Masters Theses

KK Khan

Date of Award

12-1988

Degree Type

Thesis

Degree Name

Master of Science

Major

Metallurgical Engineering

Major Professor

Carl D. Lundin

Committee Members

Charlie R. Brooks,

Abstract

2 l/4Cr-lMo steels have been used in power and chemical plants for elevated temperature high pressure components since the 1930's. Recent concerns involving submerged arc (SA) and shielded metal arc (SMA) welded steam piping during service have resulted in the inception of this study. Since welded piping may be either annealed and tempered (A6eT), normalized and tempered (N&T) or subcritically postweld heat treated (PWHT), the aim of this study was the determination of the elevated temperature properties of 2 l/4Cr-lMo weldments subjected to these heat treatments after welding.

The SA weldment fabricated for this study had a difference of chromium content (0.62%) between the base metal (2.07%) and weld metal (2.69%). The significance of this chromium difference was investigated in addition to the determination of stress rupture properties in the different heat treated conditions. Another part of this study sought to evaluate the effect of the severity of tempering on the creep strength of SMA 2 l/4Cr-lMo weld metal.

After heat treatment a "soft" carbon denuded ferritic region was found along the fusion line in the HAZ of the SA weldment because of carbon migration from the base metal to the weld metal driven by elemental differences (especially Cr) during heat treatment. This region was found to be widest for the A&T material followed by N6eT and only a grain wide when PWHT(C1.2). Hardness measurements on A&T specimens revealed a hardness approximately 35 DPH lower in the V "soft" zone compared to the adjacent enriched zone. The soft zone was found to be 15 DPH softer than base metal and 10 DPH softer than the weld metal remote from the fusion line.

SA weldment stress rupture and tensile tests conducted at 950 and 1050°F on all-weld metal specimens extracted parallel to the welding direction and composite specimens (base metal-HAZ-weld metal) extracted transverse to the welding direction revealed that none of the specimens ruptured in the "soft" zone. All transverse tensile specimens ruptured in the weld metal. Transverse creep specimens ruptured in the weld metal in N&T condition and in the base metal in the PWHT(C1.2) condition. Transverse stress rupture specimens in the A&T condition ruptured in the weld metal at 950°F, base metal at 1050°F and in the weld metal at temperatures of 1150 and 1225°F. Creep rupture testing of specimens from the SA weldment reveals superior creep properties in N&T condition when tested at 950°F and in A&T condition when tested at 1050°F.

Stress rupture and tensile testing of the SMA weldment indicates that the strength decreases with an increase in the temper parameter. Characterization of microstructure using analytical techniques indicated that stability of M2C type carbides in the various microconstituents (ferrite, bainite) dictate the elevated temperature properties of the weld metal. Microscopy of the "soft" ferrite band revealed presence of fine M2C type carbides. This "soft" ferritic region is strengthened by fine acicular M2C carbides along with interactive solid solution hardening effect of molybdenum in ferrite. In addition, adjacent regions which are stronger than the soft zone exert a constraint effect thereby preventing rupture. Thus, the zone did not contribute to creep failure. A carbide evolution sequence in the weld metal due to tempering or service exposure has been determined from this study.

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