Vacuum Sintering to Full Density

The prime object of vacuum sintering to full density is to uniformly expose all parts to sufficient temperature and time. This is typically done to produce the minimum required density and to prevent overheating and/or excessive carbide growth.

Sintering Cycles. Figure 29 shows typical sintering cycles. These sintering cycles consist of a lower temperature hold, followed by one or more high-temperature holds to densify compacts.

Fig. 29 Sintering cycles for fully dense sintering of high-speed steels.(a) British Patent 1,562,788. (b) U.S. Patent 4,063,940

Deoxidation. Heating rates to the deoxidation temperatures are not critical. Parts should be heated at the maximum practical heating rate to minimize furnace time, provided the temperature of the load does not greatly surpass the deoxidation soak temperature. Batch-type vacuum furnaces can be backfilled with inert gas or hydrogen to >46 kPa (350 torr) to improve the heating rate and temperature uniformity of the load.

The primary function of the deoxidation step is to react oxygen with carbon to form carbon monoxide, which is removed by the vacuum pumps. Oxygen, nitrogen, and other gases that may be present in the powder compact must be removed before sintering closes the interconnected porosity to the surface. Once the porosity is no longer interconnected, gas evolution results in blistering.

The deoxidation soak or hold also provides an opportunity for load temperature to become uniform. Temperature in any part of the load should not vary more than ±9 °C (±16 °F) before heating to the sintering soak. The load may be soaked at 1040 °C (1900 °F) for several hours to improve temperature uniformity without seriously affecting the microstructure.

Heating to sintering temperature is slow and closely controlled to ensure temperature uniformity. Typical ramp rates are 0.5 to 5.5 °C/min (1 to 10 °F/min).

Sintering Soak. Sintering treatments may consist of an isothermal soak or a very slow ramp (typically 3 to 33 °C/h, or 5 to 60 °F/h). Temperature uniformity is essential for successful sintering. Temperature range should not vary by more than 8 °C (15 °F) at any location within the load. Table 14 gives typical sintering temperatures for two-soak sintering cycles for several alloys.

Table 14 Typical sintering temperatures and compositions for several high-speed steels

Alloy

Sintering temperature(a)

Composition, %

Relative sinterability(b)

°C

°F

C

Cr

Mo

W

V

Co

M2

1245

2270

0.85

4.2

5.0

6.3

1.9

3

M2 (high carbon)

1240

2260

1.00

4.2

5.0

6.3

1.9

3

M3 type 2

1255

2290

1.20

4.1

5.0

6.0

3.0

2

M4

1260

2300

1.32

4.5

4.5

5.5

4.0

3

M35

1225

2240

1.15

4.2

5.1

6.4

2.0

5.0

4

M42

1220

2230

1.10

3.8

9.5

1.5

1.2

8.0

5

(a) Approximate sintering temperature for powders annealed at atmospheric pressure and pressed at 830 MPa (60 tsi).

(a) Approximate sintering temperature for powders annealed at atmospheric pressure and pressed at 830 MPa (60 tsi).

(b) 1 represents the easiest, 5 is the most difficult.

Cooling. Generally, cooling from the sintering temperature is performed as rapidly as possible by backfilling the furnace with inert gas and using forced convection cooling (also known as fan cooling or gas quenching) to minimize furnace time. Typically, the hardnesses of gas-quenched fully dense tool and high-speed steels are 50 to 60 HRC. These hardnesses are substantially lower than the austenitized and quenched hardnesses of 55 to 65 HRC because of excessive dissolution of carbides, which results in excessive retained austenite.

Nitriding. Tool steels and high-speed steels can be alloyed with nitrogen by maintaining a nitrogen partial pressure after deoxidation, but before sintering. Significant alloying does occur with partial pressure at >133 Pa (1 torr). Increasing the partial pressure increases the nitrogen content. Depending on the alloy, nitrogen contents from 4000 ppm (M2) to over 8000 ppm (T15) can be produced by backfilling to atmospheric pressure.

Nitrided cases can be produced by introducing a suitable nitrogen partial pressure after sintering has closed off the interconnected porosity. Use of nitrogen as the quenching gas during forced convection cooling does not result in any significant nitriding.

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