Abstract:
A method of heat treating a surface with a laser, successive passes of the laser over the surface having a large overlap with each individual pass applying insufficient energy to obtain the desired effect on the surface but the overlapping passes applying sufficient energy. Various patterns of laser movement may be used.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 61/846,584, filed Jul. 15, 2013, which is herein incorporated by reference in its entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    Laser heat treatment. 
       BACKGROUND 
       [0003]    Lasers are commonly used to heat treat the surfaces of metal objects. Typically, it is not desired to melt the surface. A laser is used to bring the surface to a temperature below the melting point of the metal but sufficient to cause hardening when the surface is cooled. Cooling occurs quickly when laser energy is no longer applied due to conduction into the bulk of the material. The laser energy is applied by a laser which follows a path over the material guided by a computer. Conventionally, the laser supplies sufficient energy to adequately treat the surface at least at the center of the path in a single pass. The path zigzags to cover the whole surface to be heat treated. The surface at the center of the beam is heated more than the edges, causing an uneven effect of the treatment over the surface. It is known to overlap the path to some extent but the overlap is generally small, as too great an overlap would cause overtreatment of the overlapped area. The paper “Predictive Modeling of Multi-Track Laser Hardening of AISI 4140 Steel” discloses simulations of overlaps of up to 50% but teaches away from 50% overlap as it finds 5/12 overlap to be preferable. When a percentage or fraction is used to indicate a degree of overlap, the percentage or fraction is used to indicate the width of the overlap of the beam in successive passes as a percentage or fraction of the width of the laser beam in a single pass. 
         [0004]    Lasers are also used in treatment applications which melt the surface. The paper “Three body abrasive wear of X12CrNiMo martensitic stainless steel laser alloyed with TiC” discloses 75% overlap when melting the surface of steel to alloy the steel with TiC grains. The surface is melted even with a single pass. This degree of overlap is not conventionally applied when not intending to melt the surface. 
       SUMMARY 
       [0005]    There is disclosed a method of heat treating a surface with a laser to obtain a change in the structure of the surface without melting the surface by directing a laser beam onto the surface, the laser beam illuminating at a point in time a portion of the surface, and moving the laser beam to illuminate at successive points in time successive portions of the surface, the portion of the surface and the successive portions of the surface forming a path, the path overlapping itself to pass over each point in an area of the surface to be treated more than once, the laser beam supplying insufficient energy to obtain the change in structure of the surface in a single pass. 
         [0006]    In various embodiments, there may be included any one or more of the following features: the width of the overlap of the path between successive passes may be greater than 50% of the width of the path in a single pass, at least 75% of the width of the path in a single pass, or greater than 75% of the width of the path in a single pass. 
         [0007]    There is also disclosed a method of heat treating a surface with a laser to obtain a change in the structure of the surface without melting the surface by directing a laser beam onto the surface, the laser beam illuminating at a point in time a portion of the surface, and moving the laser beam to illuminate at successive points in time successive portions of the surface, the portion of the surface and the successive portions of the surface forming a path, the path overlapping itself to pass over each point in an area of the surface to be treated more than once, the width of the overlap of the path between successive passes being greater than 50% of the width of the path. 
         [0008]    In various embodiments, there may be included any one or more of the following features: the width of the overlap between successive passes may be at least 75% of the width of the path, or greater than 75% of the width of the path. The surface to be treated may be steel. The change in the structure of the surface may be the formation of a martensitic grain structure. The path may follow a zig-zag route, a route with stacked line passes, a looping route, or a route with S-pattern motion. 
         [0009]    These and other aspects of the device and method are set out in the claims, which are incorporated here by reference. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0010]    Embodiments will now be described with reference to the figures, in which like reference characters denote like elements, by way of example, and in which: 
           [0011]      FIG. 1  shows an example of successive passes of a laser over a surface to be treated, the laser moving in opposite directions in the successive passes; 
           [0012]      FIG. 2  shows an example of successive passes of a laser over a surface to be treated, the laser moving in the same direction in the successive passes; 
           [0013]      FIG. 3  shows an example of successive passes of a laser over a surface to be treated in a prior art method, the laser moving in opposite directions in the successive passes; 
           [0014]      FIG. 4  shows an example of successive passes of a laser over a surface to be treated in a prior art method, the laser moving in the same direction in the successive passes; 
           [0015]      FIG. 5A  shows an area of a surface treated according to an embodiment of the present invention; 
           [0016]      FIG. 5B  shows a cross section of the surface of  FIG. 5A ; 
           [0017]      FIG. 6A  shows an area of a surface treated according to a prior art heat treating method; 
           [0018]      FIG. 6B  shows a cross section of the surface of  FIG. 6A ; 
           [0019]      FIG. 7A  shows a surface exposed to a single pass of a laser according to an embodiment of the present invention; 
           [0020]      FIG. 7B  shows a cross section of the surface of  FIG. 7A ; 
           [0021]      FIG. 8A  shows a surface exposed to a single pass of a laser according to a prior art heat treating method; 
           [0022]      FIG. 8B  shows a cross section of the surface of  FIG. 8A ; 
           [0023]      FIG. 9  shows the area of surface illuminated by a laser at multiple points of time as collectively forming a path of the laser; 
           [0024]      FIG. 10  shows an overlap between successive passes of a laser showing what is meant by a percentage of overlap; 
           [0025]      FIG. 11  shows an embodiment in which the surface is divided into segments, each segment having zig-zag passes with 100% overlap; 
           [0026]      FIG. 12  shows an embodiment in which the surface is divided into segments, each segment having stacked line passes with 100% overlap; 
           [0027]      FIG. 13  shows an embodiment with looping motion; and 
           [0028]      FIG. 14  shows an embodiment with looping motion with passes in neighbouring strips looping in opposite directions; 
           [0029]      FIG. 15  shows the embodiment of  FIG. 14  in a context where the looping motion allows the heat treatment to avoid obstacles in the surface; 
           [0030]      FIG. 16  shows an embodiment with S-pattern motion; and 
           [0031]      FIG. 17  shows an embodiment with zig-zag motion or stacked line passes in strips, with some overlap between strips. 
       
    
    
     DETAILED DESCRIPTION 
       [0032]    In standard laser heat treating a laser is used with a small amount of overlap  12  between successive passes  14 , as shown in  FIGS. 3 and 4 . If a large amount of overlap is used without decreasing the power or increasing the travel speed the surface of the metal melts. The melting of the surface damages the products and as laser heat treatment is done after all of the machining is completed, this is an issue. Our process gets around this issue by allowing the energy to soak into the part. We do this by using multiple lower-energy passes  14  with large overlaps  12 , as shown in  FIGS. 1 and 2 . The key to the process is that each individual pass does not provide enough energy to fully heat treat the steel, but it is the combination of the overlapping passes which causes the heat treatment. If we were to run our process in a straight line without the overlap we would cause very little hardening in the metal; this can be seen in  FIG. 7B .  FIG. 7A  shows a single pass  14  over a surface  10  to be treated in the disclosed method. The hardened region  16  is very small if it exists at all. This differs from conventional heat treatment where if the beam was ran in a straight line over the steel it would still provide full hardening in the area which the laser run over; this can be seen in  FIG. 8B .  FIG. 8A  shows a single pass  14  over a surface  10  to be treated in the prior art. The hardened region  16  has the intended depth for a full heat treatment with the single pass. 
         [0033]    Our process can work with various laser motion controls, be it zig-zag, stacked line passes, looping motions, or S-pattern motion. Each consecutive pass provides a pre-heat for the next pass.  FIG. 1  shows the disclosed method using a laser motion control in which the laser travels in opposite directions in successive passes, and  FIG. 2  shows the disclosed method using a laser motion control in which the laser travels in the same direction in successive passes.  FIG. 3  shows a prior art method using a laser motion control in which the laser travels in opposite directions in successive passes, and  FIG. 4  shows a prior art method using a laser motion control in which the laser travels in the same direction in successive passes. 
         [0034]    The process disclosed uses a higher travel speed or lower laser energy output (or a combination of both) than is used in standard laser heat treatment. By increasing the speed or lowering the laser power output we are enabling ourselves to use a greater overlap than is possible in conventional laser heat treatment. By using a greater overlap we are able to insure that the areas we heat treat do not have areas of shallow hardening.  FIG. 5A  shows a surface  10  treated using this method.  FIG. 5B  shows a cross section of the surface of  FIG. 5A  showing what the hardened region  16  looks like with our process. There are areas of shallow hardening  18  only at the ends.  FIG. 6A  shows a surface treated using a prior art method.  FIG. 6B  shows a cross section of the surface of  FIG. 6A  showing a hardened region  16  with shallow hardening  18  at the overlaps between passes as well as at the ends. When  FIG. 5B  is compared to  FIG. 6B  it becomes evident that our process is superior as these areas of shallow hardening can be detrimental to certain products which require full hardening. 
         [0035]    Referring to  FIG. 9 , at a point in time a laser illuminates an area  20  on the surface. At further points in time the laser illuminates portions of the surface  20   a ,  20   b  and  20   c . The portions illuminated by the laser at successive points of time define a path  22 . The path  22  has a width indicated by line  24 .  FIG. 4  shows the laser illuminating a circular portion of the surface but it may illuminate different shapes, for example, a rectangular portion of the surface. The laser may be continuous or pulsed. 
         [0036]    Referring to  FIG. 10 , the path of the laser returns to a portion of the surface that it has previously illuminated, each time it does so being referred to as a pass over that portion of the surface. The path has a width as indicated by line  24  on each of the passes. The passes have an overlap indicated by line  26 . The degree of overlap is represented as a percentage, where the percentage refers to the ratio of the width of the overlap to the width of the path expressed as a percentage. In  FIG. 10  as shown, the width of the overlap is 75% of the width of the path so the percentage of overlap is 75%. In an embodiment, the width of the path may be different on one pass than on another pass. In this case, the percentage of overlap may be considered to be the ratio of the overlap to the larger of the widths of the path. In an embodiment, a pass and a successive pass may be passes of respective different paths formed by corresponding different lasers. 
         [0037]    The process can be used for heat treatment of metal products in any industry, including but not limited to, for example, oilfield equipment or automotive parts. 
         [0038]    As the heat from each laser pass dissipates over time, it is preferred that each pass over a point in the surface occur within a relatively short time frame. For a sufficiently large surface, there may not be enough time for the laser to traverse the full width of the surface before the heat from a pass dissipates, and so the width of the surface can be divided into strips  30 , the path of the laser traversing the width of a strip in each pass and each strip being treated in turn.  FIGS. 11-15  and  17  show various embodiments of this principle. In  FIGS. 11 and 12  the strips are further divided into segments 32, each segment being treated with two passes of the laser with 100% overlap. In  FIG. 11  the overlapping passes have motion of the laser in opposite directions (zig-zag motion) and in  FIG. 12  the overlapping passes have motion of the laser in the same direction (stacked line passes). It should be noted that the zig-zag motion and stacked line passes can also be used with less than 100% overlap and without dividing the strip into segments 32, for example, with successive passes proceeding from one end of the strip to the other each one having less than 100% overlap with the preceding pass, as shown in  FIG. 17 . In  FIG. 13  an embodiment is shown in which the laser has a looping motion giving a curved shape to each pass  14 . In  FIG. 14 , the passes  14  in the second strip curve in an opposite direction to the passes in the first strip.  FIG. 15  shows an example context in which the embodiment of  FIG. 14  might be used, showing the curved passes  14  avoiding poles  34 ; curved passes could also be used to avoid any other gap in the area of the surface to be treated.  FIG. 16  shows passes  14  having an S-pattern motion, shown here in a context where the S-pattern motion allows avoiding two poles within a single strip. The S-pattern motion can be used with multiple strips (not shown) as with the other motions. In  FIGS. 15-17  numerals  36  beside the passes indicate an example time order in which the passes might be made.  FIG. 17  shows an embodiment with zig-zag motion or stacked line passes, with an overlap  38  between strips  30 . There can also be an overlap between strips with other motions, including the motions of  FIGS. 11 and 12  where there could be an overlap between segments as well as between strips. 
         [0039]    Immaterial modifications may be made to the embodiments described here without departing from what is covered by the claims. 
         [0040]    In the claims, the word “comprising” is used in its inclusive sense and does not exclude other elements being present. The indefinite articles “a” and “an” before a claim feature do not exclude more than one of the feature being present. Each one of the individual features described here may be used in one or more embodiments and is not, by virtue only of being described here, to be construed as essential to all embodiments as defined by the claims.