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;
• a plurality of third boundary surfaces generated by a plurality of alternating current frequencies applied to said first and second current electrodes, each one of said third boundary surfaces positioned at a unique, frequency-dependent depth δ below said external surface where
• 51 δ = 1 μ ⁢ ⁢ σ ⁢ ⁢ π ⁢ ⁢ f ,
• μ is the material magnetic permeability, σ is the material conductivity and f is one of said alternating current frequencies, each one of said third boundary surfaces at one of said frequencies positioned at said unique depth between said first and second boundary surfaces;
• and
• each one of said sample volume elements being bounded by a region enclosed by said first and second boundary surfaces, one of said third boundary surfaces and said external surface of said material.

A thermal processing system for control of a solute subsurface concentration profile at an external surface of a conductive material, said system comprising:

• a heat treating furnace having a gas composition controller and temperature controller;
• a solute sensor probe for sequestering and sampling a plurality of defined subsurface volume elements in a conductive material through application of a plurality of alternating current frequencies;
• a supervisory controller for storing process parameters, receiving input signals from said gas composition and temperature controllers and said 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 measurement of solute subsurface concentration profiles in a conductive material comprising the steps of:

• providing a solute sensor probe for sequestering and sampling a plurality of defined subsurface volume elements in a conductive material through application of a plurality of alternating current frequencies;
• contacting said conductive material with said sensor probe;
• introducing a first alternating current frequency to said probe;
• measuring a first resistance at a first frequency;
• applying a second alternating current frequency to said probe;
• determining a second resistance at said second frequency;
• computing a difference layer resistance and resistivity;
• detecting a first solute concentration from said computed first resistivity;
• calculating a depth for said first and second frequency measurements and a depth of said first difference layer;
• assigning said solute concentration to said first difference layer computed depth;
• and
• incrementing said first current frequency and repeating said introducing, said measuring, said applying, said determining, said computing, said detecting, said calculating and said assigning steps until the computed resistivity at adjacent difference layers are equal.