Service Experience with duplex stainless steels in the chemical industry

The 22% Cr, 5% Ni and 3% Mo grade of duplex stainless steel is by far the most used duplex grade in the chemical process industry. Duplex steels have conquered a portion of about 6% of all highly alloyed materials used to combat corrosion in chemical industry. The main areas being chloride containing aqueous media with pH values not to low-duplex steels are used there for their high stress corrosion resistance. Other fields of application are hot caustic soda at relatively low temperatures where nickel or nickel base alloys can be replaced, and media where high nickel contents are unfavourable to the corrosion resistance. Some literature findings showing other possible fields of special application of ?standard”? 22/5/3 and also of so called superduplex grades are critically evaluated.     

Super duplex stainless steels

Abstract

This paper presents an overview of duplex stainless steels (DSS) with particular emphasis on super DSS, i.e. steels containing sufficient amounts of chromium, molybdenum, and nitrogen to produce a pitting resistance equivalent greater than 40. Duplex stainless steels have an attractive combination of mechanical and corrosion properties and are thus suitable for many marine and petrochemical applications, particularly where chlorides are present. The paper covers the following aspects of DSS: physical metallurgy, mechanical properties, corrosion properties, metallurgy of welding, machinability, and applications. A large number of references to the literature are given to aid the reader who is interested in acquiring a deeper understanding of the behaviour of this family of steels.

MST/1685     

Welding duplex stainless steels

An account is given of some of the duplex steels now available, their properties and in particular means of welding them.     

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.     

Magnetostriction of duplex stainless steels

Abstract

To identify suitable non-destructive examination techniques, the saturation magnetostriction of a number of ferritic–austenitic (duplex) stainless steel samples from various manufacturers has been measured using a dilatometric technique. The values were found to lie in two distinct ranges, which suggested that two types of material structure were present, often in the same steel type. By examination of optical micrographs of all the samples, it was confirmed that the two ranges of saturation magnetostriction corresponded to two types of structure as follows: (15·5 ±3·3) ×10^?6 (average) in the rolling direction for highly anisotropic material structure and (5·7 ± 2·2) × 10^?6 (average) for isotropic material independent of orientation and for highly anisotropic material perpendicular to the rolling direction.

MST/945     

Textures of ferritic stainless steels

Abstract

Whereas much research has been carried out on the texture development of Fe–Si steels or low carbon steels, very little attention has been given to the texture formation and investigation of the underlying mechanisms in Fe–Cr steels. Ferritic stainless steels containing between 11 and 17%Cr establish an important group of alloys owing to their good mechanical behaviour and corrosion resistance. Various industrial problems, such as roping or optimisation of deep drawability, can be tackled by means of quantitative texture analysis. Therefore, fundamental aspects concerning the inhomogeneity of the hot and cold rolled band, the origin of Goss texture after annealing, and selective particle drag during recrystallisation of alloys with finely dispersed Nb and Ti carbonitrides have been discussed.

MST/1678     

Thermodynamics of duplex stainless steels

Abstract

The thermodynamic predictions of microstructures in the multicomponent alloys used for duplex stainless steels are discussed in detail in this paper. The production of the duplex ferrite–austenite microstructure by high temperature (1323–1423 K) annealing is considered. Phase diagrams are calculated for Fe–Cr–Ni ternary alloys and the more complex multicomponent systems. It is shown that there is good agreement between the predictions of the thermodynamic models and the measurements carried out on heat treated alloys. The low temperature aging behaviour of Fe–Cr–Ni ternary alloys is discussed and the limitations of activation energy approaches to the modelling of the aging process in these alloys are shown.

MST/1201     

Flash butt resistance welding for duplex stainless steels

Duplex stainless steels were welded using flash butt resistance welding with temperature controlling system. Flash butt resistance welding is consisting of two stage processes of flash action and contact resistance. First stage is flashing action. The specimen produced flashing or arcing across the interface of the two butting ends of the specimens. Fine particles of metals near the surface were burned out towards the opposing surface of the specimen irregularity and then the melted particles were deposited on the surface. The second stage is resistance welding. The solid state bonding was performed in the region around the deposited particles. The cross-sectional microstructure of the weld bond region was observed using microscopy. The microstructure showed two types of a deposited fine particles region and a solid state bonding region. The grain growth was hardly observed in the weld region and the heat-affected zone. The tensile strength and the impact energy increased with increasing heating time up to 1373 K because of increasing fine grained deposited metal.     

Increasing the wear resistance of stainless steels

The wear resistance of martensitic, austenitic and duplex stainless steels is increased by carbon and nitrogen in solid solution and in addition by carbides and nitrides, which are formed during conventional ingot metallurgy or by powder metallurgical processing. High temperature and low temperature nitriding provide a hard surface zone. Characteristic alloys and applications are discussed, which try to optimize the resistance to wear and corrosion.     

Future developments and applications of nitrogen-bearing steels and stainless steels

After considerations related with the global frame of the demand of society in the field of materials and some recalling of basic properties and principles of nitrogen alloying, possible future developments are listed and discussed.     

Mechanical behaviour of high-nitrogen stainless steels

Abstract

The application of previously published equations describing the dependence of proof stress on the nitrogen content of austenitic stainless steels has been validated and also extended to higher nitrogen contents in an experimental study of the mechanical property–structure relationships in Rex 734 steel. The proof stress is increased by nitrogen alloying through a combination of solid-solution hardening and a grain-size effect. The effective grain size of these steels is influenced by the nitrogen content through changes in stacking-fault energy, which Increases the density of twinning. In the analysis of tensile test data this effect appears as an increase in the grain-size coefficient of the Hall–Petch equation with increasing nitrogen concentration. Compression–tension low-cycle fatigue properties are similarly enhanced by a combination of nitrogen in solid solution and the influence of nitrogen on stacking-fault energy. An increase in the extent of planar slip is observed with increasing nitrogen content, and is most pronounced at low levels of fatigue strain. The increase in fatigue life observed at the strain levels examined reaches a limit at about 0·12 wt-%N both in AISI 316 grade steels and in Rex 734.

MST/453     

Antibacterial property of Ce-bearing stainless steels

Rare earth (RE) elements inhering in low toxicity have been widely used in biomedicine. The RE elements can also improve the hot workability, corrosion resistance and mechanical properties of stainless steels. However, the antibacterial mechanisms of the RE elements are still under discussion and the antibacterial property of RE-bearing stainless steels has not yet been investigated or reported so far. In this paper several Ce-bearing stainless steels were prepared, the microstructure of the steels was examined, and the antibacterial property of Ce-bearing stainless steels was investigated. It was found that Ce-rich zones were precipitated in the Ce-bearing stainless steels and the volume fraction of the Ce-rich zones increased with increasing Ce content. The Ce-bearing stainless steels showed excellent antibacterial property when the amount of Ce added was above a critical value. Compared to the conventional Cu-bearing antibacterial stainless steels, the Ce-bearing stainless steels investigated presently exhibited good antibacterial ability without any thermal aging treatment. The antibacterial mechanism of Ce-bearing stainless steels is also discussed.     

Magnetic characteristics of ferromagnetic stainless steels

The magnetic characteristics of two representatives of martensitic and one of ferritic stainless steels, i.e. SUS 403, TAF, and SUS 405, were investigated in connection with development of high-temperature and nuclear resistive sensors. The test pieces were machined out from rod materials and heat-treated in an argon atmosphere. They were placed in an electric furnace, and their magnetic characteristics were measured at room temperature and elevated temperatures. The examined characteristics were of initial magnetization curve, hystersis loop, AC permeability, Curie point, and inverse magnetostriction. The martensitic stainless steels show somewhat semihard characteristics (H_c: 5 to 15 Oe) and the ferritic one is rather soft (H_c; 2 to 5 Oe). The Curie point ranges from 720 to 750°C. The magnetization is about 15000 G at a field strength of 100 Oe. A large inverse magnetostriction is observed in SUS 403 and SUS 405. Their magnetic characteristics were observed to change with heat treatment     

Polarization characteristics of magnetoelectropolishing stainless steels

The aim of the study is to present the polarization characteristics of electropolishing process carried out in a magnetic field (MEP—magnetoelectropolishing), in comparison with the ones obtained under standard/conventional process of electropolishing (EP) conditions. The polarization characteristics have been considered as the most important factor in establishing effective treatment conditions. With this paper we would like to show the results of the studies concerning the effects of a magnetic field strength and electropolishing conditions (convection, temperature) on the electrochemical polarization characteristics. Specifically the occurrence of electropolishing EP plateau has been observed in view of optimization of magnetoelectropolishing MEP process. It has been found that, depending on the magnetic field strength, under higher potentials the current density has been growing with the increase of potential to a certain value only. Afterwards the current density remains unchanged. This behavior in the course of polarization curves obtained under high potentials has been revealed to be the effect of cyclically disclosure of sample surface and re-submerge, this resulting in the “second limiting current”.     

Mechanical sputtering of structural stainless steels

In this paper the results of an investigation of the beam modification (erosion) of stainless steel are presented. The possible similarity of the martensitic stainless steel degradation for turbine engine blades and austenitic stainless steel used as a wall of the vacuum vessel for fusion reactor application has been pointed out. Changes appearing during the interaction have been investigated on the rotating turbine blade material. Simultaneously morphological changes of the ‘target’ and the change of shape of bombarding species have been analysed.     

High-temperature hydrogen resistance of stainless steels

At elevated temperatures, the influence of hydrogen on various mechanical characteristics of martensitic and disperse-hardened austenitic steels is different. The maraging steel has better characteristics of durability and plasticity and the critical values of static and cyclic crack resistance at temperatures of 450–600°K than the austenitic steel with intermetallic hardening. As a result of the intense temperature softening, its ultimate and yield strengths are much lower than for the austenitic steel. The austenitic steel has higher resistance in terms of the threshold value ∆K _th. At room temperature, the low-cycle fatigue limit proves to be most sensitive to the action of hydrogen, whereas at 673°K, the parameter K _fc for the maraging steel decreases.