Assessment of HAZ Hot Cracking in a High Nitrogen Stainless Steel
Fully austenitic materials, such as nickel base alloys and several stainless steels, can be prone to hot cracking in the heat affected zone during welding. Such cracks, due to their small size commonly denoted as micro-cracks, were found in laser hybrid w
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Abstract Fully austenitic materials, such as nickel base alloys and several stainless steels, can be prone to hot cracking in the heat affected zone during welding. Such cracks, due to their small size commonly denoted as microcracks, were found in laser hybrid welds of a high nitrogen stainless steel (UNS S 34565 / German material no. 1.4565). Mechanical testing, such as tensile and Charpy V-notch tests as well as cyclic loading, has been carried out on hot crack afflicted laser plasma hybrid welds. The test results show that the ductility of the material is decreased, whereas the strength is not influenced at all. SEM micrographs of the fracture surfaces revealed that micro-cracks in the HAZ were associated with failure. For a thorough evaluation of the cracking mechanism, the orientation of the micro-cracks alongside the fusion line of the hybrid welds is of great importance. Thus, computer tomography using micro focus X-ray has been carried out. Using this new approach, the orientation of the cracks towards the fusion boundary was ascertained. It turned out that the critical strain emerged at specific sites of the hybrid weld. The critical strain was oriented tangentially to the fusion boundary and perpendicularly to the welding direction. X-ray cone beam computer tomography turned out as a very useful tool for the investigation of the crack distribution and hence, for getting qualitative information about the critical stress/strain distribution in real welds.
Introduction Fully austenitic stainless steels offer high performance for many applications, such as offshore technology, chemical industries and wherever high corrosion resistance and/or special mechanical, e.g. high
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ductility at cryogenic temperatures, or special physical properties, e.g. low magnetic permeance, are required. In these steels, nitrogen as an alloying element can effectively reduce the material costs if it is used as a substitute for nickel. Thus, the nitrogen alloyed stainless steels represent a very costsaving option compared to nickel base alloys. Furthermore, nitrogen additions strongly increase the yield strength of these materials [1, 2]. However, fully austenitic materials can suffer from high sensitivity to hot cracking, i.e. solidification cracking, liquation cracking and ductilitydip cracking (DDC) [3-6]. Hot cracking occurs during welding if a critical strain is imposed on an interface within a so called brittle temperature range (BTR) or ductility dip temperature range (DTR) in case the of DDC [5, 7]. In addition to solidification cracking, cracking within the BTR implies also material separation in the partially melted zone. This type of cracking is denoted as liquation cracking because it arises from low melting phases on the grain boundaries. It is the temperature as a function of time, i.e. heat input, which controls diffusion and thus dissolution and/or precipitation of low melting phases. If the peak temperature enters the BTR, then the material cannot sustain critical localised strain during
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