Effect of process parameters on surface oxides on chromium-alloyed steel powder during sintering D. Chasoglou * , E. Hryha, L. Nyborg Department of Materials and Manufacturing Technology, Chalmers University of Technology, Gothenburg, Sweden highlights < A local atmosphere microclimate is very important for sintering of PM steels. < High risk of surface oxide enclosure between 800 and 1000  C. < Coalescence and agglomeration of enclosed oxides take place during sintering. < The effect of different process parameters on the oxide reduction is examined. < A model is proposed for the occurring phenomena. article info Article history: Received 25 July 2012 Received in revised form 13 November 2012 Accepted 25 November 2012 Keywords: Powder metallurgy Sintering Oxide Oxidation abstract The use of chromium in the PM steel industry today puts high demands on the choice and control of the atmosphere during the sintering process due to its high affinity to oxygen. Particular attention is required in order to control the surface chemistry of the powder which in turn is the key factor for the successful sintering and production of PM parts. Different atmosphere compositions, heating rates and green densities were employed while performing sintering trials on water atomized steel powder pre-alloyed with 3 wt.% Cr in order to evaluate the effect on surface chemical reactions. Fracture surfaces of sintered samples were examined using high resolution scanning electron microscopy combined with X-ray microanalysis. The investigation was complemented with thermogravimetric (TG) studies. Reaction products in particulate form containing strong-oxide forming elements such as Cr, Si and Mn were formed during sintering for all conditions. Processing in vacuum results in intensive inter-particle neck development during the heating stage and consequently in the excessive enclosure of surface oxide which is reflected in less good final mechanical properties. Enhanced oxide reduction was observed in samples processed in hydrogen-containing atmospheres independent of the actual content in the range of 3e10 vol.%. An optimum heating rate was required for balancing reduction/oxidation processes. A simple model for the enclosure and growth of oxide inclusions during the sinter-neck development is proposed. The obtained results show that significant reduction of the oxygen content can be achieved by adjusting the atmosphere purity/composition. Ó 2012 Elsevier B.V. All rights reserved. 1. Introduction The powder metallurgy (PM) process is an established method used for the large-scale production of precision parts in a cost- efficient manner. The constant demands for improved mechanical properties of the produced structural parts are met with various routes by the PM industry, i.e. by careful alloying with elements such as Cr, Mo, Mn, V, Si, etc. [1]. Especially chromium is considered a very attractive choice since it increases hardenability, it is rela- tively inexpensive and easily recyclable [2]. The issue concerning the reduction in compressibility, which arises with the incorpora- tion of Cr, can be dealt with up to certain extent if Cr is added in small amounts in the pre-alloyed state [3]. Probably the most critical aspect regarding the use of elements such as Cr, Mn and Si today is their high sensitivity to oxygen which in turn results to the formation of thermodynamically stable oxides. For powder grades pre-alloyed with Cr it was shown [4,5] that the surface of the powder is covered by a thin iron-oxide layer (w6 nm) that covers up to w95% of the surface, whereas the rest of the powder surface is covered by “oxide islands” (w300 nm) that are rich in strong oxide * Corresponding author. Tel.: þ46 762315051. E-mail addresses: dimitris.chasoglou@chalmers.se, dimitris.chasoglou@ swerea.se (D. Chasoglou), hryha@chalmers.se (E. Hryha), lars.nyborg@chalmers.se (L. Nyborg). Contents lists available at SciVerse ScienceDirect Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys 0254-0584/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matchemphys.2012.11.074 Materials Chemistry and Physics 138 (2013) 405e415