Dissimilar-weld failure analysis and development program. Volume 2. Metallurgical characteristics. Final report
This volume describes the results of investigations to determine the metallurgical characteristics and microstructural evolution, in fossil-fired boiler service or after accelerated heat treatment employed in the laboratory, of dissimilar metal welds (DMWs) between low-alloy ferritic steel and 300-series stainless steel made with stainless-steel (iron-base) and nickel-base filler metals. Microstructural changes in the ferritic-steel heat-affected zone (HAZ) occur in the form of softening of the HAZ, carbon diffusion from the HAZ into the weld metal, precipitation of carbides at the weld interface and at HAZ prior-austenite grain boundaries, and oxide notching. Three failure modes were identified: (1) prior-austenite grain boundary cracking (predominant in the HAZ of iron-base DMWs); (2) weld interface cracking (predominant in nickel-base DMWs); and (3) oxide notch propagation (predominant in thin-wall tubing). The mechanism of the first two modes involves nucleation and growth of creep voids associated with the M/sub 23/C/sub 6/ and M/sub 6/C carbides. The weld interface exhibits two types of carbide morphologies. Type I is a plane of coarse, closely spaced precipitates. This morphology constitutes about 80% of the interface in nickel-base DMWs. Type II is a wide band of smaller precipitates. The Type II morphology constitutes 100% and approx.20% of the weld interface of iron-base and nickel-base DMWs, respectively. The creep voids were only associated with Type I precipitates. A method using the microhardness values of the ferritic-steel HAZ as a function of the Larson-Miller parameter in conjunction with the interfacial microstructure has also been established to simulate, in laboratory welds, the HAZ and interface carbides features seen in long-term-service welds. 66 refs., 42 figs., 5 tabs.
- Research Organization:
- Metal Properties Council, Inc., New York (USA)
- OSTI ID:
- 6352227
- Report Number(s):
- EPRI-CS-4252-Vol.2; ON: TI86920065
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
36 MATERIALS SCIENCE
BOILERS
WELDED JOINTS
FOSSIL-FUEL POWER PLANTS
SUPERHEATERS
FAILURE MODE ANALYSIS
METALLOGRAPHY
AUSTENITIC STEELS
CREEP
EXPERIMENTAL DATA
FERRITIC STEELS
HEAT AFFECTED ZONE
HEAT TREATMENTS
MATERIALS
NONDESTRUCTIVE TESTING
SERVICE LIFE
ALLOYS
DATA
INFORMATION
IRON ALLOYS
IRON BASE ALLOYS
JOINTS
MATERIALS TESTING
MECHANICAL PROPERTIES
NUMERICAL DATA
POWER PLANTS
STEELS
SYSTEM FAILURE ANALYSIS
SYSTEMS ANALYSIS
TESTING
THERMAL POWER PLANTS
ZONES
200101* - Fossil-Fueled Power Plants- Cooling & Heat Transfer Equipment & Systems
360103 - Metals & Alloys- Mechanical Properties