Powder Metallurgy Progress, Vol.7 (2007), No 4 181 STUDY OF REDUCTION/OXIDATION PROCESSES IN Cr-Mo PREALLOYED STEELS DURING SINTERING BY CONTINUOUS ATMOSPHERE MONITORING E. Hryha, L. Čajková, E. Dudrová Abstract The high affinity of chromium to oxygen is the main deterrent force for P/M parts manufacturers for the wider utilization of chromium low- alloyed powders that possess a high performance/cost ratio of the produced sintered steels. The thermodynamic calculations of the required sintering atmosphere composition/purity during sintering in nitrogen/hydrogen atmosphere (10% of H 2 ) at a temperature of 1120°C, paying special attention to carbothermic reduction processes, indicate strict requirements for the sintering atmosphere purity for such a highly sensitive prealloyed powder. Continuous atmosphere monitoring (CO/CO 2 /H 2 O content measurements), performed directly in the container with the specimens for the Fe-3Cr-0.5Mo-0.5C powder system during the whole sintering cycle, indicates pronounced carbothermic reduction at the beginning of sintering. It was shown that carbothermic reduction and formation, esp. CO/CO 2 , occurs in two different temperature ranges, emphasizing the importance of the carbon role in carbothermic reduction of chromium oxide for this system, involving both direct and indirect carbothermic reduction mechanisms. Three peaks of dew point profile can be distinguished during the sintering cycle as well. Sintering at a different atmosphere purity (dew-point of above -45 and -65°C respectively) indicate a pronounced difference in the resulting specimens oxidation, as was confirmed by metallography and SEM+EDX analysis of the fracture surfaces. Results obtained indicate sufficient specimen purity even during low-temperature sintering (at 1120°C) if proper atmosphere purity is maintained during the different sintering stages. Keywords: sintering atmosphere monitoring, carbothermic reduction, oxide reduction, chromium prealloyed powder INTRODUCTION Historically powder metallurgy was a low-cost technology for production of complex-shape parts, production of which by other technologies is sometimes impossible or economically unfeasible. Nevertheless, presence of a certain degree of residual porosity, inherent for P/M parts, leads to lower mechanical properties of iron-based sintered parts in comparison with wrought steel products. Thus the further increasing of the P/M market requires high-strength alloy steels, that in the case of powder metallurgy traditionally are alloyed with Cu, Ni and Mo [1-3] in comparison with conventional low alloy structural steels that contain mostly Cr, Mn and Si, with some addition of carbide-formers such as V Eduard Hryha, Lucia Čajková, Eva Dudrová, Institute of Materials Research, Slovak Academy of Sciences, Košice, Slovak Republic
Transcript
Powder Metallurgy Progress, Vol.7 (2007), No 4 181
STUDY OF REDUCTION/OXIDATION PROCESSES IN Cr-Mo PREALLOYED STEELS DURING SINTERING BY CONTINUOUS ATMOSPHERE MONITORING
E. Hryha, L. Čajková, E. Dudrová
Abstract The high affinity of chromium to oxygen is the main deterrent force for P/M parts manufacturers for the wider utilization of chromium low-alloyed powders that possess a high performance/cost ratio of the produced sintered steels. The thermodynamic calculations of the required sintering atmosphere composition/purity during sintering in nitrogen/hydrogen atmosphere (10% of H2) at a temperature of 1120°C, paying special attention to carbothermic reduction processes, indicate strict requirements for the sintering atmosphere purity for such a highly sensitive prealloyed powder. Continuous atmosphere monitoring (CO/CO2/H2O content measurements), performed directly in the container with the specimens for the Fe-3Cr-0.5Mo-0.5C powder system during the whole sintering cycle, indicates pronounced carbothermic reduction at the beginning of sintering. It was shown that carbothermic reduction and formation, esp. CO/CO2, occurs in two different temperature ranges, emphasizing the importance of the carbon role in carbothermic reduction of chromium oxide for this system, involving both direct and indirect carbothermic reduction mechanisms. Three peaks of dew point profile can be distinguished during the sintering cycle as well. Sintering at a different atmosphere purity (dew-point of above -45 and -65°C respectively) indicate a pronounced difference in the resulting specimens oxidation, as was confirmed by metallography and SEM+EDX analysis of the fracture surfaces. Results obtained indicate sufficient specimen purity even during low-temperature sintering (at 1120°C) if proper atmosphere purity is maintained during the different sintering stages. Keywords: sintering atmosphere monitoring, carbothermic reduction, oxide reduction, chromium prealloyed powder
INTRODUCTION Historically powder metallurgy was a low-cost technology for production of
complex-shape parts, production of which by other technologies is sometimes impossible or economically unfeasible. Nevertheless, presence of a certain degree of residual porosity, inherent for P/M parts, leads to lower mechanical properties of iron-based sintered parts in comparison with wrought steel products. Thus the further increasing of the P/M market requires high-strength alloy steels, that in the case of powder metallurgy traditionally are alloyed with Cu, Ni and Mo [1-3] in comparison with conventional low alloy structural steels that contain mostly Cr, Mn and Si, with some addition of carbide-formers such as V Eduard Hryha, Lucia Čajková, Eva Dudrová, Institute of Materials Research, Slovak Academy of Sciences, Košice, Slovak Republic
Powder Metallurgy Progress, Vol.7 (2007), No 4 182 and Mo. The reason of such economically inexpedient utilizing of alloying elements currently employed in P/M, the price of which (above 40,000 $/ton for Ni and 50,000 $/ton for Mo [4], respectively) is above 30 times higher in comparison with chromium (above 1,500 $/ton [4]), is connected first of all with the high oxygen sensitivity of Cr, Mn, Si and V which require high-purity sintering atmospheres [5-17]. The second negative impact of utilizing such alloying elements is the expected lower compressibility of such pre-alloyed powders; however, recent studies [8,19] indicate that low content of such alloying elements has no such detrimental effect on the compressibility of powders pre-alloyed with Mn and Cr, if proper morphology and fraction composition of the pre-alloyed powders are maintained. Modern powder grades pre-alloyed with chromium are now among the most attractive powders that have become widely utilized for the production of highly loaded high-performance structural components, such as hubs and gears [20-22].
High affinity of chromium for oxygen can be “bypassed” by a lowering of the chromium activity, which can be done by introducing chromium in the pre-alloyed state, where the alloy element is presented in solid solution and its activity is roughly equal to its content. In this case the main problem is manufacturing such pre-alloyed powders with a low content of mainly oxygen and carbon, which is not very easy by conventional water atomization. Even using a good vacuum annealing process for chromium pre-alloyed powders in the eighties [23,24], in spite of the excellent mechanical properties, did not provide high utilization of the powder grades at that time due to technical and economical drawbacks. Nevertheless these first pre-alloyed powder grades offer the challenges of chromium based pre-alloyed powders and, Höganäs AB at the end of the 90`s introduced a pre-alloyed steel powder grade, containing 3% of Cr and 0.5% of Mo, designated Astaloy CrM [25]. Water atomization with a subsequent reduction anneal under defined conditions results in an appropriately low oxygen content and Astaloy CrM is now available on the market at a comparatively low price.
During the sintering of powder metallurgy structural steels, protective atmospheres of special compositions are used, depending on starting powders and the required properties of sintered materials. Sintering atmospheres have to reduce residual surface oxides in order to improve the metallic contact between adjacent particles, protect sintered components from oxidation and prevent carburization/decarburization of the material. For these reasons, proper choice and careful control of the sintering atmosphere are important technological parameters for obtaining good quality sintered products. The choice of gas must take into account possible reactions between the gas, the sintered material and the furnace (heating elements, furnace lining). These reactions depend on temperature and pressure, and are numerous, because gases used in a commercial production contain gases such as H2, H2O, CO, CO2, O2, N2 etc. The changes in the free enthalpy ΔG of oxide formation are a measure of how readily the material oxidises. From the free enthalpy values at a given temperature, the pressure balance between reducing/oxidizing gases (p(O2); CO/CO2, H2/H2O) can be obtained and is summarized in the Richardson-Ellingham diagram. Values below the equilibrium curves mean prevention of the metal oxidation and indicate the boundaries of gas composition variation during the sintering process to prevent oxidation throughout any production stage. For this reason, control of the sintering atmosphere composition is very important because on-line control of sintering process gives a full picture of processes that take place during sintering and is useful for assuring the consistent production of any alloy system.
Proper measurements and control of the sintering atmosphere is a good instrument for establishing real conditions at which specimens are sintered at each sintering stage and gives possibility to find most problematic stages of sintering and find a way how to avoid
Powder Metallurgy Progress, Vol.7 (2007), No 4 183 them. Proper measurement of the sintering atmosphere composition is not so easy [6-8], active microclimate in the pores rather than atmosphere in the furnace determine the state of the system regarding the reducing/oxidising equilibrium.
The main aim of the study presented in this work is identifying the various phenomena arising during the sintering cycle and resulting in changes of exhaust gas composition and study of the main reactions between the specimen and the sintering atmosphere. Continual monitoring of the sintering atmosphere gives the possibility to establish extension and importance of chemical reactions at a certain sintering stage at real sintering conditions. Information obtained in this way is important not only from scientific points of view for clarifying the chemical processes in the sintered compact, but also from an engineering point of view for adjusting the required atmosphere purity level and flowing, sintering temperature, and cooling rate as a minimum for the prevention of sintered products from oxidation. The next very powerful approach for study of the specimen behaviour during different stages of the sintering process is thermoanalytical study using dilatometry, differential thermal analysis and thermogravimetry. Coupling these methods with mass-spectroscopy makes it possible to establish chemical reactions between the sintered specimen and sintering atmosphere and are good established in the works Danninger and Leitner [26,27] for the different powder systems.
THERMODYNAMIC CALCULATIONS Thermodynamic calculations are necessary prior to the establishment of sintering
conditions in order to find the requisite conditions for proper sintering of powder metallurgy parts, alloyed to elements highly sensitive to oxygen, such as chromium or manganese [6-8,12-14]. For the calculation of the compositional requirements for a protective atmosphere, it must be implicitly assumed that the system may be brought to an equilibrium or steady state condition. In the system with a continuously flowing atmosphere the steady state condition is the one most frequently achieved. This is done by supplying atmosphere at a rate sufficient to overcome any compositional changes that might occur by reaction of the atmosphere with the metal surface. But it is obvious that this situation is not possible with porous metal materials, since their surface extends into the interior volume and continuous replenishment of the atmosphere in contact with this inner surface is physically impossible under normal furnace conditions. The situation inside the parts is totally different, in these regions equilibrium can be established only between local carbon, manganese and oxygen and residual water vapour. Thus the effect of the atmosphere on the processes inside the parts can be important at relatively low densities.
The thermodynamic calculations concerned with the reduction reactions between the sintering atmosphere and metal oxides were carried out using the Outocumpu HSC Chemistry 5.0 database. Theoretical evaluation of required sintering atmosphere composition for the prevention of chromium alloyed steels from oxidation during every stage of sintering was performed on the basis of reaction energies calculations [5,7,8,28]. A simple algorithm for the sintering gas composition calculations at defined temperature was developed in the work of Mitchell [7] and was used in this work as well.
Maximum pressures of active gases, which can be tolerated in the system, can be calculated by considering free enthalpy changes ΔG0 at absolute temperature T. For the reduction of Cr2O3 by graphite, the following reactions have to be considered (ΔG in cal·mol-1):
Kp=P(CO)2/P(CO2) → P(CO2)=P(CO)2/Kp=(0.195)2/516.084=7.368*10-5 (bar) These calculations show that at 1393 K for Cr2O3, Cr and C to be in equilibrium
with a gaseous atmosphere, this atmosphere must be of the composition: P(CO)= 0.195 bar P(CO2)= 7.368*10-5 bar (4) P(O2)= 1.097*10-19 bar The following reaction between carbon and water vapor has to be considered as
well: C+H2O=CO+H2 (5) ΔG1393
0=32325-34.16*1393=-15259.88 (cal) lgKp= ΔGT
0/(-4.575*T)= -15259.88/(-4.575*1393)=2.394 → Kp=248.009; and for the sintering atmosphere of composition 90%N2/10%H2: Kp=[P(CO)*P(H2)]/P(H2O) → P(H2O)=[P(CO)*P(H2)]/Kp=[0.195 *0.1/248.009]=
=7.862*10-5(bar) Water partial pressure P(H2O)= 7.862*10-5 bar corresponds to a dew-point near
-44°C. This indicates that a dew-point lower than -44°C is required for reduction of chromium oxide in the 90%N2/10%H2 atmosphere at 1120 ºC.
Powder Metallurgy Progress, Vol.7 (2007), No 4 185
EXPERIMENTAL PROCEDURE The studied specimens were prepared on the base of Cr-Mo pre-alloyed powder
Astaloy CrM (Fe-3Cr-0.5Mo) with admixing of 0.5% C; natural graphite (Kropfműhl UF4) was used as the carbon carrier. Cylindrical specimens (∅10×12mm) were pressed at 600 MPa to the density of ~7 g/cm3 using only die-wall lubrication (amide-wax in carbon tetrachloride). This was done to avoid polluting, causing a closing of the filtering system during atmosphere monitoring by products of lubricant decomposition. The temperature profile in all experiments: heating rate 10°C/min up to 1120°C, dwell at 1120°C for 30 min and then cooling at a rate of 50°C/min down to 200°C. The sintering atmosphere monitoring setup, see Figs.1 and 2, was installed on the base of laboratory tube furnace LAC LHR A-type with a quartz tube. As the work was focused on the high-precision analysis of sintering atmosphere, special attention was focused on the used process gases composition and purity. For this reason, as processing gas there was used a specially ordered gas mixture prepared from calibration gases of composition 10% N2/90% H2, purity 5.3, with the composition deviation <0.1%. To prevent gases entrapping from the surrounding atmosphere, reduction valves with stainless steel membranes and PTFE or stainless steel tubing with stainless unions were used. Nevertheless inlet tube drying to such a low dew-point, as is required for such experiments (above -60÷-75°C), requires a long time for blowing-through the inlet system and furnace due to the huge amount of absorbed water vapor at the pipe-lines inner surface and some leakage from ambient air. This problem was solved by two-stage drying using a liquid nitrogen dryer (freezer, constructed from thin brass tubes, immersed in liquid nitrogen, through which process gases are blown) and on the next stage atmosphere drying was performed utilizing zeolite molecular sieve dryers, see Fig.2. A more complicated part of atmosphere monitoring setup is the sampling system that starts directly in the container with the specimens. A ferritic stainless steel container and sampling tubes were utilized for atmosphere sampling. The sampled atmosphere was analyzed employing non-dispersive infra-red CO and CO2 analyzers on the base of GasCard II Plus sensors; dew-point in the container with the specimens was monitored using the highly-sensitive ceramic Michel Cermet II sensor. All sensors were connected to a PC, and using specially prepared software, continuous monitoring of CO2/CO/H2O with a very small step (200 ms) was performed. To obtain the pronounced peaks after establishing starting atmosphere conditions, flow-rate was lowered to 0.5 l/min. The inlet dew-point was ~-65-75°C, measured by hygrometer Super Dew Shaw. A more detailed description of the used sintering atmosphere monitoring setup is presented elsewhere [8].
Two different batches of specimens were sintered – those sintered during atmosphere monitoring, marked A, and specimens of the same composition sintered at the same conditions, the difference is only in higher flow-rate – above 4 l/min or 0.5 m/sec for the employed tube cross-section (in comparison with 0.5 l/min for specimens A), and marked as B. Mechanical properties of differently sintered specimens were compared on the basis of rupture strength values (RFR), obtained using the specially designed for such small specimens “button test”, described elsewhere [8]. For microstructural analysis the specimens were polished to 1 μm finish and observed under optical microscope Olympus GX 71 in non-etched and etched (picral+nital) states. Fractography study and SEM+EDX analysis of the fracture surfaces was performed on the scanning electron microscope JEOL JSM-7000F equipped with INCA X-sight energy-dispersive X-ray analyzer on the fresh surfaces directly after the “button test”. Microhardness tests were carried out using a semiautomatic microhardness tester LECO LM 700AT.
Powder Metallurgy Progress, Vol.7 (2007), No 4 186
Fig.1. Sintering atmosphere monitoring setup.
Fig.2. Scheme of atmosphere monitoring setup.
5
RESULTS AND DISCUSSION
Atmosphere monitoring Different approaches can be used for evaluation of oxidation/reduction behavior of
materials in sintering atmospheres, based on the measurements of the pressure balance between oxidizing/reducing gases (p(O2), p(CO2)/p(CO), p(H2O)/p(H2)). By measuring any of the presented couple of gases, the trend of oxidizing/reducing reactions between the material and sintering atmosphere can be established by using the well-known Ellingham-Richardson diagram, see Fig.3. Of course, measurement of oxygen partial pressure is the most optimal and elementary method [29,30], which gives sufficiently reliable results. Measuring p(CO)/p(CO2) has a lot of disadvantages caused by the necessity of gases cooling before measurements and two different partial pressures (CO2 and CO) have to be measured to obtain the same result, that of course leaves a negative footprint on the accuracy of such measurements. In the case of p(H2)/p(H2O) ratio measurements only one measurement of water vapor content (or dew-point) is required, if a good-quality calibrated processing gas is used with a known hydrogen content (10% in our case), but some
Powder Metallurgy Progress, Vol.7 (2007), No 4 187 problems exist as well, connected with response time and sensitivity of some hygrologs. On the other hand, measurements of only oxygen partial pressure gives only information about oxidizing/reducing conditions in the system at a defined temperature, whereas measurements of partial pressure ratios p(CO)/p(CO)/ and p(H2)/p(H2O) give the possibility to evaluate the trend and intensity of the oxidizing/reduction reactions in the systems sintered material - sintering atmosphere. This is interesting not only from scientific point of view, but also for industrial production for avoiding oxidizing reactions, by adjusting proper flow and atmosphere purity at a defined sintering stage, and due to this assuring the consistent production of sensitive materials. On the basis of speculations stated above, continual measurements of dew-point and partial pressures of active gases p(CO2) and p(CO) were carried out for evaluation of the intensity and importance of two different reduction processes –hydrogen and carbothermic reduction at a defined sintering stage. In Figure 4 the results of atmosphere monitoring can be seen, together with a temperature profile during the sintering of Astaloy CrM+0.5% C.
Fig.3. Ellingham - Richardson diagram.
Powder Metallurgy Progress, Vol.7 (2007), No 4 188
Fig.4. Atmosphere monitoring (smoothed graphs) for Astaloy CrM+0.5%C.
Dew-point profile: The first peak of the dew point profile has a maximum above ~194°C and is linked with humidity removal from the samples. The largest second peak in the container begins at above 350°C and extends up to 530°C with a maximum near 433°C. This peak is attributed to a lot of different processes which overlap at this temperature range: • lubricant removal, that has a maximum at ~ 350°C (even if specimens were pressed
using only die-wall lubrication, some residuals of the lubricant are presented on the specimen surface and open pores);
• moisture, absorbed by compact constituents; • desorption of the physically bonded water; • decomposition of the hydroxides – one of the largest contributions; • starting of reduction of the surface iron oxides which is the most important for inter-
particle development of necks. The third wide peak starts at 1030°C and will reach a maximum at ~16 min of
isothermal sintering and then begins to decrease, that is rather surprising result because it was not observed during degassing experiments of this system [31]. In comparison with an iron-based system, this high-temperature peak in a pre-alloyed system is much more pronounced, and can be caused by higher oxygen content of the powder. If this peak is associated with the reduction of the surface iron oxides, then it has to have a maximum at the beginning of the sinter-holding. For this reason, some contributions from the reduction of internal iron oxides is present here as well; additionally this shift of the peak can be caused by some worsening of the analyzing system response, connected with drying of the sampling tubes after water vapor content increasing before isothermal sintering. It is important to note the presence of a pronounced peak during cooling stage above 285°C; this peak is observed at the same temperature as the carbon dioxide peak and will be described below.
Powder Metallurgy Progress, Vol.7 (2007), No 4 189
Carbon monoxide profile: The first peak of carbon monoxide was observed at above 324°C, at the same temperature as the first carbon dioxide peak – equilibrium between CO/CO2, and is linked with absorbed gases and graphite oxidation. It is important to note that some amounts of these gases were produced by lubricant decomposition, presented on the surface of the specimens in a rather large amount as well. The second group of more pronounced peaks was observed at above 450°C – graphite oxidation, reduction of surface iron oxides to carbon dioxide. But the most important are high-temperature peaks. First the CO peak with a maximum at above 802°C is connected with the reduction of surface iron oxides by plain graphite. The second and largest peak rises near ~900°C and has a maximum value at the beginning of sinter-holding – reduction of the oxides from internal pores which are at least to some extent open to the surface, and the reduction of the internal iron-containing oxides, as was observed during interrupted sintering experiments [8]. This is a direct carbothermic reaction caused by the reduction of oxides by dissolved carbon (that begins to dissolve in the iron matrix at above 800°C). A carbon monoxide peak during cooling stage at near ~730°C was observed for this system as well, linked with the Bouduard equilibrium between CO and CO2 in this temperature range.
Carbon dioxide profile: The first peak of the CO2 profile was observed at above ~173°C and is connected with removal of gases absorbed by the particles surface. The largest carbon dioxide peak in the container with a maximum near 323°C is attributed to absorbed gases; lubricant decomposition also has a contribution to this peak, but very important is a contribution from the iron carbonate decomposition. Small peaks can be observed in the range 600 – 800°C as well, which are linked with the Bouduard equilibrium between graphite, CO and CO2. During the cooling stage a pronounced peak of carbon dioxide was observed at the same temperature range as well, connected with the shift of the Bouduard equilibrium to CO2 formation – oxidizing conditions during cooling stage, as was registered for CO-content measurement as well. An intensive peak of carbon monoxide was observed at above 285°C, accompanied by a water vapor peak at the same temperature as well, indicating oxidizing conditions concerned with the shift of the equilibrium in the CO/CO2/H2/H2O gas mixture to oxidising gases formation (CO2 and H2O respectively).
Characterization of sintered specimens Carbon content of materials A and B is presented in Table 1. Sintering atmosphere
purity (caused by different flow rate in this case) strongly influences the carbon content – for specimens sintered at the same temperature and the same inlet atmosphere, purity carbon content for the well sintered specimen is more than 75% higher than for the same specimens sintered during atmosphere monitoring, where the local atmosphere conditions around the specimens were worse – this is the consequence of more intensive carbothermic reduction reactions during sintering with poor local sintering atmosphere purity, caused by insufficient removal of the reaction products.
Tab.1. Properties of test materials.
Material Green density, [g/cm3]
Sintered density, [g/cm3]
Carbon content, [wt.%]
Rupture strength, [MPa]
Microhardness, HV0.025
Hardness, HV10
A 6.93 6.94 0.21 723 362 ± 47 205 B 6.93 6.92 0.37 845 465 ± 68 247
Rupture strength value is also largely influenced by local sintering atmosphere
purity, but the difference is not so high – about 15%, especially if taking into account such
Powder Metallurgy Progress, Vol.7 (2007), No 4 190 large difference in the carbon content between specimens, which results in weaker microstructure in the case of specimen A.
Microstructures The presented as-polished microstructures of specimens A, see Fig.5, clearly
indicate the presence of oxides around the pores, on the grain boundaries and inside the base powder particles – internal oxides. Material B shows a much purer microstructure, rarely observed oxides are located inside the base powder particles and do not have a huge influence on the mechanical properties as the oxides in material A.
The etched microstructure of material A shows that it consists mostly of upper bainite with a microhardness of above 362 HV0.025, see Fig.6, in contrast to material B, the microstructure of which is more heterogeneous and is composed of a martensite-bainite mixture (martensite portion is above 20%), with a microhardness 586±51 HV0.025 in the martensitic regions and 424±46 in the bainitic ones; presented in Table 1 are values averaged along the sample. The difference in the microstructure of materials A and B is caused by a difference in carbon content – much higher carbon content in the case of material B results in martensite presence and a higher portion of lower bainite.
Fig.5. As polished microstructure of materials A (left) and B (right), showing presence of oxides at grain boundaries and in the matrix for material A in comparison with material B,
where oxides were rarely observed inside base matrix particles.
Fig.6. Etched microstructure (picral + nital) of materials A (left) – upper bainite and B
(right) – martensite-bainite mixture.
Powder Metallurgy Progress, Vol.7 (2007), No 4 191
Fractography Fractographic investigations of the fracture surfaces, obtained after the “button
test” (tensile loading), confirmed higher oxidation of material A in comparison with material B, see Fig.7 and Fig.8, respectively. Inter-particle ductile fracture, initiated mostly by point contaminations within inter-particle connections, is the main failure mechanism in both cases.
Fig.7. Fracture surface of material A together with a high magnification image of inter-
particle necks, where rather large contaminations (up to 3 μm) are observed. EDX spectra indicate a very high chromium and oxygen content in observed contaminations.
Powder Metallurgy Progress, Vol.7 (2007), No 4 192
The distinguishing characteristic of the observed contaminations within inter-particle necks of the specimen A in comparison with inclusions, observed in material B – its their higher size in the case of specimen A. High-resolution SEM coupled with EDX analysis indicate a very high chromium and oxygen content in such contaminations with the presence of some Si and V, indicating that the observed contaminations are complex refractory oxides where chromium oxide is dominant.
Fig.8. Fracture surface of material B together with a high magnification image of inter-
particle necks, where some fine contaminations (<0.5 μm) are observed.
DISCUSSION According to the careful study of surface products formations on the chromium
pre-alloyed powders by H. Karlsson and L. Nyborg [32-34], the surface of the chromium pre-alloyed powder is covered by a surface oxide layer, inhomogeneous in thickness. This surface oxide layer is formed by a continuous iron oxide layer with a thickness of above 5.5-7 nm with the presence of some particulate compounds (size above 20-100 nm) with a high content of strong oxide forming elements such as Cr, Mn and Si. The essential precondition for the formation of noticeable sintering necks is the removal of this surface oxide layer on the powder particles. A thin iron-rich layer is thermodynamically less stable and is easily reduced by hydrogen from the sintering atmosphere at the beginning of heating – second extended peak on the dew-point profile during atmosphere monitoring at the temperature interval of 350 – 530°C (see Fig.4). Above 500°C the dew-point begins to decrease, indicating less intense reduction of the iron oxide layer by hydrogen, but still remains high (~-35°C) which points to a continuation of the reduction process. As the reducing activity of hydrogen decreases with the temperature increasing, it is very important to have another reducing agent at higher temperatures during the heating stage at these still oxidizing conditions. Thanks to the Bouduard equilibrium, at temperatures higher than 720°C (see Fig.3) the reducing activity of carbon increases and it begins to play a dominant role in the reducing of surface oxides due to carbothermic reduction. Really, the first pronounced high-temperature CO peak in this system was observed at above 800°C, and is connected with the reduction of surface iron oxides by plain graphite, reduction of particulate refractory oxides is not possible at such low temperature by carbothermic reduction, at present conditions. It seems that a pronounced reduction of the surface refractory oxides (especially Cr2O3 which is prevailing on the surface of the base powder) starts above 900°C, when the second and largest carbon monoxide peak starts, and has a
Powder Metallurgy Progress, Vol.7 (2007), No 4 193 maximum value at the beginning of the isothermal sintering. Certainly, if taking into account the sharp increase in CO content and its maximum value, and compared with thermodynamic calculations presented in the theory section – measured ~0.003 bar (see Fig.4), it is much lower than the maximum allowed 0.195 bar according to calculations, see eqn.4, which points to the high reducing potential of the sintering atmosphere at this temperature range for chromium oxides, located on the surface (with the activity aCr2O3 ~1). On the basis of eqn.2, a maximum allowed carbon monoxide partial pressure for the reduction of chromium oxide during the complete sintering cycle can be calculated, and a comparison of the calculated curve with the experimentally measured one, see Fig.7, shows much more optimistic results – reduction of chromium oxide starts in this system above 790°C and takes place during the entire sinter holding and even cooling stage up to above 830°C, see Fig.9. Nevertheless, even a visual inspection of specimen A indicates its oxidation, and this emphasizes the critical importance of the second product of carbothermic reduction – carbon dioxide, because only the p(CO)/p(CO2) ratio gives correct information about the reducing ability of a sintering atmosphere, as is presented in the Ellingham-Richardson diagram, see Fig.3. The measured carbon dioxide content at the beginning of the sintering is above ~1 ppm, which is much lower than the calculated one, see eqn.4, underlining reducing conditions for chromium oxide at the beginning of the sintering. Nevertheless, calculation of the maximum allowed carbon dioxide level in the system for the reduction of chromium oxide on the basis of eqn.3 for the entire sintering cycle, and taking the ratio between theoretically calculated p(CO)/p(CO2) and their comparison with the ratio of experimentally measured p(CO) and p(CO2), see Fig.10, gives more pessimistic results – chromium oxides in the used sintering conditions is in reducing conditions only after the first 8 minutes of sinter-holding, and this result can explain the higher oxidation of the specimen.
Fig.9. Experimental (gray) and theoretical (black) curves of carbon monoxide content
during the sintering of material A, indicating that according to CO-content reduction take place above 790°C during heating up to and above 830°C of cooling stage, which points to a rather extended reduction time (theoretical curve is calculated for pure Cr2O3). The green
arrow indicates the range where reduction can take place.
Powder Metallurgy Progress, Vol.7 (2007), No 4 194
Fig.10. Experimental (gray) and theoretical (black) curves of p(CO)/p(CO2) ratio during
the sintering of material A, indicating that reduction takes place only above 8 minutes after beginning of the isothermal sintering, which points to a much shorter reduction time, as
can be evaluated only by CO-content measurements [theoretical curve is calculated for the pure Cr2O3]. A green arrow indicates range where reduction can take place.
Despite everything, according to presented results one thing is clear – reduction of chromium oxide in the used atmosphere conditions is possible as a minimum during sintering for the surface of the specimen, and it must be taken into account that local atmosphere conditions in the pores determine oxidation/reduction processes within the specimen, and the situation here is much better due to more intensive carbothermic reduction processes. Nevertheless, one of the main contributions to the high temperature CO peak is caused by reduction of the oxides from internal pores, which are at least to some extent open to the surface, and the reduction of the internal iron-containing oxides. The dew-point profile shows some increasing during sinter holding as well, caused by some reduction processes, but it has to be taken into account the that water vapor content is all the time in relation with carbon monoxide at higher temperatures according to the so called “water reaction”, eqn.5, that is the main reason of such an increase in the dew-point level. During the cooling stage, oxidation of the specimen is evident in all monitored exhaust gas profiles.
The presented results of the atmosphere monitoring are in good agreement with the degassing experiments performed by Danninger et al. [16, 26-27, 30], carried out for the same powder system [16, 31], where the authors emphasize that the full benefits of this systems can be achieved if high-temperature sintering is used and more complete oxide reduction occurs.
Metallographic (see Fig.5) and fractographic (see Fig.7) observation of sintered specimen A confirm high oxidation of the specimen. A considerable quantity of point inclusions are observed within the dimples of inter-particle ductile fracture facets, which are evident on the light optical micrographs as well, which are complex refractory oxides with a dominant content of chromium oxide according to SEM+EDX analysis. This observation, together with atmosphere monitoring results, indicates that these contaminations originate from particulate compounds, which were observed on the initial
Powder Metallurgy Progress, Vol.7 (2007), No 4 195 surface of the base powder [17,32-34]. As was discussed above, the surface iron oxide layer was easily reduced by hydrogen and then carbon. Reduction of the chromium oxides at conditions used for specimen A became possible only at higher temperatures, when the inter-particle connections begins to develop – between 900 – 1000°C for such alloyed systems [8,16]. With temperature increasing, inter-particle necks grew considerably, and such point inclusions are “closed” inside these necks. The size of such inclusions is determined by sintering atmosphere purity near the specimens, or to put it more precisely – by local “microclimate” composition [6-8,11]. They can considerably grow in size if conditions during the heating stage are strongly oxidizing – slow heating at poor atmosphere purity and a huge influence of the de-lubrication stage [8], and then they are closed inside inter-particle necks, where the reduction is hampered even at favorable conditions during sinter holding due to kinetic reasons. Specimen B, sintered at the same inlet atmosphere purity shows much lower oxidation which is caused by a higher flow-rate that “blow off”, all formed during reduction exhaust gases, leading to a huge improvement of local atmosphere purity. This is especially critical during the heating stage when the pores are open and the refined local sintering atmosphere, as a minimum, prevents powders from further oxidation by produced “exhaust gases” and growth of the fine oxides present. Even if they are further closed inside inter-particle necks, their size is considerably smaller (<0.5 μm in the case of material B in contrast to the above 3 μm in the case of material A) and are not so critical, which results in improvement of mechanical properties [12-14,17]. Internal refractory oxides, observed in the base powder particles [17], are observed in both the sintered specimens as well; their reduction needs much higher temperatures – above 1250°C according to [16], but in any case they have no considerable influence on the final mechanical properties. The presented results indicate that as-sintered carbon content is largely influenced by atmosphere purity as well – much higher carbon loss for the specimen sintered at poor atmosphere conditions was registered.
CONCLUSIONS Thermodynamic calculations of the maximum allowed partial pressures of active
gases (O2, CO, CO2, H2O) in the nitrogen/hydrogen (10% H2) sintering atmosphere were carried out for the reduction of chromium oxide as the main constituent of the refractory oxides, presented on the initial powder particles surface of Astaloy CrM. Comparison of the calculated ratio of p(CO)/p(CO2) with the experimentally obtained one during atmosphere monitoring during the sintering of Astaloy CrM+0.5% C clearly shows favorable conditions for carbothermic reduction of the surface chromium oxides as a minimum during isothermal sintering at 1120ºC. Huge improvement in the purity of specimens was obtained by an increase of atmosphere flowrate to above 4 l·min-1, that corresponds to 0.5 m·s-1 – the commonly used flowrate in industrial furnaces, caused by refinement of the local “microclimate” inside open pores during the very important temperature range between 900-1000ºC of the heating stage, when the inter-particle connections begin to form and there is a huge risk of “closing” non-reduced surface oxides within inter-particle necks, which is difficult to reduce afterwards due to kinetic reasons, even at favorable conditions for their reduction, as was confirmed by metallographic and fractographic analyses.
Poor atmosphere conditions, especially during heating stages used for the specimen A as was observed by atmosphere monitoring, result in a larger oxidation of the specimens. A high portion of formed oxides are located inside inter-particle necks (with a size above 3 μm) and result in lower mechanical properties.
Reduction processes in the Astaloy CrM+0.5%C system according to the results of continuous atmosphere monitoring can be divided into three main steps – reduction of the
Powder Metallurgy Progress, Vol.7 (2007), No 4 196 surface iron oxide layer by hydrogen up to 500ºC, carbothermic reduction of surface iron oxides which are in contact with plain graphite at above 800°C and direct carbothermic reaction (oxides from internal pores, reduction of the internal iron-containing oxides and surface chromium oxides) caused mostly by reduction of oxides by dissolved carbon, which starts near ~900°C and reaches a maximum at the beginning of isothermal sintering.
Continuous atmosphere monitoring (CO/CO2/H2O/O2 content measurements) during the whole sintering cycle, performed as close as possible to the sintered material, is a powerful instrument for avoiding oxidation of the sintered material by adjustment of the required atmosphere purity/flowrate according to the base material used, and gives the possibility to obtain a high-quality final product with the desired mechanical properties.
Acknowledgement The authors gratefully acknowledge VEGA 2/6209/26 and Höganäs Chair III
grants for the financial support of this work.
REFERENCES [1] Schatt, W., Wieters, KP.: Powder Metallurgy, Processing and Materials. Shrewsbury :
European Powder Metalurgy Association, 1997 [2] Höganäs Handbook for Sintered Components. Höganäs : Höganäs AB, 2004 [3] Šalak, A.: Ferrous Powder Metallurgy. Cambridge International, 1995 [4] LEM Report, LEM, April 2007 [5] Beiss, P. In: Hőganäs Chair in Powder Metallurgy. Workshop Sintering Atmospheres.
Vienna, September 10-11, 1999 [6] Mitchell, SC.: The Development of Powder Metallurgy Manganese Containing Low-
Alloy Steels. PhD. Thesis. University of Bradford, 2000 [7] Mitchell, SC., Cias, A.: Powder Metallurgy Progress, vol. 4, 2004, no. 3, p. 132 [8] Hryha, E.: Fundamental Study of Mn Containing PM Steels with Alloying Method of
both Premix and Pre-alloy. PhD. Thesis. IMR SAS, 2007 [9] Čajková, L.: Strength and toughness of modern high-strength sintered steels. PhD.
Thesis. IMR SAS, 2007 [10] Danninger, H., Pöttschcher, R., Bradac, S., Šalak, A., Seykammer, J.: Powder
Metallurgy, vol. 48, 2005, p. 23 [11] Hryha, E., Dudrová, E.: Materials Science Forum, vol. 534-536, 2007, p. 761 [12] Bergman, O., Lindqvist, B., Bengtsson, S. In: Proc. of Powder Metallurgy World
Congress and Exhibition PM 2006. Busan, Korea, September 2006, p. 280 [13] Bergman, O.: Powder Metallurgy, vol. 50, 2007, no. 3, p. 243 [14] Frykholm, R., Johansson, P., Bergman, O. In: Sintering-2005. Grenoble, France,
September 2005, p. 1 [15] Engstrom, U., Bergman, O., Milligan, D., Klekovkin, A. In: Proc. of Euro PM 2007.
Vol. 1. Toulouse, 15-17 October 2007, p. 35 [16] Kremel, S., Danninger, H., Yu, Y.: Powder Metallurgy Progress, vol. 2, 2002, no. 4, p.
211 [17] Ortiz, P., Castro, F.: Powder Metallurgy, vol. 47, 2004, no. 3, p. 291 [18] Ortiz, P., Castro, F.: Materials Science Forum, vol. 426-432, 2003, p. 4337 [19] Hryha, E., Dudrová, E., Bengtsson, S. In: Proc. Euro PM2007. Vol. 3. Toulouse, 15-17
October 2007, p. 3 [20] Sigl, LS., Rau, G. In: Proc. of Euro PM 2007. Vol. 1. Toulouse, 15-17 October 2007, p.
385 [21] Dizdar, S., Johansson, P. In: Proc. of Euro PM 2007. Vol. 1. Toulouse, 15-17 October
Powder Metallurgy Progress, Vol.7 (2007), No 4 197
2007, p.129 [22] Takemasu, T., Koide, T., Takeda, Y., Bengtsson, S. In: Proc. of Euro PM 2007. Vol. 3.
Toulouse, 15-17 October 2007, p. 413 [23] Minegishi, T., Unami, S., Furukimi, O., Komamura, K.: Advances in Powder
Metallurgy and Particulate Materials, vol. 5, 1992, p. 53 [24] Ogura, K., Takajo, S., Yamato, N., Maeda, Y., Morioka, Y.: Metal Powder Report, vol.
42, 1987, p. 292 [25] Hull, M.: Powder Metallurgy, vol. 41, 1998, no. 4, p. 232 [26] Danninger, H., Gierl, C., Kremel, S., Leitner, G., Jaenicke-Roessler, K., Yu, Y.:
Powder Metallurgy Progress, vol. 2, 2002, no. 3, p. 125 [27] Danninger, H., Kremel, S., Leitner, G., Jaenicke-Roessler, K., Yu, Y. In: PM World
Conference, API/MPIF. Vol. 13. Orlando, Florida, June 2002, p. 291 [28] Gaskell, DR.: Introduction to the Thermodynamics of Materials. 4th ed. New York :
Taylor&Francis, 2003. 618 p. [29] Lindquist, B., Kanno, K. In: Proc. of 2002 Wold Congress on Powder Metallurgy,
Orlando. Vol. 13. Princetown : MPIF, 2002, p. 278 [30] Franzo, C., Gomez-Acebo, T., Ortiz, P., Calero, JA., Castro, F. In: Proc. of Euro PM
2007. Vol. 3. Toulouse, 15-17 October 2007, p. 169 [31] Danninger, H. In: Proc. Of Hagener Symposium 2006. Hagen, 23-24 November 2006,
p. 21 [32] Karlsson, H.: Role of Surface Oxides in Sintering of Chromium-Alloyed Steel
Poweder. PhD. Thesis. Chalmers University of Technology, 2005 [33] Karlsson, H., Nyborg, L., Berg, S.: Powder Metallurgy, vol. 48, 2005, no. 1, p. 51 [34] Karlsson, H., Nyborg, L., Bergman, O. In: PM World Conference. Vol. 3. Vienna,
![LOW-TEMPERATURE SINTERING AND MICROWAVE DIELECTRIC ...€¦ · and promotes the densification by liquid phase sintering [12-14]. However, over sintering would cause abnormal grain](https://static.documents.pub/doc/80x56/5fb3b4a9e5540561916e2940/low-temperature-sintering-and-microwave-dielectric-and-promotes-the-densification.jpg)
