Powder Metallurgy Progress, Vol.8 (2008), No 2 115

MODELLING OF SINTERING ATMOSPHERE AND MICROSTRUCTURE DEVELOPMENT OF CHROMIUM ALLOYED STEEL

S. Hatami, E. Hryha, L. Nyborg, D. Nilsson

Abstract In order to take advantage of recent achievements in furnace technology and monitoring equipments, it is necessary to have a rigorous understanding of the chemical reactions that take place inside the sintering furnace. If overlooked, there is a risk for insufficient control of the sintering condition and the outcome will be scatter in PM parts’ final alloying content and mechanical properties. At the same time, end user demands require tight tolerances in product performance for sintered goods. In this study, different mixtures of iron based chromium prealloyed Astaloy CrM powder with the nominal composition of 3 wt.% Cr and 0.5 wt.% Mo were studied. The investigated powder mixtures were the plain powder, admixed with lubricant and admixed with lubricant and graphite (0.45 wt.%). Specimens were sintered in N2 and 10%H2/90%N2 atmospheres at 1120°C. Chemical reactions that are possible to occur in the above mentioned atmospheres and between the aforementioned powders mixtures are thoroughly examined from a thermodynamic point of view. Calculations have been performed to find the equilibrium partial pressures of the atmosphere constituents using a thermodynamic software package, HSC Chemistry 6.0. In addition, by means of modern thermodynamic and kinetic modeling software, JMatPro 4.0, the carbon activity for the alloying systems in question has been calculated. The simulation concept used for microstructural predictions of chromium alloyed sintered steel, developed by Nyborg et al. is applied to the alloying systems studied. Keywords: modelling, controlled atmosphere, chromium alloyed steel

INTRODUCTION Considering homogenous distribution of Cr and Mo in austenite, the carbon

activity is significantly reduced [1]. The combined effect of Cr, Mo and C in solution will then be enhanced possibilities for bainite and martensite formation and thereby associated higher strength. Chromium activity is lowered when chromium is added by means of prealloying as compared to added as ferro-chromium powder, which consequently reduces its sensitivity to oxidation [2]. Two different types of oxides have been found in Astaloy CrM powders: surface and internal oxides [3]. The surface oxide is composed of particulate features of a size ranging up to 100 nm containing strong oxide forming elements such as Cr, Mn and Si; and a continuous iron oxide layer of about 7 nm in thickness in between [4]. Approximately equal parts of the total oxygen content of the powder are from the internal

Sepehr Hatami, Eduard Hryha, Lars Nyborg, Department of Materials & Manufacturing Technology, Chalmers University of Technology, Gothenburg, Sweden Daniel Nilsson, Höganäs AB, Höganäs, Sweden

Powder Metallurgy Progress, Vol.8 (2008), No 2 116 and the surface oxides, respectively [4]. The internal oxides are (100-500 nm) spherical and are Cr and O rich of the type M2O3 [3]. In contrast to internal oxides that do not have any significant influence on the final mechanical properties of the sintered component, the surface oxides are barriers for diffusion and will retard proper neck formation during sintering [5-8]. Thus, if not reduced, these oxides can profoundly reduce the mechanical properties [6]. In PM parts the final mechanical properties and microstructure are closely related to the precision of the sintering atmosphere control [1,7,9]. Thus, for proper control of the sintering atmosphere a rigorous understanding of the interactions between the green part and the atmosphere is a necessity. In this study, the reactions between the powder mixtures and the above mentioned atmospheres are examined from a thermodynamical point of view.

EXPERIMENTAL The base powder was Astaloy CrM with the chemical composition given in

Table 1. In this investigation three different powder mixtures were used, see Table 2. Hereafter, each powder mixture will be referred to by its corresponding sample number, see Table 2. The green bodies were thin discs with a diameter of 25 mm and height of 2 mm, compacted at 600 MPa to a density of 6.9-7.0 g/cm3. The sintering experiments were carried out under pure N2 and 10%H2/90%N2 mixture with a dew-point of about -45 to -50°C. Sintering was performed in an industrial belt furnace at 1120°C for 30 minutes. The cooling rate was 1°C/s from 800°C to 300°C. Analyses of as-sintered oxygen and carbon content were performed using LECO instruments. Metallographic observations were carried out using optical microscopy.

Tab.1. Chemical composition of Astaloy CrM powder [wt.%].

C O - tot Cr Mo Fe <0.01 0.15 3.00 0.50 bal.

Tab.2. Powder mixtures used in this investigation [wt.%].

Sample Material 1 Astaloy CrM 2 Astaloy CrM + 0.8 AW 3 Astaloy CrM + 0.8 AW + 0.45C

*AW = Amidewax, lubricant

RESULTS AND DISCUSSION The oxygen and carbon content of the sintered samples are given in Table 3.

Tab.3. Oxygen and carbon content of samples sintered in pure N2 and 10%H2/90%N2.

N2 atmosphere 10%H2/90%N2 atmosphere Sample Number O [wt.%] C [wt.%] O [wt.%] C [wt.%]

1. Astaloy CrM 0.22 0.02 0.16 0.03 2. Astaloy CrM + 0.8 AW 0.19 0.07 0.14 0.03 3. Astaloy CrM+ 0.8 AW + 0.45 C 0.10 0.38 0.10 0.40

Powder Metallurgy Progress, Vol.8 (2008), No 2 117

As-polished and etched microstructures of specimens 1-3, sintered in nitrogen atmosphere, are presented in Figs.1-2. The same microstructures for the sintered in 10%H2/90%N2 atmosphere specimens 1-3 are presented in Figs.3-4.

Accurate activities (a) of the alloying elements are obtained using the modelling software, JMatPro. It should be noted that JMatPro assumes a homogenous distribution of elements and the calculations are made for solid material. At 1120°C for sample 3 the activities of Fe, Cr, Mo, C and O are 0.95, 0.04, 0.01, 0.11 and 2.2 x 10-9, respectively (Fig.5a.).

Fig.1. a) Sample 1, O = 0.22 wt.%; b) Sample 2, O = 0.19 wt.%.

Fig.2. Sample 3, a) polished, O = 0.10 wt.%; b) etched, C = 0.38 wt.%.

Fig.3. a) Sample 1, O = 0.16 wt.%; b) Sample 2, O = 0.14 wt.%.

Powder Metallurgy Progress, Vol.8 (2008), No 2 118

Fig.4. Sample 3, a) polished, O = 0.10 wt.%; b) etched, C = 0.40 wt.%.

The calculations done in the proceeding sections are only valid for the isothermal sintering. Comparing the oxygen concentrations, see Table 3, and observing the micrographs of the polished specimens in Fig.1 and Fig.2, it can be deduced that samples 1 and 2 have been oxidised while sample 3 has been reduced. Pure N2 with as low a DP as -45 °C should not be oxidising nor decarburising [7]. Therefore, the increase in the oxygen content for sample 1 and 2 is related to the oxidation that takes place during cooling from the sintering zone. To find the equilibrium partial pressure of oxygen, P(O2), the calculations can be performed as follows. By means of the software, HSC Chemistry, the Gibbs free energy change (ΔG) and the equilibrium constant (K) at 1120°C can be calculated:

232 34

32 OCrOCr +⇔ → ΔG = 515428 J → K = 4.709x10-20 (1)

It is also possible to calculate (K) by making use of the calculated activity of Cr (from JMatPro) and allocating 1 for the activity of Cr2O3;

32

32

234

)(1

)()()4.(

OCr

OPCrooK ×= (2)

The ΔG value is related to the equilibrium constant through LnKRTG ×−=Δ (3),

where R is the universal gas constant and T is absolute temperature. Using eqns. (2) and (3) and knowing the ΔG value, the equilibrium P(O2) at

1120°C is calculated to be 3.48 x 10-13 Pa (3.44 x 10-18 atm). In sample 3, the oxygen content has decreased from 0.15 to 0.10 wt.%. This reduction in oxygen content is due to the presence of admixed graphite in the powder mixture. The carbon from the admixed graphite will be in solid solution in austenite at the sintering temperature and can react with the available oxygen according to:

COOC 22 2 ⇔+γ → → K = 3.905x10JG 4691454 −=Δ 17 → P(CO) = 12.868x103 Pa (0.127 atm) (4).

The CO produced from eq. (4) can then react with O2 and form CO2: 22 22 COOCO ⇔+ → → K = 1.322x10JG 3232585 −=Δ 12 → P(CO2) = 27.35 Pa

(2.71x10-4 atm) (5) These values can be checked by using the Bouduard equilibrium reaction:

COCCO 22 ⇔+ γ → → K= 5.435x10JG 729436 −=Δ 2 (6)

Powder Metallurgy Progress, Vol.8 (2008), No 2 119

By considering direct carbothermic reduction of chromium oxide by dissolved carbon the same values of P(CO)=0.127 atm and P(CO2)=2.71x10-4atm were obtained:

COCrCOCr 2342

32

32 +⇔+ γ → → K = 1.839x10JG 462837 =Δ -2 (7)

232 2342

32 COCrCOOCr +⇔+ → → K = 6.226x10JG 1921698 =Δ -8 (8)

The carbon content in sample 3 has decreased from its original value, 0.45 to 0.40 wt.%, see Table 3. Therefore an efficient oxide reduction is anticipated. Figure 2a illustrates a clean polished surface, indicating low oxide content. This confirms the above statement. Whereas, in Fig.1, due to absence of required amount of carbon, oxides (black marks) can be easily found. Similar calculations, as above, can be done for the 90%N2-10%H2 atmosphere. However, hydrogen addition plays an influential role in the reduction/oxidation behaviour during sintering, especially during the heating stage. The reduction of the surface iron oxide layer by hydrogen begins at above ~350°C, before the carbothermic reduction that in turn takes place at above 720°C [8, 11]. This leads to two important results – reduction of the oxides at lower temperatures, that means lower re-oxidation during the heating stage, and lower carbon loss because of surface iron oxide layer is reduced before by hydrogen. Thus, the following reactions should in addition be considered:

OHOH 222 22 ⇔+ , , 22 HCOOHC +⇔+γ OHCOHCO 222 +⇔+and

OHFeHOFe 2232 323 +⇔+ , . OHCrHOCr 2232 323 +⇔+For Astaloy CrM powder the surface oxide layer is formed by principally Fe2O3

with thickness of above 7 nm [4]. Surface bound oxygen, XS, in metal powder can be estimated by [10]:

twSX pS ρ= (9), where Sp is specific surface area (65 m2kg-1), ρ – oxide density (ρFe2O3=5200 kg·m-3), t – oxide thickness and w – weight fraction of the oxygen in the oxide (0.3 for Fe2O3).

According to (9) for this powder, the surface bound oxygen is 0.07 wt.%. Remaining oxygen is linked to the surface refractory oxides and internal complex oxides that are not reduced by hydrogen at the applied sintering conditions. Thus, it is possible to remove 0.07 wt.% of oxygen only by hydrogen at lower temperatures, consequently leading to a lower carbon loss in hydrogen containing atmosphere (0.05%), as compared to pure N2 (0.07%). Due to semi-closed porosities and microclimates inside the pores not all surface oxides are reduced at lower temperatures by hydrogen. Thus, carbon loss is slightly higher in pure N2. Re-oxidation during cooling causes sample 3 specimens (in both atmospheres) to contain equal amount of oxygen.

The etched micrograph of samples 3, sintered in pure N2, illustrates a bainitic-martenistic microstructure, Fig.2b. According to the methodology developed by Nyborg et al. [12] for simulation of microstructures and mechanical properties of chromium alloyed PM steels with JMatPro, the similar microstructure is predicted for an alloy system same as sample 3 with a cooling rate equal to the furnace’s cooling rate (1°C/s), Fig.5b. The only required input parameters for the simulation are the grain size and the austenitisation temperature which are given as 140 μm and 1120°C (sintering temperature), respectively.

Powder Metallurgy Progress, Vol.8 (2008), No 2 120

Fig.5. JMatPro simulations for sample 3 in N2, left) activities of the elements, right) CCT.

CONCLUSIONS Thermodynamic calculations using accurate activities of the elements for the steel

alloyed with 3 wt.% Cr demonstrate that the oxygen partial pressure should be below 3.48 x 10-13 Pa (3.44 x 10-18 atm) in order to have reducing conditions during sintering at 1120°C. • Accurate values of the carbon monoxide and carbon dioxide level during isothermal

sintering were calculated using HSC Chemistry and JMatPro softwares. • These values are required for proper control of the sintering atmosphere. • Thermodynamic calculations showed importance of hydrogen additions in sintering

atmosphere for reduction of the surface iron oxide layer leading to lower re-oxidation during heating stage and lower final carbon loss.

• JMatPro shows to be a successful tool in predicting the microstructure of Cr alloyed PM steel.

REFERENCES [1] Mosca, E. In: Sintering Theory and Pract. Intensive Short Course. EPMA, 1996, p. 97 [2] Danninger, H. et al.: Powd. Met., vol. 48, 2005, p. 23 [3] Oritz, P. et al.: Powd. Met., vol. 47, 2004, no. 3, p. 291 [4] Karlsson, H. et al.: Powd. Met., vol. 48, 2005, no. 1, p. 51 [5] Bergman, O.: Powd. Met., vol. 50, 2007, no. 3, p. 243 [6] Olefjord, I. et al.: Powd. Met., vol. 31, 1988, no. 1, p. 33 [7] Beiss, P. In: Hoganas Chair in Powder Metallurgy Workshop, Sintering Atmospheres,

1999. [8] Danninger, H. et al.: Powd. Met. Prog., vol. 2, 2002, no. 3, p. 125 [9] Bocchini, GF.: Powd. Met. Prog., vol. 4, 2004, no. 1, p. 1

[10] Bracconi, P., Nyborg, L.: Appl. Surf. Sci., vol. 133, 1999, p. 129 [11] Hryha, E. et al.: Powd. Met. Prog., vol. 7, 2007, no. 4, p. 181 [12] Nyborg, L. et al. In: 2008 World Congress on PM and Particulate Materials, p. 1200