A study of residual stresses in low alloy steel theta ring castings.

Residual stresses generated during the casting and heat treatment of a low alloy steel, BW2 have been studied using ta theta ring so that temperature differentials could be varied using different tie bar sizes. Residual stresses have been measured using centre hole drilling and tie bar sectioning techniques. Centre hole drilling was shown to be sensitive to surface preparation methods. Stresses induced by drilling were accounted for in measured stresses and drilling stresses were found to be greater in cast than annealed samples. Cast theta rings have been shown to have compressive residual stresses, becoming less compressive or tensile as the tie bar width was reduced. Tie bar sectioning producedexpansion which increased with increasing tie bar width, although there was no direct correlation between tie bar stress and width. Results from both techniques have been explained using factors contributing to residual stress formation. The S-shaped runner contained residual stresses and its removal altered residual stress levels.Heat treatments have been found to produce different amounts of stress relief according to the geometry and thermal cycle imposed. In a uniform section theta.ring normalising and tempering relieved stresses in the tie bar but not in the outer ring, and the tie bar contained no stresses after sectioning. For the non-uniform section theta ring annealing made stresses more compressive whilst normalising and tempering generated stresses due to differential cooling, and the tie bar contained compressive stresses after sectioning. Maximum Von Mises equivalent in the cast theta ring was shown to be 25% of the yield strength and reduced further after heat treatment.A computational model based on finite difference has been used to simulate solidification of a tie bar across the width. Temperature gradients computed along this bar axis were shown to be small and are believed not to significantly contribute to residual stresses.