Solidification Structures in Low-Alloyed Powder Metallurgy Steels Obtained through Permanent Liquid Phase Sintering Using a Master Alloy Containing Boron

Abstract

Powder metallurgy (PM) steels containing boron form an attractive group of alloys because of the important densification that can be achieved through permanent liquid phase sintering (LPS). However, upon solidification, such liquid phase is known to form borides or borocarbides. Recent works have shown that some alloying elements have significant interactions with the LPS of PM steels containing boron. More specifically, it is suspected that the concentration of prealloyed molybdenum influences the formation of boride/borocarbide upon cooling at the end of the sintering cycle. Therefore, the main objective of this work is to describe the relationship that exists between the concentration of prealloyed molybdenum and the crystal structure of the boride/borocarbide eutectic component that typically forms in PM steels containing boron. A master alloy made of iron-manganese-nickel-boron-carbon was utilized to introduce boron, thus providing enhanced sintering through LPS. Characterization in optical and scanning electron microscopy combined with electron-backscattered diffraction and energy-dispersive X-ray spectrometry (EDS) revealed that increasing the prealloyed molybdenum content not only increased the volume fraction of liquid phase but also modified the morphology and the nature of the boron-rich eutectic. Changing the prealloyed molybdenum content from 0.5 to 0.85 wt.% transformed the discontinuous M2B boride to a continuous M23(C,B)6 borocarbide phase, causing a drastic decrease in strength despite the higher densification observed at 0.85 wt.% molybdenum. The effect of molybdenum on the LPS process of boron PM steels is undeniable and was found to occur after the initial formation of the liquid phase. Indeed, differential scanning calorimetry revealed no difference in the endothermic melting peaks temperature for both concentration of molybdenum.

Characterisation of the mechanisms taking place during liquid phase sintering of PM boron steels with the help of artificial intelligence

“…Figure 10 presents the evolution of density and TRS as a function of holding time. As previously observed in similar materials, after a complete sintering cycle the Fe-0.85Mo-9MA specimens suffered from the typical embrittlement observed in boron-containing PM steels [15].…”

“…The proportion of solidified liquid phase is lower than in the 0.85Mo counterpart (Figure 5(a)), and the smaller average area of individual regions after 30 minutes of sintering (Figure 6(a)) are elements that contribute to the higher strength values. Moreover, it was also found in a previous work that chemistry of the boron-rich phase in Fe–0.5Mo–9MA formed upon solidification of the liquid phase renders this chemical composition less susceptible to embrittlement [15].…”

“…The amount of MA used in this work was based on previous experiments on similar materials. An optimum boron content of 0.09 wt-% was shown to maximise strength when pre-alloyed Mo content was 0.50 wt-%, but caused aggravated fragilization due to the solidified liquid phase in the form of a brittle eutectic structure when the base powder contained a larger fraction of Mo (0.85 wt-%) [15]. The nominal chemistry of the PM steels investigated, designated Fe–0.5Mo–9MA and Fe–0.85Mo–9MA in the rest of this text, are therefore Fe–(0.50; 0.85)Mo–1.4Mn–1.1Ni–0.4C–0.09B.…”

“…Previous work showed that a MA with a chemical composition Fe-15.8Mn-12.2Ni-4.4C-1.0B (in wt-%) can be used in combination with an iron base powder pre-alloyed with 0.5 wt-% Mo to perform LPS and obtain a PM steel having high mechanical properties, without showing the embrittlement often observed in those materials [15]. These results also showed that an increase of 0.35 wt-% Mo in the iron base powder had a major influence on the nature and proportion of the solidified boron-rich liquid phase, which in turn adversely affected the mechanical properties.…”

“…Regarding permanent LPS in iron and steels, boron is a well-known alloying element that can be used to promote the formation of a liquid phase through a eutectic reaction meeting the densification criteria mentioned above [6–10]. However, this metallurgical system suffers from a typical drawback frequently observed with permanent LPS: the solidification of the boron-rich liquid into a brittle eutectic structure located at grain boundaries that can greatly reduce the strength of components made with PM boron steels [6,9–15].…”