manganese in ferrous powder metallurgy · ellingham diagram). they refer to chemically pure...

Powder Metallurgy Progress, Vol.1 (2001), No 1 41

MANGANESE IN FERROUS POWDER METALLURGY

A. Šalak, M. Selecká and R. Bureš

Abstract The thermodynamic analysis of Mn-O, Mn-O-H and Mn-O-C systems, important for sintering Fe-Mn-C PM steel, and the conditions for reduction and conversion in these systems was performed. The effect of dry milling of electrolytic manganese and of ferromanganese on oxygen content is given. The data about the sublimation of manganese during sintering and its consequence on sintering and alloying, and the properties of manganese steel are presented. The self-cleaning effect of manganese for sintering atmosphere, as a combined effect of high affinity of manganese for oxygen, and of high vapour pressure as a basic condition for sintering of manganese steel, are given. Experimental examples concerning the effective sintering and alloying of Fe-Mn-C steel in α-iron area and under industrial conditions, in spite of the thermodynamic requirement for sintering atmosphere with a very low dew point, are given. Keywords: manganese steel, thermodynamics, oxygen, sublimation, sintering

INTRODUCTION Manganese is a basic and alloying element in wrought structural and austenitic

manganese steels. Manganese has a high hardenability and is much cheaper than common alloying elements in powder metallurgy. In spite of this, manganese in powder metallurgy is generally not used as a common alloying element. Manganese could be a suitable substitute for carcinogenic and allergenic nickel and copper due to the recycling problems. Sintered manganese steel is possible to prepare from admixed powders only. The water atomised and in hydrogen annealed Fe-2.9Mn-0.3C powder exhibited microhardness of 245 HV0.02 [1]. Since the year 1948, about 250-300 papers concerning sintered manganese steel in journals and in conference proceedings have been published. The commercial application appears restricted to <1.5% Mn alloys [2].

During this time period, practically all physical and technological factors with possible effect on properties of sintered manganese steel were investigated. These were the iron powder grades, manganese carriers and amounts added, special low melting and carbide master alloys, compacting pressure and methods, sintering temperature and time, different sintering atmospheres with different dew point etc.

High affinity of manganese for oxygen and consequently, difficult reducibility of the oxides was mainly considered as a problem for sintering manganese steel parts. The requirement for sintering atmosphere with a dew point, according to the equilibrium conditions for Mn-O system at a sintering temperature under industrial sintering conditions, was not realisable. In spite of this, in most published papers the requirements for atmosphere purity were not fulfilled and in some of them the dew point was not declared. In

Andrej Šalak, Marcela Selecká, Radovan Bureš, Institute of Materials Research of SAS, Košice, Slovakia

Powder Metallurgy Progress, Vol.1 (2001), No 1 42 reality, the sintering of investigated manganese steel was successful, in cases when the dew point of the atmosphere was higher than required, as well.

Nevertheless, the tensile strength values of Fe-Mn-C systems up to ~900 MPa corresponding with other mechanical properties were obtained as shown in Table 1. The presented data are ranged in the time sequence of publishing, and the highest tensile strength value obtained under given conditions as a criterion was selected. The basic factors, affecting the properties of sintered manganese steels in a proportional extent, including the dew point of the sintering atmosphere, were included in the data. The high tensile strength values were obtained also in the cases when the dew point of the atmosphere was not adhered to. It means that many other factors affect the mechanical and toughness properties of sintered manganese steel to a greater extent then the purity of the sintering atmosphere.

In this paper, the main physico-chemical data of manganese – oxygen system, and some experimental data concerning the sintering and properties of manganese steels in an atmosphere of common purity, are summarised.

Tab.1. The highest tensile strength and other properties of sintered manganese steels obtained under given experimental conditions presented in mentioned references. Compacting pressure 600 MPa. a - properties of Fe-Mn-C alloys; b - experimental conditions

a)

Property No. Alloy Fe- Sintered density

[g.cm-3] Rm

[MPa] TRS

[MPa] Hardness A

[%] KC [J]

1 4Mn-0.8C 6.95 670 - 260 HV - - 2 5Mn-0C 6.60 270 610 - - - 3 5Mn-0C 6.80 360 720 - - - 4 6Mn-0C 6.79 600 - 90 HRB 3.2 - 5 2Mn-0.8C 6.97 585 - 92 HRB - - 6 4.5Mn-0.4C 7.12 886 - 179 HV 4.0 21 7 5.5Mn-0.45C 6.93 675 - 155 HV 0.8 17 8 4.5Mn-0.4C 6.62 565 - 157 HV 10 9 3Mn-0.8C 7.10 750 - 215 HV 2.3 -

10 2Mn-0.3C 7.11 495 - 117 HV 6.2 47 11 4Mn-0.3C 7.01 966 - 275 HV 2.5 - 12 3Mn-0.25C 7.00 590 - 190 HV 2.0 - 13 4Mn-0.8C 6.80 480 932 170 HV - - 14 3Mn-0.8C 6.66 527 1091 239 HV 0.6 5.6 15 3Mn-0.8C 6.35 445 1016 193 HV 0.3 5.6 16 4Mn-0.3C 7.06 644 1113 162 HV - 11.6 17 1Mn-0.1C 6.80 335 - 100 HV - 26 18 3Mn-0.9C 6.94 652 1184 148 HV - 12 19 3Mn-0.9C 6.92 706 1231 169 HV - 12 20 2Mn-0.4C 7.29 1570 - 550 HV 2.0 -

Powder Metallurgy Progress, Vol.1 (2001), No 1 43 b)

Experimental No. Sintering [°C/min]

Atmosphere; Dew point [°C]

Powder grade Mn - Carrier

Ref.

1 1200/240 H2; getter Hametag MnO2 3, 4 2 1100/60 H2 MH100.24 E 5 3 1300/60 H2 MH100.24 E 5 4 1200/50 c. a.; getter Atomet 28 E 6 5 1200/50 c. a.; getter Atomet 28 E 6 6 1120/180 c. a.; -30 Hametag FeMnC 7 7 1120/180 c. a.; -30 RZ FeMnC 7 8 1300/1, HF H2 Hametag FeMnC 8 9 1150/60 H2; -40 Carbonnyl E 9

10 1160/90 H2; getter ASC100.29 E 10 11 1280/75 H2; getter ASC100.29 E 10 12 1180/60 H2; -30 NC100.24 FeMn 11 13 1180/60 H2; <-40 NC100.24 FeMn 12 14 1200/60 H2; -40 NC100.24 FeMn 13 15 1200/60 H2; -60 NC100.24 FeMn 13 16 1120/120 c. a.; -33 ASC100.29 FeMnC 14, 15 17 1120/60 c. a.; -33 SC100.26 FeMnC 15 18 1120/90 c. a.; -33 SC100.26 FeMnC 16 19 1120/90 c. a.; -33 NC100.24 E 16 20 1250/60 20H2/80N2 Carbonnyl FeMn 17

Iron powder grade: Hametag - eddy milled; RZ - air atomised. Manganese carrier: FeMnC - high carbon ferromanganese, FeMn - medium carbon ferromanganese, E - electrolytic manganese. HF - high frequency sintering; c. a. - cracked ammonia. No.20 MIM, heat treatment

PHYSICO-CHEMICAL REGULARITIES OF Mn-O SYSTEM

Thermodynamics of Mn-O system Manganese oxides are formed in air at 400 to 1200°C according to parabolic law.

The Mn3O4 and MnO exist at a temperature higher than 800°C [18]. These Mn-oxides, in terms of the sintering of manganese steels, are of the greatest importance.

The basic reaction for manganese and oxygen is given by the equation: 2Mn + O2 = 2MnO (1)

All considerations concerning the reducibility of manganese oxides during the sintering of mixed manganese steel are related to MnO only. Equilibrium conditions for this reaction in the presence of oxygen and in the H2/H2O mix are shown in Table 2.

Tab.2. Equilibrium data for Mn/MnO in O2 and in gas mix H2/H2O (Ellingham-Richardson diagram) [19]

Temperature [°C] 600 700 800 900 1000 1100 1200

PO2 [Pa] 10-33 10-29 10-25 10-24 10-19 10-17 10-15

H2/H2O 6·107 7·106 106 4·105 105 5·104 104

Dew point [°C]

-102 -90 -77 -70 -60 -54 -40


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Predominance Diagram for Mn-H-O System

Mn

MnO

MnO2

Mn2O3

Mn3O4

Fig.1. Predominance diagram for Mn-H-O system [20]

Fig.2. Graphical determination of the equilibrium dissociation pressure PO2 for FeO and MnO at 1120°C [21]

It follows from these data, that hydrogen containing atmospheres, to prevent the oxidation of manganese or to provide a reduction of MnO, should have a dew point lower than given in Table 2. It would be very hard to fulfil these requirements for the purity of the sintering atmosphere of manganese steel, namely under industrial conditions. Figure 1 illustrates these data for Mn-H-O system as a dependence of partial oxygen content on temperature. These data are complemented also by Fig.2, the equilibrium dissociation oxygen partial pressure for FeO and MnO at 1120°C. Figure 3 indicates that neither endothermic gas, dissociated ammonia nor N2 - based atmospheres, at different dew points, can reduce the formed oxides or avoid a further oxidation of manganese or chromium at 1120°C, which is the common sintering temperature.


Fig.3. Oxygen potential in endothermic gas, dissociated ammonia and N2-based atmospheres at different dew points for chromium and manganese (indicated temperatures refer to the dew point of the atmosphere) [22]

It is necessary to note that all mentioned reactions for Mn-O-H systems from the thermodynamic point of view are calculated for a unit activity of the system (Richardson – Ellingham diagram). They refer to chemically pure compounds including hydrogen. In the real systems, the chemical activity of e. g. ferromanganese grades is much lower because they contain different impurities, mainly iron. The places in the lattice of MnO can be substituted by FeO and vice-versa (unlimited solid solubility).

Shown in Figs. 4 and 5 are the phase stability diagrams, elaborated on the basis of thermodynamic analysis for reduction and conversion reactions of manganese oxides with H2(g), CO(g) and C(s) for Mn-O-H and for Mn-O-C system.

It follows from this analysis: − reduction of MnO→Mn by neither H2(g), CO(g) nor C(s) occurs below 1280°C − reduction of MnO is possible only with C(s) at temperature above 1280°C − reduction both of Mn2O3→Mn at >1000°C and of Mn3O4→Mn at >1100°C with C(s)

can take place − reduction of MnO(g) in H2(g) and CO(g) can take place.

According Ref. [23], the reduction of MnO (the most stable manganese oxide) to Mn occurs with carbon-graphite (solid) at a temperature over 1200°C through the Mn-carbide (Mn5C2) as follows from the overall reduction reactions for this process:

5MnO + 7C = Mn5C2 + 5CO (2) 2MnO + Mn5C2 = 7Mn(g) + 2CO (3)


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Fig.4. Mn-O-H phase stability diagrams at 800, 1200 and 1300°C [20]

Fig.5. Mn-O-C phase stability diagrams at 800, 1200 and 1300°C [20]

During the sintering of manganese steel, the conditions for this reaction are not formed because at any sintering temperature over 900°C, solid carbon in a sintered manganese steel part could be present. From this point of view, if the manganese particles in the compact are the carrier of the Mn-oxide (oxide surface film), this oxide cannot be MnO since under sintering conditions used it is not reducible.

Manganese vapour pressure and sublimation One physical properties of each element is vapour pressure in dependence on the

temperature. The vapour pressure of manganese has a decisive effect on sintering and alloying manganese steel [24]. Validity for α and δ manganese in dependence on temperature can be expressed by the equation:

log p = A·T-1 + B·logT + C·T + D [Pa] (4)

Powder Metallurgy Progress, Vol.1 (2001), No 1 47 where A, B, C, D are constants (-14920; -1.96; 0; 18.32) for manganese, T is temperature (K), p vapour pressure (Pa) [25].

The calculated manganese vapour pressure values and for some others used in powder metallurgy alloying elements are given in Table 3. The vapour pressure values of manganese at the temperatures used for sintering metal powder parts by solid state diffusion, are highest compared with others in powder metallurgy mostly using alloying elements. The manganese vapour volume generated by sublimation from all manganese particles contained in a 1 cm3 compact, and the time for manganese sublimation from 1 particle size 15 μm is given in Table 4. The consequence of this phenomenon is that already during the heating period the sublimation starts, it means the formation of manganese vapour from the manganese particles in the compact.

Tab.3. Vapour pressure values for Mo, Ni, Fe, Cr, Cu and Mn [26]

Temperature [°C] 900 1000 1100 1200 1300

Element

Vapour pressure [Pa] Mo 2.03·10-17 4.07·10-15 3.73·10-13 1.85·10-11 5.66·10-10

Ni 3.95·10-6 1.17·10-5 2.11·10-4 2.68·10-3 2.25·10-2

Fe 2.99·10-6 6.47·10-4 8.85·10-3 8.40·10-2 5.93·10-2

Cr 1.08·10-5 2.36·10-4 3.26·10-3 3.13·10-2 2.24·10-1

Cu 4.23·10-4 6.10·10-3 5.94·10-2 4.23·10-1 2.33·100

Mn 3.80·10-1 3.23 19.88 98.69 366.70 Vapour pressure of manganese at 600°C is 2.88·10-5, at 700°C 1.33·10-3 and at 800°C 2.95·10-2 [Pa]

Tab.4. Volume of manganese vapour generated by sublimation from all manganese particles contained in 1 cm3 compact Fe-3% Mn (0.21 g Mn; density of 7.0 g.cm-3) and the time of manganese sublimation from 1 particle size 15 μm at different temperature [27]

Temperature [°C] 600 700 800 900 1000 1100 1200

Manganese vapour volume [cm3]

270 303 336 366 399 429 465

Time of sublimation [s] 5.7·103 1.3·102 6.1 0.5 0.06 0.01 0.002

It is evident from the given data that the manganese vapour volume, generated by manganese particle sublimation in the compact to be evaluated, is considerably higher than the pore volume in the compact. The sublimation of manganese at ~700°C was technically proved. It is the temperature at which the interparticle necks in the compact start to form at an observable rate [28]. The sublimation of manganese from fine particles at a higher temperature takes place almost immediately. The values calculated for pure manganese are, of course, higher than the real values when using ferromanganese or an other master alloy because of lower activity of the material system.

Manganese vapour formed in dependence on temperature as shown before, firstly fills in the compact's open pores, and simultaneously condenses on the surface of all iron particles. The iron powder particles become then alloyed by manganese, starting from the surface, by all solid state diffusion mechanisms in dependence on the substructure and physical properties of iron powder and sintering conditions.


Fig.6. Microstructure of high frequency sintered Fe-4Mn-0.3C steel. Air atomised powder RZ, particles size <0.16 mm. Compacting 600 MPa; sintering 1100°C for 1 min, hydrogen, heating rate ~1000°C/min. Nital etched

Fig.7. Microstructure of conventionally sintered Fe-4Mn-0.3C steel. Air atomised iron powder RZ, particle size <0.10 mm. Compacting 600 MPa; sintering 1100°C for 10 min, hydrogen. Nital etched

The consequence of the condensation of manganese vapour on the surface of iron particles in a compact is shown in Figs.6 and 7. On the surface of all iron particles a continual layer alloyed with manganese and carbon was formed. The diffusion of manganese into the interior of the iron particle occurs from the total surface, not only from contact point of a starting manganese particle with an iron particle. The microhardness of the alloyed layers was in the range of 250 to 400 HV0.05, which corresponded to pearlite and bainite. The microhardness of ferrite was in the range of 150 to 190 HV0.05. It is an example of solid - gas alloying system.

Self-cleaning effect of manganese for sintering atmosphere A portion of manganese vapour escapes during sintering from the compact into the

surrounding sintering atmosphere where it reacts with the oxygen. The missing manganese from the sintered parts was confirmed by the manganese weight loss as given in Table 5.

The product of this reaction is a very fine dispersed oxide MnO as shown in Fig.8. By this reaction, the high vapour pressure of manganese (physical property) and high affinity of manganese for oxygen (chemical property) according to the equation (1), is expressed. The equilibrium conditions for Mn–O system over parts being sintered by this reaction, as self-cleaning effect of manganese in vapour form for the sintering atmosphere, are formed.

Tab.5. Manganese loss [% mass] in tension test bars compacted at 590 MPa and sintered at 1120°C in cracked ammonia and at 1200°C and 1300°C in a vacuum (1.33 Pa) under laboratory conditions. Manganese carrier - high carbon ferromanganese [29]

Sintering conditions [°C/h] Manganese added [% mass] 1120/3

cracked ammonia 1200/2 vacuum

1300/2 vacuum

2.5 0.37 - 0.62 0.19 0.20 3.5 0.41 - 0.62 0.56 0.60 4.5 0.60 - 0.68 1.06 1.38 5.5 0.84 - 1.28 0.62 0.35


Fig.8. The partial pressures of sublimating manganese in flowing hydrogen under isothermal conditions as a function of the distance from the part surface: A – the presence of an inactive gas; A1, A2 – the presence of oxygen in the gas oxidising the manganese vapour in very fine dispersed oxide MnO; B1, B2 - the corresponding oxygen partial pressure gradients. Fluxes are presented schematically below [27]

Fig.9. Scheme of a double steel box for sintering of PM parts under getter. 1 – basic box, 2 – cover box, 3 – parts, 4 – getter powder (alumina + 5% graphite)

If sintering in an atmosphere with a low oxygen partial pressure is performed, the

reaction Mn(g) with O, in a greater distance from the surface of the parts, occurs. If sintering in an atmosphere with high oxygen partial pressure is performed, the reaction under the formation of a green film – MnO on the surface of parts can take place.

The formation of MnO by this reaction was proved by the sintering of the Fe-Mn-C samples in a double steel box under a getter mix as shown in Fig.9. The inner and outer walls of the cover steel box above the getter mix surface were covered by a green film, which is MnO (pure chemical compound).

SINTERING OF Fe-Mn-C STEEL

Oxygen in Fe-Mn-(C) compacts Up to this time, the problem of sintering manganese steels is joined predominantly

with oxygen in the starting manganese powder and with its possible oxidation during sintering in the compacts by oxygen in the sintering atmosphere.


The basic and most problematic carrier of oxygen content is starting manganese powder. Manganese as elemental (electrolytic) or in the form of ferromanganese, also after crushing by a producer and storage for a long-time at an ambient temperature and humidity, remains silver-grey, without a measurable surface oxide film. The reaction Mn with O2 at an ambient temperature is very slow, similar to that for Cr-O system.

The change in the appearance of a manganese carrier from silver-grey to black occurs during dry milling in a ball or vibration mill to fine powder at a temperature not exceeding 30-40°C. The milling of manganese carrier to fine powder, which corresponds to crushing or cleavage under the formation of new oxygen free metal surfaces, is the source for oxygen content increase with decreasing particle size (specific surface) as shown in Table 6. The mean oxygen content in high and medium ferromanganese and electrolytic manganese milled in air and in nitrogen (<40 μm) on the basis of many analyses (LECO and BALZER apparatus), was in the range of 0.30 to 1.4%. The oxygen content in fine manganese powder (<40 μm) is affected by specific surface and therefore by particle size distribution in dependence on milling conditions. The oxygen content of high carbon ferromanganese powder (<40 μm) milled in air and stored for 22 years in a plastic vessel (¾ full, air), was 0.90 %. The oxygen content of ~1% in fine manganese or ferromanganese powder milled in nitrogen or in air, can be accepted as a characteristic. It is possible to store the manganese powder in a commonly closed vessel in air.

Tab.6. Oxygen content in medium carbon ferromanganese (1.1%C, 84.1% Mn) crushed or milled in a ball mill under nitrogen in dependence on the particle size (apparatus LECO TC-336, AGH Krakow)

Particles size [μm] *300-500 <100 160-100 100-63 63-45 <45 Oxygen [% mass] 0.08 0.64 0.17 0.25 0.68 1.32

*single pieces – crushed. No chemical standard for the analysis is available.

Only a weak reflection of MnO2 on the surface of air-milled ferromanganese particles (<45 μm) by X-ray-diffraction analysis was detected. It is necessary to presume that the black coloured film on the milled manganese particle surface is formed also by abrasion material coming as wear product from milling steel bodies and, in a small portion, from hard manganese carrier being milled. The MnO oxide is green and the milled manganese powder is black. Therefore MnO cannot form the black film on the manganese powder surface. There are no data in literature about the real existence of MnO on fine manganese powder used for alloying in powder metallurgy. From this point of view, the thermodynamic analysis of mentioned systems should be oriented to the stable Mn3O4 oxide. It will be necessary in advance to analyse the real manganese oxide form existing in Fe-Mn-C systems in starting state and under the sintering conditions used.

Iron powder as prevailing part of the iron – manganese mixture brings to the powder system a decisive content of oxygen (~0.15-0.20%). Manganese in a form of medium ferromanganese in Fe-4% Mn powder mixture increases the oxygen content by 0.05% to ~0.25%.

Sintering of Fe-Mn-C steel in Fe-α phase Shown in Fig.10 is the microstructure of a sample which demonstrates the alloying

of the iron powder matrix by manganese at 875°C. A layer alloyed by manganese and by carbon on the surface of all iron particles in the compact of the same character, as shown in Figs.6 and 7, was formed. Grain boundary diffusion of manganese in a larger iron particle

Powder Metallurgy Progress, Vol.1 (2001), No 1 51 was recorded. The interior of the iron particles was ferritic. This character of the alloying of iron powders by manganese gas phase at sintering at higher temperature was also stated in Ref. [31]. It is proof that under a given extreme low sintering temperature the manganese and iron particles in the compact were not covered with an oxide film. The reduction of oxides on the starting iron and manganese particles in the compact, at lower temperature than used in this case, proceeded in cracked ammonia. The oxidation of manganese particles in the compact was not observed in spite of the fact that the dew point of the sintering atmosphere did not fulfil the requirement according to equilibrium condition (-70°C) for Mn-O at the temperature used.

The tensile strength of manganese steels, based on different iron powder grades, sintered at 875°C for 30 or 80 min in cracked ammonia is shown in Fig.11. These data show also an increase in tensile strength for all samples, compared to the tensile strength of pure iron sintered at 1120°C, which is about 180 MPa. The differences in tensile strength values for iron powder grades used, are the consequence of their shape, substructure and microstructure properties caused by the production method and by manganese vapour alloying [7].

These and previous data confirm that the sintering and alloying of Fe-Mn-C steel at this low temperature proceeded because the oxides on the starting iron and manganese particles in the compact at lower temperature were reduced in cracked ammonia.

Fig.10. Microstructure of a Fe-4Mn-0.3C sample sintered at 875°C for 30 min in cracked ammonia (dew point -30°C). Compacting at 600 MPa. SC100.29 iron powder [30]. Nital etched.

Fig.11.Tensile strength of Fe-4.5Mn-0.3C steels sintered at 875°C in cracked ammonia (dew point -30°C). Compacting at 600 MPa. High carbon ferromanganese. Iron powder grade: 1 - Hametag (eddy-milled), 2 – RZ (air atomised), 3 – ASC100.29, 4 – SC100.26, 5 – NC100.24. 1 and 2 sintering for 80 min, 3 to 5 sintering for 30 min [30,32]


Sintering in continuous furnace The heating for all sintered powder systems is a very important period of the

sintering process for the final properties of the material. The formation of the interparticle necks and the sublimation of manganese start during heating in α-iron area. As follows from the thermodynamic data, to prevent the oxidation or to reduce the MnO during this period, a sintering atmosphere with the dew point <-100°C should be used. By using a sintering atmosphere with the dew point of -60°C during heating up to the temperature 1000°C, it would be not possible to prevent partial oxidation of manganese particles.

It is necessary to distinguish between laboratory (stationary) and industrial (continuous) sintering conditions. When sintering in a laboratory sintering furnace, in spite of the mentioned requirement for the purity of the atmosphere, during the whole process an atmosphere with a constant dew point is supplied. The sintering of parts in an industrial furnace proceeds with a gas counterflow. It means that at the entry of a steel box with green parts into the furnace there is a gas outlet. The flowing atmosphere in this direction is continually contaminated by the reduction products (H2O, CO2) coming from the previous boxes with parts being sintered. This atmosphere is less pure than at the sintering temperature in the middle part of the furnace and less than at the inlet. The manganese particles in the compacts should be even more oxidised compared with the laboratory conditions. The sintering atmosphere used enters into the furnace at the exit of sintered parts from the furnace. The cooling period of the Fe-Mn-C parts is in an another state. The activity of the e.g. (2-4)% Mn alloyed sintered steel, is lower compared to the activity of the manganese carrier particles in the compacts during heating.

Microstructure of sintered Fe-Mn-C steel The formation of the microstructure of sintered steels contrary to ingot steels is a

process of solid state diffusion of alloying elements into iron and of their mutual reaction in dependence on many factors.

Iron powder grade is one of the factors causing the heterogeneity of the microstructure of sintered manganese steel. Shown in Fig.12 is the ferrite-bainite microstructure of the Fe-2Mn-0.17C steel without any manganese oxide network. The different portion of ferrite in the microstructures in dependence on iron powder grade, and herewith on production method and on particle size, is demonstrated.

A characteristic microstructure of sintered Fe-3.5Mn-0.7C steel with higher manganese and carbon content is shown in Fig.13. A detailed micrograph of this sample part is shown in Fig.14. The interior of some large grains, which corresponds approximately with the size of the larger starting iron particles, is ferritic. The alloyed layer of a thickness of 8 to 20 μm on the surfaces of these ´iron particles´ was formed. The mean microhardness value of the interior ferrite area (1) was 147, of the surface pearlite area (2) 221 and of the bainite area (3) 391 HV0.025, and in small areas in the interior of the sample 785 HV0.025. A new sharp grain boundary between the alloyed layer and the interior was formed. The comparison of the ternary C-Fe-Mn isotherm and vertical section diagrams shown in Figs. 15 and 16, with the presented microstructure constituents in sintered manganese steel, clearly show the differences.


Iron powder grade:

a) NC100.24

b) SC100.26

c) ASC100.29

Nital etched.

Fig.12. Microstructure of Fe-2 Mn-0.17 C steel. Compacting at 600 MPa; sintering 1120°C for 60 min, cracked ammonia, dew point -30°C. High carbon ferromanganese.

Fig.13. Microstructure of sintered Fe-3.5Mn-0.7 C steel component (as-sintered 3.47% Mn-0.51% C). Weight 98 g. Industrial sintering 1180°C for 40 min, cracked ammonia, dew point -30°C. Iron powder SC100.26. Medium carbon ferromanganese. Nital etched.


Fig.14. Microstructure of the sample as in Fig.13. Arrows: non-metallic inclusions

Manganese content microanalyses in the mentioned areas, are given in Table 7 and corresponding points of measurement are shown in Fig.17. Ferritic areas are alloyed by manganese to 0.48 – 0.81% compared to manganese content in starting iron powder of <0.2%.

The Mn content of 2.98 % (analysis 2) corresponds to pearlite and of 4.59% to bainite (analysis 3) area. The manganese content in the sintered component was 3.47% what demonstrated a small loss of manganese.

Fig.15. Ternary C-Fe-Mn diagram. The isotherm section at constant 800°C (according Brewer, Chipman, Chang) [33]


Fig.16. Ternary C-Fe-Mn diagram. The vertical section at 4.5% Mn (according Walters, Wells) [33]

Diffusion of carbon into interior of analysed coarser grains was retained by

manganese in the surface alloyed layer (~3% Mn). Manganese content in carbide increases with increasing manganese content in the steel by a ratio ~4:1. The preferable faster formation of Mn3C or (Fe, Mn)3C carbide (C-Fe-Mn diagrams) in pearlite and bainite can cause this microstructure feature and the depletion of α-phase by carbon [34].

The formation of such a heterogeneous microstructure of individual grains feature, depends on particle size and form and on physical properties of the iron powder grade, on manganese and carbon content, and on sintering conditions. Mechanical properties of manganese steels presented in Table 1 are partial demonstration of it. As to e.g. friction properties of manganese steel, for the elimination of ferrite areas from the microstructure, a use of sponge or atomised iron powder particle size below 120 -100 μm will be preferable.

The sintering of Fe-3.5Mn-0.7C components in industrial furnace in an atmosphere with the dew point -30°C, without any additional Mn-oxide form in the microstructure, was proof that for the sintering of manganese steel, an atmosphere of common industrial purity is sufficient.

Powder Metallurgy Progress, Vol.1 (2001), No 1 56 Tab.7. Microanalysis of manganese content in sintered manganese steel microstructure constituents (TESCAN) - to the Fig.17

Point of measurement Content [% mass] 1 2 3 5-7 8 9-10

Mn 0.60 2.98 4.59 0.58 0.48 0.81

Fig.17. Micrograph of the sample as in Fig.13 with designation of the points of microanalysis of manganese. SEM


CONCLUSIONS From the study it is concluded:

• The most stable MnO is possible to reduce by solid carbon at a temperature >1280°C only.

• The existence of MnO on milled fine manganese particles used as addition for the alloying of powder manganese steel was not detected.

• Dry milling caused an increase in oxygen content in manganese powder. • The black film covering the fine manganese particles is formed by reducible iron and

manganese oxides. The sintering and alloying of Fe-Mn-C steel compacts in an α-iron area proved it.

• The sublimation (vapour formation) of manganese due to high vapour pressure already from low temperatures, is the main process affecting the sintering and alloying of Fe-Mn-C powder steel.

• The manganese vapour formed during the heating and sintering, condenses on the surface of iron powder particles in the compacts. Thus, alloying in a solid-gas system occurs.

• The self-cleaning effect of manganese for the sintering atmosphere is the result of the reaction of manganese vapour escaping from compacts being sintered with the oxygen in the atmosphere.

Acknowledgements This study was supported by the Scientific Grant Agency of MS SR and SAV

No.2/7228/20 and by the NATO Scientific Affairs Division within the framework of the Science for Peace Program (Project No. 972395).

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manganese in ferrous powder metallurgy · ellingham diagram). they refer to chemically pure...

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