Degassing during sintering of CrMo steels prepared from prealloyed powder
Entgasen während des Sinterns von CrMo-Stählen aus vorlegiertem Pulver
Cr alloyed sintered steels from prealloyed powders are more difficult to produce than their Mo, Ni, or Cu alloyed counterparts, due to the fairly stable oxide layers covering each powder particles. Such oxide layer are present at all types of ferrous powders and have to be removed in the initial stages of the sintering process. Here, the degassing and reduction behaviour of sintered steel 3%Cr-0.5%Mo and several reference materials was studied by combined thermal and gas analysis. It is shown that carbothermic reduction of the oxide layers occurs during the heating stage, in well defined temperature intervalls. For unalloyed or Mo alloyed steels the most relevant temperature is about 700 deg C while for Cr-Mo steels at least 1000 deg C are necessary to result in reduction of the particle surfaces and formation of stable sintering contacts. The effect of the carbon content on the reduction is small if there is at least sufficient carbon to act as reducing agent, which is the case at about 0.2 % C minimum added. This observation can be shown to be in good agreement with thermodynamic estimations. Conclusions: 1) The reduction/degassing behaviour of sintered steels can be effectly by DTA/TG/MS measurements in He atmosphere. 2) For Fe-C and Fe-Mo-C, weight loss during heating occurs predominantly in the temperature range 700 deg C and 950 - 1100 deg C, the former peak being more pronounced but the latter bringing the larger total mass loss. 3) With Cr containing steel prepared from Astaloy CrM (3%Cr-0.5%Mo), the mass loss at lower temperatures is less pronounced, the degassing occuring primarily at temperatures >1000 deg C. The material shows two distinct weight loss maxima at 1000 and 1250 deg C, respectively. 4) The mass loss maxima can be clearly correlated to MS peaks for the mass 28, i.e. CO, CO2 can be found at low temperatures, about 300 - 400 deg C, and to some extent also at very high ones. At the low temperatures also the signal for m18 is quite pronounced. 5) The degassing and reduction behaviour is only slightly affected by the carbon content, even 0.2 % nominal (admixed) carbon being sufficient for effective reduction although the final (combined) carbon content is markedly smaller. 6) Carbon free compacts exhibit marginal weight loss, only in the low temperature range some signals for H2O and CO2 being found, which further confirms that the degassing/weight loss observed at T>1000 deg C is carbothermic reduction through admixed carbon. 7) For sintering of Cr containing steels made of prealloyed powders, sufficiently high sintering temperatures should be selected to ensure as complete reduction of the oxide layers as possible.