Aspectos operacionais e metalúrgicos da soldagem do aço inoxidável ferrítico com 11% de cromo

Abstract

Ferritic stainless steels in general present low weldability when compared with austenitic stainless steels. This factor has limited their applicability in welded structures. To achieve better mechanical behavior for these alloys it is necessary, besides alloy composition control, rigorous heat treatments. One example of these treatments is the Box furnace annealing, which promotes greater control of the parameters, allowing better microstructural homogeneity that in turn tends towards better mechanical characteristics of the final product. Nowadays, though, the differential is to achieve improved mechanical properties at a low processing cost. One of the processes capable of achieving lower costs is continuous furnace annealing, which achieves more productivity therefore reducing the production costs. Taking into account that welding is one of the most important material bonding processes, it is greatly important for the material to be, in some sense, able to be welded and to assure good weld quality. Thus, the present study aims to analyze, using the mechanical characteristics of the alloy, the efficiency of the continuous furnace heat treatment process, for an 11% chromium stainless steel alloy. This study also covers the process and metallurgy applied to welding of this alloy. Two types of continuous consumables for alloy welding were used for this: A solid, commercial (AWS ER309LSi), conventionally used for ferritic alloys, and an experimental tubular electrode of class AWS EC 409. A survey of the metal transfer characteristics was taken for each of these consumables in order to establish adequate parameters for welding of this experimental alloy. Once those parameters were established, two heat treatment conditions (Box Furnace and Continuous Furnace) of this alloy were welded, with two welding energy conditions, for each type of consumable. All conditions were submitted to microstructural analysis by optical microscopy and SEM, besides being evaluated by Charpy-V impact toughness. The results showed that the structure of this new alloy presents good base metal characteristics, allowing utilization of continuous furnace treatment. At the welding, though, the region submitted to high temperatures (heat affected zone HAZ) tends to form delta-ferrite. This component is the result of cold work remains and may, under certain conditions, embrittle the HAZ.