Sten Wessman

Evaluation of the WRC 1992 diagram using computational thermodynamics

Ferrite content of a stainless steel weld metal is a vital parameter for ensuring that the microstructure and thus also corrosion and mechanical properties are adequate. A rapid way of estimating ferrite content and thus microstructure is by using weld metal composition and a Welding Research Council (WRC)-1992 diagram. The ferrite-forming alloying elements are estimated to a chromium equivalent, the austenitic to a nickel equivalent and the ferrite content is estimated by help of isoferrite lines. This diagram was derived by empirical work on a large number of commercial and laboratory stainless weld metals covering a wide alloy span. The present paper uses computational thermodynamics, i.e. Thermo-Calc, to evaluate the influence of temperature and key stainless alloying elements. The alloying range for duplex stainless weld metal was studied and the WRC 1992 diagram was compared with results from calculations. The chromium and nickel equivalents were evaluated and coefficients for Mo, Nb, C, N and Cu assessed. The results showed that while the coefficients for Mo and N proved accurate, the values for Nb, C and Cu would require an adjustment. Alternative diagrams with adjusted chromium and nickel equivalents and iso-ferrite lines in both ferrite numbers and volume percent were suggested.

Computational Thermodynamics Study Of The Influence Of Tungsten In Superduplex Stainless Weld Metal

Tungsten is used by some duplex stainless steel producers for partial substitution of molybdenum, both elements enhancing the corrosion resistance of duplex stainless steel. The negative aspect of both molybdenum and tungsten alloying is that they increase the tendency to precipitate intermetallic phases, which may have a detrimental effect on corrosion and mechanical properties. The temperature region for intermetallic phase precipitation is about 700–1000 °C, depending on alloy composition, and the time for precipitation is within minutes for superduplex steels.There has been scientific discussion on the relative effects of Mo alone or Mo-W on intermetallic precipitation behaviour in superduplex steels for the past two decades. While the base material response to ageing and precipitation of intermetallic phases has been satisfactorily assessed, weld metal has proved more of a challenge. The main reason for this is that welding is a very complex process introducing many parameters to the assessment which not have to be considered in studies of base material. For example superduplex weld metal typically solidify fully ferritic but may in case excessive nitrogen is added solidify as a mixture of ferrite and austenite. The solidification mode may vary also between weld passes as a consequence of minor variations in composition. Ferritic solidification is the preferred mode, giving the well known Widmanstätten austenite, which forms in the solid state during cooling. Mixed mode solidification gives a vermicular appearance, which is known to increases the tendency to intermetallic formation. A comprehensive study using computational thermodynamics was done to investigate this matter. This study included equilibrium calculations, Scheil-Gulliver solidification simulations, and calculations of the driving force for intermetallic phase precipitation and further a study of diffusion of Mo and W in these alloy systems. The different approaches were applied on model superduplex weld metals with nominal compositions matching commercial superduplex fillers available today. The principal conclusion is that all thermodynamic calculations clearly indicates the W containing filler to show a more pronounced sensitivity to heat treatments by precipitation of intermetallic phases.

Effect of Sigma Phase Morphology on the Degradation of Properties in a Super Duplex Stainless Steel

Sigma phase is commonly considered to be the most deleterious secondary phase precipitating in duplex stainless steels, as it results in an extreme reduction of corrosion resistance and toughness. Previous studies have mainly focused on the kinetics of sigma phase precipitation and influences on properties and only a few works have studied the morphology of sigma phase and its influences on material properties. Therefore, the influence of sigma phase morphology on the degradation of corrosion resistance and mechanical properties of 2507 super duplex stainless steel (SDSS) was studied after 10 h of arc heat treatment using optical and scanning electron microscopy, electron backscattered diffraction analysis, corrosion testing, and thermodynamic calculations. A stationary arc was applied on the 2507 SDSS disc mounted on a water-cooled chamber, producing a steady-state temperature gradient covering the entire temperature range from room temperature to the melting point. Sigma phase was the major intermetallic precipitating between 630 °C and 1010 °C and its morphology changed from blocky to fine coral-shaped with decreasing aging temperature. At the same time, the average thickness of the precipitates decreased from 2.9 µm to 0.5 µm. The chemical composition of sigma was similar to that predicted by thermodynamic calculations when formed at 800–900 °C, but deviated at higher and lower temperatures. The formation of blocky sigma phase introduced local strain in the bulk of the primary austenite grains. However, the local strain was most pronounced in the secondary austenite grains next to the coral-shaped sigma phase precipitating at lower temperatures. Microstructures with blocky and coral-shaped sigma phase particles were prone to develop microscale cracks and local corrosion, respectively. Local corrosion occurred primarily in ferrite and in secondary austenite, which was predicted by thermodynamic calculations to have a low pitting resistance equivalent. To conclude, the influence of sigma phase morphology on the degradation of properties was summarized in two diagrams as functions of the level of static load and the severity of the corrosive environment.

High-Temperature Phase Equilibria of Duplex Stainless Steels Assessed with a Novel In-Situ Neutron Scattering Approach

Duplex stainless steels are designed to solidify with ferrite as the parent phase, with subsequent austenite formation occurring in the solid state, implying that, thermodynamically, a fully ferritic range should exist at high temperatures. However, computational thermodynamic tools appear currently to overestimate the austenite stability of these systems, and contradictory data exist in the literature. In the present work, the high-temperature phase equilibria of four commercial duplex stainless steel grades, denoted 2304, 2101, 2507, and 3207, with varying alloying levels were assessed by measurements of the austenite-to-ferrite transformation at temperatures approaching 1673 K (1400 °C) using a novel in-situ neutron scattering approach. All grades became fully ferritic at some point during progressive heating. Higher austenite dissolution temperatures were measured for the higher alloyed grades, and for 3207, the temperature range for a single-phase ferritic structure approached zero. The influence of temperatures in the region of austenite dissolution was further evaluated by microstructural characterization using electron backscattered diffraction of isothermally heat-treated and quenched samples. The new experimental data are compared to thermodynamic calculations, and the precision of databases is discussed.