Estudo dos efeitos das condições de usinagem na microestrutura da superfície de um aço inoxidável super duplex.

Abstract

Stainless steels super duplex are widely used in the oil and gas sector for having a high performance in very aggressive corrosion environments, these also have superior mechanical proprieties to austenitic stainless steels. The combination of alloying elements like chrome, nickel and molybdenum provides to these steels a high hardening coefficient and a low thermal conductivity. These properties causes problems in machining theses kind of steels, because it causes the formation of built up edges on the cutting tool the heat is not dissipated efficiently either, causing the temperature at the cutting edge to rise significantly, thus activating diffusional phenomena and accelerating the tool wear. Knowing that in many carbon steel alloys machining can causes changes in microstructure near the machined surface, generating a hard surface or even with phase transformation. It was decided to study if this phenomenon can happens also in stainless steel alloys super duplex, and with that was chosen the UNS S32760 alloy. The study was done by varying the parameters depth of cut, feed and cutting speed to generate different loads applied in the material, besides that, an apparatus was set up to monitor the current during the machining to estimate the cutting power and the force applied to shear the material. In addition, two tests were performed with high cutting speeds to verify if the heat generated by removing the material would be sufficient to cause some kind of microstructural modification in the material. For microstructural analysis was used optical microscopy and scanning electron microscope, and only optical microscopy for tool analysis. With the analysis of the tools, the objective was to determine which conditions would be more efficient comparing the tool wear, to generate a cost reduction when machining this kind of material. It was observed that, increasing the cutting power and the applied load in the material did not cause any effect in the microstructure of it. On the other hand, in the tests with high cutting speeds, was observed that near the machined region there was an imbalance in the percentages of ferrite and austenite, causing the amount of austenite phase to increase. Showing that the predominant factor in machining to cause microestrutural changes in UNS S32760 is the cutting temperature, that when reaching high enough values can induce the nucleation of the austenite from ferrite.