Study on the Mechanism of Heterogeneous Catalysis ( 3 ) — The Catalysis of W , Mo , S and SiO 2 on Carbon Dissolving and Mechanism of Carbon Dissolving in the Iron-Graphite Compact

This is an article that validates Electron Cyclic Donate-Accept Catalysis Mechanism-ECDAM or Electron Orbital Deformation-Recovery Cycle Catalysis Mechanism-EODRM. In this paper, the influence of compact density, sintering temperature and time, vacuum degree, atmosphere and alloying element on the carbon dissolving rate into γ-Fe were studied. The experiment results have been reported. The way that the carbon is dissolved into γ-Fe was discussed. Gas phase composition and temperature play an important role on the carbon dissolving rate. The carbon gasification reaction is the determining step of dissolving carbon process. K2CO3, W and Mo are a catalysts, and their active size are K2CO3 > W > Mo. S and SiO2 are a poison, and their poison size are SiO2 > S. The experiment results are completely consistent with the original judgment of ECDAM.


Introduction
Fe-C alloy sintered steel is the most essential and important material of iron based P/M.This conventional production process of P/M sintered steel part consists of admixture, pressure forming and sintering.In order to improve the mechanical strength of the sintered steel, the alloy elements of Cu, Ni, W, Mo etc. which is not easy oxidizing are often added to admixture powder.In order to improve the processing performance and to reduce the mold wear, the sulfur is usually added to admixture powder.During sintering, the particle graphite dissolves in iron, and then sintered steel has formed.So, in what mechanism does graphite dissolve into iron?With respect to this problem of carbon transfer, there are only two possibilities.One is that the carbon directly dissolves into iron, and other is that the carbon indirectly dissolves into iron.The directly or contact dissolution should be  The transfer of carbon relies on the carbon gasification reaction.In the compact, the iron is in close contact with carbon, the contacting reaction of iron with carbon is entirely possible to take place.So the direct dissolve and indirect dissolve, which one is the dominant one?If indirect dissolution is the main one, then what is about the role of alloying elements or catalysts?This is the purpose of this study.In fact, the main purpose of this subject is to validate the ECDAM or EODRM.

Experiments
Iron powder used in this study is electrolytic iron powder.Its chemical composition and sieve analysis were listed in Table 1 and Table 2. Synthetic graphite powder was chosen, its chemical and physical properties were listed in Table 3.Some alloy elements and compound additives such as tungsten powder (average particle size around 1 μm), molybdenum powder (average particle size around 2.6 μm), potassium carbonate (around 7.8 μm, chemical purity), silica powder (around 0.1 μm) and sulfur powder (around 9.3 μm) were added respectively into the iron-graphite compacts.The amount of graphite added to the iron powder was 0.8%, and the amount of additive added to graphite powder was 15% -18% of graphite.
In process, first, the additives such as W, Mo etc. is mixed with graphite, and then mixed with iron powder.In order to eliminate the interference of lubricants, it will not be used.The mixed iron powder is pressed in steel die to densities 5.8 and 7.0 g/cm 3 .
The size of cylindrical compact is 10 mm dia.and 10 mm height.
The sintering process is carried out in a high temperature quartz tube furnace.
There are four kinds of sintering atmosphere; a) Vacuum, the vacuum degree is 4 × 10 −5 torr.
b) Vacuum, the vacuum degree is 1.2 × 10 −2 torr.c) Protective atmosphere.Nitrogen 25% + Hydrogen 75%.And its dew point was about −15˚C, the gas flow rate was 2 l/min.The sample was exposed to the protective atmosphere.
d) In the package or cover boat.The sample was closed in package.It is cut off from protective atmosphere.The atmosphere in the package would be self-generated by chemical reaction of remnant oxygen with graphite within the compact pores as well as air gap in package.
 Metallographic microscope was applied to estimate the amount% of pearlite on the friction mirror face which is corroded by 5% nitric acid alcohol.Based on national standard, the amounts of Pearlite are measured by professional technicians.The black field of vision, empty except, shows Pearlite (contained carbon 0.8%), and the white field of vision shows Ferrite (contained carbon 0.08%).The more pearlite content, the more dissolved carbon.

Results and Discussion
1) Direct and indirect dissolving of carbon in the sintering process?
From Figure 1 and Figure 2, it can be seen that the dissolving rate of carbon of high density compacts (7.0 g/cm 3 , porosity 11%) is slower than that of low density, 5.8 g/cm 3 (porosity 26%).The differences are more remarkable for the samples sintered in the covered boat.If the carbon absorbing by iron took place by solid-solid reaction, hence it would be favour for the high density compacts, due to the contact points between particle graphite and iron are more than that of low density.But the experimental results are just the opposite.This clearly shows that the dissolution of carbon mainly relies on indirect dissolution, which relies on carbon gasification reactions.The carbon gasification reaction becomes the process control step.
From Figure 3, although the vacuum degree and compact density are higher, however, the rate of dissolution of carbon in a with tungsten compact is still higher than that of unadded tungsten compact, and the tungsten still has appeared the catalytic active, the result show that the gas phase in porosity still plays an important role in carbon dissolving process.Journal of Materials Science and Chemical Engineering    It is worth noting that, from Figure 4, photo (d) and (e), on the one hand it is obvious that S and SiO 2 appear clearly poison effects, and on the other hand it is obvious that there is no direct dissolving, because of it can't see the pearlite in the field of vision.But the author believes that direct dissolution is completely excluded, this judgment needs to be further research.
2) Effects of sintering atmosphere and time on the dissolving carbon rat.
Figure 5 shows the effects of different sintering atmospheres on the dissolving rate of carbon.In N 2 -H 2 atmosphere, it can be seen that the dissolving rate is the fastest during the initial period.In high vacuum, p = 4 × 10 −5 torr, the dissolving rate of carbon is lowest than that the low vacuum, p = 2 × 10 −2 torr.These results shown clearly that indirect dissolving was the main way of carbon dissolving, and the carbon gasification reaction still played an important role.Because the point of contact between iron and graphite, after all, not much.
From Figure 1, Figure 5, Figure 6, it can be seen that the dissolving rate of carbon is higher fast in the N 2 + H 2 atmosphere during sintering process, and the  From Figure 2, Figures 5-9, it appears that repeated experiments did not appear decarburization phenomenon in the package sintering.This may be very valuable to avoid hydrogen decarburization for P/M factory which is to use the dissociated ammonia as a protective gas.
From Figure 9, when the sintering time exceeds 60 minutes, there was no significant difference between adding and not adding catalyst to pearlite content formed.From Figure 4, sintering temperature 980˚C, sintering time 8 minutes, sintering in package, on the K 2 CO 3 catalyst, the photograph shows clearly that the whole field of vision seen in metallographic microscope are almost pearlite (empty except).It shows clearly that 0.8% K 2 CO 3 has good catalytic active.The catalytic active size is arranged: K 2 CO 3 > W > Mo.
Dautzenberg etc [1] have studied the catalysis of carbonates in Fe-C alloy compact sintering, it has found that the additions of 2% K 2 CO 3 was not catalytic active, and think that the carbon is dissolved in the iron in a direct dissolving.Tanaka etc.
[2] have also studied the catalytic effect of carbonates in iron-graphite compacts sintering, they have obtained similar results.But it's completely different from that Hong [3] get. Author considers that the different experimental results are caused by inappropriate sintering temperature (1100˚C high temperature), time (long time), and atmosphere.The wrong experimental method will be to get unreliable experimental results

3) Catalysis of alloy elements on the carbon dissolving
From Figure 7, Figure 8, Figure 9 and photograph Figure 4, it can be seen that the K 2 CO 3 , W, Mo. have a significant catalytic effect on carbon dissolving during sintering process.On the contrary, the S and SiO 2 had showed a retarding effect on it, and SiO 2 is more poison than that the S.Although the sulfur has been completely gasified at 980˚C, the contact with carbon is far better than that of the solid SiO 2 .However, due to the ENV (estimate: more than that the S (X S = 2.58), so poison of silica is naturally greater than that the S.
In iron based P/M parts production, in order to reduce mold wear, the sulfur is always added to the admixture; However, sulfur is a poison that will slow down the dissolution rate of carbon and the formation of pearlite.Author considers that the three high (high temperature, high pressure and high energy consumption) situation appeared of ammonia production are likely to be caused by improper application of Al 2 O 3 as promoter [8].

Conclusions
1) The experiment results have demonstrated that the carbon dissolving into γ-Fe takes place mainly by indirect dissolving during the sintering process of iron-graphite compacts, i.e., the carbon gasification reaction plays an important role.And it is a determining step of dissolving process, even in high vacuum sintering.
2) Hydrogen in the protective gas N 2 + 3H 2 , which is both a catalyst and a decarburizing agent, can significantly increase the rate of carbon dissolving and significantly accelerate decarburization or reduce Pearlite content.
3) Sintering temperature plays an important role on the process.The rate increases considerably by raising sintering temperature.
Sintering under high vacuum conditions, the dissolution rate is significantly reduced.

Figure 1 .
Figure 1.Variation of pearlite%-sintering time for two densities sintered in N 2 -H.

Figure 2 .
Figure 2. Variation of pearlite%-sintering time (min.)for two densities sintered in the covered boat.

Figure 8 .Figure 9 .
Figure 8. Variation of pearlite%-sintering time for S and SiO 2 additives sintered in covered boat.900˚C, 980˚C, d = 7.0 g/cm 3 χ −∆ value, the greater the poisoning.In terms of χ −∆ value, Al 2 O 3 is much more poison to iron catalysts than carbon.

Table 3 .
Chemical composition and physical properties of graphite.
[7]xperiment has proved that Al 2 O 3 was poison to Ru catalyst.At the same time, Du[7]experiment has also proved that the Al 2 O 3 was poison to carbon gasification, for this reason, the ENV of Al 2 O 3 should be greater than 2.55.
[8]for Fe based ammonia synthesis catalyst, the catalyst produced in the whole world contains always 2.5% -3.0%Al 2 O 3 as a structural promoter.Author[8][9] has repeatedly pointed out that Al 2 O 3 is a poison to iron catalyst, because the molecular ENV of Al 2 O 3 is much greater than that of iron.Let's see