Surface Chemical Characteristics of Chromium-alloyed Steel Powder and the Role of Process Parameters during Sintering

Doctoral thesis, 2012

Powder Metallurgy (PM) is a cost efficient method suitable for sustainable production of structural parts that have strict dimensional tolerances and complex geometries. It has been a common practice to utilize Ni and Cu as alloying elements in PM steels, but the high and fluctuating prices of Ni and the concerns around it associated with health hazards as well as the difficulty in recycling Cu lead the PM industry to search for effective alternatives such as Cr and Mn. However the use of Cr has been met with scepticism due to its affinity to oxygen which can lead to the formation of stable oxides on the powder surface. Such oxides can act as barriers for the development of sinter necks and as a consequence they are of major importance for the production of structural components through the press-and-sinter route. Thus particular attention is required in order to control the surface chemistry of the powder which is the key factor for the successful sintering and production of PM parts. The research work presented in this thesis was aimed at acquiring fundamental knowledge concerning the surface chemistry of metal powder and how it changes during the heating/sintering stage and finally how and why different process parameters can affect the reduction/oxidation mechanisms and thus the mechanical performance of the sintered part. High-resolution analytical techniques (scanning electron microscopy with X-ray microanalysis, X-ray photoelectron spectroscopy and Auger nanoprobe analysis) were used to investigate the powder surface characteristics with regard to composition, morphology, size and distribution of surface oxides. It was revealed that the Cr-alloyed powder is predominantly covered by a homogeneous (~6 nm thick) Fe-oxide layer up to ~94% whereas the rest is covered by fine particulate features with size below 500 nm which were rich in strong oxide forming elements such as Cr, Mn and Si. Hence, most of the contacts between the metal particles comprising the compacted powder will be comprised of Fe-oxide rather than the more stable oxides. Sintering trials on water atomized steel powder grades pre-alloyed with Cr using varying process parameters showed that the most critical stage is during the heating between 800-1000ºC due to the risk of enclosure of surface oxide inside the developing inter-particle connections and further transformation of any residual Fe-based oxides into the more stable Cr-Mn-spinels which at elevated temperatures can coalesce into larger agglomerates. The balance between the developed “microclimate” and the mass transport phenomena is the decisive factor for the efficient reduction of the surface oxide and is greatly affected by the existing conditions such as the atmosphere composition and temperature as well as the different process parameters like the heating rate and green density. Provided that proper pre-cautions are taken in this respect, good mechanical performance of the produced component can be achieved.