Metallurgical Analysis of Cracked Redundant Valves for Propellant Flow Control in the Attitude Orbital Control System of
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CASE HISTORY—PEER-REVIEWED
Metallurgical Analysis of Cracked Redundant Valves for Propellant Flow Control in the Attitude Orbital Control System of a Satellite Abhay K. Jha • A. Venugopal • K. SreeKumar
Submitted: 25 November 2009 / in revised form: 17 March 2010 / Published online: 8 April 2010 Ó ASM International 2010
Abstract Attitude Orbital Control System (AOCS) thrusters, used in Indian satellites employ Latchable Series Redundant Valves (LSRV) for propellant flow control. The valve body has a major component called bobbin, which comprises four parts, three parts made of AISI 304L stainless steel and one part of AISI 446 stainless steel. About 150 bobbins were electron beam welded and one, from a group of thirty bobbins, developed a crack in the HAZ of one of the face welds. Detailed analysis revealed that the crack was due to stress-assisted cracking phenomenon. The stress induced by press fitting of a part in between other two parts of valve was primarily responsible for the cracking. Keywords Stress-assisted cracking Press-fitted components
was given a functional test for the valves. Of the 30 solenoid valves tested, one developed a crack in the heat affected zone (HAZ) of one of the weld faces. A schematic sketch of the bobbin, indicating the location of crack is shown in Fig. 1. The valve body was basically fabricated in four parts, hereafter referred as part 1, part 2, part 3, and part 4 as indicated in Fig. 1. Parts 1, 3, and 4 were fabricated out of AISI 446 stainless steel, whereas AISI 304L stainless steel was the material selected for part 2. Part 2 was in the shape of ring and was press fitted within the slot provided between the part 1 and 3. The process of press fitting was followed by the EB welding between parts 1 and 2, parts 2 and 3, as well as radial welding between parts 3 and 4. The crack was found in the HAZ of the parent material (AISI 446), very near to the weld of part 2 and part 3. The weld was an autogenous EB weld between AISI 304L (part 2) and AISI 446 (part 3). The cracked bobbin was analyzed in detail to understand the cause of cracking.
Introduction Attitude Orbital Control System (AOCS) thrusters, used in Indian satellites employ Latchable Series Redundant Valves (LSRV) for propellant flow control, were fabricated using AISI 304L and 446 stainless steel components that are joined with three Electron Beam (EB)-welded joints (two on face and one radial weld). The valve body had a major component called bobbin. About 150 bobbins were EB welded at a work center. A set of 30 bobbins
A. K. Jha (&) A. Venugopal K. SreeKumar Materials Characterisation Division, Materials and Metallurgy Group, Vikram Sarabhai Space Centre, Indian Space Research Organisation, Trivandrum 695 022, India e-mail: [email protected]
Material AISI Type 304L rods (20-mm diameter) were used for fabrication of part 2, whereas parts 1, 3, and 4 were fabricated out of 36-mm diameter AISI 446 stainless steel rod. The chemical composition of the AISI 446 steel was in agreement with the supplier’s test cer
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