Systems and methods for monitoring or controlling the ratio of hydrogen to water vapor in metal heat treating atmospheres

Abstract
Systems and methods for monitoring a heat treating atmosphere derive from at least one sensor placed in situ in the atmosphere a process variable, which is indicative of the ratio of gaseous hydrogen H2 (g) to water vapor H2O (g) in the atmosphere. The systems and methods use the process variable, e.g., to control the atmosphere, or to record or display the process variable.
Description




FIELD OF THE INVENTION




This invention relates generally to the monitoring and/or controlling of the ratio of hydrogen to water vapor in metal heat treating furnaces.




BACKGROUND OF THE INVENTION




In heat treating or thermal processing of metal and metal alloys, metal parts are exposed to specially formulated atmospheres in a heated furnace. Usually, the atmosphere contains the gaseous species hydrogen H


2


(g) and water vapor H


2


O (g). For example, the atmosphere can comprise a mixture of nitrogen N


2


, hydrogen H


2


, and water vapor (steam) H


2


O. Alternatively, the atmosphere can comprise an exothermic-based atmosphere, generated by an external exothermic generator to contain a mixture of carbon monoxide CO, carbon dioxide CO


2


, nitrogen N


2


, hydrogen H


2


, and water vapor H


2


O.




The hydrogen to water vapor ratio in these atmospheres (in shorthand, called the H


2


/H


2


O ratio) can affect the metal parts being processed and therefore should be monitored. The magnitude of the H


2


/H


2


O ratio at a given temperature relates to the presence or absence of oxidation. More particularly, based upon thermodynamic considerations, oxidation of metal parts at a given temperature occurs when the H


2


/H


2


O ratio of the atmosphere is lower than the H


2


/H


2


O ratio at which equilibrium of the metal to its oxide at that temperature exists, which in shorthand will be called the equilibrium ratio.




The equilibrium ratio for a given metal at a given temperature for a given type of atmosphere can be approximated using, e.g., an Ellingham diagram (see Gaskell,


Introduction of Metallurgical Thermodynamics


, p. 287 (McGraw-Hill, 1981). The actual H


2


/H


2


O ratio of the furnace atmosphere is usually determined by using remote gas analyzers. Remote gas analyzers individually measure percent hydrogen content and the dew point of the atmosphere, which is a measure of the water content. From these two measured quantities, the H


2


/H


2


O ratio of the sampled furnace atmosphere can be ascertained by conventional methods.




Remote sensing of percent hydrogen content is accomplished using conventional thermal conductivity analyzers. These analyzers are generally well suited for sensing H


2


content in simple, binary gas atmospheres, containing a mixture of H


2


and N


2


gases. However, conventional thermal conductivity analyzers are not as well suited to sense H


2


content in more complex exothermic-based atmospheres, where carbon monoxide and carbon dioxide are also present with nitrogen.




In addition, the process of remote gas sensing can itself create significant sampling errors, which lead to erroneous readings. Remote gas sampling requires withdrawing atmosphere gas samples out of the furnace through gas sampling lines. The analysis is performed at ambient temperatures, and not at the temperature present in the furnace, so the sample must be cooled. These physical requirements for remote analysis introduce sampling errors, which are difficult to eliminate.




For example, error may arise due to leaks in the gas sampling line. Another error may also arise due to alteration of the gas chemistry caused either by soot formation during cooling (which is governed by the reaction: CO+H


2


=C+H


2


O), or by a water gas shift in the atmosphere (which is governed by the reaction: H


2


O+CO→CO


2


+H


2


), both of which alterations are a function of the sampling flow rate. Furthermore, in the case of high dew point atmospheres, condensation of water in the gas sampling lines can occur, leading to erroneous sensing results. All or some of these errors can occur at the same time.




The dew point of an exothermic-based atmosphere is usually measured when the atmosphere is produced by a separate external generator. However, this measured dew point does not relate to the dew point of the atmosphere once it enters the heated environment of the furnace itself. This is because, exothermic-based atmospheres are cooled to reduce their water content before introduction into a heated furnace environment. The cooling leaves the atmosphere in a non-equilibrium condition in reference to carbon dioxide CO


2


and water H


2


O. When reheated to thermal processing temperatures inside the furnace, these gases react to reach equilibrium, generating water to prescribe a new dew point and percent carbon dioxide content, according to the reaction: CO


2


+H


2


=CO+H


2


O.




For these reasons, there is a need for more direct and accurate systems and methods to ascertain the actual H


2


/H


2


O ratio in atmospheres during the thermal processing of metals and metal alloys. There is also a need for systems and methods to apply the ascertained H


2


/H


2


O ratio for control and for record keeping purposes.




SUMMARY OF THE INVENTION




One aspect of the invention provides systems and methods for monitoring a metal heat treating atmosphere by generating a computed H


2


/H


2


O ratio for the atmosphere as a function of temperature and oxygen partial pressure P


02


.




In a preferred embodiment, the P


02


is sensed in situ by a zirconia oxygen sensor. The temperature is likewise sensed by an in situ thermocouple. The in situ oxygen sensor and thermocouple are installed in the thermal processing furnace in direct contact with the gas atmosphere. This obviates sampling errors that are inherent in remote gas sampling techniques.




Another aspect of the invention provides systems and methods that make beneficial use of the computed H


2


/H


2


O ratio. For example, the systems and methods control the thermal processing atmosphere based, at least in part, upon the computed H


2


/H


2


O ratio, e.g., by controlling the mixture of gases in the atmosphere. As another example, the systems and methods record or display the computed H


2


/H


2


O ratio, or both.




Another aspect of the invention provides systems and methods for monitoring a metal heat treating atmosphere by deriving from at least one sensor placed in situ in the atmosphere a process variable indicative of the H


2


/H


2


O ratio. The systems and methods make use of the process variable, e.g., by displaying the computed H


2


/H


2


O ratio, recording the H


2


/H


2


O ratio, or by using the H


2


/H


2


O ratio as a process variable to control the atmosphere.




Other features and advantages of the inventions are set forth in the following Description and Drawings, as well as in the appended Claims.











BRIEF DESCRIPTION OF THE DRAWINGS





FIG. 1

is a schematic view of a system for heat treating metal, which includes a processing module for deriving a H


2


/H


2


O ratio as a function of in situ temperature and a voltage signal from an in situ oxygen sensor;





FIG. 2

is a side view, with portions broken away and in section, exemplifying one of the types of in situ temperature and oxygen sensors, which can be coupled to the processing module shown in

FIG. 1

;





FIG. 3

is a schematic view of a furnace for annealing electric motor laminations, which is controlled by one or more processing modules as shown in

FIG. 1

;





FIG. 4

is a representative screen of a graphical user interface to display information processed by the processing module for the furnace shown in

FIG. 3

;





FIG. 5

is a screen of the data shown in

FIG. 4

, with the data recorded for a selected heat treating zone of the furnace in a trend format; and





FIG. 6

is the screen of the data shown in

FIG. 4

, with the data displayed for a selected heat treating zone of the furnace in a unit data format.




The invention may be embodied in several forms without departing from its spirit or essential characteristics. The scope of the invention is defined in the appended claims, rather than in the specific description preceding them. All embodiments that fall within the meaning and range of equivalency of the claims are therefore intended to be embraced by the claims.











DESCRIPTION OF THE PREFERRED EMBODIMENTS




I. Systems and Methods for In Situ Monitoring and Control of the H


2


/H


2


O Ratio





FIG. 1

shows a system


10


for heat treating metal and metal alloys. The system


10


includes a furnace


12


, in which the metal or metal alloys are heat treated, i.e., thermally processed.

FIG. 1

schematically shows the furnace


12


for the purpose of illustration, as the details of its construction are not material to the invention. Representative examples of specific types of furnaces will be described later.




The furnace


12


includes a source


14


of a desired atmosphere, which is conveyed into the furnace


12


. The contents of the atmosphere are selected to achieve the desired processing objectives. One important objective is the monitoring or control of the H


2


/H


2


O ratio, e.g., either to prevent oxidation or to cause an oxide to form.




The furnace


12


also includes a source


16


of heat for the furnace


12


. The source


16


heats the interior of the furnace


12


, and thus the atmosphere itself, to achieve the temperature conditions required to create the desired thermal reactions. Representative temperature conditions will be described in detail later. A temperature sensor S, e.g., a thermocouple, is electrically coupled to a furnace temperature controller


26


, which is itself coupled to the heat source


16


. The furnace temperature controller


26


compares the temperature sensed by the sensor S to a desired value set by the operator (using, e.g., an input device


28


). The furnace temperature controller


26


generates command signals based upon the comparison to adjust the amount of heat provided by the source


16


to the furnace


12


, to thereby maintain the desired temperature.




The system


10


includes a processor


18


for monitoring or controlling the H


2


/H


2


O ratio of the atmosphere at the temperature maintained in the furnace


12


. According to one aspect of the invention, the processor


18


includes no remote gas analyzers. Instead, the processor


18


includes only an in situ temperature sensor


20


and an in situ oxygen sensor


22


. The processor


18


also includes a microprocessor controlled processing function


24


, which is electrically coupled to the temperature and oxygen sensors


20


and


22


.




The oxygen sensor


22


can be variously constructed. In

FIG. 2

, the oxygen sensor


22


is of the type described in U.S. Pat. No. 4,588,493 (“the '493 patent”), entitled “Hot Gas Measuring Probe.” The '493 patent is incorporated into this Specification by reference.




The oxygen sensor


22


is installed through the wall


30


in the furnace


12


. The oxygen sensor


22


is thereby exposed to the same temperature and the same atmosphere as the metal parts undergoing processing.




As

FIG. 2

shows, the oxygen sensor


22


includes an outer sheath


32


, which, in the illustrated embodiment, is made of an electrically conductive material. Alternatively, the sheath


32


could be made of an electrically non-conductive material.




The sheath


32


encloses within it an electrode assembly. The electrode assembly comprises a solid, zirconia electrolyte


34


, formed as a hollow tube, and two electrodes


36


and


38


.




The first (or inner) electrode


36


is placed in contact with the inside of the electrolyte tube


34


. A reference gas occupies the region where the inside of the electrolyte


34


contacts the first electrode


36


. The oxygen content of the reference gas is known.




The second (or outer) electrode


38


, which also serves as an end plate of the sheath


32


, is placed in contact with the outside of the electrolyte tube


34


. The furnace atmosphere circulates in the region where the outside of the electrolyte


34


contacts the second electrode


38


. The furnace atmosphere circulates past the point of contact through adjacent apertures


40


.




A voltage E (measured in millivolts) is generated between the two sides of the electrolyte


34


. The voltage-conducting lead wires


42


(+) and


42


(−) are coupled to the processing function


24


. Alternatively, when an electrically non-conductive sheath


32


is used, internal lead wires (not shown) are coupled to the second electrode


38


to conduct the voltage E to the processing module


24


.




Other types and constructions for the oxygen sensor


22


can be used. For example, the oxygen sensor


22


can be of the type shown in U.S. Pat. No. 4,101,404. Commercial oxygen sensors can be used, e.g., the CARBONSEER™ or ULTRA PROBE™ sensors sold by Marathon Monitors, Inc., or ACCUCARB® sensors sold by Furnace Control Corporation. Some oxygen sensors are better suited for use in higher temperature processing conditions, while other oxygen sensors are better suited for lower temperature processing conditions.




In the illustrated embodiment, the temperature sensor


20


takes the form of a thermocouple. Preferably, the temperature sensor


20


is carried within the electrolyte tube


34


, e.g., by a ceramic rod


35


. In this arrangement, the ceramic rod


35


includes open interior bores


37


, through which the reference gas is introduced into the interior of the electrolyte tube


34


. The lead wire


42


(+) for the oxygen sensor


22


passes through one of the bores


37


, and the other lead wire


42


(−) for the oxygen sensor


22


is coupled to the sheath


32


. The lead wires


39


(+) and


39


(−) for the thermocouple sensor


20


pass through the other bores


37


, to conduct the thermocouple voltage outputs to the processing module


24


.




By virtue of this construction, the temperature sensor


20


is exposed to the same temperature conditions as the furnace atmosphere circulating past the point of contact of the electrolyte


34


and electrodes


36


and


38


. This is also essentially the same temperature condition as the metal parts undergoing treatment.




Alternatively, the temperature sensor


20


can comprise a separate sensor, which is not an integrated part of the oxygen sensor


22


. The thermocouple S, used in association with the heat source


16


, can also be used to sense temperature conditions for use in association with the oxygen sensor


22


.




The magnitude of the voltage E (mv) generated by the oxygen sensor


22


is a function of the temperature (sensed by the temperature sensor


20


) and the difference between the partial pressure of oxygen in the furnace atmosphere and the partial pressure of oxygen in the reference gas. The voltage E (mv) can be expressed as follows:










E


(
mv
)


=

0.0496





T
×
log








P
02



(
Ref
)



P
02







(
1
)













where:




T is the temperature sensed by the temperature sensor (in degrees Kelvin °K).




P


02


(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


02


is the partial pressure of oxygen in the furnace atmosphere.




The magnitude of P


02


(Ref) is known. The quantity P


02


can thereby be ascertained as a function of T (which the in situ temperature sensor


20


provides) and E (which the in situ oxygen sensor


22


provides).




The expression of P


02


derived from in situ outputs of E and T can be reexpressed as a new expression of the H


2


/H


2


O ratio of the atmosphere.




More particularly, at a given temperature under equilibrium conditions, the partial pressure of oxygen P


02


is related to the reaction upon which the H


2


/H


2


O ratio is based, as follows:












H
2



(
g
)


+


1
2




O
2



(
g
)




=


H
2



O
(
g
)






(
2
)













The thermodynamic equilibrium constant K


2


for Equation (2) is given by the following expression:










K
2

=


P


H
2


O




P

H
2




P

O
2


1
/
2








(
3
)













where:




P


H2O


is the partial pressure of water.




P


H2


is the partial pressure of hydrogen.




The thermodynamic equilibrium constant K


2


can also be expressed exponentially as:






K


2


=exp


−ΔG




2


°/RT   (4)






where:




ΔG


2


° is the standard free energy equation for Equation (2).




R is the gas content of the atmosphere.




T is the temperature of the atmosphere in degrees Kelvin.




By combining Equations 1, 3, 4, and the thermodynamic expression for ΔG


2


°, an expression for the ratio P


H2


/P


H2O


as a function of E and T is obtained, as follows:






P


H2


/P


H






2






O


=10


[(10.081E-12,880.1)/(T°K)+3.2044]


  (5)






where:




E is the millivolt output of the in situ oxygen sensor


22


.




T°K is the temperature sensed by the in situ temperature sensor


20


(in degrees Kelvin).




The processing function


24


includes a resident algorithm


44


. The algorithm


44


computes P


H2


/P


H2O


as a function of E and T, according to Equation (5).




To supply the input variables E and T to the algorithm


44


, the processing function


24


is electrically coupled to the lead wires


42


(+) and


42


(−) of the oxygen sensor


22


and the lead wires


39


(+) and


39


(−) of the temperature sensor


20


. The electrical inputs E and T are supplied to the algorithm


44


, which provides, as an output, the quantity P


H2


/P


H2O


as a function of E and T, according to Equation (5). The output expresses the magnitude of the H


2


/H


2


O ratio.




Unlike prior systems, the system


10


requires no measurement of the hydrogen content or dew point by remote sensing at ambient temperatures to derive the H


2


/H


2


O ratio. The system


10


can thereby be free of remote sensors. The system


10


relies solely upon in situ sensing to derive the H


2


/H


2


O ratio. The system


10


thereby eliminates errors associated with remote gas sensing, as previously described.




The processing function


24


outputs the calculated H


2


/H


2


O ratio for further uses by the system


10


. The H


2


/H


2


O ratio output can, e.g., be displayed, or recorded over time, or used for control purposes, or any combination of these processing uses.




For example, in

FIG. 1

, the system


10


includes a display device


48


coupled to the processing function


24


. The display device


48


presents the derived H


2


/H


2


O ratio for viewing by the operator. The display device


48


can, of course, show other desired atmosphere or processing information. Alternatively, or in combination, a printer or recorder


50


can be coupled to the processing function


24


for showing the derived the H


2


/H


2


O ratio and its fluctuation over time in a printed strip chart format.




In a preferred embodiment, the processor


18


further includes an atmosphere control function


46


. The atmosphere control function


46


includes a comparator function


52


. The comparator function


52


compares the derived H


2


/H


2


O ratio to a desired control value or set point, which the operator can supply using, e.g., an input


54


. Based upon the deviation between the derived H


2


/H


2


O ratio and the set point, the atmosphere control function


46


generates a control signal to the atmosphere source


14


. The control function


46


generates signals, to adjust the atmosphere to establish and maintain the derived H


2


/H


2


O ratio at or near the set point. The control function


46


is also coupled to the device


48


to show other atmosphere or processing information. In this way, the processor


18


works to maintain atmosphere conditions optimal for the desired processing conditions.




The system


10


can take various forms. The following description presents an illustrative arrangement and use of the system


10


for the purpose of controlling processing conducted for the purpose of annealing steel laminations, e.g., laminations contained in electric motors.




II. Monitoring and Control of Atmospheres for Annealing Steel Laminations





FIG. 3

shows in schematic form a furnace


56


specially configured for annealing steel laminations used in electric motors.

FIG. 3

generally shows these laminations as work


166


.




The furnace establishes three different processing conditions


58


,


60


, and


62


. The first condition


58


is for annealing. The second condition


60


is for cooling prior to blueing. The third condition


62


is for blueing after cooling. Each processing condition


58


,


60


, and


62


serves a different purpose. Therefore, each condition


58


,


60


, and


62


requires a different atmosphere and temperature environment.




The furnace


56


can be variously constructed. The furnace


56


can, e.g., comprise a batch furnace, such as a bell-type furnace, a box furnace, or a pit furnace. In this arrangement, different atmosphere and temperature conditions are cyclically established in a single furnace chamber.




Alternatively, the furnace


56


can comprise a continuous furnace of a roller hearth, pusher, or mesh belt construction. In this arrangement, the furnace is compartmentalized into two or more processing chambers. The atmosphere and temperature conditions are controlled in the chambers to establish the conditions


58


,


60


and


62


.





FIG. 3

typifies a continuous furnace arrangement, wherein the conditions are established in three sequential zones


58


,


60


, and


62


. The work


166


is transferred from one zone to another by a suitable work transport mechanism


64


, like a mesh belt or rollers, for processing.





FIG. 3

is meant to show a typical continuous furnace in simplified, schematic form, without all the structural detail which is known by those skilled in heat processing. For example, the furnace


56


may also include burnout and gas purge regions before the first zone


58


. Also, the first and second zones


58


and


60


may coexist at opposite ends of a single chamber, which may, in turn, be separated by an additional gas purge region from the third zone


62


, which occupies its own distinct chamber. There are many different types of possible furnace configurations. Understanding or practicing the invention do not depend upon and are not limited by such structural details.




A. The Annealing Zone




In the annealing zone


58


, high temperature conditions are maintained, e.g., 1400° F. to 1550° F. A temperature sensor S is coupled to a temperature controller


72


for the annealing zone


58


. The temperature controller


72


is coupled to a source


74


of heat for the zone


58


. Based upon temperature signals received from the temperature sensor S, the controller


72


operates the heat source


74


to maintain the zone


58


at the desired temperature.




Further, a source


66


supplies an atmosphere to the annealing zone


58


of the furnace


56


. The atmosphere is established and maintained to serve two purposes.




As a first purpose, the atmosphere provides a reducing atmosphere, which prevents oxidation of iron present in the steel laminations. In addition, the atmosphere minimizes internal oxidation of more active elements, like silicon and aluminum, present in the steel laminations. A reducing atmosphere is characterized by the presence of hydrogen H


2


and water H


2


O in sufficient proportions, given the temperature, to reduce the presence of iron oxide. The presence of a reducing atmosphere in the annealing zone


58


prevents the formation of iron oxide on the surface of the steel laminations and minimize internal oxidation within the steel laminations.




As a second purpose, the atmosphere in the annealing zone


58


provides a decarburizing atmosphere. A decarburizing atmosphere removes carbon from the laminations. This is important to improve the magnetic properties of steel. More specifically, carbon causes aging and magnetic core losses in the laminations.




The decarburizing reaction desired in the annealing zone


58


is given by the following reaction:








C


+H


2


O=CO+H


2


  (6)






where






C


represents the carbon in solution in the ferrite structure of iron.




H


2


O is water vapor.




CO is carbon monoxide.




H


2


is hydrogen.




The source


66


can generate the atmosphere for the annealing zone


58


in various ways.




For example, the source


66


can provide a mixture of nitrogen N


2


and hydrogen H


2


(which will be in shorthand called a “N


2


+H


2


atmosphere”). The N


2


+H


2


atmosphere is inherently free or essentially free of water vapor.




Alternatively, the source


66


can provide an exothermic-based atmosphere. This atmosphere is produced by mixing air with a fuel, like natural gas or propane, in an external apparatus, as before described. This atmosphere includes, in addition to nitrogen N


2


and hydrogen H


2


, carbon monoxide CO, carbon dioxide CO


2


, and water vapor.




Based upon Equation (6) and kinetic considerations, for a given atmosphere and temperature, the rate of removal of carbon (i.e., decarburization) is proportional to the partial pressure of water P


H2O


in the atmosphere. At a given temperature, increasing the dew point of the atmosphere (by increasing the water vapor content) increases the rate of decarburization. However, increasing the water vapor content without proportionally increasing the hydrogen H


2


content will decrease the H


2


/H


2


O ratio, causing oxide formation. A balance must therefore be struck between decarburization and oxidation.




In the N


2


+H


2


atmosphere, the water vapor content is inherently very low. Steam is added to increase the water vapor content and change the dew point. For a given temperature, as steam is added to the atmosphere, the dew point increases and, with it, the rate of decarburization.




In an exothermic-based atmosphere, the magnitude of the inherent water vapor content is affected by the air-to-fuel ratio. At a given temperature, the introduction of more air, to raise the air-to-fuel ratio, increases the water vapor content and dew point, and vice versa. With these increases, the rate of decarburization increases, as well.




In the annealing zone


58


, in addition to the need for decarburization, the H


2


/H


2


O ratio must be kept high enough to provide a reducing atmosphere, to prevent oxidation of iron and minimize internal oxidation of the more active elements in the laminations. Increasing the water vapor content of the atmosphere to increase decarburization, without proportional increases in the hydrogen H


2


content of the atmosphere, decreases the H


2


/H


2


O ratio, driving the atmosphere toward an undesirable oxidizing condition.




In the N


2


+H


2


atmosphere, the amount of hydrogen is usually kept at a generally constant magnitude. The constant amount of hydrogen limits the maximum dew point that can be obtained at a given atmosphere.




In an exothermic-based atmosphere, increases in water vapor content are accompanied by decreases in the hydrogen H


2


content.




In either situation, the optimum range of H


2


/H


2


O ratios to prevent oxidation, yet be as decarburizing as possible at a given temperature, is constrained. For this reason, the accurate measurement and control of the H


2


/H


2


O ratio is critical to assure desired results.




According to the invention, an in situ oxygen sensor


68


and temperature sensor


70


are placed in the annealing zone


58


of the furnace. The sensors


68


and


70


are preferable part of an integrated assembly, as

FIG. 2

shows. For example, an ACCUCARB® Oxygen Sensor, Model AQ620-S-1 (Furnace Control Corporation) can be used, as it is well suited for use in high temperature conditions.




Both the oxygen and temperature sensors


68


and


70


are further coupled to a processing module


78


for the annealing zone


58


. The resident algorithm


44


, already described, is installed in the processing module


78


.




An output of the processing module


78


is coupled to an atmosphere controller


76


. An output


80


of the controller


76


is, in turn, coupled to a controllable valve


82


, which is operatively coupled to the atmosphere source


66


for the annealing zone


58


.




For a nitrogen-based atmosphere, the valve


82


controls the rate at which steam is introduced into the nitrogen-based atmosphere. In an exothermic-based atmosphere, the valve


82


controls the air-to-fuel ratio of the atmosphere. In both arrangements, operation of the valve


82


affects the water vapor content of the atmosphere in the annealing zone


58


.




A desired set point H


2


/H


2


O ratio for the annealing zone


58


is entered into the atmosphere controller


76


by the operator through an input


84


. The desired set point H


2


/H


2


O ratio is selected to maintain a desired reducing atmosphere condition at the processing temperature maintained in the annealing zone


58


.




The processing module


78


receives the electrical E (mv) signal from the oxygen sensor


68


and T (mv) signal from the temperature sensor


70


residing in the annealing zone


58


. Based upon these inputs, the algorithm


44


of the processing module


78


derives as an output the H


2


/H


2


O ratio. This output is conveyed to the atmosphere controller


76


.




The atmosphere controller


76


also includes the comparator function


52


, as before described. The comparator function


52


compares the derived H


2


/H


2


O ratio to the set point. The comparator function


52


preferable conducts a conventional proportional-integral-derivative (PID) analysis. The PID analysis takes into account the difference between the derived magnitude and the set point, and also integrates the difference over time. Based upon this analysis, the atmosphere controller


76


derives a deviation, which is converted to a control output. The controller


76


conveys the control output to the valve


82


, based upon the magnitude of the deviation, to keep the deviation at or near zero.




When the deviation indicates that the derived H


2


/H


2


O ratio exceeds the set point, the controller


76


operates the valve


82


to lower the magnitude of the H


2


/H


2


O ratio in the atmosphere in the annealing zone


58


, i.e., by increasing the water vapor content. In the N


2


+H


2


atmosphere, the valve


82


increases the flow rate of steam into the atmosphere of the annealing zone


58


. In an exothermic-based atmosphere, the valve


82


increases the air-to-fuel ratio of the external generator.




When the deviation indicates that the derived H


2


/H


2


O ratio for the annealing zone


58


is lower than the set point, the controller


76


operates the valve


82


to raise the magnitude of the H


2


/H


2


O ratio in the annealing zone


58


, i.e., by decreasing the water vapor content. In the N


2


+H


2


atmosphere, the valve


82


decreases the flow rate of steam into the atmosphere of the annealing zone


58


. In an exothermic-based atmosphere, the valve


82


decreases the air-to-fuel ratio of the external generator.




It should be appreciated that other corrective action can be taken based upon the deviation. The foregoing description is intended to exemplify one type of corrective action.




In this way, the processing module


78


provides a process variable indicative of the H


2


/H


2


O ratio in the annealing zone


58


, based solely upon in situ sensing by the temperature sensor


70


and the oxygen sensor


68


, to control the atmosphere in the annealing zone


58


. The in situ sensing reflects the actual H


2


/H


2


O ratio of the atmosphere within the furnace, and eliminates the errors of remote sensing.




An output


86


of the controller


76


and an output


87


of the processing module


78


are coupled to a device


88


that displays or records or stores in memory the calculated H


2


/H


2


O ratio and other operating conditions in the annealing zone


58


on a real time basis. Details of a preferred display will be described later.




B. The Cooling Zone




The work


166


(i.e, the laminations) is carried by the transfer mechanism


64


from the annealing zone


58


into the cooling zone


60


. The cooling zone


60


establishes a region where gradient cooling can occur between the high temperature of the annealing zone


58


and the lower temperature of the blueing zone


62


.




In the cooling zone


60


, the temperature is typically under 1000° F., which corresponds to the lowest temperature that wustite (FeO) is stable and therefore will not form on the work


166


. The purpose of the zone


60


is to allow the laminations to gradually cool before entering the blueing zone


62


, to thereby prevent stress to the annealed laminations without wustite formation.




The temperature gradient can be established in various ways. For example, as

FIG. 3

shows, a temperature sensor S can be coupled to a temperature controller


96


for the cooling zone


60


, to operate a heat source


98


to maintain a desired temperature gradient in the zone


60


. Alternatively, the cooling zone


60


may not be directly heated, thereby establishing a region where gradient cooling can occur between the annealing zone


58


and the blueing zone


62


.




The cooling zone


60


may comprise a separate chamber in the furnace


56


physically separated from the annealing zone


58


and/or the blueing zone


62


. Typically, however, the annealing zone


58


and the cooling zone


60


share opposite ends of a common chamber within the furnace


56


.




In this arrangement, when a N


2


+H


2


atmosphere with added steam is supplied to the annealing zone


58


by the source


66


, the cooling zone


60


can itself be served by a separate source


90


, which supplies a N


2


+H


2


atmosphere, but without added steam. This provides a reducing atmosphere to prevent oxidation of the iron and minimize internal oxidation of the more active elements like silicon and aluminum in the laminations, as they cool.




Alternatively, in this arrangement and when an exothermic-based atmosphere is supplied by the source


66


to the annealing zone


58


, no separate source


90


of atmosphere communicates with the cooling zone


60


. In this arrangement, the exothermic-based atmosphere present in the annealing zone


58


flows into the cooling zone


60


. This also provides a reducing atmosphere to prevent oxidation of the iron and minimize internal oxidation of the more active elements like silicon and aluminum in the laminations, as they cool.




In either situation, an in situ oxygen sensor


92


and a temperature sensor


94


are preferably placed in the cooling zone


60


of the furnace


56


. The sensors


92


and


94


are preferable part of an integrated assembly, as

FIG. 2

shows. For example, an ACCUCARB® Oxygen Sensor OXA20-S-0 (Furnace Control Corporation) can be used, as it is well suited for use in lower temperature conditions. The oxygen and temperature sensors


92


and


94


are coupled to a processing module


102


for the cooling zone


60


.




The processing module


102


includes the resident algorithm


44


, already described, to generate the H


2


/H


2


O ratio output. An output


111


of the module


102


is coupled to a device


112


that displays or records or stores in memory the computed H


2


/H


2


O ratio for the cooling zone


60


on a real time basis. In this way, the sensors


92


and


94


monitor the H


2


/H


2


O ratio in the cooling zone


60


.




When the separate source


90


supplies a N


2


+H


2


atmosphere to the cooling zone


60


(or when the atmosphere in the cooling zone


60


can otherwise be separately controlled, e.g. by providing a segregated cooling zone


60


), the H


2


/H


2


O ratio of the processing medule


102


is conveyed to an atmosphere controller


100


. An output


104


of the controller


100


is, in turn, coupled to a control valve


106


. The control valve


106


controls the source


90


to directly provide an atmosphere in the cooling zone


60


to achieve a desired H


2


/H


2


O ratio.




In this arrangement, a desired set point H


2


/H


2


O ratio for the cooling zone


60


is entered into the atmosphere controller


100


by the operator through an input


108


. The desired set point H


2


/H


2


O ratio is selected to maintain a desired reducing atmosphere condition at the temperature maintained in the cooling zone


60


. As the equilibrium H


2


/H


2


O ratio for a given reducing atmosphere increases with decreases of temperature, the set point H


2


/H


2


O ratio is likewise increased in the cooling zone


60


, as compared to the set point of the annealing zone


58


.




In this arrangement, the atmosphere controller


100


for the cooling zone


60


operates in the same fashion as the atmosphere controller


76


for the annealing zone


58


. Based upon the electrical E (mv) signal from the oxygen sensor


92


and T (mv) signal from the temperature sensor


94


, the processing module


102


derives the H


2


/H


2


O ratio of the atmosphere in the cooling zone


60


according to the resident algorithm


44


. The H


2


/H


2


O ratio is conveyed to the atmosphere controller


100


, where the resident comparator function


52


compares the derived H


2


/H


2


O ratio to the set point to generate a deviation. The atmosphere controller


100


generates a control output to the valve


106


based upon the deviation, to keep the deviation at or near zero. In this way, the controller


100


maintains the H


2


/H


2


O ratio of the atmosphere of the cooling zone


60


at or near the set point. An output


110


of the atmosphere controller


100


can also be coupled to the display device


112


, to show various processing conditions.




When an exothermic-based atmosphere is present in the cooling zone


60


, or when there is otherwise no separate controllable atmosphere source


90


for the zone


60


, indirect control of the H


2


/H


2


O ratio in the cooling zone


60


can be achieved by monitoring of the H


2


/H


2


O ratio by the sensors


92


and


94


. For example, the set point H


2


/H


2


O ratio for the annealing zone


58


can be adjusted, based upon the monitored computed H


2


/H


2


O ratio for the cooling zone


60


, to obtain a balance of oxidation-free conditions in both annealing and cooling zones


58


and


60


.




In either way, the processing module


102


provides a monitored H


2


/H


2


O ratio and/or a process variable for the cooling zone


60


, indicative of the H


2


/H


2


O ratio, based solely upon in situ sensing by the temperature sensor


94


and the oxygen sensor


92


.




C. The Blueing Zone




The transfer mechanism


64


carries the work


166


(i.e., the laminations) from the cooling zone


60


and into the blueing zone


62


. The work


166


has, by now, cooled to below the temperature at which wustite (FeO) can form. If needed, a temperature sensor S can be coupled to a temperature controller


120


for the blueing zone


62


, to operate a heat source


122


to maintain the zone


62


at the desired temperature.




A source


114


supplies an atmosphere into the blueing zone


62


. Unlike the annealing and cooling zone


58


and


60


, the atmosphere introduced into the blueing zone


62


purposely provides an oxidizing atmosphere. The oxidizing atmosphere produces desired forms of iron oxide on the surface of the laminations. Still, the temperature of the blueing zone


62


prevents the formation of wustite (FeO) in the oxidizing atmosphere of the blueing zone


62


, which is highly undesired.




In the illustrated embodiment, the source


114


supplies steam to the blueing zone


62


to provide the oxidizing atmosphere. Alternatively, an exothermic-based atmosphere with water vapor content can be used.




As in the annealing and cooling zones


58


and


60


, an in situ oxygen sensor


116


and temperature sensor


118


are placed in the blueing zone


62


of the furnace


56


. The sensors


116


and


118


are preferable part of an integrated assembly, as

FIG. 2

shows. For example, an ACCUCARB® Oxygen Sensor OXA20-S-0 (Furnace Control Corporation) can be used, as it is well suited for use in the lower temperature conditions of the blueing zone


62


(e.g., 800° F. to 1000° F.).




The oxygen and temperature sensors


116


and


118


are likewise coupled to a processing module


126


for the cooling zone


62


. The processing module


126


includes the resident algorithm


44


already described. An output


133


of the processing module


126


is coupled to a device


134


that displays or records or stores in memory the H


2


/H


2


O ratio for the blueing zone


62


on a real time basis. In this way, the sensors


116


and


118


monitor the H


2


/H


2


O ratio in the blueing zone


62


.




When a steam atmosphere is supplied to the blueing zone


62


, a reaction creating a desired form of iron oxide Fe


3


O


4


can be expressed as follows:






4 H


2


O+3 Fe=3 Fe


3


O


4


+4 H


2


  (7)






The hydrogen H


2


content in the blueing zone


62


is typically low (compared to the rich hydrogen H


2


nitrogen-based or exothermic-based atmospheres in the annealing and cooling zones


58


and


60


). As a result, the desired H


2


/H


2


O ratio for the blueing zone


62


is typically several orders of magnitude smaller than the desired (i.e., set point) H


2


/H


2


O ratio for either the annealing or cooling zones


58


and


60


.




From Equation (7), it can be appreciated that effective control of the formation of H


2


in the blueing zone


62


, to thereby maintain the desired low H


2


/H


2


O ratio, can not be achieved by controlling the introduction of a steam (H


2


O) atmosphere. From Equation (7), it can be seen that more effective control of the reaction to reduce the formation of H


2


can be achieved, e.g., by reducing the temperature of the blueing zone


62


, to thereby slow the reaction; or by adding a gas, e.g., nitrogen N


2


, to dilute the steam to provide less water vapor to react and form H


2


; or by reducing the number of parts in the blueing zone


62


, thereby reducing the formation of hydrogen H


2


.




Likewise, should a higher H


2


/H


2


O ratio be desired in the blueing zone


62


, Equation (7) shows that the H


2


content can be increased by adding H


2


or a H


2


and nitrogen N


2


mixture to the blueing zone


62


.




When an exothermic-based atmosphere with water vapor content is supplied to the blueing zone


62


, the air-to-fuel ratio of the external generator can be controlled (as already described) to provide the desired oxidizing gas atmosphere.




It can therefore be appreciated that the ability to monitor the H


2


/H


2


O ratio in the blueing zone with the in situ sensors


116


and


118


is advantageous, as it makes possible the direct control of the H


2


/H


2


O ratio in the blueing zone


60


. For example, the H


2


/H


2


O ratio output of the processing module


126


can, if desired, be conveyed to an atmosphere controller


124


for the blueing zone


62


. An output


128


of the controller


124


is coupled to a suitable control mechanism


130


. For a steam atmosphere, the control mechanism


130


controls the reaction expressed in Equation (7) to control the H


2


content in the blueing zone


62


. For an exothermic-based atmosphere, the control mechanism


130


affects the air-to-fuel ratio of the external generator to control the H


2


content in the blueing zone


62


.




A desired set point H


2


/H


2


O ratio for the blueing zone


62


is entered into the atmosphere controller


124


by the operator through an input


132


. The controller


124


includes the resident comparator function


52


, already described. The desired set point H


2


/H


2


O ratio is selected to maintain a desired oxidizing atmosphere condition at the temperature maintained in the blueing zone


62


.




The controller


124


for the blueing zone


62


can therefore, if desired, operate in the same fashion as the controller


76


for the annealing zones


58


. Based upon the electrical E (mv) signal from the oxygen sensor


116


and T (mv) signal from the temperature sensor


118


in the blueing zone


62


, the processing module


126


derives the H


2


/H


2


O ratio according to the resident algorithm


44


. The comparator function


52


of the controller


124


compares the derived H


2


/H


2


O ratio for the atmosphere of the blueing zone


62


to the set point, to generate a deviation. The controller


124


generates a control output to the valve


130


based upon the magnitude of the deviation, to keep the deviation at or near zero, thereby maintaining the H


2


/H


2


O ratio in the atmosphere of the blueing zone


62


at or near the set point. An output


131


of the atmosphere controller


124


can also be coupled to the display device


134


to show various processing conditions.




In this way, the processing module


126


provides a process variable for the blueing zone


62


indicative of the low H


2


/H


2


O ratio, based solely upon in situ sensing by the temperature sensor


118


and the oxygen sensor


116


, to control the atmosphere in the blueing zone


62


.




III. Graphical User Interfaces




In the illustrated embodiment (see FIG.


4


), the devices


88


,


112


, and


134


are consolidated to provide an interactive user interface


136


. The interface


136


allows the operator to select, view and comprehend information regarding the operating conditions within any of the zones


58


,


60


, or


62


of the furnace


56


. The interface


136


also allows the operator to change metal heat treating conditions in one or more zones of the furnace


56


.




The interface


136


includes an interface screen


138


. It can also include an audio or visual device to prompt or otherwise alert the operator when a certain processing condition or conditions arise. The interface screen


138


displays information for viewing by the operator in alpha-numeric format and as graphical images. The audio device (if present) provides audible prompts either to gain the operator's attention or to acknowledge operator actions.




The interface screen


138


can also serve as an input device, to input from the operator by conventional touch activation. Alternatively or in combination with touch activation, a mouse or keyboard or dedicated control buttons could be used as input devices.

FIG. 4

shows various dedicated control buttons


140


.




The format of the interface screen


138


and the type of alpha-numeric and graphical images displayed can vary.




A representative user interface screen


138


is shown in FIG.


4


. The screen


138


includes four block fields


142


,


144


,


146


, and


148


, which contain information, formatted in alpha-numeric format. The information is based upon data received from the associated heat and atmosphere controllers, relating to processing conditions within a given zone of the furnace


56


.




The first field


142


displays in alpha-numeric format a process variable (PV), which is indicative of the H


2


/H


2


O ratio derived by sensing from the in situ sensors residing the atmosphere of the furnace zone. The value displayed in the first field


142


comprises the H


2


/H


2


O ratio derived by the resident algorithm


44


.




The second field


144


displays in alpha-numeric format the set point value SV for the H


2


/H


2


O ratio for the given zone. The value displayed is received as input from the operator, as previously explained.




The third field


146


displays in alpha-numeric format the deviation DEV derived by the comparator function


52


of the algorithm


44


. The deviation DEV displays the difference between the process variable PV and the set point SP.




The fourth field


148


displays in alpha-numeric format the percent output (OUT), which reflects the magnitude of the control correction commanded by the PID analysis to bring the process variable PV to the set point SP. For example, when a valve controls the steam content, an OUT equal to 83.5% (as

FIG. 4

shows) indicates that the valve is 83.5% open.




The screen


138


also includes two graphical block fields


150


and


152


. The fields


150


and


152


provide information about the processing conditions within a given zone of the furnace


56


in a graphical format.




The first block field


150


includes a vertically oriented, scaled bar graph. A colored bar


154


graphically shows the magnitude of the process variable PV relative to the set point on the bar graph. An icon


156


marks the set point value within the scale of the bar graph.




The second block field


152


includes a horizontally oriented, bar graph scaled between 0 and 100. A colored bar


158


graphically depicts percent output (OUT), which is the magnitude of the control correction commanded by the PID analysis to bring the process variable PV to the set point SP, as before explained.




As

FIG. 4

shows, the screen


138


also includes various other an alpha-numeric block fields


160


,


162


, and


164


displaying status information. The block field


160


identifies the mode of atmosphere control, e.g., AUTO (for automatic control by the processing module) or MAN (for manual). The block field


162


identifies the furnace zone to the displayed information pertains. The operator is able by selection of a control button


140


to select the particular zone


58


,


60


, or


62


for viewing information on the screen


138


. The block field


164


contains date and time stamp.




By selection of another control button


140


, the operator is able to change the set point for the zone


58


,


60


, or


62


then visible on the screen


138


.




By selection of another control button


140


, the operator can select among different display options for viewing information relating to the selected zone. For example, the operator can select a trend display (see FIG.


5


), which graphically displays the variation over time of selected processing conditions, e.g., PV, E, and T. As another example, the operator can select a real time data display (see FIG.


6


), which records instantaneous unit data values for selected processing variables, e.g., high and low measured temperatures, the highest and the current E (mv) output of the oxygen sensor, and the lowest and the current H


2


/H


2


O ratio derived.




Due to different temperature and atmosphere conditions, the magnitudes of the H


2


/H


2


O ratio-based values change for different processing zones. As before explained, for example, the magnitude of the H


2


/H


2


O ratio for the blueing zone


62


can be several orders of magnitude less than the magnitude of the H


2


/H


2


O ratio in the annealing or cooling zones


58


or


60


. The considerable difference in scale of the magnitudes can lead to confusing differences in the presentation of H


2


/H


2


O ratio-based values for the different furnace zones. To maintain consistent display proportions numerically and graphically, the processing module applies a scaling factor to the H


2


/H


2


O ratio-based values for the blueing zone


62


for display on the screen


138


. The scaling factor shifts the small absolute magnitudes of the H


2


/H


2


O ratio-based values for the blueing zone


62


by, e.g., several orders of magnitude, for display purposes. In this way, the display of data for the blueing zone


62


has the same “look and feel” as the display of data for the annealing zone


58


or the cooling zone


60


. The exponential scale factor can be displayed, e.g., as part of the real time data display (see FIG.


6


).




The graphical user interface


136


shown in

FIGS. 4

to


6


can be realized using a HONEYWELL™ VPR-100 Controller with standard or advanced free form math capability (Honeywell, Inc.).




The features of the invention are set forth in the following claims.



Claims
  • 1. A heat treating system for a metal part comprisinga heat treating furnace, an atmosphere source for supplying a preselected gas atmosphere to the furnace, a heat source to heat the heat treating furnace including a furnace temperature controller coupled to the heat source to maintain the preselected gas atmosphere inside the furnace at a preselected temperature not greater than about 1000 degrees Fahrenheit, a temperature at which wustite (FeO) forms on the metal part, an oxygen sensor located in situ in the furnace in contact with the preselected gas atmosphere, the oxygen sensor providing a first electrical input that varies according to oxygen content of the preselected atmosphere, a temperature sensor located in situ in the furnace in contact with the preselected gas atmosphere, the temperature sensor providing a second electrical input that varies according to temperature of the preselected atmosphere, a processor incuding a processing function to generate a computed ratio of gaseous hydrogen H2 (g) to water vapor H2 O (g) for the preselected atmosphere as a function of the first and second electrical inputs.
  • 2. A system according to claim 1 and further including an output for the computed ratio.
  • 3. A system according to claim 2 wherein the output is coupled to a device for displaying the computed ratio.
  • 4. A system according to claim 2 wherein the output is coupled to a device for recording the computed ratio.
  • 5. A system according to claim 2 wherein the output is coupled to a controller for the atmosphere source.
  • 6. A system according to claim 1 wherein the processor includes an atmosphere control function comprising a comparator to compare the computed ratio to a selected set point and generate a deviation, andfurther including an output for the deviation.
  • 7. A system according to claim 6 wherein the output is coupled to a controller for the atmosphere source.
  • 8. A system according to claim 1 wherein the heat treating atmosphere comprises an H2/N2 atmosphere.
  • 9. A system according to claim 1 wherein the heat treating atmosphere contains CO, CO2, H2, and H2O.
  • 10. A system according to claim 1 wherein the heat treating atmosphere comprises a steam atmosphere.
RELATED APPLICATION

This application is a divisional of application Ser. No. 09/218,390 filed Dec. 22, 1998.

US Referenced Citations (22)
Number Name Date Kind
3844770 Nixon Oct 1974 A
3920447 Schroeder et al. Nov 1975 A
4158166 Isenberg Jun 1979 A
4285742 Bowes et al. Aug 1981 A
4288062 Gupta et al. Sep 1981 A
4485002 Wunning Nov 1984 A
4588493 Blumenthal et al. May 1986 A
4992113 Baldo et al. Feb 1991 A
5137616 Poor et al. Aug 1992 A
5211820 Poor et al. May 1993 A
5231645 Uno et al. Jul 1993 A
5352344 Gohring et al. Oct 1994 A
5385337 Schultz Jan 1995 A
5393403 Sasabe et al. Feb 1995 A
5496450 Blumenthal et al. Mar 1996 A
5498299 Schmidt Mar 1996 A
5498487 Ruka et al. Mar 1996 A
5556556 Blumenthal et al. Sep 1996 A
5666631 Polizzotti et al. Sep 1997 A
5772428 Van Den Sype et al. Jun 1998 A
6591215 Blumenthal et al. Jul 2003 B1
6612154 Blumenthal et al. Sep 2003 B1
Non-Patent Literature Citations (17)
Entry
Trade Brochure Furnace Control Corporation “Lamination Annealing Control Systems”, Undated.
Chen, Yong Chwang, Automatic Control of the Carbon Potential of Furnace Atmospheres without Adding Enriched Gas., Metallurgical Transactions, vol. 24B, Oct. 1993, pp. 881-888.
Weissohn, Von K.H., Sauerstoffmebzellen zum Regein von Ofenatmospharen, warme Gas International, Band 21, Oct. 10, 1983, pp. 436-437.
Weissohn, K.H., Oxygen Partial Pressure Measurements with a Zirconium Oxide Probe, warme Gas International, Jun. 6, 1990, pp. 331-342.
Sastri and Abraham, Atmosphere Control in Heat Treatment Furnaces Using Oxygen Probes, Tool & Alloy Steels, Apr. & May 1986, pp. 155-162.
Armson et al., Electrochemical Sensors for Heat Treatment Atmoshere Monitoring, CKN Group Technological Centre, Wolverhampton England pp. 905-918.
Controlled Atmos. Annealing of Steel Using Oxygen Sensors to Control and Monitor Atmosphere Compan. Japanese Patent Abstract, JP 7988711 Dialog Search.
Oxygen Sensor for Annealing, Japanese Patent Abstract JP 60-9447.
Apparatus for Oxygen Analysis, Japanese Patent Abstract, JP 91-330656.
Blumenthal, R.N. “Control of Endothermic Generators-A Technical Comparison of Endothermic and Nitrogen/Methanol Carrier Atmosphere”; ASM International, pp. 19-25 Copyright 1996.
ASM Handbook Volume 4 Heat Treating Circa 1998.
Robert, Marc et al; “Automatic prevention of decarburization through ALNAT FC” processing pp. 75-78.
Robert, Marc “Carbon flux method controls N2-hydrocarbon decarb/recarb Heat Treating” Sep. 1990 pp. 22-24.
Stanescu, Mircea “Principal annealing atmospheres for steel rod and wire” pp. 79-83.
Powers, Charles et al., “Process Control for short Time Cycle Spheroidize Annealing”.
”Ispen unviels new sensor technology” Advanced Materials & Processes” Oct. 1998 pp. 128-129.
“Ispen International Inc.; Advanced Materials Processes” Oct. 1998 pp. 122-123.