Thermal-mechanical working of wrought non-hardenable nickel alloy

The weld zone in Hastelloy X nickel superalloy is thermal mechanically worked by cold working the weld zone to reduce its thickness by about 5-40%, and by annealing at 1120.degree.-1175.degree. C. for one hour to cause recrystallization. Low cycle fatigue properties of laser and gas tungsten arc weld zones are substantially increased.

DESCRIPTION 
Background 
1. The present invention relates to thermal-mechanical processing of welded 
wrought nickel alloys. 
2. A related application, Ser. No. 440,673 "Contour Forming Conical Shapes" 
filed on even date herewith by Sanborn et al discloses how conical 
segments of combustor liners may be formed from the alloy Hastelloy X, for 
use in gas turbine engines. Part of the disclosed procedure involves 
making a longitudinal butt weld which runs circumferentially in the 
finished segment. Being so positioned, the weld is subject to particularly 
high stress when the segment is used in a gas turbine engine. As is well 
known, the properties of a cast weld zone will be different from those in 
the remainder of the wrought structure. 
Gas turbine engine combustor liners are particularly susceptible to low 
cycle fatigue failures. Hastelloy X, a nickel-base alloy, has been used in 
the construction of combustor liners for many years. The properties of 
welded Hastelloy X and the processing necessary to produce optimum 
properties are well characterized. However, it is also known that welded 
Hastelloy X, even after the post weld heat treatments, has weld zone 
properties which are substantially inferior to wrought Hastelloy X. For 
this reason, the design and construction of combustor liners heretofore 
has taken this inferiority into account. 
While there has been a well recognized desire to raise weld properties, the 
configuration of the combustor liners has not permitted more than the 
normal annealing heat treatments. Sheet metal burners, nominally 1.3 mm 
thick, are susceptible to excessive distortion when the assembly is heat 
treated at a temperature greater than 1100.degree. C. 
Now, an improved combustor liner design has been invented, as described in 
the patent application Ser. No. 227,317 "Combustor Liner Cooling Scheme" 
filed Jan. 22, 1981 by Dierberger; a novel way of constructing the 
combustor liner is disclosed in the aforementioned application of Sanborn 
et al. (The disclosures of both the foregoing applications are hereby 
incorporated by reference.) The new design and method of fabrication 
disclosed in the other applications provide the need and opportunity for 
improving the properties of welded Hastelloy X. 
DISCLOSURE OF THE INVENTION 
An object of the invention is to provide in a welded nickel base alloy, 
such as Hastelloy X, properties in the weld zone which are equivalent to 
the properties of the wrought material. A further object is to thermal 
mechanically process welded Hastelloy X, to improve its low cycle fatigue 
properties. 
According to the invention, a weldment made of wrought Hastelloy X is cold 
worked after welding so that the weld zone is reduced in cross sectional 
area by at least 5%, preferably 20%. The weld zone is then subjected to 
annealing at 1120.degree.-1175.degree. C. for one hour. In the invention 
there is an interrelation between reduction in area and the annealing 
temperature. Excess reduction in area or excess annealing temperature can 
result in excessive grain growth which is associated with low fatigue 
life. On the other hand, a combination of low reduction in area and low 
annealing temperature will produce poor metallurgical structure and poor 
fatigue life. 
For typical strain range conditions, the invention provides fatigue 
properties in the weld zone which are equivalent to unwelded Hastelloy X 
sheet. Fatigue life will be increased three times over that which results 
in the absence of the invention. 
The foregoing and other objects, features and advantages of the present 
invention will become more apparent from the following description of 
preferred embodiments and accompanying drawings.

BEST MODE FOR CARRYING OUT THE INVENTION 
The invention is described in terms of the welding of the alloy Hastelloy X 
(by weight percent 20.5-23 Cr, 0.5-2.5 Co, 17-20 Fe, 8-10 Mo, 0.2-1.0 W, 
0.05-0.15 C, 0-0.01 B, 0-1.0 Mn, 0-1.0 Si, 0-0.04 P, 0-0.030 S, balance 
Ni) which is a wrought product commonly available in sheet and mill forms 
under Aerospace Material Specification (AMS) 5536, AMS 5754 and 
Specification PWA 1038 of Pratt & Whitney Aircraft, East Hartford, Conn. 
Nominally, the alloy consists of 22 Cr, 1.5 Co, 18 Fe, 9 Mo, 0.6 W, 0.1 C, 
balance Ni. And in the present invention the Hastelloy X sheet stock is 
made to the aforementioned PWA 1038 specification which means it is the 
same as AMS 5536 except that the average grain size is finer, of the order 
of ASTM 8 compared to ASTM 4 for typical AMS 5536. Specifically, the 
invention is addressed to the forming of a weldment from butt welded 
pieces of material, in accord with the aforementioned Sanborn et al 
application. 
Various means may be used to make a linear butt weld including gas tungsten 
arc (GTA) and laser welding (LW). Subsequent to welding, all workpieces 
are stress relieved by heating to 1150.degree. C. for 8 to 15 minutes. 
Next the workpiece is cold rolled so that the cross sectional area of the 
weld zone is reduced by about 20%. By cold rolling is meant rolling at a 
temperature of less than 650.degree. C., typically ambient temperature. 
Subsequent to the cold reduction step, the weldment is subjected to an 
annealing heat treatment for one hour. The purpose of this heat treatment 
in the 1120.degree.-1175.degree. C. range is to produce recrystallization 
in the weld. However, significant grain growth must be avoided for the 
reasons set forth below. When properly performed, the annealing will 
result in a fully recrystallized weld zone having an average ASTM E-112 
grain size range of ASTM No. 4-8 (0.0898 to 0.0224 mm in nominal 
diameter). 
FIG. 1 shows the relationship between the reduction in cross sectional area 
of the weld zone and the annealing temperature insofar as 
recrystallization and grain growth are concerned. Generally, annealing 
temperatures less than about 1100.degree. C. are insufficient to cause 
recrystallization and those greater than about 1200.degree. C. produce 
excessive grain growth. From FIG. 1 it is seen that when the reduction in 
area is low, then relatively higher annealing temperatures are needed to 
obtain recrystallization. On the other hand, if the reduction in area is 
too great, then there may be somewhat more tendency for excessive grain 
growth (although it is well recognized temperature is the most influential 
variable). 
Excessive grain growth is associated with poor fatigue properties as our 
test data in FIG. 2 demonstrate. PWA 1038 material was annealed at 
different temperatures to produce grains ranging in size from coarse to 
fine. Fatigue life decreased by about a factor of two when the higher 
annealing temperatures caused heavy grain growth. While heavy reduction in 
area promotes good recrystallization dynamics, there are problems 
associated with it. First, cracking of the workpiece can occur upon great 
reduction in area unless intermediate annealing steps are employed (which 
are to be avoided for cost reasons). Secondly, as reference to the Sanborn 
et al application will show, there is a criticality in the reduction in 
area related to the configuration of the particular conical workpiece 
which is useful for combustor liners. As the Sanborn et al application 
indicates, the percentage reduction in area for their particular workpiece 
will be between 0-40%. Thus, it is a question as to what combinations of 
cold working and heat treatment are useable. 
FIG. 2 shows how the 870.degree. C. fatigue life is increased when grain 
size decreases as a result of thermal mechanical processing. (Note that in 
the figure, finer grain size is on the right, associated with bigger ASTM 
Number.) 
FIG. 3 shows the fatigue life for 870.degree. C. fully reversed bending of 
1.27 mm thick strip stock fatigue specimens with fully machined weld 
zones. It is seen that as-welded Hastelloy X has substantially inferior 
properties compared to the unwelded baseline material which was in the PWA 
1038 as-received mill annealed condition. However, for the specimens which 
were cold rolled to about 36% reduction and then annealed, it is seen that 
the properties are substantially improved, to be equal or better than the 
unwelded material. 
FIG. 4 provides additional data which enables a further distinction to be 
made based on 870.degree. C. reversed bending fatigue testing with 
.+-.0.3% strain for at least three specimens at each condition. First, it 
is seen that the as-welded specimens have a first set of properties, with 
the GTA welded specimens being inferior to laser welded specimens. Cold 
reduction and annealing increases the properties of both welds and there 
is indication of the superiority of the laser welding procedure. 
Table 1 indicates that heat treatments in the 1120.degree.-1150.degree. C. 
range are preferred because the higher 1175.degree. C. heat treatment is 
starting to cause coarse grain in the base metal and GTA weld. The Table 1 
1175.degree. C. data and FIG. 4 data tend to show that laser welding is 
superior to GTA welding. 
Accordingly, the foregoing data indicate that if cold reduction is omitted, 
fatigue life will be inadequate regardless of heat treatment. At least 
about 5-8% cold work is needed to promote recrystallization in combination 
with heat treatment at the higher end of the range, at about 1175.degree. 
C. With heavier reduction in area, at about 40%, lesser temperatures of 
about 1120.degree. C. may be used. Regardless of heavy cold reduction, 
temperatures less than about 1120.degree. C. produce inadequate 
recrystallization. Preferably the weld is cold reduced 20-35% and the 
annealing temperature is 1150.+-.15.degree. C. Typically, the weld zone is 
annealed for one hour. Lesser or greater time may be used, provided the 
grain size is measured to ascertain that ASTM 4-8 is obtained. 
The invention will be useable with fusion welding methods in addition to 
GTA and LW, including plasma arc welding, electron beam welding, gas metal 
arc welding, and so forth. In the preferred embodiment, we disclose cold 
rolling to reduce the cross sectional area of the weld zone. 
TABLE 1 
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Grain Size in Thermal Mechanically 
Worked Hastelloy X Alloy 
ASTM 
Cold One Hour Grain Size 
Type of 
Reduction Heat Treatment 
Weld Base 
Weld % .degree.C. Zone Metal 
______________________________________ 
GTAW 0 none (a) 8.3 
LW 0 none (a) 8.1 
GTAW 36 1120 8.6 9.4 
LW 36 1120 8.2 9.0 
GTAW 36 1150 7.8 8.0 
GTAW 36 1175 5.3 3.3 
LW 36 1175 8.1 4.3 
______________________________________ 
Grains too elongated to use an ASTM grain size number 
By weld zone we mean the region in the metal where fusion has taken place 
as well as the adjacent heat affected zone. Practically speaking, when the 
cross sectional area of the weld zone is reduced, the adjacent unaffected 
base metal will be also reduced to varying extent. Generally, this is not 
undesirable. Rolling to reduce the weldment cross section is preferred in 
longitudinal stock, such as in the Sanborn et al invention. But other 
techniques for reducing cross section including forging, swaging and so 
forth may be used in the alternative and for other material forms. 
We expect that our specific invention will be useable with alloys which 
have essentially the same composition as Hastelloy X, and we believe the 
general principles of our invention will be applicable to other nickel and 
cobalt base alloys. 
Although this invention has been shown and described with respect to a 
preferred embodiment, it will be understood by those skilled in the art 
that various changes in form and detail thereof may be made without 
departing from the spirit and scope of the claimed invention.