Bildung und Einfluß von α'-Martensit auf die Spannungsrißkorrosion von Chrom-Nickel-Stählen

Formation of martensite and influence of the latter on stress corrosion cracking of chromium nickel steels Martensite formation depends from alloy composition, deformation and undercooling. By contrast to an industrial alloy corresponding to (DIN) X 10 CrNiTi189 a pure Fe 19 Cr 10 Ni alloy becomes partially martensitic upon deformation and quenching to ?190 °C. Undercooling and subsequent deformation give rise to the addition of deformation and quenching martensite. The formation of α′-martensite is largely suppressed by addition of 2% Mo. Since corrosion resistance in aqueous MgCl₂ solutions increases with the α′-martensite contents, cracks grow predominantly along the martensite plates. Addition of molybdenum reduces stress corrosion resistance because of the suppression of martensite formation. In this type of alloy cracking is intercrystalline nature.     

Einflußgrößen der H-induzierten Spannungsrißkorrosion bei niedriglegierten Stählen

Parameters influencing the hydrogen-induced stress corrosion cracking of low alloy steels Wet H₂S corrosion of steel leads to surface blistering, internal cracking at sharp edged inclusions (HIC), and in stressed specimens to transgranular micro cracks transversal to the main stress. The latter grows to SCC. HIC resistant material displays high purity of non-metallic inclusions. The critical stress level for SCC of lien pipe steels with yield strengths up to 600 N/mm² is proportional to the yield strength. In H²S containing media different grades of low alloy and rolled steels show no correlation between life time and yield strength. In general, the susceptibility for H-induced SCC increases with decreasing pH and with cathodic polarisation, whereas the temperature dependence displays a minium of life time at 20°C.     

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 Fe²⁺/Fe₃O₄ transition.     

Wasserstoff-induzierte Spannungsrißkorrosion von Stählen durch dynamisch-plastische Beanspruchung in Promotor-freien Elektrolytlösungen

Hydrogen induced stress corrosion cracking of steels subjected to dynamic loading involving plastic deformation in promotor free electrolytic solutions Plain carbon steels and low alloy steels suffer internal cracking and a relatively high embrittlement when they are subjected to dynamic loading involving plastic deformation in any type of electrolytic solution where there is a simultaneous cathodic hydrogen evolution. These conditions can be encountered in service in the case of cathodic polarisation and free corrosion in acids if the mechanical stresses lead to plastic deformation, e.g. at notches. There is an upper limit to the potential range in which internal cracking occurs. This limit is independent of the yield strength (300 to 500 N mm^?2) of the materials tested and lies at U_H = ?0.5 V in oxygen free salt waters. It is more negative in oxygen bearing electrolytic solutions or in alkalaine media. Materials containing hard transformation products such as martensite and bainite are more susceptible to cracking in the regions of these hard transformation products. There is only a small decrease in susceptibility with increasing temperature. Ultra high strength, quenched and tempered steels with yield strengths > 1000 N mm^?2 undergo hydrogen induced stress corrosion cracking even when they are subjected to static loading in the elastic region. The critical potential is very negative and shifts to less negative values with increasing strength of the material. Stainless steels with stable austenitic microstructure are resistant to this type of corrosion. However, if the mechanical deformation can lead to the formation of martensite surface cracking and brittle fractures occur. The hydrogen induced damage decreases with increasing temperature.     

Dehnungsinduzierte Spannungsrißkorrosion an Stählen

Strain-induced stress corrosion cracking of steels Characterization of mechanical influencing factors and their effects on the corrosion of low alloy and high alloy steels. Stress- and strain-induced corrosion of steels. Influence of strain on the occurrence of stress corrosion cracking of low alloy steels in nitrate-, carbonate- and sodium hydroxide solutions and of high alloy steel in magnesium chloride solution. Significance of critical strain-rates.     

Die Ermittlung der Beständigkeit gegen Spannungsrißkorrosion (SpRK) von hochlegierten Sonderstählen in chloridhaltigen wäßrigen Medien

Determination of resistance to stress corrosion cracking (SCC) of high-alloy special steels in chloride-containing aqueous media The 18 Cr 10 Ni(Mo) based stainless steels have been continually improved by raising the Cr, Ni and Mo contents. The behavior of these high-alloy steels towards SCC was determined in test media generally used in practice since the question of the resistance to stress corrosion cracking (SCC) had still remained unanswered to a large extent. SCC tests on U-bend samples in boiling 62% CaCl₂ solution showed a good differentiation depending on the Ni and Mo contents. With increasing Ni content, the susceptibility of special high-alloy steels to SCC is shifted towards longer service lives, alloys containing ≧ 42% by weight of Ni being resistant. High-Mo special alloy steels are more resistant to SCC than low-Mo special alloy steels. These results could be confirmed by tests carried out on circular cross section samples in boiling 62% CaCl₂ solution under constant load and potentiostatic control. The free corrosion potentials recorded for 25% Ni special alloy steel and Ni-based alloys are within the potentiostatically determined range of insusceptibility to SCC. The high-Mo special alloy steel X 2 NiCrMoCu 25 20 6 (1.4529) shows the same critical SCC potential on the anodic side as the Ni alloy NiCr21 Mo (2.4858). Superferrit X 1 CrNiMoNb 28 4 2 (1.4575) and austenitic ferritic steel X 2 CrNiMoN 22 5 (1.4462) showed that the SCC behavior was unsatisfactory in both tests as in the case of steel X 10 CrNiMoTi 18 10 (1.4571). Tests in boiling 4 m NaCl showed no SCC, not even under the aggrevated test conditions in the test set-up. The great influence of the oxygen content was demonstrated in tests carried out in the autoclave with defined oxygen and chloride concentrations. The resistance of the steels to SCC decreases under air-saturated conditions (8 … 10 ppm O₂) whereas the chloride concentration (200 and 2000 ppm Cl^?) does not exercise an important influence. U-bend samples should be given preference to Erichsen samples for SCC tests. SCC break characteristics could be determined metallographically and by scanning electron microscope.     

Wasserstoffinduzierte Korrosion von niedriglegierten Stählen in wäßrigen Medien

Hydrogen induced corrosion of low alloy steels in hydrous media The absorption of hydrogen from media can lead to cracking in steels with a ferritic structure. The interaction of absorbed hydrogen and steel is governed by the hydrogen activity and by stress- and structure-dependent threshold values. Different types of hydrogen-induced corrosion are illustrated by means of tests performed on lowalloy structural steels. Surface blistering and internal cracking can occur on the one hand without external stresses. In the case of hydrogen-induced stress corrosion cracking on the other hand a differentiation is made between corrosion systems in which constant loading displays already cracking and systems in which the cracking is tied to slow strain rates within a critical range.     

Spannungsrißkorrosion von hochlegierten Manganstählen in wäßrigen Chloridlösungen

Stress corrosion cracking of high alloy manganese steels in aqueous chlorides In tensile tests made without applied current in aerated solutions a stabilization of the austenitic structure by increasing Mn and N contents yields increased times to failure. The potential-time-to-failure curves determined by potentiostatic tensile tests reveal a compley joint action of constitution and passivation behaviour of the steels. The intercrystalline stress corrosion cracking of the steel X 40 MnCr 19 with chromium carbide precipitations at the grain boundaries can be attributed to an electrochemical differentiation of the chromium-depleted grain boundary region. Steels of this type are characterized by a pronounced sensitivity to intercrystalline stress corrosion cracking the precipitation annealed state, and by a certain sensitivity to transcrystalline corrosion cracking after solution annealing. Low carbon Mn steels containing up to 4 % Cr are susceptible to transcrystalline stress corrosion cracking irrespective of the heat treatment. As to the temperature dependence of times-to-failure, constitution and layer formation have different effects. Increasing the Cr content to 8 % gives rise to a transition from stress corrosion cracking to pitting type corrosion. In terms of electron optics, an increased chromium content gives rise to a changed dislocation pattern, so that there may be an effect of the type of gliding processes on stress corrosion, The increased stress corrosion resistance of MnCr steels containing at least 8% Cr may be due to the lower height Of the gliding step and to an increasing tendency to repassivation of damaged surface layers.     

Untersuchungen der Spannungsrißkorrosion austenitischer Stähle durch saure Chloridlösungen bei Niedrigen Temperaturen

Investigation of stress corrosion cracking of austenitic steels in acid chloride solutions at low temperatures Tests were carried out on materials 1.4301, 1.4571, 1.4439 and 1.4558 in cold hydrocloric acid (c(Cl^?) = 1.5 mol/L and c(H⁺) = 1 mol/L, a few tests also at 0.1 and 0.01 mol/L). Chronopotentiostatic tests yielded data on active corrosion, passivity and pitting corrosion. CERT tests (10^?6s^?1, a few tests also at 2 · 10^?7s^?1) showed superposition of general corrosion on stress corrosion under free corrosion condition, while the rest potential was relatively negative in the active range. Oxygen purging has only a minor effect. The extent of cracking decreases with decreasing c(H⁺). In the case of cathodic polarisation straining induced surface notches occur which can be attributed to hydrogen induced effects. In the case of anodic polarisation pittings are generated without any crack initiation. Characteristic features of stress corrosion increase with decreasing strain rate. It follows from the results that high acid concentrations are necessary for stress corrosion cracking in the active state to occur. CERT tests cannot be used as an accelerated test for this kind of stress corrosion cracking. Few CERT tests carried out in warm NaCl solution did not show any stress corrosion cracking though these solutions are known to cause stress corrosion cracking after long periods. Furthermore, predamage in the form of pitting does not alter the situation.     

Untersuchungen zur Spannungsrißkorrosionsbeständigkeit nichtrostender Stähle mit unterschiedlichem Verformungsgrad

Investigations on stress corrosion cracking resistivity of stainless steels with different degree of cold straining Within the context of the elaboration of a new approval for stainless steels for civil engineering applications the question arised whether cold straining could adversely affect the resistance against stress corrosion cracking. By means of experimental tests with different stainless steels it should be clarified whether cold straining increase the stress corrosion cracking susceptibility under high chloride loads like e.g. in swimming hall atmospheres. The investigations were carried out using the high-alloyed steels 1.4462 and 1.4529. As reference material with known susceptibility against stress corrosion cracking the steel 1.4301 was used. The tests were performed using U-bent and tension specimens with saturated MgCl₂ salt spots. Indications for an increase in stress corrosion cracking susceptibility of the cold drawn steel were not found.     

Einflüsse des Phosphors auf die Interkristalline Spannungsrißkorrosion von Kohlenstoffstählen

Influence of phosphorus on the intergranular stress corrosion cracking of carbon steels The effects of phosphorus on the intergranular stress corrosion cracking were studied for steels with 0.15% C and 0.4 or 2% Mn, the phosphorus contents were 0.003, 0.03 and 0.05% P. Constant strain rate tests were conducted at constant potentials in 55% Ca(NO₃)₂ at 75°C, in 5 N NH₄NO₃ at 75°C, and in 33% NaOH at 120°C. The strain rate was 10^?6/s. Different grain boundary concentrations of phosphorus were established by varying the annealing time at 500°C, they were determined by fracturing the samples in UHV and analyzing intergranular fracture faces by Auger-electron spectroscopy. In the nitrate solutions the toughness, i.e. resistance against stress corrosion cracking of the steels is somewhat decreased with increasing P-content within the range of the corrosion potential, that is ?300 mV_H to ?50 mV_H in NH₄NO₃ and ?80 mV_H to ?50 mV_H in Ca(NO₃)₂. It is shown that this effect is caused by phosphorus segregated at the grain boundaries. At potentials above ?50 mV_H the relative toughness is very low for all tested steels, also without stress intergranular attack is observed. In NaOH the steels are most sensitive against stress corrosion cracking at ?700 mV_H, here the phosphorus content or segregation shows no effect. At potentials varied in both directions the toughness increases and a somewhat negative effect of phosphorus becomes visible. In all tested electrolytes the effect of phosphorus is restricted to a small potential range. There is no effect of phosphorus in the range of the minimal toughness, thus the steels are sensitive anyway, with and without phosphorus segregation. Therefore low phosphorus contents in carbon steels cannot guarantee resistance against stress corrosion, most decisive are the potential and the electrolyte.     

Zur Spannungsrißkorrosion austenitischer Chrom-Nickel-Stähle

Contribution to the stress corrosion cracking of austenitic chromium nickel steels The authors present a summarizing treatment of the transcrystalline stress corrosion cracking of chromium nickel steels in chloride solutions. The factors having a bearing on the susceptibility to this type of corrosion during operation, temperature and p_H of the medium. Residual stresses may be due to heat-input (during welding) or to grinding which, in addition, may give rise to notch effects. The medium generally used to test stress corrosion susceptibility (MgCl₂ solution) acts according to different mecanisms, depending on its concentration which, in term, has an influence on p_H and the boiling temperature. Quite generally it may be said that stress corrosion cracking always presupposes the existence of a lower of stresses, the amount of these stresses depending from the structural factors and from the surface condition.     

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.     

Allgemeine Beständigkeit von Schweißnähten unter Berücksichtigung der Spannungs- und Schwingungsrißkorrosion

General resistance of weld seams with a view to stress corrosion cracking and corrosion fatigue The corrosion of welds is due to thermal effects during welding which give rise to structural changes and, frequently, compositional changes in the transition zone. The welded material is rapidly cooled and may thus be heterogeneous and may present residual stresses resulting in increased susceptibility to selective and stress corrosion. The manganese content is of high importance in low alloy steels, as well as residual martensite or austenite embedded in a ferrite matrix. Low ferrite contents are generally beneficial because they counteract high temperature cracking; however, ferrite contents should be hept below 10% in order to prevent the formation of a continuous network giving rise to selective corrosion. Corrosion susceptibility may also be produced by carbide or carbonitride precipitation in austenitic and ferritic steels and nickel base alloys. Weld zones in aluminium alloys are attacked in rare cases (e.g. by HNO₃) and the susceptibility of Ta, Zr and Ti depends from the properties of the protective atmospheres.     

Untersuchung der Spannungsrißkorrosion von Baustählen in flüssigem Zink

Investigation into stress corrosion cracking of unalloyed steels in liquid zinc This investigation has been done in order to get further informations of stress corrosion cracking in galvanized steel. The samples were made of unalloyed steel with increased contents of carbon (up to 0,24%), silicon (up to 0,30%) and copper (up to 0,4%). Welded and notched samples have been equally tested. One batch of the samples was preloaded with 70, 80, 90 and 100% of yield point and then dipped into hot zinc (450°C). Another batch was tested in liquid zinc with constant strain rates of 5 · 10^?4, 5 · 10^?5, and 5 · 10^?6 · s^?1 during 250 h. None of the tested samples have been destroyed by stress corrosion cracking.     

Korrosionsprobleme in chlorhaltigen Medien: Lösungsmöglichkeiten durch einige nichtrostende Spezialstähle

Corrosion problems in chloride containing media: possible solution by some stainless special steels The increasing water pollution forces the chemical industry to use water with increasing chloride content for cooling and other purposes. This trend brings about increasing corrosion danger, in particular pitting, stress corrosion cracking and corrosion fatigue as well as crevice corrosion. The present paper deals with some steels characterized by resistance to these specific corrosion phenomena. A steel containing (%) 21 Cr., 7.5 Ni, 2.5 Mo, 1.5 Cu, to 2 Mn, to 1 Si and 0.06 C is particularly resistant to stress corrosion cracking. It contains 30 to 50% ferrite in an austenitic matrix. Even in Mg chloride solutions it may be kept under a load of 7 kg/mm² without stress corrosion occurring (with a steel of the 18 10 CrNiMo type the admissible load is only 2 kg/mm²). A steel containing (%) 25 Ni, 21 Cr, 4.5 Mo, 1.5 Cu, to 1 Si, to 2 Mn, and 0.02 C has a broad passivity range and is resistant to general corrosion in acid reducing media and phosphoric acid of all concentrations. A ferritic steel containing (%) 26 Cr. 1 Mo and minor additions of C, Mn, Si, Cu, Ni and nitrogen is resistant to stress corrosion cracking in neutral chloride solutions and general corrosion in oxidizing and neutral media, even against hydrogen sulfid and organic acids; it is beyond that lergely resistant to pitting in chloride solutions.     

Einfluß der Oberflächenbeschaffenheit auf die Beständigkeit nichtrostender,austenitischer Chrom-Nickel-Stähle gegen transkristalline Spannungsrißkorrosion

Influence of surface conditions on the resistance of stainless austenitic bromium nickel steels to transcrystalline stress corrosion Grinding weld seams may result in an increased susceptibility to stress corrosion cracking of the areas treated in that way. This susceptibility may be eliminated by an ultimate pickling step; the thickness of the layer to be removed is 0.15 to 100 μm, depending on surface roughness. The susceptibility to stress corrosion cracking of the ground zones can be determined, however, only in the boiling 42% MgCl₂-solution; no stress corrosion cracking could be produced in solutions containing from 10 to 3O% MgCl₂ or 10 to 2O% and even 40% CaCl₂. Another possibility to eliminate stress corrosion cracking susceptibility is an ultimate sand blasting which produces compressive residual stresses in the ground surface.     

Radiation defects and hydrogen in austenitic and austenitic-martensitic steels under low-temperature neutron irradiation

The experimental data concerning the effect of hydrogen (300 appm), radiogenic helium, and low-temperature neutron irradiation (77 K) on the properties of the promising austenitic 16Cr15Ni3Mo1Ti and austenitic-martensitic 16Cr9Ni3Mo steels have been reported. It has been found that hydrogen saturation causes an increase in the yield stress, with this increase being larger in the martensitic than in the austenitic phase. The yield stress of both steels increases substantially after exposure to fast neutrons. The variation of the yield stress of the two-phase steel and its phase components under low-temperature neutron irradiation has been estimated. The displacement cascades begin overlapping under irradiation at a fluence larger than 1.5 × 10¹⁸ cm⁻².     

Inter- und transkristalline Spannungsrißkorrosion austenitischer Mn- und CrNi-Stähle in Meerwasser

Intercrystalline and transcrystalline stress corrosion cracking of austenitic Mn and CrNi steels in seawater The MnCr steels which were originally used for the construction of special ships turned out to be susceptible to intercrystalline stress corrosion cracking in seawater; later on, transcrystalline corrosion susceptibility was found, too; this latter type of corrosion appears in cold seawater and is not due to a sensilizing annealing. This type of corrosion was also found with austenitic CrNi steels in chloride solutions of higher temperature, not, however, at room temperature. The author has made an effort at defining the susceptibility regions of the particular austenitic steels. According to the results obtained it is necessary always to take account of the possibility of stress corrosion cracking when Mn based austenitic steels are used, while austenitic CrNi steels can be considered to be resistant to this type of corrosion. Sensilizing annealing, too, has a bearing on the Mn containing steels only, while the influence of temperature, potential, specimen diameter and stress does not reveal any difference between the two types of steels.     

Einfluß der Oberflächenbehandlung nichtrostender Stähle auf deren chemische Beständigkeit insbesondere gegen Spannungsrißkorrosion

Influence of the surface treatment of stainless steels on their chemical resistance, in particular to stress corrosion cracking The stress corrosion resistance of austenitic stainless steels shows a pronounced dependence from the surface treatment. Grinding with a coarse material makes the surface very susceptible to this type of corrosion. The susceptible can be largely removed by subsequent picking, provided the treatment removes a layer about 3 μm in thickness. Grinding affects the structure to a depth of about 200 μm, but particular conditions appear to prevail in the above mentioned thin surface zone, so that in particular transcrystalline stress corrosion may occur. The susceptibility to stress corrosion cracking can be tested with a solution containing (%) 0.06 acetic acid, 1 acetaldehyde and 100 ppm Cl ions (as CuCl₂) when the corrosion susceptibility is to be evaluated in comparative terms. Pitting corrosion by mixed acid, too, can be largely prevented by deep picking of ground material.