Identification of MnCr2O4 nano-octahedron in catalysing pitting corrosion of austenitic stainless steels

Pitting corrosion of stainless steels, one of the classical problems in materials science and electrochemistry, is generally believed to originate from the local dissolution in MnS inclusions, which are more or less ubiquitous in stainless steels. However, the initial location where MnS dissolution preferentially occurs is known to be unpredictable, which makes pitting corrosion a major concern. In this work we show, at an atomic scale, the initial site where MnS starts to dissolve in the presence of salt water. Using in situ ex-environment transmission electron microscopy (TEM), we found a number of nano-sized octahedral MnCr2O4 crystals (with a spinel structure and a space group of Fd (3) over barm) embedded in the MnS medium, generating local MnCr2O4/MnS nano-galvanic cells. The TEM experiments combined with first-principles calculations clarified that the nano-octahedron, enclosed by eight {1 1 1} facets with metal terminations, is "malignant", and this acts as the reactive site and catalyses the dissolution of MnS. This work not only uncovers the origin of MnS dissolution in stainless steels, but also presents an atomic-scale evolution in a material's failure which may occur in a wide range of engineering alloys and biomedical instruments serving in wet environments. (C) 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Mn ion dissolution from MnS:a density functional theory study

The dissolution of MnS inclusions could induce pitting corrosion in stainless steels, but its dissolution mechanism is poorly understood at the atomic scale. With the help of ab initio molecular dynamics calculations, one inevitable step in the dissolution of MnS is studied by simulating the process of one Mn ion leaving the surface. The reaction mechanism is determined to contain three steps with two large barriers and a small one, leading to two slow steps in the Mn ion dissolution. Comparing to the Na ion dissolution from NaCl, the barriers of the Mn ion dissolution are much larger, which is a reflection of their different electronic structures.

Modelling the correlation between processing parameters and properties of maraging steels using artificial neural network

An artificial neural network (ANN) model is developed for the analysis and simulation of the correlation between the properties of maraging steels and composition, processing and working conditions. The input parameters of the model consist of alloy composition, processing parameters (including cold deformation degree, ageing temperature, and ageing time), and working temperature. The outputs of the ANN model include property parameters namely: ultimate tensile strength, yield strength, elongation, reduction in area, hardness, notched tensile strength, Charpy impact energy, fracture toughness, and martensitic transformation start temperature. Good performance of the ANN model is achieved. The model can be used to calculate properties of maraging steels as functions of alloy composition, processing parameters, and working condition. The combined influence of Co and Mo on the properties of maraging steels is simulated using the model. The results are in agreement with experimental data. Explanation of the calculated results from the metallurgical point of view is attempted. The model can be used as a guide for further alloy development.

Nitride-strengthened reduced activation ferritic/martensitic steels

Two nitride-strengthened reduced activation ferritic/martensitic (RAFM) steels with different Mn contents were investigated. The experimental steels were designed based on the chemical composition of Eurofer 97 steel but the C content was reduced to an extremely low level. Microstructure observation and hardness tests showed that the steel with low Mn content (0.47 wt.%) could not obtain a full martensitic microstructure due to the inevitable δ-ferrite independent of cooling rate after soaking. This steel showed similar room temperature strength and higher strength at 600 °C, but lower impact toughness, compared with Eurofer 97 steel. Fractography of the Charpy impact specimen revealed that the low room temperature toughness should be related to the Ta-rich inclusions initiating the cleavage fracture. The larger amount of V-rich nitrides and more dissolved Cr in the matrix could be responsible for the strength being similar to Eurofer 97 steel. In the second steel developed from the first steel by increasing the Mn content from 0.47 wt.% to 3.73 wt.%, a microstructure of full martensite could be obtained.

The preparation of highly ordered single layer ZSM-5 coating on prefabricated stainless steel microchannels

An elegant way to prepare catalytically active microreactors is by applying a coating of zeolite crystals onto a metal microchannel structure. In this study the hydrothermal formation of ZSM-5 zeolitic coatings on AISI 316 stainless steel plates with a microchannel structure has been investigated at different synthesis mixture compositions. The procedures of coating and thermal treatment have also been optimized. Obtaining a uniform thickness of the coating within 0.5 mm wide microchannels requires a careful control of various synthesis variables. The role of these factors and the problems in the synthesis of these zeolitic coatings are discussed. In general, the synthesis is most sensitive to the H2O/Si ratio as well as to the orientation of the plates with respect to the gravity vector. Ratios of H2O/Si=130 and Si/template=13 were found to be optimal for the formation of a zeolitic film with a thickness of one crystal at a temperature of 130 degreesC and a synthesis time of about 35 h. At such conditions, ZSM-5 crystals were formed with a typical size of 1.5 mu mx1.5 mu mx1.0 mum and a very narrow (within 0.2 mum) crystal size distribution. The prepared samples proved to be active in the selective catalytic reduction (SCR) of NO with ammonia. The activity tests have been carried out in a plate-type microreactor. The microreactor shows no mass transfer limitations and a larger SCR reaction rate is observed in comparison with pelletized Ce-ZSM-5 catalysts; (C) 2001 Elsevier Science B.V. All rights reserved.

Phase transformations in maraging steels

Research on the kinetics of precipitate formation and austenite reversion in maraging steels has received great attention due to their importance to steel properties. Judging from the literature in recent years, research into maraging steels has been very active, mainly extending to new types of steels, for new applications beyond the traditional strength requirements. This chapter provides an in-depth overview of the literature in this area. In addition, the kinetics of precipitate formation are analysed using the Johnson–Mehl–Avrami (JMA) theory.

Fracture surface, impact energy and hardness of Ni-free high-Mn steels

Two manganese steels were investigated: Fe-19.7%Mn (VM339A) and Fe-19.7%Mn stabilized with 0.056%C, 0.19%Ti and 0.083%Al (VM339B). The toughness of VM339A was higher than VM339B, but VM339B had higher hardness. Tempering does not affect the toughness of the alloys. SEM images of the fracture surface for both the alloys revealed ductile fractures. A further alloy with a lower manganese content, Fe-8.46%Mn-0.24%Nb-0.038%C, and thus even lower cost than the conventional 3.5Ni cryogenic steel, was tested for its impact toughness after heat treatment at 600°C, giving promising results.