Manufacture of torsion bars

A method of making high performance torsion bars from austenitic stainless steels which comprises the steps of fabricating a blank complete with splined ends and torsion working said blank below the Md temperature of the material to a stress value above 80-90% of its ultimate. Prior working may be done in tension at below the Md temperature, and subsequent precipitation hardening may be added to the basic process.

BACKGROUND OF THE INVENTION 
The present invention relates to manufacture of stainless steel products 
and has particular reference to manufacture of high performance torsion 
bars for motor vehicles. 
The high performance features described below have great value for military 
vehicles such as tanks which are designed to be capable of maneuvering 
over large obstructions at increasingly higher speeds while still 
maintaining acceptable dynamics for the crew. 
The desired properties of a high performance torsion bar are a specified 
torsional stiffness (angle of twist for a given length and twisting 
moment) and the highest possible cylic angle of twist consistent with 
survival of a specified number of applications of that twist. 
To meet these requirements, the optimal material will have the lowest 
Young's modulus for its class and the highest cyclic elastic strain 
capability for the specified cyclic life. The elastic strain capability is 
usually enhanced by inducing a beneficial state of residual stress in the 
bar by twisting it beyond its yield point. This twisting leaves the bar 
ends permanently twisted under no torque. Although the larger the 
permanent twist the larger the beneficial effect, for each material there 
is a safe upper limit above which the material is damaged and no benefit 
accrues. With zero permanent twist, the allowable elastic strain increases 
with increasing cyclic strength. 
It will be seen that among steel bars, the proposed procedure produces a 
steel with 10-20% reduction in Young's modulus with a high cyclic elastic 
strain capability because of its very high cyclic strength aided by an 
order of magnitude higher permanent twist than is possible with heat 
treated steels. 
A number of metals display what is known as a martensitic transformation. 
The martensitic transformation is a rearrangement of the crystallographic 
structure without any change in the chemical composition of the crystal 
structure and results in a material characterized by new mechanical 
properties. The transformation is diffusionless. Moreover, such 
transformation in materials in which they occur are spontaneous at certain 
temperatures. For instance, as the temperature of the material is dropped, 
a temperature point will be reached where a martensitic transformation 
will commence occurring spontaneously. This temperature is known as the Ms 
temperature. The martensitic transformation will progress further as the 
temperature of the material continues to be dropped until at a certain 
temperature, generally known as the Mf temperature, there will be maximum 
spontaneous martensitic transformation, that is, as much martensitic as 
can be formed will be formed. It has been found that the martensitic 
transformation can be started above Ms temperature if the material is 
plastically deformed, that is, if irreversible mechanical work is put into 
the material. However, there is a maximum temperature above which no 
martensitic transformation will occur even if deformation takes place. 
This temperature is known as the Md temperature. Moreover, it has been 
found that at temperatures below the Ms temperature, the martensitic 
transformation can be made to progress further than it normally would 
spontaneously, provided the material is mechanically deformed at such a 
temperature. 
In view of this knowledge and finding, we have discovered that with torsion 
springs made of materials which exhibit a martensitic transformation, it 
is desirable to deform the spring blank at a temperature below the Md 
temperature, and, preferably, close to the Ms temperature and most 
preferably at or slightly below the Ms temperature. When the deformation 
of such blanks is performed at such temperatures, the blank will gain in 
strength not only due to the inelastic stretching of the material but also 
due to the crystallographic transformation of the material to the 
generally stronger martensitic phase. 
This invention exploits the known fact that austenitic stainless steels are 
capable of extremely high strains with a concurrent permanent increase in 
ultimate and yield strengths when worked at temperatures where martensitic 
transformation takes place, i.e. between the Md and Ms temperatures. For 
many of the popular grades of stainless steel the Md temperature is near 
that of dry ice (-100.degree. F) while the Ms temperature is very low and 
comparable to that of liquid nitrogen (-320.degree. F). However, the 
novelty of the invention lies in the particular sequence of operations it 
employs to strain harden the material at the martensitic transformation 
temperatures. 
The essential steps for making a torsion spring according to the invention 
are these: 
1. Determine stress-strain relationships in tension and in torsion at the 
temperature at which the working will take place, e.g. the Md temperature 
of the material to be used. Preferably specimens of the actual batch of 
raw material to be used will be so tested. Also, determine the change in 
density undergone by this material during martensitic transformation. 
2. Using this data establish the required dimensions of the workpiece prior 
to straining such that the strained article will have the desired 
dimensions without subsequent machining. 
3. Fabricate the torsion bar blank according to the dimensions determined 
in step 2. Preferably the heavier end regions are created by upsetting the 
ends of the bar. The end splines are made by forming or machining grooves 
into the upset ends prior to straining operations. 
4. Place the blank into an environment which brings its temperature below 
the Md temperature thereof. 
5. Torsion load the blank to a stress level between the yield point and 
ultimate. The direction of twist must correspond to that which the torsion 
bar will experience when installed and in operation. 
For this procedure to be of value, the bar must be made to substantially 
final dimensions, including the spline region, taking into account the 
decrease in density which occurs during the martensitic transformation. 
Machining the spline region after forming would adversely affect this bar 
by removing portions of the strengthened structure. It should be 
recognized that the spline bearing region will yield and deform somewhat. 
The dent produced by a loose mating spline will be straight except for the 
effect produced by small variations in elastic springback along the length 
of each spline. The deepest region of the spline cavity where no bearing 
occurs may twist with respect to the straight dent made by the external 
spline tooth. 
The extensive straining in torsion with subsequent metallurgical changes in 
the grain structure, produces a substantially higher final strength than 
that achievable by cold working procedures normally used to harden these 
stainless steels. The yield strength is also increased substantially, and 
most significantly, a piece fabricated by this process achieves a high 
endurance limit. Although strength or endurance of this magnitude is 
exhibited by other steels, it is the unique combination of low cost, 
flexibility, beneficial higher residual stress, high strength, and high 
endurance obtained by this process, combined with the inherent corrosion 
resistance of the material, which produces a superior torsion bar. 
A particularly significant point of superiority exists at the splines, 
where stress concentrations are responsible for many premature fatigue 
failures in existing bars. The toughness and notch resistance of bars made 
by the new process, aided by large regions of high beneficial residual 
stresses in the spline reduce the likelihood of this type of failure. 
In one alternative, the spline region could be worked by compression, 
twisting or bending to strengthen them prior to cutting the splines and 
final twisting. 
In other alternatives, some refinements to the basic procedure may be added 
if desired. For example, prior to the torsion working, the workpiece may 
be worked in tension at a martensitic transformation temperature to 
strengthen the spline regions before forming them on the bar ends. The 
tension load applied should be about 80-95% of the ultimate. 
Also, after torsion working, the article could be precipitation hardened, 
for example by aging in a heated oven and subsequent cooling to room 
temperature without quenching. For a material such as ASTI 304 or ASTI 301 
the torsion bar would be heated for 20 hours at 780.degree.-790.degree. F. 
The value of precipitation hardening depends on the degree of cold work; 
for severe cold work, precipitation reduces the resulting strength.

The torsion bar 10, shown in FIG. 1, is an elongated steel bar having a 
working length 11 and splined ends 12, 13 which fit into cooperating 
fixtures (not shown) such as might be found in a motor vehicle suspension. 
The initial step in the manufacture of the torsion bar 10 in accordance 
with the present invention is to determine the stress-strain 
characteristics of the raw material to be used. These are determined by 
well known methods for the material in torsion and in tension, if desired, 
at or near the Ms or Md temperature of the material. FIG. 2 shows typical 
stress-strain characteristics for a material such as ASTI 304 stainless 
steel (without specific values for stress and strain). Curve I represents 
loading in tension, Curve II represents loading in torsion. The tensile 
curve is true-stress vs true-stain. The torsion curve is nominal stress vs 
strain. It will be seen that Curve I initially rises linearly, reaches an 
elastic limit at A, yields with small increases in stress to B, then rises 
substantially linearly to a knee C reaching an ultimate value at D, at 
which point the material ruptures. Curve II has a substantially similar 
shape except that the torsional strain which the material withstands after 
the knee C' before rupture of the bar at D' is many times the strains 
withstood in tension. Curves I and II are separated for clarity and no 
significance should be ascribed to their relative positions in FIG. 2, 
which Figure is intended to depict only the shape of characteristics. 
With the information of curve II and density change measurements in hand, a 
blank 14 of that material (e.g. stainless steel ASTI 301 or 304) as shown 
in FIG. 1 is designed and fabricated, and placed into the jaws 23, 24 of a 
torsion machine (the rest of the machine is not shown) as seen in FIG. 3. 
The blank 14 comprises a central portion 11 and ends 12, 13 into which 
splines have been cut, formed or rolled. Preferably the blank 14 is made 
by upsetting the ends of a long bar to make the greater diameter ends 12, 
13 rather than by machining away material from a rod of greater diameter 
in the interest of conservation of materials. 
A cryostat 19, in FIG. 3 surrounds the blank 14 and rests on the lower jaw 
24 sealing the aperture 18 in the cryostat 19. The cryostat 19 is filled 
with alcohol and dry ice 25 to bring the bar 14 near its Md temperature. 
The jaws 23, 24 are rotated relatively to one another, to twist the 
portion 11 of bar 10 and to work that portion cryogenically under torsion. 
This working, done in the region between C' and D' of the curve II, to 
impart the desired strength to the bar 10 is most effective at the surface 
of the bar where the more intensive working takes place due to its greater 
radius. 
The direction of torsional stress applied during working must conform to 
the direction of stress encountered during operation. Since torsion bars 
on the right and left hand suspension of a motor vehicle are exposed to 
torsional stresses of opposite character, the torsion bars are made, using 
either clockwise or counterclockwise torsion stresses as appropriate. 
Since the strengthening in this case is concentrated on the surface (the 
greater strains take place at a greater radius) it is imperative that the 
blank 14 can be designed initially to require minimum machining after 
forming in order that the strengthened material is not removed. To this 
end it must be recognized that a decrease in density (about 3.0%) occurs 
during martensitic transformation and proper steps must be taken to 
include that decrease in the design calculations. If the spline region is 
machined prior to strengthening by working, the spline region will deform 
more than when machined after initial strengthening. 
This deficiency of excessive twisting may be alleviated somewhat by working 
the spline regions by pulling, compressing, twisting or bending so as to 
strengthen them prior to cutting the splines and the final twisting of the 
bar. 
FIG. 4 shows an alternative prior step in the process involving initial 
tension working. The blank 14' comprises a central portion 11, 12, 13 
(corresponding to the portions 11, 12, 13 of FIG. 1) stretch end grips 15, 
16 and a conical transition piece 17 between the grips 13 and 15. 
Preferably this blank is also made by upsetting and forming the ends of a 
long bar into the respective sections 12, 13, 15, 16 and 17 rather than by 
machining away material from a rod of considerably greater diameter than 
the central portion 11. 
The stretch grip 16 of blank 14' is fed through the aperture 18 in the 
bottom of cryostat 19, and the cryostat 19 set to rest on the conical 
piece 17 which effectively plugs up the aperture 18. Stretch grips 15 and 
16 are placed in the jaws 20, 21 respectively of a stretching machine (not 
shown) and the cryostat 19 is filled with either dry ice or with liquid 
nitrogen 22 to bring the work piece 14' to a temperature near its Md or Ms 
temperature respectively. 
The tensile machine (not shown) drives jaw 20, 21 apart thereby applying 
tension to the blank 14' which tension is increased gradually to a value 
of 80-90% of the yield stress, point D on Curve I. The blank 14' is 
removed from the stretcher and returned to room temperature. 
An alternative grip which lends itself to more rapid capture in the machine 
is shown in FIG. 4A. Here, the stretch grip 15 terminates in a T-section 
26. The grip 20a includes a socket 27 for capturing the section 26, and 
the cryostat wall is attached directly to the grip 20a. The T section may 
enter the socket from above and be locked therein by twisting, or the 
T-section may enter the socket from the front and slide therein. The 
particular arrangement used is not critical to the practice of this 
invention. 
In yet another alternative, the jaws 20, 21 may apply compressive, twisting 
or bending forces to the spline regions 12, 13 rather than tension for the 
initial cold working. 
Now, the torsion bar 11, 12, 13 is cut out from the blank 14' and the ends 
12, 13 are machined to have splines appropriate for the holding sockets in 
which they are to be placed. Typically there may be more than 50 splines 
and the circumference, about 0.020 inch deep. 
After the step illustrated in FIG. 4, and described in connection 
therewith, the torsion bar (having been cut free of the blank 14') is 
placed in the cryostat of FIG. 3 and torsion worked as described earlier. 
After the torsion working the bar may be further strengthened by 
precipitation hardening by aging at 780.degree.-790.degree. F for 20 hours 
and subsequent slow cooling to room temperature without quenching. 
It should be recognized that the methods here described are not limited to 
torsion springs but that other articles can benefit from the procedure 
here described. Propeller shafts for small boats, being primarily stressed 
torsionally in operation are particularly attractive subjects, for 
example. Retaining pins for tank treads is another example of a product 
which can be made by the method here described. 
It is known that some austenitic stainless steels have Ms temperatures near 
absolute zero while others have Ms temperatures near that of dry ice. Some 
non-standard stainless steels even transform at room temperatures. It 
should be understood therefore that the terms Ms and Md temperatures in 
the claims appended hereto should be given their widest interpretation so 
as to cover all these situations. Although we prefer to use steels with Ms 
temperatures somewhat below the boiling point of liquid nitrogen 
-320.degree. F and Md temperatures near that of dry ice -100.degree. F we 
do not want to be limited to those steels.