Golden, J. L.

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Copyright © 2009 Boeing. All rights reserved.
Failure Analysis of a 
Gas-Nitrided 15-5PH 
Steel
J. L. Golden, Ph.D.
ASM Houston Chapter / Failure Analysis Seminar
Houston, TX
03 April 2012
https://ntrs.nasa.gov/search.jsp?R=20120003598 2019-08-30T19:39:21+00:00Z
ASM Houston Failure Analysis Seminar J.L.Golden 03 April 2012  2
Engineering, Operations & Technology | Boeing Research & Technology
Copyright © 2009 Boeing. All rights reserved.
Materials & Fabrication Technology
Failure Analysis of Gas-Nitrided 15-5PH Steel
• Acknowledgements
• All Boeing and Boeing subcontractor activity conducted 
under NASA contract
– NAS15-10000
• Intense Team Effort, conducted from October 2007 through 
January 2010.
– NASA JSC ES4
– NASA MSFC EM30
– NASA KSC NE
– NASA Engineering & Safety Center (NESC)
– Boeing
– Lockheed-Martin
– ATK
– Numerous Supportive Subcontractors
ASM Houston Failure Analysis Seminar J.L.Golden 03 April 2012  3
Engineering, Operations & Technology | Boeing Research & Technology
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Materials & Fabrication Technology
Failure Analysis of Gas-Nitrided 15-5PH Steel
Summary of Presentation
• Brief Background on the Failure
• International Space Station; Solar Alpha Rotary Joint
• “What do you do when you cannot get at the hardware?”
• Failure Analysis Approach
• Sampling Failed Surface and Microscopy/Metallography
• Review of All Acceptance Data
• Sampling of Production Witness Specimens
• Sampling on Engineering Development and Spare Parts
• Production of Test Articles for Mechanical Test in an Attempt 
to Duplicate Flight System Failure
– Use as high a fidelity as is reasonable
– Nitrided 15-5PH steel (same steel spec., same heat treatment, 
same nitriding process and processor).
– IVD gold coating on 440C (roller simulation with same process 
and processor)
– Comparable mechanism kinematics
ASM Houston Failure Analysis Seminar J.L.Golden 03 April 2012  4
Engineering, Operations & Technology | Boeing Research & Technology
Copyright © 2009 Boeing. All rights reserved.
Materials & Fabrication Technology
Failure Analysis of Gas-Nitrided 15-5PH Steel
ISS 2007 Configuration: Solar Alpha Rotary Joint (SARJ) Locations
Port SARJ
Starboard SARJ (failed)
ASM Houston Failure Analysis Seminar J.L.Golden 03 April 2012  5
Engineering, Operations & Technology | Boeing Research & Technology
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Materials & Fabrication Technology
Failure Analysis of Gas-Nitrided 15-5PH Steel
The Solar Alpha Rotary Joint (SARJ) is used to rotate 
the outboard Solar Array Wings (SAW)  and transfer 
generated power to the ISS pressurized elements
SEGMENT
(P3/S3)
SEGMENT
(P4/S4)
SARJ
ASM Houston Failure Analysis Seminar J.L.Golden 03 April 2012  6
Engineering, Operations & Technology | Boeing Research & Technology
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Materials & Fabrication Technology
Failure Analysis of Gas-Nitrided 15-5PH Steel
Nitrided 15-5PH 
Race Ring
Inn
er
 
45
Datum A
O
uter
 45
Race Ring
Trundle Bearing 
Assembly
Inboard Arm
Camber Pivot
Fixed Arm
Trundle 
Mounting Clamp
Camber Pivot
Camber Pivot
440C Stainless 
Rollers
w/Au plating
ASM Houston Failure Analysis Seminar J.L.Golden 03 April 2012  7
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Materials & Fabrication Technology
Failure Analysis of Gas-Nitrided 15-5PH Steel
Initial Examination of Starboard SARJ Race Ring Damage
ASM Houston Failure Analysis Seminar J.L.Golden 03 April 2012  8
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Materials & Fabrication Technology
Failure Analysis of Gas-Nitrided 15-5PH Steel
• Physical Characteristics of the Failure Debris
• Particle Size Distribution
s-SARJ Debris Total Surface Area vs. Particle Size 
0
50000
100000
150000
200000
250000
300000
350000
400000
450000
0 100 200 300 400 500 600 700 800
Feret Diameter (µm) - Bin
T
o
t
a
l
 
S
u
r
f
a
c
e
 
A
r
e
a
 
(
m
2
)
ASM Houston Failure Analysis Seminar J.L.Golden 03 April 2012  9
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Materials & Fabrication Technology
• Physical Characteristics of the Failure Debris – Cont.
• Particle Size Distribution – Both views are 100X
Failure Analysis of Gas-Nitrided 15-5PH Steel
Representative area of
“fine” debris (< 100 µm)
Representative area of all debris;
broad particle size distribution
ASM Houston Failure Analysis Seminar J.L.Golden 03 April 2012  10
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Materials & Fabrication Technology
• Physical Characteristics of the Failure Debris – Cont.
• Particle Types – Agglomeration Particles
Failure Analysis of Gas-Nitrided 15-5PH Steel
Nitrided 15-5 PH agglomerated fragment from Tape 31 imaged under high vacuum.  
Reference magnifications:  100X (left), 500X (right)
ASM Houston Failure Analysis Seminar J.L.Golden 03 April 2012  11
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Materials & Fabrication Technology
• Physical Characteristics of the Failure Debris – Cont.
• Particle Types – “Large Chip” Particles
– Original surface typically visible on one side (right photo)
– Low cycle subsurface fractures on the other (left photo)
– Thickness typically at or near nitride layer thickness
Failure Analysis of Gas-Nitrided 15-5PH Steel
ASM Houston Failure Analysis Seminar J.L.Golden 03 April 2012  12
Engineering, Operations & Technology | Boeing Research & Technology
Copyright © 2009 Boeing. All rights reserved.
Materials & Fabrication Technology
Failure Analysis of Gas-Nitrided 15-5PH Steel
• Physical Characteristics of the Failure Debris – Cont.
• Fracture Surfaces on “Chip” Particles – Continued
• Largest particle located 
was ~3mm x ~5 mm.
• Particle thickness was 
about 150-170 µm, 
essentially equal to the 
nitride layer thickness.
• Numerous subsurface 
fracture initiation sites 
were observed, indicating 
a low-cycle fracture 
phenomenon took place.
ASM Houston Failure Analysis Seminar J.L.Golden 03 April 2012  13
Engineering, Operations & Technology | Boeing Research & Technology
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Materials & Fabrication Technology
• Physical Characteristics of the Failure Debris – Cont.
• “Chip” Particles In Cross-Section
Failure Analysis of Gas-Nitrided 15-5PH Steel
Typical large “chip” cross section showing:
• Maximum thickness essentially equivalent to nitride layer thickness, indicating 
failure is occurring near the case/core interface;
• Evidence of residual “white layer”, in excess of the 8 µm maximum required.
• Fracture lines are transgranular, showing no preference for the intergranular 
networking observed near the surface.
ASM Houston Failure Analysis Seminar J.L.Golden 03 April 2012  14
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Materials & Fabrication Technology
• We observed Discontinuous Intergranular Separations 
(DIGS) in the nitride layer on our 15-5PH steel
• Once the decision was made to surface harden the SARJ race 
rings, the gas nitriding process was geared toward producing 
a nitride layer of 150 µm minimum in a relatively short time (36 
hours).
– Principal means used to accomplish these goals was with the 
use of an activator.
• Our team’s theory is that nitrogen supersaturation of the 15-
5PH steel surface occurred, which on cooling led to 
separations forming along some grain boundaries in the outer 
50 µm of the surface.
• Differences in DIGS concentration appears to be related to the 
amount of activator used during the heat treatment.
– Thirty percent more activator was used on the Starboard SARJ 
race ring than was used for the Port race ring.
Failure Analysis of Gas-Nitrided 15-5PH Steel
ASM Houston Failure Analysis Seminar J.L.Golden 03 April 2012  15
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Materials & Fabrication Technology
• How did we miss the DIGS originally?
• They were right in front of us, but not recognized as an issue.
• Micrograph below is from original test coupon inspection.
Failure Analysis of Gas-Nitrided 15-5PH Steel
CASE COREWHITE
LAYER
INTER-
GRANULAR
NETWORKING
DIGS
ASM Houston Failure Analysis Seminar J.L.Golden 03 April 2012  16
Engineering, Operations & Technology | Boeing Research & Technology
Copyright © 2009 Boeing. All rights reserved.
Materials & Fabrication Technology
Failure Analysis of Gas-Nitrided 15-5PH Steel
Nitride Control Coupon (NCC) –
Nitrided in same furnace as the flight 
SARJ race ring, at fixed locations .
Metallurgical Evaluation of SARJ Race Ring 
S/N 4
Sample Coupon (Polished, Etched and View 
@1000X)
Networking
Intergranular separations
(appear like cracks)
ASM Houston Failure Analysis Seminar J.L.Golden 03 April 2012  17
Engineering, Operations & Technology | Boeing Research & Technology
Copyright © 2009 Boeing. All rights reserved.
Materials & Fabrication Technology
Failure Analysis of Gas-Nitrided 15-5PH Steel
Separation 
/ void
Networking
ASM Houston Failure Analysis Seminar J.L.Golden 03 April 2012  18
Engineering, Operations & Technology | Boeing Research & Technology
Copyright © 2009 Boeing. All rights reserved.
Materials & Fabrication Technology
Failure Analysis of Gas-Nitrided 15-5PH Steel
Polished metallographic sample exhibiting DIGS. 
The degree of porosity and the type scaffolding 
structure remaining inside each DIGS varies and at 
times appears as a nanostructure.
ASM Houston Failure Analysis Seminar J.L.Golden 03 April 2012  19
Engineering, Operations & Technology | Boeing Research & Technology
Copyright © 2009 Boeing. All rights reserved.
Materials & Fabrication Technology
• What happened to the SARJ “as-designed” lubrication 
system?
• The sole lubrication between trundle rollers and the nitrided 
race ring was 1500-2000Å of gold deposited on the 440C 
rollers by ion plating.
• Ground testing (in retrospect, inadequate and misleading) 
indicated that this lubrication design was sufficient to protect
the nitrided 15-5PH steel race ring surface.
• In addition, the ground environment was undermining the 
lubrication system by deteriorating the plating adhesion (see 
the following charts).
• Once the transfer-film lubricant was lost, the kinematics of the 
SARJ mechanism coupled with very high friction coefficients 
pulled subsurface stresses up into the nitride layer, resulting 
in nitride layer fractures and loss of the nitrided surface.
Failure Analysis of Gas-Nitrided 15-5PH Steel
ASM Houston Failure Analysis Seminar J.L.Golden 03 April 2012  20
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Copyright © 2009 Boeing. All rights reserved.
Materials & Fabrication Technology
Carbon Tape Adhesion
Post-Test
Failure Analysis of Gas-Nitrided 15-5PH Steel
ASM Houston Failure Analysis Seminar J.L.Golden 03 April 2012  21
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Materials & Fabrication Technology
SE  
SARJ TBA Tape Pull Specimen Analysis
TBA 1045, Datum A, Test #2, Carbon Tape
• Gold coating was easily removed from 
trundle bearing roller surfaces using 
carbon tape, and was generally 
segmented into strips of dimension 
consistent with the as-machined surface 
finish.
Failure Analysis of Gas-Nitrided 15-5PH Steel
ASM Houston Failure Analysis Seminar J.L.Golden 03 April 2012  22
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Copyright © 2009 Boeing. All rights reserved.
Materials & Fabrication Technology
BSE  
BSE  
Prelim SARJ TBA Tape Pull Specimen Analysis
SE  
SE  
TBA 1005, Outer 45, Test #1, SEM
Iron Oxide 
(by 
elemental 
analysis)
Failure Analysis of Gas-Nitrided 15-5PH Steel
Outer 
Surfaces
Inner 
Surfaces
ASM Houston Failure Analysis Seminar J.L.Golden 03 April 2012  23
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Copyright © 2009 Boeing. All rights reserved.
Materials & Fabrication Technology
Failure Analysis of Gas-Nitrided 15-5PH Steel
 Freshly gold-plated 440C after 21 days 
of 95%RH at 125°F.
 Adhesion failure mode is consistent 
with that observed with trundle bearing 
rollers.
ASM Houston Failure Analysis Seminar J.L.Golden 03 April 2012  24
Engineering, Operations & Technology | Boeing Research & Technology
Copyright © 2009 Boeing. All rights reserved.
Materials & Fabrication Technology
• Summary of Failure Analysis
• The nitride surface of the SARJ race rings had unrecognized 
subsurface flaws susceptible to fracture initiation.
• The lubrication system selected in design was not robust 
enough to maintain friction control.
• Once friction control was lost, subsurface loads moved up 
into the interface of and within the nitride layer.
• Spalling probably initiated at DIGS, generating particles which 
exacerbated the surface stresses, leading to spalling of the 
nitride layer in a short period of time.
• Weak magnetic forces in the microgravity environment held 
released particles within in the mechanism, leading to 
crushing of the larger “chip” particles into fines and eventual 
agglomeration of those fines.
Failure Analysis of Gas-Nitrided 15-5PH Steel
ASM Houston Failure Analysis Seminar J.L.Golden 03 April 2012  25
Engineering, Operations & Technology | Boeing Research & Technology
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Materials & Fabrication Technology
• What has happened to the SARJ mechanisms?
• We decided to lubricate using a MoS2 + PTFE filled vacuum-
stable grease.
• We cleaned the damaged Starboard race ring using the same 
grease to help capture particulate.
• We replaced all of the Starboard SARJ trundle bearing 
assemblies.
• After several test runs of increasing duration plus analyses to 
determine the structural viability of the damaged race ring, the
Starboard SARJ returned to nominal operation early 2010.
– Performance is characterized as excellent.
• We applied the same grease to the Port SARJ as soon as we 
could, and believe that we saved it from the same eventual 
failure mode.
Failure Analysis of Gas-Nitrided 15-5PH Steel
ASM Houston Failure Analysis Seminar J.L.Golden 03 April 2012  26
Engineering, Operations & Technology | Boeing Research & Technology
Copyright © 2009 Boeing. All rights reserved.
Materials & Fabrication Technology
Failure Analysis of Gas-Nitrided 15-5PH Steel
ISS 2010
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Failure Analysis of a Gas-Nitrided 15-5PH Steel

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