Masters Theses

Date of Award

8-1981

Degree Type

Thesis

Degree Name

Master of Science

Major

Metallurgical Engineering

Major Professor

Carl D. Lundin

Abstract

In order to assess the metallurgical significance of weld discontinuities, numerous studies were undertaken. These experiments included mechanical testing, non-destructive testing, metallographic, and fractographic examination, and hydrogen determinations. Tensile, bend and impact tests on weld metals E6010, E7018, E9018B3L, and E11018 in both as-welded and PWHT conditions were conducted and evaluated. E8010 low-cycle tension-compression fatigue specimens tested by NBS were included in the evaluations. Interrupted tensile testing was employed to determine the strain level necessary to cause fracture adjacent to a discontinuity.

A new approach was taken for introducing discontinuities into test weldments. It was found that the introduction of discontinuities into welds by controlling welding procedure and environment was difficult and discontinuity sizes and distribution were not representative of those encountered in practice. Am unskilled welder was employed to fabricate the test weldments using an acceptable welding procedure. The resulting weldments contained a random distribution of small discontinuities as a result of 'unintentional operator errors. The discontinuity sizes and distributions occurring were observed to be more representative of those encountered in industry than those produced by a skilled welder using good welding practice.

Mechanical tests confirmed that the presence of minor discontinuities had little effect upon strength or impact toughness but ductility was reduced. The reduction in ductility was attributed to the formation of low ductility regions or "haloes" surrounding the discontinuities. The haloes were easily detected on the fracture surfaces of the tensile and bend test specimens. The haloes are caused by the interaction of strain and the hydrogen present in the cavities forming or associated with weld discontinuities, Ferritic materials are especially susceptible to the effects of this interaction between strain and hydrogen. Haloes were also noted in concert with fatigue fractures and are considered to contribute to premature failures.

Metallographic and fractographic examination revealed a variety of microstructures and microscopic fracture modes. Haloes can occur in all weld metals, regardless of microstructure but their occurrence depends upon the level of hydrogen, the amount of plastic strain, and the local stress concentration at the discontinuity. The fracture mode in the haloes was observed to be primarily quasi-cleavage. Varying degrees of microscopic ductility were apparent in the halo fracture areas. It is postulated that this is due to variations in hydrogen content within the discontinuity and the local stress concentration present during the initiation of the halo fracture. A mechanism for initiation and propagation of fracture to cause haloes is derived.

The level of plastic strain required to initiate halo fractures in hydrogen containing welds was found to be as low as 1.0% when determined by the interrupted tensile test methods. Incipient halo fractures in the interrupted tensile test specimens were detected by radiographic methods. When fatigue conditions prevail, the level of plastic strain required to initiate a halo fracture was observed to be at least as low as 0.45%.

The occurrence of haloes in welds containing discontinuities could affect the integrity of weldments exposed to conditions where halo formation could occur. The formation of haloes requires the presence of discontinuities, hydrogen, and strain in the weld metal. Haloes may be eliminated if proper care is exercised during welding to reduce or eliminate the conditions leading to halo formation. Discontinuities and hydrogen are not easily eliminated, but the strain imposed on the weld metal may be reduced by proper joint design and/or use of overmatching weld metals. Minor amounts of discontinuities can be tolerated in welds provided that the weld metal strength overmatches that of the base material.

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