Propeller Manufacture



Propeller Manufacture
The basic metallurgy of ship propellers has been the single most important factor in manufacturing. Propeller material has evolved from the medium-strength manganese bronze alloy (448 MPa [65000 psi] minimum tensile) through short eras of nickel-manganese-bronze & Superston to today's primary alloy, nickel-aluminium-bronze, whose higher strength (586 MPa [85000 psi] minimum tensile) & greater wearability have contributed greatly to expanding the limits of propeller size & horsepower absorption.

The need for material change became more apparent when the higher rpm's & greater hp (kW) accelerated the stress corrosion fatigue phenomenon already known to exist in manganese bronze. While nickel-aluminium-bronze alleviated the material problem with its superior physical characteristics, it presented manufacturers with a new set of problems which had to be addressed in order to insure those qualities. It requires not only a higher pouring temperature (1166 versus 1010 deg C [2130 versus 1850 deg F]), but also several ladle additions to remove oxides & increase fluidity, lengthy nitrogen degassing to remove hydrogen, & more specific gating & risering techniques.

The greater sizes & weights for propellers designed in the past decades have also focused on another major casting concern associated with the increased thickness that is necessary in the blade fillet area to support the additional stresses. Since tests have shown that the tensile strength of the cast propeller material is reduced as section thickness is increased & that the tensile strength in a given cross section decreases from the surfaces to the centreline - both due to increased grain growth in the weaker zones - it becomes apparent that excess material removal in the blade fillet area can be a great detriment to the overall quality, average strength, &, ultimately, the life span of the propeller.

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