The determinations of yield stress, ultimate tensile
strength, ductility (%Elongation) and impact toughness of 70/30 cupronickel (NES 824) in the as-cast and HIPped condition were carried out using standard laboratory equipment: a J.J tensile testing machine and a Charpy impact testing machine. The strength and ductility
evaluations were carried out on cylindrical test specimens in accordance with BS500. The Metric A specimen
illustrated in Figure 71 is recommended for the testing of cast material. The tapered shoulders of the specimen
reduce the build up of stresses at sharp contours and induce fracture to occur within the gauge length of the specimen. The impact toughness specimens were rectangular sections containing a 45° notch as shown in Figure 72, the notch acting as a stress concentrator.
Mechanical property investigations have been carried out on cast 70/30 cupronickel test bars and material taken from weld plates of NES 824 and encompassed:
(i) MECHANICAL PROPERTY EVALUATION - TEST BAR MATERIAL
Determination of the effect of HIPping on the as-cast properties of NES 824. Cast test bars were subjected to hot isostatic processing for a sustain period of 4 hours at temperatures of 850°C, 950°C and 1025°C with argon gas
pressures of 103MPa. These investigations were carried out courtesy of H.I.P Limited and lead to the HIP temperature of 950°C being selected as the commercial processing
temperature for 70/30 cupronickel (NES 824) (173,213). The HIP pressure and sustain time were based on previous
experience gained from the HIPping of steel (214).
(ii) MECHANICAL PROPERTY EVALUATION - PLATE MATERIAL
A similar investigation to (i), in order to determine the effect of HIPping on the as-cast mechanical and
microstructural properties of NES 824 plate material. This investigation was carried out on encapsulated and HIPped sections of NES 824 weld plate material from which Metric A and Charpy impact specimens were machined. The HIP processing conditions included 4 hours at temperatures of 850°C, 950°C and 1025°C and argon gas pressures of
103MPa.
(iii) EFFECT OF HIP TIME ON PROPERTIES
Determination of the effect of HIP time on the mechanical properties of cast NES 824 HIPped at a
temperature of 950°C and gas pressures of 103MPa. The HIP time was varied to include 45 minutes, 2, 4 and 6 hours.
In addition to investigating the HIP time required to
optimise properties at a standard temperature and pressure, the effect of HIP time on the extent of homogenisation
(reduction in segregation/coring) of the cast structure was examined, using Scanning Electron Microscopy (SEM) together with quantitative ZAF4 EDAX analysis. The extent of coring can significantly effect the corrosion resistance of these alloys and therefore HIPping was expected to have a
beneficial effect.
(iv) HIP RECOVERY OF CAST 70/30 CUPRONICKEL PLATES
Determination of the effect of HIPping to recover cast plate sections of NES 824 which encompassed:
(a) Class I material which contained <5% porosity; and
(b) plates which contained shrinkage and microporosity;
in order to examine the change in material properties if any, during the closure of the pores. The plates were
encapsulated using standard techniques developed by Infutec Diffusion Bonding Limited (200) and HIPped at a temperature of 950°C with argon gas pressures of 103MPa for a sustain period of 4 hours.
(v) HIP RECOVERY OF LARGE CASTING DEFECTS
Determination of the effect of HIPping using standard commercial conditions for 70/30 cupronickel to recover a large casting containing a severe shrinkage defect as
illustrated in Figure 73. After encapsulation and HIPping the cast section was cut in half through the original
defect, one half of the casting being used in these investigations to compare the mechanical and
microstructural properties of HIPped material taken from sound and recovered defect areas. The second half of the casting was used in similar investigations carried out by the Ministry Of Defence.
(vi) THERMAL SIMULATION OF A HIP CYCLE
Investigations carried out by VSEL (215) on material subjected to a thermal simulation of a commercial HIP cycle, that is, heated to the treatment temperature of 950°C at a rate of 10-15°C/minute, holding for a period of
4 hours followed by cooling at a rate of 7-10°C/minute. These investigations revealed that stress relieving
material treated in the above manner for 2 hours at 475°C had a beneficial effect in regards to the strength
characteristics of 70/30 cupronickel. Thus, it was considered necessary to determine the effect of stress relieving on the properties of NES 824 HIPped under the conditions commercially developed for this material.
3.8 HOT TENSILE DATA FOR 70/30 CUPRONICKEL
The determination of high temperature yield stress and flow stress data for 70/30 cupronickel were carried out on an Instron tensile testing machine. The specimen was mounted in the grips between a load cell and a moving cross head. The load cell was connected to a chart recorder which registered the applied load and the extension in the
specimen. The cross head controlled the strain rate which was kept constant at 0.2cm/minute. The testing arrangement is shown in Figure 74, which involved a moveable furnace mounted on the front panel of the machine. The furnace could be moved out of the test zone to allow the insertion of the grips and the specimen assembly, after which the furnace could be returned to its original position. The grips were attached to the load cell and the cross head by universal joints. A resistance wound furnace coupled with a variable transformer was used to raise the specimen
temperature. The furnace was packed with Koal Wool at the top and the bottom to prevent heat losses and to control the specimen temperature to the desired accuracy. A
temperature variation of ± 3°C was observed at a specimen temperature of 950°C, which was within the range specified by BS3500. A chromal-alumel thermocouple was attached to the specimen surface to monitor the specimen temperature. The furnace was calibrated and the relationship between temperature and voltage obtained is shown in Table 30 and Figure 75. In addition to the temperature calibration, the furnace was calibrated to determine the position and extent of its hot zone. The relationship between temperature and distance from the top of the furnace is shown in Figure 76. This enabled the test specimens to be positioned in the furnace between 110-170mm from the top of the furnace, which ensured good temperature control along the gauge
length of the test specimen.
A cylindrical test specimen in accordance with BS500 as shown in Figure 77 was selected as a suitable hot tensile specimen, and using the test arrangement described
previously tensile tests were carried out within the
temperature range 300-1025°C. The specimen was stabilised at the test temperature and held for 20 minutes prior to the test being carried out. Care was taken to avoid
stressing the specimen during the stabilisation period, by adjusting the cross head, so that no load was generated on the specimen at any time.