Microstructure and Mechanical Properties of Metal Powder Treated AISI-430 FSS Welds

Microstructure and Mechanical Properties of Metal Powder Treated AISI-430 FSS Welds

M.O.H. Amuda, F.T. Lawal, M. A. Onitiri, E. T. Akinlabi, S. Mridha
DOI: 10.4018/IJMMME.2018100104
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An innovative yet simple technique for the inoculation of the weld pool of commercial AISI 430 Ferritic Stainless Steel (FSS) with metal powders for grain refinement is discussed. Aluminum or titanium powder in varying amounts introduced into the weld pool via a powder preplacement technique was melted under a tungsten inert gas (TIG) torch. This strategy of inoculating the welds offers dual benefits of grain refinement and constriction in the weld geometry. The addition of the metal powders constricts the HAZ by as much as 50% of the conventional weld providing a grain refinement index (GRI) of about 0.8 in titanium powder treated welds. It equally emerged that weld property is not influenced by the grain size alone but equally by the amount of delta ferrite in the microstructure.
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Nickel free FSS are attractive alternative to the expensive austenitic and duplex grades in chloride and caustic environments due to their excellent stress corrosion cracking, pitting and crevice corrosion resistances in these environments (Wing, Song, Sun, Li & Zhai, 2009). In certain instances, they provide high temperature oxidation resistance comparable to the austenitics (Anbazhagan & Nagalakshmi, 2002). Recently, the ferritics have also received attention consequent upon their superior performance under irradiation (Silva, Lima & Campos, 2007). Thus, they are suitable for application in heat transfer systems, condenser tubing for fresh water plants, railway vehicle and several other areas. However, hitherto, the wide application of the ferritics in areas that requires welding has been very low. This is because the welding of the ferritics particularly the first-generation grades is associated with many problems. These problems include lower mechanical property in the weld section as well as increased susceptibility to intergranular corrosion due to precipitation of grain boundary chromium carbides. The loss in mechanical property is due to the combined effect of grain coarsening and higher hardness in the weld zone owing to the influence of the welding heat (Amuda & Mridha, 2010; Amuda & Mridha, 2011).

Though, the austenitics also experience grain coarsening, the level is higher in the ferritics; at times, it can be as high as ten orders of the grain size of the base metal (Lippold & Kotecki, 2005). Extensive grain growth occurs in the HAZ of the weld, impairing the load bearing strength of the joint. Weld section with equiaxed grains exhibit excellent mechanical properties (Mohandas & Reddy, 1997). It follows therefore that mechanical properties of welded ferritics can be improved if refined and equiaxed grains are formed in the weld section. Furthermore, equiaxed grains also assist in reducing solidification cracking in the welds (Reddy & Meshram, 2006).

There are many techniques of grain refinement in fusion welds. However, the conventional technique of post weld thermal treatment is not feasible in the FSS grade due to the absence of phase transformation or allotropic modification in the alloy (Reddy & Meshram, 2006). Other techniques such as low energy input, inoculation with heterogeneous nucleants, gas tungsten arc (GTA) pulsing, oscillation and electromagnetic stirring as well as addition of elemental powder in electrode flux coatings are useful for refining weld grains. A review on these procedures is available in the literature (Amuda & Mridha, 2010). Attempts have been made to refine grain structure in FSS welds by the addition of elements that have similar atomic diameter to iron such as Al, Cu and V with promising findings (Cavazos, 2006).

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