Failure of 17-4 PH Stainless Steel Bolts on a Titan Space Launch Vehicle
L. Raymond and E.G. Kendall, Failure of 17-4 PH Stainless Steel
Bolts on a Titan Space Launch Vehicle,
93 (No 1) Jan 1968 as published in
Source Book in Failure Analysis, American Society for Metals, 1974, p 105–107
Several broke at the shank, and failure was attributed to stress-corrosion cracking. But results could not be duplicated in laboratory with salt-solution immersion tests until the real culprit was established: the secondary effect of galvanic coupling—hydrogen embrittlement.
Bolts; Galvanic corrosion; Spacecraft
(Precipitation-hardening stainless steel), UNS S17400
Hydrogen damage and embrittlement
Fig. 1 Success of the mission depends greatly on the reliability of high-strength
stainless steel fasteners.
Analysis of service failures is complicated because several mechanisms
operate simultaneously or sequentially. For example, a structural component
in a space launch vehicle can be simultaneously subjected to stress, a marine
environment, and complex fatigue loads and frequencies.
A series of bolts on thrust control valves for a Titan vehicle failed
at Cape Kennedy (Fig. 2
were made of 17-4 PH in the H900 condition — a martensitic, age-hardening
alloy. In the H900 condition (solution treated and aged 1 hr at 900 F), the
material had nominal strengths of 180,000 psi yield and 200,000 psi tensile.
The alloy's composition: 0.07 C max, 15.5 to 17.5 Cr, 3.00 to 5.00 Ni, 3.00
to 5.00 Cu, 0.15 to 0.45 Cb+Ta, and trace elements of 1.00 Mn max, 1.00 Si
max, 0.04 P max, and 0.03 S max.
Fig. 2 Thrust control valve bolts failed because of hydrogen embrittlement.
Preventing Future Failures
Laboratory findings indicate that the most fool-proof method of preventing
such failures without sacrificing any notched yield strength is to age the
bolts for one more hour at 1,000 F. Anodized aluminum in the flange interface
will minimize corrosion — it will also inhibit galvanic corrosion only
if the bolts are inserted carefully and the anodized layer is not abraded.
Failure analysis established that the composition was within specification,
as were hardness and tensile properties. Fracture was intergranular; there
was no evidence of grinding, burning, mechanical damage, or other microstructural
The significant observation was that the failure of the bolt was located
in the vicinity of a 7075-T6 aluminum alloy flange which corroded —
evidenced by white, chalky corrosion products. Rust spots (pits) were observed
along the shank about
in. from the head. At this point, the bolt emerges
from the flange. The formation of rust spots could be prevented by covering
bolts with a suspension of graphite in grease (Lox-Safe).
It became apparent that the presence of aluminum accelerated the stress-corrosion
cracking of the stainless. Aluminum is anodic to stainless in the galvanic
series — it can be used as the sacrificial anode in restricting the
corrosion of the steel. In fact, the 7075-T6 alloy has the same potential
as cadmium, and it should protect stainless in the same manner as cadmium
The only possible explanation of the failures was hydrogen evolving
at the cathode (the stainless steel bolt) and causing hydrogen embrittlement.
Role of Aluminum
In a laboratory test, 7075-T6 was coupled to one of each of a series
of duplicate test samples loaded with a stress ring and exposed to salt solution.
The specimens broke overnight. To verify results, the remaining uncoupled
specimens were exposed to the salt-solution immersion cycle for two more weeks.
No failures or rust spots were observed. The aluminum alloy was then coupled
to these specimens, and they failed within 24 hr.
Test results established that the critical nature of the failures was
related to the aluminum, not the alloy itself. Other tests eliminated the
possibility of crevice corrosion introducing pits which lead to stress-corrosion
cracking. Serious pitting can be initiated in stainless beneath substances
lying on the surface, independent of the chemistry of the object in question.
Once pitting starts, serious localized corrosion attack can set up within
the stainless. Once attack starts, failure can occur rapidly under the influence
To establish the susceptibility of the 17-4 PH fasteners to hydrogen
embrittlement, V-notched bolts were charged cathodically with hydrogen for
15 min in a 4% sulfuric acid electrolyte at 0.02 amp per sq in. The specimens
were immediately cleaned and loaded to 10,000 lb in a tensile testing machine.
Failure took place in about
hr in the bolts that were heated at 900 F for 1 hr.
The results established the susceptibility of 17-4 PH to hydrogen embrittlement.
Simulation of Bolt Failures
Actual bolts removed from the installation at Cape Kennedy were also
used as specimens which were loaded with 10,000 lb for 1,000 hr to establish
“fail” or “no fail” criteria. Environmental cycle
consisted of immersion in a 3.5% NaCl solution for 10 min of every hour. When
bolts aged 1 hr at 900 F were coupled to the aluminum alloy, they failed in
hr — the same failure time recorded for the hydrogen-charged
specimens statically loaded in air. It meant that the absorption of hydrogen
in the stressed bolt coupled to the aluminum alloy was comparable in severity
to that caused by electrolytic charging.
Bolts aged 4 hr at 900 F had longer times to failure, averaging from
three to four days. Bolts aged 1 hr at 1,000 F did not fall in tests up to
The aluminum alloy, after failure is simulated in laboratory tests,
was white and chalky. The stainless steel had surface rust spots in the area
covered by the aluminum alloy. These test results duplicated the appearance
of failures in service.
A simple method of eliminating hydrogen embrittlement in the 17-4 PH
bolts is to age them an additional 1hr at 1,000 F. Heat treatments which promote
toughness also add resistance to hydrogen embrittlement. The almost linear
relationship between hardness and toughness is shown in
. Each point on the graph represents the data
obtained by using various combinations of time and temperature. Therefore,
it can be concluded that the impact energy is a function of the hardness independent
of the specific variable of the aging treatment (Fig. 4
). Such behavior is not common to bolts made from
low-alloy steels. It can be explained on the basis that the notched-yield
to the unnotched yield strength ratio increases as the hardness of the 17-4
Fig. 3 The relationship between hardness and toughness of 17-4 PH is almost
Fig. 4 Behavior of bolts with different aging treatments was studied under
varying loads (see table). Permanent plastic deformation occurs at a load
slightly above 11,000 lb regardless of heat treatment. Failure was in the
Insulation is another way of eliminating hydrogen embrittlement. When
a suspension of graphite in grease (Lox-Safe) was applied at the stainless-aluminum
interface, there were no failures after 1,000-hr exposure — again consistent
with the observations made at Cape Kennedy. The grease was an insulator between
the dissimilar metal couples, and it eliminated the galvanic cell reaction.
Even bolts heat treated to maximum hardness did not fail as long as the grease
Nickel-cadmium plating gave inadequate protection because the layer
of cadmium was porous and allowed the diffusion of hydrogen. Cadmium plating
for protection against stress-corrosion cracking may not inhibit hydrogen
Anodizing the aluminum effectively inhibits hydrogen embrittlement.
The aluminum contact with stainless steel is only slightly attacked electrolytically
because the flow of current is limited by polarization. The effect was very
pronounced when aluminum pieces were galvanic couples to the stainless in
the laboratory tests. The aluminum pieces were given a commercial anodizing
treatment by the chromic-acid process. Anodized aluminum pieces showed no
sign of general corrosion (the chalky white residue), while untreated pieces
were attacked extensively under identical conditions of environment and time.
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