Journal of Materials Processing Technology 212 (2012) 977–987 Contents lists available at SciVerse ScienceDirect Journal of Materials Processing Technology jou rnal h om epa g e: www.elsevier.com/locate/jmatprotec On-line control of processing atmospheres for proper sintering of oxidation-sensitive PM steels E. Hryha a,∗ , E. Dudrova b,1 , L. Nyborg a,2 a Department of Materials and Manufacturing Technology, Chalmers University of Technology, Rännvägen 2A, SE - 412 96 Gothenburg, Sweden b Institute of Materials Research, Slovak Academy of Science, Watsonova 47, 043 53 Koˇ sice, Slovakia a r t i c l e i n f o Article history: Received 17 May 2011 Received in revised form 28 October 2011 Accepted 9 December 2011 Available online 16 December 2011 Keywords: Prealloyed powder Sintering Processing atmosphere Atmosphere monitoring Oxide reduction Carbothermal reduction a b s t r a c t Changes of atmosphere composition during sintering of water atomized powder prealloyed with Mn and Cr (up to 2% of both) were studied. Increasing sensitivity to atmosphere purity with increasing alloying elements content was registered. Continuous monitoring of sintering atmosphere composition (CO/CO 2 /H 2 O) indicates three critical stages during the heating up to final sintering temperature: the importance of rapid atmosphere purification after lubricant decomposition and removal; the reduc- tion of the iron oxide layer by hydrogen at temperatures up to ∼500 ◦ C and by carbon at temperatures around ∼720 ◦ C; the reduction of the spinel oxides on the powder surface at above 900 ◦ C and further reduction of thermodynamically stable surface oxides and mixed internal oxides close to the sintering temperature. The measured ratio of CO/CO 2 indicates favorable thermodynamic conditions for reduc- tion of stable oxides as (Cr,Mn) x O y close to sintering temperature (1120 ◦ C) for the applied sintering conditions. The experimental results were confirmed by modeling the metal–gas interactions using the thermodynamic/thermochemical softwares ThermoCalc and HSC Chemistry. The modeling indicates the significance of maintaining a sintering atmosphere with high reducing potential during heating stage for minimizing oxidation before high-temperature carbothermal reduction starts. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Conventional powder metallurgy for components production starts from the preparation of powder mixture, where powder is mixed with lubricant, carbon and other alloying elements. A com- pact of defined shape is formed by densification of the powder mixture with high pressure (usually about 600 MPa). The compact is sintered then in order to strengthen powder compact. The advan- tages of this powder metallurgical processing are the possibility to produce complex shaped products that are difficult or impossible to obtain by other method, efficient utilization of raw material (near net shape components production), lower energy consumption and overall production time due to fewer shaping operation that makes powder metallurgy an inestimable working route (Schatt and Wueters, 1997). However, further increase in powder metal- lurgical applications demands improved high-strength alloy steels. These alloys are preferably based on the utilizing of conventional ∗ Corresponding author. Tel.: +46 31 772 1255; mobile: +46 762 838255; fax: +46 31 772 3872. E-mail addresses: hryha@chalmers.se, eddigrig@yahoo.com (E. Hryha), edudrova@imr.saske.sk (E. Dudrova), lars.nyborg@chalmers.se (L. Nyborg). 1 Tel.: +421 55 7922442; fax: +421 55 7922408. 2 Tel.: +46 31 772 1257; fax: +46 31 772 3872. low-cost alloying elements like Cr, Mn and Si. One of the most critical issues associated with modern Cr- and Mn-alloyed pow- ders is their high oxygen sensitivity. Campos et al. (2003) studied admixed Cr–Mn PM steels and found that the presence of man- ganese changes significantly the behavior of surface oxide layer. Hryha (2007) confirmed that during heating up to sintering temper- ature the powder may be contaminated by oxides if the reduction potential of the atmosphere applied is unsatisfactory. Owing to this fact, it is of crucial importance to not only employ good enough sin- tering conditions in terms of protective atmosphere to protect the sintered gods from oxidation, but also further monitor this atmo- sphere to ensure that residual oxides are largely reduced to improve the metallic contact between adjacent particles and control car- burization/decarburization of the steel. For these reasons proper choice and careful control of the sintering atmosphere is an impor- tant technological parameter for obtaining good quality sintered products. The choice of gas must take into account possible reac- tions between the gas, the sintered material, the furnace lining and heating elements. Therefore interactions in the furnace depend on the temperature and pressure and are numerous because of high- activity of the gases used in commercial production such as H 2 , H 2 O, CO, CO 2 , O 2 , N 2 , etc. According to the definition, controlled atmospheres are the single gas or mixture of gases, whose composition ensures a 0924-0136/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2011.12.008