Real-time carbon sensor for measuring concentration profiles in carburized steel

A solute sensor probe for sequestering and sampling a plurality of defined subsurface volume elements in a conductive material, said sensor probe comprising:

• a first voltage electrode for contacting a first portion of an external surface of said material, said first voltage electrode defining a first boundary surface of each of said sample volume elements;
• a second opposing voltage electrode for contacting a second portion of said external surface, said second voltage electrode in coaxial alignment with said first voltage electrode, said second voltage electrode defining a second boundary surface of each of said sample volume elements;
• a first current electrode for contacting a third portion of said external surface, said first current electrode positioned adjacent to said first voltage electrode and in coaxial alignment with said first and second voltage electrodes;
• a second opposing current electrode for contacting a fourth portion of said external surface, said second current electrode positioned adjacent to said second voltage electrode and in coaxial alignment with said first current electrode and said first and second voltage electrodes;
• said first and second current electrodes being adapted for receiving an alternating current source, said current source configured for providing a plurality of alternating current frequencies to said electrodes;
• said first and second voltage electrodes being adapted for measurement of a plurality of voltage potentials established between said voltage electrodes by application of said alternating current to said current electrodes at said plurality of said frequencies;
• a plurality of third boundary surfaces of each of said sampled volume elements, each one of said third boundary surfaces positioned at a unique, frequency-dependent depth δi below said external surface where
• δ
• =
• 1
• μ
• ⁢
• ⁢
• σ
• ⁢
• ⁢
• π
• ⁢
• ⁢
• f
• ,
• μ is the material magnetic permeability, σ is the material conductivity and fi is one of said alternating current frequencies, each one of said sampled volume elements being bounded by a region enclosed by said first and second boundary surfaces, one of said frequency-dependent third boundary surfaces and the external surface of said material;
• and
• a solute concentration correlation means for determining the solute concentration of each of said sampled volume elements from a measured resistivity of each sampled volume element, said resistivity of each volume element being derived from said applied current and said measured voltage at one of said plurality of frequencies.

A system for measuring solute concentration profiles in a conductive material during thermal processing, said system comprising:

• a solute sensor probe for measuring solute concentration profiles in a conductive material by sequestering and sampling a plurality of defined subsurface volume elements, said sensor probe comprising:
• at least one first electrode for contacting a first portion of an external surface of said material, said first electrode defining a first boundary surface of each of said sampled volume elements;
• at least one second opposing electrode for contacting a second portion of said external surface, said second electrode in coaxial alignment with said first electrode, said second electrode defining a second boundary surface of each of said sampled volume elements;
• said first and second electrodes being adapted for receiving an alternating current source, said current source configured for providing a plurality of alternating current frequencies to said electrodes;
• said first and second electrodes being adapted for measurement of a plurality of voltage potentials established between said electrodes by application of said alternating current at said plurality of said frequencies;
• a plurality of third boundary surfaces of each of said sampled volume elements, each one of said third boundary surfaces positioned at a unique, frequency-dependent depth δ1 below said external surface where
• δ
• =
• 1
• μ
• ⁢
• ⁢
• σ
• ⁢
• ⁢
• π
• ⁢
• ⁢
• f
• ,
• μ is the material magnetic permeability, σ is the material conductivity and fi is one of said alternating current frequencies, each one of said sampled volume elements being bounded by a region enclosed by said first and second boundary surfaces, one of said frequency-dependent third boundary surfaces and the external surface of said material;
• and
• a solute concentration correlation means for determining the solute concentration of each of said sampled volume elements from a measured resistivity of each sampled volume element, said resistivity of each volume element being derived from said applied current and said measured voltage at one of said plurality of frequencies;
• a heat treating furnace having a gas composition controller and temperature controller;
• a supervisory controller for storing process parameters, receiving input signals from said gas composition and temperature controllers and said solute sensor probe, and generating reference signal outputs to said gas composition and temperature controllers;
• a process model algorithm for providing said supervisory controller with initial process parameters to generate initial reference signal outputs for said temperature and gas composition control;
• a rule-based control algorithm for controlling said supervisory controller by monitoring said sensor probe output, temperature and gas composition, and instructing said supervisory controller to modify said process parameters by adjusting reference signal output to said gas composition and temperature controllers;
• and
• a PID controller for receiving said supervisory controller reference signal output and controlling said furnace temperature and gas composition.

A method for measuring solute concentration profiles in a conductive material by sequestering and sampling a plurality of defined subsurface volume elements comprising the steps of:

• providing a solute sensor probe comprising:
• at least one first electrode for contacting a first portion of an external surface of said material, said first electrode defining a first boundary surface of each of said sampled volume elements;
• at least one second opposing electrode for contacting a second portion of said external surface, said second electrode in coaxial alignment with said first electrode, said second electrode defining a second boundary surface of each of said sampled volume elements;
• said first and second electrodes being adapted for receiving an alternating current source, said current source configured for providing a plurality of alternating current frequencies to said electrodes;
• said first and second electrodes being adapted for measurement of a plurality of voltage potentials established between said electrodes by application of said alternating current at said plurality of said frequencies;
• a plurality of third boundary surfaces of each of said sampled volume elements, each one of said third boundary surfaces positioned at a unique, frequency-dependent depth 6, below said external surface where
• δ
• =
• 1
• μ
• ⁢
• ⁢
• σ
• ⁢
• ⁢
• π
• ⁢
• ⁢
• f
• ,
• μ is the material magnetic permeability, σ is the material conductivity and fi is one of said alternating current frequencies, each one of said sampled volume elements being bounded by a region enclosed by said first and second boundary surfaces, one of said frequency-dependent third boundary surfaces and the external surface of said material;
• and
• a solute concentration correlation means for determining the solute concentration of each of said sampled volume elements from a measured resistivity of each sampled volume element, said resistivity of each volume element being derived from said applied current and said measured voltage at one of said plurality of frequencies;
• contacting said conductive material with said sensor probe;
• applying a first alternating current at a first frequency to said probe electrodes;
• measuring a first resistance at said first frequency;
• applying a second alternating current at a second frequency to said probe electrodes;
• measuring a second resistance at said second frequency;
• computing a first difference layer resistance for said first and second resistances;
• calculating a first difference layer resistivity for said difference layer;
• computing a depth for said first and second resistance measurements and for said first difference layer;
• calculating a first solute concentration from said computed first difference layer resistivity;
• assigning said solute concentration to said first difference layer computed depth;
• and incrementing said first and second current frequencies and repeating said applying a first current, said measuring a first resistance, said applying a second current, said measuring a second resistance, said computing a first difference layer resistance, said calculating a first difference layer resistivity, said computing a depth, said calculating a solute concentration, and said assigning said solute concentration steps until the calculated resistivity at adjacent difference layers are equal.