Aerospace coatings – a specialist field

Abstract

An insight is provided into the performance requirements for many of the surface coatings used by the aerospace industry, which cover a wide spectrum of paints and other materials. Examples of the latter include dry film lubricants, attrition coatings and even special wax polishes for aircraft use.     

Improving 410NiMo weld metal toughness by PWHT

Highlights of the present study is the importance of choosing suitable temperatures for two stage PWHT to achieve desirable toughness in the weld metals produced by ER 410NiMo filler wire. Weld pads prepared using this filler wire was used for extensive metallurgical characterization of the weld metal. Results indicate by choosing appropriate temperatures for the PWHT, it is possible to obtain toughness in the weld metal which is comparable to the toughness reported for the base metal of similar composition. Good toughness of the weld metal is attributed to the presence of retained austenite in the weld metal. Two stage PWHT that gave excellent toughness for the weld metal was employed for repair of cracked shrouds of a steam turbine in a nuclear power plant. The metallurgical characterization of the mock up weld pad prepared prior to actual repair confirmed that microstructure and hardness of the weld metal are similar to those obtained during the welding procedure development.     

Effect of component thickness on lifetime and oxidation rate of chromia forming ferritic steels in low and high pO2 environments

Abstract

Long term oxidation tests were carried out with a high-Cr ferritic steel at 800°C and 900°C in simulated cathode and anode gas of a solid oxide fuel cell (air and an Ar/H₂/H₂O mixture respectively). It was found that with decreasing sample thickness the life time of the steel decreases due to breakaway phenomena. This effect is caused by faster exhaustion of the chromium reservoir from the bulk alloy in the case of thinner components. During air exposure the oxidation rates increase with decreasing specimen thickness and this has to be taken into account in the calculation of the Cr-reservoir exhaustion. This thickness dependence is not found during the exposures in simulated anode gas. Hence, especially for thin walled components, the oxidation rates in anode gas are substantially smaller and thus the life times are longer than during air exposure. The differences in oxidation behaviour in the two environments are discussed on the basis of scale formation mechanisms involving microcrack formation in the surface oxide scale and depletion of major and minor alloying additions in the bulk alloy.     

Characterisation of amorphous phosphate coatings onto carbon steel surfaces

Abstract

This study continues the authors’ investigation of the newly developed product KAF-09OF and concerns the preparation and characterisation of amorphous phosphate coatings on carbon steels. KAF-09OF is a liquid concentrate of ammonium and alkaline phosphates, stabilising additives – buffers, softeners, activators (both inorganic and organic), as well as surfactants – a non-ionic and an anionic detergents. Gravimetric and polarisation methods, X-ray fluorescence analysis, transmission electron microscopy, X-ray photoelectron spectroscopy and atomic force microscopy have been used to determine the effect of pH (2–6), temperature conditions (20–80°C) and agitation rate (0–2000 rev min^?1) on the process of phosphating using this developed product. Coating thicknesses, compositions, structures and passivation characteristics in the phosphating baths with different concentrations of KAF-09OF have been analysed.     

Corrosion of furnace wall materials in waste-wood fired power plant

Abstract

Corrosion tests were performed with four different materials exposed at the furnace wall in a power boiler burning recycled wood, with the aim of evaluating coatings to reduce the corrosion. The nickel base Alloy 625 and the iron–chromium–aluminium alloy Kanthal APMT had the lowest corrosion rates followed by the stainless steel 310S. The low alloy steel 16Mo3, from which the walls are constructed, had the highest rate. Different corrosion mechanisms were found to occur according to the alloy type. Thermodynamic modelling showed that chlorine gas exists at extremely low levels under the prevailing conditions and the hydrated form is thermodynamically favoured.     

Brazing of UNS S32101 and UNS S32304 lean duplex stainless steels and UNS S32750 super duplex with AWS A5.3 BNi-7 (Ni–Cr–P) filler metal

Summary

Weld metals solidified in the ferritic-austenitic solidification mode (FA mode) have dual phases of ferrite and austenite in their as-solidified condition, where ferrite exhibits different morphologies depending on the chemical composition and welding conditions. This paper describes an investigation of the effect of the solidification and transformation sequence on the formation of final ferrite morphologies. Austenite is formed through either a eutectic reaction or peritectic reaction at the dendrite boundaries after the primary formation of ferrite. During the eutectic formation of austenite, the <100>_δ direction of the primary ferrite and the <100>_γ direction of the eutectic austenite are parallel to each other and lie along the growth direction of the primary dendrites. However, any specific lattice plane relationship between the two phases is not identified. During cooling after solidification, the austenite extends into the primary ferrite via solid-state transformation, and the final morphology of the ferrite is vermicular without any coherent orientation relationship between the primary ferrite and eutectic austenite. During peritectic formation of austenite, the Kurdjumov-Sachs orientation relationship is established between the primary ferrite and peritectic austenite, and the <100>_γ direction of the peritectic austenite is not parallel to the growth direction of the primary dendrites. During cooling after solidification, the primary ferrite transforms into austenite, and the final morphology of the ferrite is lathy, since the primary ferrite and peritectic austenite have a favourable coherent orientation.     

High strength,high elongation steels obtained by hot deformation and quenching and partitioning

Abstract

An effective means to achieve energy savings, environmental protection and improved safety in automobiles is to explore higher strength steels having low cost and better processability and service performance. Two routes to develop advanced high strength steels combining ultrahigh strength with appropriate plasticity and characterised by low cost and good quality are proposed: hot stamping followed by quenching and partitioning (HS-Q&P) and deformation induced ferrite transformation followed by Q&P (DIFT-Q&P).     

Metallurgical special features of the production of welded joints in dispersion‐filled metallic composite materials

Investigations were carried out on the special features of the structure and properties of electron beam welding joints in 150 mm thick plates of pearlitic steel 16GNMA and 160 mm plates of 12Kh2MFA steel. It is shown that using vacuum as the shielding medium results in the efficient removal of gases from the weld pool and reduces the content of alloying elements with high vapour tension.     

Strain-induced martensitic transformation in stainless steels: A three-dimensional phase-field study

A three-dimensional elastoplastic phase-field model is developed to study the microstructure evolution during strain-induced martensitic transformation in stainless steels under different stress states. The model also incorporates linear isotropic strain hardening. The input simulation data is acquired from different sources, such as CALPHAD, ab initio calculations and experimental measurements. The results indicate that certain stress states, namely uniaxial tensile, biaxial compressive and shear strain loadings, lead to single variant formation in the entire grain, whereas others, such as uniaxial compressive, biaxial tensile and triaxial strain loadings, lead to multivariant microstructure formation. The effects of stress states, strain rate as well as temperature on the mechanical behavior of steels are also studied. The material exhibits different yield stresses and hardening behavior under different stress states. The equivalent stress is higher at low strain rate, whereas a higher elongation is obtained at high strain rate. The deformation temperature mainly affects the hardening behavior of the material as well as the transformation, i.e. martensite volume fraction decreases with increasing temperature. Some of the typical characteristics of strain-induced martensite, such as the formation of thin elongated martensite laths, shear band formation and nucleation of martensite in highly plasticized areas, as well as at shear band intersections, are also observed.     

Materials for flexible riser systems: problems and solutions

Criteria to be satisfied in the selection of materials for unbonded flexible risers are first outlined. The paper then describes how three main types of thermoplastic and two classes of steel meet these requirements, and briefly discusses selection criteria and materials used for ancillary equipment for flexible riser systems