Effect of long-term service exposure at elevated temperature on microstructural changes of 5Cr-0.5Mo steels

Journal Article · Fri Aug 01 04:00:00 EDT 1997 · Metallurgical and Materials Transactions. A, Physical Metallurgy and Materials Science

DOI:https://doi.org/10.1007/s11661-997-0252-x· OSTI ID:540990

Das, S; Joarder, A ^[1]

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1. National Metallurgical Lab., Jamshedpur (India). Materials Characterization Div.

Effects on long-term service exposure at elevated temperature on microstructural changes have been studied for both virgin and service-exposed process heater tube pipes of 5Cr-0.5Mo steels used in oil refineries. Samples selected for this study had experienced a nominal temperature range of 450 C to 500 C for about 20 to 25 years. Two different initial virgin microstructures were taken and designated by steel A and steel B. The virgin microstructure of steel A exhibited fine platelets of fibrous or hairlike M{sub 2}C carbides within the ferrite grains and occasionally irregularly shaped M{sub 23}C{sub 6}, both along the grain boundaries and at the grain interiors, and very few spheroidally shaped M{sub 3}C, either along the grain boundaries or at the grain interiors. The size, shape, position, distribution, and type of carbides in virgin steel A changed significantly due to 220,000 hours of service exposure in the temperature range of 450 C to 500 C. Massive M{sub 23}C{sub 6} carbides precipitated along the grain boundaries. In addition, regular geometrically shaped M{sub 23}C{sub 6} carbides, such as hexagonal, square, and triangular type, were observed to form at the grain interiors. The virgin steel B microstructure exhibited predominantly M{sub 23}C{sub 6} carbides, either along the grain boundaries or at the lath boundaries. Occasionally, fine platelets of M{sub 2}C carbides were also observed within the laths. The position, shape, distribution, and type of carbides did not change significantly due to 172,000 hours of service exposure in the temperature range of 450 C to 500 C. The average interparticle spacings of the carbides increased from 0.35 to 1.2 {micro}m due to 172,000 hours of exposure.

• Sponsoring Organization: USDOE
• OSTI ID: 540990
• Journal Information: Metallurgical and Materials Transactions. A, Physical Metallurgy and Materials Science, Journal Name: Metallurgical and Materials Transactions. A, Physical Metallurgy and Materials Science Journal Issue: 8 Vol. 28; ISSN 1073-5623; ISSN MMTAEB
• Country of Publication: United States
• Language: English

Related Subjects

36 MATERIALS SCIENCE
CARBIDES
FERRITE
GRAIN BOUNDARIES
MICROSTRUCTURE
MORPHOLOGY
SERVICE LIFE
STEEL-CR5MO
TEMPERATURE RANGE 0400-1000 K