Proceedings of the 28th ASM Heat Treating Society Conference
October 20–22, 2015, Detroit, Michigan, USA
Copyright © 2015 ASM International®
All rights reserved
asminternational.org
Applying and Specifying Metallurgical Engineering in the Production
of Heavy Truck Axle Shafts
Steven C. Heifner, PE
Sypris Technologies, Inc., Louisville, KY, USA
Steven.Heifner@sypris.com; 502 774 6215
•
Abstract
The American Trucking Associations reports that “In 2014,
trucks moved 9.96 billion tons of freight, or 68.8%” of all
freight tonnage transported domestically. [1]. "Spending in the
U.S. logistics and transportation industry totaled $1.33 trillion
in 2012, and represented 8.5% of annual gross domestic product
(GDP)."[2] Truck axle shafts for decades have been made from
induction hardened carbon steel with 0.4% to 0.5% carbon.
Associated metallurgical engineering of steel procurements,
forging, processing, and applied machining, impacts axle shaft
production and performance. This paper, and the associated
presentation at the ASM 2015 Heat Treat Society (HTS)
Conference and Exposition, reviews metallurgical principles
and controls currently applied to heavy truck axle shaft use and
production in North America. Basic metallurgical engineering
principles and controls, as historically and currently applied and
specified, plus potential opportunities for increasing
engineering value optimization, are reviewed. In particular,
case depth, surface hardness, microstructure, grain size,
chemical compositional interactions, procurement, processing,
metallurgical and overall engineering characterizations and
achievement targets are discussed.
Engineering impacts people and the world we live in. The
designs and services provided by engineers require awareness,
impartiality, and equity. Engineers serve many masters,
including both our employers and our customers. In fact,
engineers serve the world. Heavy trucks, especially in North
America, and also around the globe, impact each of us every
day because transported freight makes our lives and our current
society possible.
The Heavy Truck Axle Shaft in Perspective to
the Entire Truck
There are many vehicles that travel the roads in North America.
“The Federal Highway Administration (FHWA) is an agency
within the U.S. Department of Transportation that supports
State and local governments in the design, construction, and
maintenance of the Nation’s highway system (Federal Aid
Highway Program).” “Through financial and technical
assistance to State and local governments, the Federal Highway
Administration is responsible for ensuring that America’s roads
and highways continue to be among the safest and most
technologically sound in the world.” [4] This includes oversight
of the vehicles that travel the highway system in North
America.
The Role of an Engineer
An engineer uses knowledge of science to create something
useful, typically something that can be sold. Engineers by
vocation are value oriented. Most engineers deal with
homogeneity in their designs and their associated creation and
application. Material and metallurgical engineers are more
focused on, and trained in, understanding imperfections and
performance limits in implementing and deploying engineered
solutions, such as truck axle shafts.
Heavy trucks are those vehicles classified as Class 8 and above
as shown in Fig. 1. [5] Heavy trucks on North American roads
are becoming longer, and at times, employing more axle shafts.
In addition, those axles’ shafts are continuing to be more
heavily loaded in order to move more freight to support our
society and our ever expanding population. All vehicles,
including heavy trucks, are striving for greater fuel efficiency,
and thus corresponding vehicle weight reductions by using less
material mass to handle expected loadings. The balance of
procurement costs and operating savings are constantly
monitored by fleet operators, truck assemblers, and component
manufacturers. Longer vehicle life also means more cycles of
loading before vehicle replacement, putting more demands on
the materials, such as the steel in truck axle shafts and the
associated processing to make that steel a truck axle shaft.
Degrading highway infrastructure is also increasing potential
vehicle loadings. This increases demands on the materials,
specifications and the processes used in engineered product
solutions. Trucks and associated truck axle shafts, as all items
The Fundamental Canons of the Code of Ethic of the National
Society of Professional Engineers (NSPE) are [3]:
“Engineers, in the fulfillment of their professional duties, shall:
•
•
•
•
•
Conduct themselves honorably, responsibly, ethically,
and lawfully so as to enhance the honor, reputation,
and usefulness of the profession.”
Hold paramount the safety, health, and welfare of the
public.
Perform services only in areas of their competence.
Issue public statements only in an objective and
truthful manner.
Act for each employer or client as faithful agents or
trustees.
Avoid deceptive acts.
398
in our economic driven society, need to continually become less
wasteful and generate more value.
system adopted in Nazi Germany. Standardized transportation
is a major component of military readiness, and has become
increasingly more critical to commercial success and value
creation since the Roman Empire.
The Industrial Revolution harnessed steam to do work, and that
work included movement. One aspect of that movement
became transportation. Metal was needed to harness and direct
the power of steam into useful work. Iron, and then steel,
became ever more in demand, in particular for railroad tracks,
railroad wheels, and railroad axles. The application of
metallurgy, in many forms, was critical to transportation
progress. As is currently the case with heavy truck axles, getting
more life from each railroad axle, was an economic necessity.
Steel processing techniques used to make railroad axles were
subsequently transferred to motor transportation designs as road
networks grew. These road networks, and vehicles using them,
became increasingly to the rapidly expanding North American
economy after World War 2.
Making a Truck Axle
In the past fifty (50) years, since the creation of the US
Interstate Highway System, the manufacture and processing of
steel trucks axle shafts has changed very little. The process
involves procuring steel, forging a spline on one end and a
wheel connecting flange on the other end. The forging is then
machined as necessary and induction hardened and tempered to
obtain optimal material performance properties in the axle
shaft. The outer fibers of the axle shaft experience the greatest
torsional stress, thus a hardened martensitic case with a ductile
pearlite and ferrite, or bainite, core is created through induction
hardening heat treatment. This process flow is illustrated in Fig.
2.
Figure 1: Federal Highway Administration (FHWA) Vehicle
Classifications graphic;
http://onlinemanuals.txdot.gov/txdotmanuals/tri/images/FHW
A_Classification_Chart_FINAL.png
Without an axle, one wheel has little value, and a pair of wheels
has even less. Specified standard axle widths were a critical
component of the success of the Roman Empire. If you wanted
to drive a Roman road, your wheels needed to fit into the road
ruts created by the standard axle widths of previous travelers.
These same axle widths greatly influenced the first railroads
centuries later. However, it also led to businesses working with
engineers to create various track widths, or gauges, to
monopolize and control freight transportation in selected areas
along particular lines of commerce.
INPUT
Raw Material (Steel)
Cut or Shear to Working Size
Forge Spline
Forge Flange
Scale Removal - Blasting
Machining
Induction Hardening
Tempering
Straightening
Axle Shaft Shipment
OUTPUT
This plethora of railroad gauges, and the freight chaos and
associated economic waste it caused, led to development of
road width standards during the previous century by
governmental predecessors of the current Federal Highway
Administration. Specifications, or standards, did not exist prior
to invention of the typewriter, subsequently put to use during
World War 1 to help create safe housing for armament workers.
Production was being lost when temporary housing collapsed
killing workers, and discouraging new workers from coming
forward. Great value was associated with these first typewritten
specifications regarding worker housing which soon expanded
throughout the military, and in the coming decades into all
aspects of North American and world business, especially after
World War 2. The interstate highway system concept was a
post-war import to North America modeled on the standardized
Figure 2: Process Map for Making a Truck Axle Shaft.
Raw Material
Steel is a relatively inexpensive material that can be easily
altered in shape by forging, modified by machining into a
particular dimensional design, and heat treated as necessary to
optimize material performance and engineering usefulness.
154X plain carbon steel is the primary material used to make
399
heavy truck axle shafts in North America. At times, more
carbon is added, or other alloying elements modified, often
altering the hardenability to an H or RH grade. Hardenability is
important because it allows induction heat treatment to give the
outer fibers of the axle shaft the strength desired.
composition of the material, the grain size, and the severity of
quench. It is important to note that the maximum achievable
hardness in steel is determined by carbon content while
hardenability is determined by the alloy chemistry, which also
includes carbon. Hardenability is key to achieving the desired
case depth in a heavy truck axle shaft, and will be discussed
more in the Induction Hardening section of this paper.
As with most steel in use in the world today, the steel material
now being produced and in use is recycled steel scrap processed
by a mini-mill using electric arc furnace (EAF) steel processing.
Steel is the most recycled material in the world today, however
this recycling is concentrating tramp and residual elements in
the steel product being produced, increasing material
hardenability potential over the past five decades, and thereby
at times impacting subsequent axle shaft processing activities.
Micro alloyed and high strength low alloy steels (HSLA) are
created by using these same tramp and residual elements.
Hot rolled steel is the primary material used to make heavy
truck axle shafts, but at times, turned and polished steel is
requested by customers. Such attention to surface condition is
a fatigue design consideration. If crack initiation can be
delayed, fatigue life of the truck axle shaft will be extended.
Fatigue cracks can initiate on the outer fibers of the axle shaft,
or at the internal case-core interface when the internal torsional
shear stress level exceeds material strength of the internal nonhardened core material (as explained by Greg Fett [13]).
In North America, ASTM A29 [7], along with other referenced
ASTM standard specifications, are universally employed for
steel production. Other parts of the world have similar national
or regional standards, but also often embrace ASTM standards.
Each ladle of steel made is assigned a heat code by the steel
mill. A certified material test report (CMTR) from a typical
steel mill reports: procurement references, applicable
specifications, dimensional information, reduction ratio, ladle
chemistry (ASTM A29), calculated Jominy hardenability
(ASTM A255 [8]), microcleanliness (ASTM E45 [9]), grain
size (ASTM E112 [10]), possibly but not typically mechanical
properties (ASTM A370 [11]), general and specific comments
per procurement requirements, approval signatures, and
processing dates.
Failures within the spline or flange are material condition issues
controllable by, and thus the responsibility of, the axle shaft
manufacturer. Failures in the shaft body and associated
transition areas are typically associated with shock loadings
during vehicle operation. A shock load is a sudden and powerful
force applied against a component that can cause the component
to crack and the halves to separate from each other. An example
of this would be, spinning wheels that suddenly find traction.
Damage occurs because a shock load overstresses the axle shaft
beyond its material strength, and this can damage or destroy the
shaft immediately or shortly later. Often an axle shaft damaged
by a shock load will continue to operate but then fatigue fail
some time later after accumulating more cyclic stresses.
Finding a rough, crystalline finish on failure surface of
fractured axle shaft is indicative of an axle shaft damaged by a
shock load. Thus, axle shaft failures can be progressive or
immediate.
Steel chemistry can include the following elemental checks: C,
Mn, P, S, Si, Al, Cu, Ni, Cr, Mo, Sn,, N, V, Cb, B, Ca, W, Ti,
Pb, Co, As, Sb, Zr, and Bi. C, Mn, P, S, Si, Al, M and Fe, are
the primary components of steel composition. The other noted
elements are tramp or residual items. Aluminum content is a
critical alloy necessary to create a killed steel with a consistent
prior austenitic grain size of 5 or finer historically[6], and
currently observed to be in the range of a grain size of 7 to 9
which is associated with improved steel cleanliness, and other
steel making, process controls.
Progressive failure is cracking of a shaft following the initial
shock followed by crack growth and torsional fatigue failure.
Immediate failure is from a single sudden shock; i.e. a torsional
shear failure. Shock load failures are initiated by the vehicle
operators, not the material condition of the axle shaft. If the
resulting axle shaft fracture is at a 45-degree angle to the axle
shaft centerline, that fracture is called a torsional tensile failure
because of an additional axle shaft loading dimension. Early life
axle shaft failure events can also include crack initiation during
axle shaft straightening, quench cracks during induction
hardening, or insufficient case depth achieved during induction
hardening.
In addition, it is typical to calculate and report the DI or ideal
diameter. As explained so well by Daniel H. Herring [12]: “A
quantitative measure of a steel’s hardenability is expressed by
its DI, or ideal diameter, value. This abbreviation comes from
the French phrase “diamètre idéal” and refers to the largest
diameter of steel bar that can be quenched to produce 50%
martensite in its center. The quench rate of the bar is assumed
to be infinitely fast on the outside; that is, it has sufficient
quench severity so the heat removal rate is controlled by the
thermal diffusivity of the metal and not the heat transfer rate
from the steel to the quenchant. Typically, water or brine
provides these infinitely fast quench conditions. The larger the
ideal diameter value, the more hardenable is the steel.” “DI
values are an excellent means of comparing the relative
hardenability of two materials as well as determining if it is
possible to harden a particular cross section (or ruling section)
of a given steel”. DI values are influenced by the chemical
Typically, axle shaft failures occur at the transition from flange
to the shaft body, or less often, at the transition from the shaft
body to the spline. These transition failure sites are desirable
because they act as fuses protecting the rest of the truck
transaxle. A high residual stress state in the outer fibers of the
axle shaft is strong mitigator of fatigue crack initiation, and
early fatigue failure. Balancing axle shaft strength and axle
shaft toughness, that is, the impact or shock resistance of that
axle shaft, is necessary to achieve performance and prevent
failure of a given heavy truck axle shaft. Well-controlled
400
processing of the starting raw material is necessary to achieve
the axle shaft’s full performance potential.
In order to balance all these factors, forgers need a greater
processing temperature range than heat treaters, typically plus
or minus 50oF [20 oC], or even plus or minus 100oF [50 oC].
Reducing allowed variances, such as processing temperature, or
material inputs (such as a grain size) can be done, but at a cost
and after repeated forging production process rebalancing.
Spline Forging
Raw material is sheared or cut to length, then one end is heated
and forged for some distance (short or long; 3” [ 70 mm ] to 12”
[ 300 mm ]) to be subsequently machined and then splined by
rack rolling or hobbing, to form a spline for insertion into the
transaxle. Forging involves heating metal and striking to form
a desired product. Such metal working, or blacksmithing, has
formed the foundation of metallurgy for centuries. Making a
spline does not take much metal movement, especially
compared to forming the flange, but the principles and material
property limitations are the same. The steel needs to be hot
enough to flow but not so hot to damage or burn the steel.
Blacksmiths judged the formability by its heated color with
their eyes, today we have pyrometers and electronic logic
systems. 154X steel is typical forged between 1800oF [ 1000 oC
]and 2250oF [ 1280 oC ]. Heating 154X steel above 2300oF [
1300 oC ] begins to damage the steel by insipient melting.
Above about 1550oF [ 900 oC ] the ferrite and pearlite
microstructure of the bar transforms to austenite. Alloying,
temperature, and time at temperature impact the metal grain
size. 154X is a fine grained, aluminum killed steel. Forgers
want the metal to flow easily and welcome higher temperature
levels (in addition to fine grained steel).
Flange Forging
The other end of this cut bar is forged into a wheel flange.
Flange formation on a truck axle shaft from a rolled steel bar
takes more metal movement than creating the spline end. Metal
can be moved only so far during each processing hit. While
spline ends are created in one or two hits, flange formation
typically takes 3 to 5 hits, or passes. During each hit the metal
comes in connect with the die, or anvil, losing heat at the contact
surface. In addition, atomic friction during metal movement
creates adiabatic energy somewhat increasing internal metal
temperature. Thus a somewhat non-uniform starting heated
zone on a bar end is becoming even more non-uniform in each
subsequent forging process step.
The piece being forged cannot get too hot. This can result in
insipient melting (burning) of the steel. Neither can it get too
cold and critically slow metal flow. The metallurgy throughout
the flange being formed is impacted by these temperatures and
flows in relation to time. Fortunately, computer modeling is
helping us to better understand these temperatures and flows in
an engineering perspective rather than our historic
blacksmithing perspective.
Forging engineers strive with their process design to create a
desired property and dimensional output. By balancing their
starting raw material, with subsequent processing activities.
This finally results in the deployment of a high quality axle in a
truck driving safely down the road. This balancing requires
understanding, specifying and applying sound metallurgical
principles in the forging operation.
154X steel heated above 1750oF [1000 oC] is fully austenitic
and flows readily in the forging die. After forging, the piece is
typically slow cooled in still air to form a ferrite and pearlite
microstructure with an associated hardness of 20 to 30 HRC.
The pearlitic/ferritic microstructure is desired for subsequent
machining and induction hardening.
1800oF [1000 oC] and 2250oF [1280 oC] is a rather wide
temperature range. Heat treaters typically strive to maintain
plus or minus 25oF [10 oC] during processing operations. The
benefits of reducing variances to standardize output is well
known in our modern Six Sigma world. Higher temperatures
require more energy to achieve, and energy is an ever escalating
cost. However, forging at lower temperatures loads the forging
equipment more with associated potential equipment damage
and repair costs, decreasing value creation. Also, steel expands
differently at different temperatures thermally thereby
impacting the forging die sizing necessary to achieve the final
desired dimensional product output. Precision dimensional
forging can reduce subsequent machining and associated costs,
creating production system value. Achieving temperature
uniformity in the portion of the bar being forged is impacted by
the metal chill effect created by the rest of the unheated baron
one side of the bar heat zone. and more exposed surface area at
the cut end (end effect). There are temperature variances
throughout the heated zone being forged. All this balancing
impacts metallurgical properties through the heated zone during
forging and during (and after) subsequent cooling.
Scale Removal - Blasting
In addition, to moving metal at temperature during forging, the
heated surface of the hot zone is also interacting with the
surrounding atmosphere forming an oxide scale. More
temperature at the surface leads to more scale formation, and
also potentially carbon migration and decarburization at the
heated surface. Scale may or may not break free during each
subsequent forging pass, including the heating prior to forging.
This scale impacts the ability of a pyrometer to correctly
measure the surface temperature of a heated piece of metal.
Also, this scale impacts subsequent machinability of this forged
product, therefore this oxide scale is typically removed by steel
grit blasting after forging, which adds cost. Scale can also be
removed with high pressured water spray as well. Preventing
scale by forging in an inert atmosphere has been considered and
attempted, but adding an inert atmosphere also adds significant
cost.
Shot peening rather than shot blasting can add residual stresses
at the surface of a truck axle shaft, dramatically improving
401
fatigue performance, however the extra time and shot loading
associated with peening adds substantial cost.
depth for induction processes than published in hardenability
curves because more and quicker cooling is available. As
explained by M.A. Grossman in 1942 [15], and as documented
and specified within Table 5 of ASTM A255 [8], achievable
surface hardness is limited by material carbon content that
during initial contact with quench media almost instantaneously
forms martensite at the outer surface. Hardness achievement
away from the spline declines with distance because the
location at depth cannot cool as fast (soley by conduction)
because the cooling quench media is at the outer surface
(allowing for both conduction and convective cooling). Thus a
case depth profile in a heavy truck axle shaft is created.
Engineers use their knowledge to create something useful,
something valuable, that a customer is willing to purchase,
however the physical world limits what can and cannot be done
for reasonable cost. The level of scale removal, or surface
cleaning, must be agreed upon and specified by impacted
parties. Customers are unwilling to absorb additional cost
unless a performance benefit is achievable and demonstrated.
Thus the level of scale removal required and specified is
dependent on the economic value associated with that scale
removal.
Conduction transfers energy, as represented by temperature,
into a part, and conduction must transfer energy back out from
inside the part during quench cooling. As with forging, it is
desirable to heat a part enough to austenite the microstructure
(above about 1550ºF [900 oC]) sufficiently to allow the outer
case to transformation to austenite while limiting temperature
and associated heating exposure and phase transformation in
the core. Maintaining the lowest austenization temperature
possible also makes cooling, or quenching, the heated axle shaft
easier and correspondingly less costly than a process requiring
additional heat exchangers, cooling towers, and fluid
management.
Machining
The cleaned forging now must be altered into the final
dimensional configuration envisioned by the transaxle design
engineers.
Material properties variances can impact
subsequent processing such as machining, but the 154X steel
heavy truck axle shaft raw material is readily processed using
standard machining tools and techniques. It will not be
attempted in this paper to cover these standard machining
techniques.
Machining tolerances must take into consideration subsequent
processing. Induction hardening heat treatment in the next
processing step (along with subsequent tempering) will alter
part dimensions due to thermal expansion. Most significantly
the axle shaft will become longer during hardening. Spline and
other dimensions are affected to a much lesser extent. So once
again understanding, applying, and specifying material
properties is critical to axle shaft processing. The metallurgical
engineer must work with both the design engineer and the
production engineer to achieve system balance and the desired
dimensional outcome.
It is important to have an axle shaft core hard enough to oppose
applied torsional stresses that decreases from maximum at the
outer surface to zero stress at the center of the core, with no
quench cracks to act as fatigue initiation sites. Over hardening
the core does not benefit axle shaft performance, but under
hardening at the core-case interface can limit axle shaft
performance as it can becomes a fatigue crack initiation site.
Certain customers request greater hardness levels, especially in
transition zones, to ensure sufficient case depth hardness and
associated material strength. Others expect no heat exposure in
the core. Verification of hardness currently requires destructive
testing (sectioning). Destructive testing is a lagging, not a
leading, indicator, that your induction hardening heat treatment
process is working and producing good parts. This raises the
question, where should a hardened axle shaft be tested? How
many hardness tests are necessary, and need to be
correspondingly specified? Sample quality and preparation is
critical for obtaining meaningful hardness measurements.
Testing has a cost, and more testing than is statistically required
has decreasing value and reduces profits unnecessarily.
Induction Hardening
Heavy truck axle shaft processing time after forging, is 80%
machining and 20 % induction hardening. Induction hardening
creates a strong, hard case over a more ductile, softer inner core.
Induction hardening changes a 154X heavy truck axle shaft
surface hardness from 20 to 30 HRC to 50 to 60 HRC. Hardness
achieved decreases from the surface to the core of the shaft,
however the induction hardening pattern also varies
longitudinally. There is hardening flare into the flange, that
transitions into the shaft body, which transitions again into the
spline before running out at, or slightly before, the end of the
spline. The transitions zones, as previously mentioned, can vary
slightly or significantly from adjacent hardened zones if
hardening operations are not carefully managed.
Induction hardening scanners incorporate electrical,
mechanical, and fluid systems to create a magnetic field, and
then move a part through that field to austentize/heat the part
and then quench/cool the part to produce a martensitic
surface/case, while limiting phase transformations in the part’s
core. This is a complex balancing act. As previously noted,
reducing variances aids process control, however the raw
material, the forging, the machining, and now the induction
scanner operation itself are all sources of potential variance.
Hardenability curves present potential achievable hardness
during a Jominy end quench [14], and thus are one dimensional.
Induction scanners for truck axle shafts operate in two
dimensions, during a single-shot induction hardening operation,
or three dimensions, during a two-turn coil induction scanning
operation. Designers demand greater than average hardness at
402
Should a hardening operation be adjusted to compensate for
previous variances, or should the previous variances be
eliminated so the hardening operation can remain constant? If
one axle shaft fit all vehicles it might be a single hardening
operation, but this is not reality. Unique solutions are costly to
create and maintain, and actually introduce more variance into
any system. Ideally a heavy truck axle shaft of consistent
construction and dimensions is moved through a consistent
magnetic field, heated and then cooled, to create a consistent
metallurgical and dimensional output..
Heavy truck axle shafts are heavy (45 to 65 pounds each), and
packing for shipment must be designed to control this weight
and also protect the axles hafts, especially the spline and flange
design interfaces. Finished parts can be exposed to the elements
before and during shipment and in storage, and can begin
rusting. Customers expect any and every delivered axle shaft to
be ready use directly upon receipt or whenever there is a need
to use that axle shaft. Adding rust protection may add value or
be just an added cost to and a shaft that has also been forged,
machined, and hardened.
Tempering
Axle Shaft Insertion into Trucks and Society
During induction hardening, the micro expansion of martensitic
needles created in the case impart a beneficial compressive
residual surface stress. Untempered martensite, however, is
susceptible to environmental hydrogen and can be subject to
catastrophic fatigue failure. Blunting the lath martensite needles
through tempering is typically employed. Part exposure to a
temperature of 300oF [200 oC] to 500oF [300 oC] for one to one
and half hours minimum is typically specified to temper a steel
heavy truck axle shaft. This hold time is based upon the heater
treat’s maxim of one hour per inch of thickness. Heavy truck
axle shafts are usually 1-7/8” [ 47 mm ] in shaft diameter, and
usually no greater than 2-1/4” [ 57 mm ]in diameter, with
hardened case depths typical 0.5” [ 12 mm ] thick, up to 0.7” [
18 mm ] thick. Having a case depth thickness less than an inch
may allow for lesser temper hold times to achieve a desired
temper. Higher tempering temperatures and longer hold times
will reduce 154X mechanical properties slightly, therefore the
lowest temperature and time exposure is considered a potential
cost savings and value generator. In addition, induction
hardening in-process controls occur directly after hardening
rather than after tempering so the production line is not
excessively idled, (or not generating parts or value). Warm
parts are easier to straighten (if required) during the next axle
shaft processing step.
Finished axle shafts are typically shipped to a transaxle
assembler, who may or may not be the final truck assembler.
Trucks are ubiquitous to our societal and economic existence.
We have invested in roads and must have vehicles to travel
those roads to deliver almost everything that makes our lives
possible. Engineering is a team activity. We must work with
others to make our solutions work. Making things work better
is what value is all about.
Conclusions
Metallurgical engineering value concepts and knowledge
applied during today's production of heavy truck axle shafts
include:

Steel is a relatively inexpensive material that be easily
altered in shape by forging, modified by machining
into a particular dimensional design, and induction
hardened as necessary to optimize material
performance and engineering usefulness.

Customers, are unwilling to absorb additional costs
unless a performance benefit is achievable and
demonstrated. Trucks and associated truck axle shafts
need to continually become more value generating and
less wasteful.

Most engineers deal with homogeneity in their designs
and their placement in the world. Material and
metallurgical engineers are more focused on and
trained in understanding imperfections and
performance limits in implementing and deploying
engineered solutions, such as truck axle shafts.

Hardenability is key to achieving the desired case
depth in a heavy truck axle shaft. Controlling this, and
other metallurgical and process inputs can assure a
consistent output. There is no value in performing
surface hardness verification tests during axle shaft
production.

Aluminum content is necessary to create a killed steel
with a consistent prior austenitic grain size of 5 or
finer. It is unclear how additional grain size
specification requirements impacts truck axle shaft
performance and usefulness.
Straightening
Ideally, straightening any axle shaft would never be necessary,
however, exposing hot metal to a cooling solution can cause
part warping. The non-uniform application of cooling solutions
or quench to a non-uniform heat surface is a major contributor
to warping. Excessive warping or distortion will compromise
the transaxle design and its operating efficiency and cannot be
allowed to compromise the axle shaft or the vehicle. Improper
straightening can also crack the outer surface and create fatigue
initiation sites. Non-destructive inspection, such as magnetic
particle inspection, of straightened parts is sometimes
mandated. Such inspection is time (and value) consuming.
Preparation for Shipment
The heavy truck axle shaft is now complete, however it has
generated no value until it is sold and delivered to a customer.
Sometimes customers require their axle shafts to be blasted
again after induction hardening, and sometimes they do not.
403

If fatigue initiation can be delayed, fatigue life of the
truck axle shaft should be extended. Improper
straightening, and other processing missteps, can
create fatigue initiation sites limiting axle life.

Non-destructive inspection, such as magnetic particle
inspection, is time and value consuming. When
specified, one must consider how this non-destructive
inspection is improving axle shaft performance.

Dimensional achievement in balance with achievable
metallurgical properties is required during axle shaft
forging. Forgers need a greater processing temperature
range than heat treaters.

Destructive testing is a lagging, not a leading
indicator, of metallurgical quality.
[3] National Society of Professional Engineers (NSPE), 1420
King Street, Alexandria, Virginia 22314 , phone number
703 684 2800; “Code of Ethics” webpage;
http://www.nspe.org/resources/ethics/code-ethics.
[4] U.S Department of Transportation, Federal Highway
Administration website; http://www.fhwa.dot.gov/about/.
[5] Traffic Recorder Instruction Manual, Texas Department of
Transportations, Released February 01, 2012, Appendix A:
Vehicle Classification Using FHWA 13-Category Scheme;
http://onlinemanuals.txdot.gov/txdotmanuals/tri/images/F
HWA_Classification_Chart_FINAL.png.
[6] Mishra, B., Steelmaking Practices and Their Influence on
Properties, Metals Handbook Desk Edition, ASM
International, 1998, p. 174–202.
[7] ASTM A29 / A29M-12e1, Standard Specification for
General Requirements for Steel Bars, Carbon and Alloy,
Hot-Wrought, ASTM International, West Conshohocken,
PA, 2012, www.astm.org.
[8] ASTM A255-10(2014), Standard Test Methods for
Determining Hardenability of Steel, ASTM International,
West Conshohocken, PA, 2014, www.astm.org.
[9] ASTM E45-13, Standard Test Methods for Determining
the Inclusion Content of Steel, ASTM International, West
Conshohocken, PA, 2013, www.astm.org.
[10] ASTM E112-13, Standard Test Methods for Determining
Average Grain Size, ASTM International, West
Conshohocken, PA, 2013, www.astm.org.
[11] ASTM A370-14, Standard Test Methods and Definitions
for Mechanical Testing of Steel Products, ASTM
International, West
Conshohocken,
PA, 2014,
www.astm.org.
[12] Herring, D.H, "THE HEAT TREAT DOCTOR:
Fundamentals of Heat Treating: Ideal Diameter," Industrial
Heating,
September
1,
2005,
page
18-20,
IndustrialHeating.com.
[13] Fett, G.A., Induction Case Hardening of Axle Shafts,
Induction Heating and Heat Treatment. Vol 4C, ASM
Handbook, ASM International, 2013, p 160–172.
[14] Kirkaldy, J.S., Quantitative Prediction of Transformation
Hardening in Steels, Heat Treating, Vol 4, ASM
Handbook, ASM International, 1991, p 20–32.
[15] Grossman, M. A., Hardenability Calculated from
Chemical Composition, AIME Transactions, Vol 150,
1942, pp. 227–259.
Acknowledgments
My friend, Matt Yaksic, for expanding my knowledge
regarding forgings.
My company, Sypris Technologies, for supporting my
professional association and involvement with ASM
International and the Heat Treat Society.
References
[1] "Trucking Revenues Top $700 Billion for the First Time
According to New Report” press release from American
Trucking Associations, 950 North Glebe Road, Suite 210,
Arlington, VA 22203-4181, dated May 11, 2015;
http://www.trucking.org/article.aspx?uid=70210058bb81-44df-a565-492f899fc139.
[2] U.S Department of Commerce, SELECTUSA website,
“The Logistics and Transportation Industry in the United
States" webpage; http://selectusa.commerce.gov/industrysnapshots/logistics-and-transportation-industry-unitedstates.html.
404