engin - heat treatment for stronger aerospace gears

8/2/2019 ENGIN - Heat Treatment for Stronger Aerospace Gears

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24 GEAR SOLUTIONS JULY 2006 gearsolutionsonline.com

It is well established that carburiza-

tion of low alloy steels promotes

compressive residual surface stress

upon quenching, and that compres-

sive surface stresses enhance fatigue

life. Building upon these principles, a

U.S. Army sponsored project is in prog-

ress to improve helicopter gear fatigue

life through innovative quenching and

the achievement of deeper compressive

surface stress. The Army has established

a goal to improve the power density andlife of helicopter transmissions. Using

Pyrowear 53 alloy steel, notched test

bars and full test gears have been heat

treated by carburizing, quenching, deep

freezing, and tempering. The quench

methods examined were conventional

oil quenching and intensive quenching.

Bending fatigue results for these pieces

will be discussed in conjunction with heat

treatment finite element simulation and

X-ray analysis of combined heat treatment

residual and gear loading stresses.

BackgroundThe steel alloy Pyrowear 53 is being

increasingly used in helicopter transmis-

sion gear applications for the U.S. Army,

based primarily on its resistance to tem-

pering at high temperatures and excellentfatigue strength. These attributes are criti-

cal in attack helicopter applications where

the transmission assembly is required to

function under severe conditions wherein

loss of gear lubrication can occur. The

assembly must be able to operate witho

breakdown for a 30-minute time period

in the absence of internal lubrication or

cooling, and thus the need for a highly

temper resistant alloy. Pyrowear 53 ha

a unique alloy content, as indicated in th

alloy chemistry data presented in Table

The alloy content for this material is

specifically designed to achieve resis-

tance to softening at high temperatures

and retain hot hardness in the carburizecase, while maintaining high core impact

strength and fracture toughness.

A variety of innovative processing

techniques continue to be advanced to

C

0.10

Mn

0.35 1.00 1.00 2.00 3.25 2.00 0.10

Si Cr Ni Mo Cu V

TABLE 1: PYROWEAR ALLOY 53 BASE COMPOSITION

By A.M. Freborg, B.L. Ferguson, Z. Li, D.X. Schwam, and B.J. Smith



2/15

gearsolutionsonline.com JULY 2006 GEAR SOLUTIONS

enhance material performance through

manufacturing, processing, and finishing.

For applications where part life is limited by

fatigue, significant life enhancement can be

realized by introducing compressive stress-

es in the part surface, and by eliminating

stress concentration factors.

Of primary interest in this project was

the investigation and application of a novel

heat treatment process called Intensive

Quenching to facilitate enhanced residual

surface compressive stresses, with con-

sequent material fatigue life improvement

[1-3]. Developed by Nikolai Kobasko, the

Intensive Quenching (IQ) process is an

alternative way of quenching steel parts to

achieve deep residual compression in part

surfaces. The technology is based in part

on the achievement of a large thermal gradi-

ent in the part by rapid surface cooling. In

non-carburized parts the process has been

shown to provide an extremely rapid and

uniform transformation to martensite in the

part surface layers while the core remains

austenitic. This condition creates a very

hard shell on the part that is under a state

of compression. As the hot austenitic core

cools and thermally contracts, the level of

surface compression is deepened signifi-

cantly. When the core subsequently trans-

forms from austenite and expands there

is some reduction in the level of surface

compression, but the final level of surface

compression in the IQ treated component

remains much higher than that of a conve

tionally quenched component [4-7].

For this program the premise for adapt-

ing this technology to carburized Pyrowear

53 was explored, with the goal of achievin

comparable or improved surface compres-

sion enhancement as witnessed in non-

carburized material. While carburization is

designed to achieve surface compression

in quenched parts by delaying formation o

surface martensite, the potential for

intensive quenching to foster additional

enhancement was a key factor of this

investigation.

An ongoing project sponsored by the

U.S. Army and headed by Deformation

Control Technology, Inc., is

providing important insights into

an innovative quenching process.

The authors wish to acknowledge the support of B. Smith and E. Ames of the U.S. Army AATD for their support of this work though the SBIR projec

#W911W6-05-C-0017. Appreciation is also expressed to B. Hansen of Sikorsky Aircraft Corp.; S. Rao of the Gear Research Institute; T. Krantz of th

Army Research Laboratory at NASA-Glenn; and J. Powell, M. Aronov, and N. Kobasco of IQ Technologies in Akron, Ohio. Program results describin

work in progress were presented at the American Helicopter Society (AHS) International Conference in May 2006.

ACKNOWLEDGEMENTS:



3/15


Heat Treating and Test Loading

Simulation

As with the notch bar, an analytical

approach was taken for the test gear heat

treatment assessment with respect to

both quench flow and fixturing design. A

proprietary fixture and channeled flow sys-

tem was developed for intensive quench

process, and a comparative heat treat-

ment simulation study conducted to com

pare the predicted residual stress profile

between the intensive and oil quenched

gears.

The resulting magnitude and distributi

of predicted residual stresses for both

quenching processes are shown in the

contour plot illustrated in Figure 15. The

figure illustrates the residual stress con

tours though the mid-plane cross section

of the gear.

While both quench methods are pre-

dicted to produce nearly identical stress

in the tooth itself, both magnitude and

distribution of the compressive stress in

the root are markedly increased by inten

sive quenching. The quantitative differen

es are clearly seen in direct comparison

of stress profiles at three locations in th

tooth/root cross section (Figure 16).

Here again, as in the notch bar speci-

mens, heat treatment simulation provide

useful predictive data concerning magni-

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Upper Coaxial Pipe

for Directing Intensive

Quench to Fixtured Gear

PHYSICAL CONFIGURATION OF INTENSIVE

QUENCHING SYSTEM FOR QUENCHING THE

PYROWEAR 53 TEST GEAR

FIGURE 20:

FIGURE 21:

LOCATIONS FOR X-RAY RESIDUAL STRESS

PROFILES IN OIL AND INTENSIVE

QUENCHED TEST GEARS

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Process No. Gears Tests/Gear Total

Standard OilQuench

12 6 72

IntensiveQuench

12 6 72

TABLE 5: TEST MATRIX FOR PHASE IA


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tude and distribution of the gear residual

stresses. Figures 17-19 show the predict-

ed residual stress profiles between the

oil and intensive quenched simulations

at the three locations shown in Figure16. While the tooth section (A) shows

consistent residual compression between

the two quenches (Figure 17), surface

and subsurface compression at the tooth

base (B) (Figure 18) and root center (C)

(Figure 19) show significant increases in

residual compression to a depth of 1.5

mm (0.059). This depth is slightly below

the carburizied case depth of ~0.050.

For the tooth base the predicted increase

is from 71 ksi to 102 ksi, and for the cen-ter root from 46 ksi to 90 ksi.

Based on the modeling results, physical

test gear specimens were subsequently

processed through both oil and intensive

quenching. Based on the heat transfer,

quench flow characteristics, and process

sensitivity determined from the DANTE

simulation work, an intensive quench fix

turing system was designed for the test

gear by IQ Technologies of Akron, Ohio.

The system was designed to be readilyintegrated within pre-existing IQ process-

ing equipment, demonstrating ready

adaptation of the technology within a

typical heat treatment manufacturing cel

An illustration of the system is shown in

Figure 20.

FIGURE 22:

COMPARISON OF PREDICTED VS. MEASURED RESIDUAL

STRESS PROFILES AT TWO LOCATIONS IN TWO HEAT

TREATED PYROWEAR 53 TEST GEARS.

FIGURE 23:

COMPARISON OF RESIDUAL STRESS PROFILES AT TOOTH BASE

IN UNLOADED (HEAT TREAT RESIDUAL STRESS ONLY) AND

LOADED (COMPOSITE STRESS) CONDITIONS.

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Material Characterization for Heat Treatment AnalysisProper mechanical and kinetics material property data are critical to

the accurate application of any simulation technology to process mod-

eling. For heat treat simulation, the required mechanical characteriza-

tion includes material stress-strain behavior by phase over the range

of stain rates and temperatures encountered during a given heat treat

process. Linkage with corresponding phase transformation models

then provides the general capability for the overall material model. The

heat treatment simulation software DANTE links both the mechanical

behavior model and the phase transformation kinetics models to accu-

rately predict the material response to heat treatment, specifically with

respect to metallurgical phase volume fraction, residual stress, and

distortion [8-10].

In defining material behavior, DANTEs material model incorporates

both rate dependent and independent yielding, kinematic and isotropic

hardening, and recoveryall as functions of temperature and metal-

lurgical phase. Isothermal, strain rate controlled tension, and com-

pression tests were run to characterize the stress-strain behavior of

Pyrowear 53 as a function of temperature and carbon level. From these

stress-strain data the mechanical parameters for DANTEs mechanical

model were determined and entered into the steel database.

The function of the kinetics model is to define the phase transforma-tion behavior of the given steel within the heating and cooling tempera-

tures and rates of the process. For the DANTE software, a complex set

of differential equations is used to describe the phase transformation

behavior for both diffusive (i.e. austenite formation or austenite decom-

position to ferrite, pearlite, or bainite) and non-diffusive (i.e. austenite

decomposing to martensite) transformation processes. This set of

equations is then coupled with the equations governing the mechanical

and thermal behavior of the material being subjected to the process

scenario under consideration. Mechanical property data for the carbu-

rized Pyrowear 53 material were obtained by tensile and compression

tests conducted at a variety of strain rates and temperatures, for vary-

ing carbon levels [11].

Phase transformation data for the kinetics models are obtained prin-

cipally by dilatometry. For the Pyrowear material samples were through-

carburized to four carbon levels (0.1, 0.3, 0.5, and 0.8 percent by

weight), and then provided to Oak Ridge National Laboratory for testing

using their high speed quenching dilatometer.

Steel phase transformation kinetics are most commonly presented

in the form of isothermal Time-Temperature-Transformation (TTT) or

Continuous Cooling Transformation (CCT) diagrams. For the subject

Pyrowear steel, Carpenter Technology Corporation was able to provide

a simple TTT diagram and critical temperatures. From this figure,

several important heat treating characteristics of Pyrowear 53 were

evident. The supplied data indicated that the steel is highly resistant

to diffusive phase transformation, with the ferrite/pearlite nose occur-

ring at 704 C (1300 F) after quickly cooling from the austenite rang

and holding for 15 minutes. This helps to explain the high hardenabil

of this steel. The specified core martensite start temperature is 510

C (950 F), and the associated carburized case martensite start tem

perature is 130 C (265F) [1,2]. Consequently, phase transformatio

kinetics characterization for simulation focused primarily on the auste

ite-to-martensite transformation. The material kinetics parameters we

implemented into the DANTE steel database after mathematical fittin

of dilatometric data obtained through testing a series of carburized

samples on ORNLs quenching dilatometer [11].

Sound material data, linking mechanics with phase transformation

predictive capability, is essential for accurate implementation of simu

tion technology to process modeling. The robust design of the DANTE

material constitutive model, coupled with the detailed Pyrowear 53

material characterization data, provided a solid foundation for the pro

cess simulation work required in this project.

Objectives and Program OutlineThe primary objective of this program was to demonstrate the

potential for improving the bending fatigue strength of Pyrowear

53 steel used for helicopter transmission gears by heat treat-

ment. The Armys Rotorcraft Force Modernization Fleet requires asubstantial increase in main gearbox power density, with minima

impact on the gearbox interface.

In lieu of redesigning gears and increasing gearbox size and

transmission weight, the program focused on achieving the highe

power density requirement through application of an innovative

heat treating process. To achieve this objectiveand demonstra

technical, engineering, and commercial feasibility for the innova-

tionan evaluation program consisting of a combination of pro-

cess simulation and physical experiments was defined. The prin-

cipal goal was to establish two independent material populations

based solely on differences in heat treatment, with simulation,

physical testing, and bending fatigue test results used to charac

terize improvement, based principally on the effect of enhanced

residual surface compressive stresses. The program is outlined

as follows:

Define heat treatments to be examined (isolation of heat treati

effects with respect to residual stress)

Establish testing program for feasibility assessment

Predictive materials engineeringsimulate heat treatments

Processing and testing of sample coupons

Evaluate results

Assess process sensitivity and enhance process control

Evaluation and testing of refined process

Implement analysis and physical testing in full gear component

FIGURE 1: FIGURE 2:

Carburized Surface

MODIFIED V-NOTCH SAMPLE CONFIGURATION SCHEMATIC OF TEST SAMPLE SHOWING CARBURIZED SURFACE


6/15


Analysis and Testing ProgramProgram Phase IFeasibility Assessment

To demonstrate the potential for improving the bending fatigue

strength of Pyrowear 53 steel for helicopter transmission gears

by heat treatment, 25 kg (55 lbs.) of 60.3 mm (2 3/8) diameter

Pyrowear 53 bar stock was donated from DCTs inventory. A general

processing and testing program for the material was then estab-

lished, as summarized in Table 2.

Sample Processing and Preparation

Rectangular test bar blanks were machined from the bar stock

with the length of the test bar being coincident with the bar stock

rolling direction. In discussion with gear engineering experts at the

Army Gear Research Laboratory at NASA-Glenn and Bell Helicopter, amodified V notch geometry for the three-point bending fatigue sam-

ple was defined; the notch geometry was consistent with notched

flexure fatigue specimens used by Bell Helicopter. The modified V

notch has a 60 included angle with a 1.16 mm radius (0.0455) to

simulate a typical gear tooth root geometry. The test specimen con-

figuration is shown in Figure 1.

To most accurately capture the combined carburizing and geometry

effects of a Pyrowear gear, the notched test bars were carburized

only on the top surface (including the notch), as shown schemati-

cally in Figure 2. To accomplish the localized carburization, the sides

and bottom surfaces of the bending fatigue samples were masked

with copper plating. This is standard practice for blocking carburiza-

tion of selected surfaces on aerospace parts.

Process Simulation

Prior to physical heat treating, a series of heat treat models were

run to characterize the respective processes to be applied to the

test pieces. The use of a simulation tool provided a rapid, non-

destructive, and cost-effective means of assessing both the internal

metallurgical behavior during and after processing as well as the

mechanical response, in terms of residual stress, hardness, and

dimensional change. For this task, the heat treatment simulation

software DANTE was used to characterize the carburization and two

quench hardening processes selected for the program.

The Pyrowear 53 material was carburized to a carbon level of 0.80

Process

Route

Heat

TreatSurface

BendingFatigueSamples

Metallurgical

Testing

1

2

OilQuench

18

18IntensiveQuench

Microstructure,Microhardness,

X-ray

Microstructure,Microhardness,

X-ray

Milled &Superfinish

Polished

Milled &Superfinish

Polished

FIGURE 3:

3

2

1

FINITE ELEMENT MESH OF TEST SAMPLE

TABLE 2: TEST MATRIX FOR PHASE I FEASIBILITY ASSESSMENT


7/15


The intensive quench is applied axially to the gear using spe-

cially controlled, high-velocity water flow. A fixture with the aus-

tenized gear is sealed against a vertical coaxial pipe (shown in

Figure 20), which contains specially designed baffles to direct the

quench across the various gear surfaces and through the teeth.

Precise timing of quench application and shut-off is essential.

discussed in Table 5, for the planned single tooth bend testing

a total of 12 test gears were oil quenched, and 12 gears were

intensively quenched using the fixturing system described in

Figure 20.

CROSS SECTION CONTOUR MAP OF RESIDUAL STRESS IN OIL

QUENCHED GEAR FOR UNLOADED AND LOADED CONDITIONS

CROSS SECTION CONTOUR MAP OF RESIDUAL STRESS IN INTENSIVE

QUENCHED GEAR FOR UNLOADED AND LOADED CONDITIONS

FIGURE 24: FIGURE 25:

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COMPOSITE STRESSES IN SINGLE TOOTH BENDING (OIL QUENCH) COMPOSITE STRESSES IN SINGLE TOOTH BENDING (INTENSIVE QUENCH)


8/15


After processing, one of each gear type (oil and IQ) was

tested in X-ray dif fraction to assess both residual stress mag-

nitudes and variability relative to the simulation predictions.

Residual stress profiles were taken at two locations for each

gear, with the profile made at the center root location (Position

C in Figure 16). The X-ray testing orientations taken on the

gears are shown schematically in Figure 21.

The measured X-ray residual stress profiles from each gear

are compared with their corresponding simulation predictions in

Figure 22. The predicted residual stress profiles for both oil and

intensively quenched gears displayed excellent agreement with

the stresses calculated from X-ray measurements. Also, the

intensively quenched gear exhibited ~40% increase in residual

compression over the baseline oil quench to a depth of 0.050.

Based on the modeling and X-ray test resultsas well as the

results of residual stress/three-point bending fatigue evaluation

conducted in the early par t of this investigationresults of the

single tooth bending fatigue testing currently underway are

predicted to show strong promise for intensive quenching

providing substantial enhancement in bending fatigue perfor-

mance over the baseline oil quenching process.

An additional engineering utility of the DANTE heat treatment

simulations is the ability to analyze composite stresses in thegear. Stresses generated in gear loading are of course affected

by the residual stresses generated in the heat treatment.

Quantitive assessment of the composite stress is not straight-

forward, and has typically been done under the assumption that

the stress states are additive.

Simulation results illustrating this interaction for both the

oil intensively quenched gears are shown in Figures 23-25.

The nonlinear response of residual stress at the tooth base

is clearly evident in the comparitive stress plots presented

in Figure 23. The graph compares the pre-loaded tooth base

residual stress profile with that under a 900 lb. tooth load. The

simulation provides important data concerning the non-lineral

response, as well as information concerning depth relation-

ships. From a design standpoint, this type of analysis provides

an impor tant tool in engineering residual stresses to meet

complex loading requirements.

Additional insight into behavior of the composite stresses can

be gained through examination of cross-sectional contour maps

such as those shown in Figures 24 and 25. For example, in

the oil quenched gear (Figure 24) one can see localized tensile

regions below the tooth. This region increases in both volume

and magnitude when the tooth is subjected to loading. In con-

trast, the intensively quenchedz gear (Figure 25) shows a much

smaller sub-tooth tensile region, which does not increase

significantly during loading. Such characterization is valuable

in understanding potential fracture path and failure mode.

Therefore extending heat treatment simulation to include sub-

sequent loading represents an impor tant engineering design

milestone.

ConclusionsThis study proved the feasibility of improving bending fatigue

strength by altering the hardening process. The intensive quench-

ing process produced a deeper compressive stress state after

CONTINUED ON PAGE 50

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the worst performance with near 50 percent residual stress drop-off at

both outer edges. Also, the stacked configuration displayed a wide regio

in the bar interior with tensile stress on the order of 330 MPa, extendin

through the bar thickness. The magnitude and spread of these tensile

stresses are not as pronounced in configurations A and B. Thus from a

residual stress standpoint, expectations were for configuration A to sho

superior bending fatigue performance as compared with the oil quenche

samples (see oil stresses in Figure 5). Configuration B would be expect

ed to show inferior, or at best similar performance to the oil quench.

Configuration C samples were not physically evaluated.

Physical Testing of Simulated Quench Configurations

In continuing the notch bar fatigue characterization, 84 additional

notch bar samples were prepared using a carefully planned design-of-

experiments approach to physically quantify the findings revealed in th

DANTE simulations. Table 4 details this second notch bar testing plan

To facilitate the normal and parallel directional flow, a new fixture w

developed for the notch bars for use in the quench processing equip-

ment. The fixture was a stainless steel cylinder in which the notch bar

was positioned diametrically. During austenitization, the entire fixture

assembly (with sample) was heated. Transfer of the fixture into the

quenching unit was rapid and highly consistent, with the entire assem

bly easily placed within the IQ water flow tube. Comparative X-ray andsimulation residual stress calculations are presented in Figure 11.

As in the review of the surface stress simulation predictions, exami

nation of the residual stress profiles would also indicate superior

bending fatigue performance of the IQ-face samples (config. A) over

the oil quenched and parallel quenched specimens (config. B). Bendin

fatigue test data for this evaluation is shown in Figure 12. The new da

showed a marked reduction in scatter, as both overall notch surface

quality was improved and process stability was enhanced. Most impor

tantly, the bending fatigue test data parallels exactly the expected per

Notch Bar Bending Fatigue PerformanceGround Notch with Tighter Process Control

1250

1500

1750

2000

2250

2500

10,000 100,000 1,000,000 10,000,000

Oil

IQ-Face

IQ-Parallel

(2)

(6)

MaxLoad(lbs)

Cycles to Failure

Gear Specifications

No. Teeth 40

Dia. Pitch 10

PressureAngle 20

Pitch Dia. 4.0

Base Cir.Dia. 3.76

Outside Dia. 4.18

Root Dia. 3.75

Tooth Thick 0.11

RESIDUAL STRESS PROFILE COMPARISON AFTER HEAT TREAT

THROUGH TOOTH/ROOT CROSS SECTION

B

A

B

C

A

B

C

REFERENCE DIRECTIONS POINTS IN TOOTH/ROOT CROSS SECTION

TABLE 4: TEST MATRIX FOR PHASE IA

3-POINT BENDING FATIGUE DATA FOR PHASE IA NOTCH BAR EVALUATION

Oil Quench

Intensive Quench

FIGURE 12:

FIGURE 13:

TEST GEAR FOR SINGLE TOOTH BENDING FATIGUE EVALUATION

FIGURE 16:

SCHEMATIC OF SINGLE TOOTH BENDING FATIGUE TEST SET-UP

FIGURE 14:

FIGURE 15:

Process Configuration No. Samples

Oil Immersion Quench 20IQ Quench Normal to

Notch Face (A)

20

IQ Quench Parallel to

Notch Face (B)

20


10/15


formance as predicted in the simulations.

The IQ-faced quenched samples showed

a clear maximum cyclic load limit of 1,825

lbs., whereas the samples quenched in the

standard oil quench practice showed a clear

limit at 1,650 lbs. Also consistent with the

simulation stress predictions and X-ray calcu-

lations were the IQ-parallel quench samples,

which performed slightly worse than the oil

quenched material.

PROGRAM PHASE IIIMPLEMENTATION

INTO FULL GEAR COMPONENT

With the strong feasibility demonstrated

in Phase I and Phase IA for achieving the

bending fatigue life increase goal, the next

phase was initiated to extend the evalu-

ation to full gear components. Through

consultation with both the U.S. Army AATD

and military helicopter OEMs, a simple

spur gear design was selected as the test

component for the second phase of this

project. Configuration and summary gear

specifications are shown in Figure 13.

Prior to quench hardening, the gears

were vacuum carburized selectively on the

teeth surfaces, with copper plating mask-

ing the balance of the gear as shown in

the related figure.

Process and Testing Plan

The effect of intensive quenching on th

bending fatigue strength of the Pyrowear

53 gear is currently being evaluated by

single tooth bend testing. The configurati

of the test apparatus is shown schemati-

cally in Figure 14. In the test, a cyclic loa

is applied to two teeth via a movable upp

anvil and a stationary lower anvil, with th

gear remaining fixed by a shaft support.

For this tooth bending test, the evaluation

plan shown in Table 5 was developed to

quantitatively assess single tooth bending

fatigue improvement.

Radial Stress Profile-Gear Tooth from Simulation

-50

-40

-30

-20

-10

0

10

20

30

0 0.01 0.02 0.03 0.04 0.05

Intensive Quench

Oil Quench

S

tress,

ksi

Depth, in

Hoop Stress Profile-Tooth Root from Simulation

-120

-100

-80

-60

-40

-20

0

20

40

0 0.02 0.04 0.06 0.08 0.1

St

ress,

ksi

Depth, in

Intensive Quench

Oil Quench

Hoop Stress Profile-Center Root from Simulation

0 0.02 0.04 0.06 0.08 0.1 0.12

Intensive Quench

Oil QuenchStress,

ksi

Depth, in

-120

-100

-80

-60

-40

-20

0

20

40

RESIDUAL STRESS PROFILE COMPARISON

(RADIAL STRESS) AT POSITION (A) IN TEST

GEAR; FROM SIMULATION


(HOOP STRESS) AT POSITION (B) IN TEST


FIGURE 17: FIGURE 18: FIGURE 19:


(HOOP STRESS) AT POSITION (C) IN TEST


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11/15


seemingly insignificant, produced a distinct

variation in the surface residual stress pro-

file across the notch surface.

Simulations showed that the surface

residual stress below the notch could vary

from 600 MPa (87 ksi) at the outer edge

to 45 MPa (6.52 ksi) at the center. This

critical finding illustrates both the utility

and importance of a simulation design tool

in understanding and optimizing a manu-

facturing process such as heat treatment.

With this information, modifications to the

IQ valve system were made accordingly.

Future plans also include computer control

with data recording of operation and water

application.

Coupling improved flow control with

enhanced notch surface quality achieved

by grinding, the next variable examined

was the orientation of the par t relative

to the quench flow. Here again, process

simulation with the DANTE design soft-

ware demonstrated critical value. With thegoal being optimal application of intensive

quenching, three new configurations were

examined (each configuration is illustrated

schematically in Figure 8).

DANTE simulations were conducted for

each of these configurations to determine

sensitivity of stress magnitude and distri-

bution as a function of intensive quench

flow orientation. As in the flow timing

diagnostic work previously discussed,

profiles of the resulting residual stresses

across the surface notch were compared

for each case. The plot in Figure 9 dis-

plays the results, while Figure 10 shows

contour maps of the final residual stress

through the notch cross section for each

case. What appears immediately evident

from the simulations is that, as seen with

flow timing application, the orientation of

the part relative to flow also has a signifi-

cant effect on residual stressspecifi-

cally the stress distribution.

Configuration A (normal flow) displayed

the most consistent and uniform residual

stress, with minimal variation in stress

magnitude across the length of the notch.

With flow parallel to the notch in process-

ing a single sample (configuration B),

stress magnitude remains relativelyuniform from the leading edge through

the center, but drops off rapidly at the

trailing edge by nearly 50 percent (225

MPa vs. max. compression of ~450 MPa).

The stacked configuration (C), with both

outer edges essentially insulated, showed

(A)

(B)

(C)

SAMPLE QUENCH CONFIGURATIONS

EXAMINED IN PHASE IA

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COMPARISON OF PREDICTED SURFACE RESIDUAL STRESS ACROSS

THE ROOT OF THE SAMPLE NOTCH.

-700

-600

-500

-400

-300

-200

-100

0

100

200

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6

Oil X-Ray Calc

IQ-side X-Ray Calc

IQ-Face X-Ray Calc

Oil DANTE

IQ-Side DANTE

IQ-Face DANTE

Res

idualStress,

MPa

Depth, mm

PREDICTED CROSS SECTION RESIDUAL STRESS CONTOURS FOR 3 QUENCH CONFIGURATIONS

COMPARATIVE X-RAY AND SIMULATION PREDICTED RESIDUAL

STRESS PROFILES IN NOTCH BARS

FIGURE 9: FIGURE 11:

FIGURE 10:

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percent and an effective case depth of 0.5 mm. The DANTE simula-

tion employed a carburization cycle with applied carbon potential as

prescribed directly from the gear OEMs. The 3-D mesh for the simu-

lation is shown in Figure 3.

Simulations were also performed to assess the probable effectson residual stress and hardness between oil and intensive quench-

ing processes. Table 3 summarizes the process steps for each of

the evaluated quenching operations after carburization. Simulation

of the quenching was performed through application of heat transfer

coefficients for the oil or intensive quenching, and the subsequent

cryogenic treatment. The DANTE tempering model was employed for

the temper operation. The simulations indicated a marked enhance-

ment in both surface and subsurface residual compression for

intensive quenching. Therefore, physical processing and testing was

initiated on the Pyrowear bend test coupons. Predictions of hard-

ness and residual stress are compared against measured values in

the next section.

Physical Characterization

Upon completion of the heat treatments on the Pyrowear notch

bar specimens, microhardness profiles were measured at the sam-

ple notch, beginning at a depth of 0.005 (0.13 mm). A plot com-

paring the resulting hardness profiles for the two heat treatments

is presented in Figure 4; also included are the hardness profiles

predicted by the heat treat simulations. Here one sees the most

pronounced improvement in hardness at the surface and subsur-

face to a depth of about 0.02 (0.5mm).

Both a conventional oil quenched and an intensively quenched

notched bar sample were sent to Lambda Research for surface

and internal residual stress characterization. A combination X-ray

diffraction/chemical etching technique was used to measure lattic

strains and then calculate the longitudinal residual stress as a fun

tion of depth from the notch root. Measurements were taken at 0

mm (0.008) increments, to a depth of 1.2 mm (0.047). The sim

lation and the measured test results display excellent agreement,as shown in the plot presented in Figure 5. The project methodolo

proved the value of process simulation as a predictive design tool

To assess the bending fatigue resistance of the carburized and

heat treated Pyrowear 53 notch bar samples, three-point bend-

ing fatigue tests were conducted using a servo-hydraulic testing

machine at Case Western Reserve University. The machine was

operated using load control, with the minimum to maximum load

ratio being 0.1 so that the notch was under constant cyclic ten-

sion. This condition assured that no slippage or sample move-

ment occurred during testing, at least up to the point of large ram

displacement due to cracking. To stop the test quickly after crack

development, strain gages were applied to the samples at the

notch root.

Eighteen oil quenched and 17 intensively quenched fatigue

samples were tested in bending fatigue to compare the effect of

the two heat treat processes on the resulting fatigue resistance.

Figure 6 shows the fatigue test data for the two quenched condi-

tions. A test was stopped after the number of cycles exceeded 10

and declared a runout. Rupture of the strain gage occurred when

crack began to extend, and the test machine would automatically

stop and the sample was declared a failure. Failed samples were

bent, not broken. While there is scatter evident in the data, Figure

shows that the resistance to bending fatigue is higher for the inte

sively quenched test bars than for the conventionally quenched oil

test bars.

Statistical analysis of the data was performed to verify that

the apparent improvement in bending fatigue resistance due to

intensive quenching was real [13]. Following a relationship used to

compare the bending fatigue strength of carburized gear teeth, raw

data were transformed to allow comparison of projected lives at a

normalized load. In this case, the normalized load was selected a

1,500 pounds, and a Weibull distribution was fit to the transforme

data. A comparison of these data at both the 10- and 50-percent

lives showed a statistical difference between the intensively and

quenched and oil quenched test bars (see Table 3) [13]. The anal

sis demonstrates the benefit of deeper residual compressive stres

COMPARISON OF PREDICTED AND MEASURED

HARDNESS AT THE TEST SAMPLE ROOT

PREDICTED VS. MEASURED RESIDUAL STRESS PROFILES AT

THE V-NOTCH AS A FUNCTION OF DEPTH.

FIGURE 4: FIGURE 5:

Intensive Quench

Test Bar

Oil Quench

Test BarWeibull Shape Parameter 0.458 0.665

10% Life(103Cycles)

50% Life(106Cycles)

25 21

1.5 0.36

Ratio of 10% Lives (IQ/OQ) 1.2

Statistical Significance 98%

TABLE 3: STATISTICAL ANALYSIS OF TRANSFORMED FATIGUE DATA


14/15


produced by intensive quenching on bending fatigue resistance. At

the 50-percent life level, the ratio of improvement was 4.2, with

a 98-percent confidence level. However, at the lower 10-percent

life level, the ratio dropped to 1.2, with just a 60-percent statisti-

cal confidence. The relatively low Weibull shape parameter for bothquenched conditions is indicative of scatter in the test data. The dif-

ference in the significance of the life data comparison is also indica-

tive of the test data scatter. One probable source of scatter was the

surface finish of the notch and the fact that grinding was not per-

formed after the milling operation to shape the notch.

PHASE IAPROCESS SENSITIVITY AND REFINEMENT

Simulation in Process Sensitivity Assessment

The data scatter seen in the

initial notch bar testing neces-

sitated additional investigation

into IQ process variables and

sensitivity. The DANTE predic-

tive heat treatment software

tool provided significant insight

into the process sensitivity,

particularly with respect to the

effect of variation in water flow

and application time.

The initial quench configura-

tion used for the notch bar

samples was a fixture system

in which the water flow was

directed parallel to the longitu-

dinal direction of the sample,

as shown schematically in

Figure 7.

Using the DANTE software, DCT executed a series of heat treat-

ment simulations to characterize the sensitivity of resulting residual

compressive stress in the sample with respect to water flow applica-

tion and timing. It was found that full flow must be achieved within

1.0 seconds from initial quench application to develop the tem-

perature gradient required to achieve the deep compressive residual

stresses. This information is crucial because it demonstrates quan-

titative sensitivity data for quenching this type of root geometry. The

shift in thermal gradient caused by an initial reduced flow, although

3-POINT BENDING FATIGUE DATA FOR CARBURIZED AND

HARDENED PYROWEAR 53 NOTCHED TEST BARS.

PHASE I QUENCH CONFIGURATION

FIGURE 7:

IGURE 6:


15/15

heat treatment than conventional oil quenching, and this resulte

in improved bending fatigue strength.

In addition to actual test data this study showed the benefit o

using accurate numerical simulation of the carburization and ha

ening processes to assess the nature of the differences betwee

the processes, and to predetermine the quenching conditions

required to achieve the goal of deeper residual compression, th

improved resistance to fatigue. Finally, the utility and importanc

of considering composite (heat treatment residual and loading)

stresses in assessing gear loading was also shown. Further wor

in this area by DCT includes expansion of the simulation capabi

ties to include both mechanical shot and laser shock peening

effects in further enhancing composite residual stresses in a

carburized gear. Preliminary work in this area is also underway

within a related U.S. Army sponsored project.

ABOUT THE AUTHORS:

REFERENCES1) N.I. Kobasko and V.S. Morganyuk, Numerical Study of Phase Changes,

Current and Residual Stresses in Quenching Parts of Complex Configuration,

Proceedings of the 4th International Congress on Heat Treatment of Materials,

Berlin, Germany, 1 (1985), 465-486.

2) N.I. Kobasko, Intensive Steel Quenching Methods. Theory and Technology of

Quenching, Springer-Verlag, New York, N.Y., 1992, 367-389.

3) N.I. Kobasko, Method of Overcoming Self-Deformation and Cracking During

Quenching of Metal Parts, Metallovedenie and Termicheskay Obrabotka Metallo

(in Russian), 4 (1975), 12-16.

4) M. Hernandez et al., Residual Stress Measurements in Forced Convective

Quenched Steel Bars by Means of Neutron Diffraction, Proceedings of the

2nd International Conference on Quenching and the Control of Distortion, ASM,

(1996), 203-214.

5) M.A. Aronov, N.I. Kobasko, J.A. Powell, J.F. Wallace, and D. Schwam, Practical

Application of the Intensive Quenching Technology for Steel Parts, Industrial

Heating Magazine, April 1999, 59-63.

6) A.M. Freborg, B.L. Ferguson, M.A. Aronov, N.I. Kobasko, and J.A. Powell,

Intensive Quenching Theory and Application for Imparting High Residual Surfac

Compressive Stresses in Pressure Vessel Components, Journal of Pressure

Vessel Technology, 125 (2003), 188-194.

7) B.L. Ferguson, A.M. Freborg, and G.J. Petrus, Comparison of Quenching

Processes for Hardening a Coil Spring, Advances in Surface Engineering,

Metallurgy, Finishing and Wear, SAE (01) 1373, (2002).

8) D. Bammann et. al., A Material Model for Carburizing Steels undergoing Phase

Transformations, Proceedings of the 2nd International Congress on Quenching

and Control of Distortion, (Cleveland, OH: ASMI, 1996), 367-375.

9) B.L. Ferguson, A.M. Freborg, G.J. Petrus, and M.L. Callabresi, Predicting the

Heat Treat Response of a Carburized Helical Gear, Gear Technology (2002),

20-25.

10) B.L. Ferguson and A.M. Freborg, A Design Tool for Tuning and Optimizing

Carburizing and Heat Treat Processes, Technical Paper 2002-01-1475, SAE

(2002).

11) B.L. Ferguson and A.M. Freborg, Software to Predict Distortion of Heat Treate

Components, U.S. Army SBIR - Phase II Final Report, USAAMCOM TR 02-D-46

(2002).

12) Product Technical Data, Carpenter Technology Corporation, 2001.

13) T. Krantz, Army Research Laboratory, Private Communication, June 2004.

CONTINUED FROM PAGE 37

A.M. Freborg, B.L. Ferguson, and Z. Li are with Deformation

Control Technology, Inc., which is based in Cleveland, Ohio. Go

online to [www.deformationcontrol.com]. D.X. Schwam is with

Case Western Reserve University, and B.J. Smith is with the U.S

Army AATD, which is located in Fort Eustis, Virginia.

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