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- Title
Failure Analysis of Fuel-Film Cooling Assembly of Reaction Control System for Satellite.
- Authors
Shifa, Madni; Tariq, Fawad
- Abstract
The present case study discusses the failure investigation conducted on fuel-film cooling assembly (FFCA) of prototype reaction control system (RCS), which was failed prematurely during static hot-fire test after 15 s of operation. The kerosene fuel is injected from small holes on to the nozzle convergent for cooling. Upon dismantling, circumferential cracks were observed in the weld joint of austenitic stainless steel. Stainless steel grade 304 is known for its satisfactory weld joint with grade 308 stainless steel filler, and no cracks were observed during dye penetrant test on weld. However, unexpected, and sudden failure of this component warrants detailed analysis to identify the root cause of the failure. Visual inspection, macro/micro examination via optical and scanning electron microscope, optical emission spectroscopy and microhardness survey were carried out on both failed weld joint as well as on samples taken from un-cracked weld for comparison. Visual observations show signs of heat tint near the cracked region, which indicates that the assembly was exposed to high temperatures. Macroscopic examination revealed weld metal cracking, which was found to be brittle transgranular fracture upon microscopic examination. Research findings established that the high welding heat input in manual TIG welding and non-equilibrium cooling conditions resulted in poor weld quality containing high delta ferrite content in the weld metal. Although limited amount of delta ferrite is considered beneficial for avoiding hot cracking in austenitic stainless steel weld joints, weld containing higher delta ferrite content is prone to embrittlement at elevated temperatures. During the hot-fire test of RCS, the FFCA was heated to high temperature, which caused transformation of delta ferrite into hard sigma phase. SEM–EDS analysis and microhardness measurements have confirmed the formation of sigma phase and extensive carbide network in weld deposit. Thermal stresses combined with metastable delta ferrite to sigma phase transformation resulted in weld metal embrittlement, which eventually fractured in brittle manner.
- Subjects
STAINLESS steel welding; EMBRITTLEMENT; FAILURE analysis; AUSTENITIC stainless steel; OPTICAL microscopes; GAS tungsten arc welding
- Publication
Journal of Failure Analysis & Prevention, 2024, Vol 24, Issue 1, p300
- ISSN
1547-7029
- Publication type
Article
- DOI
10.1007/s11668-023-01840-3