Mechanical Properties of Metal Castings Quenched After Varying Time Periods

Based on faculty observation that metallic castings fabricated by pre-clinical students display variable burnishability, it is hypothesized that differences in casting alloy mechanical properties result from inconsistent time duration before quenching.

Objectives:  To measure Vickers hardness (VHN), yield strength (YS), and modulus of elasticity (E) of noble metal castings quenched after varying time periods.

Methods:  Specimens of the alloy, Ney ORO B2, in the shape of a cylindrical rod, 27.5 mm long x 1.6 mm in diameter, were individually cast using acrylic patterns and subjected to quenching in a room-temperature tap water after bench-cooling for 0, 5, 10, 15, and 20 minutes (n=3).  VHN numbers were obtained on 3 mm sections after embedding in acrylic and polishing with standard metallographic procedures.  The remaining rod lengths were subjected to 3-point bending at a crosshead speed of 2 mm/min until a deflection of 2 mm occurred.  YS was determined using a 0.2 mm offset and E was calculated from the linear portion of the bending curve.  The data were analyzed using ANOVA (a=.05).

Results:  Mean and standard deviation values of Vickers hardness, yield strength, and modulus of elasticity in bending for different quenching times were as follows: 

No significant differences (p>0.05) were found.

Conclusion:  Delay in quenching of the alloy tested did not appreciably affect the mechanical properties of the castings.  Further investigation is required to reconcile the observation of variable burnishability.