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The Center for Heat Treating Excellence
(CHTE) at Worcester Polytechnic Institute
(WPI) in Massachusetts is conducting a
cutting-edge research project aimed at
measuring the surface hardness and case depth on carburized
steels for process verification and control. The results will enable
companies to improve the quality of heat-treated products faster
and more cost-effectively.
“This is the first time a project like this is being undertaken,” ex-
plains Lei Zhang, CHTE researcher and Ph.D. candidate at WPI. “Our
focus is on developing new measurement techniques that will en-
hance product quality.”
According to lead researcher Richard Sisson, Jr., George F. Fuller Pro-
fessor of Mechanical Engineering at WPI, and CHTE director, the heat
treating industry needs rapid, accurate nondestructive techniques to
measure surface hardness and case depth on carburized steels for
process verification and control. “Current measurement methods re-
quire destructive testing with traveler specimens that cannot always
represent the configurations of the production part, nor the associated
subtleties of thermal history, carbon atmosphere, and geometry influ-
enced diffusion. Our research will eliminate much of the guesswork.”
Another industry challenge with the traveler specimen-measure-
ment method is that it often requires periodic sectioning of pro-
duction parts to validate the hardness and case depth after car-
burization, especially for critical shaft and gear teeth configura-
tions. “This method is labor intensive, expensive, and susceptible
to operator error, as well as counterintuitive to the end result of
high quality, usable heat-treated parts,” explains Sisson.
A key challenge of the project is to distinguish between hard-
ness and residual stress, because most techniques currently used
to measure case depth are not only sensitive to hardness distri-
bution, but also to residual stress distribution.
Nondestructive techniques being evaluated initially include eddy
current, meandering winding magnetometer (MWM), Barkhausen
noise testing, and alternating current potential drop.
The project focuses on four tasks:
• Identify nondestructive techniques to measure surface
hardness and case depth and develop a fundamental
understanding of their mechanisms. Select nondestructive
techniques for testing based on project objective and
equipment availability.
• Select alloys and design heat treating conditions to be used
to create testing standards using the simulation method,
and fabricate and characterize the standards.
• Conduct nondestructive tests to determine the correlations
between the destructive test results and the known results
in the standard.
• Determine correlations among nondestructive test
measurements, hardness, and microstructure for the
standards. Verify the effectiveness of the respective
nondestructive test technique in industry.
Completion of the research project is expected in 2016.
ADVANCED MATERIALS & PROCESSES •
NOVEMBER-DECEMBER 2014
40
Breakthroughs in Nondestructive Measuring Techniques
HTPRO
6
About CHTE
The CHTE collaborative is an alliance between the industrial sec-
tor and university researchers to address short-term and long-
term needs of the heat-treating industry. Membership in CHTE is
unique because members have a voice in selecting quality re-
search projects that help them solve today’s business challenges.
Member research process
Research projects are member driven. Each research project has
a focus group comprising members who provide an industrial
perspective. Members submit and vote on proposed ideas, and
three to four projects are funded yearly. Companies also have
the option of funding a sole-sponsored project. In addition,
members own royalty-free intellectual property rights to precom-
petitive research, and are trained on all research technology and
software updates.
CHTE also periodically undertakes large-scale projects funded
by the federal government or foundations. These endeavors
keep members informed about leading edge technology.
CHTE current research portfolio
Other projects now in progress include:
• Improving Alloy Furnace Hardware Life
• Induction Tempering
• Gas Quench Steel Hardenability
• Cold Spray Nanomaterials (supported by ARL)
For more information about CHTE, its research projects, and
member services, visit
wpi.edu/+chte, call 508.831.5592, or
email Rick Sisson at
sisson@wpi.edu, or Diran Apelian at
dapelian@wpi.edu.
l
l
l
l
l
l
l
l
l
l
1.2
1
0.8
0.6
0.4
0.2
0
Slope ratio
0.6 0.7 0.8 0.9 1 1.1 1.2 1.3
Case depth, mm
Barkhausen noise unit used
to measure case depth. The
magnetization voltage
sweep method has been
applied, and the slope ratio
reflects the case depth
variation of the tested part.
8 Hz/20 Hz
8 Hz/128 Hz
8 Hz/250 Hz
20 Hz/128 Hz
20 Hz/250 Hz
Primary winding
Secondary
windings
l
l
l
l l l
39.0 41.0 43.0 45.0 47.0 49.0
Permeability
Meandering winding magnetometer
(MWM) used to measure surface
hardness. At a high frequency
(5 MHz), permeability measured by
the MWM sensor has good
correlation with surface hardness.
y
= 16.018
x
= 177.11
600
400
200
Microhardness,
Hv