The Precipitation and Deformation Behaviour of Superaustenitic Stainless Steel

Doktorsavhandling, 2000

The present study concerns the precipitation behaviour and the deformation characteristics of a superaustenitic stainless steel with composition Fe-24Cr-22Ni-7.3Mo-0.5N (wt%). Regarding precipitation, also materials with Mo partially replaced by W have been studied. Increased demands on materials for severe corrosive environments, both in chemical industry and in offshore industry, have lead to the development of the superaustenitic alloys. Compared to conventional austenitic stainless steels, superaustenitic materials have a superior resistance to pitting and crevice corrosion in environments containing halides. The most corrosion resistant alloys in this group show corrosion properties on a level with many nickel-based alloys. Main alloying additions to achieve the good corrosion properties are chromium, molybdenum, tungsten and nitrogen, which also contribute to improved mechanical properties. The high contents of chromium and molybdenum makes the material sensitive to precipitation of secondary phases when exposed to higher temperatures. A typical situation where such precipitation may occur is during welding. In the present study both the precipitation occurring in the heat-affected zones during welding, and precipitates in isothermally aged material have been investigated. Analytical techniques used include optical microscopy, scanning electron microscopy and transmission electron microscopy. Special emphasis is put on the crystallographic identification of precipitates and the corresponding compositions. In the heat-affected zones (HAZ), precipitatation of the intermetallic .sigma. and R phases has been found. Typical times for precipitation are below 30 seconds, based on thermocouple measurements in the HAZ. During the initial stages of isothermal ageing, various intermetallic phases, e.g. .sigma., .chi., R and Laves related phases, form in grain boundaries. The Laves related phases (C14 Laves .my. and C) all show highly distorted crystals, and intergrow in a complex manner. The most critical temperature range for precipitation is 900-1050°C. Prolonged ageing results in the formation of Cr rich phases, e.g. .my. and Cr2N, adjacent to previously formed Mo-rich intermetallic precipitates. Tungsten promotes the formation of high Mo+W containing phases (e.g. C14 Laves and .my.). The cyclic deformation characteristics of the material is strongly related to the high nitrogen content, possibly in combination with the high levels of chromium and molybdenum. The material shows a pronounced slip planarity during cyclic straining, both regarding surface slip topography as well as the developed dislocation structures. Detailed studies were performed on the cyclic hardening/softening behavior, internal and effective stresses and the relation to the dislocation structure at various fractions of the fatigue lifetime. In Coffin-Manson diagrams, a double-slope behaviour is observed. This can be related to a gradual change from single slip at low strain amplitude to multiple slip at higher strain amplitudes. The strong slip planarity most likely results from breakdown of short-range order between nitrogen and molybdenum atoms. Such ordering has been indicated by atom probe analysis.