A Comparison of Mechanical Properties and Microstructures of PM Steels With Chemical Compositions Fe-( 1-3 ) % Mn-0 . 8 % C

Mechanical properties of sintered steels containing 1, 1.5, 2, 2.5 and 3 %Mn and 0.8 %C, candidate materials for structural parts, are compared with actually used PM steels. Höganäs iron powder grade NC 100.24, low-carbon ferromanganese Elkem and graphite powder grade C-UF were used as the starting powders. Powder mixtures were prepared in a Turbula mixer for 30 minutes and “dog bone” compacts were single pressed at 660 MPa, according to PN-EN ISO 2740 standard. Sinterhardening was carried out in a semi-closed container in a laboratory tube furnace at 1120 °C and 1250 °C for 60 minutes in a mixture of 95%N2-5%H2. Microstructures consisted of pearlite and ferrite, sometimes bainite and martensite, depending on the Mn content. Yield, tensile and three point bend strengths and Vickers' microhardness were determined and metallographic observations carried out. The best combination of properties was for 2.5 %Mn steel: yield strength 620 MPa and 3.7 % elongation. The tensile properties of 2.5/3 %Mn-0.8 %C are not inferior to the best Ni-CrMo-Cu type PM steels in MPIF Standard 35.


Introduction
The interest in application of Mn in powder metallurgy is related to its beneficial effect on the strength properties of steel.Manganese is an alloying element which widens the austenite range by reducing the GS line and moving the ES line to the left (Fig. 1) in the Fe-Mn-C system [1].In addition, it enhances the coexistence of the three-phase austenitecementite-ferrite structure in Fe-Mn-C.This alloying element is inexpensive and easily accessible [2] and, unlike carcinogenic nickel, does not adversely affect the human body.Accordingly, it is extensively used in wrought steel products.
The first mention of the usage of manganese in sintered steel appeared in 1950 by F. Benesovsky and R. Kieffer [3], followed by, In 1968 and 1971, K. Mauer and H. Grewe [4].Mn steels were investigated in detail in 1975 by G. Zapf et al [2], who reported the best combination of mechanical properties (yield point, tensile strength) at ~6%Mn, Fig. 2. NoTab. in further PM Mn steel research are the contributions of A. Salak [5,6], who reported the selfcleaning effects of Mn vapour, as a major problem in PM production of Mn steels is brittleness, due to its affinity for oxygen.
This microstructural problem was identified by Mitchell et al [7] as continuous oxide networks, which were not produced when steels were sintered in a sufficiently clean atmosphere.Renewed interest in PM Mn steels followed, as evidenced by numerous publications of e.g.E. Dudrova, E. Hryha, A. Ciaś, A. Wronski, S.C. Mitchell and M.

Fig. 1.
The effect of manganese content on the austenite region in the Fe-Mn-C system [1].

Fig. 2.
The effect of: a) type of manganese carrier and its content on mechanical properties (yield point, tensile strength) of sintered Fe-Mn materials, b) carbon content of steels with 2%Mn,both sintered at 1280 °C [2].
Manganese is added to the powder mixture generally either as electrolytic powder or, very often, as ferromanganese.When added as a ferroalloy, however, it decreases the compressibility of the mixtures.In addition, to reduce the formation of oxides during sintering, high-purity reducing atmosphere and a low dew point [5,6] or special backfill [16] are required.Relatively recently sinterhardening has been introduced [13][14][15][16][17][18][19] and thus is worth investigating for Mn steels.Accordingly PM 1-3%Mn steels were reinvestigated, sinterhardened in a semi-closed container.The results are critically compared with conventionally sintered Ni-Cu-Mo-C materials.

Materials and Experimental Procedures
Höganäs iron powder grade NC 100.24,Elkem low-carbon ferromanganese (77%Mn, 1.3%C, particle size-below 20 µm -Eramet Norway Sauda) and graphite powder grade C-UF were used as starting powders.From the starting powders, mixtures with compositions of Fe-1.5/2.5%Mn-0.8%C), to complement data for 1, 2 3%Mn [11,12], were prepared in a Turbula mixer for 30 minutes.Following mixing, "dog bone" green compacts were single-action pressed at 660 MPa following the PN-EN ISO 2740 standard.The batch number for each processing variant was 15 samples.To minimize friction during pressing, zinc stearate was used as a lubricant and was applied on the punches before pressing each sample.The average densities of green compacts (Tab. 1) varied from 6.67 g/cm 3 (for samples contained 1 %Mn) to 6.59 g/cm 3 (for 3 %Mn samples).Following pressing, compacts were sintered at 1120 ºC and 1250 ºC (heating rate -75 °C/min) for 60 minutes in the mixture 95%N 2 -5%H 2 in a semiclosed container and cooled at a rate 66°C/min.(sinterhardening).The inlet dew point of the gas, following gas producer specification, was maintained below -60 °C.The designation, chemical composition, variants of sintering and heat treatment of the steels are presented in Tab.I.
Tab.I The designation, chemical composition, and variants of sintering and heat treatment of the steels -mean values and corrected standard deviations.

A2
Fe-1%Mn-0.8%C [16] 1250°C/SH Fe-3%Mn-0.8%C [16] 1250°C/SH 6.59±0.016.59±0.070.00 *Batch size-15 samples, SH -sinterhardened The sintered steels were physically (green and as-sintered densities -Tab.I) and mechanically (tensile, 3-point bend and apparent hardness -Tab.II and Fig. 8) tested at room temperature.Green and as-sintered densities were calculated by the geometrical method.During tensile testing, on a MTS 810 instrument, according to 10002-1 standard the crosshead speed was 1 mm/min.The 0.2 % offset yield strength was determined from the engineering stress-strain diagram.Elongation was measured using geometric method.Transverse rupture strength (TRS) was measured using ZD10-90 testing machine, following PN-EN ISO3325 standard on test samples parallel to the pressing direction.The load was applied to the surface on which the pressing punch contacted.Vickers hardness was measured on an Innovatest machine.Ten data points were taken on the length of the cross-sectional surface of the sample.Following mechanical tests, metallographic (LOM) investigations were carried out using Leica DM 4000M on areas etched with 3 % Nital.The results are the continuation of investigations [16] and were to be compared with the results of conventionally sintered 1-3%Mn-0.8%C[12][13][14][15][16][17][18][19][20].

Results and Discussion
The representative results of metallographic investigation of Fe-X%Mn-0.8%C steels are presented in Figs.4-8.The microstructure of sintered steel containing 1 %Mn consisted of pearlite and ferrite, sometimes bainite, which corresponds to average tensile strength (calculated for 15 samples) of 587 MPa ± 32 for this steel.Metallographic examination showed that microstructure of sintered steels containing 2 %Mn (Fig. 6) consisted of pearlite, bainite and martensite, which were present in the areas surrounding residual particles of ferromanganese [16].These results correspond with the CCT curves presented in Fig. 3 [13].It can be observed that bainitic or martensitic structures can be more easily obtained when Mn content is higher.It widens the range of occurrence of austenite and decreases the amount of bainite [14].Further increase of Mn content up to 3 % resulted in the creation of a more martensitic, or martensitic-bainitic, structure with addition of Mn-rich austenite.The results of mechanical properties of each batch are summarized in Tab.II and Fig. 9.For comparison purposes, data for sintered and slow-cooled alloys [12] are also presented in Fig. 9. Taking into account the Anderson-Darling and Kolmogorov-Smirnov goodness-of-fit tests, for investigated steels the number of samples in batches (15) should be satisfactory to introduce the 2-p and/or 3-p Weibull statistic to predict probability of failure of investigated steels initiated by non-interacting flaws.This analysis will be the subject of further investigations.

*Batch size -15 samples, SH -sinterhardened
There were no significant changes in density from green to sintered values (Tab.I).The densities are lower than presented in [17] and are comparable with the results published in [9].The microstructure of the steels changed with the amount of Mn: from mainly pearlite and ferrite, sometimes bainite -in Fe-1%Mn-0.8%C-through pearlitic, bainitic and martensitic, in the areas surrounding the residual particles of ferromanganese, up to martensitic or bainitic/martensitic with Mn-rich austenite for Fe-3%Mn-0.8%C.This modification is connected with the increasing influence of Mn on creation of martensitic (bainitic/martensitic) structure.
Expectedly the mechanical properties were generally somewhat better after sintering at 1250 °C (Fig. 9).The R 0.2 yield offset of sintered steel with 3 %Mn was the highest, because of the appearance of martensite in the microstructure, and hence hardness of this steel was higher than of steel containing 1-2.5 %Mn, while plastic properties were the lowest.Salak et al [17] previously reported the best combination of strength and plastic properties for manganese content of 2-2.5 %.However our plasticity was superior because of sintering in a semi-closed container [18], since, as Cias demonstrated in [13], the sintering system inhibits the formation of deleterious oxide networks.Fig. 10 presents our, slightly superior, yield strength and elongation data for sinterhardened specimens with those for similarly processed, but slow-cooled [12].It is further useful to compare all our mechanical properties results with the most demanding PM steels of MPIF Standard 35: FC-0208-60, FC-0508-60, FN-0408-55 and FLN-4205-55 with 0.2 % offset stress above 400MPa (Tab.III).For sintered steel containing 2-3 %Mn higher strength and plastic properties could be obtained than in the PM steels containing Ni, Cu and Mo.This demonstrates the possibility of replacing carcinogenic nickel and Cu with Mn structural sintered steels used today.

Conclusion
1.The increased plasticity of all our Mn steels, as compared to conventionally sintered, even in high-purity atmospheres, should be attributed to sintering in a semi-closed container.
2. Mechanical properties of sinterhardened Mn steels are only slightly superior to those of conventionally sintered.

4.
Comparison with commercial Cu and Ni steels, described in MPIF standard 35, indicates that for both sintered and sinterhardened (2.5-3)%Mn-0.8%Csteels strengths of are not inferior and plasticity superior.
5. The mechanical properties now reported are generally better than in previous publications on similar alloys.