Chemical Reactions During Sintering of Fe-Cr-Mn-SiNi-MoC-Steels With Special Reference to Processing in Semi-Closed Containers

Sintering of Cr, Mn and Si bearing steels has recently attracted both experimental and theoretical attention and processing in semiclosed containers has been reproposed. This paper brings together relevant thermodynamic data and considers the kinetics of some relevant chemical reactions. These involve iron and carbon, water vapour, carbon monoxide and dioxide, hydrogen and nitrogen of the sintering atmospheres and the alloying elements Cr, Mn, Mo and Si. The paper concludes by presenting mechanical properties data for three steels sintered in local microatmosphere with nitrogen, hydrogen, nitrogen-5% hydrogen and air as the furnace gas.


Introduction
Although Cr steels are the most promising materials for medium-to-high strength PM parts, sintering of these alloys is not easy.
The main difficulty to overcome in these steels is the classical problem of the sintering atmosphere.
These alloys are thermodynamically unstable at the sintering temperature and the process is often not repeatable.Whereas the oxides of nickel can be reduced during sintering at conventional sintering temperatures in atmospheres without strict dew point control, the oxides of chromium, manganese and silicon cannot.In well-monitored N 2 /H 2 inlet sintering atmospheres with oxygen partial pressures <10 -13 Pa, however, Cr-Mn steels have been sintered [1][2][3].
Another approach is to use semiclosed containers in a flowing nitrogen atmosphere, and rely on the water vapour -carbon reaction to produce sufficient necessary hydrogen and carbon monoxide.Their use proved successful both on a laboratory [2,[4][5][6][7] and semiindustrial scale [8].Use of semiclosed containers with labyrinth or melted glass seals for sintering PM steels makes it possible to dispense with drying of special flowing atmospheres or sintering without a flowing gas environment.Semiclosed containers substantially lower heat losses, which are up to 78% higher for furnaces with a controlled protective atmosphere than for furnaces with an uncontrolled atmosphere [9].Such technique overcomes totally safety, legislative and economic problems necessarily associated with the use of hydrogen in the workplace.In its place are the additional costs associated with the use of semiclosed containers (sintering boxes), which currently find little industrial application.Furthermore, an element may be put in the sintering box which forms an oxide which prevents the absorption of oxygen by the Cr steel being sintered.Suitable elements for this purpose are those which oxidize more readily than Cr in the nitrogen/hydrogen atmosphere.Theoretically they include Mn, Al and Si, with Mn being particularly preferred.Further, another sintering aid successfully introduced was a nascent carbon donor, e.g.naphthalene [10].
It seems therefore timely to analyse the reactions involving oxygen, hydrogen, carbon, carbon monoxide, carbon dioxide and methane during sintering.Introducing sodium carbonate, ammonium iodide and ammonium chloride (potential activators, for example salts decomposing with the formation of gaseous halides) into the semiclosed container has also been considered, but will be analysed elsewhere.

Thermodynamic and kinetic aspects of reactions involving Mn, Cr and Mo with the sintering atmosphere -theoretical background
The Ellingham-Richardson-Jeffes diagram [11,12] is useful in determining the affinity of a metal for oxygen and a combination of temperature and other reducing conditions in which metal oxides can be reduced.However, if an alloying element has a higher affinity for oxygen then that of iron, its oxidation is not always preferential, but depends on its concentration.In some alloys only one component (most reactive -Cr, Mn, Si) oxidizes -selective oxidation.Some PM alloy steels may oxidize within the alloy grain boundaries -internal oxidation.Oxidation/reduction processes strongly depend on the sintering temperature (Tab.I).The reduction of oxides in Cr steels is shifted to markedly higher temperatures than in Fe-C alloys [13][14].It is important to note that the higher the Cr-content in the materials, the better the atmosphere conditions have to be.
Tab.I Critical oxygen partial pressures and dew points for oxidation during sintering Fe-3Cr-0.5Calloy in a 90N 2 -5%H 2 atmosphere at 1120 and 1250°C (from Thermo-Calc calculations).
Sintering temperature, °C p O2 , Pa Dew point, °C 1120 4•10 -13 -28 1250 1•10 -10 -18 Carbide forming processes in alloys strongly depend on the sintering temperature.The Ellingham diagram for the transition series carbides [15] is useful in determining the affinity of a metal for carbon and a combination of temperature and other carburizing conditions in which a metal can form carbides.The formed carbide may either dissolve in metal matrix or form separate phases.In some alloys only some elements (most reactive -Cr, Mn, Fe) form carbides.
Detailed studies of the thermodynamics of reduction of oxides in the case of powders pre-alloyed with Cr and Mn have been presented elsewhere [16,17], and in this paper the roles of Cr, Mn, Mo and Si alloying elements is considered.

Manganese
A specially relevant property of manganese is the effective sublimation at relatively low temperatures, from ~ 700°C.The manganese vapour pressure in Pa is given by the following equation [16] : log p Mn = -14920T -1 -1.96logT + 18.32 (1) Thus at the (conventional, industrial) sintering temperature of 1120°C its vapour pressure reaches 19 Pa and 147 Pa at 1250°C [18].Manganese first sublimes (or evaporates), then, to a certain extent, oxidises: ) and finally condenses on the Fe powder particle surfaces.As the Ellingham-Richardson-Jeffes line for manganese lies below the lines of most metals, including chromium, iron and molybdenum, manganese vapour can be used as the reducing agent for oxides of all these metals.At sintering temperature 1120°C the standard Gibbs free energies (ΔG o = RT lnp O2 ) of formation of chromium (III) oxide and manganese (II) oxide, per mole of oxygen consumed, are: -504 kJ and -553 kJ, respectively. - 2 at 1120°C and -51 kJ/mole O 2 at 1250°C.Since the Gibbs free energy change is negative, manganese (vapour) can reduce chromium oxide.So manganese oxide is more stable than chromium oxide at sintering temperatures, and in fact all the way up to the decomposition temperatures of the oxides.
Thus from ~700°C there can be readily available manganese vapour, from the compact or intentionally introduced in a semiclosed container, to promote the direct reduction of chromium oxide -earlier in the sintering cycle than would be expected when only the direct and indirect carbothermic reduction mechanisms were available [6], as in conventional sintering of Mn-free chromium alloys.The manganese vapour can fill the pores of the compacts, picking up oxygen and added carbon.
The carbothermal direct reduction of MnO at temperatures between 1120 and 1250°C takes place only at the points of contact between oxide and graphite particles as: MnO (s) + C (s) = Mn (s, l, g) + CO (g) (3) The presence of solid carbon will maintain partial pressure of H 2 O at a low level by the reaction: C (s) +H 2 O (g) =CO (g) +H 2(g) (4) which makes the reduction of manganese oxide by hydrogen in the presence of solid carbon feasible.Using hydrogen for the reduction of manganese oxide: 5) is the decisive process for sintering in a semiclosed container.
Another overall reaction of MnO reduction by carbon to manganese carbide is possible: 7MnO (s) +10C (s) = Mn 7 C 3(s) +7CO (g) (7) which is a sum of three reactions: (10) It is suggested that the rate of the overall MnO reduction is limited by the interfacial Boudouard reaction [4,6], or by transport of CO 2 within the porous sintered compact to graphite or soot particles [7].The rate determining seemed to be the chain reaction between CO-CO 2 gas phases.Even at high sintering temperatures, when the diffusion rate is also high, the direct graphite-MnO reaction does not dominate in the reduction processes [19].
The thermal decomposition of hydrocarbons generates also hydrogen gas in situ.The presence of hydrogen in the reduction process of MnO with carbon results in the formation of methane gas.Addition of hydrocarbons into the semiclosed container results in their combustion, to provide two reducing gases, carbon monoxide and hydrogen, although some carbon dioxide and water vapour will also be produced.For example: 2CH

Chromium
The thermodynamics of the formation and reduction of chromium oxide on pure chromium powders, as well as on iron powders alloyed with chromium, have often been studied [13,14].Cr steels are difficult to sinter because Cr 2 O 3 layers are formed on the alloy surfaces.As a result of relatively negative Gibbs free energy change (ΔG) for Cr oxidation, Cr 2 O 3 is formed more easily than many other oxides.Cr 2 O 3 can be reduced by pure dry hydrogen at the standard sintering temperature 1120°C.The reduction rate of chromium oxide with H 2 is higher than that obtained with CO in the lower temperature range.
The difficulties are based on the thermodynamics of the reaction: ) with the equilibrium constant: It follows, that the partial pressures ratio H O H p p at equilibrium is very small, e.g.2.8•10 -4 at 950°C and 3.2 •10 -5 at 800°C.Therefore a reduction is only possible in hydrogen free from oxygen and water vapour, or with the continuous removal of water vapour from the reaction zone.To maintain reducing conditions in actual sintering practice of Cr containing steels, a dew point of at least -35 to -40°C is required in the furnace.Because water vapour is formed during reduction of metal oxides, a sufficient flow of gas is required to continually remove this water, as well as the water that is formed by the reaction between hydrogen and the air introduced with the parts and through furnace openings.Through careful moisture control, it is entirely possible to sinter high chromium steel (>12%Cr) directly in hydrogen at inlet dew point from -50 to -60°C , but preferably at -70°C or lower, if a sufficient gas flow is used.In a semiclosed container the reduction with hydrogen is only possible with continuous removal of water vapour, which is a product of the reaction, from the sintering microatmosphere.It is suggested that it can be attained by the reaction (2) with manganese vapour.
The carbothermal reduction process of Cr 2 O 3 was investigated by a micro-scale thermoanalytical method with quantitative in situ gas (CO) detection using carbon black, active carbon and graphite [20].Most probably, the oxide layers are the precursors of the carbide layers.The transfer of carbon to the surface of the oxide layers is realized by the CO/CO 2 mass transport mechanism.Cr 3 C 2 is the first carbide formed as a layer on the Cr 2 O 3 layer or around the Cr 2 O 3 particles.The carbothermal reduction process of Cr 2 O 3 consists of two subprocesses, firstly, the CO/CO 2 transport reactions and, secondly, the reaction of the primarily formed Cr 3 C 2 with Cr 2 O 3 .

Molybdenum
Mo has a lower affinity for oxygen than iron and thus easily reducible oxides.
Therefore, Mo PM steels can be effortlessly processed in industrial furnace sintering atmospheres with dew point below only -15°C.Oxides of molybdenum are volatile.In general, the reaction between MoO 2 and H 2 in the hydrogen reduction process can be represented by the reaction: ) At conventional sintering temperatures the reaction is thermodynamically favourable.The reaction might not take place, however, because the equilibrium constant is relatively large at these temperatures.Hydrogen reduction of MoO 2 might take place if the partial pressure of water vapour produced from reaction ( 17) is relatively low.

Silicon
Silicon has a higher affinity for oxygen than Cr and Mn.This makes impossible reduction of SiO at conventional sintering temperatures.Only at temperatures over 1900 °C, in the liquid alloy, silicon can undergo the chemical reaction with carbon: SiO 2 + 2 C → Si + 2 CO (18) The thin Si oxide layer, when formed, in spite of Cr and Mn oxides, resists oxygen diffusion and limits further oxidation of the powder particle.Difficulty in reducing oxides has historically necessitated that Si-containing PM steels be sintered at temperatures above 1250°C.Thus it is desirable to achieve the joint benefits of chromium, manganese, and silicon in one alloy system, while maintaining the ability to sinter steel components at temperatures lower than 1250°C.

Experimental verification of the effect of the sintering microatmosphere
A number of variants of processing conditions were investigated, especially for copper-free Fe-1.4Cr-1.3Ni-0.7Mn-0.2Mo-0.2Si-0.4C, Fe-3.2Mn-1.4Si-0.5Cand 3Cr-0.5Mo-0.6Csteels (detailed results to be presented elsewhere).Representative mechanical properties data are presented in Table II.All the 3 steels can be successfully sintered in semiclosed containers with nitrogen or air as the furnace gas.Generally mechanical properties are superior to those obtained by conventional sintering in a flowing N 2 -5%H 2 or H 2 atmosphere.They compare favourably with those of currently used PM structural steels [21,22] and detailed in MPIF Standard 35.Mechanical properties are enhanced when sintering takes place in a semiclosed container with ferromanganese, aluminium and/or naphthalene additives.Introducing sodium carbonate, ammonium iodide and ammonium chloride (potential activators, for example salts decomposing with the formation of gaseous halides) into the semiclosed container has also been investigated.
Tab. II Experimental results to verify the effect of the sintering microatmosphere; properties of the investigated steels, together with MPIF data for a similar PM steel.Should 1250 °C, or higher, temperatures be required for sintering components that match properties of wrought steels, since sintering can take place with air as the furnace atmosphere, use can be made of furnaces designed for sintering ceramics.The author suggests that this opens new possibilities for PM.

Conclusions
This paper reviewed several important points in the processing PM steels, including the following major items: (i) the effect of the local sintering microatmosphere and processing, involving presence of Mn on the microstructure and the basic mechanical properties of PM steels sintered in nitrogen flowing atmosphere; (ii) comparison of properties of the so-produced specimens with those sintered in hydrogen, nitrogen-5% hydrogen and MPIF Standard 35 counterparts.The following can be concluded from the study: 1. Semiclosed containers for sintering PM steels make possible generation within the container of any gaseous environment necessary for the removal of oxide films.When sintering metals having a high affinity for oxygen, combining the sintering process with a chemicothermal (e. g. carbothermal) treatment, by means of getters a low partial pressure of O 2 in a sealed volume of the container can be established.2. The important result of these studies is that the semi-closed container processing is a satisfactory technique for sinteraustempering of Fe-1.4Cr-1.3Ni-0.7Mn-0.2Mo-0.2Si-0.4C steel.3. Successful hydrogen-free PM processing of Cr and Mn steels.When sintered in a nitrogen atmosphere in semiclosed containers, they generally possess superior mechanical properties (fracture strength and ductility) in comparison with specimens sintered in flowing hydrogen and are better than those of the MPIF 35 Standard.4. Further improvement in mechanical properties can be expected if isothermal sintering at higher temperatures, e.g.1250°C, is employed.

2
The temperature must be increased to over 850°C before the reaction will take place.