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

Michael Marucci*, Alan Lawley**, Robert Causton*, and Suleyman Saritas***

*Hoeganaes Corporation, Cinnaminson, NJ 08077**Department of Materials Engineering, Drexel University, Philadelphia, PA 19104

***Department of Mechanical Engineering, Gazi University, Maltepe/Ankara, 06570, Turkey

Abstract:

Low levels of boron (0.01-0.15w/o) may induce sufficient hardenability andstrength in powder metallurgy steels to permit a decrease in the level of the alloyingelements, increase powder compressibility and reduce the as-sintered hardness. Theselean alloys may be sufficiently ductile to coin and be hardened by subsequent heat-treatment. The goal of this study was to identify the boron level in FLN2-4400 (Fe +0.85w/oMo, 2.0w/oNi, 0.3w/oC) which yields the optimal combination of strength,ductility, and hardenability. Tensile, transverse rupture, hardness, and Jominy endquench tests were performed on this alloy with six different levels of boron addedSintered strength and ductility increase up to 0.05w/oB, but decrease beyond this level,even though sintered density increases significantly. Jominy hardness traces show thatthe hardenability is not increased substantially until the concentration of boron reaches0.05w/o. The microstructures of the Jominy bars show that with an increase in boronlevel, the depth to which martensite is retained increases, but that grain boundarysegregation occurs. A level of boron ~ 0.05w/o gives the optimum combination ofstrength, ductility, and hardenability in FLN2-4400.

Introduction:

Ferrous P/M has been a major metalforming technology for more than 40 yearsduring which time data have been developed to identify mechanical properties as afunction of sintered density. Heat-treatment response, in particular hardenability, has notbeen studied extensively and data have been "borrowed" from the literature on wroughtsteels. These data are useful, but only as a first approximation, since sintered P/M partscontain pores. These pores affect the thermal properties of the steel with an attendanteffect on hardenability.

Hardenability is a measure of how much martensite is formed at a given coolingrate in a steel. The hardenability of a steel is defined as the maximum (critical) diameterof a cylinder that has a microstructure that is 50% at its center after quenching from theaustinitizing temperature1. Thus, a steel with a larger critical diameter has a higherhardenability.

The work of Saroop2 and Fuhrer3 has shown that additions of boron toAncorsteel® 85HP improve the degree of sinter and hardenability. It is not clear,

however, which effect (sintering or hardenability) dominates at different boron levels. Inwrought steels there is evidence that at very low (trace) levels, boron is a potent elementwith respect to hardenability, and at higher levels (0.10w/o) it is a sintering aid because aliquid phase, Fe2B, forms. It is important to understand what is taking placemetallurgically at specific concentrations of boron, so that these alloys can be tailored forspecific applications.

There is a need for a powder metallurgy (P/M) alloy for machine parts that can becoined following sintering to improve surface characteristics and which can be heat-treated subsequently resulting in a wear resistant part that possesses high strength.Current P/M steels that are sufficiently hardenable contain ferrite hardeners such as Crand Cu, which makes the steel too hard and brittle to coin. Removal of these ferritehardeners results in higher compressibility, reduced tool wear and improved greendensity. Previous work by Saroop2, Fuhrer3, and Causton4 has shown that small levels ofboron 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 coinedin the as-sintered state. Previous hardenability data for similar P/M grades of steel wereobtained by sintering in a laboratory furnace with a slow attendant cooling rate2. Thepresent study utilized a furnace with a cooling rate comparable to that in productionfurnaces with water-jacketed cooling zones. The goal of this study was to identify theboron 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 boronadditions to a common P/M alloy. The test matrix holds all alloying elements constantexcept for boron. Thus only the change in boron content should affect the resultingproperties.

Table I: Alloy Test Matrix

Base Powder AdmixedMo (w/o) Ni (w/o)* C (w/o) (Graphite)* B (w/o)*

1 Ancorsteel 85HP 0.85 2.00 0.30 0.002 Ancorsteel 85HP 0.85 2.00 0.30 0.013 Ancorsteel 85HP 0.85 2.00 0.30 0.034 Ancorsteel 85HP 0.85 2.00 0.30 0.055 Ancorsteel 85HP 0.85 2.00 0.30 0.106 Ancorsteel 85HP 0.85 2.00 0.30 0.15*nominal composition

The alloyed powders were premixed using Hoeganaes’ water atomized Ancorsteel85HP (dm = 95µm) as the base powder. Nickel and graphite were admixed using Inco123 (3-13 µm) and Asbury Graphite 3201 (90 w/o <10µm), respectively. Boron wasadded as a gas atomized ferroalloy containing 12.0w/oB (dm = 18µm). Without boron,

the mixes are referred to as FLN2-4400 throughout this paper. Two sets of mixes weremade, one set with 0.75w/o Acrawax C lubricant for uniaxial compaction intomechanical 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 (45 tsi) togive a green density of 7.12 g/cm3 ±0.05 in a rigid die. Jominy bars were compacted bycold isostatic pressing (CIP) at 414 MPa (60,000 psi) to a green density of 6.9 g/cm3

±0.1 in a cylindrical rubber mold.

Sintering

All test pieces were sintered in a high temperature Hayes furnace at theHoeganaes’ R&D facility, Cinnaminson, NJ. This is a pusher-type furnace with acooling capacity from >1100°C (>2000°F) to room temperature in 40 min in a pure H2

atmosphere. Sintering was done at 1120°C (2050°C), 1175°C (2150°F), and 1230°C(2250°C). These temperatures were chosen because they fall below, at, and above theFe-Fe2B eutectic temperature of approximately 1175°C (2150°F). All sintering wasperformed in pure H2 to avoid nitriding caused by sintering atmospheres that contain amixture of hydrogen and nitrogen. The samples were sintered for 30 min at temperature(45 min total time, laboratory standard).

Mechanical Testing

Tensile and transverse rupture (TRS) tests were conducted on all samples for allsintering temperatures. Five tensile and five transverse rupture specimens were tested foreach condition for a total of 90 tensile and 90 TRS pieces. The properties obtained fromthe 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 870°C (1600°F) for 30 min., andquenched according to ASTM Standard A2555. Hardness traces (HRA) were performedon the quenched bars. Density measurements on the Jominy bars were made in both thegreen and the as-sintered conditions. The microstructures of the end-quenched Jominysamples were characterized on a representative number of samples.

Chemical Analysis

Chemical analysis was performed on the sintered P/M steels to characterize thelevel/uniformity of carbon, sulfur, nitrogen, oxygen, and boron. Carbon and sulfur weremeasured by infrared oxide detection. Nitrogen and oxygen were determined by enthalpymeasurements and CO2 detection, respectively. Boron was measured by inductively

coupled plasma-mass spectroscopy (ICP/MS). Chemical analysis was also performed onthe Jominy bars following heat-treatment.Results and Discussion:

Mechanical Testing

Green density was in the range of 7.11-7.13 g/cm3 for all alloys examined. Figure1 shows how sintered density changes as the boron level is increased; sintered densityincreases as the concentration of boron increases, and as the sintering temperatureincreases. This is expected because the liquid Fe2B phase forms, wetting the grainboundaries and filling the pores. Figure 2 shows representative optical micrographs ofsamples 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 areaof the 0.10w/oB sample, pores that have been filled with liquid are evident.

Figure 1: Sintered density of FLN2-4400 + boron

Figure 2: (a.) Microstructure of FLN2-4400 + 0.00 w/oB, (b.) FLN2-4400 + 0.10w/oBboth sintered at 1230°C (2250°F), optical micrograph (2% nital/4% picraletch).

7.00

7.05

7.10

7.15

7.20

7.25

7.30

7.35

7.40

7.45

0.00 0.01 0.03 0.05 0.10 0.15

w/oB

Sintered Density (g/cm3)

1120°C (2050°F)1175°C (2150°F)1230°C (2250°F)

(a.) (b.)

Figures 3 and 4 display the tensile strength (TS) and elongation respectively, ofthe base alloy for all the boron levels evaluated. Strength and elongation increase butthen plateau at about 0.05w/oB. At higher concentrations of boron, strength decreases;this occurs at 1120°C (2050°F) and 1230°C (2250°F). At 1175°C (2150°F), the Fe-Fe2B

eutectic temperature, the strength continues to increase but elongation falls dramaticallyat 0.10w/oB. This fall in elongation is attributed to segregation of boron to the grainboundaries, which causes embrittlement of the steel. Figure 3: Tensile Strength (TS) of FLN2-4400 as a function of sintering temperature and

boron content.Figure 4: Elongation to failure of FLN2-4400 as a function of boron content and

sintering temperature.

Reviewing Figure 3, the tensile strength of the samples with 0.10 and 0.15w/oBsintered at 1175°C (2150°F) is much higher than that of the samples sintered at the othersintering temperatures. This is also reflected in the hardness levels. The test piecessintered at 1175°C (2150°F) have Rockwell hardness values (HRA) of 62 and 60 whereasthe pieces sintered at the other sintering temperatures have values around 50 HRA.However, the sintered carbon level is consistent for all conditions. This indicates that thetest specimens sintered at 1175°C (2150°F) were subjected to a different cooling ratethan the specimens sintered at 1120°C (2050°F) and 1230°C (2250°F). This anomaly isattributed to breakage of the ceramic sintering boat in the cooling zone of the sinteringfurnace resulting in a faster cooling rate.

0.0

0.5

1.0

1.5

2.0

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4.5

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0.00 0.01 0.03 0.05 0.10 0.15

w/oB

Elongation(%)

1120°C (2050°F)1175°C (2150°F)1230°C (2250°F)

400

450

500

550

600

650

700

750

800

0.00 0.01 0.30 0.05 0.10 0.15

w/oB

TS(ksi)

1120°C (2050°F)1175°C (2150°F)1230°C (2250°F)

58

65

73

80

87

94

102

109

116

TS(103 psi)

Figure 4 shows elongation values as a function of nominal boron level andsintering temperature. The elongation of the samples with 0.10 and 0.15w/oB sintered at1120°C (2050°F) is > 4.0%. This is three times the elongation of the samples with thesame concentration of boron sintered at the higher temperatures. Comparison of Figure5(a) and 5(b) explains what is happening. Both micrographs are for the same level ofboron, however the microstructures are different. The sample sintered at 1120°C(2050°F) consists of ferrite with dispersed carbides and an average grain size of 25 µm.The sample sintered at 1230°C (2250°F) consists of coarse divorced pearlite with anaverage grain size of 50 µm and with Fe2B at the grain boundaries. The ferriticmicrostructure resulting from the lower sintering temperature has lower strength becauseof the absence of well-defined pearlite. It has a higher elongation because of the smallgrain size. Strength and elongation at the higher sintering temperature are reducedbecause of the presence of coarse pearlite, combined with the larger grain size and grainboundary segregation, even though the sintered density is higher.

Figure 5: Microstructure of FLN2-4400 + 0.10 w/oB sintered at (a.) 1120°C (2050°F)and (b.) sintered at 1230°C (2250°F), optical micrograph (2% nital/4% picraletch).

Hardenability

Figure 6 shows representative Jominy hardness traces of FLN2-4400 for eachboron concentration at the three sintering temperatures. There is an error of ±2 inhardness readings and this yields a trace with noise. To decrease the noise level thetraces 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 adistance j, and F is the resulting value. A period of 3 was used for the data presented.

(a.) (b.)

(1)

Figure 6: Jominy hardness traces for FLN2-4400 (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 affecthardenability except for the samples with 0.10 and 0.15w/o B. The samples sintered atand above the Fe-Fe2B eutectic temperature [1175°C (2150°F)] exhibit traces that are ofsimilar 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 sinteringtemperature increases can be attributed to the fact that FLN2-4400 with boron levels

202530354045505560657075

0.0 0.5 1.0 1.5 2.0 2.5

Depth (in.)

HRA

1120°C (2050°F)1175°C (2150°F)1230°C (2250°F)

202530354045505560657075

0.0 0.5 1.0 1.5 2.0 2.5Depth (in.)

HRA

1120°C (2050°F)1175°C (2150°F)1230°C (2250°F)

202530354045505560657075

0.0 0.5 1.0 1.5 2.0 2.5Depth (in.)

HRA

1120°C (2050°F)1175°C (2150°F)1230°C (2250°F)

202530354045505560657075

0.0 0.5 1.0 1.5 2.0 2.5Depth (in.)

HRA

1120°C (2050°F)1175°C (2150°F)1230°C (2250°F)

202530354045505560657075

0.0 0.5 1.0 1.5 2.0 2.5

Depth (in.)

HRA

1120°C (2050°F)1175°C (2150°F)1230°C (2250°F)

202530354045505560657075

0.0 0.5 1.0 1.5 2.0 2.5

Depth (in.)

HRA

1120°C (2050°F)1175°C (2150°F)1230°C (2250°F)

(a.) (b.)

(c.) (d.)

(e.) (f.)

greater than 0.10w/o sintered above the Fe-Fe2B eutectic temperature forms a liquidphase that fills the pores. The resulting increase in sintered density increases theapparent hardness of the steel.Figure 7: Jominy hardness traces for FLN2-4400 sintered at 1230°C (2250°F).

Table II: As Sintered Chemistry

w/oCarbon* Sulfur* Oxygen** Nitrogen*** Boron****

0.00w/oB 1120oC (2050oF) 0.28 0.005 0.036 <0.0001 <0.0010.00w/oB 1175oC (2150oF) 0.28 0.005 0.030 <0.0001 <0.0010.00w/oB 1230oC (2250oF) 0.29 0.003 0.029 <0.0001 <0.0010.01w/oB 1120oC (2050oF) 0.29 0.004 0.045 <0.0001 0.0070.01w/oB 1175oC (2150oF) 0.29 0.003 0.043 <0.0001 0.0080.01w/oB 1230oC (2250oF) 0.30 0.004 0.040 <0.0001 0.0080.03w/oB 1120oC (2050oF) 0.31 0.004 0.058 <0.0001 0.0230.03w/oB 1175oC (2150oF) 0.31 0.003 0.044 <0.0001 0.0190.03w/oB 1230oC (2250oF) 0.30 0.003 0.044 <0.0001 0.0200.05w/oB 1120oC (2050oF) 0.31 0.006 0.062 <0.0001 0.0380.05w/oB 1175oC (2150oF) 0.31 0.006 0.060 <0.0001 0.0290.05w/oB 1230oC (2250oF) 0.31 0.007 0.072 <0.0001 0.0360.10w/oB 1120oC (2050oF) 0.30 0.006 0.077 <0.0001 0.0940.10w/oB 1175oC (2150oF) 0.31 0.005 0.100 <0.0001 0.1500.10w/oB 1230oC (2250oF) 0.32 0.006 0.097 <0.0001 0.1040.15w/oB 1120oC (2050oF) 0.30 0.006 0.084 <0.0001 0.1560.15w/oB 1175oC (2150oF) 0.33 0.007 0.085 <0.0001 0.1510.15w/oB 1230oC (2250oF) 0.33 0.005 0.110 <0.0001 0.158

*Infrared oxide detection, ** CO2 Detection, ***Enthalpy measurement, ****ICP/MS

A comparison of how different concentrations of boron affect hardenability showsthat 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 atall sintering temperatures. Steels with boron levels ≥0.05w/o exhibit a significantincrease in hardenability. At both temperatures the hardness does not begin to decreaseup to a distance of 11.1 mm (7/16 in.) from the water-quenched end. In the lower levelboron steels, hardness drops at a distance between 4.0-6.0 mm (3/16-1/4 in.) from thewater-quenched end. The boron concentrations quoted are nominal. Measurement of the

30

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0.0 0.5 1.0 1.5

Depth (in.)

HRA

0.00 w/o B0.01 w/o B0.03 w/o B0.05 w/o B0.10 w/o B0.15 w/o B

boron concentrations in the sintered condition by inductively coupled plasma-massspectroscopy (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 1230°C(2250°F). These micrographs illustrate how increasing boron concentration affects themicrostructure. Figure 8 shows the sample that contains no boron. At the quenched endthere 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 water-quenched end, the microstructure consists of pearlite with Ni-rich regions; the grain sizeis about 30 µm. The microstructure at 50.8 mm (2 in) from the water-quenched end is thesame as at 25.4 mm (1 in); however, the pearlite is coarser and the grain size is 40 µm.The pores are irregular with dimensions in the range of 10-15 µm. Figure 9 shows themicrostructure with 0.05w/oB. The water-quenched end consists entirely of lathmartensite. At 25.4 mm (1 in) from the water-quenched end, the microstructure consistsof a combination of bainite, fine divorced pearlite, Ni-rich regions, and small ferriteGBA’s. The structure at 50.8 mm (2 in) from the water-quenched end is pearlite withsome bainite and Ni-rich regions. The pores at this boron level are rounded with anaverage size of about 15-25 µm. Figure 10 shows the microstructure of a steel containing0.10w/oB. At all distances from the water-quenched end, the structures are the same asin the 0.05w/oB steel except that segregation of Fe2B at the grain boundaries and in thepores is evident. The other difference is that the degree of sinter is enhancedsignificantly; the pores are rounded and are 25-35 µm.

Figure 8: Microstructure of FLN2-4400 + 0.00 w/oB following Jominy quenching, (a.) atquenched end, (b.) at 25.4 mm (1.0 in.), (c.) at 50.8 mm (2.0 in.), sintered at1230°C (2250°F), optical micrograph (2% nital/4% picral etch).

The changes observed in microstructure as the boron level increases show thatboron improves the degree of sinter which improves hardness. Hardenability studies onwrought steels have confirmed that boron inhibits the onset of ferrite nucleation, allowingthe bainite reaction to take place6. It is reasonable to assume that boron behaves in asimilar manner in sintered steels. In combination, the effect of boron on sintering

(a.) (b.) (c.)

response and hardenability produces a harder steel. This results in the development of amartensitic/banitic microstructure.

Figure 9: Microstructure of FLN2-4400 + 0.05 w/oB following Jominy quenching, (a.) atquenched end, (b.) at 25.4 mm (1.0 in.), (c.) at 50.8 mm (2.0 in.), sintered at1230°C (2250°F), optical micrograph (2% nital/4% picral etch).

Figure 10: Microstructure of FLN2-4400 + 0.10 w/oB following Jominy quenching, (a.)at quenched end, (b.) at 25.4 mm (1.0 in.), (c.) at 50.8 mm (2.0 in.), sintered at1230°C (2250°F), optical micrograph (2% Nital/4% Picral Etch).

Conclusions:

Boron improves the degree of sinter in the P/M steels evaluated. Above theeutectic temperature of 1175°C (2150°F) liquid Fe2B forms which fills the pores. Above0.05w/oB the boron segregates to the grain boundaries where Fe2B forms improving thedegree of sinter. The mechanical properties are improved by the increase in sintereddensity; however, once grain boundary segregation occurs the steel becomes weak andbrittle. Boron improves hardenability by inhibiting the nucleation of ferrite at the

(a.) (b.) (c.)

(a.) (b.) (c.)

austenite grain boundaries; this allows bainite to form, increasing the depth to which thesteel hardens.

The goal of this study was to identify the level of boron that produces theoptimum combination of strength, ductility, and hardenability, utilizing a lean alloycomposition. Strength and elongation decrease beyond 0.05w/oB. This is explained byFe2B segregation to the grain boundaries. The hardness traces show that at 0.05w/oB thehardenability is comparable to, or greater than that of, steels containing 0.10 and0.15w/oB. Since the strength of FLN2-4400 + 0.05w/oB does not decrease (as it does athigher boron levels), and this alloy has a markedly improved hardenability than the steelswith a lower boron level, FLN2-4400 steel with a boron level of about 0.05w/o isoptimal. References:

1. R. E. Reed-Hill, and R. Abbaschian, 1994 Physical Metallurgy Principles. ThirdEdition, 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 FinalReport, Drexel University, Philadelphia, PA.

4. R. J. Causton, J-S., Oh, and A. Lawley, “Processing Microstructure and MechanicalProperties of Fe-B and Fe-B-C Alloys”, Advances in Powder Metallurgy andParticulate Materials – 1999, Compiled by C.L. Rose and M.H. Thibodeau, MetalPowder Industries Federation, Princeton, NJ, Vol. 2, Part 7, p.3, 1999

5. “Standard Test Method for End-Quench Test for Hardenability of Steel (A255-96).”ASTM Book of Standards-1997, Vol. 3.01, p. 25, American Society for Testing andMaterials, West Conshohocken, PA, 1997.

6. 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 specimenpreparation, material characterization, and chemical analysis.