Structural integrity of 15-5PH stainless steel flow formed pressure vessel for launch vehicle applications

Abstract

15-5 PH stainless steel is a martensitic precipitation-hardenable stainless steel (0.07C-15Cr-5Ni-3.5Cu-Fe). Presently, high strength low alloy steel, AFNOR 15CDV6 is used for the fabrication of Thrust Vector Control system of satellite launch vehicle. These pressure vessels are used for storing strontium perchlorate, intended for attitude (pitch and yaw) control of the launch vehicle during first stage. The tanks are fabricated through welding of rolled plates, machined rings, and end domes from forgings. AFNOR 15CDV6 steel is prone for corrosion and requires alumina barrier coating to protect from strontium perchlorate. Therefore, as a superior alternative, 15-5PH steel with improved corrosion resistance, simple heat treatment avoiding distortion was selected for the fabrication of pressure vessel. In order to improve the structural integrity of the pressure vessel, flow forming technology was adopted for realization of cylindrical portion of the tank, which eliminated the long seam weld joint and reduced the number of circumferential weld joints. As a part of the developmental program, the tank was designed, shells were flow formed and welded with end domes and were directly aged. The fabricated tank was subjected to a series of pressure tests followed by final burst test for estimating structural margins and ensuring quality requirements.This paper describes the design of the pressure vessel, process followed for manufacture, qualification program adopted and detailed metallurgical analysis carried out on the burst tested hardware. The 15-5PH pressure vessel has successfully withstood the proof pressures and test results are better than the predicted analysis results, indicating structural integrity of the pressure vessel for flight.