This therefore could produce unrealistic results for predication of phase transformation in real welding. studied sigma phase precipitation in a 2507 weld metal, but the chemical composition (lower Ni) and high ferrite content (78%) were different compared to the standard requirement and practice. Some information about the influence of heat treatment temperature and time on the microstructure of SDSS weld metal is available but is far from complete.
Knowledge about allowable combinations of times, temperatures, and cooling and heating rates to avoid detrimental changes of the microstructure is therefore essential for efficient fabrication and processing. The precipitation of secondary austenite, furthermore, is a well-known phenomenon occurring in multipass welds often leading to a reduction of corrosion resistance. However, in thicker structures and multipass welding, the heat input has to be balanced against the fact that sigma phase can precipitate quickly at 600–1000 ☌. To avoid the formation of nitrides and an unacceptably high ferrite content during welding, filler metals overalloyed in Ni, shielding and backing gases with N-additions, and/or higher heat input are recommended to promote austenite formation. This is typically due to excessive heating and reheating during fabrication and processing, such as cutting, heat treatment, and welding. However, a large imbalance in ferrite/austenite ratio and/or the precipitation of unwanted secondary phases such as nitrides, intermetallics, and sometimes secondary austenite may result in the degradation of properties. Super duplex stainless steels (SDSS), with a microstructure consisting of approximately equal amounts of ferrite and austenite, present an excellent combination of toughness and corrosion resistance. Results are summarized as time-temperature-precipitation and property diagrams for hardness and sensitization. Increased hardness and etching response suggest that 475 ☌ embrittlement had occurred after 10 min. Coarse and fine secondary austenite precipitated at high and low temperatures, respectively: The finer secondary austenite was more detrimental to corrosion resistance due to its lower content of Cr, Mo, and N as predicted by thermodynamic calculations. After 10 min, larger amounts of intermetallics resulted in hardness up to 400 HV0.5 and more severe sensitization at 580–920 ☌. After 1 min, intermetallics such as sigma and chi phase had precipitated, resulting in moderate sensitization at 720–840 ☌. Temperature modeling and thermodynamic calculations complemented microstructural studies, hardness mapping and sensitization testing. Super duplex stainless steel (SDSS) weld metal microstructures, covering the complete temperature range from ambient to liquidus, were produced by arc heat treatment for 1 and 10 min.
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