News Release

March 30, 2021

Development of Cleanliness Evaluation Technology for Greatly Improved
Reliability of Special Steel Products—Contributing to Smaller and Lighter Machine Parts toward Carbon Neutrality—
Sanyo Special Steel Co., Ltd. (Representative Director and President: Higuchi Shinya; Head Office: Himeji)
has developed new cleanliness evaluation technology capable of greatly improving the reliability of special
steel products.
This technology establishes a method for quickly and accurately
measuring the cleanliness of special steel (which is a measure ofsmall traces of unavoidable nonmetallic inclusions in steel) which
impacts the reliability of bearings and other machine structural parts.
By combining this method with the extreme value statistics method,
Sanyo Special Steel has made it possible to estimate the size of thelargest inclusion in steel with an unprecedented level of accuracy.
The contributions expected from this technology include thedevelopment of manufacturing technology that further improves the As carbon neutrality drives the widespread use ofcleanliness of special steel products, and the optimization of part wind power generation, bearings, which are a key
designs based on quantitative predictions of fatigue strength as component, will require an even higher level ofreliability.
derived from the size of the largest inclusion.
■ Development BackgroundThe needs for further improvements to the reliability of special steel products are becoming increasingly strong
against the backdrop of initiatives aimed at carbon neutrality, such as the push for electrified automobiles and theexpansion of wind power generation. Sanyo Special Steel has already established steel manufacturing technologythat boasts the world’s highest level of cleanliness for reducing nonmetallic inclusions in steel. However, raising that
reliability to the next level calls for the development of manufacturing technology that further reduces the size ofinclusions that can become the starting point for fatigue fracture. When developing technology, the methods andtechnology for evaluating the current status and improvement benefits with high accuracy are indispensable.
Moreover, because the probability that large inclusions will exist in a given volume is low, it is necessary to examine
a larger volume to achieve high accuracy evaluations. Nevertheless, it is difficult to quickly and accurately evaluatethe largest inclusion, including its size and type, with the current widely used methods for evaluating cleanliness,such as microscopic observation (microscopy) and ultrasonic testing. Additionally, the method for measuring thelargest inclusion on the fractured surface exposed through means of fatigue testing is ill-suited to examinations oflarge-volume test pieces because of the difficulty of fracturing test pieces unless they are small.
■ Key Points of Development
Sanyo Special Steel overcame these problems by developing the technology for quickly and accurately
measuring the largest inclusion in steel for evaluating larger volumes. This technology uses an enlarged proprietary
test piece, and a proprietary method for causing rapid fatigue fracture by combining hydrogen-induced
embrittlement with ultrasonic fatigue testing. By applying the extreme value statistics method to the measurement
results of multiple tests using this method, it is now possible to estimate the size of the largest inclusion potentiallyfound in steel with unprecedented accuracy.
It can be expected that this will allow us to help our customers optimize their part designs (see Note) through thedevelopment of manufacturing technology that affords further reliability improvements to our special steel products,
and quantitative fatigue strength predictions based on the size of the largest inclusion.
Note: Mechanical parts subject to heavy loads such as bearings and drivetrains are designed so as to avoid short-life risks posed by breakage or
the like by weighting the assumed maximum stress with a specific safety factor. Quantitative predictions of fatigue strength in special steel,the raw material for these products, improves its reliability and thus enables this safety factor to be reviewed (or, reduced), which leads tosmaller and lighter part designs.

Amidst strong calls around the world for reduced CO 2 emissions aimed at achieving carbon neutrality, we will
strive to develop and supply more reliable special steel products by making active use of this newly developed
evaluation technology, and contribute to the improved energy efficiency of automobiles and the expanded use ofwind power generation and other carbon-free power sources through the use of smaller and lighter parts.
EndFor inquiries: Sanyo Special Steel Co., Ltd., (Phone: +81 79-235-6002)

[Reference information](Table) Comparison of Existing Evaluation Method and Sanyo Special Steel’s Evaluation Technology forInclusions(Existing inclusion evaluation method)

Microscopy

Ultrasonic testing(Micro testing)(Macro testing)

1. The polished surface ofa test piece is observedwith an optical
microscope.
2. The size and otherproperties of thedetected inclusion aremeasured.
3. Based on a plurality of
the abovemeasurement results,
the size of the largest
inclusion within a given
volume is estimatedusing the extreme value
statistics method.

1. Ultrasonic waves areirradiated into steel andother materials by an
ultrasonic detector.
2. The reflected echo
waves are analyzed
and the size anddistribution of inclusions
in steel and othermaterials are detected.

Fractured surface
observation(Micro testing)

Summary ofevaluation
methodSpeed

Accuracy

Evaluation
volume1. Fatigue fracture(breakage) of the test
piece is induced byultrasonic vibration orother means, which
reveals the largest
inclusion in the evaluation
volume.
2. An electron microscopeor the like is used toanalyze the size, type,and other properties of
the inclusion on thebroken surface.
3. Based on a plurality of theabove results, the size ofthe largest inclusion
within a given volume isestimated using theextreme value statistics
method.

Fair

GoodGood

Because the observation
range is limited, an
enormous number ofobservations is required
to measure large
inclusions that have a low
probability of existence.

Quick detection of large
inclusions over a broad
range

Fatigue fracture of the test
piece reveals the largest
inclusion in the evaluation
volume on the fractured
surface.*Our proprietary
technology(Fractured surface observationof large test pieces)
1. The proprietary test piece ischarged with hydrogen.
2. Fatigue fracture (breakage)of the hydrogen-embrittled
test piece is induced byultrasonic vibration, which
reveals the largest inclusionin the evaluation volume.
3. An electron microscope isused to analyze the size,
type, and other properties of
the inclusion on thefractured surface.
3. Based on a plurality of theabove results, the size of thelargest inclusion within agiven volume is estimatedusing the extreme value
statistics method.

GoodFatigue fracture of the test
piece reveals the largest
inclusion in the evaluation
volume on the fractured
surface.*

GoodFairGoodGood

Microscopy allows foraccurate measurements.

Non-destructive
inspection does not lend
to accurate identificationof the size and type ofinclusion.
The sizes of inclusions
that can be detected arealso limited.
(Very small inclusions aredifficult to detect.)

Inclusions that appear on
the fractured surface can bemeasured accurately.

Inclusions that appear on thefractured surface can bemeasured accurately.

FairGoodFairGood

Because the observation
Macro inspections enable
Test piece must be small
Hydrogen charging makes it
range is limited, anevaluation of the entire
enough to be fractured topossible to easily fracture evenenormous amount of test
steel piece
reveal the inclusion.
large-size test pieces to reveal
piece observations isthe inclusion.
required to evaluate largevolumes.
* Repeatedly applying load to the test piece with ultrasonic vibration or other means causes fatigue fracture that originates where the largest
inclusion exists within the evaluation volume.

(Figure) Cleanliness Evaluation Process Developed by Sanyo Special SteelEnd