Eigenschaften eines neuen nichtrostenden austenitischen Stahles mit erhöhter Festigkeit

Properties of a new stainless austenitic steel with improved tensile strength The paper presented a new nitrogen-alloyed stainless austenitic steel-grade X 4 CrNiMnN 18 6 3 with improved strength compared with grade 1.4306 as concerns: mechanical properties at room and elevated temperature, intercrystalline corrosion, corrosion rate in solutions of nitric, hydrochloric and sulfuric acids, weldability. Essential increase of mechanical properties in steel-grade X 4 CrNiMnN 18 6 3 compared with traditional 18Cr8Ni steels, without change in corrosion resistance and weldability was achieved by addition of nitrogen (0,11–0,26%).     

Herstellung und Eigenschaften stickstofflegierter,korrosionsbeständiger Stähle und Sonderstähle mit niedrigen Kohlenstoffgehalten

Production and properties of nitrogen alloyed, corrosion resistant steels and special steels with low carbon contents Alloying with nitrogen has favourable influence in particular on the mechanical properties of CrNiMo steels (X 2 CrNiMoN 17 12, materials No. 1.4406, X 2 CrNiMoN 17 13 5, materials No. 1.4439 und X 2 CrNiMoN 22 5, materials No. W.-Nr. 1.4462). This comes to bear when ambient temperature and low temperature strength and toughness are concerned. With respect to the corrosion behaviour the data concerning the effect of nitrogen are contradictory. It has become clear that nitrogen improves pitting corrosion resistance; this applies, however, only to pit initiation but not to pit growth. Stress corrosion cracking is not delayed by nitrogen but different results have been obtained with different media: while the duplex steel X 2 CrNiMoN 22 5 is attacked considerably faster than the corresponding nitrogen-free steel in 42% boiling magnesium chloride solution the time-to-failure of both steels are comparable in 30% boiling MgCl₂-solution. The nitrogen alloyed steels can be welded by all known welding procedures, provided fully austenitic welding rods are used.     

Application limits and sensitisation behaviour of the manganese- and nitrogen-alloyed austenitic stainless steel P2000 (X13CrMnMoN18-14-3)

Nickel-free high-nitrogen-alloyed stainless steels like the P2000 (X13CrMnMoN18-14-3) were developed to enhance the strength and corrosion resistance of austenitic stainless steels like 304 and 316 while keeping the typical high ductility. The mechanical and corrosive properties of P2000 were investigated and compared with 304 and 316 to highlight the application opportunities of this new alloy. The microstructure of the solution-annealed condition was characterised by electron backscatter diffraction and the mechanical properties were studied by uniaxial tensile tests, Charpy impact tests and hardness measurements. The passivation behaviour was analysed using the electrochemical potentiodynamic reactivation, whereas the pitting corrosion resistance was compared by pitting potentials and pitting temperatures. However, secondary thermal influences or suboptimal heat treatment can impair the corrosion resistance due to the precipitation of secondary phases and the resulting sensitisation. Thermodynamic calculations and artificial ageing treatment in the range of 500–900°C for up to 100 h were used to determine critical time–temperature parameters for sensitisation. The microstructure of the various aged states was evaluated by scanning electron microscopy and compared with the degrading corrosion resistance characterised by the KorroPad method.     

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.     

Corrosion‐technical properties of high‐strength stainless steels for the application in prestressed concrete structures

Experience with prestressed concrete over about half a century has indicated that the corrosion resistance of conventional prestressing steel does not always satisfy, especially the prestressing steels are susceptible to chloride attack (de‐icing salts) and hydrogen (hydrogen‐induced stress corrosion cracking). On the other hand corrosion agents, such as chloride, condensation water, can penetrate in the concrete and arrive at the surface of steels. Hence, corrosion damage of prestressing steels can happen and, in the extreme cases, the prestressed concrete structure collapsed resulting from the failure of the tendon. In this paper, consideration is made to use high‐strength stainless steels as prestressing tendon with bond in concrete. The high‐strength stainless steels of qualities 1.4301 (X5CrNi18‐10), 1.4401 (X5CrNiMo17‐12‐2), 1.4436 (X3CrNiMo17‐13‐3) and 1.4439 (X3CrNiMoN17‐13‐5) with sequence of increasing austenite stability were investigated. For application in prestressing tendon with bond in concrete the cold‐drawn high‐strength stainless steel of quality 1.4401 is an optimal proposition regarding its satisfactory resistance against pitting corrosion and stress corrosion cracking (SCC) in structure‐related corrosive conditions. The lower alloyed steel 1.4301 has an insufficient resistance against the chloride‐induced corrosion because of the lack of molybdenum and the content of deformation martensite due to the strong cold‐drawing of its unstable austenitic structure.     

Kurzzeituntersuchungen zur Auswahl von rostbeständigen Cr-Stählen mit verbessertem Widerstand gegen Schwingungsrißkorrosion

Short-time investigations into the selection of stainless chromium steels with improved corrosion fatigue resistance Using the steel X 20 Cr 13 various short-time test methods have been tested for evaluating the corrosion fatigue susceptibility in concentrated air-saturated sodium chloride solution. In addition the steels X 15 CrNi17, X 35 CrMo 17, X 5 CrNiMoCuNb 14 5, X 4 CrNiCuNb 16 4, X 2 CrNiMoN 22 5 3, X 5 CrNiMoCu 25 5, X 8 CrNiMo 27 5 and X 1 CrNiMoNb 28 4 2 have been studied at room temperature of 80° C using crack propagation, constant straining and pitting corrosion methods. The two last mentioned steels are superiour under the test conditions.     

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.     

Zum Korrosionsverhalten nichtrostender austenitischer Chrom-Nickel-(Molybdän,Kupfer)-Stähle in nahezu wasserfreier Essigsäure

Corrosion of stainless austenitic steels in almost anhydrous acetic acid As-welded samples and looped specimens from 5 differently alloyed stainless steels were tested for up to 246 days in 99,5% to 99,95% acetic acid at 118°C (boiling temperature/normal pressure) and at 150°C; the chloride content was varied between < 1 and 100 ppm. Pitting corrosion – of shallow depth, however (approx. 0,1 mm) – was already observed at surprisingly low chloride concentrations. Only the following were found to be resistant to pitting corrosion:

• – stainless steels 1.4439 and 1.4539, containing approx. 4,5% molybdenum, in 99,5% acetic with < 1 ppm chloride at 118 and 150°C,
• – stainless steels 1.4439 and 1.4539 in 99,9% acetic acid with < 1 ppm chloride at 118°C, and
• – special stainless steel X 2 CrNiMoCuN 20 18 6, containing approx. 6% molybdenum, in 99,5% acetic acid with > 3, < 10 ppm chloride at 118 and 150°C.

Looped specimens and ground as-welded samples showed no sensitivity to transcrystalline, chloride-induced stress corrosion cracking at any of the concentration ranges. High surface-removal rates can be expected if air has access to the specimens; under this condition pitting corrosion and general corrosion may overlap. Contamination of acetic acid with chlorides must be prevented under all circumstances.     

Vergleichende Untersuchungen des SwRK-Verhaltens verschiedener Dampfturbinenschaufelstähle

Comparative investigations into the corrosion fatigue behaviour of different steam-turbine-blade steels Corrosion fatigue tests in NaCl-solution in the temperature range from ambient temperature to 150°C revealed a decrease of the corrosion fatigue strength of the steels X20Cr13 with 1% resp. 2.5% Mo, X4CrNiMo 16 5 1 and X2CrNiMoN 22 5 with increasing temperature; a minimum of the corrosion fatigue strength is reached at 150°C. At temperatures of about 80°C it was found an optimum corrosion fatigue behaviour of the Mo-alloyed steel X20CrMo13 2.5 in comparison with the other investigated steels. It was observed that crack initiation of the ferritic, Mo-alloyed steel is caused by Cl-induced pitting, of the soft martensitic steel by oxide inclusions and of the duplex steel by non metallic, brittle inclusions and/or by grain boundarys of the ferritic/austenitic microstructure. Crack initiation caused by pre-deformation at slip bands was observed on the duplex steel. All materials investigated failed at elevated temperatures. At temperatures higher than 80°C an optimum corrosion fatigue behaviour was not found.     

Untersuchungen über den interkristallinen Kongrenzenangriff von austenitischen Mangan-Chrom-Stählen durch Wasser und wäßrige Lösungen

Research on intercrystalline grain boundary corrosion of austenitic manganesechrome steels caused by water and aqueous salt solutions Stressed bar specimens of welded and tempered austenitic manganesechrome steel such as as X 40 MnCr 18 and X 22 MnCrNi 885, exposed to sea water, tap water or destilled water, are liable to suffer intercrystalline stress corrosion cracking. The occurrence of intercrystalline corrosion is not necessarily dependent on mechanical tensil stress. In particular, even unstressed specimens exposed to tap or distilled water are liable to show marked intercrystalline corrosion. Additional tensile stress merely has the effect of speeding up the inter-crystalline corrosion. The cause for the intercrystalline corrosion must be seen in the segregation of chrome-rich carbids at the grain boundaries and the consequent reduction in the chrome content within the grain boundary zone. It is possible to prove the segregation of a chrome-rich carbide of the structure M₇C₃ at the grain boundaries of steel X 40 MnCr 18, and the aggregation of a chrome-rich carbide of the structure M₂₃C₆ at the grain boundaries of steel X 22 MnCrNi 885. Through contact with zinc, it is possible to obtain a complete corrosion protection with the manganese-chrome steels investigated. A short circuit with passive austenitic chrome-nickel steels increases the general or selective corrosion rate through anodic polarisation. By adding chromate to the sea water, the corrosion rate is retarded. In tap water, an addition of chromate result in complete corrosion protection.     

Versprödungserscheinungen an ferritischen Chromstählen und deren Einfluß auf das Schwingungsrißkorrosionsverhalten

Embrittlement conditions of ferritic chromium steels and influence on the corrosion fatigue behaviour The corrosion fatigue behavior of two ferritic stainless steels, with 18% and 28% chromium respectively, embrittled by α'- or sigma-phase has been studied in this investigation. No effect of the α'-phase on the corrosion fatigue behaviour in a 0.5 N or 4 N air saturated soldium chloride solution at 80 °C has been found. In the 4 N solution corrosion fatigue cracking of the 18% chromium steel in the solutionized as well as in the α'-embrittled condition is initiated by pitting at nonmetallic inclusions. Sigma-phase not only decreases the mechanical properties of the 28% chromium steel by heavy embrittlement but also leads to crack initiation by pitting in the 4 N solution at the interface sigma/ferrite. The pronounced decrease of the corrosion fatigue properties of the 28% chromium steel could well be correlated with chromium depletion of the ferrite matrix by precipitation of sigma phase.     

Oberflächenbehandlung als Korrosionsschutzmaßnahme von nichtrostenden Stählen

Surface treatment as corrosion protection measure of stainless steels The pickling behaviour of several stainless austenitic steels and of one steel with ferritic/austenitic grain structure were investigated in pickling solutions of different compositions based on hydrofluoric acid. Because of uncertainties in practical applications, the influence of temperature, time and acid content on the mass loss is of high interest. In another series of experiments, aqueous solutions of citric acid were tested for their suitability as pickling chemicals for the materials X 6 CrNiTi 18 10 (AISI 321) and X 6 CrNiMoTi 17 12 2 (AISI 316 Ti). Finally, the pickling procedures based on nitric acid/hydrofluoric acid mixtures were compared with mechanical cleansing methods and with pickling procedures based on aqueous citric acid solutions as well, to elucidate their influence on the corrosion resistance of the treated materials. The valuation followed a pitting corrosion test in sodium chloride solutions of different concentrations after Herbsleb and Schwenk. Pickling with hydrofluoric acid solutions is superior to other cleansing procedures, if corrosive environments are present. The ecologically beneficial citric acid solutions are only able to remove the annealing colours from stainless steels.     

Einfluß unterschiedlicher Anlaßtemperaturen auf die Korrosionszeitfestigkeit eines molybdänlegierten 13%igen Chromstahls in chloridhaltigen Medien

Influence of various annealing temperatures on the long-term strength under corrosion of a molybdenum alloyed 13% chromium steel in chloride containing media Corrosion fatigue tests were performed at varied levels of tensile strength of the material X 20 CrMo 13 at temperatures of 150 and 200 °C in 0.01 n solution of NaCl. A considerable decrease in the resistance to corrosion fatigue could be observed with increasing tensile strength of the material. As expected, the material shows a distinct sensitivity to stress corrosion at the highest tensile strength. Additional investigations of the morphology of the fracture proved that stress corrosion mechanisms are not directly controlling the formation of fracture surfaces. Spots of pitting corrosion on the surface of the samples can be coordinated with the amount of stressing. Pitting and fracture initiation under corrosion fatigue conditions, however, cannot be coordinated. Analyses of the microstructure reveal a layer on the samples which is expected to have a good protective effect against corrosion. Obviously the course of the fatigue corrosion depends on the rebuilding time of passivity which should be influenced by alloying of molybdenum. Under the selected testing conditions, however, the alloying of molybdenum was not yet sufficient to obtain a distinct improvement of the resistance to corrosion fatigue. The investigations will he carried on with the chosen material with higher amounts of alloyed molybdenum.     

940X新型不锈钢的耐蚀性能

通过盐雾试验和点腐蚀电位测试,对比了新型940X不锈钢和9Cr18不锈钢的耐蚀性能.结果表明,在NaCl溶液中940X不锈钢的耐全面腐蚀和耐点腐蚀的性能比9Cr18的高.并结合化学成分及金相能谱分析了940X不锈钢的耐蚀机理.     

双级时效制度对6156铝合金组织和性能的影响

采用拉伸力学性能测试、电导率测定、晶间腐蚀实验及透射电镜分析等手段研究双级时效处理条件下6156铝合金的力学性能、电导率、晶间腐蚀和显微组织结构,采用正交实验优化双级时效工艺。结果表明:在本研究范围内,6156铝合金双级时效的四因素中第一级时效制度对合金的力学性能和电导率影响不大,第二级时效温度和时间是影响合金最终性能的主要因素。对于6156铝合金,最佳双级时效工艺为(175℃,6 h)+(210℃,5 h),相对于T6态,合金强度稍有降低,电导率上升,腐蚀类型也由晶间腐蚀转变为点蚀,腐蚀深度明显变浅。电镜观察结果表明:双级时效处理后,晶内析出大量的Q′相,晶界析出相球化且析出相之间的间距增大,呈断续分布,无沉淀析出带(PFZ)变宽,这种微观结构能有效提高6156合金的电导率和腐蚀性能,同时使合金具有较高的强度。     

Austenitische gegen Spannungsrißkorrosion in konzentrierten Chloridlösungen beständige Manganchromstähle

Austenitic manganese chromium steels resistant to stress corrosion cracking in concentrated chloride solutions The testing of stress corrosion susceptibility in MgCl₂ and CaCl₂ shows that the classical Austenitic CrNi and CrMnNiN steels are not sufficiently resistant to this type of corrosion. On the other hand MnCr steels exhibit good resistance even in hot solutions. The resistance, however, is considerably deteriorated by addition of Ni even in small quantities; with Ni contents up to 0.5%, however, the time to failure is still considerable. This fact is important since such nickel contents must be counted with because of production conditions in metallurgy. On the basis of experiments including investigation into mechanical properties, microstructure, metallic phases present and stress corrosion resistance (under constant tensile load) in MgCl₂ solution (35%, 115–120 °C) the authors selected out of 62 types of steels tested four experimental austenitic MnCr steels meeting the following complex of requirements: high resistance in MgCl₂ solution, sufficiently high resistance to intercrystalline corrosion, certain resistance to pitting and practically monophasic (austenitic) structure.     

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.     

Plasma-Auftragschweißen mit CrNiMoN-legierten Duplex-Werkstoffen

Plasma arc surfacing with CrNiMoN-alloyed duplex stainless steels This paper presents results of researching the Plasma-Hot-Wire-Surface-Welding process with duplex stainless steels X2CrNiMoN22-5-3 and X2CrNiMoN25-7-4. This includes the determination of the microstructure as well as the mechanic-technological and corrosion properties of the layers. The welding parameters have been optimized first in order to produce claddings of a thickness between 2–3 mm. Primary, the welding process influence regarding the degree of dilution on the ferrite-austenitecontent has been investigated. The rate of cooling t_12/8 and t_8/5 have been evaluated during duplex claddings of one and two layer. The results correspond to rate of cooling of PHA-weldings with duplex stainless steels without any dispersion. These results were confirmed by metallographic investigations of one and two layer duplex claddings. Layer analysis shows the homogeneity of the layer composition. Bending tests with normal and side bending samples of one and two layer revealed the excellent bonding between substrate and cladding. In the chemical corrosion experiments the claddings show approximately the similar pitting behaviour as the by rolling manufactured reference material. The surface quality of the X2CrNiMoN22-5-3 claddings is significant to the pitting behaviour. One and two layer claddings of X2CrNiMoN25-7-4 shows, independent of their initial conditions, the same pitting resistance as the reference material. It has been proved by SEP 1877 and EPR (electrochemical potentiodynamic reactivation) tests, that PHA-claddings are resistant against intercrystalline corrosion.     

Tribokorrosion von nichtrostenden Stählen

Tribocorrosion of stainless steels To research tribocorrosive processes a test instrument was developed which allows the simultaneous recording of tribological and electrochemical characteristica in aggressive solutions with solid particles. Therefore the stainless steels X5Cr12, X30Cr12 and X5CrNi25 – 6 were moved oscillating in quartz containing sodium sulphate solution and sulphuric acid with a definite surface pressure over a period of 16 h. Corrosion potential-time curves, current transients taken at a constant potential and current density-potential curves were carried out with and without mechanical load to characterize the electrochemical system. The surface removal rates were determined from the mass loss at free corrosion and at polarization in the passive region. These data were compared with those of only corrosion and only wear. So it was possible to determine the electrochemical part of the tribocorrosion process and to calculate the mechanical one. With help of the concept of Matsumura and Oka [18] synergetic effects could be calculated. The total synergism was determined for three steels in two different solutions and at two different potentials.     

Hochkorrosionsbeständige Sonderstähle für Rauchgas-Entschwefelungsanlagen

Highly corrosion resistant special steels for flue gas desulfurisation plants Highly corrosion resistant stainless steel grades have been proved under the severe corrosion conditions existing in flue gas desulfurisation scrubbers (FGD). Besides general corrosion pitting, crevice corrosion and eventually stress corrosion cracking can occur. Thus highly alloyed special steels must be used. Steel grades with a minimum content of 2.75% Mo are essential. At higher chloride levels and decreasing pH-values higher alloyed stainless steels containing up to 6% Mo are necessary. Some of these special steels are described in view to their composition and mechanical properties; their corrosion behavior has been tested under laboratory and field conditions. The use of nitrogen alloyed grades has been shown of remarkable advantage. Nitrogen additions enhance the mechanical properties and structure stability. Furthermore the precipitation of deleterious intermetallic compounds during heat treatment will be delayed by nitrogen additions, thus e.g. multi layer weldings can be carried out with higher security in view to corrosion resistance and mechanical properties. Materials selection for the different scrubber systems will be illustrated by examples. Up to now experiences about stainless steel components in FGD plants are taken into consideration. Welding with distinctly higher alloyed filler metal at the medium-touched side has been well proved in view to adequate corrosion properties.