Tony Shen

The Boeing Company, Mesa, Arizona

Timothy Krantz

Glenn Research Center, Cleveland, Ohio

Jason Sebastian

QuesTek Innovations, Evanston, Illinois

Advanced Gear Alloys for 

Ultra High Strength Applications

NASA/TM—2011-217121

November 2011

AHS 2011–000285
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 7115 Standard Drive

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Tony Shen

The Boeing Company, Mesa, Arizona

Timothy Krantz

Glenn Research Center, Cleveland, Ohio

Jason Sebastian

QuesTek Innovations, Evanston, Illinois

Advanced Gear Alloys for 

Ultra High Strength Applications

NASA/TM—2011-217121

November 2011

AHS 2011–000285

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135

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Acknowledgments

The Enhanced Rotorcraft Drive System (ERDS) program is performed under a Technology Investment Agreement (TIA) between 

Boeing and the U.S. Army Aviation Applied Technology Directorate. Authors of this paper like to thank the following individuals: 

Mark Traylor, Bob Filler, Justine Chandler of Boeing; Jim Wright and Rajesh Prasannavenkatesan of QuesTek; and NASA Glenn 

Research Center, Army Research Laborarory, Vehicle Technology Directorate (for adminstrative and technical support).
NASA/TM—2011-217121 1 

Advanced Gear Alloys for Ultra High Strength Applications

Tony Shen

The Boeing Company

Mesa, Arizona 85215 

Timothy Krantz

National Aeronautics and Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Jason Sebastian

QuesTek Innovations

Evanston, Illinois 60201 

Abstract

Single tooth bending fatigue (STBF) test data of UHS 

Ferrium C61 and C64 alloys are presented in comparison with 

historical test data of conventional gear steels (9310 and 

Pyrowear 53) with comparable statistical analysis methods.

Pitting and scoring tests of C61 and C64 are works in 

progress. 

Boeing statistical analysis of STBF test data for the four 

gear steels (C61, C64, 9310 and Pyrowear 53) indicates that 

the UHS grades exhibit increases in fatigue strength in the low 

cycle fatigue (LCF) regime. In the high cycle fatigue (HCF) 

regime, the UHS steels exhibit better mean fatigue strength 

endurance limit behavior (particularly as compared to 

Pyrowear 53). However, due to considerable scatter in the 

UHS test data, the anticipated overall benefits of the UHS 

grades in bending fatigue have not been fully demonstrated. 

Based on all the test data and on Boeing’s analysis, C61 has 

been selected by Boeing as the gear steel for the final ERDS 

demonstrator test gearboxes. In terms of potential follow-up 

work, detailed physics-based, micromechanical analysis and 

modeling of the fatigue data would allow for a better 

understanding of the causes of the experimental scatter, and of 

the transition from high-stress LCF (surface-dominated) to 

low-stress HCF (subsurface-dominated) fatigue failure.

Additional STBF test data and failure analysis work, 

particularly in the HCF regime and around the endurance limit 

stress, could allow for better statistical confidence and could 

reduce the observed effects of experimental test scatter.

Finally, the need for further optimization of the residual 

compressive stress profiles of the UHS steels (resulting from 

carburization and peening) is noted, particularly for the case of 

the higher hardness C64 material.

Introduction

The Boeing Company has conducted development effort as 

part of the Enhanced Rotorcraft Drive System (ERDS) 

program under a Technology Investment Agreement (TIA)

between Boeing and the U.S. Army Aviation Applied 

Technology Directorate. The ERDS program goals consist of 

design, fabrication, and demonstration testing of critical drive 

system technologies required to achieve the program goals for 

the Army’s Current/Future Force fleet of rotorcraft. The 

specific goals of the ERDS are listed below.

1. 40 percent increase in drive system transmitted horse 

power-to-weight ratio

2. 15 dB reduction in drive system generated noise

3. 30 percent reduction in drive system production cost

4. 30 percent reduction in drive system O&S cost

5. 75 percent automatic detection of critical mechanical 

component failures. 

In support of the increased power-to-weight ratio (or power 

density) and reduced O&S cost goal, this test program is to 

investigate four Advanced Gear Alloys and heat treatment 

optimization processes required. The gear alloys were selected 

(Pyrowear 675 (Carpenter Technology Corporation), 

Aermet 100 (Carpenter Technology Corporation), Ferrium 

C61 and Ferrium C64 (QuesTek Innovations, LLC)) after a 

preliminary evaluation of a larger material matrix that had 

consisted of, in addition, CCS42L, LESCO53, 32CDV13 and 

XD15NW ferrous alloys. Pyrowear 675 is a high strength 

stainless steel designed for gear application. Aermet 100 is an 

ultra high strength steel which has potentials for gear 

application. C61 and C64 are secondary hardening grade gear 

steels, products of QuesTek’s Computational Design, 

Development and Application of High-Performance Gear 

Steels (see Section 2.0 for details). One of the four candidate 

alloys was to be down-selected and further characterized in the 

carburized and hardened state using three types of gear tests: 

Gear Tooth Scoring Test and Gear Tooth Pitting and STBF 

Tests. Core mechanical properties of all four alloys were to be 

assessed for data base purposes. This same down-selected 

alloy was to be used in fabricating a gearset for the AH-64 

Intermediate and/or Tail Rotor Gearbox for installation and 

testing during a 200-hr bench test. In order to fairly evaluate 

the strengths of the Advanced Gear Alloys, the typical heat 

treat and fracture toughness values are referenced in Table 1. 
NASA/TM—2011-217121 2 

Comparison of core mechanical properties in conjunction 

with hardness profile data and microstructure including 

carbide distribution facilitated the down-selection process. 

Four mechanical tests to characterize the core mechanical 

properties are: static test fatigue (smooth and notched) test; 

fracture toughness test; and fatigue crack growth rate test. The 

materials were provided by Boeing-Mesa in bars with a size 

range of 5.25 to 6.5 in. in diameter. These bar materials were 

heat treated to represent a pseudo-carburized condition in 

order to meet the tensile (or equivalent hardness) requirements 

in Table 1. Details of the heat treat specifications were agreed 

to between Boeing-Mesa and Boeing-qualified heat treat 

suppliers who provided a fully-documented process to BoeingMesa. Test specimens were then prepared per test plan and 

drawings at the Boeing-qualified suppliers. Core mechanical 

properties test matrix is summarized in Table 2. 

Due to test laboratory issues of unforeseeable nature, the 

properties test was partially completed at the time of preparing 

this paper. Therefore only the available test results are 

presented in Table 3, and the full test results will be reported 

at a later date.

Table 3 summarizes the test results of the static test all four 

alloys. C61, C64 and Pyrowear meet the requirements in all 

categories, whereas Aermet 100 does not meet the ultimate 

tensile strength requirements. 

Figure 1 illustrates the fatigue crack growth test result in a 

da/dN versus delta K diagram with the linear portion shown 

and threshold value noted in the legend block at two different 

stress ratios (for C61 alloy only).

Computation Design, Development and 

Application of High Performance Gear 

Steels

Ferrium C61and C64are two new alloys designed by 

QuesTek Innovations LLC that are used for power 

transmission applications. Both alloys utilize an efficient 

nanoscale M2C carbide strengthening dispersion within a NiCo lath martensitic matrix (Ref. 3). Utilizing their suite of 

computational models, QuesTek designed these alloys 

considering the intricate interplay of critical design factors 

including martensitic matrix stability (Ms temperature); 

strengthening carbide thermodynamic stability and formation 

kinetics; matrix cleavage resistance; and embrittling phase 

thermodynamic stability. 

QuesTek’s design methodology yielded a number of 

attractive material properties for C61 and C64. These 

properties yield performance features such as the following:

Greater Core Strength

C61 and C64 exhibit core steel tensile strengths (UTS) of 

229 ksi or more, which is a 35+ percent increase versus

conventional gear steels. Higher tensile strength allows 

significant reductions in part size and weight, particularly 

where structural components are integrated with gearing into 

single components.

Greater Surface Fatigue Resistance

The alloys demonstrate increased contact fatigue and 

bending fatigue performance. Generally speaking, increasing 

the surface hardness without creating embrittling features 

(such as interconnected primary carbides) increases surface 

fatigue resistance. Increased surface fatigue resistance can 

enable either smaller, lighter power transmission units or 

higher power throughput in a given unit size.

High Surface Hardenability, designed to use HighTemperature, Low-Pressure (i.e., vacuum) Carburization 

Methods C61 and C64 were specifically designed to achieve 

high surface hardenability and use high-temperature, lowpressure (i.e., vacuum) carburization and gas quenching 

processing methods. These processing methods can permit 

significant reductions in manufacturing costs and schedules 

due to:

• Shorter processing times at higher carburizing 

temperatures.

• Elimination of the secondary hardening and oil quench 

process steps, which eliminate associated costs of 

custom press quench dies, liquid quenchants, transfer 

mechanisms, hydraulic systems, etc.

• Reduction of excess grinding, excess stock removal 

waste and part scrap waste, due to reduced part quench 

distortion and avoidance of intergranular oxide (IGO) 

formation inherent in a pre-oxidation step.

The high hardenability of Ferrium C61 when compared to 

9310 is illustrated in Figure 2. 

Greater High Temperature Survivability

C61 and C64 exhibit increased thermal stability versus AISI 

9310 or Pyrowear 53, because they were designed to be 

tempered at 900 or 950 °F (up to 500 °F hotter than AISI 9310 

or Pyrowear 53). This increased thermal stability is expected 

to result in better survivability in “oil-out” or low lubricant 

situations, and endure other high-temperature operating 

conditions.

Ferrium C61 was designed to provide a carburized surface 

hardness of 60-62 Rockwell C Hardness (or HRC), similar to 

conventional gear steels such as Pyrowear 53 (59-64 HRC) 

and AISI 9310 (58-64 HRC), but deliver ultra-high core 

strength and excellent fracture toughness. C61 has wear 

properties, toughness (~130 ksi√in), and surface fatigue 

properties that are similar to those of current commercial 

alloys. However, C61’s typical core hardness of 49 to 50 HRC 

far exceeds Pyrowear 53’s core hardness of 36 to 44 HRC.

C61 can offer an attractive combination of properties for 

integral power shaft and gearing applications, where both the 

core strength and the fatigue strength of an alloy are critical.
NASA/TM—2011-217121 3 

As an example, C61 is being evaluated in a U.S. Army Small 

Business Innovation Research (SBIR) program for main rotor 

shaft applications on the Boeing-designed CH-47 Chinook 

helicopter. Main rotor shafts are among the largest, heaviest, 

and highly-loaded single components on rotorcraft. With 

improved core properties, use of C61 in lieu of the currentlyused carburized 9310 should be able to reduce the weight of 

the main rotor shaft on the CH-47 by 15 to 25 percent without 

requiring significant changes in the production and 

manufacturing process of the component. See Figure 3 for 

data developed under this SBIR program demonstrating the 

superior axial fatigue life performance of C61 versus 9310. 

C61 material is commercially available from Latrobe 

Specialty Steel, who operates under a license from QuesTek.

QuesTek anticipates licensing additional producers in order to 

establish a robust, competitive market for C61. The 

composition of C61 is covered under U.S. Patent Number 

6,176,946 B1. SAE Aerospace Material Specification 6517 for 

C61is pending (expected in 2011). 

QuesTek developed C64 under a U.S. Navy Small Business 

Technology Transfer (STTR) program to achieve higher 

surface hardness (63-64 HRC) than C61 while retaining 

superior core hardness and strength (230 ksi UTS, 200 ksi

YS), fracture toughness (85 ksi•√in.), high allowable operating 

temperature due to higher tempering temperatures, and other 

manufacturing benefits. A higher surface hardness for C64 

provides better surface fatigue resistance (particularly pitting 

fatigue) relative to C61. C64 material is also commercially 

available from Latrobe Specialty Steel, who operates under a 

license from QuesTek. Vacuum carburized surface hardness 

profiles of C64 show a marked increase in surface hardness 

over C61 with a carburized surface microstructure that is 

substantially free of primary carbides.

Single Tooth Bending Fatigue Testing 

This part of the test is to provide component level test data 

for the determination of comparative design bending fatigue 

strength of Advanced Gear Alloys Test Specimen.

The STBF test gears are defined in NASA Drawing no.

CD851944. One test gear consists of five groups of gear teeth, 

and each group consists of three gear teeth. A sketch with 

dimensions is illustrated in Figure 4, though actual clocking of 

the five groups of teeth is not reflected. 

All gear specimens were rough machined from the same 

heat of material, heat treated in the same lot. Specimens were 

finish machined as a single lot. Vacuum carburization and 

hardening were performed in accordance with Boeingprovided instructions (Boeing to conduct trials with supplier to 

determine process uniformity). Shot peeing of the gear tooth 

roots were performed per Boeing Process Specification HP4-

36 except as noted in Table 4. 

Prior to the start of testing, all test gears were dimensionally 

inspected per the requirements of CD851944, including such 

measurements as fillet radius and thickness. All parts also 

underwent magnetic particle inspection. The representative 

sample of each alloy and condition was measured for residual 

stress, core hardness, case hardness, effective case depth, 

involute tooth profile, and for tooth surface roughness. 

All gears were serialized and marked with tooth numbers. 

These serial numbers and tooth numbers were used throughout 

testing and noted on all test results and post-test inspections. 

Test matrix is summarized in Table 5. The STBF specimens 

comprise 5 test gears for a total of 25 data points. Each group 

of gear teeth was used for one and only one test run. 

All fatigue testing was coordinated and performed by the 

Army Research Laboratory, Vehicle Test Directorate (ARLVTD) and NASA at the Glenn Research Center. Testing was 

conducted using a standard fatigue test system and specialized 

fixture for gear tooth fatigue testing shown in Figure 5. The 

test apparatus features an alignment fixture which was used 

for closely controlled positional and angular adjustments. The 

load cell was positioned between the alignment fixture and the 

top grip. The output from the load cell was monitored for test 

control and test documentation. Testing was done on two 

nominally identical 20,000 lb capacity load frames, running at 

approximately 10 Hz. 

The test fixtures were checked for proper alignment before 

conducting the test program. A check on the performance of 

the fixture alignment was made by “bluing” the gears prior to 

testing. The contact patterns created by the load rod contact 

areas provided a clear indication that uniform load 

distributions on both gear teeth (test tooth and reaction tooth) 

had been achieved. 

While the contact patterns provide a good check of the 

fixture alignment, the test fixturing must also be free of 

friction effects. The upper and lower rods (Fig. 6) are located 

in the test fixture via brass bushings. If there exists any 

frictional forces at these interfaces, then the loads on the test 

teeth might differ from the load sensed at the load cell that is 

located between the loading rod and grip of the loading frame 

(external to the gear fixture) To check for friction effects, 

special loading rods were instrumented with four strain gages 

on each rod and a careful calibration procedure was 

completed. Strain gage data was recorded while loading a test 

gear while at the same time recording the output from the test 

machine load cell. It was discovered that one of the two test 

frames required an adjustment to one of the bushings to make 

certain tooth loads equaled the loads sensed at the load cell. 

Some fatigue tests were conducted before making the 

adjustment to the bushing. The calibration information 

provided by the strain-gaged loading rods was combined with 

load vs. deflection data that is recorded during all fatigue tests.

In this manner precise and accurate loads on the gear tooth 

could be determined even for those tests prior to bushing 

adjustments. 

It was desired to compare data from the present experiments 

to previous tests conducted using AISI 9310 and Pyrowear 53 

gears. The gears from these tests of AISI 9310 and Pyrowear 

53 were done using 5.33 diametral pitch with 3/8 in. To 

compare to the current work using 8 diametral pitch, 1⁄4 in.

face width gears, the maximum fillet stress was calculated 
NASA/TM—2011-217121 4 

using a plane-stress finite element analysis. The finite element 

model geometry in the fillet region was adjusted to match 

closely the actual manufactured gear geometry. This was done 

by plotting the finite element model to appropriate scale and 

overlaying this image onto photographs of test gear fillets 

capture using a digital microscope. Strain gage experiments 

using the 5.33 diametral pitch gears validated the finite 

element modeling technique with analytical and measured 

stress values in agreement to within 1 percent. These stresses 

are due to operating loads; no attempt is made in this work to 

include residual stresses for purposes of comparing single 

tooth bending fatigue data. 

The STBF tests were conducted using unidirectional 

loading. Testing was executed in load control. The gear was 

positioned to provide load on the test tooth at the theoretical 

highest point of single tooth contact as defined by Boeing 

engineering. The load was cycled from a small minimum load 

to the maximum load desired for the given fatigue test with a 

target R ratio of 0.02. The load range was maintained at a 

constant value throughout the test, utilizing a sinusoidal 

waveform. 

Previous Pyrowear 53 STBF and 9310 STBF tests were 

used as a basis to determine the initial load levels. Loads were 

estimated to target lifetimes in the range of 10E3 to 10E6 

cycles to failure. Subsequent test load levels were chosen 

based on initial results.

Crack initiation was assumed to occur when the loading rod 

stroke has increased 2 percent relative to the stroke for the 

new gear tooth. Experience showed that at this point a crack 

has initiated with a size on the order of the case depth. Testing 

machine parameters might be set up to allow a somewhat 

larger increase of the stroke (more than 2 percent), and the 

crack initiation time was determined by examination of the 

test data. Each test was discontinued upon reaching this set 

increase in stroke. If a test completes 10E6 cycles without 

fatigue crack initiation, the test was terminated and considered 

a run-out. Tests were performed at room temperature.

For each test, ARL-VTD documented the test stand number, 

test gear serial number, test tooth number, cyclic life to failure, 

load and stress levels, and stress ratio (R). ARL also provided 

a general description of the test setup, load control parameters, 

“failure” definition, and example photographs of the final 

condition of the tested specimen. In addition to these 

parameters, Boeing examined all test gears and determined the 

fracture origin. Representative samples of each alloy were 

sectioned and analyzed for microstructure and amount of 

retained austenite. Additional post-test evaluation was 

performed as dictated by results. These tests included 

verifying proper microstructure, characterizing fracture 

topography, and verifying chemistry. All results were 

compiled with the original gear measurements. After all 

testing was completed the test gears were protected, packaged, 

and returned to Boeing Mesa. After Boeing Mesa completed 

all post-test evaluation and documentation, the specimens 

were stored in a designated area.

Test gears made of C61 and C64 alloys were subjected to 

comprehensive lab evaluation. The core hardness, effective 

case depth and surface hardness, tooth crack image (optical 

micrograph), and crack origin location (scanning electron 

micrograph) was compared with conventional gear alloys such 

as 9310 and Pyrowear 53 from previous test programs (Refs. 1 

and 2).

Figures 7 to 12 contain hardness profile measurement data 

during the carburization cycle development stage and for 

representative C61 and C64 test gears, respectively as 

referenced in Table 6, at a tooth root fillet radius location for 

specific tooth number within each test gear. Microhardness 

traverses were performed at the pitch line and root fillet radius 

on each metallographic cross-section which provided the case 

depths and case hardnesses at 0.002 in. depth interval. For 

C-61 data, it shows that a peak hardness of 61-62 HRC is 

achieved at, or very near, the surface while the measured core 

hardness is 49 HRC, referenced in Table 6, roughly 

corresponding to an ultimate tensile strength of 246 ksi at 

deeper than 0.085 in. below the surface. For C64 data, it 

shows that a peak hardness of 62 to 63 is achieved at, or very 

near, the surface while the measured core hardness is 48 HRC, 

referenced in Table 6, roughly corresponding an ultimate 

tensile strength of 238 ksi at deeper than 0.085 in. below the 

surface. 

Measured values for average core hardness are: 40 HRC 

(P53), 37 HRC (9310), 49 HRC (C61), 48 HRC (C64), 

respectively. Representative core hardness and case depth (at 

60 HRC for C61 and at 62 HRC for C64 test gears) are 

summarized in Table 6. The microhardness traverses show 

that comparably in the case layer (1) for C61 at HRC 60, the 

case depth is in the range of 0.023 to 0.027 in. at the pitch line 

and of 0.015 to 0.019 in. at the root fillet for C61 and (2) for 

C64 at HRC 62, the case depth is in the range of 0.021 to

0.025 in. at the pitch line and of 0.012 to 0.017 in. at the root 

fillet.

Bending fatigue origin location for each tooth of 

representative test gears selected for lab investigation is 

contained in Table 7. 

A vast majority of the teeth which cracked during testing 

were not completely liberated from the body of the specimen.

An abrasive cutoff saw was utilized to excise the cracked 

teeth. Optical photomicrographs illustrating metallurgical 

microstructure of representative test gears associated with 

Table 7 are shown in Figures 13 and 14, for C61 and 

Figures 15 and 16 for C64, respectively. Isolated nonnetworked case carbides were observed.

To reveal the nature and morphology of bending fatigue 

failure of the representative test gears, scanning electron 

micrographs (SEM) were taken, and are shown in Figures 17

and 18 for C61 test gear and Figures 19 and 20 for C64 test 

gear referenced in Table 6. Each fracture surface was 

ultrasonically cleaned in a mild detergent solution during the 

SEM preparation for examination. A vast majority of fatigue 

origins are located at tooth root fillet radius surface, and few 
NASA/TM—2011-217121 5 

are at subsurface inclusion as represented by C61-1009 tooth

nos. 2 and 3 and C64-4012 tooth no. 5. 

Test Results, Post-Test Lab Data and 

Discussion

Raw data after Goodman corrected to R = 0.1 are plotted for 

all four groups of STBF test gear alloys (C61, C64, Pyrowear 

53 and 9310), as shown in Figure 21. 

Boeing performed statistical STBF test data reduction of all 

four groups of test gears. The results are summarized in the 

Table 8 that summarizes both mean and mean minus three 

standard deviation endurance limits along with standard 

deviation and coefficient variation. Fitting equation for STBF 

test data is shown below. 

{ ( ) ( ) [ ] }d

max 11 ×+−×−+×= Nccbba

For C61 test data, for example, the values for constants are 

as follows: a = 457; b = 0.62, c = 0.001. d = –0.4 where max 

(ksi) is max bending stress at minimum root radius and N is 

number of cycles to failure or run-out. In the high cycle 

fatigue (HCF) region of the S-N diagram, using the endurance 

limit level as the parameter, C61 and C64 are shown to 

perform better than 9310 and Pyrowear 53. It is noted that 

STBF mean endurance limit values for both C61 and C64 

alloys are close to each other, 289 ksi (C61) and 281 ksi (C64) 

respectively, due to, in part, the core heat treat hardness 

converted to strength of C61 (246 ksi) and C64 (238 ksi), 

referenced in Table 6 and Table 8 and Figure 22, being close 

to each other. However this higher performance diminishes 

when three standard deviation is accounted for since both 

newer alloys exhibit much higher coefficient of variation, C61 

(6.5 percent) and C64 (6.9 percent) compared with 

conventional gear alloys, 9310 (4.6 percent) and Pyrowear 53 

(3.9 percent), and therefore C61 and C64 are bracketed by 

9310 and Pyrowear 53 at the endurance level, as shown in 

Figure 23. This leads to an observation whether a tooth 

bending fatigue strength plateau may be approached at a core 

heat treat level of 230 to 240 ksi for C61 and C64, and even at 

a higher level at 280 ksi for stronger alloys which far exceed 

160 ksi for 9310 and Pyrowear 53. 

The mean value S-N diagram is presented in Figure 22 and 

the mean minus three standard deviation S-N diagram is 

presented in Figure 23. It is noted that at the low cycle fatigue 

(LCF) region, below 10,000 cycles, both new alloys exhibit 

higher performance than incumbent alloys. This aspect of 

comparison is important when the entire S-N curve is used for 

flight load related life calculation for a gear component in that 

alloys with higher LCF strength impart higher life for high 

load maneuvers and other flight conditions. 

As a result of this material test, C61 is planned to be tested

in an Apache Intermediate Gearbox assembly single mesh 

configuration in conjunction with an Apache Tail Rotor 

Gearbox assembly with near-net-forged, isotropic super 

finished spiral bevel gear and pinion mesh.

Conclusion and Recommendation

The Boeing Company has conducted development effort as 

part of the Enhanced Rotorcraft Drive System (ERDS) 

program under a Technology Investment Agreement (TIA) 

between Boeing and the U.S. Army Aviation Applied 

Technology Directorate. A new class of ultra high strength 

(UHS) grades has been evaluated by analysis and lab testing 

under the ERDS program. The investigated UHS alloy steels 

included Ferrium C61 and C64, secondary hardening steels 

that were offered for significant improvements in both core 

(gear shaft) and case (gear tooth) properties that require higher 

performance in strength and toughness. 

Single tooth bending fatigue (STBF) test data of UHS 

Ferrium C61 and C64 alloys are presented in comparison with 

historical test data of conventional gear steels (9310 and 

Pyrowear 53) with comparable statistical analysis methods.

Pitting and scoring tests of C61 and C64 are works in 

progress. 

Boeing statistical analysis of STBF test data for the four 

gear steels (C61, C64, 9310 and Pyrowear 53) indicates that 

the UHS grades exhibit increases in fatigue strength in the low 

cycle fatigue (LCF) regime. In the high cycle fatigue (HCF) 

regime, the UHS steels exhibit better mean fatigue strength 

endurance limit behavior (particularly as compared to 

Pyrowear 53). However, due to considerable scatter in the 

UHS test data, the anticipated overall benefits of the UHS 

grades in bending fatigue have not been fully demonstrated. 

Based on all the test data and on Boeing’s analysis, C61 is 

planned for further demonstrator testing. 

In terms of potential follow-up work, detailed physicsbased, micromechanical analysis and modeling of the fatigue 

data would allow for a better understanding of the causes of 

the experimental scatter, and of the transition from high-stress 

LCF (surface-dominated) to low-stress HCF (subsurfacedominated) fatigue failure. Additional STBF test data and 

failure analysis work, particularly in the HCF regime and 

around the endurance limit stress, could allow for better 

statistical confidence and could reduce the observed effects of 

experimental test scatter. Finally, the need for further 

optimization of the residual compressive stress profiles of the 

UHS steels (resulting from carburization and peening) is 

noted, particularly for the case of the higher hardness C64 

material. 

References 

1. Report No. 1L1G-TR-1.2-09001, “Single Tooth Bending Fatigue 

Test Report,” submitted to U.S. Army Aviation Applied 

Technology Directorate (AATD), Aviation and Missile 

Command (AMCOM), 30 July 2009 (Agreement no. W911W6-

06-2-0006) 
NASA/TM—2011-217121 6 

2. Report No. 1L193-FR-06001, “Qualification of Ausform 

Finishing Process for the Manufacturing of Aerospace Gearing,” 

submitted to Office of Naval Research (ONR), 30 April 2006 

(Agreement no. N00014-99-3-017). 

3. AGMA Technical Paper 09FTM14, “Design, Development, and 

Application of New High-Performance Gear Steels,” J.A. Wright, 

Ph.D., J.T. Sebastian, Ph.D., C.P. Kern, and R.J. Kooy, P.E., 

QuesTek Innovations LLC; also presented at Fall 2009 AGMA 

Technical Meeting; reprinted in Jan./Feb. 2010 Gear Technology 

magazine. 

TABLE 1.—TYPICAL VALUES FOR CORE MECHANICAL

PROPERTY OF ADVANCED GEAR ALLOYS

Materials Specification. Heat treatment

UTS/YS

(ksi) typ

Fracture toughness

KIC

(ksi (in.)

0.5) typ

Pyrowear 675 AMS 5930 185/160 125

Ferrium C61 --------------- 240/225 130

Ferrium C64 --------------- 230/200 85

Aermet 100 AMS 6532 285/250 100

TABLE 2.—TEST MATRIX FOR CORE MECHANICAL PROPERTIES 

Test type ASTM 

standard

Test load 

ratio

Number of specimens

Pyrowear 675 Ferrium C61 Ferrium C64 Aermet 100 Total

Static E 8 ----------- 6 6 6 6 24

Fatigue smooth E 466 R = –1.0 20 20 20 20 80

R = 0.10 20 20 20 20 80

Fatigue notched E 466 R = –1.0 20 20 20 20 80

R = 0.10 20 20 20 20 80

Fracture toughness E 399 ----------- 6 6 6 6 24

Crack growth E 647 R = 0.1 2 2 2 2 8

R = 0.8 2 2 2 2 8

Hardness -------- ----------- Each tensile specimen blank (not a separate specimen)

TABLE 3.—ULTIMATE TENSILE STRENGTH, YIELD STRENGTH, ELONGATION TO 

FAILURE AND REDUCTION IN AREA FOR C61, C64, PYROWEAR 675 AND AERMET 100

UTS, 

ksi

YS,

ksi

E, 

%

RA, 

%

KIC,

ksi-in.

1/2

C61

Average value of measured data 249 225 16 69 140

Supplier Data Sheet 240 225 15 68 130

S-basis (AMS 6517) 225 208 13 60 ----

C64

Average value of measured data 238 201 20 78 73

Supplier Data Sheet 230 200 18 75 85

Pyrowear 675

Average value of measured data 188 152 23 74 ----

Supplier Data Sheet 185 154 20 75 125

Aermet 100

Average value of measured data 265 242 16 68 ----

Supplier Data Sheet 285 250 14 65 100

A basis (AMS 6532) 275 235 10 55 ----

TABLE 4.—GEAR TOOTH ROOT SHOT PEEN REQUIREMENT

Boeing Process Specification: HP4-36 except the followings:

A Stray shot allowed over all areas

B Use ASR 110 shot per AMS2431/1D (45-52 HRC) to an intensity of 0.008 to 0.012 at 200% coverage

C Fixture one almen strip tangent to the bottom of the tooth root. Strips to represent tooth flanks are not required

D Surface roughness requirements (Table 5) apply prior to shot peening
NASA/TM—2011-217121 7 

TABLE 5.—SINGLE TOOTH BENDING TEST MATRIX

Configuration Part number Tooth surface 

roughness

No of data 

points

1 Drawing no. CD851944 16 Ra Max 25

TABLE 6.—HARDNESS DATA FOR C61 AND C64 STBF TEST GEARS

Specimen Case depth at 60 HRC, in. (C61)

Case depth at 62 HRC, in. (C64)

Core hardness, 

HRC

Pitch line Root fillet

C61-1009 0.023 0.015 49

C61-1012 0.027 0.019 49

C61-1018 0.027 0.015 49

C64-4007 0.025 0.012 48

C64-4011 0.021 0.015 48

C64-4012 0.024 0.017 48

TABLE 7.—C61 AND C64 STBF TEST GEARS 

FATIGUE ORIGIN LOCATION

Specimen Tooth Origin location

C61-1009 

1 Fillet surface

2 Subsurface inclusion

3 Subsurface inclusion

C61-1012 

1 Fillet surface

2 Fillet surface

3 Fillet surface

C61-1018 

1 Fillet surface

2 Fillet surface

3 Fillet surface

4 Fillet surface

C64-4007 

1 Fillet surface

2 Fillet surface

3 Fillet surface

C64-4011 1 Fillet surface

2 Fillet surface

C64-4012 

1 Fillet surface

2 Fillet surface

3 Fillet surface

5 Subsurface inclusion

TABLE 8.—SUMMARY OF MEAN AND MEAN-3 σ FOR STBF TEST DATA

Description Endurance limit, 

ksi

Standard dev., 

ksi

Coefficient variation,

%

Mean Mean-3 σ σ C.V.

9310 279.6 240.6 13.00 4.65

Pyrowear 53 253.8 224.1 9.90 3.90

C61 289.1 233.1 18.67 6.46

C64 281.3 223.0 19.40 6.90
NASA/TM—2011-217121 8 

Figure 1.—Fatigue crack growth test results for Ferrium C61. 

*Ferrium C61 is a secondary hardening steel that will increase hardness after tempering. (Typical core hardness 

after tempering is 47 to 50 HRC.) 

Figure 2.—Hardenability comparison (Jominy end quench test) for 9310 and Ferrium C61. 

1.00E-08

1.00E-07

1.00E-06

1.00E-05

1.00E-04

1.00E-03

1.00E-02

1 10 100 1000

DELTA K, KSI-IN^0.5

da/dN, INCCH/CYCLE

C61-1 (R=0.1), DeltaK(th)=3.23 C61-2(R=0.1), DeltaK(th)=2,60

C61-3(R=0.8), DeltaK(TH)=2.187 C61-4(R=0.8), DeltaK(th)=2.15
NASA/TM—2011-217121 9 

Figure 3.—C61 exhibits improved axial fatigue performance relative to 9310 (smooth bar axial fatigue 

testing, shot peened specimens; arrows denote runouts) in this data developed under U.S. Army 

Contract no. W911W6-09-C-0001. 

Figure 4.—Single tooth bending fatigue test gear. 
NASA/TM—2011-217121 10

Figure 5.—Overall view of MTS 810 servo-hydraulic test system.
NASA/TM—2011-217121 11

Figure 6.—Example gear test assembly showing specimen and loading 

arrangement. 
NASA/TM—2011-217121 12

Figure 7.—Hardness transverse during the carburization cycle development for C61. 

Figure 8.—C61-1018 root fillet hardness traverse data. 
NASA/TM—2011-217121 13

Figure 9.—C61-1012 root fillet hardness traverse data. 

Figure 10.—Hardness transverse during the carburization cycle development for C64. 
NASA/TM—2011-217121 14

Figure 11.—C64-4011 root fillet hardness traverse data. 

Figure 12.—C64-4012 root fillet hardness traverse data. 
NASA/TM—2011-217121 15

Figure 13.—Core microstructure of C61-1012 root area. Nital etch. 480X. 

Figure 14.—Case microstructure of C61-1012 root area. 480X (carburized surface at top). 
NASA/TM—2011-217121 16

Figure 15.—Core microstructure of C64-4011, root area. Nital etch. 480X. 

Figure 16.—Case microstructure of C64-4011, root area. 480X (carburized surface at top).
NASA/TM—2011-217121 17

Figure 17.—SEM fracto-micrograph of C61-1012 Tooth no. 2 showing bending fatigue 

crack origin at surface. 

Figure 18.—SEM fracto-micrograph of C61-1009 Tooth no. 2 showing bending fatigue

crack origin at a subsurface inclusion. 
NASA/TM—2011-217121 18

Figure 19.—SEM fracto-micrograph of C64-4011 Tooth no. 2 showing bending fatigue

crack origin at surface. 

Figure 20.—SEM fracto-micrograph of C64-4012 Tooth no. 5 showing bending fatigue

crack origin at a subsurface inclusion. 
NASA/TM—2011-217121 19

ERDS Advanced Gear Steel Fatigue Test Results 

STBF Data, R = 0.1, Failure Data Comparison 

Figure 21.—STBF raw data plot on S-N diagram for all four alloys.

ERDS Advanced Gear Steel Fatigue Test Results 

STBF Data, R = 0.1, Mean S-N Curve Comparison 

Figure 22.—STBF Mean S-N Curve for all four alloys. 

ERDS Advanced Gear Steel Fatigue Test Results 

STBF Data, R = 0.1, M-3σ S-N Curve Comparison 

Figure 23.—STBF Mean minus 3 Standard Deviation S-N Curve for all four alloys.

 

 

200

220

240

260

280

300

320

340

360

380

1.E+04 1.E+05 1.E+06 1.E+07

Cycles

Max Bdg Stress (ksi)

9310_Failures P53_Failures C61_Failures C64_Failures

 

 

200

250

300

350

400

450

500

1.E+03 1.E+04 1.E+05 1.E+06 1.E+07

Cycles

Max Bdg Stress (ksi)

9310_Mean P53_Mean C61_Mean C64_Mean

 

 

200

220

240

260

280

300

320

340

360

1.E+03 1.E+04 1.E+05 1.E+06 1.E+07

Cycles

Max Bdg Stress (ksi)

9310_M-3sig P53_M-3sig C61_M-3sig C64_M-3sig
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Advanced Gear Alloys for Ultra High Strength Applications 

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Shen, Tony; Krantz, Timothy; Sebastian, Jason 

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WBS 659877.02.03.AR16.01 

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13. SUPPLEMENTARY NOTES

14. ABSTRACT

Single tooth bending fatigue (STBF) test data of UHS Ferrium C61 and C64 alloys are presented in comparison with historical test data of 

conventional gear steels (9310 and Pyrowear 53) with comparable statistical analysis methods. Pitting and scoring tests of C61 and C64 are 

works in progress. Boeing statistical analysis of STBF test data for the four gear steels (C61, C64, 9310 and Pyrowear 53) indicates that the 

UHS grades exhibit increases in fatigue strength in the low cycle fatigue (LCF) regime. In the high cycle fatigue (HCF) regime, the UHS 

steels exhibit better mean fatigue strength endurance limit behavior (particularly as compared to Pyrowear 53). However, due to

considerable scatter in the UHS test data, the anticipated overall benefits of the UHS grades in bending fatigue have not been fully 

demonstrated. Based on all the test data and on Boeing’s analysis, C61 has been selected by Boeing as the gear steel for the final ERDS 

demonstrator test gearboxes. In terms of potential follow-up work, detailed physics-based, micromechanical analysis and modeling of the 

fatigue data would allow for a better understanding of the causes of the experimental scatter, and of the transition from high-stress LCF 

(surface-dominated) to low-stress HCF (subsurface-dominated) fatigue failure. Additional STBF test data and failure analysis work, 

particularly in the HCF regime and around the endurance limit stress, could allow for better statistical confidence and could reduce the 

observed effects of experimental test scatter. Finally, the need for further optimization of the residual compressive stress profiles of the UHS 

steels (resulting from carburization and peening) is noted, particularly for the case of the higher hardness C64 material. 

15. SUBJECT TERMS

Gear; Steels; Bending fatigue 

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