Eigenschaften des schweißbaren,hoch korrosionsbeständigen Stahles X 2CrNiMoN 25 22

Properties of the weldable and corrosion resistant steel X 2 CrNiMoN 25 22 In view of the limited application of con- ventional CrNiMo steels the authors have investigated potential applications of the above mentioned steel. An optimum com- position would be (%) 0.02 C, 25Cr, 22 Ni, 2 Mo and 0.1 N. The low carbon content is a vital factor and the nickel content must not exceed 22% because higher contents would give rise to very pronounced precipitation. The new steel ist largely resistance to boil- ing nitric acid it is considerably superior to conventional steels, The high break-through potential is indicative of high pitting resist- ance. Improvements are equally remarkable with respect to mecanical properties and weldability     

Einfluß der Kerbgeometrie auf das Dauerschwing- und Schwingungsrißkorrosionsverhalten des Duplexstahles X 2 CrNiMoN 22 5 3

Influence of notch geometrie on the fatigue and corrosion fatigue behaviour of duplex stainless steel X 2 CrNiMoN 22 5 3 The fatigue behaviour of the ferritic-austenitic steel X 2 CrNi MoN 22 5 3 (German number of materials 1.4462) is investigated under cyclic tension load with a frequency of 25 Hz. Four different notch shapes with stress concentration factors varying from α_k = 1. 0 to 3.9 are researched. The experiments are carried out in laboratory air and in 3% NaCl-solution with pH = 7, both at room temperature. In air the fatigue limit of smooth specimens amounts to 490 N/mm². An increase of the stress concentration factor causes a reduction of the fatigue limit to 285 N/mm² for α_k = 3.9. The corrosion fatigue limits in 3% NaCl-solution for N = 10⁷ are 20% lower than the corresponding values in air. The calculated fatigue stress concentration factors for air and corrosive environments reveal nearly identical results. The recordings of the free corrosion potential show characteristic results. For all specimens, which do not fail under the mechanical-corrosive load, the potential remains in the passive state during the whole test duration. Higher mechanical loaded specimens announce their rupture by a sudden decrease of the potential to negative values. For equal fatigue life the remaining time until rupture of the specimen is considerable longer for higher stress concentration factors.     

Die Beeinflussung des Schwingungsrißkorrosionsverhaltens des Stahles X 20 Cr 13 durch einen Molybdänzusatz in chloridhaltigen Medien

Influence on the fatigue corrosion behaviour of the steel X20Cr13 in chloride containing media by molybdenum additions The fatigue corrosion behaviour of a 13% Cr steel (used for turbine blades) has been studied in terms of its dependence on carbide shape and molybdenum additions in aqueous NaCl solutions of various concentrations at temperatures between room temperature and 200°C. Further tests in autoclaves were carried out in order to study the surface layers formed above 150°C without mechanical loads. Addition of 1% molybdenum yields a considerable increase of corrosion fatigue strength at room temperature, while there is no influence attributable to different shapes of graphite precipitations. At 150°C the corrosion fatigue strength is largely independent from sodium chloride concentration (0,1 to 27%); this means that very small quantities of sodium chloride are sufficient to produce heavy corrosion in wet steam at > 106 cycles. A range particularly suitable for such tests is the load range between ± 150 and ± 200 N/mm² at 150°C, because these conditions approach those encountered in practice. The passivating layers are very thin (interference phenomena). Considering the increase of corrosion resistance produced by 1% Mo it is expected that 2.5% Mo will produce a further improvement, in particular with regard to pitting corrosion since rupture generally starts at the pits.     

Untersuchung des SwRK-Verhaltens des hochreinen martensitaushärtenden Stahles X5CrNiCuNb 17 4 PH im Vergleich zum weichmartensitischen Stahl X4CrNiMo 16 5 1 ESU in chloridhaltigen wäßrigen Medien. Teil 2: SwRK-Untersuchungen und Rißeinleitungsmechanismen

Investigations of the corrosion fatigue behaviour at a super pure martensitic stainless steel X5CrNiCuNb 17 4 PH in comparison to the soft martensitic stainless steel X4CrNiMo 16 5 1 ESR in chloride containing aqueous media — Part 2: Corrosion fatigue tests and crack initiation mechanisms The following report concerns the study of the corrosion fatigue behaviour of the soft martensitic steel X4CrNiMo 16 5 1 ESR and the precipitation hardened X5CrNiCuNb 17 4 PH in sodium solution in the temperature range between 20° and 150 °C and the determination of their general corrosion properties and the mechanism of crack propagation. Their corrosion fatigue limits were compared with each other. A comparision was also made between an electro-slag-remelted soft martensitic steel and a charge without an ESR aftertreatment. Microfractographical fracture and crack path investigation were carried out for interpretation of the experimental results. It was observed that in both super pure steels (soft martensitic and precipitation hardened) the oxidic inclusions are not responsible for the crack intiation, as it was found in the non ESR treated steels. In the 17-4 PH steel copper containing inclusions in the crack initiation areas were observed. In concentrated sodium solution pitting corrosion was found at both steels.     

SwRK-Versuche und Rißeinleitungsuntersuchungen am neuentwickelten Duplexstahl X 3 CrMnNiMoN 25 6 4 im Vergleich zum bekannten ferritisch-austenitischen Stahl X 2 CrNiMoN 22 5

Investigation into corrosion fatigue and crack initiation with the newly developed duplex steel X 3 CrMnNiMoN 25 6 4 by comparison with the known ferritic-austenitic steel X 2 CrNiMoN 22 5 The majority of low pressure steam turbine damages are caused by corrosion fatigue. This kind of damage occurs primarily in the Wilson-zone of steam turbine machines. In spite of careful feed water washing there is still a build-up of damaging chloride-containing water impurities on the turbine blades. During the last twenty years the standard material X 20 Cr 13 often was and is still used for industrial turbine applications, although the corrosion fatigue resistance was insufficient concerning the increasing operation conditions of moder steam turbine machines. Modifications of the chemical composition of the steel and further material development did not reach a satisfying corrosion fatigue resistance till today. Recent investigations of duplex stainless-steels, especially the steel X 3 CrMnNiMoN 25 6 4 (A905; austenite/ferrite = 50/50) show both a good corrosion behaviour and a sufficient strength of this type of steels. In the present investigations the dependence on the corrosion fatigue limit on the pH-value, chloride concentration, temperature and the behaviour of corrosion cracking (crack initiation) is evaluated.     

Auswirkungen des elektrochemischen Probenpotentials auf das SwRK-Verhalten des Duplexstahles X 2 CrNiMoN 22 5 3 in NaCl-Lösung

Effect of electrochemical potential on the corrosion fatigue behaviour of duplex stainless steel X 2 CrNiMoN 22 5 3 in NaCl-solution The influence of the electrochemical potential on the corrosion fatigue behaviour of the nitrogen-alloyed ferritic-austenitic steel X 2 CrNiMoN 22 5 3 (german material-number 1.4462) is investigated in 3% NaCl-solution with pH = 7 at room temperature. Smooth and notched (α_k = 2,0) specimens are tested under cyclic tension load (σ_μ/σ₀ = 0) with a frequency of 25 Hz up to 10⁷ cycles. The fatigue limits for both specimen geometries in corrosive environment at free corrosion potential reach only about 78% of the according values in air. In potentiostatic controlled corrosion fatigue tests the fatigue life respectively the fatigue limit is higher than under free corrosion. The chosen cathodic (?300 mV_SHE) and anodic (+ 700 mV_SHE) protection potentials cause important improvement to 500 N/mm² or 510 N/mm² for smooth specimens (fatigue limit in air: 490 N/mm²). For notched specimens there is an improvement to values of 98% respectively 91% of the air fatigue limit. The recordings of the polarisation current show a sudden increase of the current at the fatigue crack initiation and a further increase during crack propagation. ESCA-analytical results of the passive films for different potentials verify the better corrosion fatigue behaviour for the tested potentials.     

Schwingungsrißkorrosionsverhalten des austenitischen Stahles X 6 CrNiMoTi 17 12 2 in 3%iger NaCl-Lösung unter Umlaufbiegebelastung

Corrosion fatigue behaviour of the austenitic steel X 6 CrNiMoTi 17 12 2 in 3% NaCl-solution under rotating bending load The fatigue behaviour of steel X 6 CrNiMoTi 17 12 2 (German material-number 14571, comparable to AISI 316 Ti) is investigated under rotating bending load with a frequency of 50 Hz. The fatigue strength in inert medium (thermostat oil) at room temperature is 315 N/mm². In 3% NaCl-solution of pH 7 at the same temperature the corrosion fatigue limit for N = 10⁷ reached only 94% of the mentioned value. A higher temperature (70 °C) caused a further reduction to 89% and an additional decrease of the pH-value (pH 3) lowers the dynamic strength to 86% of the reference value. The free corrosion potential, recorded during all corrosion fatigue tests, remains in the passive region for all specimens, which do not fail under the mechanical-corrosive load. The potential of specimens, which fail under a higher mechanical load, announces their rupture by a sudden decrease to negative values, which starts at least 3·10⁴ cycles (10 minutes) before.     

Untersuchung des SwRK-Verhaltens des martensitaushärtenden Stahls X 5 CrNiCuNb 17 4 PH im Vergleich zum weichmartensitischen Stahl X 4 CrNiMo 16 5 1 ESU in chloridhaltigen wäßrigen Medien. Teil 1: Korrosionsuntersuchungen und SpRK-Untersuchungen zur Optimierung der Wärmebehandlung bezüglich der SpRK-Beständigkeit

Investigations of the corrosion fatigue behaviour at a super pure martensitic stainless steel (X 5 CrNiCuNb 17 4 PH) in comparison to the soft martensitic stainless steel X 4 CrNiMo 16 5 1 ESR in chloride containing aqueous media — Part 1: Corrosion investigations and stress corrosion tests to optimize the heat treatment according to the stress corrosion resistance The aim of this investigation was to improve the reliability of operation and the economy of corrosion fatigue stresses of structural elements, especially at elevated temperatures, by optimizing the material. This investigation is of great interest both to the industry and to related fields. It concerns the influence of a higher degree of purity achieved by the secondary metallurgical aftertreatment of soft martensitic steel X4CrNiMo 16 5 1 ESR (ESR-electroslag remelting) in comparison to the corrosion fatigue behaviour of the precipitation hardening steel X5CrNiCuNb 17 4 PH, which was specially heat-treated with respect to stress corrosion cracking resistance. The stress corrosion was investigated for all heat-treatments of the 17-4 PH in 22% NaCl(pH3) solution. The precipitation hardening steel was most resistant to stress corrosion in concentrated NaCl-solution after a three-stage heat-treatment. There was no improvement of corrosion fatigue resistance after metallurgical aftertreatment of soft martensitic steel compared to the untreated material. This is due to the instable passive behaviour of the material which led to crack initiation, especially during the 150°C experiments, at chloride-induced places of pitting. The investigation of the electrochemical corrosion behaviour of both materials showed that the pH-value hardly influences corrosion restistance. An increase of the salt content leads to higher pitting induction. At temperatures of 80°C in a saturated NaCl-solution the material showed no corrosion resistance. In potentiokinetic investigations, a direct transition from the active area to the pitting potential was observed. In accordance with both the corrosion fatigue and the stress corrosion cracking investigations, it was found that pitting at the martensite precipitator starts primarily around Cu-containing or oxidic inclusions.     

Selektive Korrosion von Duplexstahl. Teil 1: Aussagekraft herkömmlicher und neuartiger Methoden zur Untersuchung des Korrosionsverhaltens von Duplexstahl X2CrNiMoN22‐5‐3 unter besonderer Berücksichtigung der Mikrostruktur

Selective corrosion of duplex stainless steel. Part 1: Corrosion behaviour of duplex stainless steel X2CrNiMoN22‐5‐3 with special consideration of the microstructure This paper is dealing with investigations on interrelations between microstructure and corrosion behaviour of duplex stainless steel X2CrNiMoN22‐5‐3. With the application of conventional methods like Strauß‐ and Huey‐test it is not possible to describe correctly the corrosion behaviour. In contrary, by means of the development of the electrochemical potentiodynamic reactivation (EPR) method it is possible to give a reliable proof of corrosion susceptibility. The knowledge about the corrosion of duplex stainless steel was intensified by fundamental investigation of interrelations between precipitations and corrosion behaviour. By means of numerous investigations on the microstructure by means of SEM and TEM secondary phases were analysied and quantified and depletion areas created by precipitations were characterized.     

Selektive Korrosion von Duplexstahl. Teil 2: Lokale Korrosionserscheinungen an Duplexstahl X2CrNiMoN22‐5‐3 unter Einwirkung von Chloriden und mechanisches Verhalten in Abhängigkeit vom Gefügezustand

Selective corrosion of duplex stainless steel. Part 2: Localized corrosion manifestations under exposure of chlorides on duplex stainless steel X2CrNiMoN22‐5‐3 and mechanical behavior in dependence of the microstructure In completion to part 1 of this paper this part deals with interrelations between localized corrosion manifestations, mechanical properties and the microstructure of duplex stainless steel X2CrNiMoN22‐5‐3. The pit formation on duplex stainless steels is substantially determined by the distribution of the alloying elements within single phases and by defects in the oxide layer. The positive properties of molybdenum become ineffective at thicker oxide layers due to the fact, that molybdate, which is responsible for inhibition of pitting, can not be formed. Depletion areas caused by precipitations are preferential attack places for corrosion. The influence of chlorides for duplex stainless steels in rolled and welded conditions is characterized by a logarithmic dependence.     

Die Lochkorrosionsanfälligkeit des Vergütungsstahles X20Cr13 in Natriumchloridlösungen

Pitting corrosion susceptibility of the heat treatable steel X20Cr13 in sodium chloride solutions For blades in steam trubines it is common to use the heat-treatable steels with 13% Cr (X20Cr13). Cracking of blades in the region of the turbine, where the steam starts to get wet, have often been referred to be caused by corrosion fatigue. The fatigue cracks sometimes start at inclusions in the material, but more often at pits caused by pitting corrosion. Pitting is brought about by chloride containing blade deposits. To examine the pitting behaviour of the steel in NaCl-solutions we measured the mixed-potential, the potentiostatic piting potential, the mass-loss and the pits per area as depending on temperature (20–80°C), pH (5–7–9), oxygen content (20 μg/kg up to saturation with oxygen) and chloride content (8×10^?5 mole/l up to saturation with NaCl 5,4 mole/l). The results show that steel X20Cr13 suffers pitting corrosion because of it's low Cr content in all technical possible NaCl solutions down to a Cl content of 10^?5 mole/l at the mixed potential.     

Entwicklung und Aufbau einer Meßmethode zur realitätsnahen Beurteilung des Spaltkorrosionsverhaltens am Beispiel von Aluminiumwerkstoffen

Development and construction of a measuring technique for a realistic evaluation of the crevice corrosion behaviour by way of aluminium alloys Aluminium alloys represent a high percentage of materials used in aircraft. Complex designs such as those found in the aircraft industry illustrate a number of constructively determined geometrical crevices. As always the crevice corrosion behaviour of materials is difficult to reconstruct and evaluate in a laboratory. The authors present an experimental set up which reflects the situation in constructively determined crevices. It will be shown, with the use of selected aluminium alloys, that a quite good evaluation of crevice corrosion behaviour of materials is possible. The acidification and enrichment of selected species can be observed very well and their reaction on crevice corrosion can be semi‐quantitatively estimated. The interpretation of the corrosion results is overall improved.     

Korrosionsschwingfestigkeit der Stahlgußsorten G-X5CrNi 13 4 und G-X5CrNi 17 4 für Laufräder von Wasserkraftmaschinen und Pumpen

Fatigue strength of steel types G-X5CrNi 13 4 and G-X5CrNi 17 4 used for blades of water-power turbines and pumps In this project the influence of different corrosive media on the fatigue behaviour of stainless cast steel alloys with 13 and 17% chromium content for hydraulic machines was investigated in the high cycle range (10⁶ < N < 10 ₈). Hereby the possible range of media was covered by using tap water with 28 ppm chlorides, 10 mMol NaCl, as well as 1% NaCl and in the case of the higher alloyed steel additionally artificial sea water without and with cathodic protection. Further the influence of high mean stresses due to rotation of runners was also analysed. The results obtained on the loss of fatigue strength in dependence of corrosive media, mean stress and cycles to failure enable an aimed material's selection as well as an appropriate design and increase the reliability and security of water power machines.     

Einflüsse von Werkstoff und Verarbeitung auf die Spannungsrißkorrosion von Spannstählen

Influence of material and processing on stress corrosion cracking of prestressing steel In prestressed concrete constructions the highstrength prestressing steels perform essential bearing effects. The alkaline layer of concrete or mortar protects the steels against corrosion and guarantees a permanent load capacity. If the corrosion protection as a result of poor workmanship is not guaranteed from the beginning, or is lost because of lacks of construction in the course of time, or the steels are predamaged during handling, stress corrosion cracking and failure of steel and construction may occur. Also an application of unsuitable materials (prestressing steel, injection mortar, concrete) can alone or in combination with the before mentioned influences favour stress corrosion cracking. In the contribution the correlations and typical failures are discussed.     

Einfluß des Elektronenstrahl- und Schutzgasschweißens auf das Korrosionsverhalten von X5 CrNiMo 18 10

The influence of cathode ray and inertgas arc welding on the corrosion behaviour of X 5 CrNiMo 18 10 When subjected to the Strauss test, the specimens welded by these welding processes are found to be not prone to intercrystalline corrosion. With the Huey test, on the other hand, the weld produced by inert-gas arc welding is much more prone to corrosion than the weld produced by cathode ray welding or the base metal. This is probably due to the fact that, because of the high energy density of the cathode ray, a greater homogeneity of the structure is obtained. In particular, the ferrite content, becomes Lower than with arc welding where the heat concentration is lower. In practice, however, ferrite formation is of comparatively little importance. This is because selective corrosion will only occur if the homogeneous austenite of the 18/18 steels is likewise showing marked corrosion, e. g. in certain mixtures of reducing and oxidizing acids (hydrochloric or hydrofluoric-acid with nitric acid).     

Der Einfluß des Prüpotentials und der Wärmebehandlung auf das Korrosionsverhalten niob- und titanstabilisierter ferritischer Chromstähle mit rund 17% Chrom

The influence of test potential and heat treatment on the corrosion behaviour of ferritic chromium steels stabilized with niobium or titanium By means of corrosion-chemical, electrochemical, metallographical and electronmicroscopical investigations, the influence of the electrode potential and the heat treatment on the corrosion behaviour of steels of the X 8 CrTi 17 and X 8 CrNb 17 has been determined. With ferritic 17 pC chromium steels, a distinction must be made between two different types of grain boundary corrosion:

• (1) Grain boundary corrosion on steels which were quenched at high temperatures and therefore sensitized, unstabilized or understabilized, due to the segregation of chrome-rich carbides at the grain boundaries, causing a chromium reduction in their vicinity (typical inter-crystalline corrosion), and
• (2) Grain boundary corrosion on steels quenched at high temperatures but fully stabilized, due to the chemical dissolution of the type MX carbonitrides segregated at the grain boundaries during quenching (carbide corrosion).

The heat treatment conditions conducive to carbide corrosion were determined, and the correlation of this type of corrosion with the potential was ascertained by potentio-static tests in sulphuric acid and compared with the behaviour of synthetic carbides. The different corrosion behaviour of the ferritic chromium steels quenched at high temperature and stabilized with niobium and titanium, respectively, is attributed to the different chemical dissolution rates of the carbo-nitrides segregated. The findings also provide an explanation of the corrosion behaviour of sensitization-annealed, stabilized austenitic chrome-nickel steels in acid solution.     

Zur Korrosionsreaktion des Eisens in Salzsäure,Schwefelsäure und Perchlorsäure

Contribution to the corrosion reaction of iron in hydrochloric, sulfuric and perchloric acids At pH values between 0 and 4, even under circulation, a linear relationship results between log j_corr and the pH value, the slope of the corrosion straight line being between ?0.23 and ?0.27. The dissolution process is predominantly under cathodic control by hydrogen ion discharge; at pH values above 3 the diffusion inhibition of the hydrated proton is the rate controlling step. The corrosion potential in sulfuric and perchloric acids decreases by about 50 mV as the pH is increased, while a different behaviour is observed in hydrochloric acid; this behaviour may be explained in terms of the chloride ion adsorption. In the acids of higher concentration the activation energy is higher than in those of lower concentrations; this phenomenon may eventually be ascribed to a change in the mechanism of the corrosion reaction.     

Einfluß von Beschichtungen auf das Schwingungsrißkorrosionsverhalten des Chromstahls X20Cr13

Influence of coatings on the corrosion fatigue behaviour of 13% chromium steel The influence of coatings on the corrosion fatigue behaviour of 13% chromium steel has been studied. There have been selected different coating systems: Barrier coating (enamel), diffusion coatings, (aluminizing, chromizing) and anodic coating, (aluminium, zinc, tin, cadmium). The corrosion fatigue limits of coated with uncoated specimens in neutral NaCl-solution are compared. Salt-concentrations were 0,01 and 22% (? 0,38 M) NaCl at 80°C and 150°C. The tests were carried out with alternating tensions and a constant frequency of 50 Hz. Only the use of andic coatings improved the corrosion fatigue behaviour of the chromium steel.     

Korrosion und Deckschichtaufbau von X5 CrTi 12, Werkst.-Nr. 1.4512, bei Beanspruchung in Salpetersäure

Corrosion resistance and formation of protective surface films on X5 CrTi 12 in nitric acids Austenitic chromium-nickel steels predominate in the nitric acid industry, since the stainless ferritic chromium-steel grades are more difficult to process and insufficiently resistant to corrosion in hot nitric acid. Longitudinally welded tubing made of X5 CrTi 12, Material No. 1.4512, has been manufactured in large quantities for the motor vehicle industry in recent years. In view of the low cost and the clear relationship between temperature and corrosion of metallic materials in nitric acid, tests were performed to ascertain whether such tubing can be used to transport cold or only slightly warm (sub-)azeotropic nitric acid. In azeotropic nitric acid which does not contain additional oxidizers, corrosion rates < 0,01 g/(cm² · h) with negligible intercrystalline attack can be expected at 40°C; consequently tubes made of X5 CrTi 12 under these conditions thoroughly can be used. The surface of the tubing first turns light brown and finally blue black. These colour effects are attributable not only to the thickness of the oxide layer caused by exposure to nitric acid, but also to the depth-dependent variation in the composition of the layer and the natural colour of the oxides.     

Einsatz des Wirbelstromprüfverfahrens zur zerstörungsfreien Bestimmung der Angriffstiefe von Korrosionsschäden an Flugzeug Tragflächenbeplankungen

Application of eddy-current method for nondestructive determination of corrosion attack depth at aircraft wing skins A NDI test system for the determination of the depth of corrosion attack in the form of shallow pit formation and pitting is presented. The system is based on the eddy-current method, whereby the signals of a specially developed probe are converted to an image representation, which can he evaluated quantitatively at a PC-screen. The described test system was developed for the field of in-service inspection of aeroplanes. As a first practical operation, the system was tested in co-operation with Daimler-Benz Aerospace at TORNADO aeroplanes. All the described results can be transferred to other test situations with comparable damage mechanisms. Because of the Skin-effect of the eddy current, this method is limited to surface-near areas however. If the test object comes as metal sheets, depth evaluation of corrosion attack on the hidden surface is also possible (even in multi-layered structures).