Creep behavior and deformation mechanisms in a nanocluster strengthened ferritic steel

Mechanically alloyed, nanostructured ferritic steels represent a class of alloys that can display high resistance to radiation and creep deformation, which are derived from the presence of nanoclusters, precipitates and solute segregation to the grain boundaries. The creep responses for a 14YWT nanostructured ferritic steel were measured over a range of temperatures and stress levels. The stress exponent was observed to vary non-linearly with applied stress; stress exponents were found to decrease with decreasing stress approaching unity at low stress. Transmission electron microscopy studies clearly demonstrated that creep deformation proceeds by a dislocation glide within nanoscale grains and that glide dislocations are attracted to and pinned by nanoclusters. In light of these observations, a new model of the creep response, inspired by the Kocks-Argon-Ashby model, is developed to explain the low creep rates and small stress exponents that are exhibited by these alloys.     

Rate-determining processes for corrosion fatigue crack growth in ferritic steels in seawater

The results of recent experiments on measuring corrosion fatigue crack propagation rates in structural steel immersed in seawater with and without cathodic polarisation are reported. These include measurements of electrochemical potentials near the growing crack tip. The experiments have been designed to learn more about the mechanisms and rate-determining processes influencing the rate of crack growth. From these and other results in the literature, deductions are made about the relative importance of mechanical and electrochemical limitations on the rate of crack propagation.     

Embrittlement of ferritic chrome steels in welding

Conclusions High-temperature heating characteristic of a weld joint in the area of overheating of the heat-affected zone causes an increase in the parameters and the static distortions of the crystalline lattice of chrome steels with an increase in nitrogen content. At the same time, the inelastic effects, the Snoek and Kester peaks, increase, which is caused by the introduction of nitrogen into the crystalline lattice and its interaction with the fields of stresses and dislocations.After imitation of the structure of overheating steels contaminated with nitrogen have a high impact strength, which is an indication of the weak influence of supersaturation of the crystalline lattice on the tendency of chrome steels toward embrittlement in welding.An increase in the carbon content has a negative influence on the impact strength of chrome steels after high-temperature welding heating. In comparison with steels contaminated with nitrogen there is practically no increase in the lattice parameters of such steels. After high-temperature heating the static distortions of the lattice decrease. There is also a decrease in the peaks of internal friction caused by migration of carbon atoms along the interstices of the crystalline lattice into the zones of stresses and their interaction with the dislocations.Embrittlement of ferritic chrome steels as the result of carbide precipitates is also possible after short-time heating in the 550–850°C range, which is indicated by the decrease in the crystalline lattice parameters and also the maximum in internal friction caused by carbon atoms. In heating to 400–550°C nitrides are also formed together with carbides.The effects revealed after short-time heating make it possible to relate the mechanism of embrittlement of ferritic chrome steels in welding to strengthening of the heat-affected zone metal as the result of precipitates of finely dispersed carbonitrides.Central Scientific-Research Institute for Machine Building Technology Scientific Production Association. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 1, pp. 41–45, January, 1984.     

The influence of alloy diffusion processes on the corrosion of ferritic and austenitic stainless steels

Sulphidation has been studied in austenitic steels AISI 310 and 447 and in ferritic steel AISI 446 at temperatures between 550°C and 700°C. The kinetics of mass gain change at about 650°C from a linear, spalling-controlled type at lower temperatures to a diffusion-controlled type above this temperature. At the higher temperatures internal sulphidation is observed. The explanation for the mass gain rate constants measured is that diffusion in the alloy is rate controlling. This is substantiated by a new high diffusivity-path-enhanced diffusion model described in a parallel paper. The high diffusivity paths are shown to be intergranular and intragranular sulphides, in ferritic and austenitic alloys, respectively. These sulphides nucleate on existing carbides. In cases where the aggressiveness of the atmosphere is not high, as is the case here, this alloy diffusion controlled corrosion mechanism can be suppressed by using austenitic as opposed to ferritic matrices and reducing the concentration of relatively unstable carbides like M₂₃C₆ in the material.     

Hydrogen embrittlement of ferritic steels: Observations on deformation microstructure,nanoscale dimples and failure by nanovoiding

While hydrogen embrittlement of ferritic steels has been a subject of significant research, one of the major challenges in tackling hydrogen embrittlement is that the mechanism of embrittlement is not fully resolved. This paper reports new observations and interpretation of fracture surface features and deformation microstructures underneath the fracture surface, providing a mechanistic view of failure catalyzed by hydrogen. Linepipe grade ferritic steels were tested in air with electrochemically pre-charged hydrogen and in high-pressure H₂ gas. The fracture surface features were studied and compared using high-resolution surface-sensitive scanning electron microscopy, and the deformation microstructures just beneath the fracture surfaces were studied using transmission electron microscopy. Significant dislocation plasticity was observed just beneath both ductile and quasi-brittle fracture surfaces. Further, the dislocation activity just beneath the fracture surfaces was largely comparable with those observed in samples tested without hydrogen. Evidence for hydrogen-enhanced plastic flow localization and shear softening on the sub-micron scale was observed very near the final fracture surface (<2 μm) in the tensile samples. The quasi-brittle fracture surfaces were found to be covered with nanoscale dimples 5–20 nm wide and 1–5 nm deep. Based on analyses of conjugate fracture surfaces, most of the nanodimples appear to be “valley-on-valley” type, rather than “mound-on-valley” type, indicating nanovoid nucleation and growth in the plastically flowing medium prior to ultimate failure. Based on these observations, an alternative scenario of plasticity-generated, hydrogen-stabilized vacancy damage accumulation and nanovoid coalescence as the failure pathway for hydrogen embrittlement is proposed.     

Properties of weldments in high-strength ferritic heat-resistant steels

In recent years, the preservation of natural resources and the reduction of the pressure on the environment have become important tasks. In fossil-fuelled power plants, efforts have been made to improve the efficiency in electricity generation by raising the temperature and pressure of steam. The main conditions for the steam used with the ultra-super critical pressure (USC) boiler of the most recent design in Japan have reached 600°C and 31 MPa. To realise these steam conditions, it is essential to use materials with excellent high-temperature creep strength in the areas subjected to the highest temperature, such as steam and heat conduction pipes. It is not an exaggeration to say that these conditions had not been achievable until high-strength ferritic steels and austenitic stainless steels were developed.¹ Incidentally, in pressure-resistant sections of a boiler like the above-mentioned pipes are welded structures of a large scale, and hence, not only base materials but also the properties of weldments affect the life and reliability of pressure-resistant sections. This article deals with the properties required for welded joints of high-strength ferritic heat-resistant steels which have been used for the most recently designed boilers for fossil power plants. It also describes the creep strength properties at the most important joints and the issues of welding consumables for high-strength materials, together with future tasks.     

Postbreakaway oxidation kinetics of two ferritic steels

The time needed to reach breakaway oxidation, weight gain at the breakaway point, and the postbreakaway oxidation rate are three important variables which, along with others, play an important role in causing boiler tube failure. In this paper an attempt has been made to study the behavior of two ferritic steels-21/4Cr-1Mo and 9Cr-1Mo. Postbreakaway oxidation kinetics of these two alloys have been studied at temperatures in the range 900–1100°C in pure oxygen for a short duration (maximum of 3 hr). No breakaway was observed at 900°C under these conditions. Postoxidation kinetics are linear at first, followed by a slower oxidation rate. The results have been substantiated by the postoxidation studies using SEM, EDAX, and X-ray diffraction.     

Mechanical alloying of lanthana-bearing nanostructured ferritic steels

A novel nanostructured ferritic steel powder with the nominal composition Fe–14Cr–1Ti–0.3Mo–0.5La₂O₃ (wt.%) was developed via high energy ball milling. La₂O₃ was added to this alloy instead of the traditionally used Y₂O₃. The effects of varying the ball milling parameters, such as milling time, steel ball size and ball to powder ratio, on the mechanical properties and microstructural characteristics of the as-milled powder were investigated. Nanocrystallites of a body-centered cubic ferritic solid solution matrix with a mean size of approximately 20 nm were observed by transmission electron microscopy. Nanoscale characterization of the as-milled powder by local electrode atom probe tomography revealed the formation of Cr–Ti–La–O-enriched nanoclusters during mechanical alloying. The Cr:Ti:La:O ratio is considered “non-stoichiometric”. The average size (radius) of the nanoclusters was about 1 nm, with number density of 3.7 × 10²⁴ m^?3. The mechanism for formation of nanoclusters in the as-milled powder is discussed. La₂O₃ appears to be a promising alternative rare earth oxide for future nanostructured ferritic steels.     

Special features of thermal and deformation processes in welding high-strength steels with different edge preparation

Investigations were carried out into the special features of thermal and deformation processes in single- and twin-arc welding of joints with different edge reparation. On the basis of the experimental results it was established that deformation of all types when using the constricted shapes of the welding gap is lower than when using the standard welding gaps. The level of strain in the component can be additionally reduced by the application of twin-arc welding. The investigation results also show that in the final stage of welding it is necessary to correct the welding conditions in the immediate vicinity of the edge of the welded component in order to eliminate the superheating effect caused by the reflection of the heat flow from the welding edge.     

Creep deformation in multilayered and microlaminate materials

Multilayered and microlaminate foils offer a convenient specimen geometry for studying creep deformation in fine-grained materials. Multilayered foils can have large total thicknesses, allowing for ease of handling and testing, while being composed of very small grains that can be controlled and maintained in the individual layers. Uniaxial creep studies have been performed on two model systems, metal/metal multilayers and metal/intermetallic multilayers, revealing both conventional and non-conventional behavior in these materials. These two studies demonstrate the benefits of studying creep deformation of multilayer materials through uniaxial tensile tests of free-standing specimens, specifically the ability independently to vary temperature, applied stress, strain and grain size. Such techniques can be applied to a variety of metal/metal and metal/intermetallic systems and enable investigation into the high-temperature deformation of fine-grained materials. For more information, contact A.C. Lewis, The Johns Hopkins University, Department of Materials Science and Engineering, Baltimore, MD 21218; lewisac@jhu.edu.     

Heat treatment of damping steels with ferritic structures

Alloyed ferritic steels are primarily used as soft, magnetic materials. This article concerns the structural conditions necessary for obtaining a high (elevated) damping capacity of these steels. Regimes of damping heat treatment are discussed and systematized.Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 2, pp. 31 – 33, February, 1995.     

Silicon coating on ferritic steels by CVD-FBR technology

Silicon protective coatings were deposited on ferritic steels (9-12% Cr) by chemical vapour deposition by means of fluidized bed reactor (CVD-FBR). The process was performed at temperatures below 580 °C with the use of Silicon donator powder and hydrogen chloride (HCl) of activator. Thermodynamic calculations were made before the experimental study to investigate the conditions for the formation of gas precursors for the deposition of the Si coating. The samples were examined by means of optical microscopy (OM), Scanning Electron Microscopy (SEM), Energy Dispersion Spectroscopy (EDS) as well as X-ray diffraction, and the results, show the formation of dense, homogenous and thin coatings consisting of Fe₃Si.     

Etching of oxide scales on chromium ferritic steels

Abstract

The oxide scale formed on chromium ferritic steels (21/4–9% Cr) by reaction with water/steam in the temperature range 200–570°c consists of an inner Fe/Cr spinel layer and an outer layer of magnetite. These layers are often indistinguishablefrom one another when the scale is sectioned and viewed under an optical microscope. Possible etching techniques, able to reveal the two layers (often without affecting the metal surface), have been investigated. Successful techniques are listed and the mechanisms involved are discussed in relation to current knowledge of oxide dissolution and passivation of metal surfaces.