Shot Peening of Pressure Vessels

Author:  Sheehan, J. P.
Source:  IIT Research Institute B1000B1 - Report No. 4
Doc ID:  1964007
Year of Publication:  1964
Abstract:  
There has been an increasing demand in recent years for ultra-high strength, light-weight pressure vessels for applications that require 100% reliability in service. Two examples of these requirements are solid fuel missile motor cases and high-pressure gas cylinders for rocket boosters and aircraft. Such pressure vessels are manufactured with utmost care, and yet because they contain weldments and possibly other undetectable metallurgical flows that could lead to catastrophic fracturing, they must be overdesigned to insure reliability in service. Super high strength steels capable of being heat-treated to strength levels in excess of 300,000 psi are available for these applicaitons, but because of their marginal or poor fracture toughness, they are generally heat-treated to strength levels far below their maximum capability. Shot peening is a process that has been used commercially for many years for improving the strength and fatigue resistance of gear teeth, drive shafts, axles, and other heavily stressed components. In shot peening, the surface layers of the material are heavily cold worked to a predetermined depth, and this produces a state of residual compressive stresses on the surface which helps to resist the applied service stresses. In pressure vessels, the majority of failures that occurred in service or in proof testing were found to originate at the surface where perhaps some type of mechanical or metallurgical flaw caused a stress concentration or weakness in the metal. It has been suggested that surface decarburization might be helpful in preventing these failures by providing an increase in toughness and ductility at the region where flaws are likely to exist. However, this process has been subject to dispute because it is difficult to control and can cause a serious loss of strength if carried too far. It is suggested that shot peening might also be helpful in improving the reliability of pressure vessels. Generally, the higher the strength of the vessel, the smaller is the size of flaw required for catastrophic failure; also, the lower is the applied stress to initiate failure. Therefore, with shot peening, because of the residual compressive stress, a surface flaw will "see" a smaller applied stress than is actually present, and for this reason larger flaws can be tolerated than in a vessel that has not received a shot peening treatment. To determine how much improvement can be obtained in the strength and service reliability of pressure vessels by shot peening treatments, the following program was conducted. This is the final report and covers the work carried out from July 2, 1963 to July 31, 1964.