Decomposition of ferrite in commercial superduplex stainless steel weld metals; microstructural transformations above 70
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STAINLESS steel with approximately equal volume fractions of ferrite and austenite is termed duplex stainless steel (DSS). The term is derived from the Latin word duplex, the meaning of which is twofold. Such steels possess a favorable combination of mechanical strength and corrosion resistance in chloride ion–containing environments. A superduplex stainless steel (SDSS) is commonly defined as the subset of a DSS that shows a pitting-resistance equivalent (PRE) above 40. The PRE is an empirical number used to estimate the resistance to pitting corrosion in chloride environments calculated according to the relation:[1] PRE ⫽ wt pct Cr ⫹ 3.3 ⫻ wt pct Mo ⫹ 16 ⫻ wt pct N
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The resistance to pitting corrosion in the base material can be optimized by selecting a diffusion-annealing temperature such that the concentration of alloying elements corresponds to equal pitting resistance in ferrite and austenite. However, the situation in the weld metal is more complex, as diffusion annealing is impracticable. The properties of the weld metal are largely influenced by the solidification mode and subsequent cooling of the welding pass as a consequence of the absence of controlled recrystallization and homogenization. The solidification
A. GREGORI, Senior Project Leader, is with Metallurgy and Corrosion Group, TWI Ltd., Cambridge CB1 6AL, United Kingdom. J.-O. NILSSON, Manager, Department of Physical Metallurgy, R&D Center, AB Sandvik Steel, S-811 81 Sandviken, Sweden, is Adjunct Professor, Physics Department, Chalmers University of Technology, S-412 96 Gothenburg, Sweden. Contact e-mail: [email protected] Manuscript submitted July 24, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
mode depends on the composition of the alloy, and small changes can result in different microstructures. Solidification during fusion welding occurs epitaxially and progresses from a condition of good mixing of alloying elements to one in which mixing is controlled by diffusion.[2] The resulting structure can be classified into “normal duplex” microstructures consisting of primary ferrite grains with intergranular and intragranular austenite mainly precipitated in a Widmansta¨tten manner and microstructures with both “normal duplex” and “vermicular” regions, in which the ferrite has a lath-like or vermicular morphology. A ferritic-austenitic mode rather than a fully ferritic mode characterizes the solidification of vermicular regions. This change in solidification mode occurs at approximately Creq/Nieq ⫽ 1.8.[3] Below this value, the solidification is entirely of the ferritic-austenitic type. Moreover, because of the metastable nature of the weld, which may be regarded as a small chilled casting, the delicate balance of alloying elements can be disturbed more easily than the base metal by the formation of precipitates. Therefore, it is quite obvious that the welded joints are often decisive of the performance of a larger structure. In DSS, the most common precipitates are hexagonal Cr2N[4] and intermetallic precipitates such
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