Stress Corrosion Tests Research Articles

炭酸ガスレーザによるステンレス鋼薄板の溶接

Recentry, laser machining has developed rapidly and its achievements have become a center of attraction. However, there are very few researches which deal with laser machining hitherto. Under such a present situation, the butt welding of thin stainless plate (SUS 304) by CO2 gas laser, was chosen as a subject of this study, that is, the influences of welding conditions (welding velocity, distance from focus of laser lens to surface of test piece) on shape and size, strength of welding part were examined. Conclusions obtained may be summarized as follow: (1) Relation among welding conditions and shape, strength of welding part was clarified. (2) It is found that the optimum welding velocity for butt welding of SUS 304 (0.2 mm thickness) in the range of this experiment is about 400-600 mm/min. (3) Judging from cracking appeared by stress corrosion test, it is considered that the residual stress of welding part, causedby CO2 laser welding is above 12 kg/mm2.

304ステンレス鋼のH2SO4-NaBr水溶液による応力腐食割れ

The influence of both strain-induced α′ martensite in Type 304 steel and NaBr concentration in a solution of 2 kmol/m3 H2SO4 at 368 K on stress corrosion characteristics, was examined. The results show that the α′ and NaBr concentration were major variables affecting the susceptibility to cracking. As the amount of α′ was increased or the NaBr concentration was reduced, the crack density increased and the time-to-fracture decreased gradually and either further increase in α′ or further decrease in NaBr concentration led to general corrosion. The attack tended to proceed along the stress-induced α′ martensite, which was anodically more active than the γ phase in the test solutions. Discussion covered the problem concerning the correlation between susceptibility to stress corrosion and the inhibiting action of bromide ions on the basis of the results of anodic polarization and weight loss measurements, and of the results of the above stress corrosion tests.

The predictability of stress corrosion cracking susceptibility of steels in acetate solutions from potentiodynamic polarization curves

Stress corrosion cracking susceptibilities of mild and low alloy steels in acetate solutions, assessed using slow strain-rate testing techniques, correlate well with those predicted on the basis of data from fast and slow sweep rate potentiodynamic polarization curves. Accurate prediction of cracking susceptibility necessitates allowances being made for changes of solution pH which occur during stress corrosion and polarization tests. A predictive technique which allows for these pH changes has been developed, and when implemented yields polarization data that indicates the influence of potential, steel composition and solution temperature on cracking susceptibility. The stress corrosion cracking of steels in acetate solutions is intergranular and occurs only when the electrochemical conditions correspond to those of a predicted potential-pH domain that is associated with the Fe 2+/Fe 3O 4 transition.

Assessing the possibility of stress corrosion cracking during acid cleaning

During chemical cleaning large volumes of inhibited acids are pumped through boilers and there is a possibility of stress corrosion cracking occurring under certain conditions. This paper examines four material/environmental combinations encountered in practice: mild steel in inhibited hydrochloric acid, mild steel, a 9%Cr-1%Mo steel and a sensitized austenitic stainless steel in inhibited citric acid. The electrochemistry of each is examined, various predictability tests performed, and these compared to the results of slow strain-rate stress corrosion tests. No stress corrosion occurred with the material-environmental-stress conditions which should be encountered in practice; however, cracking could be induced to occur by out of specification conditions. The results of the predictability tests did not correlate with the observed cracking susceptibilities. However this lack of agreement could be explained with a detailed knowledge of the electrochemistry involved in each system.

Slow strain rate stress corrosion testing at elevated temperatures and high pressures

Slow strain rate stress corrosion cracking experiments have been performed on single phase and duplex phase 304 stainless steels at 290°C. Environmental variables included chloride concentrations (0–1000 ppm), oxygen concentration (0–2 ppm) and potential ( ϕ corr to + 500 mV vs Ag/AgCl). These experiments have shown that s.c.c. resistance is relatively unaffected by Cl − if O 2 concentrations approach zero. However, at 2 ppm O 2 concentration, there is a large decrease in resistance with increasing Cl − concentrations. Anodic polarization of the steels during straining in solutions containing 100 ppm Cl − and 0 ppm O 2 showed a threshold potential for s.c.c. at ∼ 500 mV more noble than the corrosion potential (− 650 ± 60 mV vs Ag/AgCl at temperature).

The influence of plastic straining on localized and general corrosion of stainless steels

Stress corrosion cracking of stainless steels is dependent upon different factors: (a) plastic deformation, the dissolution rate of (b) passive and (c) depassivated unfilmed metal, and (d) the repassivation rate. The particular concurrence of these four processes creates the possibility of cracking. The kinetics of these phenomena seem to be the determining factors for cracking to occur. Stainless steels of several different compositions and structures (ferritic, austenitic and duplex), as well as steels of varying Mo and Si composition, have been examined by observing the influence of straining rates on depassivation, and on rates of dissolution and repassivation. Average crack propagation rates have been obtained. Structure exerts an important influence, as deformation modes are very different for alpha, gamma and duplex stainless steels. In certain ferritic steels, containing Ni and/or Cu, deformation modes such as mechanical twinning may cause very strong localized depassivation, which initiates cracking. In environments other than MgCl 2, such as dilute solutions of NaCl, repassivation kinetics seem to prevent crack propagation in spite of initial depassivation. It seems nearly impossible to define a standard stress corrosion test applicable to any metal/solution couple. A safer approach would be to define a methodology for investigating stress corrosion, putting to use the different experimental techniques relevant to each of the four processes listed above, on the alloy immersed in the actual environment, or in an environment of very similar properties.

Comportement mecanique en milieu inerte et en milieu chlorure chaud d'aciers inoxydables austenitiques: Influence de certains elements d'alliage (Ni, Mo, Ti)

The purpose of this paper is to relate some experiments which show correlations between mechanical and electrochemical parameters in S.C.C. of austenitic stainless steels. A part of the results has been analysed in various publications 1, 2, 3. To know the very particular mechanical behaviour of stainless steels it is necessary to study the stress elongation curves at different elongation rates and to know the creep behaviour of the steels. In stress corrosion test (MgCl 2 − 44% − 153°C) under dead load, the elongation time curves allow us to distinguish simply the incubation time for the propagation time of stress corrosion cracks. The differences between incubation time of the different steels are consistent with the difference of repassivation kinetics and mechanical behaviour. The study of the rates of cracks propagation allow us to show the beneficial effect of Ni and Mo on the S.C.C. of austenitic stainless steels.

Shot peening to prevent the corrosion cracking of austenitic stainless steels

Rockwell International and Metal Improvement Company have conducted a joint test program to develop shot peening as a technique for preventing corrosion cracking in austenitic stainless steels. Initial laboratory-scale tests demonstrated the feasibility of preventing stress corrosion cracking by shot peening to impose compressive stresses on the surface of the work pieces. Conventional U-bend test specimens, when peened, survived 1000 h tests in the boiling 42 pct magnesium chloride stress corrosion test. Unpeened reference specimens commonly fractured within one or two hours in this test. Component tests were conducted to demonstrate the practicality of the peening process for sizes and shapes that typify components in a reactor piping system. Pipe sections and cold worked, hexagonal tubes were tested. In all components, unpeened sections developed stress corrosion cracks within a few hours in the magnesium chloride test; in contrast, the shot peened surfaces survived hundreds of hours. It was discovered at Rockwell International that intergranular corrosion can be prevented in austenitic stainless steels by severe shot peening prior to exposure to sensitizing temperatures. For this purpose, the surfaces must be severely cold worked by the shot peening to break up surface grains and grain boundaries. Two nondestructive testing techniques show promise as methods for measuring the stresses or cold work imparted on the surface of the work piece by peening. In one method, eddy currents are used to measure differences in electrical properties induced by cold working of metals. Another is a magnetic technique which measures the changes in magnetic properties due to the transformation of austenite to ferrite by cold working.

Stress corrosion testing and data analysis

AbstractExperiments have been carried out on structural materials to investigate the effect of pre‐straining on the rupture strength. The effect of sodium chloride on aluminium and stainless steel and that of ammonium hydroxide on brass and copper has been analysed. It hat been found that the rupture strength decreases with increasing pre‐strain. A stress corrosion index has been defined and this index has been found to depend on the initial pre‐strain; a linear relation exists with the percentage of pre‐strain.

高張力鋼における応力腐食き裂の発生および成長に及ぼす前オーステナイト粒径の影響

Stress corrosion tests were conducted on the blunt-notch compact tension specimens of quenched-and-tempered SNCM 8 steel having several different prior-austenite grain-sizes in a 3.5% NaCl solution environment. The effects of grain-size on the nucleation and growth of stress corrosion cracks were analysed from a viewpoint of fracture mechanics. The susceptivity of material to stress corrosion cracking was characterized by five quantities, i. e., the time to crack nucleation at the notch root, tn, the threshold stress intensity factor, KISCC, the stress corrosion fracture toughness, KSC, the crack growth rate, (da/dt)II, and the stress intensity factor, (K)II, at the middle point of the region II in crack growth curves.The results obtained are summarized as follows:(1) The crack nucleation life tn was determined by the stress intensity factor K0, the yield strength σY and the notch root radius ρ astn=C[2K0/(σY√πρ)]mwhere C and m are constants independent of prior-austenite grain-size. The value of effective notch root radius obtained by substituting K0=KISCC and tn=240hrs into the above equation was found to increase with grain size.(2) When the plastic zone ω was smaller than the grain size d, the KISCC value was found to take a nearly constant value of 35 to 45kg/mm3/2. For the cases of ω/d>1, the values of KISCC and (K)II increased with decreasing yield strength while the value of (da/dt)II decreased. The larger grain sized material showed lower (K)II and higher (da/dt)II values when compared at the same yield strength.(3) The values of KISCC and (K)II were higher and (da/dt)II was lower in thin specimens than in thick specimens. The amount of microbranching of cracks was suggested to be a significant clue to the effects of grain size and specimen thickness on growth kinetics.

Creep and stres corrosion cracking of austenitic stainless steel in boiling 35% magnesium chloride solution

The stress-corrosion crack initiation and propagation in two austenitic stainless steel wires was studied using a 35 wt.%MgCl 2 solution at constant electrode potential. The results of stress corrosion tests were compared with those of creep tests in 35%MgCl 2 and in n-octane at 125°C. Both the SCC rate vs applied stress curve and the primary creep rate vs applied stress curve were found to exhibit a maximum followed by a minimum. This behaviour was probably associated with the mechanism of the plastic deformation of austenitic stainless steels, the competition between film failure and film repair and the effects of surface films on plasticity.

Creep and stress corrosion cracking of austenitic stainless steel in boiling 35% magnesium chloride solution

The stress-corrosion crack initiation and propagation in two austenitic stainless steel wires was studied using a 35 wt.%MgCl 2 solution at constant electrode potential. The results of stress corrosion tests were compared with those of creep tests in 35%MgCl 2 and in n-octane at 125°C. Both the SCC rate vs applied stress curve and the primary creep rate vs applied stress curve were found to exhibit a maximum followed by a minimum. This behaviour was probably associated with the mechanism of the plastic deformation of austenitic stainless steels, the competition between film failure and film repair and the effects of surface films on plasticity. † Present address: The Ohio State University, Fontana Corrosion Center, Columbus, Ohio 43210, U.S.A.

極低ひずみ速度応力腐食試験機の試作と2, 3の試験結果

極低ひずみ速度応力腐食試験機の試作と2, 3の試験結果

Flaw Growth in High Strength AI-Zn-Mg-Cu Alloys Exposed to Stress Corrosion Environments

Abstract Marine atmosphere and synthetic sea water stress corrosion tests on smooth and precracked specimens from 7075, 7050, and 7049 alloy plates (32 and 76 mm thick) show that for a given strength level, alloys 7050-T7X and 7049-T7X have superior short-transverse stress corrosion resistance (SCR) to 7075-T7X. At typical strength levels above the minimum of 7075-T6, for example, SCR of these alloys is considerably better than that of 7075-T76, and approaches that of 7075-T73. Alloys 7050-T73651 and 7049-T7351 have SL threshold stress intensities (KISCC) of about 24 MPa √m at a yield strength of 450 MPa. The corresponding KISCC value for 7075-T7X at this strength level is about 19 MPa √m. In both environments, KISCC is independent of plate thickness, per se; absolute strength level appears to be the controlling factor, KISCC being inversely proportional to yield strength. Examination of fracture surfaces and metallographic cross sections showed a considerable amount of crack blunting, crack jogging, crac...

Marine Atmosphere Stress Corrosion Tests on Precracked Specimens from High-Strength Aluminum Alloys: Effect of Corrosion-Product Wedging

Abstract Stress corrosion tests on precracked double-cantilever beam specimens from 2000- and 7000-series aluminum alloys have been in progress for up to seven years at a marine-atmosphere exposure site near Daytona Beach, Fla. One of the most significant results of these tests concerns the effects of specimen self-loading (because of exfoliation and corrosion-product wedging) on the relative crack growth resistance of different alloys. Self-loading is apparently affected by copper content but is not limited to copper-bearing alloys. Copper-free alloy 7039-T64, for example, showed self-loading effects after 3 to 4 years' exposure. Naturally aged 2XXX alloys and peak-aged, copper-containing 7XXX alloys were most sensitive to self-loading, usually showing the effects within six months. However, cracks in 7075-T73, a stress corrosion resistant alloy, also continued to propagate at rates that were not insignificant after several years' exposure, whereas alloy 2024-T851 appeared more resistant to corrosion-product wedging than 7075-T73.

A comparison of constant-load and constant-deflection stress-corrosion tests on precracked DCB specimens

A comparison is made between measurements of stress-corrosion crack propagation made by a constant-load procedure and by a constant-deflection procedure. Precracked double cantilever beam specimens from 7075 aluminum alloy plate were used. The specimens were oriented in such a way that cracking would begin in the short-transverse plane and would propagate in the rolling direction. The specimens were subjected to a buffered salt-chromate solution and a 3.6% synthetic sea salt solution. The measurements were made optically with a binocular microscope. Stress intensities and crack lengths were calculated and crack velocities were obtained. Velocity was plotted against the average calculated stress intensity. Good agreement between the two methods was found for the salt-chromate solution, although some descrepancies were noted for the artificial sea salt solution.

Tapered Tensile Specimen for Stress Corrosion Threshold Stress Testing

AbstractA new type of specimen for determining stress corrosion threshold stress is described. The specimen has a tapered gage length that provides longitudinal stress distribution with a maximum to minimum stress ratio of 1.85. After a constant load stress corrosion test, the surface of the specimen is inspected and the gage length is longitudinally sectioned. In long-time tests the minimum stress at maximum diameter at which cracking is found approaches the threshold value. Applicability of the specimen has been confirmed by tests of several materials in different environments. The specimen can also be used for the testing of stress-induced pitting.

The relationship between microstructure, deformation behavior, and stress corrosion cracking resistance of an age-hardened Ni-base alloy

A gamma prime strengthened nickel-base alloy was given five different heat treatments to form microstructures ranging from solution annealed to overaged. Stress corrosion tests in 50 pct NaOH and undeaerated water solutions at 316°C showed that material that was overaged sufficiently to produce homogeneous plastic deformation possessed greatly increased resistance to stress corrosion cracking. The results suggest that the stress corrosion resistance of other nickel-base alloys could be significantly improved by suitable heat treatments.

Ａｌ‐８％Ｍｇ合金の強さと応力腐食に及ぼす加工熱処理の影響

The tensile and stress corrosion tests were made on the Al-8%Mg alloy aged at 200°C for various periods after aging at 150°C for 24hr followed cold rolling by 0.30 and 50%. The stress corrosion behavior was tested in the 3.5% NaCl solution by loading of 80% of 0.2% proof stress of the alloy with the anodic current of 5mA/cm2. The two-step aging is not effective, but the thermomechanical treatment is effective in strengthening of the alloy. The effect of thermomechanical treatment is due to a synergetic effect of refining the structure of precipitates and presence of dislocations. The stress corrosion susceptibility is improved especially in the overaging state. This is also due to refinement of the precipitates in the matrix. It seems that presence of stabilized dislocation structure slows down propagation of cracks by affecting the mechanism of embrittling of the grain boundary area.

Analysis of specimen-fixture interaction for slow crack-growth studies

The self-loading procedures used in stress-corrosion tests have important effects on the associated stress intensities. This study examines those effects and uses finite element calculations to generate curves of stress intensity as a function of flaw size for several specimen types in common use. By means of a procedure based on compliance, the resulting data are recombined into a form suitable for crack-growth studies. A reliable method that uses these results is suggested here for measuring flaw size during long-term tests. A testing method of considerable versatility based on commonly used fracture specimens is discussed, and experimental experience with the method is described.