Patent Publication Number: US-8979422-B2

Title: Cover having a wear-resistant surface and method for the production thereof

Description:
FIELD OF THE INVENTION 
     Background Art 
     The present invention relates to a cast-iron cover fitted with at least one abrasion-resistant surface as defined in the preamble of claim  1  and to a method for manufacturing such a cover. 
     Covers, caps/gratings, other objects used in spot or linear water sprinkling as well as tree-protecting gratings respectively tree-protecting systems have long been used in street/road construction. Preferably such components are made of cast iron since such covers may be subjected to high traffic loads. 
     For safety reasons, such covers also must be anti-skidding. One way to attain the required skid resistance is controlling the surface roughness using composite materials. Accordingly the cover&#39;s top side is coated with a resin-bonded or concrete-bonded granulate. Preparing such covers is costly because casting them must be followed by further finishing procedures. 
     SUMMARY OF THE INVENTION 
     The covers of the state of the art incur frequently the drawback of short surface life. Vehicles or pedestrians moving over them significantly abrade them, thus jeopardizing traffic safety. 
     Accordingly it is the purpose of the present invention to create a cover which is fitted with a traffic-bearing respectively contact surface that shall be resistant to the said abrading loads. This invention also shall be economical and easily implemented. 
     As regards a cast-iron cover comprising at least one abrasion-exposed surface, the invention stipulates that said abrasion-exposed surface is fitted at least segment-wise with a wear-resistant layer and that said wear-resistant layer constituted in part by white solidification of the cast iron. 
     The cover of the invention applies equally well to a manhole, an intake grating, a cap for street/road intakes of a tree protecting system. By forming a comparatively thin wear-resistant layer constituted by a white solidified cast iron, the service life of such covers may be extended considerably. The solidification of iron carbon alloys by means of the metastable system assures that the wear-resistant layer shall be harder than the remaining cast iron which is solidified cast gray iron. 
     Nor need all of the abrasion-exposed surface be made of white solidified cast iron. Instead only part of said surface need consist of white solidified cast iron. Mixed cast iron containing white solidified and gray solidified iron zones next to each other is, just as applicable. 
     Formation of a white solidified structure zone (chill depth) assures that the cover of the invention as a result shall better withstand the abrasive stresses than a conventional cast iron cover. Such a cover of the invention moreover can be made more economically. Nor is the abrasion-exposed surface mandatorily situated at the cover&#39;s top side. Illustratively the contact area with the frame also is exposed to higher abrasive loads. 
     In a particularly advantageous design of the invention, the Brinell hardness of the wear-resistant layer shall be larger than 275 HBW and less than 350 HBW. Surface abrasion is already precluded optimally within this range of hardnesses. If the hardness exceeds 350 HBW, the wear-resistant layer becomes brittle. Impulsive loads then gradually destroy said layer, degrading the entire cover. 
     In a further feature of the invention, the traffic surface is fitted with a skid-inhibiting means created by a fine cast iron structure. As a result, in combination with the said chilled wear-resistant layer, the surface roughness can be created directly during the casting procedure. The structure so attained is well protected against abrasion at the cast iron surface and even years of use later provides skid inhibition at the cover&#39;s surface. 
     In the ideal case, said structure is characterized by a fluting height of 2 to 8 mm. When a cover is fitted with such a preferred fluting height, the entire structure may be made of chilled cast iron. In this manner the cover&#39;s service life is extended even further. 
     A method of the invention to manufacture a cast iron cover includes the following stages: 
     1. Making a shaped body for said cover, 
     2. Making an activation layer within the shaped body, 
     3. Filling-in cast iron in a manner it is chilled at least partly at the activation layer&#39;s contact surface. 
     The method of the invention meets the criteria to create a cover constituted solely of cast iron and being fitted at its traffic surface respectively at the contact surface between the cover and the frame with a durable, abrasion-resistant wear-resistant layer. 
     For that purpose, and in a first stage, a shaped body is made and frequently is constituted of clay-bonded molding sand. The molding sand&#39;s main components are quartz sand, casting clays, various carbon carriers and water. Using an appropriate model enclosed by a frame, namely the so-called bottom box, the molding sand is compacted into a casting-ready mold and in this manner it constitutes the shaped body. 
     Next an activation layer is constituted within the shaped body that then is filled with the casting iron which shall chill at its contact surface with the activation layer. Said white solidification does not take place through all of the cast component, but only at its surface facing the activation layer. In the process the wear-resistant structural components such as iron carbide or ledeburite are constituted as crystals in the cast iron. Thereupon this layer may serve as the wear-resistant layer, its thickness ideally being up to 5 mm. The wear-resistant layer so made is hard enough to withstand for many years the stresses of daily traffic. 
     Illustratively the activation layer may be made of chemical elements or chemical compounds assuring the crystalline formation of iron carbide, ledeburite or other wear-resistant structural components at the casting iron&#39;s contact surface. Tellurium for instance is such a chemical element. This design of the invention makes possible a uniform activation layer surface. The particles of the chemical compounds can be distributed arbitrarily within the mold. 
     In especially preferred manner, the chemical elements resp. chemical compounds are introduced into the mold by laying, spraying, soaking, padding or strewing. In this way nearly all suitable chemical compounds and chemical elements are easily applied to the mold. As a result, the activation layer can be matched to every surface of the shaped body. Therefore the method of the invention also allows making complex geometry covers. 
     Another and equally advantageous embodiment mode of the invention stipulates that the activation layer be constituted in the shaped body by means of inserted cooling elements or inserted chilling elements. The white solidification also may be carried out by rapidly cooling the filled-up cast iron. Rapid cooling at the activation layer is implemented by the cooling or chilling elements, again resulting in a white solidification. This procedure offers the advantage of circumventing the use of expensive chemicals such as tellurium. Moreover, preparing the mold sand is simplified and hence more economical because no chemical residues need be removed from it. 
     The shaped body preferably is set up in a molding box consisting of an upper box and a lower box. To constitute the shaped body, the upper and lower boxes are filled separately from each other with molding sand. In the next operational step a ramming cap moves over the upper box respectively the lower box and compacts the molding sand into a hard, casting-ready shaped-body half. Thereupon the upper and lower boxes are superposed and secured by a locking means against upward forces during casting. Now the shaped body shall be ready for casting in the closed molding box. 
     Following casting the halves can be easily separated and the blank can be removed. 
     In advantageous manner, the activation layer may be constituted in the shaped body at either of the shaped body halves. During this procedural stage, the upper and lower boxes are not yet connected to each other. 
     Preparing the activation layer in this manner offers the advantage that the shaped body halves fitted with the activation layer may be checked directly for defects. Accordingly costly casting defects are averted and the manufacture of the activation layer is simplified. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further features, particulars and advantages of the invention are defined in the claims and are implicit and/or explicitly stated in the description below of illustrative embodiment modes in relation to the appended drawings. 
         FIG. 1  shows a cover fitted with a continuous wear-resistant coating, 
         FIG. 2  shows a cover fitted with a wear-resistant zone, and 
         FIG. 3  shows a cover of the invention within the molding box. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows a cover  10  of the invention fitted with the associated frame  18 . A wear-resistant layer  30  is constituted at the traffic surface  20 . However the wear-resistant layer  30  does not run through the full thickness of the cover. The rim supports  12  and the ribs  14  of the cover  10  also are coated only at their top side with a constant, thin wear-resistant layer  30 . Also, the thickness of the white solidified ear-resistant layer  30  is approximately constant across the full top surface. This feature also applies equally for both the zones of the fluting troughs RT and the zones of the fluting peaks RK. The fluting height RH subtended between the fluting trough RT and the fluting peak RK does not affect the thickness of the wear-resistant layer  30 .  FIG. 1  also shows the feasibility of constituting a wear-resistant layer  30  at said cover&#39;s contact surface  16  touching the frame  18 . 
       FIG. 2  shows a similar embodiment mode of the invention. This design merely differs from that of  FIG. 1  in that the wear-resistant layer  30  is constituted in a wear-exposed zone VB. As a result, the costly manufacture of the wear-resistant layer  30  is eliminated in zones experiencing lesser abrasions. In this embodiment mode too, the support zones (contact surfaces  16 ) of the cover  10  are fitted with a wear-resistant layer  30 . 
       FIG. 3  shows a cover  10  of the invention while still in the molding box  40 . The cover  10  together with the wear-resistant layer  30  rests in the lower box  42 . The rim supports  12  as well as the ribs  14  run as far as into the upper box  42 . By means of its molding sand  48 , the molding box  40  encloses the entire cover  10 . An activation layer  50  is constituted in the lower box  44 . The wear resistant layer  30  of the ulterior traffic surface was solidified by chilling at the contact surface  60  of the activation layer  50 . By means of an input funnel  49  and a runner and gate system  46 , the cast iron enters the shaped body. Following cast iron solidification, the upper box  42  and lower box  44  can be separated from each other along the model parting axis A. The cover  10  now may be removed from the mold box  40 . 
     The invention is not restricted to one of the above described embodiment modes, but may be modified in many ways. Illustratively the upper and lower casting boxes may be interchanged during the casting procedure. Again, the position of the activation layer may be varied. 
     All features and advantages implicit in and explicit from the claims, description and drawing, inclusive design details, spatial configurations and procedural steps, may be inventive both per se as in arbitrary combinations. 
     LIST OF REFERENCES 
     
         
           10  cover 
           12  rim support 
           14  ribs 
           16  contact surface 
           18  frame 
           20  traffic surface 
           30  wear-resistant layer 
           40  molding box 
           42  upper box 
           44  lower box 
           46  runner and gate system 
           48  molding sand 
           49  casting input funnel 
           50  activation layer 
           60  contact surface 
         RT fluting trough 
         RK fluting peak 
         RH fluting height 
         VB wear-resistant zone 
         A model parting axis