Abstract:
The invention relates to surface hardening of steel workpieces using laser beams and more particularly to laser hardening steel cutting rules. The method comprises applying a first laser beam of a first intensity and focused to a first focal point to a selected surface area of the metal workpart and subsequently applying a second laser beam having a second intensity and focused to a second focal point to the selected surface area. Application of the first laser beam heat treats the selected surface area to a predetermined depth, thereby increasing the surface hardness. Application of the second laser beam relieves internal stresses produced by the heat treating while retaining the increased hardness.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to surface hardening of steel workpieces. In particular, the present invention is a method of hardening selected surface areas of steel cutting instruments, such as cutting rules or knife blades, using laser beams to perform both surface hardening and stress relief of the workpiece.  
           [0003]    2. Description of the Related Art  
           [0004]    Typically, hardening of metals has been performed by carburizing, induction heating and, more recently, laser heat-treating. In conventional gas carburizing methods, a steel workpiece is heated in an atmosphere of a selected gas. Materials from the gas dissolve in the surface of the workpart becoming part of the crystalline structure. For example, a steel workpart is heated in an atmosphere of CO 2  causing minute amounts of carbon to be liberated on the surface of the hot metal and to dissolve in the metal. A subsequent heat treatment to form a martensitic microstructure on the surface produces a hard surface. A martensitic microstructure is formed by heating the steel above the critical temperature—the temperature at which the steel changes phases from a ferrite or cementite microstructure to an austenite microstructure—and rapidly cooling, or quenching, the steel to form a new microstructure phase, martensite. Martensite is the hardest of the steel microstructure phases.  
           [0005]    However, the rapid cooling required to produce martensite also induces internal stresses within the microstructure that make the martensite brittle. Therefore, a subsequent tempering process is required to relieve these internal stresses. Tempering typically entails heating the steel to a temperature below the critical temperature for several hours. Heating the steel below the critical temperature avoids inducing a microstructure phase change back to austenite, but also reduces some of the hardness of the martensite. The hardness reduction is the result of some of the carbon particles trapped in the martensite being released. Thus, the microstructure before tempering appears as untempered martensite and after tempering the microstructure appears as tempered martensite.  
           [0006]    Some drawbacks are present in surface hardening by carburizing. One such drawback is that it is difficult to surface harden only selected areas of the workpart. In order to only harden selected areas, those surfaces not to be hardened must be masked. The masking prevents those surfaces from being subjected to the gas atmosphere, thereby preventing hardening of the masked surface. The masking process is often difficult, time-consuming and unreliable due to the intense heat of the carburizing process. Another drawback of carburizing is controlling the depth of the hardened surface. Carburizing typically requires post-processing machining, such as grinding, in order to obtain the desired hardened case depth. Carburizing also requires an additional tempering process after the quenching process in order to stress relieve the part. Such a stress relief process typically entails placing the workpiece in an oven, often for a period of several hours. This significantly increases both the cost and the amount of time to process the workpiece.  
           [0007]    Another known method of surface hardening steel workparts is induction heating. In induction heating, the steel workpart is placed within an induction coil. An electrical current is passed through the induction coil which induces secondary currents to flow along the surface of the workpart. The secondary current flow causes the surface of the workpart to be preferentially heated. As the electrical current in the induction coil is increased, the surface of the workpart is heated above the critical temperature, thus causing a microstructure phase change to austenite. When the workpart is rapidly cooled, or quenched, a martensitic microstructure is formed. Thus, when only a shallow surface of the part is heated above the critical temperature and is rapidly quenched, only the shallow surface is transformed into a martensitic microstructure while the remainder of the part remains unchanged. This shallow surface of martensite forms the hard surface.  
           [0008]    However, the rapid cooling induces internal stresses that cause the steel part to become brittle. Therefore, a subsequent tempering process is required to relieve the internal stresses.  
           [0009]    Induction heating has some of the same drawbacks as carburizing. Namely, it is difficult to harden only selected surface areas and the steel workpart requires a post-hardening tempering process that is costly and time-consuming.  
           [0010]    Additionally, shallow hardened case-depths are difficult to achieve with induction hardening. Typically, the case depth is controlled during induction hardening by producing a higher frequency current in the induction coil. However, common induction heating machines present limitations on the highest frequency available. Common induction heating machines have a frequency limit of about 1 MHz. However, if a case depth of 0.004-0.006 was desired, an induction machine frequency of approximately 10 MHz would be required. Such a machine is costly and commonly only available in Europe.  
           [0011]    Induction heating has been the most common method of producing steel cutting rules. Steel cutting rules produced by induction heating generally provide good bendability properties, thereby allowing the rules to be formed into a number of shapes. However, induction heated rules generally have low durability properties, thereby requiring frequent replacement. Additionally, induction heated steel cutting rules require air or liquid quenching during the heat-treating process which causes thin rules to warp and further requires tempering to relieve internal stresses. The tempering process typically lowers the surface hardness previously obtained during the heat treating step. Therefore, common induction hardened rules are typically hardened to only about 55 R c .  
           [0012]    Another known method of surface hardening is laser heat-treatment. Various types of lasers are available for heat treating workpieces, including continuous wave CO 2  lasers. Laser heat treatment using a CO 2  laser typically entails applying an absorbent substance, such as black oxide or phosphate coatings, to the surface area of the part to be heated. This coating reduces reflection of the laser beam and focuses the energy of the laser beam to the area to be hardened. The laser beam is then focused, via a lens or the like, which generates an intense energy flux that rapidly heats the surface.  
           [0013]    One distinct advantage of laser heat treatment is that the laser beam may be controlled to heat the surface of the metal piece above the critical temperature to a depth of only a few thousandths of an inch or less. Controlling the depth of the heating to this shallow level allows for self quenching. That is, no liquid or air quenching is required. Self-quenching is accomplished by conduction due to the mass and temperature disparity between the portion of the workpart not heated by the beam and the small depth of the surface heated above the critical temperature by the beam. The heat on the surface is quickly transferred to the unheated portion thereby quenching the heated surface. However, the self-quenching process has been taught to be undesirable for thin parts such as knife blades and therefore air or liquid quenching has been particularly advisable. Air or liquid quenching is required due to the insufficient mass of the part to facilitate the conduction. The addition of such air or liquid quenching increases both the cost and the processing time.  
           [0014]    One such method of laser-treating steel workparts is disclosed in U.S. Pat. No. 4,304,978. This patent teaches laser heat treating a flat part, such as a knife or blade, by focusing a laser beam perpendicular to the major flat surface of the part using a cylindrical lens. The width of the beam is adjusted according to the desired width of the part to be heated. The part is then moved through the laser or the laser may be moved along the part to heat the surface. U.S. Pat. No. 4,304,978 teaches that thin parts, such as a knife blade, requires gas quenching to prevent melting of the part. Therefore, one shortcoming of U.S. Pat. No. 4,304,978 is that the laser treated part, such as a knife blade, is not self quenching.  
           [0015]    Therefore, it is desirable to provide a method of hardening a steel cutting rule or knife blade so as to obtain equivalent or superior ductility properties as common induction heated rules, but with superior wear resistance. It is also desirable that the method provide for self quenching of the cutting rule or knife blade to reduce processing time and cost.  
           [0016]    Further, it is desirable to provide a method of stress relieving the heat treated cutting rule that reduces the processing time and cost without weakening the metal part.  
         SUMMARY OF THE INVENTION  
         [0017]    The present invention addresses the foregoing shortcomings of conventional steel hardening techniques by providing a method of surface hardening metal workparts while maintaining the untempered martensitic microstructure and relieving internal stresses, thereby removing brittleness usually characterized with untempered martensite but maintaining the hardness. Additionally, the present invention provides self-quenching of thin workparts, such as cutting rules or knife blades. The present invention accomplishes the above while also producing hardened cutting rules with comparable ductility properties to that of current cutting rules, but with superior durability properties.  
           [0018]    The present invention accomplishes the foregoing by providing a process of surface hardening metal workparts by heat treating and stress relieving the parts using laser beams. The process entails first heat treating the parts using a narrowly-focused laser beam and subsequently stress relieving the parts using a laser beam of a lower intensity.  
           [0019]    The heat treating process is controlled by adjusting the laser beam intensity in order to obtain a desired case depth, preferably a shallow case depth. The process does not require the parts to be air or liquid quenched since the process results in self-quenching of the parts.  
           [0020]    Subsequent to the heat treating process a stress relief process is performed. The stress relief process consists of subjecting the part to the laser beam a second time, usually at a lower intensity than that used in the heat treating process. The stress relief process is controlled so as to only perform stress relief and not to temper the microstructure of the parts. The resultant microstructure after stress relief appears as untempered martensite but without the brittleness usually accompanying untempered martensite.  
           [0021]    In one aspect of the invention, metal workparts are surface hardened using laser beams to perform both heat treatment and stress relief of the part. Prior to heat treating, a laser beam is configured to obtain the desired hardness results. After configuring the laser beam, a metal workpart is subjected to the laser beam to perform the heat treatment process. The workpart is preferably passed through the laser beam; however, the laser beam may be traversed across the workpart surface. The heat treating process is performed such that the parts are self-quenching. That is, no air or liquid quenching is required. The heat treating process forms a hard martensitic layer having a microstructure of untempered martensite. Internal stresses created in the untempered martensite layer make the untempered martensitic layer brittle, thereby requiring stress relief.  
           [0022]    Subsequent to the heat treating process the workpart is stress relieved by being subjected to a laser beam a second time. The laser beam is reconfigured to obtain the desired results for performing stress relief. The workpart is then subjected to the laser beam for stress relief either by passing through the laser beam or by the laser beam traversing the surface of the part. The resultant microstructure after stress relief appears as untempered martensite. However, the internal stresses have been relieved. Therefore, the hardness of the martensitic layer has been retained but the brittleness has been eliminated.  
           [0023]    In another aspect of the invention thin workparts such as steel cutting rules or knife blades are surface hardened. The process entails first heat treating and subsequently stress relieving the cutting rule. Prior to the heat treating process, a laser beam is configured to obtain the desired hardness results. During the heat treating process the cutting rule is fed through the laser beam vertically, in an upright position, such that only the cutting tip of the cutting rule is subjected to the laser beam for hardening. The tip of the cutting rule is hardened by the laser beam to form a shallow hardened case of only a few thousandths of an inch.  
           [0024]    The cutting rule is subsequently stress relieved by being subjected to the laser beam a second time. The laser is reconfigured to obtain the desired results for performing stress relief. The cutting rule is then passed through the laser beam, thereby performing the stress relief. The microstructure of the hardened surface after heat treatment but before stress relief appears as untempered martensite. After being subjected to stress relief, the microstructure maintains its appearance as untempered martensite. However, the internal stresses have been relieved, thereby eliminating brittleness.  
           [0025]    The process may provide for additional steps such as cleaning the cutting rule prior to the heat treatment process and application of a corrosion inhibitor after the stress relief process. Further, an additional step of applying a laser beam absorbent substance to the surface area to be heat treated may be required depending on the type of laser being used. For example, a continuous wave CO 2  laser beam would require a laser beam absorbent substance, whereas a YAG laser would not require application of the laser beam absorbent substance.  
           [0026]    The resultant laser hardened cutting rule performs with the bendability properties of known cutting rules. However, the durability, wear-resistance, characteristics are greater than commonly known cutting rules.  
           [0027]    This brief summary has been provided so that the nature of the invention may be understood quickly. More complete understanding of the invention may be obtained by reference to the following detailed description of the preferred embodiments thereof in connection with the attached drawings.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0028]    [0028]FIG. 1 is a top perspective view of a laser hardening process according to the present invention.  
         [0029]    [0029]FIG. 2 is a side perspective view of a laser hardening process according to the present invention.  
         [0030]    [0030]FIG. 3 is an enlarged view of a central laser processing station.  
         [0031]    [0031]FIG. 4A is an enlarged side view of the laser beam-cutting rule interface.  
         [0032]    [0032]FIG. 4B is an enlarged front view of the laser beam-cutting rule interface.  
         [0033]    [0033]FIG. 5 is an enlarged view of the interface shown in FIG. 4B.  
         [0034]    [0034]FIG. 6A is a top view of a typical steel rule spring coil.  
         [0035]    [0035]FIG. 6B is a sectional view of a typical steel rule spring coil.  
         [0036]    [0036]FIG. 6C is a sectional view of a typical cutting rule after having a beveled edge machined on one side.  
         [0037]    [0037]FIG. 7 is a flow diagram for a laser hardening process.  
         [0038]    [0038]FIG. 8 is a flow diagram for a laser hardening process.  
         [0039]    [0039]FIG. 9A is a photograph of a cross-section of a steel cutting rule before heat treating.  
         [0040]    [0040]FIG. 9B is a photograph of a cross-section of a steel cutting rule grain microstructure after laser heating but before stress relief.  
         [0041]    [0041]FIG. 10 is a photograph of a cross-section of a steel cutting rule grain microstructure after being both laser heat treated and stress relieved.  
         [0042]    [0042]FIG. 11 is a photograph of a cross-section of a steel cutting rule after being surface hardened by induction heating.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0043]    Referring now to the drawings, a detailed description of the preferred embodiments according to the present invention will be described.  
         [0044]    [0044]FIG. 1 is a top perspective view and FIG. 2 is a side perspective view of a laser hardening process setup according to one aspect of the present invention. As seen in FIG. 1 and FIG. 2, a laser heat-treating process setup for laser hardening a steel cutting rule contains a central laser processing station  1 , a start/finish station  2  and an intermediate takeup station  3 . Located between start/finish station  2  and central processing station  1  are cleaning station  4 , laser absorbent applying station  5 , laser absorbent drying station  6 , and corrosion inhibitor applying station  7 . Located along a path connecting start/finish station  2 , cleaning station  4 , laser absorbent applying station  5 , drying station  6 , central laser processing station  1  and takeup station  3  are a means for guiding steel cutting rule  14  through the various processing stations, such as guide rollers  11 .  
         [0045]    Start/finish station  2  and takeup station  3  provide a means for retaining cutting rule  14  during the laser hardening process. The function of stations  2  and  3  is to retain cutting rule  14 , whatever the form, during the laser hardening process and to maintain tension in the cutting rule between stations  2  and  3  throughout the process. In the present embodiment, cutting rule  14  is in the form of a spring coil  16 , therefore, a means compatible for retaining spring coil  16  will be described. In the present embodiment, start/finish station  2  and takeup station  3  preferably contain a spindle  12  and drive motor  13  connected via a shaft or the like. Spindle  12  contains a means for winding and unwinding spring coil  16  such as slot  15 . Drive motors  13  are preferably common electrically driven motors. Drive motors  13  do not drive cutting rule  14  during the laser hardening process. Rather drive motors  13  maintain tension in cutting rule  14  between stations  2  and  3  during the laser hardening process by each applying an opposing rotational force to their respective spindles. Drive motors  13  are preferably controlled by a common programmable linear controller (PLC)  33 .  
         [0046]    Cleaning station  4  provides a means for cleaning cutting rule  14  at the initial stages of the laser hardening process. Cleaning is sometimes necessary to remove dust and dirt particles that may cause defects in the hardened surface. In the present embodiment, cleaning station  4  preferably contains cleaning pads  17  connected to actuating device  18 . Cleaning pads  17  are preferably covered with a soft cloth material  19 , such as cheesecloth or the like. Actuating device  18  preferably actuates in a vise-like manner applying a clamping force between cleaning pads  17 .  
         [0047]    Laser absorbent applying station  5  provides a means for applying a laser beam absorbent substance, such as a water-soluble ink or black oxide, to a selected surface area of cutting rule  14 . In the present embodiment, laser absorbent applying station  5  preferably contains an applicator  20 , such as a roll-on applicator, an ink reservoir  21 , and a means for supplying the ink from ink reservoir  21  to applicator  20 , such as tube  22 . Ink reservoir  21  preferably contains a water soluble ink  23  and is preferably pressurized.  
         [0048]    Drying station  6  provides a means for drying the laser absorbent substance applied in station  5 . In the present embodiment, drying station  6  is preferably comprised of a series of air nozzles  24 . Air nozzles  24  supply compressed air  25  for drying of the ink applied in the laser absorbent applying station  5 .  
         [0049]    Corrosion inhibitor applying station  7  provides a means for applying a corrosion inhibiting substance  27 , such as a rust preventative oil, to the surface of cutting rule  14 . In the present embodiment cleaning station  4  is reconfigured as corrosion inhibitor applying station  7  after the laser hardening process and before the stress relief process. In station  7 , cleaning pads  17  are removed and replaced by oil applying pads  26 . Pads  26  are soaked in a corrosion preventative oil such that when cutting rule  14  passes between pads  26 , the oil is wiped onto the surface of cutting rule  14 . Station  7  also preferably contains an oil drip system  37  for supplying additional oil to pads  26  during the process. Drip system  37  preferably contains a pressurized oil reservoir pressurized to cause oil to drip from an outlet in the reservoir onto pads  26 .  
         [0050]    Central laser processing station  1  provides a means for laser heat treating and laser stress relieving cutting rule  14 . In the present embodiment, central laser processing station  1  preferably comprises a laser beam producing device  8 , a laser beam focusing device  9 , and drive motors  10   a  and  10   b . Laser beam producing device  8  is preferably a 1,000 watt continuous wave CO 2  laser beam producing mechanism. As a substitute for a continuous wave CO 2  laser, a YAG laser may be used or any other type of laser that reaches a level of at least 500 watts continuous wave may be used. Use of a YAG laser eliminates the need for application of the water-soluble ink solution adding station, i.e. stations  5  and  6 . However, a YAG laser may create a safety hazard, requiring special equipment not necessary for the use of a continuous wave CO 2  laser. Central laser processing station  1  may also contain a means for supplying an assist gas for facilitating the laser heating process, such as assist gas nozzle  34 . Assist gas nozzle  34  may provide a gas such as nitrogen to the interface of the laser beam and the surface of the cutting rule being hardened in order to facilitate the hardening process.  
         [0051]    Laser beam focusing device  9  preferably comprises an optical device  30  and an adjustable height optical device support  31 . It has been found that when optical device  30  is a plano/convex lens, optimum laser hardening results are achieved. Optical support  31  preferably contains a linear translation mechanism  29  that provides controlled vertical translation of optic  30 . Vertical translation of optic  30  provides a means for controlling the focal point of laser beam  28  which will be described in more detail below. Translation mechanism  29  is preferably a shaft having a sliding frictional lock collar or other similar arrangement. Translation mechanism  29  also preferably contains a means for measuring the translation, such as a micrometer. Translation mechanism  29  may also be a motorized translation device, such as a ball screw actuator, and may also be computer controlled.  
         [0052]    Drive motors  10   a  and  10   b  provide rotational power to drive wheels  32 . Drive wheels  32  provide a frictional force for feeding cutting rule  14  through the laser hardening process stations. Drive wheels  32  are preferably made of a substance such as rubber. Drive motors  10   a  and  10   b  are preferably common electrically driven motors synchronously controlled by a common programmable linear controller  33 . Controller  33  provides a proper feed rate for performing both the heat treating and stress relief processes on cutting rule  14 .  
         [0053]    Having obtained the processing stations setup according to the foregoing, a description will now be made of the laser hardening process for laser hardening a steel cutting rule. Prior to performing the laser hardening process, central laser processing station  1  is configured to obtain the desired laser hardening results, and the steel cutting rule is prepared and installed in the laser processing setup.  
         [0054]    Referring now to FIGS. 3, 4 a ,  4   b , and  5  a detailed description will be made of the laser beam settings and adjustment according to one aspect of the present invention.  
         [0055]    As seen in FIG. 3, central laser processing station  1  comprises a laser beam producing device  8  and laser beam focusing device  9 . Laser beam producing device  8  is preferably a 1,000 watt continuous wave CO 2  laser that produces a D-mode laser beam  35 . Laser beam focusing device  9  comprises optic  30  and adjustable height optic support  31 . Optic  30  is preferably a 1½ inch diameter, 5 inch focal length plano/convex optic and is connected to optic support  31 . Optic support  31  preferably contains a linear translation mechanism  29  that provides a means for focusing laser beam  28 . In the present embodiment, translation mechanism  29  preferably contains a shaft and sliding collar having a frictional lock and a means for measuring the translation, such as a micrometer. Upon actuation of translation mechanism  29 , optic  30  translates vertically along a Z axis, thereby providing for adjustment of the focal point of laser beam  28 . The focal point reference origin O is preferably the cutting edge surface of cutting rule  14 . Utilizing cutting surface O as a reference, a focal distance F may be obtained.  
         [0056]    The laser beam power setting and focal point are first established for the heat treating process. The power settings for the heat treating process of the present embodiment preferably comprise a laser beam power setting of between 500 to 550 watts. The 50 watt range is used as a variable for adjusting the hardened surface case depth. It should be noted that there is an almost limitless number of options available to obtain a desired hardness result. For example, the laser power setting and focal point may each be independently varied to obtain a desired hardened surface case depth. Additionally, the rate at which the workpart is fed through the laser beam may also be varied in order to obtain a desired result. Therefore, the laser power settings, focal point, and feed rates described herein have been found to produce the optimum results for the present invention. As seen in FIG. 4A, the laser beam focal point referenced from surface O is optimally 0.010 inch as designated by dimension F. For the heat treating process the laser beam dimensions L and W, as seen in FIGS. 4A and 5, are approximately 1¼ inch and 0.010 to 0.012 inch, respectively. The resultant beam has a substantially parabolic shape as denoted by P in FIG. 4A.  
         [0057]    After having obtained the laser hardening process configuration and laser beam adjustments according to the foregoing, a steel cutting rule raw material is prepared for the laser hardening process. As seen in FIGS. 6A and 6B, the steel cutting rule raw material commonly comes in a spring coil  16  form. The cutting rule raw material commonly has a rectangular cross-section, as seen in FIG. 6B. The preferred dimensions of the steel cutting rule raw material according to the present embodiment are a thickness T of 0.021 to 0.042 inches and a height H of two inches or less. However, thicknesses up to 0.084 inch may also be used. The preferred material for the cutting rule according to the present embodiment is AISI 1050 spring steel. The preferred body hardness of the raw material is 33-35 R c  and has a grain structure consisting mainly of tempered martensite with as much as 10-15% bianite. However, other material types and sizes may also be utilized. As seen in FIG. 6C, a beveled edge  36  is machined on one side of the steel cutting rule raw material. Beveled edge  36  may be machined by common methods such as grinding or forming. Having machined beveled edge  36 , steel cutting rule coil  16  is now ready for installation in the laser hardening process system.  
         [0058]    As seen in FIG. 1, steel cutting rule coil  16  is installed in start/finish station  2 . Steel cutting rule coil  16  is installed on spindle  12  with free end  37  on the innermost portion of coil  16  installed in slot  15  on spindle  12 . The outermost free end  38  of spring coil  16  is fed through the various processing stations and into slot  15  of spindle  12  in intermediate takeup station  3 . The portion of steel cutting rule  14  initially fed through the processing stations is not subjected to the laser hardening process. Rather, it is excess material, known as lead, to be discarded after the laser hardening process.  
         [0059]    Upon commencing the laser hardening process, steel cutting rule  14  is fed through the laser hardening process by drive motor  10   a  with drive motor  10   b  being idle. The speed of drive motor  10   a  is controlled by programmable linear controller  33  and is preferably set to a feed rate of about 125 feet per minute. It has been found that a feed rate of 125 ft./min. coupled with the previously described laser beam settings of 500-550 watts with a 0.010 focal distance from origin O provide the optimum laser hardening results. However, as previously described, the feed rate may be varied according to a desired hardness result. Drive motors  13  in stations  2  and  3  are also controlled by controller  33 . Drive motors  13  apply opposing rotational forces to their respective spindles  12  to maintain tension in cutting rule  14  during the laser hardening process.  
         [0060]    The first step of the laser hardening process is to clean the cutting rule in cleaning station  4 . In the present embodiment of the invention, steel cutting rule  14  passes between cleaning pads  17  in cleaning station  4 . Actuating device  18  supplies a clamping force between pads  17  sufficient to supply wiping of steel cutting rule  14  but not excessive such as to cause binding of steel cutting rule  14 . Steel cutting rule  14  is wiped clean by cloth  19  attached to cleaning pads  17 . Although described in terms of the present embodiment, alternate embodiments for cleaning station  4  may be used. For example, cutting rule  14  may be cleaned by air curtains or a spray nozzle which dispenses a cleaning solution rather than being wiped by cloth  19 . After passing through cleaning station  4  steel cutting rule  14  next passes through laser absorbent applying station  5 .  
         [0061]    In laser absorbent applying station  5 , a laser beam absorbent substance such as a water-soluble ink or black oxide is applied to a selected surface area of steel cutting rule  14 . In the present embodiment of the invention, a water-soluble ink solution is applied to cutting edge O of steel cutting rule  14 . The ink solution is applied by an applicator  20 , such as a roll-on or drip applicator. Applicator  20  is connected to an ink reservoir  21  containing a water-soluble ink  23 . Ink reservoir  21  is preferably pressurized by an external pressure source, such as compressed air, to a pressure of approximately 5 psi (pounds per square inch). Pressurization of ink reservoir  21  is preferably sufficient to cause the water-soluble ink  23  to flow to applicator  20  at a desired rate in order to effect optimum application of the water-soluble ink solution  23  to the selected surface of cutting rule  14 . Pressurization of ink reservoir  21  is preferably controlled by programmable linear controller  33 . As steel cutting rule  14  passes through applying station  5 , water-soluble ink solution  23  is applied by applicator  20  to the selected surface area of steel cutting rule  14  to be hardened. Although described in terms of the present embodiment, alternate laser absorbent materials and application methods may also be used. For example, a black oxide or phosphate coating may be applied rather than ink. Additionally, the laser absorbent material may be applied by an alternate applying means such as a spray nozzle. After application of the water-soluble ink solution, steel cutting rule  14  next passes through drying station  6 .  
         [0062]    Drying station  6  contains a means for drying the laser absorbent substance applied in station  5 . In the present embodiment of the invention, drying station  6  preferably contains a series of air curtains  25 . Air curtains  25  preferably comprise compressed air supplied by a series of air nozzles  24 . The air pressure supplied to nozzles  24  is preferably regulated to approximately 80 psi and is preferably controlled by programmable linear controller  33 . The air pressure supplied by nozzles  24  is preferably sufficient to dry water-soluble ink solution  23  but insufficient to cause removal of the ink solution from the surface. Alternate methods of drying the laser absorbent substance may also be employed. For example, a heat source may be applied to the laser absorbent substance in order to dry it. After passing through drying station  6 , steel cutting rule  14  next passes through central laser processing station  1 .  
         [0063]    Having obtained the laser power settings of 500-550 watts and focal point of 0.010 inch from cutting edge surface O according to the foregoing description, steel cutting rule  14  is heat treated by passing steel cutting rule  14  beneath laser beam  28 . The laser beam intensity at the cutting rule surface is sufficient to cause a shallow depth of the cutting rule surface to be heated above the transformation temperature, thereby changing the phase of the steel to austenite. After passing through laser beam  28  and being transformed to austenite, the shallow surface area is rapidly cooled by self-quenching, thereby transforming the steel phase to martensite. The resulting martensite layer formed by the laser heat treating process preferably has a hardness of at least 60 R c  and a case depth of about 0.004 to 0.006 inches. A shallow depth of 0.004 to 0.006 inch has been found to provide optimum surface hardness and ductility properties. However, case depths between 0.001 to 0.010 may be obtained by varying the laser power settings, focal point and feed rate. The hardened surface provides increased wear resistance, thereby increasing the longevity of cutting rule  14  and reducing the cost of requiring frequent replacement of the cutting rule. Furthermore, the hardened surface depth is shallow enough that the cutting rule maintains its ductility properties, thereby allowing the cutting rule to be bent or formed into a number of shapes after being laser hardened.  
         [0064]    [0064]FIG. 9A is a photograph of a cross-section of a steel cutting rule prior to being subjected to the foregoing laser heat treatment process. FIG. 9B is a photograph of a cross-section of a steel cutting rule after being subjected to the foregoing laser heat treatment process. As seen in FIG. 9B, the tip of the cutting rule has been heated and contains a grain microstructure that has an appearance of untempered martensite. The heat treated surface area is depicted by the white area in the tip of the cutting rule. It should be noted that the laser heat treatment process of the present invention results in a uniform grain structure throughout the heat treated tip area. In contrast, FIG. 11 is a photograph of a cross section of a common induction hardened steel cutting rule having a non-uniform heat treated tip. As seen in FIG. 11, a grayish area in the middle of the heat treated tip has not been heat treated, thereby resulting in a non-uniform heat treatment process. This results in a lower surface hardness than that achieved by the foregoing laser heat treatment process.  
         [0065]    The hard untempered martensitic layer formed by the foregoing laser heat treatment process contains internal stresses that make the hardened surface brittle. In order to remove the internal stresses, a stress relief process must be performed. The stress relief process for the present invention is described in more detail below. The next processing station after cutting rule  14  passes through laser processing station  1  for the heat treatment process in intermediate takeup station  3 .  
         [0066]    In the present embodiment of the invention, intermediate takeup station  3  winds steel cutting rule  14  back into the form of a spring coil. This is accomplished by drive motor  13  in station  3  applying a rotational force to spindle  12 , thereby causing steel cutting rule  14  to wrap around spindle  12  forming coil  16 . Although described in terms of a coil winder, takeup station  3  may provide for an alternate method to takeup the steel cutting rule after the laser hardening process has been accomplished. After all of the steel cutting rule has passed from station  2  to station  3 , the process is reversed to perform stress relief of the laser hardened surface.  
         [0067]    In the present embodiment of the invention, prior to performing the stress relief, central laser processing station  1  is reconfigured to perform the stress relief and cleaning station  4  is reconfigured into corrosion inhibitor applying station  7 .  
         [0068]    Central laser processing station  1  is reconfigured by adjusting the laser beam power setting and by adjusting the focal point of the laser beam. The laser beam power setting for performing the stress relief is preferably set to about 80 watts below the power setting for the heat treating process. For example, a power setting of 500 watts for heat treating would require a power setting of about 420 watts for stress relief. The focal point of the laser beam is adjusted by adjusting translation mechanism  29 , thereby adjusting the distance of optic  30  from the laser hardened surface O. The focal point of laser beam  28  for the stress relief process is preferably set to 0.170 inch from cutting surface O, thereby defining dimension F, as seen in FIG. 4A. It has been found that the 80 watt power setting differential coupled with the 0.170 inch focal distance from cutting edge O provides for the optimum stress relief results. However, as previously discussed, the power settings, focal point and feed rate may be varied as desired to achieve a desired result.  
         [0069]    Cleaning station  4  is reconfigured into corrosion inhibitor applying station  7  by removing cleaning pads  17  and installing corrosion inhibitor applying pads  26  in place of cleaning pads  17 . Corrosion inhibitor applying pads  26  are preferably soaked in a corrosion preventive oil prior to installation onto actuating device  18 . Corrosion inhibitor applying station  7  also preferably contains a reservoir  39  of corrosion preventive oil and a means for supplying the oil from the reservoir  39  to applying pads  26 . Reservoir  39  is also preferably pressurized similar to reservoir  21  in applying station  5  and the pressurization is preferably controlled by programmable linear controller  33 . The pressurization is preferably controlled to provide a predetermined continuous drip rate of the corrosion preventive oil from reservoir  39  to oil applying pads  26 . Supplying a continuous drip of oil from reservoir  39  to pads  26  ensures that pads  26  remain soaked with the oil and thereby ensuring the oil is applied to cutting rule  14 .  
         [0070]    After having reconfigured stations  1  and  7 , cutting rule  14  is prepared for the stress relief process. Free end  37  of cutting rule  14 , now contained on intermediate takeup station  3 , is fed through the processing stations and back onto spindle  12  in start/finish station  2 . Free end  37  is installed in slot  15  of spindle  12  such that upon application of a rotational force by drive motor  13  to spindle  12 , cutting rule  14  is wound back into the form of a coil  16 .  
         [0071]    Upon commencement of the stress relief process, steel cutting rule  14  is fed through the processing stations by drive motor  10   b  while drive motor  10   a  remains idle. Drive motor  10   b  is preferably controlled by programmable linear controller  33  and is set to provide a feed rate of about 155 feet per minute. It has been found that a feed rate of 155 ft./min. coupled with the laser settings of 80 watts below the heat treating power setting and a 0.170 focal distance, provide optimum stress relief results. However, as mentioned, these variables may be adjusted in order to achieve a desired result.  
         [0072]    During the stress relief process steel cutting rule  14  first passes through central laser processing station  1 . The selected surface area of steel cutting rule  14  which was previously hardened during the heat treating step is now subjected to laser beam  28  a second time to perform stress relief. Laser beam  28 &#39;s intensity is set such that only stress relief is performed while retaining the previously hardened untempered martensite microstructure. One objective of the present invention is to relieve the internal stresses in the untempered martensite layer formed in the heat treating step without substantially reducing the hardness of the untempered martensite.  
         [0073]    Typically, the stress relief process relieves internal stresses by releasing some of the carbon particles trapped in the microstructure when the untempered martensite was formed. The release of these carbon particles from the microstructure reduces the hardness of the untempered martensite and also changes its microstructure appearance to tempered martensite. However, in the present invention, the laser beam power setting and focal point are established such that the internal stresses are relieved but the microstructure retains its appearance as untempered martensite.  
         [0074]    [0074]FIG. 10 is a photograph of a cross-section of a steel cutting rule after being subjected to the foregoing stress relief process. As seen in FIG. 10, the previously heat treated tip of the cutting rule retains its appearance as untempered martensite. However, since the internal stresses have been relieved, the brittleness has been removed. The resultant steel cutting rule has a surface hardness of at least 60 R c , about 5 R c  higher than conventional cutting rules, but has equivalent ductility properties since the brittleness has been removed. After being stress relieved in central laser processing station  1 , steel cutting rule  14  moves on to corrosion inhibitor applying station  7 .  
         [0075]    Upon entering corrosion inhibitor applying station  7 , steel cutting rule  14  passes between corrosion inhibitor applying pads  26 . Pads  26  are preferably soaked in a corrosion preventive oil. Actuating device  18  applies a clamping force between pads  26 , such that as cutting rule  14  passes between pads  26 , corrosion preventive oil is wiped onto the surface of cutting rule  14 . As the stress relief process proceeds, reservoir  39  is pressurized by an external pressure source preferably to about 5 psi. The pressure is sufficient to provide a continuous drip of oil contained within the reservoir to be applied to pads  26 , thereby maintaining saturation of pads  26 . Although the present embodiment employs a wipe-on method of applying corrosion preventive oil, alternate methods such as a spray or immersion bath application may also be employed.  
         [0076]    Upon completion of the stress relief process, steel cutting rule  14  is wound into the form of a spring coil  16  and is retained in start/finish station  2 . Spring coil  16  is then removed from station  2  and is now ready for use in its final form.  
         [0077]    In another aspect of the invention the foregoing laser hardening process is utilized in laser hardening a metal workpart. The metal workpart is not limited to the form of a steel cutting rule but may be in any form such as a shaft or a flat plate. As seen in FIG. 7, the metal workpart is surface-hardened by being processed through central laser processing station  101 . Central laser processing station  101  may be similar to central laser processing station  1  according to the foregoing description. Central laser processing station  101  performs both a laser heat treat process and a stress relief process similar to the foregoing description. The laser beam configuration, such as the power settings and focal point, are adjusted according to the foregoing description in order to perform both the laser heat treatment and the stress relief process on the metal workpart. The metal workpart may be passed through central laser processing station  101  by means such as a conveyor belt or other similar means or central laser processing station  101  may be traversed across a stationary metal workpart. The metal workpart is first heat treated by being subjected to the laser beam, thereby forming a hard surface layer having an appearance of untempered martensite. The metal workpart is then stress relieved by being subjected to the laser beam a second time similar to the foregoing description. After the stress relief process, the metal workpart microstructure retains its appearance as untempered martensite. However, internal stresses have been relieved, thereby removing brittleness.  
         [0078]    The metal workpart may also be subjected to additional processing steps, such as cleaning station  104 , laser beam absorbent applying station  105 , laser beam absorbent drying station  106  and corrosion inhibitor applying station  107 , as seen in FIG. 8. Stations  104  through  107  as seen in FIG. 8 are similar to stations  4  through  7  according to the foregoing description. Accordingly, the metal workpart may be cleaned by methods such as a dry cloth wipe, spray on cleaning solution or being subjected to an air curtain. Also, a laser beam absorbent substance may be applied to the surface area of the metal workpart depending on the type of laser beam used in the laser hardening process. The laser beam absorbent substance may be applied by various methods, such as roll on or spray on application. Additionally, the laser beam absorbent substance may require drying, such as being subjected to a series of air curtains or a heat source. Further, a corrosion inhibiting substance such as oil may be applied to the surface of the metal workpart after being subjected to the stress relief process.  
         [0079]    Although the present invention has been illustrated with reference to certain preferred embodiments, it will be appreciated that the present invention is not limited to the specifics set forth therein. Those skilled in the art readily will appreciate numerous variations and modifications within the spirit and scope of the present invention, and all such variations and modifications are intended to be covered by the present invention, which is defined by the following claims.