Research on the Influence of Manganese Content of Physical and Chemical Characteristics Iron-Based Sintered Products

In this paper are presented results obtained from the elaboration of different types of sintered steels alloyed with manganese content of manganese featuring up to 5% (Fe-0Mn, 1Mn Fe, Fe-2Mn, Fe-3Mn, Fe-4Mn, Fe-5Mn). The main physico-chemical characteristics determined for sintered steels alloyed with manganese in the paper refer to the apparent density and fluidity mixtures, but also the dimensional changes, respectively the mass variation in sintering. Also in this paper it is presented the microstructure of sintered steels Fe-Mn, with contents of 1÷5% Mn.


Introduction
As a result of the analysis of the initial stage of the research related to obtaining sintered steels with a low level of manganese alloy and especially the advantages and disadvantages of making prealloyed manganese powders [1,3], in this work we used manganese as the alloying element in ferrous sintered products and we chose to introduce it as a mixture of unalloyed dust core and ferromanganese powder.The way in which it is alloyed with manganese significantly determines the conditions of its difusion within the iron crystal lattice, thus influencing the structure and, implicitly, the properties of the sintered pieces.
The possibility of improving compressability of the mixture of iron powder and ferromanganese powder as compared to that of the prealloyed manganese powder is based on a decrease of deformability of the particles of the latter because of the significant hardening effect of manganese over the ferrite [4,5].
Specialized texts fail to mention the existence of any study concerning the possibility of getting prealloyed powders of the Fe-Mn-C type with a manganese level beyond 4% [6,8].

Materials and Methods
The experimental research we presented in this work aimed mainly at determining the influence of the manganese and carbon levels over the physical and chemical characteristics of iron-based sintered products and at emphasizing the possibility of using manganese as the alloying element in sintered steels.As a means of introducing manganese into the structure of the steel we chose to mix the iron powder with the ferromanganese powder.In order to blend the carbon concentration into the mixture we chose a brand of manganese having a low carbon level (< 0,1% C).
The materials we used for the experiments were: Taking into consideration the favorable effect of cold hardening the powder has over the sinterability of powder mixtures [9,10], we ground the fine ferromanganese powder (< 40 µm) with the granulometric composition in Table I (delivery state) in a grinder.
In order to prevent the ferromanganese from oxidizing during the grinding process, we introduced argon in the grinding barrels.After a relatively brief grinding time (10 min.)we noticed the fact that the ferromanganese powder becomes pyrophoric, as a result of its specific surface increasing because of the reduced dimension of its particles and the high compatibility between the manganese and the oxygen, a phenomenon which was emphasized for both manganese and ferromanganese powders [11,12].Elaborating and handling potentially pyrophoric powders involve taking appropriate actions in order to prevent their self-ignition.These actions consist of using grinding lubricants which create protective layers or post-grinding chemical passivation (controlled superficial oxidation).In our research, in order to avoid pyrophoricity resulting from grinding ferromanganese powders and to obtain a fine powder, with a minimal oxygen level, we used zinc stearate of 3% concentration as a grinding lubricant.
We thus avoided the need to apply any further passivation resulting from controlled superficial oxidation [13].Due to the addition of the zinc stearate, we were able to handle the powder we obtained after 20 min of grinding without any particular issues.The average size of its particles was below 40 µm, and they consisted of aggregations of very fine particles (Fig. 1).
We pressed the pills in an uniaxial, bilateral way by applying a static force (the accuracy of measuring the pressure force was ± 5daN), in a matrix made of hardened steel, with a cavity aperture of 11.28 mm, corresponding to a 1 cm 2 section.The pressing pressure was between 100 MPa and 800 MPa.We used 7 tests (pills).
We used formula (1) to determine the concentration of the manganese powder mixed with the iron powder used for elaborating the weakly sintered manganese-alloyed material (1÷5)% Mn, based on the material balance sheet: where: [EP] is the concntration of the powder with the bearing element (Fe-Mn) within the [Fe+(Fe-Mn)] mixture, [%Mn] -the nominal concentration of the main alloying element (Mn) within the sintered material, [%Mn] FeMn -the concentration of the main alloying element (Mn) within the powder in which the iron is the bearing element (Fe-Mn).
For the comparative studies regarding the was to emphasize the influence of manganese on the sinterability of iron powders and on the properties of the resulting steels, there have been various variants of mixtures, consisting of DP 200 iron powder and P40 (Fe20-Mn80) ferromanganese powder, with a nominal manganese level of 0,1, 2, 3, 4 and 5 %.
We used a Turbula space homogenizer in order to mechanically homogenize pulverous components.The enclosure had a 60 rot/min rotative speed.We allowed 15 minutes for the metallic pulverous components [Fe + (Fe-Mn)] to homogenize, after which we added 0.6 % zinc stearate as a lubricant and allowed another 20 minutes for the mixture to continue.The total amount of mixtureed powders was 200 g, which filled 1/3 of the volume of the homogenizer enclosure.We must mention the fact that, when obtaining the Fe + (Fe-Mn) ground powder mixturees in order to make the test tubes, we took into account the amount of zinc stearate we introduced during the grilling process, by appropriately decreasing the amount of zinc stearate we normally add.
The tubes sinterizing was made at a 1150 °C, and 1250 °C, respectively.We used a Pruffer sinterising oven, with a fireproof bell mouth, with automated temperature adjustment at soaking durations of 60 and 120 minutes.We used hydrogen of minimum 99.999 % purity level, with its dew point at 40 °C as a lubricant.The test tubes chilling speed was approximately 30 °C/min.In order to prevent the test tubes from decarbonizing during the sinterizing process because of their interaction with the protective atmosphere, we covered them with a "trap" mixture of alumina and 1 % graphite.

Results and discussion
The aim of the study regarding the sinterability of Fe-Mn-C materials was the influence of the manganese level in the mixture of iron powder and ferromanganese powder.The chemical composition, apparent density and ability to flow of the pulverous mixtures are shown in Tab.II.
In order to study the reaction of powder mixtures to the pressing process we increased the variation curves for density and tightness of the pressed pills according to the compacting pressure.In Tab.III we stated the values of the experimental results on the basis of which we incresed the compressibility curves, and the influence of the compacting pressure on the density of the compacted pills resulting from powder mixtures Fe+(Fe-Mn) in Fig. 2. The fact that the radial dimension of the pressed pills along with the manganese concentration of the powder mixture is due to a lower level of deformability of the ferromanganese powder and implicitly of the Fe + (Fe-Mn) powder mixtures.

Tab. II
The experimental research also aimed at the influence of the manganese concentratio (0 ÷ 5%) on the sinterability of the pressed pills resulting from iron powders, at different temperatures and at different sinterizing durations.As we well know from classic metallurgy, low manganese alloyed steels are construction improvement materials.Due to this aspect we also studied the influence of adding 0 ÷ 1.25 % graphite on the sinterability of the 2 % manganese alloy.We established the sinterability of the alloys we studied by studying the density variation and by determining the size contraction during the sinterizing process.
The value of the radial size contraction of the 600 MPa pressed test tubes.sinterized in hydrogen.at temperatures ranging between 1150 °C and 1250 °C.for 60 and 120 minutes. is presented in Tab.IV.
After analyzing the information in Tab.IV we can notice the fact that the increase in the manganese level coincides with a significant change in the values of radial size variations of the alloys we studied during the sinterizing process.
Thus. while there is barely any significant influence in the case of a manganese level of up to 1%. this influence significantly increases in case of manganese levels of up to 3 ÷ 5.The radial size variation of the sintered tests is also sensibly influenced by the increase in temperature and sinterizing time.
The size changes of the tests during the sinterizing process are directly influenced by their mass variations.resulting from the manganese evaporation and devaporation processes.by the reduction of the iron oxides and the elimination of the pressing lubricant.
The effects of manganese over the microstructure of the sintered steels we studied.resulting from Fe+(Fe-Mn) powder mixtures are emphasized by some typical examples in Fig. 3 and Fig. 4. We must mention the fact that, although these alloys contain no carbon, there is a certain amount of it (0.01 ÷ 0.06 % C) in their structure because of its presence in the ferromanganese and in the iron powder.The more the manganese concentration within the alloy increases, the larger the pearlite concentration becomes, and the finer its structure becomes.In the case of alloys consisting of 4 -5 % manganese, besides large areas of alloyed pearlite and ferrite, there also appear some acicular bainite elements (Fig. 4).The determination of the microhardness values of the structural constituents of the alloys we studied confirmed the fact that the presence of the manganese favors the formation of bainite structures even at lower chilling speeds.Microhardness levels of bainite elements range between 300 and 400 HV 0.02 .
As seen in Fig. 4. the increase of 100 °C determines.besides a more obvious pore nodulizing.the increase in the manganese difusion layer within the iron particles.Even in the case of those alloys which were sintered at 1250 °C, the structure of the material remains heterogenous.Within the large iron particles.in the ferrite areas microhardness levels range between 145 and 166 HV 0.02 , while in the pearlite areas on the outside of these particles, these values range between 218 and 243 HV 0.02 .
In order to make a more detailed structural analysis we present in Fig. 5 the structure of the 2 % manganese steel, consisting of ferrite and and of small uniform areas presumably consisting of an α solid solution.Fig. 4. Microstructure of some Fe-Mn sintered steels, with manganese contents of 1÷5 %. at 1150 ºC and 1259 ºC, (Compacting pressure 600 MPa sinterizing 60 minutes Hydrogen, dew point -40 ºC).Attack: 3 % Nital.

Conclusions
-The increase of the manganese level determines a decrease in apparent density and in the ability to flow (fluidity) of powder mixtures.This effect is due to the increase, within the mixture, of the ferromanganese powder of fine druse and large specific area, and it is well known that fine powders (< 63 µm) have a lower apparent density and ability to flow.-Adding manganese (ferromanganese) to the iron powder determines a decrease in density of raw compacted pills.The negative influence of the ferromanganese being present in the iron powder must be accounted for by the low deformability of the ferromanganese powder, its high hardness level and the large specific area of the mechanically ground powder.-The decrease in the tightness of the pressed pill coincides with an increase in the amount of ferromanganese we added to the mixture, and this decrease is more significant than that of the density of the pressed pills and results from the decrease in both raw density and theoretical density of the mixture as a result of the fact that the manganese density (7.44 g/cm 3 ) is smaller than the iron density (7.875 g/cm 3 ).The decrease in tightness is significant when subjected to compacting pressures of up to 500 MPa.At a 800 MPa compacting pressure the 1% increase in the manganese level determines a decrease in the tightness of the pressed pill by approximately 0.9 %. -The increase in radial size and mass losses during the sinterizing process, along with the increase in the manganese level lead to a decrease in density of the sintered tests as compared to the raw ones.Thus, in the caase of a 5 % manganese alloy sintered at 1150 °C for 60 min.The density is decreased by up to 3 %.The variation of the density values for the sintered tests is similar to that of the density values of the pressed pills, in the sense that their decrease coincides with an increase in the nominal manganese concentration within the alloy.-By using an electronic scanning microscope to examine the microstructure of the steels we studied we noticed he rpesence of certain white areas, of micrometrical dimensions, standing out of the basic matrix, therefore harder, which we assumed to be martensitic areas, respectively α solid solution of iron-manganese cubic martensite.-Dimensions standing out of the basic matrix therefore harder which we assumed to be martensitic areas respectively α solid solution of iron-manganese cubic martensite.
Chemical composition, apparent density and fluidity of powder mixtures of iron and ferro-manganese (0.6 % zinc stearate).Tab.IV Influence of the sinterizing parameters with hydrogen with -40 °C dew point on the size variation mass variation and sinterizing density of the Fe-Mn alloys (600 MPa compacting pressure).
Tab. III Raw Density, g/cm3, of pressed pellets of mixtures of powders studied in the pressure of compaction.Fig. 2. Influence of the compacting pressure on the density of the compacted pills resulting from powder mixtures Fe+(Fe-Mn)