Influência da composição química de gás de proteção do tipo DX no acabamento superficial de tubos de aço sem costura normalizados em fornos de atmosfera controlada

Abstract

Vallourec Soluções Tubulares do Brasil S.A (VSB) has different tunnel furnaces, with controlled atmosphere, intended to heat treatments such as: normalizing, spheroidizing, stress relief, etc. These furnaces use an exothermal protective gas (DX type), which is composed by: CO, CO2, H2, H2O, O2 (residual), CH4 e N2 (balance). In the present study, the superficial finishing of seamless cold drawn tubes submitted to a normalizing heat treatment under controlled atmosphere with a reductive protective DX gas combined with nitrogen was analyzed. By means of an industrial scale experiment, it was assessed if the adopted atmosphere was the most adequate in order to avoid scale formation and to obtain an homogeneous superficial finishing in terms of material color. The nitrogen and DX gases flow rates used complied with the minimum atmospheres changes, two changes per hour, according to good practices established for furnaces that have entrance and exit doors partially open(1) . The initial purge was not adopted considering that the atmosphere was already stabilized. The purge is used only before the first batch heat treated each week, based on the maximum admissible value established on the technical procedure. The DX gas (CO, CO2, H2 e H2O) equilibrium chemical composition was calculated, as wells as, those ones concerning the minimum and maximum compositions of each protective gas generator. One verifies that all equilibrium chemical compositions reached an iron stability region on the Fe-C-H-O diagram, for situations free for oxygen. It was observed that the heat treated tubes do not exhibit scale, nevertheless, their colors were heterogeneous (light and dark). Samples were submitted to a SEM analysis, in which it was not possible to detect a significant oxygen concentration difference between the previous heterogeneous regions. However, the X-ray diffractometry revealed the presence of iron oxides (Fe2O3 and Fe3O4) on both samples. Demonstrating that even though the scale formation was avoided, which is the most critical for customer quality, the iron oxidation was not avoided.