| 摘要 |
A composite nozzle assembly for conducting a flow of molten steel in an electromagnetic valve is provided which comprises an inner nozzle shell formed from a ceramic material having high erosion-resistance but which undergoes internal tensile stresses of a magnitude sufficient to promote cracking when subjected to the thermal stresses caused by the flow of the molten metal, in combination with an outer nozzle shell that contains and closely engages the inner nozzle shell for applying a compressive load to the inner nozzle shell upon the initial flow of the molten metal through the shell in order to counteract the thermally induced tensile stresses and thereby prevent cracking. The outer nozzle shell is preferably formed from a ceramic material having superior thermal shock properties but yet which has a thermal coefficient of expansion about the same as that associated with the ceramic material forming the inner nozzle shell to minimize stresses at the interface of the inner and outer shells after thermal steady state has been attained. Additionally, the relative thickness of the outer nozzle shell is between about three and ten times that of the inner shell. In a preferred embodiment of the invention, the inner shell is formed from zirconia, while the outer shell is formed from boron nitride. |
