Study on Inclusions Distribution and Cyclic Fatigue Performance of Gear Steel 18CrNiMo7-6 Forging

The three-dimensional morphologies of inclusions in gear steel 18CrNiMo7-6 forging were investigated by a non-destructive extraction method, and the cleanliness of radial positions was analyzed, mainly including the variation of total oxygen content and the distribution of size and quantity of inclusions. In addition, fatigue performance was tested using an ultrasonic fatigue machine to investigate the fatigue characteristics of the steel. The results show that the quantity density of inclusions per unit volume in gear steel 18CrNiMo7-6 decreases exponentially with increasing size, oxide inclusions with a size less than 8 μm account for more than 90%, while sulfide inclusions account for more than 85%. The average value of the oxygen content can reflect the level of inclusions that were evenly distributed in the molten steel, and the accumulative total oxygen content increases significantly with increasing inclusion size. The fatigue specimen failed after the stress exceeded the critical value, and fatigue failure hardly occurred when the stress was below the critical value. Meanwhile, large-sized nondeformable inclusions such as Al2O3-CaO in gear steel 18CrNiMo7-6 are closely related to fatigue failure. It is recommended that the area from the center to the 1/2 radius with low cleanliness should be avoided, while the area from the 3/4 radius to the edge with high cleanliness should be selected during the machining of the gear.

Reoxidation of Al-Killed Steel by Cr2O3 from Tundish Cover Flux

Reoxidation has long been a problem when casting ultra-low oxygen liquid steel. An experimental study of the reoxidation phenomenon caused by Cr2O3-bearing cover flux of Al-killed steel is presented here. MgO-CaO-SiO2-Al2O3-Cr2O3 tundish cover flux with various Cr2O3 contents were used to study the effects of Cr2O3 on total oxygen content (T[O]) and alumina and silicone loss of Al-killed steel at 1923 K (1650 °C). It was found that Cr2O3 can be reduced by Al to cause reoxidation, and the reaction occurs mainly within 2 to 3 min after the addition of the tundish cover flux with 5% and 10% Cr2O3 concentration. T[O] and Al loss increase with higher Cr2O3 concentration flux. Two controlled experiments were also made to investigate the oxygen transported to the steel by the decomposition of Cr2O3. It was calculated that when Al is present in steel, more than 90% of the reoxidation of Cr2O3 is caused by Al, and the rest is caused by decomposition.

Influence of steel cleanliness on drawability of fine filaments with high tensile strength

The aim of the current study is to clarify the influence of steel cleanliness on the fracture during wet wire drawing of high strength high carbon very fine filaments. Methodologies for inclusion identification and for inclusion count were developed. Ninety percent of the fractured filaments were associated with an inclusion which was SiO2, or SiO2-CaO or SiO2-MgO. The fracture of the wires resulted from debonding of the inclusion from the matrix. Inclusion density assessed from total oxygen content in the wire was successfully and reliably obtained. Inclusions exhibited a wide range of sizes but mostly around 6 μm. The inclusion density did not appear as the only parameter that affects the fracture occurrence. An important effect of size and of chemical composition of the inclusions was found. A critical size of inclusions for fracture was observed but it depends on the chemical composition of inclusions.

Inclusions in Continuous Steel Casting: Formation, Characterization, Tracking and Influences on the Quality of the Product

The formation mechanisms and characterization (distribution, size and morphology) of the inclusions during continuous casting process of steels were investigated in this study. Samples were took out from several steps of the process and then, fully characterized via metallographic techniques, microscopic investigations and elemental analysis. Results showed that the addition of ferroalloys can intensify the formation of inclusions in some steps of the steelmaking process. Also, it was confirmed that the total oxygen content of the samples can be considered as a criteria of the clarity (purity and inclusion-free) of the steels, if the samples prepared carefully.

Improvement of Hydrogen Permeability of Pd-Gd Film by Removal of Inclusion Particles – Part 1: Removal of Inclusion Particles

It was hard to make a thinner film of hydrogen permeable alloy by rolling when small nonmetallic inclusion particles with the size of 5 to 30 μm were present in the alloy. For this reason, the removal of nonmetallic inclusion particles from Pd-Gd alloy were examined using various fluoride fluxes. In the experiments on the removal of nonmetallic inclusion particles from Pd-7 to 16 mass% Gd alloys, the lowest total oxygen content was obtained by the addition of GdF3 flux.