REG NASA-LLIS-0571-1997 Lessons Learned Intergranular Corrosion of Austenitic Steels due to Nitriding Heat Treating Controls for hazardous liquid vapor vent lines Response to anoma.pdf

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1、Lessons Learned Entry: 0571Lesson Info:a71 Lesson Number: 0571a71 Lesson Date: 1997-09-23a71 Submitting Organization: JSCa71 Submitted by: L. UnderwoodSubject: Intergranular Corrosion of Austenitic Steels due to Nitriding/Heat Treating; Controls for hazardous liquid/vapor vent lines; Response to ano

2、malous conditions during a hazardous test. Description of Driving Event: There was an inadvertent release of nitrogen tetroxide (N2O4) at JSC on April 21, 1994 at Approximately 11:20 am. The release occurred in the Thermochemical Test Area (TTA) during the preparation phase of a simulated altitude t

3、est of a small bi-propellant rocket engine. The immediate trigger event was the failure of a three-way solenoid valve. This failure enabled liquid oxidizer (N2O4) to flow in a vent line and the oxidizer burner stack (which was used as a control device to burn off small amounts of N2O4 which might le

4、ak or be released from the system). As the system was being charged to the desired operating pressure of 800 psig, the leak rate increased and the burner stack was filled with liquid oxidizer and saturated oxidizer vapor. The system was initially charged to 850 psig, instead of the desired pressure

5、of 800 psig, and was vented to reduce the pressure. Venting resulted in the release of a small cloud of oxidizer from the burner stack. Although this was an indication of anomalous system operation it was mistakenly attributed to an improper setting on the methane supply valve for the N2O4 burner, a

6、nd test was continued. Other indications that the system was not operating properly were available in the control room, but were not noticed or were misinterpreted. A subsequent attempt to raise system pressure to 800 psig for the test caused the release of a large oxidizer cloud and percolated liqu

7、id oxidizer from the burner stack onto the ground area surrounding the burner, resulting in a spill.Subsequent to this incident, several fittings started leaking. Two fittings were successfully tightened, but attempts to tighten the third led to an increase in the leak rate. This fitting ( a mechani

8、cal Parker- type flareless, single ferrule fitting) was removed and sent to the materials laboratory for analysis. Leakage was attributed to the collapse of the ferrule component. The ferrule had clearly lost all of its functional integrity and separated into small fragments. All surfaces showed evi

9、dence of severe intergranular attack. Chemical analysis identified the ferrule as a 316-type stainless steel. Metallographic examination of a sample piece confirmed that gross intergranular corrosion attack of the ferrule material occurred. In addition, the examination suggested that the grain bound

10、ary Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-sensitization may have occurred in the material, increasing the susceptibility of the ferrule to intergranular corrosion. Examination of other fittings removed from the system also revealed at least

11、 slight amounts of intergranular corrosion. Discussions with the manufacturer indicated that at least some of the ferrules had been nitrided, a process which may require extended times at temperatures of from 480 degrees C to 590 degrees C. Since austenitic stainless steels becomes susceptible when

12、subjected to temperatures from 480 degrees C to 815 degrees C, the subject ferrules may have inadvertently become susceecptible during the nitriding process.Lesson(s) Learned: Improper heat treatment of stainless steel fittings used in systems containing corrosive fluids may sensitize the fitting ma

13、terials to intergranular corrosion and cause leaks. Vapor vent lines for hazardous liquid systems must be designed to accommodate potential leaks of liquid into the lines. Using three-way solenoid valves to open hazardous fluid systems to vent for pressure transducer calibration introduces an unnece

14、ssary leak path and increases risk.Recommendation(s): Periodically inspect and replace effected fittings in corrosive use. Use stainless steel fittings made of material other the 316 type stainless steel for hazardous fluid systems. ): Modify the system to enable it to safely accommodate component f

15、ailures which could result in a liquid flow to the vent system. One design of the system which should be considered is a liquid trap capable of containing the amount of liquid which could potentially leak.Evidence of Recurrence Control Effectiveness: N/ADocuments Related to Lesson: N/AMission Direct

16、orate(s): N/AAdditional Key Phrase(s): N/AAdditional Info: Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-Approval Info: a71 Approval Date: 1997-10-21a71 Approval Name: Ronald A. Montaguea71 Approval Organization: NA3a71 Approval Phone Number: 281-483-8576Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-