High Temperature Sintering

Interest in high-temperature sintering of ferrous components continues to grow in the P/M industry (see the article "High-Temperature Sintering of Ferrous Powder Metallurgy Components" in this Volume). Improvements in both production rates and properties are possible as sintering temperatures increase above 1120 °C (2050 °F). Rounding of the porosity results in increased strengths, especially impact strength. Higher diffusion rates also increase the strength and hardenability of admixed powder compositions. The greater oxide reduction that occurs at higher temperatures proves valuable in powder forging and other applications.

To demonstrate the feasibility of high-temperature sintering of ferrous materials on a production basis, a study was conducted on three iron-base compositions sintered in a nitrogen-methanol atmosphere (Ref 5). Alloys tested had the following nominal compositions: Fe-0.9C, Fe-4Ni-0.7C, and Fe-2Cu-0.9C.

Transverse rupture bars were pressed to a density of 6.8 g/cm3. Methanol was blended with nitrogen to form the following compositions: 1% carbon monoxide and 2% hydrogen, remainder nitrogen; 3% carbon monoxide and 6% hydrogen, remainder nitrogen; and 5% carbon monoxide, 10% hydrogen, remainder nitrogen. Methane additions of 0, 0.25, and 0.5% were made to each of the atmospheres. Atmosphere composition in the hot zone of the furnace was monitored by infrared analysis, gas chromatography, and dew point analysis.

Lubricant was burned off in a mesh belt furnace at 760 °C (1400 °F) for 35 min. An atmosphere of 90% nitrogen and 10% hydrogen with a dew point of -12 °C (10 °F) was used for lubricant burn off. Sintering was performed in a pusher furnace with a ceramic muffle and wound molybdenum heating elements.

A temperature cycle was selected (Fig. 11) that held the parts above 1290 °C (2350 °F) for 7 min and above 1280 °C (2340 °F) for 10 min. The maximum temperature reached was 1301 °C (2374 °F). For comparison with conventional sintering, a set of test bars was sintered at 1123 °C (2053 °F) for 32 min (Fig. 11). A single nitrogen methanol atmosphere forming 5% carbon monoxide, 10% water vapor, 0.25% methane, and the remainder nitrogen was chosen for conventional temperature sintering.

1500

p 1250

1 750

Fig. 11 Temperature profile of sintering cycles. Profile A shows the high-temperature cycle used in study. Profile B shows the conventional sintering cycle used as a standard. Source: Ref 5

The sintered bars were tested for transverse rupture strength, apparent hardness, and dimensional change. Surface and core carbon contents were determined metallographically and by combustion and thermal conductivity analysis.

Effect of Sintering Temperature. The 10 min sinter at 1290 °C (2350 °F) exhibited equivalent or slightly better properties than the longer 30 min sinter at 1120 °C (2050 °F) in a comparable atmosphere. Transverse rupture strengths were very similar, with only a 1 to 1.5% variation in the Fe-Ni-C and Fe-Cu-C alloys and 5% for the Fe-C steel (Table 9).

Table 9 Effect of sintering temperature on transverse rupture strength of test bars pressed to 6.8 g/cm3

Alloy

Transverse rupture strength at 1120 °C (2050 °F)

Transverse rupture strength at 1290 °C (2350 °F)

MPa

ksi

MPa

ksi

Fe-0.9C

693

100.5

728

105.6

Fe-4Ni-0.7C

910

132.0

924

134.0

Fe-2Cu-0.9C

975

141.5

964

139.8

Transverse rupture strength of bars sintered for 32 min at 1120 °C (2050 °F). Average transverse rupture strength for all atmosphere tests of bars sintered for 10 min at 1290 °C (2350 °F). Source: Ref 5

Metallographic analysis revealed a slightly more spheroidized pore structure and increased sintering in the 1290 °C (2350 °F) specimen. Dimensional change (Table 10) was more negative by approximately 0.2% for the Fe-C and Fe-Ni-C samples sintered at 1290 °C (2350 °F). Consequently, increased particle diffusion may have occurred at the higher temperatures even though a shorter sintering time was used. No difference in dimensional change was evident for the Fe-Cu-C samples.

Table 10 Effect of sintering temperature on dimensional change of test bars pressed to 6.8 g/cm3

Change at

Change at

1120 0C (2050 "F), %

1290 "C (2350 "F), %

Fe-0.9C

+0.25

+0.04

Fe-4Ni-0.7C

0.0

-0.2

Fe-2Cu-0.9C

+0.4

+0.4

Dimensional change of bars sintered for 32 min at 1120 °C (2050 °F) and average for all atmosphere tests of bars sintered for 10 min at 1290 °C (2350 °F). Source: Ref 5

Apparent hardness increased slightly for all three alloys at the higher sintering temperature (Table 11). Generally, the 10 min sinter at 1290 °C (2350 °F) was found to be an adequate substitute for a 30 min sinter at 1120 °C (2050 °F). This type of cycle can be used if an increased production rate that yields equivalent mechanical properties is desired.

Table 11 Effect of sintering temperature on the apparent hardness of test bars pressed to a density of 6.8 g/cm3

Alloy

Apparent hardness,

HRB,

1120 "C (2050 "F)

1290 "C (2350 "F)

Fe-0.9C

57.9

63.2

Fe-4Ni-0.7C

73.1

75.9

Fe-2Cu-0.9C

75.6

81.3

Apparent hardness of bars sintered for 32 min at 1120 °C (2050 °F) and 10 min at 1290 °C (2350 °F) in comparable atmospheres. Source: Ref 5

Effect of Atmosphere Composition. Variations in methanol (forming carbon monoxide and hydrogen) and methane additions to the nitrogen carrier gas did not result in any significant change in transverse rupture strength for any of the alloys. This is attributed to the consistent carbon content (0.6 to 0.7%) at the core of the samples for all atmospheres tested.

Dimensional change of the alloys was unaffected by the atmosphere composition. The Fe-Cu-C and Fe-Ni-C alloys were consistent for all atmospheres tested. Some variations were evident in the Fe-C compacts, although no trend was evident.

Atmosphere composition was found to significantly affect surface carbon content and apparent hardness. Methanol enrichment without methane additions resulted in the most decarburization. Increases in methanol alone did not significantly increase the surface carbon. The surface of an Fe-0.9C sample was decarburized to a combined carbon content of 0.45 to 0.55% when sintered for 10 min at 1290 °C (2350 °F) in 15% dissociated methanol. Methane additions were required to increase the surface carbon.

Although some decarburization was still evident, acceptable results were obtained with an atmosphere containing 15% dissociated methanol (5% carbon monoxide and 10% hydrogen) with 0.5% methane additions. The dissociation of methanol and the resulting carbon dioxide and water vapor levels were found to be affected by localized temperature, residence time, and availability of local catalytic surfaces. When introduced properly, the methanol dissociation was demonstrated to be very efficient at 1290 °C (2350 °F).

Concentrations of carbon dioxide and water vapor were low, and the formation of methane was too low to be detected by gas chromatography. The atmosphere formed with nitrogen-methanol compared favorably to an equivalent atmosphere formed by blending nitrogen and endothermic gas.

These results indicate that efficient dissociation of methanol was achieved and that methane additions should be used to further reduce carbon dioxide and water vapor levels. As a result, nitrogen-methanol-methane atmospheres can be considered as viable alternatives to nitrogen-hydrogen-carbon monoxide and nitrogen-hydrogen-methane atmospheres for high-temperature sintering of ferrous alloys.

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