Data processing: measuring – calibrating – or testing – Measurement system – Temperature measuring system
Reexamination Certificate
1999-02-18
2003-07-08
Wachsman, Hal (Department: 2857)
Data processing: measuring, calibrating, or testing
Measurement system
Temperature measuring system
C702S136000, C266S080000
Reexamination Certificate
active
06591215
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to the monitoring and/or control of atmospheres within heat treating furnaces.
BACKGROUND OF THE INVENTION
Steel parts can undergo a process called carburizing or neutral hardening inside a heat treating furnace. Inside the furnace, the steel parts are exposed to prescribed high temperature conditions in the presence of a specially formulated, carbon-enriched gas atmosphere.
Most heat treating atmospheres contain carbon monoxide (CO), carbon dioxide (CO
2
), methane (CH
4
), hydrogen (H
2
), and water vapor (H
2
O). The relative amounts of these gases in the atmosphere depend upon the type of carrier gas used, the processing temperatures, and the amount of enriching gas added during processing.
For example, an endothermically generated gas, produced by catalytic cracking of natural gas in the presence of air, typically contains the following nominal ranges (expressed in % by volume)of gas constituents:
CO ≈
20%
CO
2
≈
0.1% to 0.5%
H
2
≈
40%
H
2
O ≈
0.2% to 1.2%
N
2
≈
40%
CH
4
≈
0.2% to 0.8%
In gas carburization, a common commercial practice is to use an endothermic gas carrier enriched with natural gas or propane. The process variables used to monitor and control the carburization process using this type of atmosphere include (i) the carbon potential of the heat treating atmosphere (expressed as a weight percent of carbon), (ii) the temperature of the heat treating furnace, and (iii) the processing time.
For a given temperature condition, the reactions that transfer carbon to the surface of the steel part are maintained by keeping the carbon potential of the gas atmosphere within a defined range. For example, if the carbon potential of the furnace atmosphere is greater than the carbon potential of the surface of the steel parts being processed, carburization occurs, i.e., carbon is transferred from the gas atmosphere to the surface of the steel parts. Increasing the carbon potential of the gas atmosphere increases the rate of carburization. However, if the carbon potential of the atmosphere at a given temperature exceeds a critical value beyond the defined range, sooting occurs. Likewise, if the carbon potential of the atmosphere at a given temperature is less than the carbon potential of the surface of the steel parts being processed, decarburization occurs, i.e., carbon is transferred from the surface of the steel parts to the gas atmosphere.
The desired condition for neutral hardening is one in which the carbon potential of the atmosphere is equal to the carbon potential of the surface of the steel parts being processed. In this case, no carbon is transferred between the surface of the steel parts and the furnace atmosphere.
Further details regarding the concept of the carbon potential and the kinetic conditions for transfer of carbon between the surface of the steel part and the furnace atmosphere are found in Blumenthal, “Control of Endothermic Generators—A Technical Comparison of Endothermic and Nitrogen/Methanol Carrier Atmosphere,”
Heat Treating Proceedings
(16
th
ASM Heat Treating Society Conference
&
Exposition
), Mar. 19-21, 1998, pp. 19 to 25.
The carbon potential of an atmosphere with a fixed carbon monoxide concentration can be ascertained by measuring the partial pressure of carbon dioxide (P
CO2
) in the atmosphere, using infrared analysis. This, however, requires sampling the gas from the furnace atmosphere and cooling it to room temperature. Sampling errors arise, due to possible leaks in the gas sampling line, alteration of the gas chemistry due to sooting, or the water-gas shift due to cooling, or a combination of these events. These sampling errors inherent in remote gas sampling are difficult to eliminate.
For this reason, a more common method for assessing the carbon potential has entailed the use of an in situ oxygen sensor used in association with a thermocouple.
The oxygen sensor is typically installed in the heat treating furnace in direct contact with the heated gas carburizing atmosphere. This obviates the sampling errors, described above, which are inherent in remote gas sampling techniques. The sensor includes a solid electrolyte. One side of the electrolyte contacts the carburizing atmosphere to be measured. The other side of the electrolyte contacts a reference gas, whose oxygen content is known.
A voltage (measured in millivolts) E(mv) is generated between the two sides of the electrolyte. The magnitude of this voltage E (mv) is a function of the temperature (sensed by the thermocouple) and the difference between the oxygen content in the carburizing atmosphere and the oxygen content in the reference gas. The voltage E(mv) can be expressed as follows:
E
(
mv
)
=
0.0496
T
×
log
P
O
2
(
Ref
)
P
O
2
(
1
)
where:
T is the sensed temperature (in degrees Kelvin ° K).
P
O2
(Ref) is the known partial pressure of oxygen in the reference gas, which in the illustrated embodiment is air at 0.209 atm. Other reference gases can be used.
P
O2
is the partial pressure of oxygen in the heat treating atmosphere.
Knowing the oxygen content of the reference gas [P
O2
(Ref)], one can determine the oxygen content of the furnace atmosphere [P
O2
] by measuring the probe voltage [E(mv)] and the temperature T(° K). Knowing the carbon monoxide content of the carrier gas (which can be pre-set or separately measured by infrared analysis), the isothermal relationship between the oxygen probe voltage output and carbon potential can be experimentally ascertained and plotted for different temperature conditions. In this way, the carbon potential can be directly related to the oxygen probe voltage and temperature.
Further details of this relationship between oxygen probe voltage and carbon potential are found in the above-identified article by Blumenthal.
In use, a controller associated with the heat treating furnace compares the carbon potential of the furnace atmosphere to a “set point” carbon potential, which is selected to reflect a targeted carbon potential. The controller controls the addition of an enriching gas, such as natural gas, into the atmosphere to maintain the carbon potential of the atmosphere at the set point, and thereby maintain the desired carbon potential in the atmosphere.
The control of carbon potential is only meaningful when the steel being processed is in a single phase field, i.e., austenite. This single phase field occurs only at elevated temperatures and is dependent on the alloy content, the carbon content of the steel, and the temperature.
There are other, lower temperature heat treating applications, e.g., spherodize annealing. In spherodize annealing, the objective is to create a two phase region, where the microstructure of the steel being processed comprises spherical-shaped particles of iron carbide(cementite)(Fe
3
C) distributed in a matrix of alpha iron (ferrite) (&agr;-Fe). This ferrite and iron carbide microstructure produces a steel that is very ductile and easily deformable by plastic deformation. The steel fastener industry, for example, depends upon steel that is in the spherodize annealed condition.
FIG. 1
shows a typical iron-carbon binary phase diagram for hypoeutectoid and hypereutectoid plain carbon steel compositions possessing different weight percentages of carbon. The diagram shows that there is a critical temperature A
1
(about 1333° F.) at which the desired two phase ferrite and iron carbide microstructure exists for both hypoeutectoid and hypereutectoid compositions. The most commercially practical spherodize annealing rates exist at or near the temperature A
1
. This preferred region is shaded in FIG.
1
.
Below A
1
, the rate of spherodize annealing decreases with decreasing temperature for both hypereutectoid and hypoeutectoid compositions. At or below a temperature of about 1250° F., the decreased rate becomes commercially impractical.
For hypoeutectoid compositions (i.e., with weight percent carbon below about 0.8), Above the t
Blumenthal Robert N
Melville Andreas T
Furnace Control Corp.
Ryan Kromholz & Manion S.C.
Wachsman Hal
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