Room-Temperature Properties of Inconel Alloy X750 Wire for Spring Temper
No. 1 Temper wire is hardened only a small degree by the cold reduction it has undergone,
but its rigidity is sufficient to permit uniform coiling on automatic machines.
Spring-Temper wire has been cold-worked to the extent that the load-carrying capacity
of springs made from it has been significantly raised. Thus, increased cold work
gives higher strength and higher working stresses but only up to service temperatures
approaching the stress-relieving temperatures.
Spring-Temper wire has a greater proportional limit but not greater resistance to
relaxation at moderate temperatures. For instance, a No. 1 Temper spring, precipitation-treated
1350°F/16 hr, loaded at 70 ksi, relaxed only 3% in 500 hr at 800°F, whereas an identical
spring made of Spring-Temper wire and also precipitation-treated 1350°F/16 hr, relaxed
12%. This difference in relaxation is believed to be the effect of a difference
in residual stresses brought about by cold work.
Therefore, for maximum strength from cryogenic temperatures up to about 700°F, the
Spring-Temper condition, precipitation-treated 1200°F/4 hr, is recommended. For
greatest relaxation resistance (up to about 900°F), No. 1 Temper, directly precipitation-treated
(1350°F/16 hr) springs are preferable.
For service from about 900° to 1200°F, springs should be made of Spring-Temper material solution-treated plus stabilization-treated plus precipitation-treated
(2100°F/2 hr, A.C., + 1550°F/24 hr, A.C., + 1300°F/20 hr, A.C.). Material given this triple-heat-treatment will have a lower proportional limit
but maximum relaxation resistance at 900° - 1200°F under stresses less than the proportional limit.
The reason for its superior relaxation resistance stems from cold working. A high percentage
of the cold work on a wire that has been reduced 15% (No. 1 Temper) will be on its
periphery; during solution treating grains will grow in the area that was cold-worked.
On the other hand, wire cold reduced 30-65% will be cold-worked throughout and uniform-size
grains will grow throughout its cross section.
In general, the relaxation strength is greater for wire with a uniform coarse grain structure.
The relaxation of springs cold-coiled from No. 1 Temper wire precipitation-treated
at 1350°F/16 hr and that of springs cold-coiled from Spring-Temper wire and triple-heat-treated
(2100°F/2 hr, A.C., + 1550°F/24 hr, A.C., + 1300°F/20 hr, A.C.) are shown in figures
below.