Applications

The ability of HIP to produce near-net shapes has been a primary impetus behind the development of HIP P/M parts. Conventional manufacturing methods for materials with high alloy content have low process yields and typically utilize only 10 to 30% of the material purchased in the final product; the remainder becomes scrap during machining. Hot isostatic pressing to near-net shape improves material utilization significantly during part manufacturing and finish machining. A hot isostatically pressed near-net shape part normally loses only 10 to 20% during final machining. The inability to provide nondestructive inspection of complex near-net-shape parts for certification has somewhat inhibited application of this technology, particularly for turbine engine applications.

High-Speed Tool Steels. The development of gas-atomized prealloyed steel powders in the 1960s (Ref 3) led to HIP P/M tool steels. This represented the first production application of HIP for a relatively low-cost material. Hot isostatic pressing improves the microstructure of tool steels by preserving the fine grain size and carbide distribution present in the atomized powder through the consolidation process. Increased homogeneity of the fine carbides throughout the material is an added benefit. Superior tool properties result from the improved microstructure. Shape stability during subsequent heat treatment is superior in HIP material. Grindability, wear resistance, and uniformity of hardness also are improved. Additionally, cutting performance of high-speed tool steels is improved by this processing treatment, due to the increased toughness related to fine austenite grain size. New high-alloy-content steels with enhanced material properties can be produced. High-speed tool steels are generally consolidated in billet form. A HIP high-speed steel compact is shown in Fig. 20.

Fig. 20 Large-sized cylindrical highspeed steel billet. Courtesy of Crucible Materials Research Center

Nickel-Base Superalloys. Starting with development in the early 1970s, nickel-base superalloys have evolved into one of the best applications for the P/M HIP technology. More than 5000 tons (4545 metric tons) of superalloy components are currently operating in commercial and military aircraft turbine engines. Hot isostatic pressing of forging preforms represents a significant portion of the current production, but there are approximately 100,000 as-HIP parts in service as well. The use of HIP P/M consolidation for superalloys is economically attractive because of its near-net-shape capabilities. High-alloy-content superalloys can be produced with attractive properties. Superalloys strengthened by a large volume fraction of second-phase 7' undergo severe segregation during ingot solidification. Such ingots would be virtually unworkable by conventional hot-working techniques for large-size parts. The division of the melt into small powder particles during atomization eliminates macrosegregation, and microsegregation is reduced because of high cooling rates during particle solidification. Hot isostatic pressing of these powders produces a homogeneous microstructure that improves mechanical properties and hot workability.

Superalloy powders are typically made by inert gas atomization or REP. Care must be taken in processing to avoid the presence of stable nonmetallic compounds on the surface of the powder particles because they can be detrimental to the properties of consolidated products. The article "Powder Metallurgy Superalloys" in this Volume discusses the properties of many nickel-base superalloys made via the HIP P/M process. A comparison of HIP properties with other forms is given in Table 3.

Table 3 Heat treatments, grain size, and tensile properties of René 95 forms

Heat

Extruded and

Hot isostatic

Cast and wrought(c)

treatment/property

forged00

pressing(b)

Heat treatment

1120 °C (2050 °F)/1 h AC + 760 °C (1400 °F)/8 h AC

1120 °C (2050 °F)/1 h AC + 760 °C (1400 °F)/8 AC

1220 °C (2230 °F)/1 h AC + 1120 °C (2050 °F)/ 1 h AC + 760 °C (1400 °F)/8 h AC

Grain size, .''-'m (mils)

5 (0.2) (ASTM No. 11)

8 (0.3) (ASTM No. 8)

150 (6) (ASTM No. 3-6)

40 °C (100 °F) tensile

properties

0.2% yield strength, MPa

1140 (165.4)

1120 (162.4)

940 (136.4)

(ksi)

Ultimate tensile strength,

1560 (226.3)

1560 (226.3)

1210 (175.5)

MPa (ksi)

Elongation, %

8.6

16.6

8.6

Reduction in area, %

19.6

19.1

14.3

650 °C (1200 °F) tensile

properties

0.2% yield strength, MPa

1140 (165.4)

1100 (159.5)

930 (134.7)

(ksi)

Ultimate tensile strength,

1500 (217.6)

1500 (217.6)

1250 (181.3)

MPa (ksi)

Elongation, %

12.4

13.8

9.0

Reduction in area, %

16.2

13.4

13.0

Source: Ref 28, 29

(a) AC, air cooled. Processing: -150 mesh powder, extruded at 1070 °C (1900 °F) to a reduction of 7 to 1 in area, isothermally forged at 1100 °C (2012 °F) to 80% height reduction.

(b) Processing: -150 mesh powder, HIP processed at 1120 °C (2050 °F) at 100 MPa (15 ksi) for 3 h.

(c) Processing: cross-rolled plate, heat treated at 1218 °C (2225 °F) for 1 h.

Source: Ref 28, 29

(a) AC, air cooled. Processing: -150 mesh powder, extruded at 1070 °C (1900 °F) to a reduction of 7 to 1 in area, isothermally forged at 1100 °C (2012 °F) to 80% height reduction.

(b) Processing: -150 mesh powder, HIP processed at 1120 °C (2050 °F) at 100 MPa (15 ksi) for 3 h.

(c) Processing: cross-rolled plate, heat treated at 1218 °C (2225 °F) for 1 h.

Heat treatment after HIP can have significant effects on material properties as shown in Table 4. Material response to post-HIP treatment depends on the processing conditions. Near-net-shape parts also may be subject to distortion during post-HIP heat treatment. If complex shapes are required, the ceramic mold process is suitable, particularly for static parts. If a carbon or stainless steel container is used for powder consolidation, a 0.5 mm (0.02 in.) diffusion zone may surround the part. This does not cause a problem in the final part because HIP envelopes usually exceed this dimension. Hot isostatic pressing conditions are alloy dependent. Processing temperatures may be keyed to the /' solvus temperatures for purposes of grain size control in nickel-base superalloys.

Table 4 Mechanical properties of hot isostatically pressed plus conventionally forged Nimonic alloy AP1

Processing

Size

of

Solution

Tensile properties(a)

Stress rupture(b)

temperature

sample

treatment

Yield

Ultimate

Elongation,

Reduction

Notched

Plain

Elongation,

Notch

Low-

disk

point,

tensile

%

in area,

tensile

life,

%

life,

cycle

0.2%

strength

%

strength

h

h

fatigue(c),

offset

cycles

°C

°F

mm

in.

MPa

ksi

MPa

ksi

MPa

ksi

1150

2100

150

6

4 h at 1110 °C (2030 °F), air cool

971

141

1307

190

30.4

31.6

1869

271

42

30.1

195

>276,000

1150

2100

150

6

4 h at 1080 °C (1980 °F), oil quench

1120

162

1513

219

23.2

24.2

1992

289

64

15.3

159

>307,000

1150

2100

150

6

4 h at 1110 °C (2030 °F), quenched and aged(d)

1037

150

1381

200

30.4

46.7

1776

258

88

20.4

163

>214,000

1220

2230

150

6

4 h at 1110 °C (2030 °F), air cool

999

145

1328

193

28.6

32.7

1868

270

45

20.5

188

>155,000

1220

2230

150

6

4 h at 1080 °C (1980 °F), oil quench

1085

157

1463

212

23.2

23.4

1941

281

66

17.2

247

>228,000

1220

2230

150

6

4 h at 1110 °C (2030 °F), quenched and aged(d)

1052

153

1383

201

25.0

25.8

1844

267

74

16.9

315

>242,000

1150

2100

475

19

4 h at 1110 °C (2030 °F), air cool

952

138

1320

191

29.5

31.4

1521

221

85

22.9

>500

>35,000

1150

2100

475

19

4 h at 1080 °C (1980 °F), oil quench

993

144

1356

197

26.1

28.0

1785

259

113

20.3

>450

>100,000

All material aged 24 h, 650 °C (1200 °F); air cooled; 8 h, 760 °C (1400 °F); air cooled.

(d) 50% water-soluble polymeric compound, 50% water

Oxide-dispersion-strengthened superalloys also can be consolidated by HIP. Prior to processing, alloy powders, additives, and oxide dispersoids are put in a high-attrition ball mill and mechanically alloyed. This ensures fine grain size and uniform oxide distribution throughout the powder. Hot isostatic pressing produces fully dense material with these microstructural features maintained.

Titanium-Base Alloys. Powder production for titanium and titanium alloys requires special setups because of the reactivity of titanium. The hydride/dehydride process is the most common way to make titanium powders, but the particles resulting from this process are not spherical and thus do not work well for near-net-shape processing. The early method used to make spherical titanium powder was the REP. This was later supplanted by PREP to reduce contamination. Either of these processes depends on the ability to manufacture bar product of the alloy being made into powder. In the late 1980s, an inert-gas-atomizing technique was developed for titanium and its alloys (Ref 30). By the use of inert atmosphere or vacuum induction skull melting, the titanium alloy is brought to the molten state. The liquid is then poured through a metallic nozzle into a high-pressure gas stream. The metal breaks up and resolidifies as spherical titanium particles. The powder is collected in a cyclone system designed to cool the powder to prevent sintering.

There are any number of applications for titanium and titanium alloy powders. In the late 1970s and through the 1980s, the Air Force Materials Laboratory supported many programs to develop near-net shapes for military uses (Ref 31). For many reasons, this work never resulted in an ongoing production process, even though there is still some experimental work being performed currently. All of the meaningful earlier work was conducted with PREP powder. When the gas-atomized powder became available, it was used for all subsequent activities. At that time, the emphasis changed to applications needing titanium aluminide powders. Because these can be easily made by the skull-melting/gas-atomization process, the bulk of the experimental work is currently being performed in this area. The powders are now being used to manufacture metal-matrix composites and intermetallic-matrix composites. The advantages of these products are their light weight, high strength, oxidation resistance, and creep resistance at high temperatures.

Cemented Carbides. Tungsten-carbide/cobalt tools are the premier example of containerless HIP to achieve full density by removing residual porosity. Superior transverse rupture strength results from HIP. The wear performance of cutting tools at high speeds is not significantly improved, however, because this behavior is governed by the hardness of the material rather than by its fracture properties. Low cobalt content (3%) alloys can be produced by HIP to give enough toughness for use in drawing dies.

Fully dense cemented carbide can be finished to give a perfectly smooth surface, which is required for high-quality rolls, dies, mandrels, and extrusion tools. Generally tungsten-carbide/cobalt tool materials are manufactured by CIP and sintering of blended powders, followed by HIP. Typical conditions for HIP are 1290 °C (2350 °F) at 100 MPa (15 ksi) for 1 h. Cemented carbide parts produced using HIP are shown in Fig. 21.

Refractory Metals. Consolidation of refractory metals by HIP is a two-step process. Processing these materials to net and near-net shape promotes conservation of these critical resources. Niobium alloy C-103 (Nb-10Hf-lTi-5Zr) has been successfully hot isostatically pressed using a duplex cycle. Hydride/dehydride and PREP powders are consolidated in a plain carbon steel container filled with powder at 1260 °C (2300 °F) at 100 MPa (15 ksi) for 3 h. The container is then removed in a nitric acid solution and further chemically milled in a nitric-hydrofluoric acid solution to remove the alloy/container interaction layer. The material is finished in a HIP step at 1590 °C (2900 °F) at 100 MPa (15 ksi) for 3 h to a final density in excess of 99% of theoretical. Room-temperature and high-temperature (1650 °C, or 3000 °F) tensile strength and ductility properties compare favorably to wrought alloy properties. The ductile/brittle transition temperature is higher (-18 °C versus 160 °C, or 0 °F versus 320 °F, for standard products) in the HIP material due to increased oxygen content. Gas content of the hydride/dehydride material results in poorer weldability than the PREP powder. Hydrogen embrittlement also occurs in the hydride/dehydride alloy C-103. Vacuum baking at 870 °C (1600 °F) for 2 h eliminates embrittlement, and the alloy will fail in a ductile manner in tensile and Charpy tests.

Fig.

parts Ref 22

21 Tungsten-carbide/cobalt produced by HIP. Source:

Near-net shape forward bowls manufactured by consolidation of C-103 in the duplex HIP cycle are shown in Fig. 22. The diameter of the bowls was within 0.13 mm (0.005 in.) of final dimensions. The P/M net shape weighed 0.8 kg (1.8 lb). This, compared with rough forging weighing 1.7 kg (3.8 lb) and a final part weighing 0.6 kg (1.4 lb), illustrates the material savings achieved by HIP to near-net shape.

Included is a provision for parts to be low-temperature HIP to a closed porosity condition, decanned, and re-HIP usually at higher temperatures. This option can be employed when the powder/container integration (melting, alloying, contamination, etc.) is unacceptable at the preferred higher HIP temperature. This technique has been used, for example, for niobium alloys that are initially hot isostatically pressed at 1205 °C (2200 °F) in low-carbon-steel containers, decanned, and re-HIP at 1595 °C (2900 °F) to circumvent an iron-niobium eutectic reaction at 1360 °C (2480 °F).

Stainless Steels. One of the most prominent applications of the HIP P/M technology is in the area of stainless steels. Both duplex and austenitic steels have been used extensively as P/M near-net shapes in the oil and gas and petrochemical industries. For example, valve bodies, fittings, and large complex manifolds for piping systems have been successfully produced in a cost-effective manner via HIP processing. Figure 23 (Ref 32) shows some of the typical fittings that have been made from 254 SMO material. Figure 24 (Ref 32) is a valve body that weighs more than 2 tons and was made from an austenitic stainless steel. Large manifolds with integral outlets hot isostatically pressed from a superduplex stainless steel have also been put in service in an offshore oil rig in the North Sea (Ref 32). In addition to the other benefits of a HIP P/M approach, the manifold can be fabricated in far less time and avoid costly welding processes. An analysis of the cost factors showed a greater than 20% savings over a similar manifold produced from fabricated cast and wrought components (Ref 32).

Fig. 23 Tees for underwater applications in the offshore industry hot isostatically pressed in 254 SMO grade. Weight: 155 kg/pc

Fig. 24 Hot isostatically pressed valve body in austenitic stainless steel. Weight: 2 t

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