Effect of Delubrication Process Parameters on Surface Characteristics of Cr- alloyed PM steels

Utilizing chromium as an alloying element in structural powder metallurgy (PM) steel parts provides advantages in terms of profitability and recyclability but poses a challenge in terms of processing due to high chromium affinity to oxygen. From the previous investigations it is known that water-atomized chromium prealloyed PM steel powders after reduction annealing are dominantly covered by a thin Fe-oxide layer with fine particulate oxides in between [1-2], meaning good powder sinterability. However, transformation of the surface iron oxide into more thermodynamically stable ones during the consolidation can inhibit the formation of sintering necks which determine the strength of the parts. Hence, the surface chemical changes during each stage of sintering have to be evaluated in order to carefully design the process in a way that proper sintering is ensured and re-oxidation of the powder is avoided. Delubrication in PM processing precedes the high temperature sintering step and aims at efficient removal of the admixed lubricant. At the same time, the process has to be performed in a way to avoid/minimize changes in surface oxide chemistry and amount. In the present work the surface chemical analysis of chromium prealloyed steel Astaloy CrM (Fe-3Cr-0.5Mo) admixed with 0.6 ethylene bis-steramide lubricant is performed after delubrication under different processing conditions. Experimental techniques used were X-ray photoelectron spectroscopy (XPS) in combination with high-resolution scanning electron microscopy (HR SEM) and energy dispersive X-ray spectroscopy (EDX). Investigation of the surface chemical characteristics has been performed on samples delubricated in dry nitrogen atmosphere at 450 and 900\ub0C and in air at 450\ub0C. Delubricated in N2 at 450\ub0C compacts showed the best surface condition similar to that of the base powder. XPS analysis revealed the presence of much higher concentration of Cr and Mn oxides on the surface of the sample delubricated at 900\ub0C. SEM and EDX analysis confirmed the presence of particulate oxides containing the aforementioned elements indicating the growth of undesirable thermodynamically stable oxides at this temperature at the expense of the pre-existing Fe-oxide layer. After delubrication in air at 450\ub0C the thickness of the Fe-oxide layer on the powder particle surfaces increased by a few nanometers in the center and a six fold increase in the oxide layer thickness at the edge of the samples was observed. High magnification SEM imaging on the sample showed presence of regions with the oxide thickness more than 100 nm near the edge of the sample compared to initial 6 nm, characteristic for the base powder. Based on the results from this investigation it is strongly recommended to perform delubrication at~450\ub0C in an inert atmosphere since the as-received surface condition of the powders is retained and no undesirable changes occur.References:1. Chasoglou, Dimitris. PhD Thesis, Surface Chemical Characteristics of Chromium-alloyed Steel Powder and Role of Process Parameters during Sintering. s.l. : Chalmers University of Technology, 2012. ISBN 978-917385-711-6.2. Karlsson, Henrik. PhD Thesis, Role of Surface Oxides in Sintering of Chromium-Alloyed Steel Powder. s.l. : Chalmers University of Technology, 2005. ISBN 91-7291-703-2