Effect of Small Additions of Boron on the Mechanical Properties and Hardenability of Sintered P/M Steels

Transcription

1 Effect of Small Additions of Boron on the Mechanical Properties and Hardenability of Sintered P/M Steels Michael Marucci *, Alan Lawley **, Robert Causton *, and Suleyman Saritas *** * Hoeganaes Corporation, Cinnaminson, NJ ** Department of Materials Engineering, Drexel University, Philadelphia, PA *** Department of Mechanical Engineering, Gazi University, Maltepe/Ankara, 0, Turkey Abstract: Low levels of boron ( w/o) may induce sufficient hardenability and strength in powder metallurgy steels to permit a decrease in the level of the alloying elements, increase powder compressibility and reduce the as-sintered hardness. These lean alloys may be sufficiently ductile to coin and be hardened by subsequent heattreatment. The goal of this study was to identify the boron level in FLN2-40 (Fe w/oMo, 2.0w/oNi, 0.3w/oC) which yields the optimal combination of strength, ductility, and hardenability. Tensile, transverse rupture, hardness, and Jominy end quench tests were performed on this alloy with six different levels of boron added Sintered strength and ductility increase up to 0.05w/oB, but decrease beyond this level, even though sintered density increases significantly. Jominy hardness traces show that the hardenability is not increased substantially until the concentration of boron reaches 0.05w/o. The microstructures of the Jominy bars show that with an increase in boron level, the depth to which martensite is retained increases, but that grain boundary segregation occurs. A level of boron ~ 0.05w/o gives the optimum combination of strength, ductility, and hardenability in FLN2-40. Introduction: Ferrous P/M has been a major metalforming technology for more than years during which time data have been developed to identify mechanical properties as a function of sintered density. Heat-treatment response, in particular hardenability, has not been studied extensively and data have been "borrowed" from the literature on wrought steels. These data are useful, but only as a first approximation, since sintered P/M parts contain pores. These pores affect the thermal properties of the steel with an attendant effect on hardenability. Hardenability is a measure of how much martensite is formed at a given cooling rate in a steel. The hardenability of a steel is defined as the maximum (critical) diameter of a cylinder that has a microstructure that is % at its center after quenching from the austinitizing temperature 1. Thus, a steel with a larger critical diameter has a higher hardenability. The work of Saroop 2 and Fuhrer 3 has shown that additions of boron to Ancorsteel 85HP improve the degree of sinter and hardenability. It is not clear,

2 however, which effect (sintering or hardenability) dominates at different boron levels. In wrought steels there is evidence that at very low (trace) levels, boron is a potent element with respect to hardenability, and at higher levels (0.10w/o) it is a sintering aid because a liquid phase, Fe 2 B, forms. It is important to understand what is taking place metallurgically at specific concentrations of boron, so that these alloys can be tailored for specific applications. There is a need for a powder metallurgy (P/M) alloy for machine parts that can be coined following sintering to improve surface characteristics and which can be heattreated subsequently resulting in a wear resistant part that possesses high strength. Current P/M steels that are sufficiently hardenable contain ferrite hardeners such as Cr and Cu, which makes the steel too hard and brittle to coin. Removal of these ferrite hardeners results in higher compressibility, reduced tool wear and improved green density. Previous work by Saroop 2, Fuhrer 3, and Causton 4 has shown that small levels of boron increase hardenability, without the use of large quantities of carbon and/or nickel. These hardenable lean P/M alloy steels are relatively soft (~ 80 HRB) and can be coined in the as-sintered state. Previous hardenability data for similar P/M grades of steel were obtained by sintering in a laboratory furnace with a slow attendant cooling rate 2. The present study utilized a furnace with a cooling rate comparable to that in production furnaces with water-jacketed cooling zones. The goal of this study was to identify the boron content that results in optimum hardenability and mechanical properties. Experimental Procedure: Alloy Test Matrix The test matrix, shown in Table I, was established to explore the effects of boron additions to a common P/M alloy. The test matrix holds all alloying elements constant except for boron. Thus only the change in boron content should affect the resulting properties. Table I: Alloy Test Matrix Base Powder Admixed Mo (w/o) Ni (w/o) * C (w/o) (Graphite) * B (w/o) * 1 Ancorsteel 85HP Ancorsteel 85HP Ancorsteel 85HP Ancorsteel 85HP Ancorsteel 85HP Ancorsteel 85HP * nominal composition The alloyed powders were premixed using Hoeganaes water atomized Ancorsteel 85HP (d m = 95µm) as the base powder. Nickel and graphite were admixed using Inco 123 (3-13 µm) and Asbury Graphite 3201 (90 w/o <10µm), respectively. Boron was added as a gas atomized ferroalloy containing 12.0w/oB (d m = 18µm). Without boron,

3 the mixes are referred to as FLN2-40 throughout this paper. Two sets of mixes were made, one set with 0.w/o Acrawax C lubricant for uniaxial compaction into mechanical test pieces, and one set without lubricant for the cold isostatic pressing (CIP) of Jominy bars. Compaction The mechanical test samples were uniaxially compacted at 619 MPa ( tsi) to give a green density of 7.12 g/cm 3 ±0.05 in a rigid die. Jominy bars were compacted by cold isostatic pressing (CIP) at 414 MPa (,000 psi) to a green density of 6.9 g/cm 3 ±0.1 in a cylindrical rubber mold. Sintering All test pieces were sintered in a high temperature Hayes furnace at the Hoeganaes R&D facility, Cinnaminson, NJ. This is a pusher-type furnace with a cooling capacity from >1100 C (>2000 F) to room temperature in min in a pure H 2 atmosphere. Sintering was done at 1120 C (20 C),, and 12 C (22 C). These temperatures were chosen because they fall below, at, and above the Fe-Fe 2 B eutectic temperature of approximately. All sintering was performed in pure H 2 to avoid nitriding caused by sintering atmospheres that contain a mixture of hydrogen and nitrogen. The samples were sintered for min at temperature ( min total time, laboratory standard). Mechanical Testing Tensile and transverse rupture (TRS) tests were conducted on all samples for all sintering temperatures. Five tensile and five transverse rupture specimens were tested for each condition for a total of 90 tensile and 90 TRS pieces. The properties obtained from the tensile and TRS specimens include: green and sintered density, dimensional change, transverse rupture, yield strength, tensile strength, elongation, and hardness (HRA). Hardenability Sintered Jominy bars were austinitized at 8 C (10 F) for min., and quenched according to ASTM Standard A2 5. Hardness traces (HRA) were performed on the quenched bars. Density measurements on the Jominy bars were made in both the green and the as-sintered conditions. The microstructures of the end-quenched Jominy samples were characterized on a representative number of samples. Chemical Analysis Chemical analysis was performed on the sintered P/M steels to characterize the level/uniformity of carbon, sulfur, nitrogen, oxygen, and boron. Carbon and sulfur were measured by infrared oxide detection. Nitrogen and oxygen were determined by enthalpy measurements and CO 2 detection, respectively. Boron was measured by inductively

4 coupled plasma-mass spectroscopy (ICP/MS). Chemical analysis was also performed on the Jominy bars following heat-treatment. Results and Discussion: Mechanical Testing Green density was in the range of g/cm 3 for all alloys examined. Figure 1 shows how sintered density changes as the boron level is increased; sintered density increases as the concentration of boron increases, and as the sintering temperature increases. This is expected because the liquid Fe 2 B phase forms, wetting the grain boundaries and filling the pores. Figure 2 shows representative optical micrographs of samples with 0.00w/oB and 0.10w/oB, respectively sintered at the highest temperature. The boron-free sample has pores that are completely empty. In contrast, in the boxed area of the 0.10w/oB sample, pores that have been filled with liquid are evident. Sintered Density (g/cm 3 ) w/ob Figure 1: Sintered density of FLN boron (a.) (b.) Figure 2: (a.) Microstructure of FLN w/ob, (b.) FLN w/oB both sintered at, optical micrograph (2% nital/4% picral etch).

5 Figures 3 and 4 display the tensile strength (TS) and elongation respectively, of the base alloy for all the boron levels evaluated. Strength and elongation increase but then plateau at about 0.05w/oB. At higher concentrations of boron, strength decreases; this occurs at and. At, the Fe-Fe 2 B TS (ksi) 800 TS (10 3 psi) eutectic temperature, the strength continues to increase but elongation falls dramatically at 0.10w/oB. This fall in elongation is attributed to segregation of boron to the grain boundaries, which causes embrittlement of the steel. Figure 3: Tensile Strength (TS) of FLN2-40 as a function of sintering temperature and boron content. Figure 4: Elongation to failure of FLN2-40 as a function of boron content and sintering temperature. Reviewing Figure 3, the tensile strength of the samples with 0.10 and 0.15w/oB sintered at is much higher than that of the samples sintered at the other sintering temperatures. This is also reflected in the hardness levels. The test pieces sintered at have Rockwell hardness values (HRA) of 62 and whereas the pieces sintered at the other sintering temperatures have values around HRA. However, the sintered carbon level is consistent for all conditions. This indicates that the test specimens sintered at were subjected to a different cooling rate than the specimens sintered at and. This anomaly is attributed to breakage of the ceramic sintering boat in the cooling zone of the sintering furnace resulting in a faster cooling rate. w/ob 58 Elongation (%) w/ob

6 Figure 4 shows elongation values as a function of nominal boron level and sintering temperature. The elongation of the samples with 0.10 and 0.15w/oB sintered at is > 4.0%. This is three times the elongation of the samples with the same concentration of boron sintered at the higher temperatures. Comparison of Figure 5(a) and 5(b) explains what is happening. Both micrographs are for the same level of boron, however the microstructures are different. The sample sintered at 1120 C (20 F) consists of ferrite with dispersed carbides and an average grain size of 25 µm. The sample sintered at consists of coarse divorced pearlite with an average grain size of µm and with Fe 2 B at the grain boundaries. The ferritic microstructure resulting from the lower sintering temperature has lower strength because of the absence of well-defined pearlite. It has a higher elongation because of the small grain size. Strength and elongation at the higher sintering temperature are reduced because of the presence of coarse pearlite, combined with the larger grain size and grain boundary segregation, even though the sintered density is higher. (a.) (b.) Figure 5: Microstructure of FLN w/ob sintered at (a.) and (b.) sintered at, optical micrograph (2% nital/4% picral etch). Hardenability Figure 6 shows representative Jominy hardness traces of FLN2-40 for each boron concentration at the three sintering temperatures. There is an error of ±2 in hardness readings and this yields a trace with noise. To decrease the noise level the traces were plotted using a moving average given by: where N is the number of periods including the actual value, A is the actual value at a distance j, and F is the resulting value. A period of 3 was used for the data presented. (1)

7 HRA HRA HRA 25 (a.) (c.) (e.) HRA HRA HRA (b.) (d.) (f.) Figure 6: Jominy hardness traces for FLN2-40 (a.) 0.00w/oB, (b.) 0.01 w/ob, (c.) 0.03w/oB, (d.) 0.05w/oB, (e.) 0.10w/oB, and (f.) 0.15w/oB. Examination of the traces shows that the sintering temperature does not affect hardenability except for the samples with 0.10 and 0.15w/o B. The samples sintered at and above the Fe-Fe 2 B eutectic temperature [] exhibit traces that are of similar shape but are shifted upward on the plot, as seen in Figure 6(e) and 6(f). Thus, there is an increase in overall hardness. This increase in hardness as sintering temperature increases can be attributed to the fact that FLN2-40 with boron levels

8 greater than 0.10w/o sintered above the Fe-Fe 2 B eutectic temperature forms a liquid phase that fills the pores. The resulting increase in sintered density increases the apparent hardness of the steel. Figure 7: Jominy hardness traces for FLN2-40 sintered at. HRA 0.00 w/o B 0.01 w/o B 0.03 w/o B 0.05 w/o B 0.10 w/o B 0.15 w/o B Table II: As Sintered Chemistry w/o Carbon* Sulfur* Oxygen** Nitrogen*** Boron**** 0.00w/oB 1120 o C (20 o F) < < w/oB 11 o C (21 o F) < < w/oB 12 o C (22 o F) < < w/oB 1120 o C (20 o F) < w/oB 11 o C (21 o F) < w/oB 12 o C (22 o F) < w/oB 1120 o C (20 o F) < w/oB 11 o C (21 o F) < w/oB 12 o C (22 o F) < w/oB 1120 o C (20 o F) < w/oB 11 o C (21 o F) < w/oB 12 o C (22 o F) < w/oB 1120 o C (20 o F) < w/oB 11 o C (21 o F) < w/oB 12 o C (22 o F) < w/oB 1120 o C (20 o F) < w/oB 11 o C (21 o F) < w/oB 12 o C (22 o F) < *Infrared oxide detection, ** CO 2 Detection, ***Enthalpy measurement, ****ICP/MS A comparison of how different concentrations of boron affect hardenability shows that as the boron level increases, so does hardenability. This can be seen in Figure 7. However, little change in hardenability is observed in the steel with less than 0.03w/oB at all sintering temperatures. Steels with boron levels 0.05w/o exhibit a significant increase in hardenability. At both temperatures the hardness does not begin to decrease up to a distance of 11.1 mm (7/16 in.) from the water-quenched end. In the lower level boron steels, hardness drops at a distance between mm (3/16-1/4 in.) from the water-quenched end. The boron concentrations quoted are nominal. Measurement of the

9 boron concentrations in the sintered condition by inductively coupled plasma-mass spectroscopy (ICP/MS) confirms that the boron levels are close to the nominal additions (Table II). This table also gives the C, S, O, and N levels after sintering. Figures 8-10 show microstructures of the Jominy bars sintered at 12 C (22 F). These micrographs illustrate how increasing boron concentration affects the microstructure. Figure 8 shows the sample that contains no boron. At the quenched end there is a mixture of martensite and ferrite forming as grain boundary allotriomorphs (GBA); there are also light etching Ni-rich regions. At 25.4 mm (1 in) from the waterquenched end, the microstructure consists of pearlite with Ni-rich regions; the grain size is about µm. The microstructure at.8 mm (2 in) from the water-quenched end is the same as at 25.4 mm (1 in); however, the pearlite is coarser and the grain size is µm. The pores are irregular with dimensions in the range of µm. Figure 9 shows the microstructure with 0.05w/oB. The water-quenched end consists entirely of lath martensite. At 25.4 mm (1 in) from the water-quenched end, the microstructure consists of a combination of bainite, fine divorced pearlite, Ni-rich regions, and small ferrite GBA s. The structure at.8 mm (2 in) from the water-quenched end is pearlite with some bainite and Ni-rich regions. The pores at this boron level are rounded with an average size of about µm. Figure 10 shows the microstructure of a steel containing 0.10w/oB. At all distances from the water-quenched end, the structures are the same as in the 0.05w/oB steel except that segregation of Fe 2 B at the grain boundaries and in the pores is evident. The other difference is that the degree of sinter is enhanced significantly; the pores are rounded and are 25- µm. (a.) (b.) (c.) Figure 8: Microstructure of FLN w/ob following Jominy quenching, (a.) at quenched end, (b.) at 25.4 mm (1.0 in.), (c.) at.8 mm (2.0 in.), sintered at, optical micrograph (2% nital/4% picral etch). The changes observed in microstructure as the boron level increases show that boron improves the degree of sinter which improves hardness. Hardenability studies on wrought steels have confirmed that boron inhibits the onset of ferrite nucleation, allowing the bainite reaction to take place 6. It is reasonable to assume that boron behaves in a similar manner in sintered steels. In combination, the effect of boron on sintering

10 response and hardenability produces a harder steel. This results in the development of a martensitic/banitic microstructure. (a.) (b.) (c.) Figure 9: Microstructure of FLN w/ob following Jominy quenching, (a.) at quenched end, (b.) at 25.4 mm (1.0 in.), (c.) at.8 mm (2.0 in.), sintered at, optical micrograph (2% nital/4% picral etch). (a.) (b.) (c.) Figure 10: Microstructure of FLN w/ob following Jominy quenching, (a.) at quenched end, (b.) at 25.4 mm (1.0 in.), (c.) at.8 mm (2.0 in.), sintered at, optical micrograph (2% Nital/4% Picral Etch). Conclusions: Boron improves the degree of sinter in the P/M steels evaluated. Above the eutectic temperature of liquid Fe 2 B forms which fills the pores. Above 0.05w/oB the boron segregates to the grain boundaries where Fe 2 B forms improving the degree of sinter. The mechanical properties are improved by the increase in sintered density; however, once grain boundary segregation occurs the steel becomes weak and brittle. Boron improves hardenability by inhibiting the nucleation of ferrite at the

11 austenite grain boundaries; this allows bainite to form, increasing the depth to which the steel hardens. The goal of this study was to identify the level of boron that produces the optimum combination of strength, ductility, and hardenability, utilizing a lean alloy composition. Strength and elongation decrease beyond 0.05w/oB. This is explained by Fe 2 B segregation to the grain boundaries. The hardness traces show that at 0.05w/oB the hardenability is comparable to, or greater than that of, steels containing 0.10 and 0.15w/oB. Since the strength of FLN w/oB does not decrease (as it does at higher boron levels), and this alloy has a markedly improved hardenability than the steels with a lower boron level, FLN2-40 steel with a boron level of about 0.05w/o is optimal. References: 1. R. E. Reed-Hill, and R. Abbaschian, 1994 Physical Metallurgy Principles. Third Edition, PWS Publishing Company, Boston, MA. 2. G. Saroop, Microstructure, Mechanical Properties, and Hardenability of Sintered Fe- B and Fe-B-C Alloys, 2000, M.S. Thesis, Drexel University, Philadelphia, PA. 3. J. Fuhrer, Hardenability of Sintered Fe-Ni-C-B Alloys, 2000, Senior Design Final Report, Drexel University, Philadelphia, PA. 4. R. J. Causton, J-S., Oh, and A. Lawley, Processing Microstructure and Mechanical Properties of Fe-B and Fe-B-C Alloys, Advances in Powder Metallurgy and Particulate Materials 1999, Compiled by C.L. Rose and M.H. Thibodeau, Metal Powder Industries Federation, Princeton, NJ, Vol. 2, Part 7, p.3, Standard Test Method for End-Quench Test for Hardenability of Steel (A2-96). ASTM Book of Standards-1997, Vol. 3.01, p. 25, American Society for Testing and Materials, West Conshohocken, PA, D.A. Porter, and K.E. Easterling, 1992, Phase Transformations in Metals and Alloys. Second Edition, Chapman & Hall, London. Acknowledgments: The authors are indebted to M. Shiber and D. VonRohr, Drexel University, and G. Golin, T. Murphy, and P. Meidunas, Hoeganaes Corporation for assistance with specimen preparation, material characterization, and chemical analysis.

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