced various forms of damage, most involving some kind of corrosion. Some of these problems have been virtually eliminated, others are successfully managed, but a few have not yet been brought under control. This study is to evaluate the environmental effects on the susceptibility to stress corrosion cracking in Cr-Mo alloy under simulated crevice condition. The test is simulated by producing drop, wet and evaporation process. The XRF measurement is performed to investigate the amount and distribution of chromium carbide of grain boundaries. The tests are performed in the alkalic and acidic solution under various strain rates.

Abstract: 16MnR steel is one of C-Mn steels widely used for pressure vessels in China. Mechanical behaviors of 16MnR steel used as liquefied petroleum gas vessel for about 100,000hr have been investigated in this paper. The macrostructure, fracture toughness, fatigue-crack growth rate, and low-cycle fatigue properties of the samples have been studied in detail. Compared the results with that of GB6654-86 (one of Chinese national material standards) and other test data of 16MnR steel, it indicates that the mechanical properties of the long-term used 16MnR steel have not been degraded and aged.

Abstract: Environmental fatigue crack propagation of CF8M and CF8A steels used in the domestic nuclear power plants (NPPs) were investigated on the simulated pressurized water reactor (PWR) condition (temperature: 316°C, pressure: 15MPa). The test equipment for environmental fatigue (high temperature-high pressure loop, autoclave, load frame, and measurement system) was designed. As-received and 60-year aged specimens were used in the test. To compare with environmental fatigue test, another test in the air condition was performed. The fracture surfaces of specimens were difficult to verify the fracture modes such as striation, inter-granular crack and cleavage and so on. As the ferrite content of CF8M is increased, more particles on the fracture surface were peeled.

Abstract: Anhydrous ammonia is one of the medium where the steel is easy to suffer stress corrosion cracking. Many stress corrosion crack accidents of ammonia storage tanks have been reported all over the world for recent years. 16MnR low alloy steel with good composite mechanical properties is the most common used pressure vessel steel in China, it has better anticorrosion properties than the low alloy high strength steel, especially in anhydrous ammonia environments. Many stress corrosion crack researches have been done for 16MnR steel in wet hydrogen sulfide service for last decades, but less has been done in anhydrous ammonia environments. Stress corrosion tests with eight constant strain pre-cracked specimen were carried out in this work. The pre-cracked specimen were loaded with loading bolts and put into the special ammonia storage tank for tests, they were taken out from the tank periodically to measure crack length. The stress corrosion crack growth rate and the threshold stress intensity factors KISCC of 16MnR steel in anhydrous ammonia environments were measured. Based on test results, the effects of environmental factors and steel strength on stress corrosion cracking of 16MnR steel in anhydrous ammonia service were analyzed. It could be concluded that specimen cracks of the 16MnR steel in anhydrous ammonia environments almost do not grow, threshold stress intensity factors KISCC are higher than 90MPa m .

Abstract: We are developing a Small-Punch SCC testing apparatus with an in-situ observation system applied to high temperature water and high pressure. This new testing apparatus is used to evaluate the Environmentally Assisted Cracking effect. A constant loading rate was carried out with different values of speed, of water flow rate and different polishing of the sample. In BWR (Boiling Water Reactor) conditions (9MPa and 288°C) it was found that the loading rate, water flow rate and surface preparation of the sample have some influence if the test is performed during a short time (maximum 4 days). After the test, by SEM observation, studying the secondary microcracks, it is possible to find all the sequences of EAC process. The in-situ observation system permits to study the strain of the sample during the test.

Abstract: The effects of cold working and heat treatment on caustic stress corrosion cracking (SCC) of mill annealed (MA) alloy 800M in boiling solution of 50%NaOH+0.3%SiO2+0.3%Na2S2O3 were investigated by means of microstructure examination, tensile test, X-ray stress analysis, SCC testing of C-rings, Auger electron spectroscopy (AES), scanning electron microscopy (SEM) and metallography. The microstructure of alloy 800M under tested conditions was austenite. With a strain of 25% by cold working, the grains of alloy 800M became longer, yield strength (YS) and ultimate tensile strength (UTS) increased, elongation (δ ) decreased, residual stress and the susceptibility to SCC increased. With increasing temperature of heat treatment of alloy 800M with cold working, the grains became bigger , residual stress, YS and UTS decreased and δ increased, the susceptibility to SCC of alloy 800M decreased. In boiling caustic solution, SCC cracks on the surfaces of C-ring specimens polarized potentiostatically at –20mV/SCE initiated from pitting and propagated along grain boundaries. AES analysis indicated that the surface films on MA alloy 800M were enriched in nickel and depleted in iron and chromium.

Abstract: The effects of cold working and heat treatment on caustic stress corrosion cracking (SCC) of mill annealed (MA) alloy 800M in boiling solution of 50%NaOH+0.3%SiO2+0.3%Na2S2O3 were investigated by means of microstructure examination, tensile test, X-ray stress analysis, SCC testing of C-rings, Auger electron spectroscopy (AES), scanning electron microscopy (SEM) and metallography. The microstructure of alloy 800M under tested conditions was austenite. With a train of 25% by cold working, the grains of alloy 800M became longer, yield strength (YS) and ultimate tensile strength (UTS) increased, elongation (δ ) decreased, residual stress and the susceptibility to SCC increased. With increasing temperature of heat treatment of alloy 800M with cold working, the grains became bigger , residual stress, YS and UTS decreased and δ increased, the susceptibility to SCC of alloy 800M decreased. In boiling caustic solution, SCC cracks on the surfaces of C-ring specimens polarized potentiostatically at –20mV/SCE initiated from pitting and propagated along grain boundaries. AES analysis indicated that the surface films on MA alloy 800M were enriched in nickel and depleted in iron and chromium.