Patent Publication Number: US-2007095434-A1

Title: Long products, method of thermo-chemical treatment and apparatus

Description:
This patent application claims filing date of Provisional Patent No. 60/730,653 filed on Oct. 28, 2005.  
     A. Field of the Invention  
      Present invention is related to the field of long products and their thermo-chemical treatment, and more specifically to carburizing, nitriding, carbonitriding, boriding, and other similar processes of surface modification of continuous metal parts, such as wire, strip, sheet, and other long products.  
     B. BACKGROUND OF THE INVENTION  
      Long products, such as wire, strip, sheet, have dimension in one direction significantly exceeding dimensions in the other two directions. In many applications, stiffness and strength are critical properties of long products. A number of ways to control these properties have been developed, including controlling shape of the normal cross section to control stiffness and surface thermo-chemical treatment, specifically, strip carburizing and nitriding. Meanwhile, other methods of thermo-chemical surface treatment, such as, carbo-nitriding, and boriding have been widely used in processing of various metal parts. The main advantage provided by surface thermo-chemical treatment processes is related to their ability to produce a high hardness surface layer preserving viscous properties of the core material. In thermo-chemical surface treatment, a Mart is heated in a special environment, often referred to as an active media environment, containing atoms of carbon, nitrogen, their mix, atoms of boron, or other elements, to the temperature, at which diffusion processes accelerate to ensure acceptable process efficiency. After achieving diffusion of these atoms to a desirable depth, part is typically quenched and tempered.  
      Nitrding, carbo-nitriding, and boriding are typically performed in a liquid or gaseous environment, and carburizing can also be done in a solid medium. The process can be accomplished as a batch process or as a continuous process. While numerous technical solutions have been developed for batch process, continuous surface thermo-chemical treatment processes use typical approaches and equipment utilized for thermo-chemical processing of individual parts with comparable dimensions in all directions.  
      As an example, U.S. Pat. Nos. 5,192,485 and 6,074,493 teach a method of strip carburizing in a gaseous environment. They provide a furnace for continuous carburizing, in which a low carbon steel strip is heated to a temperature between temperature of pearlite transformation and 980 C. Similarly, U.S. Pat. No. 5,653,824 provides a strip carburizing method for continuous carburizing of a strip material by using a furnace through which strip is moving in a carburizing atmosphere. This furnace has a typical design for continuously heating strip with a quenching zone at the furnace exit and means to feed said strip in the furnace and accumulate strip on a take-up spool.  
      The main disadvantage of these methods is related to a relatively long processing time and expensive and complex equipment  
      U.S. Pat. No. 5,798,002 teaches a method of producing a surface layer selected from the group consisting of carbide containing surface layer and a carbon solid solution containing surface layer on a substrate selected from the group consisting of a metal and a metal containing an alloying element. The method comprised the following steps: providing a bath composed of a cold carbon-containing liquid active medium at ambient temperature; introducing heating means into the cold liquid active medium; immersing a substrate into the cold liquid active medium; heating the substrate directly by the heating means inside the liquid active medium to a corresponding processing temperature until the layer of a desired chemical composition and thickness is formed on the substrate. However, this method was developed primarily in application to processing individual parts, and not to long products, such as sheet or strip.  
      The present invention provides a solution eliminating these disadvantages of the prior art.  
     C. SUMMARY OF THE INVENTION  
      Present invention provides long products with desirable combination of stiffness, surface strength and ductility of the core, method of their surface thermo-chemical treatment allowing their efficient processing, and apparatus for its realization.  
      In one aspect, present invention provides long products with an outside layer having strength and corrosion resistance properties increased by thermo-chemical modification techniques, such as carburizing, nitriding, carbo-nitriding, and boriding. Stiffness of said long products is controlled by the shape and dimensions of cross section normal to their long axis. The cross sectional shape is formed by using roll forming or profiled die drawing operations during or right after their thermo-chemical treatment. Torsion operation can also be used to achieve final shape of the cross section before or after their surface thermo-chemical treatment.  
      In another aspect, present invention provides a method of thermo-chemical treatment comprising the following steps: 
          surface cleaning the processed material     placing material in an active media environment in a gaseous, liquid, solid form or in a mixture of these substances, i.e. gaseous-liquid, liquid-solid, gaseous-solid, or gaseous-liquid-solid form     locally heating the material at least in one region to a predetermined temperature     forming the final shape of cross section     moving material through the heater and shape forming tool processing all of the material or only selected regions     cooling the material with a predetermined rate        

      Acid solutions, such as hydrochloric acid, nitric acid, sulfuric acid or any other cleaning solutions and methods, including mechanical methods, are used to clean surface of processed material.  
      Material is exposed to active media environment. For example, carburizing is accomplished by placing material in a container with a carbonous gas, liquid, solid, or a mixture of these substances.  
      Material is locally heated to a predetermined temperature at least in one location. Heating temperature and holding time at that temperature depend on desirable depth of the processed layer.  
      In one implementation, material is heated to typical carburizing, nitriding, carbo-nitriding, or boriding temperatures that fall in a temperature interval from 560 to 950 C depending on type of surface treatment process and active media used.  
      In another implementation, material is heated to up to 500 C above typical temperatures, i.e. to temperature of up to 1450 C.  
      In still another implementation, multiple material areas are heated to desirable processing temperate.  
      Final material dimensions and shape are ensured by using metal forming operations.  
      In one implementation, material is formed to the final shape during surface thermo-chemical processing.  
      In another implementation, material is formed to the final shape after surface thermo-chemical processing is complete, but material is still hot, i.e. at temperature resulting in good material formability.  
      In still another implementation, meal is formed to the final shape after surface thermo-chemical processing is complete at ambient temperature.  
      Material is being fed into heating zone and spooled on a take up spool under controlled tension.  
      In one implementation, tension is maintained below material yield strength to avoid shape changing.  
      In another implementation, tension exceeds material yield strength to achieve desirable shape changing, for example, wire diameter reduction.  
      Cooling after surface thermo-chemical processing is done with a rate providing desirable final properties.  
      In one implementation, cooling is done with a rate exceeding critical quenching rate to a predetermined temperature at which material is tempered and then cooled to the ambient temperature.  
      In another implementation, cooling is done with a rate below critical quenching rate continuously to the ambient temperature.  
      In still another implementation, cooling is done according to a regime including cycles of heating and cooling.  
      In still another aspect, present invention provides an apparatus for thermo-chemical modification of long products. Said apparatus comprised the following blocks: means of feeding processed material into the container with active media, heating means, means of supporting processed material inside the heating zone, shape forming means, cooling means, means of applying pre-determined tension on the strip, and means of taking processed material out of the processing zone and storing it in a desirable way.  
      In one implementation, said apparatus allows simultaneous processing of more than one long product.  
      In another implementation, heating means include direct or indirect resistance heating, induction heating, plasma hag, laser heating, or a furnace used for metal heat treating. Such heating means as induction heating or direct resistance heating can be positioned inside or outside of the container with active media.  
      In still another implementation, container with an active media can be positioned inside another container with cooling means that can serve the purpose of keeping desirable temperature of active media and to cool processed material with a desirable cooling rate. In still another implementation, active media is circulated through the processing zone or agitated.  
      In another implementation, at least two different types of heating means are used. In another implementation, tension applied to the processed material is predetermined in such a way that said material is self supported.  
      In another implementation, means of material support include a conveyor or set of rollers.  
      In another implementation, shape forming means include shaped rolls, dies, or any other means of shaping material in a desirable way.  
      In another implementation, means of storing processed material can include devices for putting said material on a spool or cutting it to the pre-determined length, for example, cutting it in short fibers.  
      Present invention allows processing of long products manufactured of different metals: low carbon steel, high carbon steel, stainless steel titanium alloys, aluminum alloys, tungsten alloys, and any other metal showing improved strength, corrosion, impact strength, or any other targeted properties after thermo-chemical processing.  
      One set of characteristic features of the present invention differentiating it from prior art is related to the long products subjected to thermo-chemical processing.  
      In contrast to currently used long products subjected to surface modification processing limited to the strip, present invention broadens types of long products subjected to surface modification processing to other types, such as wire, cord, strip with a profiled cross section, or any other type of products with their stiffness controlled by dimension and shape of normal cross section.  
      In contrast to currently used long products subjected to surface modification processing, present invention provides said product with increased effective strength.  
      Another set of characteristic features of the present invention differentiating it from prior art is related to the method of thermo-chemical processing.  
      In contrast to standard methods of surface modification methods, present invention provides temperature regime of up to 500 C higher than temperatures used in standard methods.  
      In contrast to method disclosed in U.S. Pat. No. 5,798,002 that teaches various types of surface modification in liquid active media, current invention provides solutions utilizing gaseous, solid forms of active media, as well as mixtures of gas-liquid, liquid-solid, and gas-solid active media.  
      In contrast to prior art, in present invention, thermo-chemical processing is combined with shape forming operation that increases depth of the processed layer.  
      In contrast to prior art, present invention allows achieving increased ratios of depth of the processed layer to the smallest transverse dimension of the processed material. While the depth of the layer with modified structure and properties in prior art is typically only a small fraction of the part&#39;s cross sectional dimension, according to the present invention, it can be significantly larger reaching one in case of fully modified cross section. For instance, in case of a small diameter filament or thin strip and foil, the modified layer can be all through the cross section.  
      Still another set of characteristic features of the present invention differentiating it from prior art is related to the provided apparatus for thermo-chemical treatment.  
      In contrast to the existing solutions, present invention provides an apparatus capable of continuously processing more than one long product simultaneously.  
      In contrast to the existing solutions, means of continuously feeding the processed material with a predetermined tension and speed allow directionally grow microstructural components in the modified layer.  
      In contrast to the existing solutions, present a can include heating means of different types can be used simultaneously.  
      In contrast to the existing solutions, present apparatus includes shape forming means are provided.  
      In contrast to the existing solutions, present apparatus can include special supporting means, such as conveyor or set of rolls.  
      In contrast to the existing solutions, present apparatus can include product storing means allowing cutting processed product to the pre-determined length. 
    
    
     D. BRIEF DESCRIPTION OF DRAWINGS  
       FIG. 1A  is a schematic illustration of a set up for thermo-chemical processing and shape forming of a wire by using electric resistance heating and liquid active medium;  
       FIG. 1B  is a schematic illustration of a figure eight wire cross section and macrographs showing straight and twisted wire:  
       FIG. 2  is a schematic illustration of a set-up for thermo-chemical processing and shape forming of a wire by using induction heating in an active medium;  
       FIG. 3  is a schematic illustration of a set-up for thermo-chemical processing using cooled tank with active media;  
       FIG. 4  demonstrates optical micrographs showing cross section and microstructure of a carburized strip with a profiled cross section;  
       FIG. 5  shows original grating strip and rolled and carbo-nitrided grating strip;  
       FIG. 6  shows carburized wire mesh;  
       FIG. 7  shows carbo-nitrided woven wire mesh, and  
       FIG. 8  shows different grades steel wool. 
    
    
     E. DETAILED DESCRIPTION OF THE INVENTION  
      Present invention provides long products with desirable combination of stiffness, surface strength and ductility of the core, method of their surface thermo-chemical treatment allowing their efficient processing, and apparatus for its realization.  
      Desirable stiffness properties of long products are achieved by controlling shape and dimensions of the normal cross section, while strength and ductility are controlled by utilizing their thermo-chemical treatment.  
      In one aspect, present invention provides long products with an outside layer having strength and corrosion resistance properties increased by thermo-chemical treatment methods, such as carburizing, nitriding, carbo-nitriding, and boriding.  
      In one implementation, shape of the normal cross section is produced by using roll forming operation or drawing through profiled dies during or after thermo-chemical treatment.  
      In another implementation, final shape of the normal cross section is produced by using torsion operation around the long axis before surface thermo-chemical processing or after it is complete.  
      In another aspect, present invention provides a method of thermo-chemical treatment of long products comprising the following steps: 
          surface cleaning the processed material     placing material in an active media environment in a gaseous, liquid, solid form or in a mixture of these substances, i.e. gaseous-liquid, liquid-solid, gaseous-solid, or gaseous-liquid-solid form     locally heating the material at least in one region to a predetermined temperature     forming the final shape of cross section     moving material through the heater and shape forming tool processing all of the material or only selected regions     cooling the material with a predetermined rate        

      Acid solutions, such as hydrochloric acid, nitric acid, sulfuric acid, or any other cleaning solutions or methods, including but not limited to mechanical de-scaling, brush rust removal, are used to clean surface of processed material. The material can have shape of wire, multiple wires arranged in a twisted bundle or in parallel, with circular, square, hexagonal, or any other shape of cross section.  
      Material is exposed to active media environment in a gaseous, liquid, solid form, or in a mixture of these substances, i.e. gaseous-liquid, liquid-solid, or gaseous-solid form. For example, carburizing is accomplished by exposing material to a carbonous gas, liquid, solid, or mixture of these substances.  
      In one implementation, material is placed in the container with an active media substance. For instance, in carburizing, said material is placed into container with a carbonous gas, such as methane, carbonous liquid, such as carburizing oil, solid, such as graphite powder, or mixture of these substances. Examples of mixtures of gaseous liquid, liquid-solid, gaseous-solid, or gaseous-liquid-solid forms of carburizing media include, but are not limited to mixtures of carburizing oil with vaporized oil or methane gas, mixtures of carburized oil with graphite powder or carbon based grease, mixtures of graphite powder with methane, similar to fluidized furnace media, and mixture of methane, carburizing oil, and graphite powder, respectively.  
      In another implementation, active media is continuously delivered to the material during processing. For instance, liquid carburizing solutions are supplied to the heated region of material as a continuous stream. Similarly, carburizing gas is supplied to the heated portion of the material. In a particular case, this process can be combined with a welding processes to improve mechanical properties of the weld.  
      The mixture of active media agents can be prepared to different ratios of components. In the case of mixing liquid and solid components, their ratio can be from 10% to 90%, preferably in the range typical for grease containing corresponding active media agents, such as graphite and carbon based grease or boron based grease. The mixture can be applied to the material before processing. Such a coating can also be dried out before processing. In the case of mixed gaseous and liquid active media, ratio of components is controlled to achieve uniform material microstructure and properties. For example, carburizing gas can be delivered to the processed material submerged into the liquid, or in a form of vaporized carbonous liquid with a predetermined vapor pressure. Container capable of withstanding high pressures is used in the case when vapor pressure significantly exceeds ambient pressure. Such a high carburizing pressure can facilitate carbon diffusion into processed material.  
      Both in the case of thermo-chemical treatment of material in a container with a stationary active media or in case of active media continuously supplied to the heated region, used means of delivering active media, exposing material to it, and evacuating active media are designed to accommodate material movement through the container. For instance, m the case of gaseous carburizing, endo-gas, comprised 40% hydrogen, 40% nitrogen, and 20% carbon monoxide mixture is delivered to the processing zone from two opposite ends of a processing unit to reduce loss of endo-gas.  
      Material is locally heated to a predetermined temperature at least in one location. Heating temperature and holding time at dig temperature depend on desirable depth of the processed layer.  
      In one implementation, material is heated to typical carburizing, nitriding, carbo-nitriding, or boriding temperatures that fall in a temperature interval from 560 to 950 C depending on type of surface treatment process and active media used.  
      Heating can be accomplished by using any kind of furnace, direct electric resistance heating, induction heating with the inductor placed in the container or positioned outside the container, laser heating, or plasma heating. In case of using induction heating, inductor can operate on more than one frequency to achieve desirable carburizing depth. The induction coil can be placed inside the container with active media or outside said container. When wire fed through induction coil placed inside the container with active media, special means of avoiding contact between processed material and said induction coil are used. These can include ceramic tube or providing sufficient tension of the processed material to prevent its bowing and sagging during heating.  
      In another implementation, material is heated to up to 500 C above typical temperatures, i.e. to of up to 1450 C. To avoid extensive austenite grain growth, heating is accomplished by means of intensive heating, such as direct electric resistance heating, induction heating with the inductor placed in the container or positioned outside the container, laser heating, or plasma heating.  
      In still another implementation, multiple material areas are heated to processing temperatures. This is accomplished by moving the material through a set of heating means heated to predetermined temperatures.  
      Final material dimensions and shape are ensured by using metal forming operations.  
      In one implementation, material is formed to the final shape during material thermo-chemical processing. Heated to the processing temperature, material is shaped by using shape forming means, such as shaped rolls or dies. In case of electric resistance healing, contact electrodes can have shape or shape forming inserts that ensure final material shape. As an example,  FIG. 1A  shows a set-up for carburizing and shaping material into a wire with figure eight cross section by using grooved roller electrodes and electric resistance heating in a carbonous environment ( FIG. 1B ). The material moving between the electrodes is heated by passing electric current through it and is shaped at the same time. In this specific example, material has a figure eight shape. Plastic deformation occurring during final shape forming facilitates thermo-chemical treatment increasing penetration depth of carbon, nitrogen, or boron atoms. Such a shape can be further subjected to plastic deformation, for instance, twisting. In another example, material is heated by using induction heating and then shaped by using profiled rolls or die ( FIG. 2 ). In another implementation, material is formed to the final shape after thermo-chemical treatment process is complete, but material is still hot, i.e. at temperature resulting in good material formability. Using profiled rolls or a die allows one achieving desirable final shape while material is still hot without exposing shape forming means to high temperature and active media.  
      In still another implementation, material is formed to the final shape at ambient temperature after thermo-chemical treatment process is complete. This operation is similar to coining and is used when high accuracy of final dimension is required.  
      Material is being fed into processing zone and spooled on a take up spool under controlled tension.  
      In one implementation, tension is maintained below material yield strength to avoid shape changing. Material is fed into processing chamber by using wire or strip feeding-like equipment utilizing pairs of rotating grooved rolls and spooled on a take-up spool. Feeding speed and speed of winding on a take-up spool are maintained to avoid plastic deformation, for instance reduction in material cross section size.  
      In another implementation, tension exceeds material yield strength to achieve desirable shape changing, for example, wire diameter reduction. If material winding speed on a take-up spool exceeds feeding speed, changes in material size will occur in areas of local heating. For example, in case of a circular cross section wire, this will result in die-less forming, i.e. diameter reduction without using shape forming die. Increased material strength due to thermo-chemical treatment can facilitate stability of material flow in die-less process. For example, in case of carburizing, low carbon wire entering heated zone has a larger cross section than that of carburized portion exiting the heated zone. Normally in die-less drawing, such difference in wire diameters results in localized material flow and wire break in the heated zone. However, increased strength of carburized wire can prevent such plastic flow localization making process more stable. In still another implementation, wire winding is done with a controlled speed to form desirable morphology of microstructural components formed after thermo-chemical treatment. For instance in carburizing, slow winding speed in combination with small cross sectional dimensions of the processed material can result in directional growing of cementite to achieve crystal whisker-like properties, i.e. strength close to that of ideal crystal in the range of from 8,000 to 10,000 MPa. Speed of moving material through the carburizing medium can also be controlled to form high strength carbon-based surface deposits, such as carbon nano-tubes. In this case, catalytic elements can be used to serve as carbon nano-tube nucleation points or to facilitate their growth.  
      Cooling after thermo-chemical processing is done with a rate providing desirable final properties.  
      In one implementation, cooling is done with a rate exceeding critical quenching rate to a predetermined temperature at which material is tempered and then cooled to the ambient temperature. To achieve martensitic structure resulting in high strength properties, material is quenched in the medium used for thermal-chemical treatment, if this medium is a liquid, outside the heating zone. If medium used for thermal-chemical treatment does not provide cooling rate sufficiently high to achieve desirable strength properties, as, for example, in case of gaseous or solid carburizing, additional cooling means can be used, for instance by using intensively circulated gas.  
      In another implementation, cooling is done with a rate below critical quenching rate continuously to the ambient temperature. It can be accomplished in one of the local heated areas, in oil or in air depending on the material chemical content.  
      In still another implementation, cooling is done according to a regime including cycles of heating and cooling. In case when post quenching annealing/tempering is to be done more than once, for example in high speed steel, material can be thermo-cycled to achieve desirable combination of strength and ductility.  
      In still another aspect, present invention provides an apparatus for thermo-chemical modification of long products. Said apparatus comprised the following blocks: means of feeding processed material into the container with active media, heating means, means of supporting processed material inside the heating zone, shape forming means, cooling means, means of applying pre-determined tension on the strip, and means of taking processed material out of the processing zone and storing it in a desirable way.  
      In one implementation, said apparatus allows simultaneous processing of more than one long product.  
      In another implementation, heating means include direct or indirect resistance heating, induction heating, plasma heating, laser heating or a furnace used for metal heat treating.  
      Such heating means as induction heating or direct resistance heating can be positioned inside or outside of the container with active media.  
      In still another implementation, container with an active media can be positioned inside another container with cooling means that can serve the purpose of keeping desirable temperature of active media and to cool processed material with a desirable cooling rate ( FIG. 3 ).  
      In still another implementation, apparatus allows circulating active media through the processing zone or to agitate it. For example, in the case of gaseous carburizing, gas is circulated through the processing zone keeping composition of gas constant. Similarly, in the case of liquid carburizing, carbonous liquid is circulated through the processing tank to keep its composition constant.  
      In another implementation, at least two different types of heating means are used.  
      In another implementation, tension applied to the pressed material is predetermined in such a way that said material is self supported.  
      In another implementation, means of material support include a conveyor or set of rollers.  
      In another implementation, shape forming means include shaped rolls, dies, or any other means of shaping material in a desirable way.  
      In another implementation, means of storing processed material can include devices for putting said material on a spool or cutting it to the pre-determined length, for example, cutting it in short fibers.  
      While continuous thermo-chemical treatment is a preferable method of processing long products as discussed above, it also can be accomplished as a batch processes. Material in a form of a coil, such as wire coil, strip coil, or coil of any other type of long product is placed in an active media environment and heated by using furnace or induction heating.  
      Present invention allows processing of long products manufactured of different metals: low carbon steel, high carbon steel, stainless steel, titanium alloys, aluminum alloys, tungsten alloys, and any other metal showing improved strength, corrosion, impact strength, or any other targeted properties after thermo-chemical processing. Depending on the material, surface layer in the chemically modified layer providing desirable properties can include titanium carbide, nitride, or boride layer or particles, aluminum nitride or boride layer or particles, tungsten carbide, nitride, or boride layer or particles, or any other types of hard layer or particles.  
      While specific processing temperatures and time depend on the material, similar to those of steel they exceed standard temperatures to ensure reduced time and incorporate fast heating. Specifically, in the case of titanium filament, heating is accomplished to the temperature above 950 C in a liquid carburizing medium, such as silicon oil. It can be used on long products and parts produced from long products, such as dental implants, stents, spokes, and other similar products.  
      In case of aluminum wire, surface oxide film is removed before desirable surface chemistry is achieved. For instance, a wire drawing die is used to expose fresh surface to produce a surface aluminum nitride layer. Heating temperate is between 450 and 620 C depending on alloy composition.  
      In case of tungsten long products, surface modification is accomplished at temperatures between 1400 C and 2500 C. It can also be used for processing such parts, as heavy armor penetrator or other similar parts to produce hard local areas.  
     F. EXAMPLES OF SPECIFIC IMPLEMENTATION  
     Example 1  
      Low carbon steel wire (1020 grade) with diameter of 1.5 mm was carburzed by using current process. Wire sample, 100 mm long, was clamped at the ends of two copper electrodes, and submerged in a silicon oil. The wire was heated by passing electric current through it to different temperatures, held at the temperature for a specified time, and then cooled down to ambient temperatures inside the processing tank or in the air. Various microstructures were achieved When wire was heated to temperature below 950 C, maximum carbon content at the surface was less than 1.2%, and corresponding microstructure was comprised pearlite and secondary cementite network. Structure of the wire core did not change and was comprised ferrite and pearlite. When wire was heated to temperature in the range from 1200 C and 1450 C, carbon concentration at the surface as high as close to 4.5% was achieved. Corresponding microstructure was comprised pearlite and primary cementite when cooling rate controlled by adjusting electric current to the ambient temperature was slow. In case of fast cooling, structure was comprised primary cementite and martensite. Successive annealing resulted in transformation of martensite into spheroidal pearlite.  
     Example 2  
      High carbon steel wire (1090 grade) with diameter of 0.5 mm was carburized by using current process. Wire sample was continuously fed through two copper electrodes submerged in a machine oil. The wire was heated by passing electric current through it to different temperatures. Wire feeding speed and take-up speed were synchronized. Various microstructures were achieved similar to the stationary heated wire. When wire was heated to temperature below 950 C, surface microstructure was comprised pearlite and secondary cementite network, while in the bulk was comprised ferrite and pearlite. When wire was heated to temperature in the range from 1200C and 1450 C, surface microstructure was comprised pearlite and primary cementite.  
     Example 3  
      A high carbon steel wire (1080 grade) with diameter of 0.3 mm was liquid nitrided by using present method. Wire sample was continuously fed through a nitriding salts bath. A thin compound layer was produced after treatment giving wire excellent corrosion resistance properties.  
     Example 4  
      A low carbon steel strip (1020 grade) with 4×0.4 mm 2  cross section dimension was rolled between two water cooled copper disc contacts with a profiled shape. Disc contact electrodes and strip were submerged into a machine oil tank. The strip was heated with electric current applied to the strip, and forming pressure was applied to the discs to achieve desirable shape of the strip cross section. After processing, a wire with a profile and microstructures shown in  FIG. 4  was obtained.  
     Example 5  
      A low carbon steel grading strip with thickness of 0.3 mm was obtained by using rolling ( FIG. 5 ). Said grading strip was carburized in machine oil by using induction heating. The strip was completely carburized through the whole cross section.  
     Example 6  
      A low carbon steel wire mesh was carbon-nitrided by using carbon-ntriding furnace process ( FIG. 6 ) and oil quenched. Said wire mesh showed significant brittleness in as-quenched condition. Successive tempering in temperature interval from 400 to 650 C resulted in increased material ductility.  
     Example 7  
      A low carbon two layer woven mesh with wire diameter of 0.25 mm ( FIG. 7 ) was carburied by using induction heating in a graphite-machine oil mix. As-carburized material showed significant brittleness. Successive normalizing at 850 C for 5 min. with cooling in air resulted in increased material ductility.  
     Example 8  
      Different grades of steel wool produced out of low carbon steel  FIG. 8 ) were carburized by using a 40% nitrogen gas, 40% hydrogen gas, and 20% carbon monoxide gas mixture and induction heating. Significant increse in strength characteristics with acceptable ductility was obtained after tempering at 650 C.