ASM Metals HandBook Vol. 14 - Forming and Forging
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Fig. 5 Backward spinning of A-286 roll-forged tube (hardness: 200 HB max). Dimensions given in inches.
It was convenient to leave flanges at both ends and to trim these off later. The flanges prevented bell-mouthing and
permitted trimming of the portions likely to have small radial cracks.
Example 4: Explosive Forming of A-286.
A tubular workpiece was explosively formed inside a die (Fig. 6) to produce a part having an internal flange. If this part
had been produced by other methods, such a flange would have had to be welded on. A-286 sheet was rolled into a round
cylinder 405 mm (16 in.) in diameter, welded, solution treated, and descaled. Tolerance on the diameter was ±0.75 mm
(±0.03 in.).
Fig. 6 Explosive forming a case from 1.5 mm (0.060 in.) thick A-286. Dimensions given in inches.
The explosive used was gel dynamite. Six shots were used to form the workpiece. For shots 1 and 2, 15 g of explosive
was used; for shot 3, 18 g; for shot 4, 20 g; and for shots 5 and 6, 25 g. After three shots, the workpiece was solution
annealed and descaled.
Forming of Heat-Resistant Alloys
Revised by S.K. Srivastava and E.W. Kelley, Haynes International
Forming Practice for Nickel-Base Alloys
Two types of annealing treatments are used to soften the age-hardenable nickel-base alloys for forming, based on the
ductility needed for forming and, if subsequent welding is required, on the avoidance of adverse metallurgical effects
during and after welding. A high-temperature anneal is used to obtain maximum ductility and when no welding will be
done on the formed part. A lower-temperature anneal, resulting in some sacrifice in ductility, is used when the will be
welded.
For example, solution annealing of alloy 41 at 1175 °C (2150 °F) followed by quenching in water gives maximum
ductility. However, parts formed from sheet annealed in this way should not be welded; during welding or subsequent
heat treatment, they are likely to crack at the brittle carbide network developed in the grain boundaries. A lower annealing
temperature, preferably 1065 to 1080 °C (1950 to 1975 °F), results in less sensitization during welding and decreases the
likelihood of grain-boundary cracking. Formability is reduced by 10 to 20%, but is adequate for most forming operations.
Typical practice for forming nickel-base alloys is described in the following examples.
Example 5: Forming and Slotting Alloy X.
The 88 flutes in the workpiece shown in Fig. 7 were finish formed and slotted one at a time with hand indexing in a 450
kN (50 tonf) mechanical press at the rate of one piece every 14.6 min, including setup. Slots were required to be within
0.5 mm (0.02 in.) of true position.
Fig. 7 Finish forming of flutes and pi
ercing of slots one at a time in an alloy X workpiece using a mechanical
press. Hardness of the workpiece was 74.5 to 81.5 HR30T. Dimensions given in inches.
The work metal was 1 to 1.1 mm (0.04 to 0.044 in.) alloy X sheet. Before the mechanical press operations, the sheet had
been formed by a rubber-diaphragm process, electrolytically cleaned, annealed to 74.5 to 81.5 HR30T, pickled, restruck
in the forming press, and trimmed. The flutes were partially formed in this series of operations.
In choosing a method of finish forming, it was decided that the only way to form the flutes to the required shape was to
use a solid tool. The rubber-diaphragm forming process, however, was the best way to form the main contours of the part.
The flutes could not be fully formed by a conventional die alone, because the percentage elongation exceeded the limits
for alloy X (38 to 42% elongation in 50 mm, or 2 in.). By making use of the natural tendency of the blank to form
wrinkles, the flutes were preformed during rubber-diaphragm forming, but pressures were only enough to form them 75%
complete. However, the amount of elongation needed in the final die-forming operation was lowered, and definite
locations for flutes were provided; therefore, each flute could be produced in one stroke of the mechanical press. The
tooling (right side, Fig. 7) consisted of a die and a camactuated punch of high-carbon high-chromium tool steel hardened
to 58 to 60 HRC, as well as die inserts, stripper, and cam sections of lower-alloy air-hardening tool steel. The punch
pierced the slot and flattened the bulge above the flute. The stripper formed the flute when struck by the punch holder.
Example 6: Explosive Forming of Alloy 718.
Fully annealed alloy 718 (UNS N07718) sheet was used to make the flame deflector shown in Fig. 8. The sheet was rolled
onto a cylinder, with the grain direction at right angles to the long axis. A 115 mm (4.5 in.) outside diameter by 815 mm
(32 in.) long tube was gas tungsten arc welded from the cylinder using alloy 41 filler rod. The weld was made flush on the
inside, and the outside was ground flush to +0.13 mm (+0.005 in.). The tube was spun to the dimensions shown in Fig. 8,
fully annealed at 955 °C (1750 °F), and grit blasted. An outstanding characteristic of this alloy is its slow response to age
hardening, which enables it to be welded and annealed with no spontaneous hardening unless cooled slowly. Explosive
forming of the flame deflector was accomplished by three successive charges in a split die, and the workpiece was fully
annealed after explosive forming.
Fig. 8
Alloy 718 flame deflector (original sheet thickness: 1.8 mm, or 0.072 in.) produced by explosive forming
in three successive charges. Dimensions given in inches.
Forming of Heat-Resistant Alloys
Revised by S.K. Srivastava and E.W. Kelley, Haynes International
Forming Practice for Cobalt-Base Alloys
Forming the cobalt-base alloys requires more force because they are usually stronger than the iron-base and nickel-base
alloys. The cobalt-base alloys with less than 20% Ni, such as Elgiloy alloy (UNS R30003) and alloy 25, are more difficult
to form. Alloy N-155 (UNS R30155), a more formable alloy, has a tensile strength of 828 MPa (120 ksi), a 0.2% yield
strength of 414 MPa (60 ksi) and elongation of 40%. These alloys, like most of the nickel-base alloys, are age hardened
for elevated-temperature service. The practice used in forming HS-25 and N-155 parts is described in the following
examples.
Example 7: Explosive Forming of Alloy 25.
Figure 9 shows the setup used for the explosive forming of a tail-pipe ball from alloy 25 sheet. The sheet was gas tungsten
arc welded (butt) into a cylinder, and the shape was formed by three explosive charges. No annealing was done between
welding and the first two shots of explosive forming, but after the first two shots (50 g of dynamite for each), the
workpiece was withdrawn from the die, annealed at 1175 °C (2150 °F), and descaled. The workpiece was returned to the
die for further forming. The third explosive charge used 62 g of dynamite. Tolerance on diameters was maintained within
±0.25 mm (±0.01 in.).
Fig. 9
Alloy 25 (sheet thickness: 1.7 mm, or 0.066 in.) welded cylinder in position for explosive forming.
Dimensions given in inches.
Explosive forming was preferred over forming on an expanding mandrel. This is because the mandrel left flats on the wall
of the workpiece and explosive forming did not.
Example 8: Alloy N-155 Exit Nozzle Produced by Tube Spinning and Explosive
Forming.
The exit nozzle shown in Fig. 10 was produced from fully annealed 3.4 mm (0.135 in.) thick alloy N-155 sheet. The sheet
was rolled into a cylinder, with grain direction at right angles to the long axis, and was gas tungsten arc welded. The weld
was ground flush on both the inside and outside, after which the cylinder was tube spun to the various wall thicknesses
shown in Fig. 10. The workpiece was then placed in a die and explosively formed to the shape shown at right in Fig. 10.
Fig. 10 Alloy N-155 exit nozzle produced by tube spinning and explosive forming. Dimensions given in inches.
The underwater explosive-forming technique was used, with a vacuum of 3 kPa (0.03 atm) between the workpiece and
the die. The explosive charge was equal to 620 g of TNT and was placed at an average distance of 190 mm (7
1
2
in.) from
the workpiece walls. The first shot produced approximately 90% of the final shape. A second shot, using the same size
charge, completed the workpiece, after which it was fully annealed.
Forming of Heat-Resistant Alloys
Revised by S.K. Srivastava and E.W. Kelley, Haynes International
References
1.
B. Hicks, in The Development of Gas Turbine Materials, Applied Science, 1981, p 229-258
2.
S.K. Srivastava, Haynes International, unpublished research, 1985
3.
W.W. Wood et al., Report AFML 64-411, Project 8-143, U.S. Air Force Materials Laboratory, Jan 1965
Forming of Refractory Metals
Revised by Louis E. Huber, Jr., Cabot Corporation
Introduction
REFRACTORY METALS are generally worked in small quantities. Production rates are low, each piece is handled
separately, and the forming process is closely controlled.
Table 1 shows the composition of refractory alloys available as sheet. Typical conditions for bending 0.5 to 1.3 mm
(0.020 to 0.050 in.) thick sheet are given in Table 2. Tensile-forming parameters for sheet materials are summarized in
Table 3.
Table 1 Nominal compositions of refractory alloys available as sheet
Commercially pure niobium, tantalum, molybdenum, and tungsten sheets are also available.
Composition, %
Alloy
Zr
Ti Hf W
Other
Niobium alloys
Nb-1Zr 1.0
. . .
. . . . . .
. . .
FS-85 1.0
. . .
. . . 11.0
28.0Ta
C-103 . . .
1.0
10.0
. . .
. . .
C-129Y . . .
. . .
10.0
10.0
0.10Y
Nb-752 2.5
. . .
. . . 10.0
. . .
Tantalum alloys
Ta-2.5W . . .
. . .
. . . 2.5
. . .
Ta-10W . . .
. . .
. . . 10.0
. . .
Ta-8W-2Hf
. . .
. . .
2.0 8.0
. . .
Molybdenum alloys
Mo-0.5Ti . . .
0.5
. . . . . .
0.03C
TZM 0.1
0.5
. . . . . . 0.03C
Table 2 Conditions for the press-bra
ke forming of refractory metal sheet 0.5 to 1.3 mm (0.020 to 0.050 in.)
thick
Formed to a 120° bend angle in a 60° V-die at a ram speed of 254 to 3050 mm/min (10 to 120 in./min)
Minimum bend radius
(a)
Metal or alloy Forming
temperature, °C ( °F)
Test data Preferred
Springback,
degrees
Niobium alloys (annealed)
C-103, C-129Y Room <1t 1t
2-6
Tantalum alloys (annealed)
Tantalum Room <1t 1t
. . .
Ta-10W Room <1t 2t
1-5
Molybdenum alloys (stress-relieved)
Mo-0.5Ti, TZM
150 (300) 2t-5t 5t
3-8
Tungsten (stress-relieved)
(a)
t, sheet thickness
Ta
ble 3 Elongation and true strain in forming refractory metal sheet of various thicknesses and grain
directions
Results are based on testing of one heat of material for each alloy.
Thickness Elongation, % in
True strain
(c)
Alloy Condition
(a)
mm
in.
Forming
temperature,
°C ( °F)
Grain
direction
(b)
25 mm (1 in.)
50 mm (2 in.)
m
c
Niobium alloys
SR 0.76
0.030
Room L
T
18.0
6.0
14.0
4.0
0.122
0.041
0.145
0.046
C-103
A 0.76
0.030
Room L
T
30.0
26.0
24.0
21.0
0.152
0.150
0.232
0.197
Tantalum alloys
Ta-10W A 1.0 0.040
Room L
T
39.0
38.0
30.5
30.0
0.180
0.197
0.283
0.282
Molybdenum alloys
Mo-0.5Ti
SR 0.51
0.020
Room L
T
19.0
11.0
15.0
9.0
0.102
0.052
0.164
0.089
TZM SR 0.89
0.035
Room
Room
L
L
19.0
16.0
15.0
11.5
0.074
0.060
0.130
0.075
Tungsten
Tungsten
SR 0.89
0.035
595 (1100)
T 4.0 2.5 0.019
0.022
(a)
A, annealed; SR, stress relieved.
(b)
L, longitudinal; T, transverse.
(c)
ε
m
, true strain at maximum load; ε
c
, maximum true strain at maximum true stress
Forming of Refractory Metals
Revised by Louis E. Huber, Jr., Cabot Corporation
Formability
Niobium and tantalum alloys are usually formed at room temperature in the annealed (recrystallized) condition, although
the stress-relieved alloys are sufficiently ductile for most forming operations. Work hardening, especially of the stronger
alloys, often necessitates annealing after severe forming.
Strong alloys of niobium and tantalum, which are made in limited quantity, are not listed in Table 1. These alloys have
varying degrees of brittleness at low temperature, but can be formed by the same procedures used for molybdenum.
Molybdenum and tungsten are more difficult to form than niobium and tantalum, but if they are heated and certain
precautions are taken, even complex parts can be formed. The greatest difficulty in forming these metals is their tendency
toward brittle fracture (cracks and ruptures that occur with little or no plastic deformation) and delamination (a type of
brittle behavior that produces cracks or ruptures parallel to the plane of the sheet). Tungsten can be hot formed only; it is
brittle at room temperature.
At slow strain rates in tension and in bending, molybdenum and TZM alloys are ductile at room temperature, becoming
brittle at lower temperatures. However, because of the high variable strain rates and triaxial stresses produced in the usual
forming processes, these metals are usually hot formed in order to decrease the probability of brittle fracture.
Molybdenum and tungsten blanks must have prepared edges to prevent cracking and splitting during forming.
Molybdenum and tungsten are generally supplied in stress relieved condition. Recrystallization increases the ductile-to-
brittle transition temperature.
Effects of Composition on Embrittlement. Niobium and tantalum are severely embrittled by oxygen, nitrogen, and
hydrogen, even in minute amounts. However, the usual melting and processing techniques keep the metals pure enough
for good formability.
Some niobium alloys are more resistant to grain growth at high temperature than high-purity niobium. Alloys such as Nb-
1Zr and C-103 are high-strength materials that resist grain growth at high temperature. These alloys have fine grain
structure and elongate uniformly for forming and drawing operations.
Surface Contamination. The most common causes of surface contamination are failure to clean the surface properly
and failure to provide the proper atmosphere in heat treatment. Niobium and tantalum are usually acid pickled, and they
are heat treated in a vacuum or an inert gas atmosphere. Additional information on the heat treating of refractory metals is
available in the article "Heat Treating of Refractory Metals and Alloys" in Heat Treating, Volume 4 of the ASM
Handbook.
Generally, the high-strength alloys are more severely embrittled by surface contamination than the lower-strength alloys.
Molybdenum and tungsten are much less susceptible to surface contamination by oxygen and nitrogen than niobium and
tantalum.
Forming of Refractory Metals
Revised by Louis E. Huber, Jr., Cabot Corporation
Factors That Affect Mechanical Properties
The major variables that affect mechanical properties and formability are working temperature, temperature of anneals
between operations, percentage of reduction after the final anneal, and temperature of final heat treatment.
Rolling. Refractory metal sheet is generally made by hot forging or extruding billets to make sheet bars, which are rolled
to sheet at high temperature. The final rolling of niobium and tantalum alloys is done below 540 °C (1000 °F), often at
room temperature. The cold-worked sheet is given a final recrystallization anneal to improve formability and ductility.
The finish rolling of molybdenum and tungsten is done at high temperature, and final heat treatment is usually for stress
relief only. Cross-rolled sheet is generally more formable, because cross rolling makes ductility almost equal in all
directions.
Heat Treatment. In the annealed (recrystallized) condition and in the ductile range, refractory metals behave much like
steel. For example, recrystallized tungsten, although brittle at low temperature, has 35% uniform elongation and 50% total
elongation at 400 °C (750 °F). Cold working strengthens molybdenum and tungsten and makes them less formable.
Although molybdenum and tungsten are given a final stress relief, they retain their cold-worked structure.
Figure 1 shows the effect of heat treatment and strain hardening on the ductility and the ductile-to-brittle transition
temperature range of unalloyed molybdenum. The curves in Fig. 1 show that the ductile-to-brittle transition for unalloyed
molybdenum is between -18 and -45 °C (0 and -50 °F) for the stress-relieved condition, between 25 and -25 °C (80 and -
10 °F) for the stretch-strained condition, and at approximately 25 °C (80 °F) for the recrystallized condition.
Fig. 1 Effect of heat treatment and strain hardening on the ductility and ductile-to-
brittle transition
temperature range of unalloyed molybdenum sheet as determined in tensile tests. The ductile-to-
brittle
transition occurs in the temperature range in the steep portion of the ductility curves.
Transition Temperature. Niobium, tantalum, and their most frequently used alloys are readily formable, and they are
ductile at temperatures as low as -195 °C (-320 °F). Molybdenum and TZM, in the stress-relieved condition, have
transition temperatures just below room temperature. Molybdenum may fracture or delaminate at room temperature under
the high deformation rates and stresses generally encountered in forming practice. Therefore, molybdenum alloys are
generally formed at moderate-to-high temperatures. Stress-relieved tungsten has a transition temperature of 150 to 315 °C
(300 to 600 °F), so that all forming of tungsten must be done at high temperatures.
Figure 2 shows how temperature changes the strength and elongation of a typical high-strength niobium alloy with good
formability. A slight increase in temperature reduces yield strength but also reduces ductility. The ductility is lowest at
about 650 °C (1200 °F) and then increases with temperature. This reduced ductility is caused by strain aging, which is
characteristic of body-centered cubic metals.