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Materials Science and Engineering: A
Available online 21 January 2023
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Porosity is generally the major cause of the inferior mechanical properties of powder metallurgy (PM) steels. Boron (B) is an effective element for improving the sintering densification of PM steel via liquid phase sintering (LPS). To date, the microstructure and mechanical properties of B-alloyed PM stainless steels remain to be determined. The objective of this research was to study the influences of B on the LPS, microstructures, mechanical performances, and fracture mechanisms of PM 410L ferritic and 410 martensitic stainless steels. The roles of the Fe-based matrix, eutectic areas, and pores in the fracture behavior and mechanical properties were examined.
The results showed that after 1250 °C sintering, the 0.6 wt% B additive in 410L and 410 obviously induced LPS and decreased the porosity from 8.9 vol% to less than 4 vol%. In both 410L+0.6B and 410+0.6B, the B-containing compounds were identified by electron backscatter diffraction as M2B boride with a tetragonal structure. The 0.13 wt% C additive in 410L+0.6B increased the ultimate tensile strength from 420 MPa to 843 MPa but decreased the elongation from 10.4% to 2.7% and the impact energy from 21 J to 6 J due to the transformation of the Fe-based matrix from ferrite to martensite. In the combination of fracture analyses and strain distribution as a function of tensile stress, the results indicated that in 410L+0.6B and 410+0.6B, the predominant sites for strain localization and fracture were respectively the ferritic grains and intergranular eutectic areas. Furthermore, the pores played different roles in the deformation and fracture of the two B-alloyed PM stainless steels.
Powder metallurgy (PM) is a process for manufacturing nearly net-shaped metallic materials, and it has been widely used in the automobile, power, biomedical, and other industries [1,2]. Compared with other metal forming methods, PM offers numerous advantages, including the capability to manufacture nearly net-shaped products, process flexibility, low energy consumption, and low production cost [3,4]. However, PM steels made by the conventional press-and-sinter routine generally have high porosity (about 5–20%), which can obviously degrade the mechanical and corrosion performances [, , , ]. Reducing the porosity with adequate sintering is the key challenge in achieving high-performance PM materials. Several techniques, such as re-pressing re-sintering, power forging, and pressure-assisted sintering, have been applied to significantly improve the sintering densification of PM materials [, , ]. However, these techniques entail high costs and energy consumption. Liquid phase sintering (LPS) is an economical means to effectively densify a PM steel by adding minor alloying elements. Boron (B) is an effective element for inducing LPS of PM steel because B reacts with Fe to form a eutectic liquid at ∼1175 °C [, , , , ]. Lowering the sintering temperature by LPS is an energy saving means for fabricating a PM steel. Improving the sintered density of PM steel by LPS can also enhance the mechanical performances and reduce the material weight. These previous two advantages of LPS are beneficial to the environment and meet the requirements of sustainable development goals (SDGs).
To improve the LPS densification of various types of PM steels by adding B, the influences of B content, source of B, alloying element, sintering temperature, and sintering atmosphere on the LPS and microstructure have been examined [, , , , , , ]. The microstructure of B-alloyed PM steel is composed of a Fe-based matrix and intergranular eutectic areas with B-containing compounds [21,, , , , ]. The crystal structures of the B-containing compounds in the various B-alloyed PM steels are divergent. In 316L+B steel, for example, Serafini et al.  found M23(B,C)6 and M2B, where M represents the metallic element. Wu et al.  investigated the effects of alloying elements on the crystal structures of B-containing compounds in PM steels. They found that additions of 3 wt% Cr and 0.5 wt% Mo in Fe-0.4B do not change the crystal structure of M2B. However, 0.5 wt% C additive in Fe–3Cr-0.5Mo-0.4B fully transforms the B-containing compound from M2B boride to M3(B,C) boro-carbide. Moreover, in the Fe–5Mo-0.8C-0.4B steel, MoFe(C,B) was identified . Thus, the crystal structures of the B-containing compounds in various PM steels are inconsistent.
Stainless steel is a major system of PM materials, and thus the effects of B on the LPS and mechanical performances of PM stainless steels have been studied [8,15,17,19,20,22,23,27]. The B addition plays a complex role in the mechanical properties of PM stainless steel. Skałoń et al.  studied the effects of B content on the mechanical properties of PM 316L by adding an FeNiMnSiB master alloy powder. The ultimate tensile strength (UTS) of 316L was gradually increased from 235 MPa to 465 MPa with increases in B content from zero to 0.4 wt%. However, the elongation of 316L with 0.4 wt% B additive is lower than that of 316L with 0.2 wt% B additive because the intergranular eutectic areas impair the elongation. Wu et al.  reported that a 0.6 wt% B addition in PM 304L sintered at 1250 °C obviously improves the UTS from 300 MPa to 490 MPa while maintaining tensile elongation of ∼17% due to the combined effects of reduced porosity, pore rounding, and the formation of γ/M2B eutectic phase after LPS. Furthermore, the corrosion resistances of PM stainless steels can also be improved by adding B [8,15,20].
According to the above background, the role of B in LPS, the crystal structures of the B-containing compounds, and the mechanical properties of PM stainless steels are very complex. The 410L ferritic and 410 martensitic stainless steels are general grades of PM stainless steels. Unfortunately, the effects of B on PM 410L and 410 stainless steels have not been clarified to date. The main purpose of this study was thus to investigate the roles of 0.6 wt% B in the LPS densification, microstructures, mechanical properties, and fracture mechanisms of PM 410L ferritic and 410 martensitic stainless steels. The correlations between the microstructure, mechanical properties, and fracture behaviors were also examined in this study.
A commercial 410L stainless steel powder (Höganäs AB, Höganäs, Sweden) and a pure B powder were used to fabricate the 410L ferritic stainless steel with 0.6 wt% B. The composition of the 410L powder was Fe-12.5Cr–1Si–1Mn-0.03C-0.03S in wt%. Moreover, graphite powder (1651, Asbury Carbons, LA, USA) was mixed with the 410L and B powders to produce the 410 martensitic stainless steel with 0.6 wt% B. The 410L ferritic and 410 martensitic stainless steels with 0.6 wt% B were respectively designated
Sintered density and DSC analysis
To understand the generation temperature of eutectic liquid, the 410L, 410L+0.6B, and 410+0.6B green compacts were analyzed by DSC, and the results are shown in Fig. 1. The results showed that there was no endothermic peak in the 410L upon heating to 1300 °C, indicating the absence of liquid. However, adding 0.6 wt% B to 410L and 410 obviously facilitated the generation of a eutectic liquid. Fig. 1 shows that the endothermic peaks of 410L+0.6B and 410+0.6B respectively ranged from 1219°C
The effects of alloying elements on liquid generation temperature
In Sections 3.1 Sintered density and DSC analysis, 3.2 Microstructure, the results demonstrated that LPS occurred in the 410L+0.6B and 410+0.6B steels during sintering at 1250°C. Table 3 shows the relationships between the material composition and liquid generation temperature of various PM steels [20,21,23,27,34]. The temperature of binary Fe-0.4B eutectic reaction is 1170–1185 °C . Alloying elements can affect the formation temperature of eutectic liquid. The additions of 1.5 wt% Cr
The 0.6 wt% B additive in 410L and 410 facilitated the liquid generation at 1219 °C–1242 °C and 1217 °C–1242°C, respectively. After 1250°C sintering, the sintered densities of 410L+0.6B and 410+0.6B were much improved to 7.64 g/cm3 and 7.58 g/cm3, respectively. Moreover, the porosity was reduced, and the pore roundness and grain size after LPS were increased by the addition of 0.6 wt% B.
The B-containing compounds in 410L+0.6B and 410+0.6B were rich in Cr and B but deficient in Fe. In both
CRediT authorship contribution statement
Ming-Hsiang Ku: Data curation, Formal analysis, Writing – original draft, Writing – review & editing. Guo-Jiun Shu: Formal analysis, Writing – original draft. Yu-Jin Tsai: Investigation, Formal analysis, Data curation. Yi-Kai Huang: Formal analysis, Data curation. Si-Xian Chi: Investigation, Visualization. Yu-Ching Wen: Formal analysis, Visualization. Ming-Wei Wu: Conceptualization, Data curation, Supervision, Writing – original draft, Writing – review & editing, Funding acquisition, Project
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
The authors thank the National Science and Technology Council (NSTC) in Taiwan for help under project numbers MOST 107-2221-E-027-006 and MOST 108-2221-E-027-059. We are also grateful to Prof. C. S. Lin and Ms. Y. T. Lee in the Instrumentation Center at National Taiwan University for their assistance with the EBSD analyses.
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(2017)(Video) Intragranular nucleation during transformations in steels
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Powder metallurgy of stainless steel: state-of-the art, challenges and development
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