Aktivierte Randaufstickung nichtrostender Stähle

Activated Solution Nitriding of Stainless Steels The solution nitriding of the stainless steels can be optimized by a two stage process. The first stage involves an surface activation and an enrichment of nitrogen in the case due to internal nitriding. After this step at temperatures between 1070 °C and 1150 °C follows the dissolution of the chromium nitrides and a solution nitriding. Investigations of ferritic, martensitic and austenitic steels showed that this technology is superior compared to the one stage technology. The treatment time for an given layer thickness in the high temperature stage is cut in halve. The case concentration of nitrogen can be controlled by a material specific choice of the treatment temperature and the partial pressure of nitrogen. For the investigated steels the desired microstructure of the case could be achieved by partial pressures of nitrogen between 0,35 an 1 bar. The solution nitriding of ferritic-martensitic steels eneables the production of martensitic cases with a hardnesses up to 700 HV 0,3. An austenitic case with higher hardness and stability of the austenit can be produced by enrichment the surface of austenitic and ferritic-austenitic stainless steels with nitrogen.     

Nichtrostende Stähle mit TRIP/TWIP/SBIP‐Effekt

Stainless steels with TRIP/TWIP/SBIP effect Economic austenitic steels with high energy absorption capability are in the focus of worldwide research activities, whereby the steels which show TRIP, TWIP and/or SBIP effects play a crucial role. New austenitic or austenitic‐martensitic stainless steels with a high cold workability and energy absorption capability are currently developed and tested in laboratory scale at the Institute of Iron and Steel Technology at the Technical University Bergakademie Freiberg. The mechanical properties of these steels are essentially influenced by the TRIP, TWIP and SBIP effect, becoming evident in hot formed and solution annealed steels as well as in as‐cast steels. The TRIP/TWIP/SBIP effects have a significant impact on the toughness and the strength of stainless steels consisting of metastable austenite. The TRIP effect owns a paramount position since it serves for a simultaneous increase of toughness and strength. The influences of alloying elements like manganese or nickel on the TRIP effect are in the centre of the investigations at the Institute of Iron and Steel Technology. These austenitic or austenitc‐martensitic stainless steels provide the ability for new applications fields due to their excellent mechanical properties. Exemplary, in the Collaborative Research Centre SFB 799 “TRIP‐Matrix‐Composites”, financed through the Deutsche Forschungsgemeinschaft DFG, the suitability of this new class of steels for cast components in ductile and transformation strengthened high performance (metal) ceramic composite materials will be investigated.     

A review on nickel-free nitrogen containing austenitic stainless steels for biomedical applications

The field of biomaterials has become a vital area, as these materials can enhance the quality and longevity of human life. Metallic materials are often used as biomaterials to replace structural components of the human body. Stainless steels, cobalt–chromium alloys, commercially pure titanium and its alloys are typical metallic biomaterials that are being used for implant devices. Stainless steels have been widely used as biomaterials because of their very low cost as compared to other metallic materials, good mechanical and corrosion resistant properties and adequate biocompatibility. However, the adverse effects of nickel ions being released into the human body have promoted the development of “nickel-free nitrogen containing austenitic stainless steels” for medical applications. Nitrogen not only replaces nickel for austenitic structure stability but also much improves steel properties. Here we review the harmful effects associated with nickel and emphatically the advantages of nitrogen in stainless steel, as well as the development of nickel-free nitrogen containing stainless steels for medical applications. By combining the benefits of stable austenitic structure, high strength, better corrosion and wear resistance and superior biocompatibility in comparison to the currently used austenitic stainless steel (e.g. 316L), the newly developed nickel-free high nitrogen austenitic stainless steel is a reliable substitute for the conventionally used medical stainless steels.     

Mechanisch-technologische Eigenschaften nickelhaltiger Superferrite

Mechanical and Technological Properties of Nickel Containing Superferrites High chromium ferritic stainless steels with molybdenum and nickel additions, containing low amounts of interstitials, show remarkably good mechanical properties besides their excellent corrosion behaviour. Yield strengths of these materials can be more than the twofold, compared with that of austenitic stainless steels. In contrary to commercial ferritic stainless chromium steels, the Superferrites exhibit remarkable notch impact toughness also at temperatures below 0 °C. These properties as well as their permanent toughness after a welding process permit a good technological workability and, because of the high strength properties, the application of thinner dimensions in construction.     

Intensive Interstitial Strengthening of Stainless Steels

The high chromium content of stainless steel impairs the interstitial solubility of carbon in austenite at solution annealing or hardening temperature. Replacing carbon by nitrogen improves the interstitial solubility which is raised most, if carbon and nitrogen are added jointly. Respective thermodynamic equilibrium calculations have led to a new group of austenitic and martensitic steels as well as to a thermochemical surface treatment, that make use of the C + N concept to intensively strengthen stainless steels with about 0.5 to 1 mass% of these interstitials. Pressurized electro slag remelting (PESR) is required to raise the N content in the melt and to avoid degassing during solidification of martensitic stainless steels with C + N known as CRONIDUR®. For austenitic CrMn steels the C + N concept affords to dissolve about 1 mass% of these elements in the melt at atmospheric pressure and keep them in solid solution during solidification of the new CARNIT® steels. Case hardening of stainless martensitic steels with nitrogen instead of carbon, called SolNit®, combines the effect of C in the steel and N dissolved in a surface zone via heat treatment in N₂ gas of controlled pressure. The key properties and respective microstructures of these three versions of the C + N concept are discussed along with some applications.     

Nichtmagnetisierbarer warmbeständiger nichtrostender Stahl für Wälzlager

Nonmagnetic heating‐resistant stainless steel for roller bearings A low cost austenitic chromium manganese steel with about 1 mass% of carbon and nitrogen was molten under normal pressure which reveals an amazing combination of properties. Starting from a yield strength of about 600 MPa it is cold work hardened to 60 HRC. This high hardness is brought about for the first time without a martensitic microstructure which is usual for roller bearings. In addition this steel is stainless, non‐magnetic and heating resistant up to about 500 °C, i.e. a material to serve under complex loading. Manufacturing by ingot metallurgy, ESR, hot working, solution annealing and machining was carried out on an industrial scale. The investigation of the structure was carried out on several scales, beginning with the electronic structure, the TEM structure, the light optical microscopy up to macro‐etchings. In this manner an extensive understanding of the outstanding combination of properties of the steel named CARNIT was derived.     

Spannungsrißkorrosionsuntersuchungen bei hohen Temperaturen. Verhalten höherlegierter Stähle unter praxisnahen Bedingungen

Stress Corrosion Cracking at Temperature in the range of 180 °C . Tests are described to find out the stress corrosion behaviour of some stainless steels of the austenitic and ferritic-austenitic type in low concentrated aqueous chloride media with temperatures up to 180°C and pressures up to 10 bar. A sensitivity of the austenitic steels could be determined during 6 to 55 hours, the ferritic-austenitic steel X 2 CrNiMoN 22 5 however didn't suffer any SCC during 235 h with the exception of the welded state. In this way – by the use of a pressure vessel – it is possible, to carry out laboratory tests under conditions of practice.     

Super austenitic stainless steels — a promising replacement for the currently used type 316L stainless steel as the construction material for flue-gas desulphurization plant

Potentiodynamic anodic cyclic polarization experiments on type 316L stainless steel and 6Mo super austenitic stainless steels were carried out in simulated flue-gas desulphurization (FGD) environment in order to assess the localized corrosion resistance. The pitting corrosion resistance was higher in the case of the super austenitic stainless steel containing 6Mo and a higher amount of nitrogen. The pit-protection potential of these alloys was more noble than the corrosion potential, indicating the higher repassivation tendency of actively growing pits in these alloys. The accelerated leaching study conducted for the above alloys showed that the super austenitic stainless steels have a little tendency for leaching of metal ions such as iron, chromium and nickel at different impressed potentials. This may be due to surface segregation of nitrogen as CrN, which would, in turn, enrich a chromium and molybdenum mixed oxide film and thus impedes the release of metal ions. The present study indicates that the 6Mo super austenitics can be adopted as a promising replacement for the currently used type 316L stainless steel as the construction material for FGD plants.     

Zum Verhalten rost‐ und säurebeständiger Stähle unter Komplexbeanspruchung. Teil 1 Passivität und Lochkorrosionsbeständigkeit

Some Aspects on Corrosion Fatique of Stainless Steels. Part 1 Passivity and Pitting Corrosion Susceptibility Iron‐Chromium‐Nickel alloys are of special interest for many applications because of their excellent resistance to corrosion. The nature and composition of passive films formed on stainless steels depend on the prevailing conditions, viz. steel‐composition, passivation potential, aging, pH, electrolyt composition and temperature. Passive films may be damaged by local breakdown. At least two mechanisms are possible for this localisation: mechanical breakdown by slip steps and electrochemical breakdown (for e.g. by the effects of chloride ions). Because of this, steels suffer a degradation of their fatique properties when exposed to an aqueous environment. Passivation of austenitic, ferritic‐austenitic and martensitic stainless steels has been studied in different solutions using electrochemical techniques. The results clarified that for two of the investigated alloys the prediction of fracture initiation based on pitting corrosion in chlorid containing solutions is possible. (To be continued.)     

The influence of the nitrogen/nickel‐ratio on the cyclic behavior of austenitic high strength steels with twinning‐induced plasticity and transformation‐induced plasticity effects

Austenitic high nitrogen (AHNS) and austenitic high interstitial steels (AHIS) are of interest for mechanical engineering applications because of their unique combination of mechanical (strength, ductility), chemical (corrosion resistance) and physical (non‐ferromagnetic) properties. But despite their high strength values e. g. after cold deformation up to 2 GPa in combination with an elongation to fracture of 30 %, which is based on twinning‐induced plasticity (TWIP) mechanisms and transformation‐induced plasticity (TRIP) mechanisms, the fatigue limit remains relatively small. While for chromium‐nickel steels the fatigue limit rises with about 0.5‐times the elastic limit it does not at all for austenitic high‐nitrogen steels or only to a much smaller extent for nickel‐free austenitic high‐interstitial steels. The reasons are still not fully understood but this behavior can roughly be related to the tendency for planar or wavy slip. Now the latter is hindered by nitrogen and promoted by nickel. This contribution shows the fatigue behavior of chromium‐manganese‐carbon‐nitrogen (CrMnCn) steels with carbon+nitrogen‐contents up to 1.07 wt.%. Beside the governing influence of these interstitials on fatigue this study displays, how the nitrogen/nickel‐ratio might be another important parameter for the fatigue behavior of such steels.     

The effect of TiN inclusions and deformation-induced martensite on the corrosion properties of AISI 321 stainless steel

Stainless steel of type 321 is commonly used for the production of exhaust systems because of its temperature resistance and welding properties, which are better than those of AISI 304 or similar steels. AISI 321 is a titanium stabilized austenitic stainless steel, where this element is added to form carbides in order to avoid chromium impoverishment due to chromium carbide formation. Cold shaping can, in the case of austenitic stainless steel, cause the formation of deformation induced martensite, which can improve its mechanical properties, but unfortunately can also spoil its good resistance to corrosion. Titanium nitride inclusions are cathodic with respect to steels, and therefore cause their anodic dissolution. Martensite is, however, more susceptible to the corrosion than austenite in steels. The main aim of this study was to analyze the pitting corrosion and stress corrosion cracking which is initiated on prototype cold-formed outer exhaust sleeves during the testing of different cleaning procedures before chromium plating. Various microscopic methods were used to identify the initiation of corrosion and its propagation.     

不锈钢晶间腐蚀弯曲评价方法的影响因素

用嵌含有GTN延性损伤模型的ABAQUS有限元法,模拟研究了不锈钢晶间腐蚀弯曲评价方法中材料力学性能、弯曲角度和压头直径对弯曲试样塑性应变分布、延性损伤和裂纹起裂的影响规律,分析了其对晶间腐蚀弯曲评价结果的影响。结果表明：随着试样弯曲角度的增大和弯曲压头直径的减小,试样拉伸面的塑性应变增加,试样越容易产生弯曲开裂;在晶间腐蚀弯曲评价标准中,当固定弯曲角度和压头直径时,对于塑性、韧性和抗断裂综合力学性能较低的不锈钢材料,在弯曲过程中材料本身会发生开裂;因此,需要考虑材料力学性能对晶间腐蚀弯曲评价结果的影响;对于该研究中的典型的奥氏体不锈钢材料,当其弯曲断裂应变低于0．51左右时,在弯曲过程中材料本身会发生开裂,不宜用弯曲方法来评价其晶间腐蚀敏感性。     

Beständigkeit verschiedener Stähle und Nickelbasiswerkstoffe unter dem Einfluß aufnitrierend wirkender Gase in der Ammoniak-Synthese

Resistance of Various Steel Types and Nickel-Base Alloys Under the Influence of Nitriding Gases in Ammonia Syntheses Plants The paper describes test results obtained during 5 years of practical trials with heat-resisting steels, steels for hydrogen service, austenitic steels and nickel-base alloys under the nitriding operation conditions found in an ammonia plant at temperatures of more than about 400°C. During the test, nitriding rates and depths at high temperatures and under stresses were established, as well as changes in the mechanical properties as a result of nitriding. Based on the test results and the knowledge of the subject matter, recommendations and service condition limits for material selection are given. The superior behaviour of austenitic materials, especially with increasing nickel contents, could be confirmed. An attempt is made to find an explanation for the reduced resistance to hydrogen attack under the influence of nitridation.     

Zur Warmfestigkeit austenitischer Ventilstähle mit hohem Stickstoffgehalt

On the hot strength of austenitic valve steels with a high nitrogen content Exhaust valves are made of CrMnNi steels with about 1 wt% of (C + N) and additions of W, Mo and Nb. Remelting under pressure allows to raise N and do without C. After solution annealing the formation of discontinuous N‐perlite during aging is suppressed by ∼ 1 wt% of Nb whereby the creep strength is increased. The size and distribution of continuously precipitated nitrides in N‐steels are finer than those of carbides and nitrides in (C + N)‐steels. Thus the creep strength of the former is superior at 700 °C. However, at 800 °C σ‐phase leads to a breakdown of creep strength. The reason is seen in a lower stability of N‐austenite as compared to (C + N)‐austenite, which shows a higher concentration of free electrons and more short range atomic ordering.     

Grain boundary chromium depletion in austenitic alloys

It is widely accepted that thermally induced grain boundary chromium depletion in austenitic alloys such as austenitic stainless steels and Ni base alloys can lead to inter-granular stress corrosion cracking (IGSCC) of high temperature components during service. Numerous experimental studies of the phenomenon have been reported and many models have been developed to predict chromium depletion under different pre-service treatment. However, there are debates on some fundamental issues in modelling and the interpretation of experimental observations. This article attempts to clarify some of the issues through numerical calculations and examination of the literature on grain boundary chromium depletion.     

Die Werkstoffdämpfung von Stählen bei hohen Dehnungsamplituden

Damping of Steels at High Strain Amplitudes The amplitude dependent damping has been investigated in two martensitic chromium steels (X 22 CrMoV 12 1, X 20 Cr 13) and a cold worked austenitic steel (X 12 CrNiWTi 16 13) at room temperature. Due to the magneto-elastic effects, the martensitic steels show a strong amplitude dependent damping. It cound be shown that the damping is reduced remarkably by a reduction of the annealing temperature. The austenitic material shows a very high damping in the cold worked condition. Tempering at relatively low temperatures (above 100 °C) reduces damping to the low values usually expected for austenitic materials. This effect was attributed to the pinning of dislocations during tempering.     

Crack propagation behavior of solution annealed austenitic high interstitial steels

Austenitic stainless steels provide a beneficial combination of chemical and mechanical properties and have been used in a wide field of applications for over 100 years. Further improvement of the chemical and mechanical properties was achieved by alloying nitrogen. But the solubility of N within the melt is limited and can be increased in substituting Ni by Mn and melting under increased pressure. In order to avoid melting under pressure and decrease production costs, a part of N can also be substituted by C. This leads to austenitic high interstitial steels (AHIS). Within the solution annealed state strength and ductility of AHIS is comparable or even higher of those of AHNS and can be further improved by cold working. Unfortunately the endurance limit does not follow this trend as it is known from cold-worked austenitic CrNi steels. This is due to the differences of the slip behavior which is governed by the stacking fault energy as well as other near field effects. Construction components operating under cyclic loads over long periods of time cannot be considered being free of voids or even cracks. Thus the crack propagation behavior is of strong interest as well. This contribution presents the tensile, fatigue, crack propagation and fracture toughness properties of AHNS and AHIS in comparison to those of CrNi-steels. The differences are discussed in relation to microstructural characteristic as well as their alterations under cyclic loading.     

Nickel-free austenitic stainless steels for medical applications

Abstract

The adverse effects of nickel ions being released into the human body have prompted the development of high-nitrogen nickel-free austenitic stainless steels for medical applications. Nitrogen not only replaces nickel for austenitic structure stability but also much improves steel properties. Here we review the harmful effects associated with nickel in medical stainless steels, the advantages of nitrogen in stainless steels, and emphatically, the development of high-nitrogen nickel-free stainless steels for medical applications. By combining the benefits of stable austenitic structure, high strength and good plasticity, better corrosion and wear resistances, and superior biocompatibility compared to the currently used 316L stainless steel, the newly developed high-nitrogen nickel-free stainless steel is a reliable substitute for the conventional medical stainless steels.     

Grain size dependence of mechanical, corrosion and tribological properties of high nitrogen stainless steels

Austenitic stainless steels have been indispensable for the progress of technology during the last 80 years. Due to the cost of nickel and to the prospective of allergic reactions caused by this element, more and more laboratories and industries are trying to develop a new class of austenitic stainless steels with a low nickel content. In order to maintain the austenitic microstructure, nickel reduction is balanced with nitrogen addition. Nitrogen addition to austenitic stainless steels is also very effective for improving yield strength and corrosion resistance without reducing ductility and toughness. In order to further increase the strength, it is possible to combine the effect of nitrogen addition and grain refining. The purpose of this study is to examine the relationship between microstructures and mechanical, corrosion and tribological properties of a high nitrogen stainless steel with an ultrafine grained structure.