BRITTLENESS OF STRUCTURAL PM STEELS ADMIXED WITH MANGANESE STUDIED BY ADVANCED ELECTRON MICROSCOPY AND SPECTROSCOPY

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1 Powder Metallurgy Progress, Vol.8 (2008), No BRITTLENESS OF STRUCTURAL PM STEELS ADMIXED WITH MANGANESE STUDIED BY ADVANCED ELECTRON MICROSCOPY AND SPECTROSCOPY E. Hryha, L. Nyborg, E. Dudrová, S. Bengtsson Abstract Increasing mechanical properties requirements for structural parts direct modern powder metallurgy towards improved performance by adding such expensive alloying elements as Ni and Mo. The strong tendency to price increase during last decade lead to decreased competitiveness of Ni-Mo-alloyed PM parts compared to products of conventional metallurgy. Therefore, cheaper alloying elements as Cr and Mn, commonly used in cast and wrought steels, are of great interest. However, the use of these alloying elements faces some difficulties in compaction and sintering, especially in the case of high-oxygen affinity manganese. The present study deals with reasons of brittleness of PM steels admixed with manganese as medium-carbon ferromanganese powder. Two systems with the same composition of Fe-0.8Mn-0.5C, but sintered at different conditions (different sintering temperatures, atmosphere purity and cooling rates) were studied. Specimens were sintered in 90%N 2 /10%H 2 atmosphere but of different purities (DP = -40 C and -60 C) for low and high temperature sintered specimens, respectively. The results obtained indicate that the brittleness is caused by a complex effect of microstructure heterogeneity around oxidized large-sized manganese carrier residues. The highest negative impact on the mechanical properties is produced by the weakness of the boundaries of the base matrix particles around manganese carrier residuals due to segregation and reaction product formation at these boundaries. Intensive study of inter-granular decohesion facets on the fracture surface close to admixed particles by advanced characterisation techniques (XPS, Auger spectroscopy, SEM+EDX) indicates that reaction products are mostly complex refractory oxides and manganese sulphide, the composition of which is dictated by type of the manganese carrier used and sintering conditions (temperature profile, sintering atmosphere purity, etc.). Specimen sintered at lower temperature and poorest atmosphere purity show ten times higher oxygen content and worse mechanical properties due to the large ferromanganese residuals and higher portion of the intergranular decohesion facets. Keywords: sintered manganese steels, admixing, XPS spectroscopy, Auger spectroscopy Eduard Hryha, Lars Nyborg, Department of Materials and Manufacturing Technology, Chalmers University of Technology, Göteborg, Sweden Eva Dudrová, Institute of Materials Research, Slovak Academy of Sciences, Košice, Slovak Republic Sven Bengtsson, Höganäs AB, Höganäs, Sweden

2 Powder Metallurgy Progress, Vol.8 (2008), No INTRODUCTION To decrease the price of PM products, a lot of attempts were made to develop manganese containing PM steels, which impart excellent strength properties along with low price. Extensive research by Šalak [1] indicates that due to high manganese volatility, Mn vapour transport is the dominant diffusion mechanism in the sintering process of these steels. However, research around admixed manganese PM steels [2-6] indicate large difficulties in obtaining adequate mechanical properties due to the considerable oxygen affinity of manganese. Manganese sublimates at relatively low temperatures (~720 C), when thermodynamic conditions favour its oxidation. Formed oxides hamper development of inter-particle necks, leading to weak inter-particle contacts. Another problem with manganese admixed systems is inter-granular decohesion facets around ferromanganese residues, leading to high brittleness of the obtained microstructure [2,6]. The present study investigates surface products. The composition, amount, morphology and localization of reaction products on fracture surfaces of the sintered steel admixed with Mn were evaluated by means of surface and microanalysis tools XPS, AES and SEM+EDX. EXPERIMENTAL The starting materials employed for this study were commercial water atomized iron powder ASC , medium-carbon ferromanganese powder (Erachem Comilog) with particle size under 45 μm and composition in mass % of 81.39Mn, 1.48C, 0.29Si, 0.17P and 0.01S; graphite powder UF4 and amide wax (AW) powder as lubricant. Powder mixture of composition (mass %): ASC Mn+0.5C+0.8AW was prepared and homogenized in Turbula mixer for 15 min. Cylindrical specimens (Ø10 12 mm) were uniaxially pressed at 600 MPa to a green density of about 7 g/cm 3. Sintering was carried out in a laboratory tube furnace LAC LHR A-type for 30 min in an atmosphere of 90%N 2 /10%H 2. Specimen A was sintered at 1120 C in an atmosphere with a dew point - 40 C and cooling rate of 10 C/min. Specimen B was sintered at a higher temperature of 1200 C in an atmosphere with a DP ~ -60 C and cooling rate of 50 C/min. The sintered specimens were dry machined to final dimensions ( mm) and fractured (bending) directly in ultra high vacuum prior to surface chemical analysis by X-ray photoelectron spectroscopy (PHI5500) and Auger electron spectroscopy (JEOL Jamp-10S). Analyzed area during XPS analysis was about 0.8 mm in diameter. High-resolution SEM (JEOL JSM 7000F) combined with X-ray microanalysis (INCAx-sight EDX) analysis was performed on fracture surfaces of the specimens A and B after surface chemical analysis. For microstructure evaluation light optical microscope Olympus GX71 was employed. RESULTS AND DISCUSSION Example of characteristic features of fracture surfaces for specimens A and B are presented in Figs.1-3. The fracture surfaces in both cases are rather rough, showing ductile inter-particle fracture with good pronounced lines and dimples of different size, initiated by bridge porosity and non-reduced refractory oxides on the prior particles surfaces. Small portion of trans-granular cleavage facets was registered as well, being more frequent for the high-temperature sintered specimen B. The FeMn residues are surrounded by intergranular decohesion facets with a huge amount of point inclusions on them, especially in the case of low-temperature sintered specimen A, see Figs.2-3. Auger analysis of specimen A, Fig.1, indicates high sulphur, oxygen and carbon, together with very high manganese content in all analyzed places of ferromanganese residue localization. The SEM+EDS analyses of such locations in the case of both specimens, Figs.2-3, indicate high manganese content as well. The agglomeration of such point inclusions forms such called point oxides that

3 Powder Metallurgy Progress, Vol.8 (2008), No were observed even in light optical microscopy examination of the metallographic crosssection samples displayed as networks, see Fig.4. Fig.1. Auger spectra of the ferromanganese residuum on the fracture surface for the sintered specimen A. Fig.2. SEM+EDX analysis of fracture surface of specimen A in the areas around ferromanganese residue.

4 Powder Metallurgy Progress, Vol.8 (2008), No Fig.3. SEM+EDX analysis of fracture surface of specimen B in the areas around ferromanganese residue. Fig.4. Etched microstructure of specimens A (left) and B (right). High temperature sintering leads to more homogeneous microstructure around the manganese carrier particles, consisting mostly of bainite in the case of specimen B in contrast to the complex sequence of austenite-martensite-bainite-pearlite extending up to μm in the case of specimen A. Identifying the elements present on the powder surface and their chemical state was done by means of alternating XPS analysis and ion etching (argon) as well. The XPS survey spectra, recorded at different etch depth for the specimens A and B, see Fig.5, indicate that there is no significant variation in surface composition between the two kinds of specimens. The XPS spectra show strong oxygen peak (O1s) together with manganese (Mn2p) and sulphur (S2p) in both cases. Apart from these elements carbon (C1s) was found on the surface caused by absorbed species. Narrow scans with high energy resolution, see Figs.6 and 7, show more intensive iron metallic peak in the case of specimen B, indicating smaller thickness of surface oxide layer. Manganese in both cases is present in chemical compound (oxide or sulphide) even at large etch depth, indicating presence of coarse reaction products, in accordance with the SEM observations, c.f. Figs.2-3. The sulphur

5 Powder Metallurgy Progress, Vol.8 (2008), No peak is evident at all etch depths in the XPS analysis, see Figs.6-7. The XPS analyses indicate higher oxygen content at all etch depths for specimen A in comparison to that for specimen B, see Fig.8. This suggests higher amount of oxides present in the former case. Furthermore, higher manganese and sulphur contents at smaller etch depth (up to 10 nm) were observed for specimen B, which indicates higher amount of fine manganese sulphide particles ( nm) on the surface of the high-temperature sintered specimen. Hence, oxidized ferromanganese residues create much larger defect area in the case of specimen A, see Figs.2-3. The defect size is basically related to the size of manganese carrier particle, which was the same in both cases (<45 μm). Hence, the much larger defect size in the case of specimen A is caused by the complex phenomena connected with manganese oxidation at the sintering conditions used. The manganese evaporation at strongly oxidizing conditions leads to its oxidation and condensation of the manganese oxide on the surface of the surrounding matrix particles that hamper inter-particle necks development. However, high temperature sintering at good sintering conditions (specimen B) leads to partial carbothermic reduction of the complex oxides formed. Fig.5. XPS survey scan of fracture surface of specimens A(left) and B(right) respectively. Fig.6. XPS narrow scans of Fe2p, Mn2p and S2p peaks on the fracture surface of specimen A. Fig.7. XPS narrow scans of Fe2p, Mn2p and S2p peaks on the fracture surface of specimen B.

6 Powder Metallurgy Progress, Vol.8 (2008), No Fig.8. XPS analysis of fracture surface of specimens A and B. As a result, much better inter-particle connections develop in the case of specimen B. The less good atmosphere purity and lower sintering temperature in the case of specimen A lead to continuous manganese oxidation and formation of thick oxide layer around the ferromanganese particles. Slower cooling rate at low atmosphere purity (long exposure to strongly oxidizing conditions) also results in higher oxidation. All these effects result in more than 10 times higher oxygen content in the case of specimen A in comparison with B (0.307 and wt.% respectively). CONCLUSIONS The analysis of defect areas in the admixed Fe-0.8Mn-0.5C sintered steel by SEM+EDX, Auger and XPS indicate that brittleness of studied material is caused by the weakness of boundaries of the base matrix particles around the manganese carrier residuals. The presence of complex oxides and manganese sulphide was detected on the intergranular decohesion facets. The composition and morphology of these mentioned products is dictated by the type of the manganese carrier used and sintering conditions (temperature profile, sintering atmosphere purity, etc.). The sintering at higher temperature with good atmosphere purity (DP~60 C) in N 2 /H 2 atmosphere leads to significantly decreased content of oxides, but higher portion of MnS, supposed to be less harmful for mechanical properties. Acknowledgement Special thanks are extended to Prof. S. Firstov and Dr. V. Avdeev from IPMS, Ukraine for the performing of Auger analysis at their Institute. Support from Höganäs AB is gratefully acknowledged. REFERENCES [1] Šalak, A. et al.: Powder Metallurgy Progress, vol. 1, 2001, no. 1, p. 41 [2] Dudrová, E. et al.: Powd. Met., vol. 47, 2004, no. 2, p. 181 [3] Cias, A. et al.: Powd. Met., vol. 42, 1999, p. 227 [4] Danninger, H. et al.: Powd. Met., vol. 48, 2005, p. 23 [5] Mitchel, SC.: PhD Thesis. University of Bradford, 2000 [6] Hryha, E.:PhD. Thesis. Košice : IMR SAS, 2007