Speaker
Description
Measurement of cryogenic propellant pressures (liquid oxygen and liquid hydrogen) of rockets is very critical for their successful launch. These pressure measurements are carried out using integral diaphragms pressure transducers. Material is machined out from both ends of the circular rod made of precipitation hardened martensite stainless steel (APX4), such that thin circular diaphragm is formed at the centre. One surface of the diaphragm senses the input propellant pressure and the other side senses the stress induced with the help of strain gauge bonded on the surface. The diaphragm senses input pressures and gets proportionally deflected at the centre (within the elastic limits) due to the stress induced. The deflection is transmitted to the strain gauges which in turn produces a measurable electric output through wheat stone bridge network. Output electrical signals are calibrated in units of pressures. These transducers are expected to exhibit dimensional for the reliable performance throughout their useful life. The precipitated hardened martensite stainless steel used for machining pressure transducer does not contain hundred percent martensite and possesses certain amount of austenite (around 3-4 %) known as retained austenite. Martensite has strong and hard microstructure as compared with austenite which is soft, tough and ductile. This unstable retained austenite slowly gets converted to freshly formed martensite with time which has different coefficient of volumetric expansion. Differential volumetric expansions within the metal cause dimensional changes and produce dimensional instability to causing zero shift and drift in output readings. The conversion from retained austenite to martensite is faster in the presence of high residual stress. Cryotreatment is a proven technology to reduce such residual stresses. Cryotreatment is a process where the material is gradually cooled down to cryogenic temperature and exposed to the cryogenic environment for prolonged time and slowly warmed up to room temperature.In the present experimental study, the machined diaphragms were cryotreated in a dedicated cryotreatment system using liquid nitrogen as the cooling medium. Diaphragms were cryotreated gradually cooled down to 98K, maintained at this temperature for 36 hours and gradually warmed upto room temperature. After cryotreatment, they were tempered at 673K for one hour in vacuum furnace. Dimensional stability analysis of both regular and cryotreated diaphragms were carried out using Thermo Mechanical Analyser (TMA). Results indicated significant enhancement of dimensional stability of diaphragms which were subjected to cryotreatment.
Keywords: Dimensional stability, Stress, Strain gauge, Austenite, Martensite, Cryotreatment
