Proceedings of the 28th ASM Heat Treating Society Conference
October 20–22, 2015, Detroit, Michigan, USA
Copyright © 2015 ASM International®
All rights reserved
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Furnace Atmosphere Troubleshooting ; Producing the highest quality stainless steel cutlery
Richard F. Speaker – ALTEC Business Development Metal Heat Treat Specialist; Air Liquide Industrial U.S. LP;
Introduction and Background
Trouble-shooting furnace heat treatment quality issues, such as oxidation and discoloration, can sometimes be
challenging. At times, normal, conventional troubleshooting steps and techniques may not always work. This is
especially true when precise atmospheric contaminant analysis is required. This can vary based on the furnace
atmosphere, the type of furnace, and the metallurgical properties of the materials being heat treated.
This article seeks to inform the reader about the problems that can occur while heat treating in atmosphere
furnaces and the analytical equipment necessary to diagnose these problems effectively and efficiently, to avoid
expensive rebuilds and lost productivity.
Recently, at one of the world’s leading manufacturers of fine stainless steel cutlery, this very challenge was faced
during the hardening process of stainless steel cutlery blades. The stainless steel blades were heat treated in a
Lindberg ® 1 continuous belt furnace, at a hot zone temperature of approximately 1900 F in a H2/N2 based
atmosphere. The customer had converted their furnaces to the H2/N2 based atmosphere in the mid 1990s,
switching from dissociated ammonia (NH ), primarily for environmental and quality reasons.
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The customer then attempted to diagnose this discoloration/oxidation issue using tried techniques that had
worked in the past: leak testing all of the internal gas piping, muffle retort pressurization, copper strip test and
smoke infiltration. Unfortunately, these techniques proved unsuccessful in properly identifying the root cause. The
result was lost productivity and increased costs until the problem could be identified and solved.
If we recall from physics and the Ellingham Free Energy oxidation/reduction diagrams, specifically the H2/H O ratio
for chromium which is present in varying high percentages in all stainless steel, note this equilibrium ratio needs to
be about 500 to 1. In other words, it does not take much moisture or oxygen, which is converted to H O in
hydrogen-based atmospheres, to produce oxidation and discoloration of parts. Depending on the temperature at
which these oxides form, they can also be quite difficult to subsequently reduce and eradicate.
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Diagnostic Procedure
As a result of lost productivity and frustration with this on-going discoloration issue, the customer approached their
hydrogen/nitrogen gas supplier, Air Liquide Industrial U.S. LP and their ALTEC team of specialists in heat treatment
applications. Understanding this issue might require more sophisticated atmosphere analytical equipment and
expertise; the aim was to combine their own problem-solving experiences with Air Liquide’s trouble-shooting;
and together be able to effectively diagnose the cause of the problem.
The first step was choosing correct analyzers for the job. Because the furnace was using a hydrogen-based
atmosphere for bright annealing, with no hydrocarbon enrichment additives or ammonia gases, dew point and
ppm trace oxygen seemed to be the logical choice for atmosphere analysis. Further, since it is important to first
eliminate the incoming gases as a possible source of contamination, it would be best to use a low dew point
(accurate below -100 F) and a low ppm oxygen sensor (0-5000 ppm trace oxygen) for these “virgin” gases. Air
Liquide chose a ceramic sensor hygrometer ( -148 F to +68 F) and an electrolytic-based trace ppm (parts per
million) oxygen analyzer, with a non-depleting electrolytic cell, for the task. The versatility of these instruments,
which also provide quick response times when monitoring in-situ furnace atmosphere contaminants, made them a
good choice.
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Step two: confirm whether the industrial gases (H2/N2) were or were not a contributor to the problem, by
analyzing the incoming gases before they enter the furnace as a protective atmosphere. Ideally, it is best to check
the incoming gases as close to the furnace entry point as possible and work back to the gas supply source, should
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you encounter suspiciously high readings. Since the customer had already checked all of their house gas lines for
leaks, this second step was basically done to verify that the H2/N2 source gases themselves were at acceptable
contaminant levels. The key to any analysis, whether on incoming gas lines or sampling directly from the furnace, is
to allow sufficient time for purging the analyzers and then sufficient time for the readings to stabilize. Impatience
with this step can often lead to false high readings; therefore we allowed approximately eight hours of sample time
on the incoming mixed gases, to ensure stability with the final readings.
Step three: once the incoming H2/N2 mixed gas purity was validated, the next step was to take sample readings
within the Lindberg muffle (retort) itself. Preferably, this is done starting at the beginning of the hot zone of the
furnace, which is where the most damaging oxidation and discoloration can occur. This procedure was done using
these same two analyzers, with the addition of a small sampling pump for positive displacement sample extraction
@ 2 SCFH flow rate, with a 99.5 micron particulate filter to protect the analyzer cells from particulate
contamination. This allowed us to effectively test the atmosphere quality for both dew point and ppm oxygen, in
both the beginning and middle of the hot zones (1900 F). Based on the specific type of the oxidant on the surface
of the stainless blades, it appeared that the oxidation was primarily originating from hot zone oxidation.
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Results
Once stable readings, with negligible fluctuations were achieved, the results for the incoming mixed gases were
- 90 F dew point, and 0.5 ppm oxygen. Both gases appeared to be well within acceptable limits for industrial grade
nitrogen and hydrogen for both dew point and oxygen content.
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Having eliminated the incoming gas as the potential cause of the oxidation, we then focused our efforts on the
Lindberg furnace itself. Again, since the customer had already done much of the preliminary testing of the gas lines
and furnace muffle integrity, we did not expect to encounter any mechanical defects, muffle defects or water
jacket leaks on the furnace itself. The only thing left was to sample the contaminant levels of moisture and oxygen
within the furnace to see if there was something else going on that could not be detected by the methods already
employed (muffle pressure test / copper oxidation test).
Our initial findings, without changing any of the operating parameters such as temperature, belt speed, or
atmosphere (H2/N2) flow rates into the furnace, yielded telling results. The dew point in the furnace hot zone was
+ 19 F, while the oxygen levels stabilized at about 3500 ppm. From prior experience and database records
previously established on this furnace, we immediately knew that these contaminant levels were considerably
higher than typically expected, and that the stainless knife parts will oxidize at 1900 o F, under these high
contaminant levels.
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With these higher than expected contaminant readings, the next logical step was to examine the actual furnace
operating parameters. The easiest parameters to evaluate first were the overall furnace gas flow rates. Based on
the Waukee tube and float flow meters, the set points used for this test were the same set points used prior to the
furnace being taken down for maintenance and the hot zone rebuild. During the rebuild, some of the gas
regulators had been upgraded and replaced. This may have resulted in a slightly different delivery pressure to the
flow meters, which in turn could alter the actual flow rates, based on the calibration pressure established for any
given flow meter.
The main purposes of the H2/N2 atmosphere are first and foremost, to prevent air (oxygen and moisture) ingress
into the furnace muffle (retort). Typically, gas flow rates achieving a few inches water column will suffice. Second,
the hydrogen in the atmosphere provides a “getter” reducing gas capability, which should reduce the oxides on the
stainless surface and provide a bright finish. The intent is for hydrogen to combine and reduce oxygen levels, by
forming water vapor. It then becomes a matter of balancing the H2/H20 ratio to achieve “bright” parts.
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Fig. 1 Relationship of Dew Point to Furnace Atmosphere Flow Rate
With this in mind, we next wanted to see the effects of increasing the total mixed gas flow rate, not the
percentages of the gases. After some initial upward flow adjustments, we found that the atmosphere
contaminants in the hot zone started to rapidly decrease as we increased the mixed gas flow. Within an hour after
our final flow meter adjustments, we achieved a dew point of - 40 F and an oxygen level of 10 ppm oxygen. Given
this dramatic improvement, the 314 SS belt started to brighten up almost instantly. So we now felt confident that
we could achieve similar results on the production blades with these adjusted flow rates. Approximately two hours
later, the customer was once again producing bright, non-oxidized, annealed stainless steel blades.
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Fig. 2 Before and after Flow Rate changes
Conclusions
Sound engineering and experience will typically go a long way when diagnosing and solving manufacturing issues,
whether it is furnace oxidation or some other manufacturing challenge. In most cases, this process experience and
product expertise will allow the customer to intuitively identify the problem and resolve the issue quickly.
However there are still those cases where an extra set of “experienced eyes” are needed, along with the
proper analysis equipment; in this case fast-responding dew point and oxygen analyzers can sometimes
provide the missing piece to the diagnostic puzzle. Fortunately, after carefully eliminating many of the potential
variables that can cause or contribute to furnace oxidation, we were able to effectively diagnose the problem and
provide the resulting solution.
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