Jan-Olof Nilsson

σ-PHASE PRECIPITATION IN STABILIZED AUSTENITIC STAINLESS STEELS

Experimental observations of sigma-phase precipitation in two stabilized austenitic stainless steers, AISI 321 and AISI 347, aged up to 80,000 h at temperatures between 500 and 800 degrees C, are c ...

Structure, chemistry, and stress corrosion cracking of grain boundaries in alloys 600 and 690

The microstructure in six commercial batches of alloys 600 and 690 has been investigated using scanning electron microscopy (SEM), analytical transmission electron microscopy (ATEM), atom probe field ion microscopy (APFIM), and secondary ion mass spectroscopy (SIMS). The materials were also tested with respect to their resistance to intergranular stress corrosion cracking (IGSCC) in high-purity water at 365 °. Applied microanalytical techniques allowed direct measurement of carbon concentration in the matrix together with determination of grain boundary micro structure and microchemistry in all material conditions. The distribution of oxygen near a crack in material tested with respect to IGSCC was also investigated. The role of carbon and chromium and intergranular precipitates on IGSCC is discussed.

Development of commercial nitrogen-rich stainless steels

This paper reviews the development of nitrogen alloyed stainless steels. Nitrogen alloying of austenitie stainless steels started at an early stage and was to a large extent caused by nickel shortage However, direct technical advantages such as increased strength of the nitrogen alloyed steels made them attractive alternatives to the current steels It was not until the advent of the AOD (Argon Oxygen Decarburisation) process in the late 1960s that nitrogen alloying could be controlled to such accuracy that it became successful commercially on a broader scale. The paper describes production aspects and how nitrogen addition influences microstructure and the resulting properties of austenitie and duplex stainless steels. For austenitie steels there are several reasons for nitrogen alloying. Apart from increasing strength nitrogen also improves structural stability, work hardenmg and corrosion resistance. For duplex steels nitrogen also has a decisive effect in controlling the microstructure during thermal cycles such as welding.

Quantitative metallography of sigma phase precipitates in AISI 347 stainless steel – a comparison between different methods

Abstract A reliable method to perform volume fraction measurements of sigma (σ) phase in a niobium stabilised steel (AISI 347) has been developed. The most accurate results of the tested methods were obtained using backscattered electrons in a scanning electron microscope (SEM) and samples etched with oxalic acid. Both optical microscopy (OM) and SEM either on polished samples or on etched samples have been evaluated to come to this conclusion. Several etchants were also tested and careful etching with oxalic acid gave a well defined rim. The measurement of σ-phase fraction has been performed using manual point counting and digital image analysis using manual threshold. It was concluded that image analysis is usually to be preferred since it is faster and also results in higher precision The phase boundary caused by etching was evaluated, and it was found that the boundary area should be included in the measurement when using the recommended SEM method.

Microstructural Stability and Mechanical Properties of a High Nitrogen Super Duplex Stainless Steel

A time temperature transformation (TTT)-diagram with respect to the formation of intermetallie phase in the range 700-1000°C has been assessed by point counting for a 29Cr-6Ni-2Mo-0.38N super duplex stainless steel. Using a computer program developed by the authors a continuous cooling transformation (CCT)-diagram was calculated from the TTT-diagram assuming that the transformation can be described by an Avrami type equation. A comparison of impact toughness and hardness showed that toughness was a very sensitive measure of intermetallic phase formation while hardness was insensitive and showed no significant increase until the material was catastrophically brittle. It was found that Thermo-Cale could be used in a qualitative manner for predicting microstructural changes at various temperatures but was unable to predict variables such as dissolution temperature and volume percentage with accuracy.

Prediction of Properties in Type 347 Austenitic Stainless Steel after Long-Term Service at High Temperatures

The evolution of microstructure during production and elevated temperature service of type 347 austenitic stainless steel in the temperature range 700-800°C was modelled using commercial software packages such as Thermo-Calc and DICTRA and characterized using various microscopical techniques. The growth and coarsening of niobium carbonitrides and σ- phase were modelled as well as nitrogen uptake. Good agreement between predictions and microstructural observations was found.