Materials and their processing

3.1.1 CM247LC powder

Gas atomized CM247LC nickel base superalloy powders, with size up to 53 µm, were supplied by Sandvik Osprey LTD. The composition of the received powder and some properties of the powders which are important in HIPping are shown in Table 3.1 and Table 3.2 respectively. (Powders were kept in a vacuum chamber once unsealed.

Table 3.1 Chemical composition of CM247LC (wt %)

W Co Cr Al Ta Hf* Ti Mo C O Fe Nb Cu Si Ni

9.1 9.0 8.1 5.4 3.2 1.3 0.88 0.43 0.077 0.008 0.06 0.05 0.02 0.02 Bal

* The Hf level was found lower than the configuration when Osprey was preparing the melt for atomisation. Some pure Hf was added into the liquid to solve this problem before atomization.

Table 3.2 Characterization of CM247LCpowders

Tap density (g/cc) 5.90

Density of alloy (g/cc) 8.54

Tap density (% Theoretical) 69.1

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A Counter particle size analyser was used to find out the range and particle sizes present.

Analysis using scanning electron microscopy was also undertaken to assess the shape and morphology of the powders.

3.1.2 Hot Isostatic Pressing

Consolidation of the powders was achieved by hot isostatic pressing (HIPping). This method involves the application of high temperature and high pressure of Ar gas. The powder needs to be encapsulated in a sealing medium that is deformable at the HIPping conditions to be used, and has good weldability. Mild steel was used as a capsule for all HIPping work and after HIPping the steel was either pickled away using acid or machined off. Cans welded from mild steel tubes with inner diameters ranging from 7 mm to 40 mm and thicknesses of 5 mm were used as capsules for HIPping samples. Before being filled with powders, these capsules were checked by the leak detector with a detectable leak rate of < 5×10^-12 mbar l/s, as shown in Fig. 3-1. A vacuum furnace (Fig. 3-2) was used to anneal the mild steel cans at 1000°C for 1 hour in order to get rid of impurities inside the cans which could contaminate the powders.

A vibration table was used to increase the flow rate during the process of filling powder into the mild steel cans and also to achieve a consistent tap density. The cans filled with powder were then outgassed at 10^-5 mbar for 20 h using an outgassing system and were crimp-closed before being welded to ensure vacuum-tightness. The sealed cans were then placed in the HIP furnace.

The pressure and temperature were increased until the final hold conditions were reached.

The cooling rate used after the HIP cycle is about 5°C/min. The density of the powder in the can increases as consolidation proceeds during the HIP cycle.

Fig. 3-1 A photograph

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1 A photographof leak detector for the checking of capsule’s sealing quality.

Mild steel capsule

of leak detector for the checking of capsule’s sealing quality.

Fig. 3-2 Vacuum furnace for annealing mild steel cans which are used for powder HIPping to get rid of impurities inside.

The EPSI Lab HIP facility (shown in Fig. 3 operating temperature of 1450

furnace with molybdenum heating elements, a heat shield, a water and a gas compressor system. The two

provide the heating, and thermocouples are positioned at the top and bottom of the two hot zones. A computer system is used to monitor the temperature and pressure continuously inside the HIP furnace. Temperature control is within

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2 Vacuum furnace for annealing mild steel cans which are used for powder HIPping to get rid of impurities inside.

The EPSI Lab HIP facility (shown in Fig. 3-3) contains a HIP uni

operating temperature of 1450 °C and pressure up to 200 MPa. This unit consists of a furnace with molybdenum heating elements, a heat shield, a water-cooled pressure vessel and a gas compressor system. The two-zone molybdenum furnace (see

provide the heating, and thermocouples are positioned at the top and bottom of the two hot zones. A computer system is used to monitor the temperature and pressure continuously inside the HIP furnace. Temperature control is within ±3 °C throughout the working zone.

2 Vacuum furnace for annealing mild steel cans which are used for powder HIPping to

3) contains a HIP unit with a maximum C and pressure up to 200 MPa. This unit consists of a cooled pressure vessel zone molybdenum furnace (see Fig. 3-4) is used to provide the heating, and thermocouples are positioned at the top and bottom of the two hot zones. A computer system is used to monitor the temperature and pressure continuously

C throughout the working zone.

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Fig. 3-3 EPSI Lab HIP facilities. Fig. 3-4 The two zone molybdenum furnace used for HIPping.

3.1.3 Heat treatment

3.1.3.1 Powder heat treatment

In addition to HIPping, powders they were also heat treated in two ways. In the first method powder was heated in an open crucible in a vacuum furnace which was evacuated at a carefully controlled speed to ~10^-4mBar before heating up. In the second method powder was sealed in the same type of mild steel can as used for HIPping, and the powder was heated after evacuating the can to a pressure of 10^-5mBar. The two different methods were

Heating elements at top zone

Heating elements at bottom zone

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used to study the influence of the trace oxygen intake if micro leakage occurred. The ramping rate of powder heat treatment was 3°C/min.

3.1.3.2 Heat treatment of HIPped materials

Solution treatment and ageing treatments were conducted in an air furnace. The heating rate was controlled at 8 °C/min. Cooling from the solution temperature was achieved either by pulling the sample from the hot furnace into the air which is called air cooling (AC) in the rest of the thesis; or by forced air cooling (FAC), by using a fan to increase the cooling rate.

Cooling from ageing temperature is mentioned as furnace cooling (FC), by isolating the heat elements of the furnace and keeping the samples in the furnace until temperature dropped to room temperature.