Abstract:
A steel part, such as a mechanical centrifuge screen of mild steel with a nitriding layer on all exposed surfaces of the steel to form an effectively hardened substrate layer, and electroplating the nitrided steel part. The surface treatment method provides an improved wear resistance and effectively provides improved erosion and corrosion protection of the steel part.

Description:
CROSS REFERENCES TO RELATED APPLICATIONS  
       [0001]    U.S. Provisional Application for Patent 60/171,206, filed Dec. 15, 1999, with title, “Low Temperature Nitriding and Chrome Plating Process” which is hereby incorporated by reference. Applicant claims priority pursuant to 35 U.S.C. Par. 119(e)(i). 
     
    
       [0002] Statement as to rights to inventions made under Federally sponsored research and development  
         [0003] Not Applicable. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0004]    1. Field of the Invention  
           [0005]    This invention relates to a surface treatment for obtaining improved life from mechanical screens as a result of a two-step process of first nitriding as the first step, then chrome plating of the nitrided screen as the second step. Each step, individually is prior art. The combination of the two-steps resulting in an unexpectedly long screen life.  
           [0006]    2. Brief Description of the Prior Art  
           [0007]    Prior art processes for improving the life of steel parts, such as mechanical screens, is to try to improve the hardness by one of the conventional methods of nitriding of the steel such as gas nitriding, vacuum ion-nitriding, or carbo-nitriding. Experiments with these methods have proven not only to not extend the lifetime of the screens but have actually reduced the lifetime due to premature corrosive failure from the action of sulfuric acid on the nitrided parts. Traditional electroplating with hard chrome has proven to extend the lifetime somewhat by protecting the steel underneath, but the lack of support from the relatively soft steel screen eventually leads to surface stress failures which rapidly degrade to corrosive and abrasive failure. The present invention process combines these two techniques by nitriding the steel to form an effectively hardened substrate for the hard chrome plating, which in turn provides additional erosion and corrosion protection. This combination duplex coating provides more protection than either of the individual surface treatments separately, and results in an unexpectedly long screen life.  
           [0008]    Methods of nitriding is well known to the trade. Chrome plating is well known to the trade. These are considered competing processes in the trade.  
           [0009]    Mechanical centrifuge screens of mild steel are common to the trade. Equipment down time and replacement labor involved with screen wear and failure is relatively high as compared to the cost of such screens. It is very desirable to have centrifuge screens that last longer. A surface treatment which can effectively provide improved erosion and corrosion protection is an object of the present process.  
           [0010]    The surface treatment process to having centrifuge screens last longer is highly applicable to many other steel parts subjected to material wear. Examples of such medal parts include tumbler barrels and parts on earth moving equipment such as scoops, plows, front end loaders, and shovels.  
           [0011]    As will be seen from the subsequent description, the preferred embodiments of the present invention process overcome the current life limitations of mechanical steel parts subjected to wear.  
         SUMMARY OF THE INVENTION  
         [0012]    The present invention comprises the treatment of steel parts such as, but not restricted to, mild steel centrifuge screens, wherein said treatment comprises a first step of conventional prior art nitriding of the steel to form an effectively hardened substrate, followed by a second step of conventional chrome plating of the low temperature nitrided steel part. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    Drawings are not required for disclosure of this invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0014]    The present invention comprises the treatment of steel parts such as, but not restricted to, mild steel centrifuged screens, wherein said treatment comprises a first step of conventional prior art nitriding of the steel to form an effectively hardened substrate layer, the nitriding layer is in the range of 0.001 to 0. 010 inches thick. Methods of nitriding include, vacuum ion-nitriding, salt bath nitriding, and gas nitriding. Followed by a second step of electroplating the nitrided steel part by applying a layer of conventional hard chrome plating. The hard chrome plating layer is in the range of 0.001 to 0.050 inches thick.  
         [0015]    The present invention process works especially well with mild steel parts, especially mild steel centrifuge screens.  
         [0016]    This solution to the problem of short life due to high wear of metal centrifuge screens is also quite applicable to metal parts subject to wear as a general case.  
         [0017]    The present invention will further be described with reference to specific tests so that advantageous features of the invention can well be recognized. The tests were as follows.  
         [0018]    Coal Preparation Plants are required to remove water and dust particles from coal in order to prepare such coal for transport. A mild steel centrifuge screen is commonly situated in a dryer and used in this process of removing water and coal dust from the coal. A slurry of water and coal are placed into the screen. The said slurry is caused to spin centrifugally within the screen. Water and coal particles less than 100 mesh, i.e. coal dust, pass through the screen holes. The remaining larger coal particles pass through the dryer for transport. The screen experiences wear based upon the coal size (mesh), the coal hardness (grind), and the acidity (ph) of the slurry. In these tests, identical mild steel centrifuge screens were subjected to wear and durability tests, using various coal sizes, hardness, and acidity (ph). The tests consisted of using prior art screens and screens treated with the present method to the foregoing process. After the tests, the amount of wear was measured and the results determined as shown below.  
         [0019]    Test 1  
         [0020]    a.) The subject screens were operated at a throughput rate of twenty-five (25) tons of coal per hour.  
         [0021]    b.) Coal Size. 28 mesh×100 mesh.  
         [0022]    c.) ph: 6.5-7.0  
         [0023]    d. Grind: 40-42  
         [0024]    Results of Test 1  
         [0025]    The screen life of the prior art screen under these conditions was 365 hours. The screen life of the screen treated with the present method was 650 hours. The treated screen of the present invention shows an increase life of 285, hours a 78% increase.  
         [0026]    Test 2  
         [0027]    a.) The screens were operated at a throughput rate of twenty-two (22) tons of coal per hour.  
         [0028]    b.) Coal Size. {fraction (1/16)}″×100 mesh.  
         [0029]    c.) ph: 6.3-6.8  
         [0030]    d.) Grind: 42-44  
         [0031]    Results of Test 2  
         [0032]    The screen life of the prior art screen under these conditions has 380 hours. The screen life of the screen treated with the present method was 660 hours. The treated screen of the present invention shows an increase life of 280 hours, a 74% increase.  
         [0033]    Test 3  
         [0034]    a.) The screens were operated at a throughput rate of twenty-one (21) tons of coal per hour.  
         [0035]    b.) Coal Size. ⅛″×100 mesh.  
         [0036]    c.) ph: 6.4-6.8  
         [0037]    d.) Grind: 44-46  
         [0038]    Results of Test 3  
         [0039]    The screen life of the prior art screen under these conditions was 320 hours. The scree life of the screen treated with the present method was 600 hours. The treated screen of the present invention shows an increase life of 280 hours, an 88% increase.  
                                                   TABLE I                           The results are shown in Table I.                Life of Prior   Treated Screen of   % of Life Increase           Art Screen (hrs)   Present Invention (hrs)   of Present Invention                        Test I   365   650   78%       Test II   380   660   74%       Test III   320   600   88%                  
 
         [0040]    From the above tests, it will be noted that the embodiments of the present invention have remarkably superior wear resistance and it has been proved that the screens treated with the present invention method results in an increase screen life of over 70%. The results of these tests further show that said increase in screen life is consistent regardless of relevant variables, such as, coal acidity, size of coal, and hardness of coal.  
         [0041]    Although the description above contains many specificities, and reference to specific examples, these should not be construed as limiting the scope of the invention but as merely providing illustrations of some of the presently preferred embodiments of the present invention.  
         [0042]    For example, while the invention is discussed in terms of mild steel centrifuge screens, it is very applicable to mild steel parts subject to wear as a general case. Beyond that, there are many other metals, such as stainless steel where this is applicable.  
         [0043]    Thus the scope of the invention should be determined by the appended claims in the formal application and their legal equivalence, rather than by the examples given.