THE INFLUENCE OF MICROALLOYING WITH BORON ON PROPERTIES OF AUSTENITE STAINLESS STEEL X8CRNIS18-9

More recently a modified stainless steels have been used to produce various structural elements that work in complex operating conditions. Stainless steel X8CrNiS18-9 (standard EN 10088-3: 2005) is the most commonly used austenitic stainless steel due to its good machinability. This steel has high mechanical and working properties thanks to a complex alloying, primarily with the elements such as chromium and nickel. The content of sulphur present in the steel from 0.15 to 0.35% improves machinability. However, while sulphur improves machinability at the same time decreases the mechanical properties particularly toughness. The addition of sulphur, which is the cheapest available additive for free machining, will impair not only the transverse strength and toughness, but also the corrosion resistance.The aim of this work is to determine the influence of microalloying with boron on the machinability, corrosion resistance and mechanical properties the mentioned steel, but alsoto determine the effect of microalloying with boron on above steel, which is already microalloyed with zirconium, tellurium, or both elements (zirconium and tellurium) due to modification of non-metallic inclusions and improvement of properties.

Failure Analysis of a Cracked Stainless-Steel Steam-Water Separator

Currently, austenitic stainless steel has been widely used for the pressure boundary, including reactors, separators and storage tanks serviced in energy, petrochemical, chemical and food industries in view of its inherent corrosion resistance. However, the corrosion resistance may deteriorate under some circumstances such as field welding and inappropriate post-weld heat treatment. A steam-water separator serviced in a power plant was found cracking and a large amount of steam leaked outside. The cracking was located in the heat-affected zone (HAZ) of the joint on the head side of the pressure vessel. The material of the head was SUS 304 austenite stainless steel. Failure analysis was conducted to investigate the cause of cracking. The testing and measurement included chemical composition analysis, metallographic examination, fracture surface observation and deposit elements analysis. Results showed that the cracking was intergranular and stress corrosion cracking (SCC) was the primary cause of failure. During the fabrication of the separator, the HAZ of the joint was overheated by the thermal input of welding. Brittle carbides such as M23C6 precipitating at the grain boundary, resulted in a narrow belt lack of chromium nearby known as sensitization. The corrosion resistance of the austenite stainless-steel decreased obviously there, and cracking failure occurred rapidly under tensile stress. The influencing factors discussed in this paper mainly focused on material performance, post-weld heat treatment, and corrosivity of medium. Austenitic stainless steel containing stabilizing elements or with low C content was recommended for the new vessel design in order to avoid similar cracking failure.