Stainless steels suited for solution nitriding

Solution nitriding is a new heat treatment to yield a high nitrogen case on stainless steels at 1100 ± 50°C. Combining experimental results and thermodynamic calculation steels are selected to give a hard martensitic or high strength austenitic case. Especially developed steels are discussed as well as the suitability of standard grades. A martensitic case is combined with a martensitic core in steel Cr13C0.2 and with a softer ferritic‐martensitic core in steel Cr13C0.1. The nitrogen content of an austenitic case increases with the Cr/Ni ratio, e.g. in the order of Cr17Ni12Mo2, Cr18Ni10, Cr22Ni5Mo3N0.2. The duplex microstructure of the latter provides the highest yield strength in the core. It is essential to stay clear of the austenite/austenite + M₂N boundary and avoid precipitates which deteriorate the fatigue and corrosion resistance. Seventeen steels are assessed in this report.     

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.     

Struktur und Eigenschaften nichtrostender Stähle nach einer Plasmaimmersionsionenimplantation und einer Plasmanitrierung

Structure and properties of stainless steels after plasma immersion ion implantation and plasma nitriding Stainless steels can be nitrided at temperatures ≤ 400 °C to increase their hardness and wear resistance without a decreasing of their excellent corrosion resistance. Structure and properties of the surface layers produced by plasma nitriding and plasma immersion ion implantation in this temperature range were tested. There are negligible differences in the structure of the produced surface layers in spite of different interaction principles of the used technologies. However there are clear differences between the case of different steels. The case of ferritic chromium steels mainly consists of ε-nitride. Whereas the cases of austenitic and ferritic austenitic steels are characterized by expanded austenite. The corrosion resistance of the steels is reduced by nitriding only, if evident CrN-formation occurs.     

Challenges of Diffusion Bonding of Different Classes of Stainless Steels

Solid state diffusion bonding is used to produce monolithic parts exhibiting mechanical properties comparable to those of the bulk material. This requires diffusion of atoms across mating surfaces at high temperatures, accompanied by grain growth. In case of steel, polymorphy helps to limit the grain size, since the microstructure is transformed twice. The diffusion coefficient differs extremely for ferritic and austenitic phases. Alloying elements may shift or suppress phase transformation until the melting range. In this paper, diffusion bonding experiments are reported for austenitic, ferritic, and martensitic stainless steels possessing varying alloying elements and contents. Passivation layers of different compositions are formed, thus affecting the local diffusion coefficient and impeding diffusion across faying surfaces. As a consequence, different bonding temperatures are needed to obtain good bonding results, making it difficult to control the deformation of parts, since strong nonlinearities exist between temperature, bonding time, and bearing pressure. For martensitic stainless steel, it is shown that it is very easy to obtain good bonding results at low deformation, whereas ferritic and austenitic stainless steels require much more extreme bonding parameters.

     

The Effect of Hydrogen on Fatigue Properties of Metals used for Fuel Cell System

The effect of hydrogen on the fatigue properties of alloys which are used in fuel cell (FC) systems has been investigated. In a typical FC system, various alloys are used in hydrogen environments and are subjected to cyclic loading due to pressurization, mechanical vibrations, etc. The materials investigated were three austenitic stainless steels (SUS304, SUS316 and SUS316L), one ferritic stainless steel (SUS405), one martensitic stainless steel (0.7C-13Cr), a Cr-Mo martensitic steel (SCM435) and two annealed medium-carbon steels (0.47 and 0.45%C). In order to simulate the pick-up of hydrogen in service, the specimens were charged with hydrogen. The fatigue crack growth behaviour of charged specimens of SUS304, SUS316, SUS316L and SUS405 was compared with that of specimens which had not been hydrogen-charged. The comparison showed that there was a degradation in fatigue crack growth resistance due to hydrogen in the case of SUS304 and SUS316 austenitic stainless steels. However, SUS316L and SUS405 showed little degradation due to hydrogen. A marked increase in the amount of martensitic transformation occurred in the hydrogen-charged SUS304 specimens compared to specimens without hydrogen charge. In case of SUS316L, little martensitic transformation occurred in either specimens with and without hydrogen charge. The results of S-N testing showed that in the case of the 0.7C–13Cr stainless steel and the Cr–Mo steel a marked decrease in fatigue resistance due to hydrogen occurred. In the case of the medium carbon steels hydrogen did not cause a reduction in fatigue behaviour. Examination of the slip band characteristics of a number of the alloys showed that slip was more localized in the case of hydrogen-charged specimens. Thus, it is presumed that a synergetic effect of hydrogen and martensitic structure enhances degradation of fatigue crack resistance.     

Study of Formation and Reversion of the Martensitic Phase Induced by Deformation of Lean Duplex Stainless Steel

Lean duplex stainless steels consist of a low percentage of nickel and molybdenum, presenting twice as much resistance compared to austenitic stainless steel and their cost is about twice as low. However, this class of steels has microstructural instabilities, such as the formation of martensite induced by austenite deformation by cold rolling. This feature can significantly alter the properties of interest of this steel. The formation of the martensitic structure, as well as its reversion, is little studied in the steels of the austenitic–ferritic structure. The process of formation and reversal of the martensitic structure in cold rolled stainless steel duplex UNS S32304 was investigated through magnetic measurements, microhardness and X-ray diffraction analyzes. The deformation process allowed the formation of the -martensite phase from the austenite phase with an increase in the values of saturation magnetization, coercive field and micro-hardness values as well as a change in the intensity of the X-ray diffraction peaks. The heat treatment performed at \(650\,^\circ \hbox {C}\) showed an increase in the peak intensity of the austenitic phase and a decrease in the saturation magnetization values, demonstrating a possible reversal of the martensitic structure. The SEM observations after annealing the Beraha’s etched samples revealed the possibility of a martensite transformation and reversion in a Lean duplex stainless steels.     

Assessment of creep rupture properties for dissimilar steels welded joints between T92 and HR3C

Dissimilar steels welded joints, between ferritic steel and austenitic stainless steel, are always encountered in high‐temperature components in power plants. As two new grade ferritic steel and austenitic stainless steel, T92 (9Cr0.5Mo2WVNb) and HR3C (TP310HCbN), exhibit superior heat strength at elevated temperatures and are increasingly applied in ultra‐supercritical (USC) plants around the world, a complete assessment of the properties for T92/HR3C dissimilar steels welded joints is urgently required. In this paper, metallographic microstructures across the joint were inspected by optical microscope. Particularly, the creep rupture test was conducted on joints under different load stresses at 625 °C to analyse creep strength and predict their service lives, while their fractograph were observed under scanning electron microscope. Additionally, finite element method was employed to investigate residual stress distribution of joints. Results showed that the joints were qualified under USC conditions, and T92 base material was commonly the weakest part of them.     

含Cu抗菌不锈钢的工艺与耐蚀性能

与普通0Cr17铁素体不锈钢和0Cr18Ni9奥氏体不锈钢相比，含铜铁素体和奥氏体抗菌不锈钢均具有良好的冷热加工性能和焊接性能．通过提高浇铸温度，抗菌不锈钢能保持良好的铸造性能．奥氏体抗菌不锈钢的抗应力腐蚀性能比0Cr18Ni9不锈钢有很大的提高，而铁素体抗菌不锈钢比0Cr17有明显的下降．与相应的普通不锈钢相比，两种类型抗菌不锈钢的耐点蚀性能均略有下降．     

Analysis of Magnetism in High Nitrogen Austenitic Stainless Steel and Its Elimination by High Temperature Gas Nitriding

Stable austenitic structure in medical stainless steels is basically required for surgical implantation. A weak magnetism was found in a high nitrogen nickel-free austenitic stainless steel for cardiovascular stent application. This magnetic behavior in high nitrogen stainless steel was investigated by optical microscopy, X-ray diffraction (XRD), electron probe microanalysis (EPMA) and superconducting quantum interference device (SQUID). The results showed that the magnetism came from the composition segregation of ferrite formation elements such as Cr and Mo in the steel and some δ-ferrites were locally formed during the pressurized electroslag remelting process. The magnetism of high nitrogen stainless steel could be eliminated by a proper high temperature gas nitriding (HTGN).     

Solution Nitriding of Stainless Steels for Process Engineering

Solution nitriding is a controlled heat treatment of stainless steels carried out at 1100 ± 50 °C in N₂ gas. Depending on the alloy content a hard martensitic or a ductile austenitic high nitrogen case of about 2 mm in depth is formed, which reduces wear by sliding, fretting, erosion and cavitation. In contrast to carbon dissolved nitrogen does not impair the resistance to superimposed corrosion, so that solution nitriding is suitable for components subjected to the flow of aggressive fluids containing suspended particles which is often encountered in chemical and process engineering.     

Erosion behaviour of hydro turbine steels

The martensitic stainless steel (termed as 13/4) is currently being used for fabrication of underwater parts in hydroelectric projects. There are, however, several maintenance problems associated with the use of this steel. A nitronic steel (termed as 21-4-N) has been developed as an alternative with the specific aim of overcoming these problems. A comparative study has been made on the erosion behaviour of 13/4 and 21-4-N steels by means of solid particle impingement using gas jet. The eroded surfaces after erosion tests were analysed by scanning electron microscopy. It is observed that the 21-4-N nitronic steel possesses better resistance to erosion in comparison to 13/4 martensitic stainless steel. The austenitic matrix of the nitronic steel possesses high hardness, high tensile toughness and work hardening ability, which results in higher erosion resistance.     

微观组织对0Cr17Ni4Cu4Nb马氏体不锈钢的耐点腐蚀性能的影响

采用化学浸泡法和模拟闭塞电池方法研究了固溶+时效和固溶+调整+时效处理的0Cr17Ni4Cu4Nb马氏体不锈钢的耐点腐蚀性能,并与18-8型奥氏体不锈钢(316L)耐点蚀性能进行了对比。结果表明,0Cr17Ni4Cu4Nb马氏体不锈钢组织内富Cu析出相促进了点蚀坑萌生,而点蚀坑发展则与组织形貌有关。固溶+调整+时效处理的0Cr17Ni4Cu4Nb马氏体不锈钢因组织内析出富Cu相多而大,其萌生的点蚀坑密度较高,但由于马氏体板条较细,其点蚀坑尺寸和深度较小;固溶+时效处理的0Cr17Ni4Cu4Nb马氏体不锈钢因组织内析出富Cu相少而小,萌生的点蚀坑密度较低,但粗大的板条马氏体组织导致点蚀坑尺寸和深度较大。与18-8型奥氏体不锈钢耐点蚀性能对比表明,通过对0Cr17Ni4Cu4Nb马氏体不锈钢进行合理的热处理,其耐点蚀性能可与18-8型奥氏体不锈钢相当。     

Methods to overcome embrittlement problem in 9Cr–1Mo ferritic steel and its weldment

This article presents the issues that need to be addressed in ferritic steel, for their use in nuclear core, namely, the embrittlement and type IV cracking of weldment. It has been established that the ferritic steels possess a significantly higher resistance to radiation damage as compared to the present generation austenitic stainless steels and the creep behavior is satisfactory for applications up to 873 K. The major challenges that need to be addressed are the poor creep resistance of the weld joints and embrittlement of ferritic steels. This article describes the efforts taken at IGCAR to overcome the embrittlement problem by impurity control, grain boundary engineering or design of suitable thermomechanical treatments in a 9Cr–1Mo ferritic steel.     

Hydrogen-induced change in microstructure and properties of steels: 18Cr10Mn–0.4N vis-à-vis 18Cr10Ni

ABSTRACT

Cr–Mn–N stainless steels have a cost and strength advantage over conventional Cr–Ni stainless steels. In this study microstructure and mechanical property of hydrogen-charged 18Cr10Mn-0.4N was compared with 18Cr10Ni austenitic stainless steel. This is the first such study for 18Cr10Mn–0.4N austenitic stainless steel. Electron microscopy was used to compare the deformed microstructure of the uncharged and hydrogen-charged specimens. The results are discussed in view of the current knowledge on hydrogen embrittlement. The 18Cr10Mn–0.4N steel suffered higher embrittlement mainly because it absorbed moref hydrogen.     

Sintered 434L ferritic stainless steels

Stainless steel powder metallurgical parts are increasingly used in automotive, aircraft, business machines, chemical, marine, tools and hardware industries. Although some results on sintered austenitic stainless steels have been reported, there is no information on a relatively cheaper ferritic stainless steel series of alloys. In the present paper, the properties of 434L ferritic stainless steel have been comprehensively evaluated and their nature discussed.     

Mechanical and Structural Properties of Similar and Dissimilar Steel Joints

The mechanical properties of specimens from similar and dissimilar weld joints were examined. A ferritic steel (St37-2) and an austenitic stainless steel (AISI 304) were joined by the gas tungsten arc weld (GTAW) process using an austenitic filler metal. Mechanical and metallographic properties of the specimens were obtained by means of microhardness testing, tensile testing, bending fatigue testing, and light optical and scanning electron microscopy. The highest microhardness values were recorded on the ferritic–austenitic dissimilar weld joint, whereas the highest tensile strength and bending fatigue life were obtained with the austenitic–austenitic joints. Ferritic and pearlitic structures were observed in the microstructure of the ferritic–ferritic joint. The microstructures of austenitic–austenitic and austenitic–ferritic joints showed small recrystallization grains in addition to the typical austenitic and ferritic structures. Scanning electron microscopy was used to observe the fracture surfaces of the specimens and the origins of the fatigue cracks.     

Tensile and Impact Properties of Shielded Metal Arc Welded AISI 409M Ferritic Stainless Steel Joints

The present study is concerned with the effect of filler metals such as austenitic stainless steel, ferritic stainless steel and duplex stainless steel on tensile and impact properties of the ferritic stainless steel conforming to AISI 409M grade. Rolled plates of 4 mm thickness were used as the base material for preparing single pass butt welded joints. Tensile and impact properties, microhardness, microstructure and fracture surface morphology of the joints fabricated by austenitic stainless steel, ferritic stainless steel and duplex stainless steel filler metals were evaluated and the results were reported. From this investigation, it is found that the joints fabricated by duplex stainless steel filler metal showed higher tensile strength and hardness compared to the joints fabricated by austenitic and ferritic stainless steel filler metals. Joints fabricated by austenitic stainless steel filler metal exhibited higher ductility and impact toughness compared with the joints fabricated by ferritic stainless steel and duplex stainless steel filler metals.     

Influence of nitriding on corrosion resistance of martensitic X17CrNi16‐2 stainless steel

Nitriding increases surface hardness and improves wear resistance of stainless steels. However, nitriding can sometimes reduce their corrosion resistance. In this paper, the influence of nitriding on the corrosion resistance of martensitic stainless steel was investigated. Plasma nitriding at 440 °C and 525 °C and salt bath nitrocarburizing were carried out on X17CrNi16‐2 stainless steel. Microhardness profiles of the obtained nitrided layers were examined. Phase composition analysis and quantitative depth profile analysis of the nitrided layers were preformed by X‐ray diffraction (XRD) and glow‐discharge optical emission spectrometry (GD‐OES), respectively. Corrosion behaviour was evaluated by immersion test in 1% HCl, salt spray test in 5% NaCl and electrochemical corrosion tests in 3.5% NaCl aqueous solution. Results show that salt bath nitrocarburizing, as well as plasma nitriding at low temperature, increased microhardness without significantly reducing corrosion resistance. Plasma nitriding at a higher temperature increased the corrosion tendency of the X17CrNi16‐2 steel.     

Influence of the microstructure on the corrosion resistance of plasma‐nitrided austenitic stainless steel 304L and 316L

Austenitic stainless steels are widely used in medical and food industries because of their excellent corrosion resistance. However, they suffer from weak wear resistance due to their low hardness. To improve this, plasma nitriding processes have been successfully applied to austenitic stainless steels, thereby forming a thin and very hard diffusion layer, the so‐called S‐phase. In the present study, the austenitic stainless steels AISI 304L and AISI 316L with different microstructures and surface modifications were used to examine the influence of the steel microstructure on the plasma nitriding behavior and corrosion properties. In a first step, solution annealed steel plates were cold‐rolled with 38% deformation degree. Then, the samples were prepared with three kinds of mechanical surface treatments. The specimens were plasma nitrided for 360 min in a H₂–N₂ atmosphere at 420 °C. X‐ray diffraction measurements confirmed the presence of the S‐phase at the sample surface, austenite and body centered cubic (bcc)‐iron. The specimens were comprehensively characterized by means of optical microscopy, scanning electron microscopy, glow discharge optical emission spectroscopy, X‐ray diffraction, surface roughness and nano‐indentation measurements to provide the formulation of dependencies between microstructure and nitriding behavior. The corrosion behavior was examined by potentio‐dynamic polarization measurements in 0.05 M and 0.5 M sulfuric acid and by salt spray testing.     

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.)