Dilatometric investigations of Fe – Cr – Mo – C system

Sintering behavior in both high purity N2 and H2-containing atmosphere of Fe-(Cr)(Mo)-C compacts was investigated. Turbula mixer was used for preparing the mixtures of powders with different Cr, Mo and C content. Following mixing, using single-action pressing in a rigid die at pressing pressure 400 MPa, green compacts with density level 5.9±0.17 g/cm were pressed. Sintering was carried out in a horizontal push rod dilatometer Netzsch 402E at 1120 and 1250°C for 60 min. Heating and cooling rates were 10 and 20°C/min., respectively. After heating, compacts were isothermal sintered at 1120 or 1250°C for 60 minutes and cooled up to 200°C, then isothermally hold for 60 minutes and definitely cooled to the room temperature. Pure nitrogen and mixture of 5% H2-95% N2 were employed as sintering atmospheres. During investigations the influence of isothermal sintering temperature, chemical composition of sintering atmosphere, chromium, molybdenum and carbon content was followed by dilatometry. The aim of investigations was to determine transformation temperatures. It was shown that both sintering parameters and chemical composition of powder mixture has a great influence on sintering behaviour of Fe-(Cr)(Mo)-C compacts.


Introduction
Powder Metallurgy (PM) technology can be used for production of structural material parts of machines.These include cogwheels, piston rings, compressor wings, even space shuttle skin plating, and household objects.More than 90% of world PM materials are iron-based materials (Ciaś, 1992;Mazahery & Shabani, 2012;Missol, 1972).The powder metallurgy route consists mainly of four stages: powders' production, mixing, the compacts fabrication and sintering.Very often the post-sintering treatment is needed for increasing the properties of PM parts (Mazahery & Shabani, 2012).Sintered materials based on pure iron are characterised by low mechanical properties.To increase the mechanical properties, the additives have to be employed.Very often it is carbon, added in the form of graphite in the maximum amount of 0.3% (Ciaś et al., 2003).According to the carbon content, several microstructures can result: ferrite-pearlite, pearlite or pearlite with pro-eutectoid cementite (Ciaś, 1992;German, 1994).Tensile strength of carbon steels does not exceed 520 MPa, and to obtain higher properties, it is necessary to introduce alloying elements such as Cu and Ni (Ciaś, 2004;Klein et al., 1985b;Šalak & Selecká, 2012;Zapf et al., 1975).However, problems with recycling scrap metal containing copper and the high price of nickel result in the substitution of these elements in sintered steels by Mn, Mo and Cr (Ciaś, 2004;Ciaś et al., 2003;Cygan, 2013;Gac, 2012).Such sintered steels attain higher mechanical properties in comparison with Cu and Ni steels.What is more, they can be produced with lower costs (Gac, 2012;Hryha et al., 2007;Sułowski & Ciaś, 2011).
Chromium is a very important alloying element in PM steels.It increases strength, hardness and hardenability and forms hard carbides.Chromium, which has a somewhat stronger carbide-forming tendency than iron, partitions between the ferrite and carbide phases ((Fe, Cr)3C, Cr7C3 and Cr23C6).Chromium also influenced the toughness and the wear resistance of steel.
Manganese is potentially and important alloying element in sintered steels.To date it has not been exploited beyond 0.7 wt.% due to its extremely high affinity for oxygen (Klein, Oberacker, & Thummler, 1985a;Klein et al., 1985b;Šalak, 1980a, 1980b).Therefore, the use of Mn as an alloying element requires special precautions.The Mn oxides cannot be reduced during sintering at conventional temperatures and atmospheres without strict dew point control.To protect the material from oxidation, the combination of "high" sintering temperature and a "low" dew point is required, e.g. at 1120°C -55°C is required.However as was shown in (Sułowski & Ciaś, 2011), the mechanical properties of Mn and Mn-Cr-Mo steels after sintering in low-hydrogen atmospheres or in air are comparable to those obtained after sintering in rich-hydrogen atmospheres.Usually PM steels are sintered at 1120°C, but high temperature sintering (HTS) -at for example 1250°C -give the possibility of oxide reduction because the thermodynamic stability of an oxides decreasing with increasing the sintering temperature.HTS promotes also homogenization of the microstructure and leads to increasing the mechanical properties of sintered steels.
PM industry needs that during production of PM high precise parts, dimensional changes must be known and controlled very carefully to keep tolerances of sintered materials.Thus, in this paper a study of the temperature and atmosphere effect on dimensional changes of Fe-Cr-Mo-C compacts is presented.
Compacts were heated at 10°C/min to the isothermal sintering temperature of either 1120°C or 1250°C.Isothermal sintering time was 60 minutes.It should be noted that the dilatometer used could not maintain the constant cooling rate during the whole cooling period.Thus, the cooling rate from isothermal temperature down to about 380°C was 20°C/min (0.33°C/s).To obtain constant cooling rate up to room temperature, samples were isothermally hold at 200°C for 60 minutes and then cooled to the room temperature.The temperature control was accurate to  1°C.In Table 1 the scheme of dilatometric investigations is shown.As-sintered density of samples were in the range of about 5.91±0.12g/cm 3 (Table 1).Dilatometric curves were afterwards analysed by Netzsch Thermal Analysis computer program.

Results
The results of dilatometric investigation of Fe-Cr-Mo-C are presented in Figs.1-8 and divided into three parts: the effect of carbon concentration, the effect of chemical composition of sintering atmosphere and the effect of sintering temperature on dimensional changes of C containing compacts based on Astaloy CrA, Astaloy CrL and Astaloy CrM Höganäs grade powders.

The effect of carbon concentration
In Figs.1-4 dilatometric curves for steels containing different carbon concentration are presented.
From Fig. 1 can, be observed that after sintering at 1120°C in nitrogen atmosphere, the highest dimensional stability was obtained for the steel sample based on Astaloy CrA powder with addition of 0.4 wt.-% C (1A).The highest dimensional changes were recorded for steel based on Astaloy CrL powder with addition of 0.4 wt.-% C (1L).The temperature of  →  phase transformation was in the range 774°C-911°C and 827°C-904°C for steels containing 0.4 and 0.8 wt.-% C, respectively.
After sintering at 1120°C in the mixture of 5%H2-95%N2 (Fig. 2), the highest dimensional stability was obtained for the steel sample based on Astaloy CrM powder with addition of 0.4 wt.-% C (2M).The lowest dimensional stability was observed for steel based on Astaloy CrA powder with addition of 0.4 wt.-% C (2A).The increasing the carbon content in steels caused the bigger dimensional changes during the whole sintering cycle (sample 6M and 6L in the contrary to the sample 2M and 2L).The carbon content in steels based on Astaloy CrA powder has an influence on their shrinkage during  →  phase transformation.The temperature range of this transformation was 750°C-912°C and 752°C-886°C for steels containing 0.4 and 0.8wt.-%C.
After sintering at 1250°C in nitrogen atmosphere (Fig. 3), the highest dimensional stability was obtained for the steel sample based on Astaloy CrA powder with addition of 0.4 wt.-% C (3A).The highest shrinkage was recorded for steel based on Astaloy CrM powder with addition of 0.8 wt.-% C (7M).The temperature of  →  phase transformation was in the range 767°C-892°C and 751°C-891°C for steels containing 0.4 and 0.8 wt.-% C, respectively.
After sintering at 1250°C in the mixture of 5%H2-95%N2 (Fig. 4), the highest dimensional stability was obtain for the steel sample based on Astaloy CrL powder with addition of 0.4 wt.-% C (4L).The lowest dimensional stability was observed for steel based on Astaloy CrA powder with addition of 0.8 wt.-% C (8A).After sintering at 1250°C for all samples shrinkage was observed.The temperature range of this transformation was 846°C-914°C and 747°C-815°C for steels containing 0.4 and 0.8 wt.-%C.
The increase the sintering temperature had negligible effect on the  →  phase transformation temperature.The temperature range was varied from 767°C-822°C to 861°C-886°C and from 747°C-812°C to 847°C-891°C, for steels sintered in nitrogen and mixture of 5%H2-95%N2.

Conclusions
Based on present work, the following concluding remarks can be drawn: • The  →  phase transformation range for all investigated steels was similar.
• During isothermal sintering in all sample's shrinkage was observed.
• The higher carbon concentration influenced bigger shrinkage in investigated steels.
• Sintering in nitrogen caused good dimensional stability of investigated steels.
The financial support of the Ministry of Science and Higher Education under AGH contract no 11.11.110.299 is acknowledged.

Table 1 .
The scheme of dilatometric investigations.