Erweiterte Diskussionsbemerkungen zum Einfluß von Silicium und Molybdän auf die Spannungsrißkorrosionsempfindlichkeit austenitischer und austeno-ferritischer Chrom-Nickel-Stähle

Amplified discussion of the influence of Silicon and Molybdenum on the stress corrosion cracking proneness of austenitic and austeno-ferritic chrome-nickel steels Tests have been carried out in boiling magnesium chloride, calcium chloride, NaCl, water (150–200°C) and hot steam (500°C); in the two last-named cases, the tests were carried out with and without the addition of chloride and oxygen. The following materials were tested: steels with (per cent.) 17 and 17.5 Cr, 12–15.5 Ni, 0 and 2.5 Mo, 0 and 4 Si, less than 0.03 C, as well as steels with 20 and 21 Cr, 8—10 Ni, 0 and 2.5 Mo, 0 and 3 Si, 0, 1.5 CU, less than 0.05 C. Silicon has a favourable effect on the stress corrosion cracking behaviour in magnesium chloride and also in calcium chloride, but a detrimental effect in NaCl and water. Its probable effect is to inhibit the extension of the crack by crystallographic obstacles and, later, by the formation of a surface film of poor conductivity (where Mo also plays a part). The attempt is made to interpret this behaviour on the strength of electro-chemical investigations; in this connection, renewed critical reservations are made in respect of the test in magnesium chloride.     

Das Ausscheidungsverhalten von hochlegierten austenitischen Stählen mit 6% Molybdän und sein Einfluß auf die Korrosionsbeständigkeit

Precipitation behaviour of high-alloyed austenitic steels with 6% molybdenum and its influence on the corrosion resistance The high-alloy austenitic steels with 6 to 7% Mo, 20 to 21% Cr and 18 to 25% Ni are increasingly used in seawater and chemical applications. This is due to the excellent resistance to pitting and crevice corrosion in chloride-containing neutral and acidic environments. It is the high chromium and molybdenum content which provides the excellent corrosion behaviour but, at the same time favors the tendency to precipitation of intermetallic phases. Therefore, time-temperature-precipitation diagrams have been established for two steels with 6% Mo, 21% Cr, 25% Ni, 0.14 and 0.19% N and for one steel with 6% Mo, 20% Cr, 18% Ni and 0.21% N. The corresponding time-temperature-sensitization diagrams (in accordance to SEP 1877/II) and time-temperature-pitting diagrams (testing in 6% FeCl₃ solution) have been evaluated as well. Precipitation of intermetallics occurs rapidly especially in the range between 700 and 1000°C. In case of the 18% Ni steel and the 25% Ni/0.14% N steel grain boundaries are covered to a large extent with precipitates after only 15 min at 850 or 950°C. In case of the 25% Ni/0.19% N steel precipitation is considerably slower. The precipitates are interpreted to be chi-phase. After very long annealing times additionally small amounts of Laves phase appear. Neither carbides nor nitrides were observed. In spite of the rapid precipitation, sensitization in terms of the 50 m?m grain boundary penetration criterion is observed not before 0.7 h at 850 °C and not before about 2 h at 800°C in case of the 25% Ni/0.19% N steel. After about the same times of annealing also the critical pitting temperature as observed in the FeCl₃-test is dropping below 50°C. Therefore, when welding according to established rules and recommendations, no deterioration of the corrosion resistance in the heat-affected zone is to be expected. If high heat inputs will occur during manufacturing because of hot forming operations or welding of heavy sections, or if more severe test conditions are a requirement, a steel with 25% Ni and about 0.2% N (UNS N 08925, Cronifer hMo) is recommended due to its retarded precipitation and sensitization behaviour when compared to steels with only 18% Ni (UNS S 31 254). Additionally, the steel with 25% Ni has an increased resistance to general corrosion in acids. Notch impact strength of the materials under consideration is increased by the initial precipitation of the intermetallic phases and decreases only after longer times of annealing below the ductility of the solution annealed material.     

Oberflächenbehandlungsverfahren und deren Einfluß auf die Korrosionsbeständigkeit nichtrostender Stähle

Surface treatments and their influence on the corrosion resistance of stainless steel The surface treatments pickling, grinding and glass beading were investigated on several stainless austenitic steels and one ferritic/austenitic steel. The different surface treatments were used on two different prepared types of samples:

• 1 high temperature oxidized samples
• 2 welded samples

The quality of the surface treatments has been examined by means of potentiodynamic, ferric chloride, dip and spray tests made in series. The corrosion resistance was highly depending on the used treatments. All in all the examination showed that a higher corrosion resistance was achieved by pickling than by grinding or glass beading.     

Einfluß des Molybdängehaltes niedriglegierter Stähle auf die Spannungsrißkorrosion in Nitratlösungen unter CERT-Belastung

Effect of molybdenum on the stress corrosion cracking behaviour of low alloy steels in nitrate solutions under slow strain rate testing conditions CERT tests ($ \mathop \varepsilon \limits^. = 10^{ - 6} {\rm s}^{ - {\rm 1}} $) were carried out in NaNO₃ solutions (0.1… 5 mol/l and 25… 90ßC) to determine the critical parameters for stress corrosion cracking. The steels tested were 15 Mo 3, LStE 36 and two laboratory heats with 0.08% C and molybdenum concentrations of 0 and 1%. All cracked specimens were examined metallographically. Different kinds of corrosion attack with and without intergranular features were observed. The critical potential range for intergranular attack is defined by two critical potentials. The negative critical potential is about U_H = ?0.2 V. It is hardly affected by the test parameters and the material. All free corrosion potentials lie in this range. All freely corroding specimens failed by intergranular cracking in 5 M NaNO₃ at 90°C. Only the coarse grained material without Mo showed intergranular cracks at even lower concentrations of NaNO₃. The positive critical potential varied widely over 0 to 0.5 V, depending on the test parameters and the material. Furthermore, in all cases a second critical potential range of intergranular attack was found at about U_H = 0.8 V. The potential range of U_H = 0.5 to 0.8 V for resistance against intergranular attack disappears with increasing temperature, as with the test conditions according to DIN 50 915 in boiling Ca(NO₃)₂ solutions. Furthermore, this was observed for the heat without Mo at already 90°C. With respect to environmental parameters, the effect of NaNO₃ concentration is very small but the effect of temperature is markedly high. The materials can be better characterized by critical temperatures rather than by critical potential ranges. Unified ranking of the materials with respect to their resistance to intergranular attack is not possible because of its dependence on the potential. However, on the basis of the extent of the domains in which the materials are resistant, it is possible to differentiate among the materials. Their resistance to intergranular attack increases in the following order: Heat without Mo → LStE 36 → 15 Mo 3 → heat with 1% Mo. The effect of Mo is explained in terms of its association with high proportions of bainite in the microstructure.     

Einfluß einer Wärmenachbehandlung auf die mechanischen Eigenschaften und das Korrosionsverhalten nichtrostender Stähle

Influence of an ultimate heat treatment of the mechanical properties and the corrosion behaviour of stainless steels Unfavorable structures as may arise during steel processing and may impair their behaviour can be eliminated by an ultimate heat treatment. Cold hardening can be removed by heating to about 1000°C for about 1 min; dissolution of chromium carbide precipitated requires 3 min at 1030°C and dissolution of sigma phase about 10 min at 1050°C. The treatment, however, will not be successful, unless the temperatures mentioned are achieved throughout the whole workpiece and a sufficiently high colling rate is maintained after the treatment.     

Zum Einfluß von Mangan auf die Korrosionseigenschaften von austenitischen 18.10-CrNi-Stählen

Influence of manganese on the corrosion properties of austenitic 18.10-CrNi stainless steels The influence of manganese in the range of 0.25 to 1.5 mass-% on the passivation and pitting corrosion behaviour of unstabilized and Tistabilized austenitic 18/10 CrNi stainless steels is examined by determination of useful characteristical electrochemical dates using potentiodynamical polarization measurements in H₂SO₄-acidic and neutral model electrolytes. In the case of Ti-stabilized steels, a trend to an improved ability to passivation and to an increased pitting corrosion resistance is signified with increased Mn-content. This is in agreement with the austenite stabilizing effect of manganese. In the case of unstabilized steels, a significant deterioration of the passivation and pitting corrosion behaviour is observed, if the Mn-content of the steel is increased from less than 0.7 to more than 1.0 mass-%. These observations are discussed in the viewpoint of segregation of Mn-rich sulfide inclusions in the steel, which are essentially influenced by the presence of titanium in the steel.     

Der Einfluß von Cer,Zirkonium und Bor auf die Oxidationsbeständigkeit hitzebeständiger Stähle an Luft

Influence of cerium, zirconium and boron on the oxidation resistance of heat-resistant steels in air Isothermal and cyclic oxidation experiments were carried out in air to investigate the influence of the minor elements such as Cerium, Zirkonium and Boron on the oxidation resistance of heat resistant ferritic and austenitic steels like X 10Cr 18, X 10CrAl 18 and X 15 CrNiSi 20 12. In the case of cyclic experiments samples were exposed at constant temperatures for 100 h and then cooled to R. T. This cycle was repeated 10 times. The corrosion was determined as weight change and was continuously measured by a thermo-balance. The distribution of the alloying elements on the phase boundary scale/steel was examined by Scanning-Electron-Microscope. Addition of small amounts of Ce (0.3 wt-% max.) could reduce the oxidation rate in the case of isothermal and cyclic conditions. Zirkonium concentrations below 0.1 wt-% could have a beneficial effect, but at higher concentrations the oxidation rate increases with increasing amounts of Zr. Small Boron concentrations of 0.02 wt-% lead to catastrophic oxidation at temperatures above 1000°C.     

Über den Einfluß von Kohlenstoff und Stickstoff auf die Empfindlichkeit niedriglegierter Stähle gegen Spannungsrißkorrosion

The influecne of carbon and nitrogen on the sensitivity of low-alloyed steels to stress corrosion Application of stress to steels under the influence of certain media produces stress-corrosion cracking. This can be examined through experimental methods only, by which the specimens are kept under constant stress. The cracking time required for the specimen is then a measure of resistance of stress-corrosion failure. In this way it is possible to investigate the influence of alloying metals and the heat treatment on susceptibility of steels, by considering the respective strenght. Experiments were carried out to determine the effect of carbon content and the combined effect of carbon and nitrogen in a series of medium frequency and vacuum steels. It is found that increasing carbon content improves the resistance of specimens, taking also the higher strength into account. A higher nitrogen content shortens the cracking time. This shortening of resistance due to nitrogen content was much more note worthy in the case of high-carbon steels than in the case of low-carbon steels which were already sensitive in this respect. Comparing an open-hearth steel with the above steels, it seems as if there are other alloying metals which make the steels susceptible to stress-corrosion cracking. The effect of these alloying metals shall be investigated further.     

Einfluß der Elemente Phosphor,Mangan, Chrom,Molybdän und Vanadium auf das Verhalten niedriglegierter Stähle bei Spannungsrißkorrosion

Influence of elements such as phosphorus, manganese, chromium, molybdenum and vanadium on the stress corrosion behaviour of low alloy steels A series of such steels having graduated alloy addition contents were obtained experimentally in an electrical furnace and were tested under permanent tensile loads in calcium nitrate solution after different heat treatments and at different relative loads. The heat treatment has a very pronounced bearing on corrosion susceptibility: overheating produces the highest sensitivity annealing at 750°C the — relatively — highest resistance, while normalizing occupies an intermediate position. P has but little effect on corrosion susceptibility so that its influence can practically be neglected. Mn seems to have generally a beneficial effect while Cr — even in amounts up to 2% — is unable to improve corrosion resistance. In the case of V there is some improvement in some cases only while Mo generally produces increased corrosion resistance. In this connection, a new method for evaluating experimental results has been described, which has been developed for the practice of steel assessment. The experimental results confirm, that a steel may be designated ?resistant”? at a certain load when its useful life under tensile load in alkaline media is 240 hrs. Extension of the test duration does not yield an improvement in evaluation accuracy.     

Der Einfluß des Molybdängehaltes auf die Korrosion austenitischer Cr-Ni-Stähle im Aktivzustand

The influence of the molybdenum content on the corrosion rate of austenitic Cr-Ni steels in the active condition The corrosion rate of steels X 5 CrNi 189, X 5 CrNoMo 18 10, X 5 CrNiMo 18 12 and X 5 CrNiMo 17 13 has been investigated under potentiostatic conditions in the active zone in 2 n H₂SO₄ flushed with nitrogen and sulphur dioxide. The active rest potential of the steels is in the vicinity of the active-passive transition, and the corrosion rate increase at cathodic polarisation. With increasing Mo content, the corrosion rate is reduced in the active condition, but the passivation potential and the corrosion rate in the passive condition are not influenced. In the acid flushed with SO₂, the corrosion rate is increased in the active range, and the latter is extended in the direction of the electronegative potentials. With these steels, even a pre-activation of the specimens has an influence on the test results. In the active-passive transition zone, the steels in the test solution containing SO₂ are partially passive and subject to local corrosion attacks.     

Der Einfluß der intermetallischen Phase ZrV2 auf die Oxidations- und Korrosionseigenschaften von Zirkonium-Vanadium- Legierungen

The influence of the intermetallic ZrV₂ phase on the oxidation and corrosion behaviour of zirkonium vanadium alloys The good oxidation resistance of zirkonium and some of its alloys is due to the formation of an oxide layer characterized by good adhesion. Since, however, cracks are formed in such a layer upon prolonged oxidation the corrosion rate can be rather high in the later stages. An improvement of the properties of the oxide layer may be obtained by providing for the incorporation into the scale of atoms able to prevent oxygen diffusion. In the case of vanadium, however, no protective effect is obtained and the corrosion rate even increases as the vanadium content is increased. This phenomenon is due to the poor solubility of vanadium in the zirconium so that the incorporation of vanadium in the scale is inhibited during the initial stages of corrosion. The corrosion mechanism is the same in saturated vapour and NaCl-solution: there are ZrV₂-Particles which are not attacked and are embedded in a largely destroyed zirkonium matrix.     

Über den Einfluß der Austenitstabilität von 18/8-Chrom-Nickel-Stählen auf die Verformungseigenschaften und auf das Korrosionsverhalten dieser Stähle

Influence of austenite stability of 18-8 Cr-Ni-steels on the cold working and corrosion properties of these steels The martensite formed during cold working has a negative effect on uniform corrosion only when the specimen is active in the particular medium; if so, corrosion current density increases with the degree of cold working. The martensite generated by cold working reduces on the other hand the susceptibility to intercrystalline corrosion and – in amounts up tn 2% – increases the time to failure in stress corrosion cracking (15 and 45 kp/mm²). The pitting potential is not shifted by the martensite, but pit density increases with the martensite content. In the Kesternich test no negative effect of the martensite is found. It is therefore concluded that reducing the Ni content in 18-8 steels improves workability without having a pronounced deleterions bearing on corrosion behaviour.     

Über den Einfluß der Bewitterung auf das Korrosionsverhalten unlegierter und niedriglegierter Stähle

Influence of weathering on the corrosion behaviour of unalloyed and low-alloy steels Accelerated weathering tests with plain carbon steel and low-alloy steels show that the influence of weathering is of considerable importance for the atmospheric corrosion. Attention has to be paid that the steel surfaces are never wet over a longer period of time. The faster the steels will be dry, the better is the long time behaviour of these materials. Frequent short wet periods, followed by dry phases lead to the formation of an increasingly better protective layer. By favorable weathering conditions the influence of the composition of the alloys is comparatively small. Metallographic and electrochemical tests further show that the corrosion protection is not only due to the formation of a compact macroscopic layer. Local passivation effects are significant as well.     

Einfluß von Korrosion und mechanischer Belastung auf das Rißwachstum niedriglegierter,ferritischer Stähle in sauerstoffhaltigem Hochtemperaturwasser

Influence of corrosion and mechanical loading on the crack growth of low-alloyed ferritic steels in oxygenated high temperature water The mathematical and with regards to the contents main features of the mostly developed analytical model for corrosion-assisted crack growth are presented and the crack growth velocities resulting for low-alloyed ferritic materials in high-temperature water are given. Experimentally determined crack growth velocities for the ferritic material 20 MnMoNi 5 5 with two different sulfur contents as well as for the similar material 22 NiMoCr 3 7 in deionized, oxygenated (0.4 and 8 ppm O₂) high temperature water at 240°C are compared with calculated ones. The constant load experiments at different level were performed on compact tension specimens with a thickness of 50 mm (2T-CT-specimens). The experimental results show, that up to a stress intensity factor K_I of 60 MPa $ \sqrt m $ the corrosion-assisted crack advance is neither dependent on the oxygen content of the medium and the K₁-value, nor on the sulfur content of the steel. A deviation up to 3 magnitudes compared to the calculated values exists. Furthermore, an increasing crack growth velocity with decreasing test duration is observed. Between 60 and 75 MPa $ \sqrt m $ the crack growth velocity increases by several magnitudes also independent of the above mentioned parameters. Above 75 MPa $ \sqrt m $ fracture of the specimen occurs soon after loading. In this region the experimentally derived crack growth velocity fits well with the analytical model. A possible explanation for the deviation between experimental and analytical results could be seen in low-temperature creep processes at the crack tip. Results of preliminary investigations on low-temperature creep processes of the material 20 MnMoNi 5 5 in air at 240°C are presented.     

Widerstand der Schweißverbindungen kohlenstoffarmer und mit Molybdän legierter korrosionsbeständiger Stähle gegen interkristalline und Messerlinienkorrosion

Resistance to intercrystalline and knifeline corrosion of welds in low carbon stainless steels containing molybdenum Welds in steels containing (%) 0,019–0,080 C, 1,00–1,56 Mn, 0,03–0,92 Si, 0–0,026 P, 0–0,018 S, 10–13,60 Ni, 16–20,50 Cr, 0–3 Mo, 0–0,057 Ti, 0–0,87 Nb, prepared by electroslag and automatic submerged arc welding are as a general rule not susceptible to intercrystalline and knifeline corrosion; a certain susceptibility encountered with the Mo containing types can be largely eliminated by sensibilizing at 650 °C. Differences in the corrosion behaviour between base metal and weld seam could not be encountered; in some cases the weld metal turned out to be even more resistant. The test solutions used were: sulfuric acid + Cu sulfate + Cu, boiling, 48 hrs, and 65% nitric acid, boiling, 3 · 48 hrs.     

Der Einfluß des Titangehaltes auf die Aktivkorrosion ferritischer Cr-Stähle und austenitischer Cr-Ni-Stähle in Schwefelsäure

The influence of the Titanium content on the active corrosion of ferritic Cr steels and austenitic Cr-Ni steels in sulphuric acids Non-stabilized and Ti-stabilized steels of types X8Cr17, X8Crti17, X5CrNi189, X10CrNiTi189 and X5CrNiMoTi25 25 are compared with each other in respect of their behaviour in the potential range of active corrosion in sulphuric acid. With ferritic 17 per cent. chromium steels, no Ti influence was detected. With more highly alloyed austenitic Cr-Ni or Cr-Ni-Mo steels, the titanium had the effect of narrowing down the potential range of active corrosion and reducing the corrosion rate which facilitates the passivation capacity of the steels. These phenomena are particularly marked with the steel known as X5CrNiMoTi25 25. With this steel, the corrosion rate in the active-passive transition range with titanium contents from 0.41 to 0.66 per cent. and about 0.66 per cent. C ist increased in the sensitivized state. Within a narrow potential range of about 200 mV, intercrystalline corrosion is encountered which decreases with increasing Ti content and is completely prevented if the C content is reduced below 0.03 per cent. The potential range of the inter-crystalline corrosion is more negative than the test potential of the Strauss test normally used for testing the grain disintegration resistance.     

Der Einfluß von Stickstoff auf die korrosionschemischen Eigenschaften lösungsgeglühter und angelassener austenitischer 18/10 Chrom-Nickel- und 18/10 Chrom-Nickel-Molybdän-Stähle. - I. Ausscheidungsverhalten der Stähle

Influence of nitrogen on the corrosion behaviour of solution treated and annealed austenitic 18/10 chromium-nickel and 18/12 chromium-nickel-molybdenum steels. - I. Precipitation behaviour of the steels Investigations into the corrosion behaviour of CrNi steels with (weight-%) 0.019–0.029 C, 18.5 Cr, 11 Ni, 0.022–0.138 N and 0.020 to 0.031 Cr, 17.2 Cr, 13 Ni, 2.6 Mo, 0.028 to 0.130 N by the Strauß-Test, the Strauß-Test under rigid condition and the Huey-Test. The first part of this paper is devoted in particular to the analysis of structural constituents and in particular to precipitations. In the molybdenum-free steel heat treatment results in precipitation of M₂₃C₆ and Cr₂N, the precipitation of the first being independent from nitrogen content. In case of the molybdenum-bearing steels the precipitation of M₂₃C₆ is independent from nitrogen content too but is shifted toward higher temperatures. The precipitation of Cr₂N is slowed down by Mo. The precipitation of other phases (chi-, sigma- and Z-phases) is affected differently by nitrogen.     

Der Einfluß mechanischer Wechselbelastungen auf die Beständigkeit Mo-freier und Mo-haltiger Stähle gegen Spannungsrißkorrosion in Calciumnitrat-Lösung und Natronlauge

The influence of cyclic loading on stress corrosion cracking of Mo-free and Mo-bearing steels in solutions of calcium nitrate and sodium hydroxide The resistance of five unalloyed or low-alloy steels differing essentially in their Mo-contents, against SCC was investigated by electrochemically controlled SCC experiments in boiling 60 wt.-% Ca(NO₃)₂ solution (E_H = 0.15 V) and 35 wt.-% NaOH (E_H = -0.75 V). For characterizing the SCC response, threshold stresses O_G (criterion: crack depths 1 = 0) were evaluated with mechanical loading by alternating stresses (saw tooth characteristic, strain rates ε = 3.4. 10^?6 or 8.7. 10^?7s^?1, load cycle frequency 5. 10^?3 or 3. 10^?3Hz). If the threshold stresses obtained by constant load SCC experiments are near the YS or in the plastic range (Ca(NO₃)₂ solution), the threshold stresses are lowered considerably by cyclic loading. If, however, the threshold stresses obtained under constant load conditions are in the elastic range (NaOH), there is only little influence of cyclic loading on the o_G values.     

Einfluß der chemischen Zusammensetzung auf die Kinetik der atmosphärischen Korrosion von Stählen

The influence of the chemical composition on the kinetics of the atmospheric corrosion of steel A total of 19 different structural steels were exposed to urban, rural and industrial atmospheres for periods up to three years. The evaluation of the weight losses determind at certain intervals permitted the establishment of a linear relationship which can be used for assessing, within certain limits of concentration, the influence of specific alloying elements. The tests showed that the effects of the different alloys varies with the type of atmosphere and with the degree of corrosion, and may well shift from “favourable” to “detrimental”. The results are also influenced by the quantitative ratios of certain components. The corrosion behavior of the steels is consistently governed by P, S and Cu and, to a lesser extent, by C and Mn.     

Einfluß der Festigkeit unlegierter und niedriglegierter Stähle auf H-induzierte Korrosionserscheinungen bei zügiger plastischer Belastung

Influence of mechanical strength on hydrogen-induced corrosion effects on unalloyed and low-alloysteels subjected to slow strain rate tests Three steels were subjected to various heat treatments or cold working to produce 7 variants of strength. Specimens from these materials were tested in 9 aqueous solutions containing various acidic components by the constant strain rate technique under cathodic polarisation or free corrosion conditions. Hydrogen induced cracks were only observed after passing the maximum load. Hydrogen induced cracking and the decrease in the reduction of area (hydrogen embrittlement) show a good correlation with the mechanical properties of the materials. For less ductile materials the number of cracks decreased with increased embrittlement. The magnitude of hydrogen embrittlement depends on the concentration of undissociated acid in the test solution and is independent of pH value. O₂ can reduce the embrittlement. The effect of test solution composition decreases as the cathodic polarisation potential becomes more negative, because hydrogen will then be generated from H₂O. There is no correlation between the type of corrosion effects and the strength of the material, except in the case of highly sensitive high strength variants with R_m in excess of 1000 N/mm². Quenched and tempered low alloy steels, even at high strength levels, have significantly higher resistance to hydrogen embrittlement than significantly higher resistance to hydrogen embrittlement than unalloyed steels. The same is also ture for workhardened variants. For unalloyed steels, metallurgical cleanness seems to have a favourable influence. No hydrogen induced corrosion effects were observed in specimens tested at 907°C under free corrosion conditions.