Key Takeaways & Executive Findings
- •• In-situ SEM tensile testing reveals that elongated MnS inclusions induce significant anisotropy in forged high-sulfur steel, reducing tensile strength from 454 MPa (parallel) to 402 MPa (perpendicular). • Addition of 730 ppm gadolinium increases tensile strength to 468 MPa (parallel) and 446 MPa (perpendicular), effectively mitigating anisotropy. • Failure mechanism depends on loading direction: MnS inclusions fracture and detach under parallel loading, while interface debonding creates elongated voids under perpendicular loading. • Gadolinium modifies MnS into spherical Gd–S inclusions, improving the isotropic mechanical response and overall tensile performance of high-sulfur steels.
Abstract
The effect of manganese sulfide (MnS) inclusions and gadolinium–sulfide (Gd–S) inclusions on the deformation behavior of steel matrix at different stages was studied by in-situ tensile experiments using a scanning electron microscopy (SEM) at room temperature. Two in-situ tensile experiments of tensile force along the elongation direction of inclusions and perpendicular to the elongation direction were conducted. The hole-induced nucleation mechanism of different tensile directions and inclusion types during the tensile deformation process was revealed. When the tensile direction of the steel without Gd was parallel to the forging elongation direction, the tensile strength was 454 MPa. Meanwhile, long strip MnS inclusions were broken and shed, forming long strip holes perpendicular to the fracture direction. When the tensile direction was perpendicular to the forging elongation direction, the gap between long strip MnS inclusions and the steel matrix was expanded into a long strip hole parallel to the fracture direction, and the tensile strength was 402 MPa. Anisotropy of the steel was induced by long strip MnS inclusions. In the steel with a total gadolinium (T.Gd) content of 730 ppm, the tensile strength was 468 MPa when the tensile direction was parallel to the forging elongation direction. The tensile strength of the steel was 446 MPa when the tensile direction was perpendicular to the forging elongation direction. The addition of Gd in the steel was beneficial to improve the tensile properties of the steel and reduce the anisotropy of the steel.
1. Introduction
The content of Mn and S elements in high sulfur steel was high, and inclusions were mainly MnS [1]. At room temperature, the hardness of MnS inclusions was HV 170 [2], and Young's modulus was between 70 and 140 GPa [3]. The MnS inclusion had the effect of promoting chip breaking [4–5], reducing tool wear [5–6], and improving the flow zone [7], thereby improving the machinability of the steel [8–10]. However, during the steel forging process, MnS inclusions were deformed into long strips [11–12] due to good plasticity [13–14]. The tip of long strip MnS inclusions was easy to cause stress concentration [15] and reduce mechanical properties of the steel [16–17]. Therefore, although the traditional high-sulfur steel had good machinability, the effect of the number [18–19], shape [20–21], and distribution [22–23] of MnS inclusions on the mechanical properties of the steel limited the use of high-sulfur steel.
Wu et al. [24] found that transverse mechanical properties of the hot rolled steel were reduced due to the long strip-like aggregated distribution of MnS inclusions, while the spindle-like uniform distribution of MnS made the steel isotropic. Temmel et al. [25] reported that the higher the sulfur content in medium carbon steel, the more obvious the anisotropy. Maciejewski [26] investigated that when the tensile direction was parallel to the elongation direction of MnS inclusions, the sulfur content did not directly affect the tensile property of the steel. When the tensile direction was perpendicular to the elongation direction of MnS inclusions, the higher the sulfur content, the worse the tensile property of the steel. Tinoco et al. [27] demonstrated that MnS inclusions in the 38MnSiV6 steel were the crack initiation point, thus reducing the mechanical properties of the steel. Guo et al. [28] concluded that MnS inclusions in the low-density steel can be deboned with the steel matrix during the tensile process, induce the nucleation of holes, and reduce the tensile property of the steel.
Hosseini et al. [4] found that the combination between MnS inclusions and the hot-rolled steel was weak. When straining was performed along short transverse (S) directions, MnS inclusions were shed from the steel matrix. However, when straining was performed along longitudinal transverse (L) directions, MnS inclusions were broken under the action of stress. The tensile strength in the short transverse direction was lower than that in the longitudinal transverse direction. Liu et al. [29] reported that long strip MnS inclusions were more harmful to the steel when they were perpendicular to the loading direction than when they were parallel to the loading direction.
Luo et al. [30] demonstrated that the addition of rare earth metals yttrium in the steel could modify long strip MnS inclusions into small-sized spherical rare earth inclusions, which significantly improved the isotropy of the steel. Dai et al. [31] concluded that the anisotropy of the twinning-induced plasticity (TWIP) steel was reduced by adding 0.045wt% Ce. Lan et al. [32] found that the anisotropy between inclusions and the Fe–Mn–C–Al twinning-induced plasticity steel was reduced by adding Ce, thereby reducing the damage of inclusions to the steel matrix. Shi et al. [33] investigated that the shape, quantity, and distribution of inclusions in the 16Mn steel were improved by Ca and rare earth metals composite tre...
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Lifeng Zhang, Hong Wei, Yadong Wang (2025). Effect of gadolinium on the anisotropic tensile properties of a forged high sulfur steel through in-situ tensile experiment. Journal of Mineral Metallurgy and Materials Science. https://doi.org/10.1007/s12613-025-3205-8
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Frequently Asked Questions
How does gadolinium affect the tensile properties of forged high sulfur steel?
Gadolinium addition (730 ppm T.Gd) improves tensile strength in both parallel and perpendicular loading directions and reduces anisotropy by modifying MnS inclusions into less harmful Gd–S inclusions.
What is the effect of MnS inclusion orientation on tensile strength?
Long strip MnS inclusions cause anisotropy: tensile strength is 454 MPa when loaded parallel to elongation, but only 402 MPa when loaded perpendicular, due to different failure mechanisms.
What mechanism leads to hole nucleation during tensile deformation?
When loaded parallel, MnS inclusions break and shed forming holes; when loaded perpendicular, debonding at the MnS-matrix interface creates elongated holes parallel to fracture.
Can rare earth elements like gadolinium improve isotropy of high sulfur steels?
Yes, gadolinium modifies elongated MnS into small spherical Gd–S inclusions, improving isotropy and overall tensile performance.
Why is in-situ tensile testing used in this study?
In-situ SEM tensile experiments allow real-time observation of inclusion deformation, cracking, and void nucleation, revealing micro-mechanisms governing anisotropy.
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