Austenitic CrNi Research Articles

Effect of heating to remove welding stresses on sensitizing of austenitic Cr-Ni steel

1. Sensitizing at 600–650°C for 3–10 h for stress relief after welding does not induce susceptibility to ICC in standard tests in sulfuric acid solution. 2. Heating of weldments inchemical apparatus made of steel Kh18N10T operating in weakly corrosive media does not sensitize the steel.

Pitting corrosion and intergranular attack of austenitic Cr-Ni stainless steels in NaSCN

The susceptibility of I8Cr-10Ni-2Mo stainless steel to corrosion in 30%NaSCN solutionsat 74 °C has been investigated by means of potentiostatic and potentiokinetic polarization techniques. Solution heat-treated specimens of this steel show pitting corrosion in the potential range between − 200 and + 200 mV (NHE). Heat treatment at 650°C raises the rate of pitting corrosion and leads to intergranular attack. In this case corrosion may be detected over almost the entire passive and transpassive regions. In the search for compositional effects, we found that Mo shows the most pronounced effect inincreasing the corrosion resistance, whereas C has the reverse effect. The stabilizing elements Ti and Nb decrease the susceptibility to intergranular attack considerably, but lead to an increase in the rate of pitting corrosion. Experiments relating the corrosion to temperature and nature of the cation are described. Theresults are discussed in terms of structural heterogeneity and breakdown of passivity under the influence of anion adsorption.

Pitting corrosion of an austenitic Cr-Ni stainless steel in H 2SO 4 containing H 2S

Potentiostatic and potentiokinetic polarization measurements, mainly at 20 °C, have been made on 18Cr-18Ni-2Mo stainless steel specimens in H 2SO 4 solutions containing H 2S. This steel is subject to pitting corrosion in the potential range between −50 and + 200mV (nhe) which is close to the region of active metal dissolution. The occurrence of pitting is related to the incomplete passivity of the steel surface. Furthermore, the presence of H 2S has been found to be an essential condition in the formation of pits. Experiments relating pitting to pH, sulphate concentration, temperature and alloy composition, are described. Finally, an example of this type of pitting corrosion in the practice of rayon spinning is discussed.

準安定Cr-Niオーステナイトステンレス鋼の常温付近における機械的性質の特異性

Mechanical properties at ambient temperatures were studied in detail on metastable austenitic Cr-Ni stainless steels and the relations between their particularity and structural changes were discussed.The metastable austenitic steels is strengthened by the occurrence of martensite during deformation and the increment in strength corresponds to the amount of martensite. Fracture elongation shows the maximum value at the temperature or the composition, at which the following two conditions are satisfied.(1) A large amount of deformation is available in austenite before martensitic transformation.(2) Once local contraction occurs, martensite is gradually induced and the increment in strength caused by this martensite is large enough to cancel the increment in stress at the spot where the local contraction takes place.When the condition (2) is satisfied, plasticity is increased as well as strength. This phenomenon is attributed to the martensite induced during deformation and may be called “Transformation Plasticity”.The stability of austenite is indicated with the temperature at which the maximum elongation is obtained.

準安定オーステナイトステンレス鋼の逆変態とその過程に現われる異常膨張

An anomalous expansion in the dilatation curve of cold-rolled 18-8 stainless steel was incidentally found at about 530°C.The present experiments were made in order to clarify the cause of the anomalous expansion observed in the dilatation curve of some meta-stable austenitic Cr-Ni steels coldworked at various temperature, and also to clarify the behaviour of the reverse transformation of martensite to austenite in these steels, mainly by the thermal dilatation method and the micro-structural observations.When a cold worked 18-8 steel was heated at constant velocity of heating, the martensite in the steel transformed to austenite reversely step by step at the temperature range of 400-900°C.The first transformation began at 400°C, and the greater part of martensite transformed to austenite martensitically at 500-650°C, and the second at 680-760°C, the third at 800-900°C, respectively.However, the amount of transforming martensite decreased less and less as temperature raised.In such reverse transformation, the diffusionless process occurred at early stage and the diffusional process continued in after the first stage.The lath type martensite was separated along {111} plane, by growing austenite nucleated at that plane, and gradually thinned off.The second anomalous expansion was newly found in this experiment at the second reverse transformation temperature range, in addition to the anomalous first expansion previously found at the first reverse transformation range.The anomaly appeared in two phase steels consisting of the predominant martensite and retained austenite in proper phase ratio, regardless the steels were cold-worked or not.It was considered that the anomalous expansion is related to the martensite which transformed diffusionally and to the reverse transformation character of textured martensite to the certain oriented austenite.

Die ergebnisse elektrochemischer untersuchungsverfahren der spannungsrikorrosion und ihre praktische anwendung

New results were found by electrochemical investigations of stress-corrosion cracking, enlarging the knowledge of this mechanism of reaction. There is a grain activation-potential boundary (breakdown potential) at the intercrystalline stress cracking of unalloyed steel in nitrate solutions. This activation-potential decreases with increasing mechanical stresses. In alkali solutions the potential range of crack formation is very small. This stress-corrosion cracking of unalloyed steels can be prevented by anodic protection. The site of the activation-potential of austenitic Cr-Ni steels in chloride solutions depends clearly on the mechanical stresses. By cathodic protection, the protection-potential-limit for preventing transcrystalline cracks is also dependent on the mechanical stresses. With increased stressing this limit is removed to negative potential values. The rarely observed transcrystalline stress-corrosion cracking of Cu-alloyed austenitic Cr-Ni-Mo steels in pure sulphuric acid is dependent on the formation of Cu-containing protective layers, showing local failures by the influence of mechanical stresses.

バルブ鋼へのAl被覆とそのエンジンバルブへの応用

The effects of Al coating on the life of engine valve as well as various characteristics of Fe-Al alloy layer formed on valve steel by Al coating were investigated. The results obtained were as follows.(1) The thickness of alloy layer after diffusion heating varied considerably depending on the kind of base steels with the same thickness of sprayed Al layer and other experimental conditions. The thickness of alloy layer on ferritic steel was about twice as large as that of austenitic one.(2) By X-ray diffraction method, it was found that the Al content in alloy layer of austenitic steel was higher than that of ferritic one.(3) The oxidation resistance of valve steel was remarkably improved by Al coating regardless of the kind of steels. However, the rate of improvement in the resistance was greater in ferritic steel than in austenitic one.(4) The corrosion resistance of austenitic Cr-Ni steel to molten PbO was considerably improved by Al coating.(5) The thermal expansion coefficient of alloy layer was almost equal to that of austenitic steel, but it was much greater than that of ferritic one. The crack formation on alloy layer of ferritic steel subjected to the repeated cycles of rapid heating and cooling was caused by the significant difference of expansion coefficients between the alloy layer and the base steel.(6) The hardness of alloy layer was very higher than that of base steel up to 600°C, and it was not so much lowered by heating at 700°C for a long time.(7) It was found by the engine test that Al coating was not so effective for the improvement in the life of inlet valve, but it has considerable effects for the prevention of mechanical and corrosive attacks on exhaust valve, which have been hitherto inevitable.

引張加工したCr-Niステンレス鋼の低温時効による収縮現象

When plastically deformed austenitic Cr-Ni stainless steels were heated isothermally at about 130°C for a long time, the shortening effect in steels occasionally appeared. Two commercial steels, 18-9 and 18-12, were prepared to study the cause of this phenomenon, and elongated 0∼20% at both room temperature and liquid nitrogen temperature after solution treatment. Then the thermal dilatation characteristics up to 1000°C and the aging characteristics at 100°∼175°C were studied.Both steels, elongated at room temperature, were all nonmagnetic, showing no contraction in the thermal dilatation curves. On the other hand, both steels elongated at subzero temperature were all magnetic and contracted clearly. The largest contraction were observed in the about 5% elongated specimen of a subzero worked 18-9 alloy and the about 10% elongated specimen of a 18-12 alloy. The contraction decreased in both cases where the elongation of steels decreased and increased. The steels elongated considerably at room temperature showed a maximum contraction of about 0.05% after a short time aging at 125°C, but the contraction diminished gradually after a long time aging. On the contrary, the steels elongated at subzero temperature, showed generally a clear contraction which attained 0.1% in maximum and the contraction remained even after a long time aging. The working ratio and temperature at which the largest contraction appeared, varied by the stability of austenite in steels. The elongated steels in which about 10% martensite and austenite coexisted showed the largest contraction after the aging at a low temperature.

Continuous Casting Steel Slabs

A description of the plate-type mold developed by Bohler Bros, for wide slabs. Also given are details of experiments on the continuous casting of heats of rimming low-carbon steels, Al-killed steels, and austenitic Cr-Ni steels and their hot and cold rolling into strip and sheet.

The development of stress-corrosion cracks in austenitic Cr-Ni steels

Direct-loaded axially stressed specimens (0·160–0·635 cm dia.) of a variety of austenitic Cr-Ni steels were exposed to boiling 42 per cent MgCl 2 solution. The progress of cracking was followed by potential measurements and by removing specimens for examination after suitable periods of exposure. The results are comparable with those earlier obtained on 0·051 cm dia. specimens. The life of a specimen can be divided into an induction period and a period of crack propagation; the latter consists of two stages. The cracks remain fine during the first stage, during which no change in potential can be detected; this stage is relatively more important the larger the specimen and the lower the applied stress, and so was not recognized previously. The cracks open up and become branched during the second stage, which is accompanied by a fall in potential. The relative duration of the induction period and the total period of crack propagation varies considerably with the experimental conditions, particularly the size and form of the specimen and the mode of stressing, but for fully softened axially-stressed cylindrical specimens the ratio of these periods is constant for a given specimen diameter. The practical significance of these observations is discussed briefly. Sufficient corrosion current is available to account for crack-formation by purely electrochemical processes.

On the propagation of stress-corrosion cracks in metals

Mechanisms which have been proposed for the propagation of stress-corrosion cracks are reviewed, and it is concluded that the simplest are the electrochemical mechanisms proposed by Logan and by Hoar and Hines. These attribute crack formation to yielding at the crack tip, which leads to concentration of attack at that point. Mechanisms that involve mechanical stages of crack propagation provide a satisfactory explanation of a number of cases in which there is evidence that, or where it may plausibly be argued that, paths that are mechanically weak exist in the material. These mechanisms are less satisfactory in cases where the cracks do not follow pre-existing paths in the material, as it is difficult to explain why localized brittle failure should occur. The conditions under which the two electrochemical mechanisms are applicable are discussed in terms of a simple crack model, and it is shown that most, if not all, of the facts concerning stress-corrosion cracks in austenitic Cr-Ni steels can be explained qualitatively in this way; in some cases reasonable quantitative agreement is obtained. Logan's mechanism, which assumes that yielding at the crack tip merely prevents formation of a passive film, cannot be applied in this case, but it may well operate in cases where more active metals are involved. The Hoar/Hines mechanism, according to which yielding reduces the activation polarization for anodic dissolution at the crack tip, is satisfactory, and it is concluded that this is the best available explanation. Other cases of stress-corrosion cracking are discussed.

Some effects of alloy composition on the stress-corrosion behaviour of austenitic Cr-Ni steels

Stress-corrosion data obtained at Cambridge University and at Billingham on fully-softened specimens of austenitic and duplex Cr-Ni steel have been subjected to regression analysis using a computer. The results give the dependence of time to fracture on composition of the steel and applied stress. The most important compositional variables are carbon and molybdenum. With increase in amount of both elements the time to fracture passes through a minimum, and the positions of the minima are influenced by the amount of the second element and, in the case of molybdenum, by applied stress. These minima are in the ranges 0·065 per cent-0·085 per cent C and 1·5 per cent-2·1 per cent Mo. Titanium modifies the effect of carbon to some extent, but the effect is not large, while nickel has only a slight beneficial effect in the range of interest. Copper also has a minor effect, but this is not of practical importance. The results indicate that many common British alloys are close to the worst possible compositions for stress-corrosion resistance. Furthermore, changes in composition of the materials during the last 20 years have tended to reduce the resistance. However, considerable improvement can be obtained in favourable circumstances by limiting the carbon content and controlling the molybdenum content. Possible lines on which resistant alloys might be developed are discussed briefly.

Austenite系Cr-Ni不銹鋼熔着金属の亀裂感受性に及ぼす燐の影響

The effect of phosphorus on crack-sensitivity of austenitic Cr-Ni stainless steel weld metal is investigated with the segmented-circular groove test process. The test results indicate that the effect depends on ferrite content in weld metal and phosphorus is allowable up to 0.05%, 0.10%, 0.13%, 0.15% each 3%, 7%, 9%, 12% ferrite level calculated in accordance with schaeflier's data.