Thursday, January 25, 2018

Bearing Design Guide: Chapter Sixteen: Effect of the Casting Method on Bronze Alloys

          The casting method should not be ignored but given consideration of the type of service the bronze alloy will be subjected to.

           In particular, the type of load - whether steady and continuous, intermittent or with shock impact or pounding loads - the surface speeds to be encountered and other important features required to be met.

          The casting method has a definite impact on the bronze alloy such as the resulting grain size, density, hardness, mechanical and physical properties, soundness and structure.

          In general, the slower chilled or cooled casting will give rise to coarser and larger grain size. These have a profound effect on the surface qualities, coefficient of friction, wear rate or wear resistance and loads.

          The faster cooled or chilled castings result in greater density, hardness, finer grain size, improved soundness and structure.

           Referring to the illustrations on the following page, please note the finer grain sizes developed by each method of casting.

          Sand Casting: Since molten bronze is poured into a sand mold, the sand or silica having thermal insulating characteristics, will cause slow cooling or chilling of the casting in air. This slow cooling permits the grain size to grow larger, the density, the soundness and structure to be less than by other casting methods.

          Permanent Molded or Chill Casting: The thermal insulating sand is replaced by nickel steel or cast-iron dies. The metal mold quickly chills the casting and this faster solidification results in finer grain size, no interconnected porosity, finer surface finish and improved physical and mechanical properties.

          Centrifugal Casting: Molten metal is poured into a rotating steel or cast-iron die. The centrifugal force impacts the molten metal against the inside of the die, eliminating any porosity. the rotating or spinning die is then sprayed with water coolant to obtain a faster chill than the first two methods discussed the finer grain size further improves the physical and mechanical properties still further.

         Continuous Casting: The molten metal flows by gravity through a graphite die which is chilled
immediately by the cooling jacket surrounding the die. This faster cooling further reduces the grain size and results in still higher physical and mechanical strength.

          The average increase progressively in the tensile and yield strengths is about 5000 to 10,000 PSI and hardness is increased by 10 to 20 points of Brinell hardness.

          Remarks: To further enhances the physical and mechanical properties of the bronzes, extrusion and forging operations can reduce the grain size additionally. These are special processes and the four methods of casting described earlier in this chapter cannot achieve comparable mechanical and physical strength.

Casting Effect On Grain Size and Density

 you can see above, things may look the same on the outside, but they can be very different on the inside.  It is always important to really understand the final application your part will be used in so you know which casting method would be best for you.

          Well...that's it for today. I say goodbye for now. Until next time my metal loving friends...


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