No abstract

Abstract:In stainless steels, microstructural damage is caused by precipitation of chromium carbides or sigma phase. These microconstituents are detrimental in stainless steel welds because they lead to weld decay. Nevertheless, they are prone to appear in the heat affected zone (HAZ) microstructure of stainless steel welds. This is particularly important for repairs of industrial components made of austenitic stainless steel. Non-destructive metallography can be applied in welding repairs of AISI 304 stainless steel components where it is difficult to ensure that no detrimental phase is present in the HAZ microstructure. The need of microstructural inspection in repairs of AISI 304 is caused because it is not possible to manufacture coupons for destructive metallography, with which the microstructure can be analyzed. In this work, it is proposed to apply in situ metallography as non-destructive testing in order to identify microstructural damage in the microstructure of AISI 304 stainless steel welds. The results of this study showed that the external surface micrographs of the weldment are representative of HAZ microstructure of the stainless steel component; because they show the presence of precipitated metallic carbides in the grain boundaries or sigma phase in the microstructure of the HAZ. Key-words:In situ metallography; Microstructure; Heat affected zone; Austenitic stainless steel; Welding. Emprego de Réplica Metalográfica na Prevenção de Falhas Devido a Danos Microestruturais em Soldas de Aço InoxidávelResumo: Em aços inoxidáveis o dano microestrutural é causado pela precipitação de carbonetos de cromo ou de fase sigma. Estes microcomponentes são prejudiciais em soldas de aço inoxidável, porque eles levam a corrosão intergranular. No entanto, eles são propensos a aparecer na microestrutura da zona afetada pelo calor de soldas em aço inoxidável. Isto é particularmente importante para reparos de componentes industriais feitos de aço inoxidável austenítico. Réplica metalográfica pode ser aplicada para identificar a necessidade de reparos em componentes de aço AISI304 inoxidável onde é difícil assegurar que tal fase está presente na microestrutura da ZTA. A necessidade de inspeção microestrutural não destrutiva no aço inoxidável AISI 304 está associada a impossibilidade de se obter corpos de prova para análise via técnicas convencionais. Neste trabalho, propõe-se a aplicação de réplica metalográfica, a fim de identificar danos microestruturais em soldas de aço inoxidável AISI 304. Os resultados deste estudo mostraram que as micrografias da superfície externa do conjunto de peças soldadas representam a microestrutura da ZTA do componente de aço inoxidável porque mostram a presença de carbonetos metálicos precipitados nos contornos de grão ou de fase sigma.Palavras-chave: Metalografia; Microestrutura; Zona afetada pelo calor; Aço inoxidável austenítico; Soldagem.

In this work, a Super Austenitic Stainless Steel is designed supported by Thermo-Calc modeling and produced at open atmosphere induction furnace, based on 254SMO (UNSS31254). The technological challenge of this work is to prevent the sensitization of the alloy, due to its high C content (0.072%). The chemical composition includes 0.25%Nb and 2.2%Mn to stabilize the alloy and improve mechanical properties. Furthermore, two different types of solution annealing are performed in order to study the effect of the precipitates on the corrosion resistance (HD-1120 C and HD-1180 C). As main results, the microstructure of sample HD-1180 C shows an Austenitic matrix and, Nb-MX and Z-phase as secondary phases; whereas, sample HD-1120 C presents coarse Mo-rich particles formed during solidification. The designed alloy SASS-NbMn (HD-1120 C and HD-1180 C) shows slightly better mechanical properties than alloy 254SMO. In addition, the corrosion behavior of SASS-NbMn is studied by cyclic polarization in 1M NaCl solution.Sample HD-1120 C shows poor pitting resistance due the presence of Morich particles generating Mo-depleted zones. However, Sample HD-1180 C shows outstanding pitting resistance, even better than 254SMO. These promising results open the possibility to develop a new field of stabilized SASS, with comparable mechanical and corrosion properties, and lower cost than commercial SASS.

Machinability is an important property of materials, especially because it affects the manufacturing cost of products. Since there is no accepted definition of machinability, there is no accepted test for measuring it. Researchers have studied two dimensional cutting forces, chip thickness, using orthogonal cutting models, others have concentrated on comparing three dimensional cutting forces, surface roughness and power consumption in oblique cutting. All these tests converge in the necessity of determining which material has better machinability. In this paper machinability tests by other authors are reproduced, proper modifications are made, to visualize which test should be considered and which to discard in the measurement of machinability.

No abstract