Science of Sintering, 47 (2015) 61-69
________________________________________________________________________
doi: 10.2298/SOS1501061C
UDK 676.017.2; 622.785; 669.14
Chemical Reactions During Sintering of Fe-Cr-Mn-Si-Ni-Mo-CSteels With Special Reference to Processing in Semi-Closed
Containers
A. Cias
Faculty of Metals Engineering and Industrial Computer Science, Department of
Physical Metallurgy and Powder Metallurgy, AGH- University of Science and
Technology,
A. Mickiewicza 30, 30-059, Kraków, Poland.
Abstract:
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.
Keywords: Sintering, Microatmosphere, Sintered steels, Mechanical properties.
1. 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 N2/H2 inlet sintering
atmospheres with oxygen partial pressures <10-13 Pa, however, Cr-Mn steels have been
sintered [1- 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, 4-7] 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
_____________________________
*)
Corresponding author: cias@agh.edu.pl
62
A. Cias /Science of Sintering, 47 (2015) 61-69
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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.
2. 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-3Cr0.5C alloy in a 90N2-5%H2 atmosphere at 1120 and 1250°C (from Thermo-Calc calculations).
Sintering temperature, °C
1120
1250
pO2, Pa
4·10-13
1·10-10
Dew point, °C
-28
-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.
2.1. 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] :
A. Cias /Science of Sintering, 47 (2015) 61-69
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log pMn = -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:
Mn(g) + H2O(g) = MnO(s) + H2(g)
(2)
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 (ΔGo = RT lnpO2) of
formation of chromium (III) oxide and manganese (II) oxide, per mole of oxygen consumed,
0
0
are: -504 kJ and – 553 kJ, respectively. ΔGMnO − ΔGCr2O3 = - 49 kJ/mole O2 at 1120°C and –
51 kJ/mole O2 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 H2O at a low level by the
reaction:
C(s)+H2O(g)=CO(g)+H2(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:
MnO(s) + H2(g) = Mn(s) + H2O(g)
(5)
and
H2(g) + 1/2O2(g) =H2O(g)
(6)
reaction (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) = Mn7C3(s) +7CO(g)
(7)
which is a sum of three reactions:
MnO(s) + CO(g) = Mn(s, g) +CO2(g)
(8)
CO2(g) + C(s)=2CO(g)
(9)
7Mn(s, g) + 3C(s) = Mn7C3(s)
(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 CO2 within the porous sintered compact to
graphite or soot particles [7]. The rate determining seemed to be the chain reaction between
CO-CO2 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:
2CH4(g) + O2(g)→4H2(g) + 2CO(g) ΔH0 298= - 71.4 kJ.
(11)
The temperature must be increased to over 850°C before the reaction will take place.
A. Cias /Science of Sintering, 47 (2015) 61-69
64
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Soot reacts with oxygen :
2C(s) + O2(g)→CO(g) ΔH0 298= - 222kJ
(12)
The presence of water vapour results in H2 and CO formation with an endothermic reaction
with carbon:
H2O(g) + C(s) → H2(g) + CO(g) ΔH0 298 = +132 kJ
(13)
2.2. 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 Cr2O3 layers are formed on the alloy
surfaces. As a result of relatively negative Gibbs free energy change (ΔG) for Cr oxidation,
Cr2O3 is formed more easily than many other oxides.
Cr2O3 can be reduced by pure dry hydrogen at the standard sintering temperature 1120°C. The
reduction rate of chromium oxide with H2 is higher than that obtained with CO in the lower
temperature range.
The difficulties are based on the thermodynamics of the reaction:
Cr2O3(s)+3H2(g)=2Cr(s)+3H2O(g)
(14)
with the equilibrium constant:
Kp =
2
p H3 2O ⋅ aCr
(15)
p H3 2 ⋅ aCr2O3
and free energy of the reaction:
ΔG°=373,422 – 89.25T J · mole -1
It follows, that the partial pressures ratio
(16)
p H 2O
pH2
at equilibrium is very small, e.g. 2.8·10-4 at
-5
950°C and 3.2 ·10 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 Cr2O3 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/CO2 mass transport mechanism. Cr3C2 is the first carbide formed as a layer on the Cr2O3
layer or around the Cr2O3 particles. The carbothermal reduction process of Cr2O3 consists of
two subprocesses, firstly, the CO/CO2 transport reactions and, secondly, the reaction of the
primarily formed Cr3C2 with Cr2O3.
2.3. Molybdenum
Mo has a lower affinity for oxygen than iron and thus easily reducible oxides.
A. Cias /Science of Sintering, 47 (2015) 61-69
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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 MoO2 and H2 in the hydrogen reduction process can be
represented by the reaction:
MoO2 + 2H2(g) = Mo + 2H2O(g)
(17)
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 MoO2 might take place if the partial pressure of
water vapour produced from reaction (17) is relatively low.
2.4. Silicon
Silicon has a higher affinity for oxygen than Cr and Mn. This makes impossible
reduction of SiO2 at conventional sintering temperatures. Only at temperatures over 1900 °C,
in the liquid alloy, silicon can undergo the chemical reaction with carbon:
SiO2 + 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.
3. 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-0.5C and 3Cr0.5Mo-0.6C steels (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 N2-5%H2 or H2
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.
Specimen
composition
Fe-1.4Cr1.3Ni-0.7Mn0.2Mo-0.2Si 0.4C
Sintering
conditions
Open boat,
N2-5%H2
atmosphere,
sinterhardened
Sintering
temp.
Sintered
density
°C
1120
1250
g/cm3
6.83
6.85
0.2%
offset
yield
stress
Tensile
strengt
h
Strain
to
failure
Transverse rupture
strength
MPa
351
378
MPa
424
499
%
1.09
1.79
MPa
981
1124
HV
30
198
207
A. Cias /Science of Sintering, 47 (2015) 61-69
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Fe-3.2Mn1.4Si-0.5C
3Cr-0.5Mo0.6C
Semiclosed
- container,
air furnace
atmosphere
additives:
ferromanga
nese,
naphthalene
, Na2CO3,
NH4I,
sinterharde
ned
Semiclosed
- container,
N2 furnace
atmosphere;
additives:
ferromanga
nese,
Al,
NH4Cl,
NH4I,
sinterauste
mpered
500°C/1
hour
Semiclosed
container,
N2
atmosphere;
additives:
ferromanga
nese,
naphthalene
, Na2CO3,
NH4I,
sinterharde
ned
Semiclosed
container,
N2
atmosphere;
additives:
ferromanga
nese,
naphthalene
, Na2CO3,
NH4I,
sinterharde
ned
Open boat,
H2
atmosphere,
sinterharde
ned
1120
6.81
386
519
1.82
1181
218
1250
6.84
399
562
2.11
1249
223
1120
6.9
409
753
3.2
1414
272
1120
6.51
542
800
2.13
1476
253
1250
6.64
561
890
2.21
1786
291
1120
6.86
531
908
4.2
1919
348
1120
6.93
445
650
1.74
1477
270
A. Cias /Science of Sintering, 47 (2015) 61-69
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Fe-2.5Cu-(1.33.0)Ni-(0.61.0)C
Semiclosed
container,
N2 furnace
atmosphere;
additive:
naphthalene
, Na2CO3,
sinterharde
ned
Semiclosed
container,
N2
atmosphere;
additive:
ferromanga
nese, NH4I,
sinterharde
ned
Convention
al, MPIF,
Standard 35
1120
6.86
495
786
2.69
1815
380
1120
6.87
486
777
2.91
1921
287
not
specified
6.7-7.4
240380
310620
1.53.0
590-1170
-
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.
4. 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 O2 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.
A. Cias /Science of Sintering, 47 (2015) 61-69
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Acknowledgements
This work was sponsored by the Polish Ministry of Science and Higher Education
under Contract no11.11.110.158. Appreciation is also expressed to Professor A. S. Wronski
for his comments on the manuscript.
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Садржај: Синтеровање челика са садржајем Cr, Mn и Si привлачи доста пажње, како
у експерименталном тако и у теоријском смислу и предложено је процесирање у
полуотвореним контејнерима. У овом раду изнети су релевантни термодинамички
подаци а узета је у обзир и кинетика важних хемијских реакција. Оне укључују гвожђе
и угљеник, испаравање воде, угљен моноксид и угљен диоксид, водоничну и азотну
атмосферу синтеровања и легирање елемената Cr, Mn, Mo и Si. У раду су
A. Cias /Science of Sintering, 47 (2015) 61-69
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презентована механичка својства три различита челика синтерована у локалној
микроатмосфери азота, водоника, азота-5% водоника и ваздуха.
Kључне речи: синтеровање, микроатмосфера, синтерован челик, механичка својства