Anomalous Sulfur Segregation Kinetics Governed by MnS Precipitation Reaction and {110} Annealing Texture Development in Al-free and 0.1% Mn–Added 3% Si–Fe Alloys

Abstract

In 0.1 % Mn-added 3 % Si-Fe alloys, the total number of {110} grains increased with increasing bulk sulfur content and with decreasing heating rate. This is contrary to that in Mn-free alloys. During isothermal annealing, the segregation concentration of sulfur for the initial time range was lower in the alloy containing 95 ppm sulfur than that in the other alloy containing 25 ppm sulfur. Also the maximum segregation concentration of sulfur appeared at a later time in the former. Such an annealing texture development and a segregation behavior can be attributed to the MnS precipitation and the subsequent dissolution into the matrix.KEY WORDS: Mn-added 3 % Si-Fe; MnS precipitation; segregation; selective growth; annealing texture.(i.e., isothermal annealing at 1 200°C) under the same vacuum or in the same dry hydrogen and subsequently finalannealed at the temperature for 12 h. All the heat treatments were performed without any MgO coating in a quartz tube with the inner and outer diameters of 29 and 33 mm that is enclosed by a three-zone tube furnace. There are several studies [15][16][17][18][19] reporting surface segregation behaviors of solutes in the 3% Si-Fe alloy strips. Due to the active evaporation of solutes at a higher temperature that attacks the electron gun, most of these studies were performed in-situ below 1 000°C in AES (Auger electron spectroscope). In the present study, the surface segregation behavior of the solutes was, therefore, investigated using an ion-sputtering technique.9) That is, the strips were first annealed, cooled and then placed in AES for the analysis. The primary beam energy was 2 keV and the sputtering rate corresponded to 5.7 nm of SiO 2 /min. AES spectra were obtained every 15 s during ion-sputtering. Differential peak heights of S 150 and Fe 703 eV were used. In each case, the differential maximum peak heights of S 150 eV were mostly observed after ion-sputtering for 45 s and these peaks were chosen as the segregation concentrations accumulated at the free surface during isothermal annealing. One example obtained by such an ion-sputtering technique is shown in Fig. 1. All the differential peak heights of S 150 eV were normalized with the peak height of Fe 703 eV which was obtained after ion sputtering for about 5 min. Here, the carbon and oxygen peaks are mainly due to the air contamination after annealing and are not considered in the analysis. ODFs (orientation distribution functions) for the cold-rolled strips and an etch-pit method for the annealed strips were used for texture analyses. The etchant in the etch-pit method is composed of distilled water 80 mLϩH 2 O 2 20 mLϩthree or four droplets of HCl solution. After etching, the annealed strips were cleaned in 15% nital solution for a short time and subsequently in alcohol before drying. Finally, the number of {110} grains in the specimen dimension of 5 mm in width and 100 mm in length and the deviation angle between the ͗001͘ crystal direction placed on the {110} grains and the rolling direction were measured using a m...

Dependence of Elevated Temperature Intergranular Cracking on Grain Size and Bulk Sulfur Content in TP347H Austenitic Stainless Steels

“…5(b), the rupture life in the larger grain size showing intergranular cracking was, however, rather shorter in the LS steel than the HS steel. Such a phenomenon can be understood on the basis of the previous researches 12,14) which report the role of MnS precipitates acting as a strong sink of free sulfur segregating to the interfaces.…”

“…9(c) of the HS steel than Fig. 9(a) of the LS steel, 14) because the incoherent interface in the HS steel acts as a strong sink of the dissolved sulfur. Such the lower grain boundary segregation concentration causes the longer time to intergranular failure in the HS steel than the LS steel, as shown in Fig.…”

“…13) The hot ductility is recovered due to not only the matrix softening arising from the coarsening of the Nb(C,N) precipitates but also the decrease in segregation concentration of sulfur at the grain boundaries which is accompanied by the MnS precipitation. Because the interface between the MnS precipitates and the neighboring matrix acts as a strong sink of the free sulfur segregating towards the grain boundaries, 14) the formation of a highly dense MnS precipitates within the matrix increases the recovery rate of the hot ductility. 12) It is the purpose of this research to clarify effects of grain size and bulk sulfur content on elevated temperature intergranular cracking occurring in TP347H austenitic stainless steels.…”

“…14,[23][24][25][26][27][28][29][30] Schematic diagrams for explaining the difference in segregation concentration of sulfur in the LS and HS steels are shown in Fig. 9.…”

“…9. These are based on Fe-Ni-(Mn,Ti) steels 23,24) age-hardened by the coherent θ-MnNi or η-Ni 3 Ti precipitates and Fe-3%Si steels 14) showing the MnS precipitation reaction. In order to simplify the explanation, it is assumed that the bulk sulfur content of the LS steel corresponds to the solubility of sulfur at the rupture test temperature T o .…”