-
e begin with an interesting tale of what hap-pens if we naively
follow along. Just ask the four young Oysters who become enthralled
with the seemingly idle chatter of the Walrus and the
Carpenter, ending up as the main course at dinner.
The time has come, the Walrus said,To talk of many things:Of
shoes and ships and sealing wax Of cabbages and kings And why the
sea is boiling hot And whether pigs have wings.
The Walrus and the Carpenter, Louis Carroll, Through the
Looking-Glass and What Alice Found There, 1872.
Atmosphere gas carburizing is a process so familiar to most heat
treaters it is too often taken for granted. We trust our
oxygen-probe readings to keep us safe, and we expect the outcome of
the process to never change. But occasionally we get in trouble,
and when we do, valuable lessons emerge. Lets learn more. We will
start by looking at various external and internal factors that can
affect the carburizing process, uncover issues related to process
and/or equipment variability, discover where the pitfalls might lie
and talk about what we can do to avoid them.
Part LoadingMany times, variation in case depth and other
carburizing prob-lems can be traced back to how parts are loaded in
baskets and x-tures. Loading arrangements generally fall into one
of two broad categories: weight-limited or volume-limited. In
either case, when loading parts in furnace baskets or onto racks,
our rst instinct is to maximize loading ef ciency. However, as heat
treaters must also be concerned with proper part spacing (i.e.
positioning parts within the load for optimal heat transfer),
atmosphere circulation, temperature uniformity and heat extraction
during quenching (to minimize distortion). And while trial and
error is often the most prudent path, we must also take into
consideration: Furnace-induced factors (often a function of the
style of fur-
nace in use). Being aware of the process limitations induced
by
a given design is an invaluable aid when things go wrong. Part
geometry and orientation factors. We need to ask ourselves
questions such as, How much space should be left between parts?
and Is random loading (Fig. 1) or nesting possible or even
prudent?
For example, bearing races of various diameters a typical
volume-limited load con guration are often nested inside one
another, producing an optically dense workload that is dif cult to
uniformly heat in many cases. In this instance, the cycle must be
adjusted to allow enough time for the interior parts to reach
temperature. Here, the furnace fan (type, speed, rotational
di-rection, location) plays a signi cant role in the heating
process. Fasteners are another example of where random loading in
either continuous or batch-type (Table 1) units is most often used
to handle the sheer volume of parts to be run. In this case,
atmo-sphere penetration throughout the load, cleanliness of the
parts
entering the furnace and allowing adequate time at temperature
are considerations that must be factored into the process. If parts
are not bulk loaded, a good rule of thumb is that the gap around a
part should be no less than 25% and no greater than 75% of the
parts envelope diameter (Table 2).
Atmosphere Gas Carburizing Case Studies, Lessons Learned (Part
1)
Daniel H. Herring | 630-834-3017 |
[email protected]
The Heat Treat Doctor
epyseWW
24 September 2012 - IndustrialHeating.com
Table 1. Part surface area to load-size relationship for typical
integral-quench furnaces
Load Size (width length height) mm (inches)
Maximum Part Surface Area m2 (ft2)
600 x 900 x 600 (24 36 24) 16.723.2 (180250)
760 x 1200 x 760 (30 48 30) 27.937.2 (300400)
900 x 1200 x 900 (36 48 36) 37.246.4 (400500)
900 x 1800 x 900 (36 72 36) 58.067.4 (625725)
Table 2. Part spacing requirements for typical batch loading
Part Diameter Horizontal Spacing (inside)Vertical Spacing
(inside)
mm inches mm inches mm inches
) 25 ) 1 619 0.250.75 1319 0.50.752550 12 1338 0.51.5 1925
0.751
5075 23 1957 0.752.25 2538 11.5
75100 34 5775 2.253 3850 1.52
* 100 * 4 * 75 * 3 * 50 * 2
This month we begin a podcast conversation called the IH Monthly
Prescription with The Heat Treat Doctor. Every month,Dan Herring
sits down with IHs editor, Reed Miller, to talk technical. If you
have a topic you would like them to discuss, drop us an e-mail at
[email protected]. Find the podcast on our website or use
the Mobile Tag on page 26. IH Monthly Prescription is sponsored by
SECO/WARWICK Corp.
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Part CleaningAlthough atmosphere gas carburizing demands only a
moderate level of cleanliness (compared to many other processes or
indus-tries), contamination, such as cutting oils and residues left
on parts, can cause signi cant problems both in our equipment (Fig.
2)and on the parts themselves. Carburizing and carbonitriding tend
to be far more forgiving with respect to the amount of
contamina-tion (e.g., oils, water, cleaning residues, etc.) that
can be tolerated without interfering with case development and the
quality of the resultant microstructure. Still, it is important to
remember that cleaning must be done to at least a level appropriate
for the in-tended application.
Carburizing Process Problems and Their SolutionsInadequate Case
DepthNot achieving the desired case depth (Fig. 3) can be due to a
num-ber of factors, some of which are carburizing at too low a
carbon potential (i.e. too lean a furnace atmosphere), partial or
complete decarburization of the part surface from air in ltration
due to a leaky furnace, processing at the wrong temperature perhaps
due to malfunctioning or improperly located thermocouples, retained
austenite in the case region or a slack quench. Steps that can be
taken to correct these maladies include in-creasing the carburizing
potential of the furnace atmosphere (par-ticularly if boost/diffuse
carburizing is being performed), changing
the carburizing process (e.g., carburizing and slow cooling
fol-lowed by a subcritical anneal prior to reheat and quench),
subzero treatments and selecting the proper tempering
temperature.
Shallow Case or No Case DepthProducing shallow case depth or
areas where there is no case devel-opment points to incomplete
surface preparation prior to carburiz-ing, the presence of surface
contaminants or possibly the misap-plication of selective
carburization methods (i.e. stop-off paints or poorly adhering
copper plate). Another area of concern is how the parts are being
received from upstream operations. Dirty dunnage and suspect
transport methods may add a level of contamination (e.g., rust)
that is unacceptable to the carburizing process. Solutions to these
problems include controlling the cleaning process, cleaning the
parts washer as well as replacing its solution on a frequent basis,
and handling parts with clean gloves.
Coming UpIn part 2, we will discuss problems associated with
retained aus-tenite, decarburizing/de-alloying, intergranular
oxidation, case leakage, case cracking/separation, case crushing,
untempered/tempered martensite effects and other issues. IH
References1. Herring, D. H., How to Load Parts in Furnace
Baskets, Heat Treating
Progress, November/December 2003.2. Herring, D. H., Its Time to
Clean Up Our Act!, Industrial Heating,
January 2008.3. Weires, Dale J., Gear Metallurgy, Effective Heat
Treating and Harden-
ing of Gears Seminar, SME Short Course, 2007.4. Mr. Darwin
Behlke, Twin Disc, Inc., private correspondence.
26 September 2012 - IndustrialHeating.com
Fig. 1. Example of random loading of fasteners on a mesh belt
prior to carbonitriding
Fig. 2. Internal furnace contamination sodium deposits in the
form of a glassy coating
Fig. 3. Low case hardness[3]
HRC 60
HRC 50 Required
Possible decarburization or retained austenite
Case too lean or tempered too highH
ardn
ess
Case depth
Fig. 4. Damaged gear teeth due to lack of adequate
carburization
Areas without case
Go directly to this months podcast by using this Mobile Tag.
-
e continue the discussion started last month on atmosphere
carburizing, namely the problems we encounter in the heat-treat
shop and the solutions that must be implemented to achieve
a successful outcome. Lets learn more.
Carbides and Carbide NecklacesThe formation of grain-boundary
(i.e. massive) carbides and carbide necklaces (Fig. 1) has been the
subject of a great deal of study but one that is directly related
to process variables that are out of control. These include too
high a carbon potential of the atmosphere during the boost portion
of the cycle, insuf cient diffusion time, too short a soak time at
temperature and
hardening from too low a temperature, to name a few.
Fortunately, the formation of carbides can be minimized by steps
such as controlling the carbon potential, adding more diffusion
time to the recipe and changing the hardening temperature (or
time). This is one of the reasons metallurgists are so concerned
about verifying the oxygen (carbon) probe readings by use of a
three-gas analyzer to determine the actual CO value, performing
shim-stock testing to determine actual surface carbon and taking
dew-point measurements to compare with historical information.
Retained AusteniteAustenite that does not transform to
martensite upon quenching is called retained austenite (RA). RA
occurs when steel is not quenched to its Mf (martensite nish)
temperature (i.e. low enough to form 100% martensite). Since the Mf
drops below
room temperature in alloys containing more than 0.30% carbon,
signi cant amounts of untransformed (retained) austenite may be
present, intermingled with martensite at room temperature (Fig. 2).
Causes for high percentages of RA include a carbon potential that
is too high and direct quenching from carburizing temperature.
Leaning out the carbon potential, slow cooling followed by a
sub-critical anneal (optional), and reheating and quench from a
lower hardening temperature are solutions as well as introducing a
subzero treatment, typically in the range of -62 to -100C (-80 to
-150F). RA is problematic because it is metastable. Stress,
elevated temperature or time will cause RA to transform into
untempered martensite. In addition, a volume change (increase)
accompanies this transformation and induces a great deal of
internal stress in a component, increasing the likelihood of
cracking.
Decarburization and DealloyingIf a steel part is exposed to
elevated temperatures in the presence of air (Fig. 3), carbon will
be depleted from the surface of the part (i.e. decarburization)
and/or alloying elements such as manganese and chromium will be
oxidized at the surface (i.e. dealloying). These effects generally
occur when air leaks are present in the equipment, an improper
carbon potential (too low) is used during the hardening process for
the alloy in question, when preheating in air prior to loading into
a protective atmosphere furnace is done above 370C (700F), or when
parts are hardened without adequate atmosphere protection. Proper
furnace maintenance, including checking radiant tubes for pinhole
leaks and periodic pressure testing, combined with proper
atmosphere control typically eliminate equipment variables related
to this problem. Copper plating or selective
Atmosphere Gas Carburizing Case Studies, Lessons Learned (Part
2)
Daniel H. Herring | 630-834-3017 |
[email protected]
The Heat Treat Doctor
eawso
f lWW
16 October 2012 - IndustrialHeating.com
This month we begin a podcast conversation called the IH Monthly
Prescription with The Heat Treat Doctor. Every month,Dan Herring
sits down with IHs editor, Reed Miller, to talk technical. If you
have a topic you would like them to discuss, drop us an e-mail at
[email protected]. Find the podcast on our website. IH
Monthly Prescription is sponsored by Praxair.
Fig. 1. Bearing race corner exhibiting retained austenite (white
areas)
Fig. 2. Bearing race corner exhibiting retained austenite (white
areas)
Fig. 3. Total decarburization on a steel part surface
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stop-off paints (if used) must be adherent and properly
applied.
Intergranular OxidationIntergranular oxidation (IGO) and
inter-granular attack (IGA) are commonly associated with oxygen
present during the carburizing portion of the cycle. In atmosphere
carburizing, some IGO/IGA is unavoidable, typically 0.013 mm
(0.0005 inches) or less, but can negatively affect mechanical
properties such as bending fatigue life. Corrective action involves
improved atmosphere control, being sure that the furnace is
leak-free and/or switching to an alternative carburizing method
such as low-pressure "vacuum" carburizing. Post-heat-treatment
solutions often involve grinding of the surface to remove this
effect.
Low Case HardnessLow hardness in the carburized case (Fig. 4) is
often caused by carburizing with a carbon potential that is too
lean, higher than normal amounts of RA, partial decarburization, a
slack quench or over tempering. The surface-hardness drop can
typically be corrected by using one of the following methods:
increasing carburizing boost time (e.g., higher carbon potential in
the atmosphere); carburizing, slow cooling, sub-critical annealing
(optional), reheating and quenching from a lower hardening
temperature; introducing a subzero treatment; and/or selecting the
correct tempering temperature.
Selected Carburization and Case LeakageDuring carburizing, a
variety of stop-off paints and/or copper-plating methods (i.e.
masking techniques) may be used to selectively carburize certain
component areas. If these techniques prove faulty, the carburizing
atmosphere can leak under the protective layer. Causes include
surface contamination or improper surface prep-aration (i.e. oils,
greases, dirt remaining on the surface) leading to blisters or
irregularities; in-adequate drying time; attempting to paint in too
high a relative humidity atmosphere; improper copper-plating
methods (e.g., adherence issues such as aky surfaces, too thin a
layer of cop-per); and overly aggressive blasting after plating.
Selecting the proper stop-off technique and material for the job,
preparing surfaces properly,
18 October 2012 - IndustrialHeating.com
Fig. 4. Low case hardness Fig. 5. Case/core separation in a gear
tooth
HRC60
HRC50
Required
Case too lean or tempered
too high
Possible decarburization or retained austenite
Hard
ness
Case Depth
allowing adequate drying time, performing a low-temperature bake
at 150C (300F), using controlled cleaning (after and prior to
carburizing) and baking of parts after copper plating will ensure a
proper outcome. When post-nital-etch checking of gears, for
example, suspect areas appear as irregular, dark-gray indications
in an area that should be light gray in appearance. Case
Cracking/Case Separation/Case CrushingOccasionally, cracks (Fig. 5)
are found to occur within the case (typically originating in the
sub-surface). This phenomenon is known as case/core separation (or
case cracking/case separation) and often leads to case crushing
(Fig. 6) the inability of the case to support the applied load. In
gears, this is not to be confused with pitting, a form of surface
fatigue failure of a gear tooth. Microcracking near massive
carbides is also reported to cause case cracking. Case/core
separation is often due to improper part geometry (e.g., thin and
thick sections on the same component) and/or carburizing to a case
depth that is too deep. Eliminating high carbon concentrations at
edges and in corners, allowing adequate stock allowance (for
possible post-heat-treat material removal) and selecting the proper
carburized case depth are all ways to help eliminate this
phenomenon.
Tempering Effects The question is often asked of a carburized
part, should the tempering temperature be selected to achieve the
targeted hardness in the case, the core or both? As it turns out,
the case is much more sensitive to the tempering temperature
selected than the core. Tempering temperature, time at temperature
and, in some instances, cooling rate after tempering are important
factors to consider. The goal is to produce a tempered-martensite
structure in the carburized-case region while maintaining proper
surface hardness.
Other IssuesFor the most part, the problems with atmosphere
carburizing are well known as are their solutions. It is the enemy
we know, which is somehow a comforting thought. Control of process-
and equipment-induced variables combined with a robust
quality-assurance program will avoid the problems discussed here as
well as others that might arise. So, there you have it. Enough
information about carburizing problems/solutions to avoid the
pitfalls of taking the process for granted and assuming nothing can
go wrong. Remember, the old oyster in the oyster bed remained where
he was and didnt wander off with the Walrus and the Carpenter.
Experience kept him off the dinner table. IH
References and Fig. 6 available online