Advances in Physical Metallurgy and Processing of Steels. The Influence of Nitrogen on Microstructure and Properties of Highly Alloyed Stainless Steel Welds.

Abstract

The influence of nitrogen on the microstructure and properties of austenitic and duplex stainless steel welds is reviewed. Three aspects are covered: intermetallic phase formation, nucleation and growth of austenite in ferrite and nitrogen balance in TIG welding.The effect of nitrogen on the precipitation of deleterious intermetallic phases is analysed for duplex and austenitic steels by thermodynamic means and the differences in response of nitrogen alloying illuminated. Also the replacement of molybdenum by tungsten is discussed.Nitrogen alloying made the duplex grades weldable. The duplex steels, designed to comprise approximately equal amounts of ferrite and austenite, solidify in a ferritic mode and a rapid transformation of ferrite to austenite is a prerequisite. Nitrogen has a beneficial effect on the austenite reformation in the HAZ (Heat Affected Zones) on cooling. This has been confirmed by both experiments and calculations. Attempts have been made to improve the nucleation of austenite and thus to increase the possibilities for welding by more demanding techniques which involve rapid cooling through the critical temperature range 1 350°C to 1 000°C.Nitrogen is added to many of the modern stainless steels because of its favourable effects on both strength and corrosion resistance. This implies that the welding procedure must consider the risk of nitrogen escape from the weld pool. Avoiding nitrogen losses may be accomplished by applying e.g. nitrogencontaining shielding gas. Applying balanced shielding gases is generally successful but on occasion reproducibility problems may arise. Correcting measures require insights in the process and the mechanisms controlling the nitrogen fluxes in TIG welding. These can be illuminated using a simple process model that accounts for alloy composition, shielding gas nitrogen content, weld pool shape and size. It is suggested that the main reasons for lack of reproducibility may be found in the batch to batch variation in surfactant content that controls the weld pool shape via the Marangoni effect and surface mass transfer reactions.

Effects of Copper Addition on the Precipitation of Chromium Nitrides and the Associated Pitting Corrosion Resistance of the Hyper Duplex Stainless Steels

“…Hertzman 18) showed by thermodynamic calculations that the driving force for the precipitation of the · phase and » phase in super austenitic stainless steel (SASS) and super duplex stainless steel (SDSS) was reduced by nitrogen addition. The effect of nitrogen on the Cr and Mo activities was suggested to be the determining factor for precipitation of the · and » phase.…”

Effect of carbonitride precipitates on the abrasive wear behaviour of hardfacing alloy

“…5b. All their sizes are smaller than 1 mm and reach a nanoscale, and have good strengthening effect to the matrix [6,12,14]. The average hardness of the samples was made as shown in the Table 3.…”

“…The martensitic stainless steel is a popular hardfacing material in weld hardfacing techniques because of the good properties of wear and corrosion resistances [2][3][4]. To further develop wear resistance and welding performances of the martensitic stainless steel, carbon content of the steels is decreased through nitrogen replacing part of carbon, which can have a carbonitride hardening effect on the matrix [4][5][6]. Niobium and titanium have been used as the most effective carbonitride alloying elements, which can produce much hard precipitation of carbonitride.…”

Formación de fase sigma en uniones soldadas de acero inoxidable súper dúplex fundido

“…En este contexto, el comportamiento de los aceros dúplex puede ser significativamente afectado por el proceso de soldadura [7][8][9] . Por otra parte, el contenido y la naturaleza de los elementos aleantes, principalmente cromo y molibdeno, así como también la presencia de una matriz ferrítica aumentan la susceptibilidad a la fragilización cuando estos materiales se exponen a altas temperaturas (813-1.173 K) o, en su defecto, a enfriamientos lentos en su proceso de manufactura o por tratamientos térmicos [10][11][12] .…”