THE EFFECT OF HEAT TREATMENT PARAMETERS ON THE MECHANICAL PROPERTIES AND MICROSTRUCTURE OF Mo-Cr Ni-Mn SINTERED STEEL

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1 ISSN NDT days 2016 / Дни на безразрушителния контрол 2016 Year /Година XXIV r Number/ Брой 1 (187) June/Юни 2016 THE EFFECT OF HEAT TREATMENT PARAMETERS ON THE MECHANICAL PROPERTIES AND MICROSTRUCTURE OF Mo-Cr Ni-Mn SINTERED STEEL ЕФЕКТЪТ НА ПАРАМЕТРИТЕ НА ТЕРМООБРАБОТКА ВЪРХУ МЕХАНИЧНИТЕ СВОЙСТВА И МИКРОСТРУКТУРАТА НА СИНТЕРОВАНА СТОМАНА НА ОСНОВАТА НА Mo-Cr Ni-Mn Fiał Ch,.MSc (Eng) 1, Ciaś A., D.Sc. (Eng) 1, Sułowski M., Ph.D. 1 AGH University of Science and Technology cfial@agh.edu.pl Abstract: The effect of heat treatment on the microstructure and mechanical properties of Mo-Cr-Ni-Mn sintered steel were investigated. Pre-alloyed Astaloy CrL iron powder, ferromanganese, elemental nickel and graphite powders were the starting powder mixture. After mixing in a Turbula mixer for 60 minutes, each powder mixture was single-action compacted at 660 MPa to produce green compacts (according to PN EN ISO 2740). Following pressing in a rigid die, compacts were sintered in N 2 furnace atmosphere in a specially designed semi-closed container at 1120 or 1250 C for 60 minutes. The chemical composition of the sintering atmosphere was modified by adding ferromanganese and/or activator into the container. Following sintering, three types of heat treatment in nitrogen atmosphere were carried out: sinteraustempering at 350, 400 and 500 C for 60 minutes; cooling at 60 C/minute and tempering at 200 C for 60 minutes; cooling at 10 C/minute (cooling rate used in industrial sintering). The best combination of strength and plasticity for steel for both sintering temperatures were achieved after sinteraustempering at 500 C. KEYWORDS: POWDER METALLURGY, SINTERING, SINTERAUSTEMPERING, SEMI-CLOSED CONTAINER, MECHANICAL PROPERTIES 1. Introduction Currently, the widely used technique for obtaining sintered steel components in the powder metallurgy is sinterhardening technique.. This process is an alternative method of heat treating steel products in place of the traditional cycle: austenitizing, oil quenching and tempering. Sinterhardening process has several advantages, among others, reduction the manufacturing cost and avoidance of contamination with oil quenching parts. The structure of sintered steel after the sinterhardening operation consists of martensite, or a mixture of martensite and bainite [1]. An alternative technique to sinterhardening appears to be sinteraustempering [2], in which the steel is cooled down rapidly from the sintering temperature to the bainitic region, then isothermally annealed to complete the bainitic transformation and subsequently cooled to room temperature all in one operation. In sinteraustempering the target structure is lower bainite. Unfortunately, the main disadvantage of this process is the relatively high temperature (~ 500 C) of annealing, encouraging oxidation. To eliminate this problem, Girardini et al. [2] suggested the use of a vacuum furnace. However, due to the significant cost of this solution, Ciaś [3, 4] and Fiał et al. [5-7] proposed a more affordable way - the use of semi-closed container [5-7] with a getter addition [3] and N 2 atmosphere. In this paper the results of research of Mo-Cr-Ni-Mn PM steels, sintered in semi-closed container in pure nitrogen atmosphere using sinteraustempering, sinterhardening and - for comparison - cooling from sintering temperature at 10K/minute (cooling rate used in industrial sintering) are presented. 2. Experimental procedures To make the powder mixture for 34HNM PM steel, the following starting powders were used: prealloyed iron Astaloy CrL (Höganäs), Norwegian low carbon ferromanganese (Elkem ASA/Eramet/Camilog Manganese Co.), graphite grade ultra fine (Höganäs), elemental Ni. From the powders, by Turbula mixing for 60 minutes, the mixture was prepared (Table 1). After mixing, the powders were single-action compacted into ISO 2740 dogbone tensile test bars. The compaction was made in a steel die with zinc lubricated walls at 660 MPa. The average density (d 1 ) of green compacts was 6.77 g/cm 3, with deviation of Sintering was carried out in semi-closed container [5-7], in a laboratory horizontal tube furnace at 1120 C or 1250 C for 60 minutes in flowing nitrogen. Five processing variants were then investigated: isothermal annealing at 500 C, 400 C, 350 C sinteraustempering (SAT), rapid cooling from sintering temperature and tempering at 200 C sinterhardening (S+H), cooling at 10K/min (S+C). For each variant 5 samples were prepared. Conditions of sintering and post-sintering heat treatment and their designation are presented in Table 2. The as-sintered average density (d 2 ) was 6.72 g/cm 3 with deviation of Table 1. Chemical composition of powder mixtures (mass %) Mixture Fe C Mo Cr Ni Mn 34 HNM The samples were mechanically tested on a MTS tensile-testing apparatus at a rate of 1 mm/min. and in three-point bending on ZD-10 tester using a jig with a span of 28.6 mm, at a crosshead rate of 2mm/min. The elongation value was based on crosshead 256

2 displacement of the testing machine during the tensile test. Microstructures were examined using: light microscopy Leica DM 4000M. Hardness was measured on an Innovatest Nexus Series tester. 3. Results The results of mechanical testing are shown in Table 3. Yield strength refers to 0.2% offset. Metallographic investigations were carried out on 3% Nital etched samples. The characteristic microstructures are shown in Fig.1-10 where: B bainite, N nickel, P pearlite, M martensite. a) SAT500, 450, 350_LT; SAT500, 450, 350_LT microstructure consists of lower bainite with some martensite in Ni rich areas (+some austenite/ni residuals); b) S+H_LT, S+H_HT: microstructure with bainite, martensite and Ni rich areas; c) S+C_LT, S+C_HT: the microstructure is heterogenous, consisting of bainite (from CrL powder). Some Ni-rich areas are visible. Martensite also can be observed as some pearlite areas. Table 2. The description of sintering and post-sintering heat treatment conditions Sintering temperature/time/atmosp here 1120 C/60 minutes/n C/60 minutes/n 2 Heat treatment type Time Temperature Designation 500 C SAT500_LT Isothermal annealing C SAT400_LT minutes 350 C SAT350_LT Sintering + tempering 200 C S+H_LT Cooling rate [10K/min] - - S+C_LT 500 C SAT500_HT Isothermal annealing C SAT400_HT minutes 350 C SAT350_HT Sintering + tempering 200 C S+H_HT Cooling rate [10K/min] - - S+C_HT Table 3. Mechanical properties of sintered steels, mean values Variant UTS TRS YS A HV [MPa] [MPa] [MPa] [%] 30 SAT500_LT ,2 244 SAT400_LT ,7 213 SAT350_LT ,6 216 S+H_LT ,6 223 S+C_LT ,4 218 SAT500_HT ,4 231 SAT400_HT ,7 227 SAT350_HT ,7 219 S+H_HT ,1 209 S+C_HT ,

3 Fig. 1. Characteristic microstructure of steel after SAT500_LT Fig. 2. Characteristic microstructure of steel after SAT400_LT Fig. 3. Characteristic microstructure of steel after SAT350_LT Fig. 4. Characteristic microstructure of steel after S+H_LT Fig. 5. Characteristic microstructure of steel after S+C_LT Fig. 6. Characteristic microstructure of steel after SAT500_HT 258

4 Fig. 7. Characteristic microstructure of steel after SAT400_HT variant Fig. 8. Characteristic microstructure of steel after SAT350_HT variant Fig. 9. Characteristic microstructure of steel after S+H_HT Fig. 10. Characteristic microstructure of steel after S+C_HT 4. Discussion sinteraustempering variant are characterized by a bainitic structure, what is the main purpose of this treatment. The mechanical properties data presented in Table 3 show that the best strength properties (UTS and TRS) regardless the sintering temperature were obtained after sinteraustempering in 500 C (LT: UTS = 753MPa, TRS = 1414MPa, HT: UTS = 837MPa, TRS = 1365). At the lower sinteraustempering temperature the mechanical properties of tested steels are smaller. Property values after sinterhardening are lower than after sinteraustempering, but the differences are small. The worst mechanical properties of tested steel were achieved after S+H The largest elongation was obtained after sinteraustempering at 500 C for both sintering temperatures, while the values for others SAT variants according the sintering temperature - are almost equal. The hardness values of tested steels are similar - the differences between processing variants do not exceed 40 units of HV30. The microstructures, regardless of processing variant after sintering, are characterized by similar morphological aspects. The percentage of microstructural components like bainite, martensite and the possible small presence of pearlite and austenite in the non-dissolved particles of Ni has only changed. All steels after 5. Conclusions It is possible to use the semi-closed container PM technique in sinteraustempering and sinterhardening processing. Tested Mo-Cr-Ni-Mn PM steel has been processed successfully. The results shows that the tested steel after sinteraustempering variant has a slightly better or similar combination of strength and ductility than the tested steel after other variants sinterhardening and sintering with cooling [10K/min]. After the sinteraustempering operation at the appropriate temperature, it is possible to obtain a bainitic structure in the tested steel.

5 5. There are the basis for further experiments using semiclosed container and sinteraustempering operation of PM steel. Acknowledgements The financial support of the Polish Ministry of Science and Higher Education under the contract No is gratefully acknowledged. 6. References [1] James W.B. What is Sinter-Hardening - Advances in Powder Metallurgy and Particulate Materials MPIF, Princeton, NJ, USA, 1998 [2] Girardini L., A. Molinari, G. Locatelli, G. Tonini: Vacuum heat treatment of components for automotive applications - Metallurgia Italiana, 2, 2006, 26 [3] A. Ciaś: Effect of local sintering microatmosphere on mechanical properties of Fe-3Cr-0.5Mo-0.6C steel - Powder Metallurgy, 56 iss.3, 2013, 231 [4] A. Ciaś: A novel method of sintering hybrid steels in an improved semiclosed container system - Science of Sintering, 45, 2013, 379 [5] Fiał Ch., E. Dudrova, M. Kabatova, M. Kupkova, M. Selecka, M. Sulowski, A. Cias: Sinteraustempering of two Mo-(Cu)-(Cr)- (Ni)-(Mn)-C steels in a semi-closed container in flowing nitrogen - Archives of Metallurgy and Materials, 60 iss.2a, 2015, 783 [6] Fiał Ch., E. Dudrova, M. Kabatova, M. Kupkova, M. Selecka, M. Sulowski, A. Cias: The effect of heat treatment on the microstructure and mechanical properties of sintered Fe-2Cu- 1.5Mo-0.5C and Fe-0.2Mo-1.5Cr-1.5Ni-0.8Mn-0.4C steels - Powder Metallurgy Progress, 14 iss.3, 2014,137 [7] Fiał Ch, E. Dudrová, M. Kabátová, M. Kupková, M. Selecká, M. Sułowski, A. Ciaś: Comparison of fracture of Fe 0.2%Mo 0.8%Mn 1.5%Cr 1.5%Ni powder metallurgy steel processed in a semiclosed container in flowing nitrogen: sintered, sinterhardened and sinteraustempered - Powder Metallurgy 15 iss SS, 2015,