Effect of processing conditions on microstructural features in Mn–Si sintered steels

Abstract

• Oxide distribution in steels containing oxidation-sensitive elements• Mn, Si introduced in a master alloy powder, mixed with a base iron powder• Selective oxidation of Mn and Si on iron grain boundaries Sintering of steels containing oxidation sensitive elements is possible if such elements are alloyed with others which present lower affinity for oxygen. In this work, a master alloy powder containing Fe-Mn-Si-C, specifically designed to create a liquid phase during sintering, has been used for such purpose. The effect of processing conditions such as sintering temperature and atmosphere was studied with the aim of describing the microstructural evolution as well as the morphology and distribution of oxides in the sintered material, evaluating the potential detrimental effect of such oxides on mechanical properties.Chemical analyses, metallography and fractography studies combined with X-ray photoelectron spectroscopy analyses on the fracture surfaces were used to reveal the main mechanism of fracture and their correlation with the chemical composition of the different fracture surfaces.The results indicate that the main mechanism of failure in these steels is brittle fracture in the surrounding of the original master alloy particles due to degradation of grain boundaries by the presence of oxide inclusions. Mn-Si oxide inclusions were observed on intergranular decohesive facets. The use of reducing atmospheres and high sintering temperatures reduces the amount and size of such oxide inclusions. Besides, high heating and cooling rates reduce significantly the final oxygen content in the sintered material. A model for microstructure development and oxide evolution during different stages of sintering is proposed, considering the fact that when the master alloy melts, the liquid formed can dissolve some of the oxides as well as the surface of the surrounding iron base particles.

Novel multiuse liquid phase promoter for PM components

“…On the one hand, the wetting process is assisted by the metallothermic reduction of oxides in the presence of high-oxygen affinity elements, such as the silicon in the composition of the drop promoter. 17 On the other hand, the hydrogen from the sintering atmosphere enhances the wetting conditions, 2,22,23 since this element promotes the reduction of iron oxides at 400°C. 23,24 When carbon is introduced into the wetting system through the substrate composition (wetting system 2), the kinetics of spreading is accelerated, decreasing both the contact angle and the spreading time.…”

“…Cu was selected due to its good wettability under reducing conditions; Fe to have a better control of the liquid infiltration in the steel matrix; Ni to increase the mechanical properties and Si as melting point depressant . Besides, Si contributes to the reduction of Fe oxide layer through metallothermic reduction, thus enhancing the removal of oxides, the liquid wetting, and the sintering process . With the aim of analyzing the features of the promoted liquid phase, the wetting behavior of LP was evaluated with the sessile drop technique following the experimental setup described in, using a small ingot of LP as the drop promoter on wrought Fe and Fe‐C substrates and sintering in N 2 ‐10H 2 .…”

“…2,[13][14][15][16] Besides, Si contributes to the reduction of Fe oxide layer through metallothermic reduction, thus enhancing the removal of oxides, the liquid wetting, and the sintering process. 17 With the aim of analyzing the features of the promoted liquid phase, the wetting behavior of LP was evaluated with the sessile drop technique following the experimental setup described in, 18 using a small ingot of LP as the drop promoter on wrought Fe and Fe-C substrates and sintering in N 2 -10H 2 . The evolution of the contact angle was monitored along the trials thanks to image acquisition and analysis with the Drop Shape Analysis DSA100 software.…”

Effect of heating rate and process atmosphere on the thermodynamics and kinetics of the sintering of pre‐alloyed water‐atomized powder metallurgy steels

“…Traditionally, PM steels were alloyed with elements such as nickel, copper, and molybdenum due to the relatively low stability of their oxides, compared with the iron which ensured that sintering complications could be kept to a minimum . However, modern development of advanced PM steels has shifted away from these elements in favor of elements such as chromium, manganese and silicon, due to their lower cost and lower volatility during purchasing . However, a major drawback with the use of these new elements is their relatively high affinity to oxygen, when compared with the iron matrix .…”

“…Most previous research into this area dealt with the influence of different gaseous atmospheres (and their components) and admixed carbon (graphite) on the reduction/oxidation processes in PM steels . Hence, the focus was placed primarily on the thermodynamics and processing conditions that ensured the removal of surface oxide.…”