Tool Materials

Both part and powder producers have made significant progress in developing higher strength P/M steels to compete with fully dense wrought steels and heat treated cast irons. The combinations of strength and ductility make these alloys more difficult to machine in the as-sintered condition requiring higher performance tool materials. However, tests show that these free-machining additives alone do not always improve machinability of higher strength materials, as seen in Table 20 for Ancorsteel 85HP. A series of cutting tests were conducted (Ref 21) to assess whether changes to tool material, tool geometry, or coating could improve cutting performance in this alloy. The drill test used the same conditions as previous tests; however, this test examined higher performance cobalt high-speed steels, the effects of titanium nitride coatings, and different tool geometries, such as parabolic flutes or "split points."

Table 20 Drill life for Ancorsteel 85HP: 2% Ni, 0.5% graphite

Iron Composition MnS (MnX), wt%

_0 0 (0.35) 0 (0.50) 0.10 (0.25) 0.15 (0.35) 0.35 (0) 0.50 (0)

Holes to Failure 1 1 2 2 3 4 2

Compaction to 7.0 g/cm3. Sintering at 1121 °C (2050 °F), 75% H/25% Ni. Machining in 0.125 in. drill, 3000 rpm, 0.0006 in./revolution

The results indicate that when combined, improvements to tool material, coating, and drill design improve drill life (Table 21). Individual improvements to high-speed steel, flute form, or coating had relatively little effect upon drill life under the test conditions employed. However, when combined, drill life increases significantly from an initial two holes for the standard drill to 85 holes for the optimum drill.

Table 21 Effect of drill type upon drill life when cutting Ancorsteel 85HP: 2% Ni, 0.5% graphite

Steel type

Coating

Flute form

Point

Drill life

Base HSS

None

Standard

135 °

2

M42

None

Standard

135°

2

Base HSS

TiN

Standard

135°

1

HSS

None

Parabolic

135°

1

Cobalt HSS

None

Parabolic

135°

3

M7HSS

TiN

Parabolic

135°

44

Cobalt HSS

TiN

Split

85

Compaction to 7.0 g/cm3. Sinter at 1121 °C (2050 °F), 75% hydrogen/25% nitrogen. Machining at 0.125 in. drill, 3000 rpm, 0.006 in./revolution feed

Attempts to improve drill life further by using solid carbide drills have been unsuccessful. The solid carbide drills appear to be too brittle for the test conditions and drilling system employed. The test results did show clearly that drilling of very high-

strength P/M steels can be improved using similar improvements in tool materials and design to those used for high-strength wrought steels.

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