Masters Theses

Sher Mohammed

Date of Award

11-1981

Degree Type

Thesis

Degree Name

Master of Science

Major

Metallurgical Engineering

Major Professor

Carl D. Lundin

Committee Members

E. E. Stansbury, C. R. Brooks

Abstract

This research is directed toward the effect of welding procedures on the fabrication of low alloy steels (SA 508 and SA 533) for pressure vessel use. During fabrication of Low Alloy Steels used in Light Water Reactor construction, preheat for welding, and in some instances, the maintenance of preheat after welding, is employed to avoid fabrication problems. The employment of preheat is aimed primarily at the elimination of HAZ cracking, which is normally attributed to cold cracking due to hydrogen and the low temperature transformation of austenite to martensite. It is beneficial to achieve a higher fracture toughness in the weld HAZ relative to chat of the adjacent base material and it has been observed that untempered HAZ regions normally show lower toughness; but a properly PWHT HAZ shows superior toughness, especially if the structure is martensitic. In addition to modifying the metallurgical structure, PWHT is applied to reduce or eliminate the residual welding stresses. Materials containing carbide forming alloy elements often exhibit precipitation of the alloy carbides within the grains of the weld HAZ during PHWT and serve to strengthen the matrix in comparison to the grain boundary. The creep strain necessary to relieve the residual stresses is forced into the weakened grain boundary region and consequently extensive grain boundary deformation may induce cracks in the HAZ. This problem of reheat cracking can be eliminated by controlling the compositions of the materials.

In order to approach the preheat problem, the continuous cooling transformation and isothermal transformation behavior has been determined dilatometrically and by resistivity measurements. This documentation of CCT and I-T behavior is unique in that it includes the full spectrum of cooling and hold behavior and is extended to weld HAZ situations. The two HAZ regions correspond to coarse-grained region (2400°F peak) which lies adjacent to the fusion zone and the refined region (1650°F peak) in the outer extremeties of the HAZ. The transformation products resulting from exposure to peak temperature of 2400 and 1650°F were determined metallographically. The result of CCT behavior showed that the higher peak temperature (2400°F) causes the bainite nose to shift to significantly longer times. It was also observed for the coarse-grained region, that the CCT behavior for both steels is identical. Comparative energy input/thickness values were also calculated for equivalent cooling rates.

The reheat cracking tendency for both steels was evaluated by metallographic studies of simulated HAZ structures subjected to PWHT cycles and simultaneous restraint. Both SA 533 and SA 508 cracked intergranularly. The stress rupture parameter (this is the product of the stress for a rupture life of ten (10) minutes and the corresponding reduction of area) calculated for both steels showed that SA 508 was more susceptible to reheat cracking than SA 533.

Cold cracking tests comprised of the Battelle Test and University of Tennessee's Hydrogen Susceptibility Test were conducted. These tests indicated that for SA 508, a higher preheat temperature is required to avoid cracking than for SA 533. Further, the Hydrogen Susceptibility Test results show that SA 508 composition is more susceptible to hydrogen embrittlement than is the composition of SA 533.

The charpy impact toughness was determined for the various HAZ transformation products by simulating the HAZ structures. It was found that tempered martensite is far superior to tempered bainite in regards to the HAZ toughness. For an untempered martensitic structure, a post-weld hold at 600°F was injurious to the as-welded HAZ toughness, but superior toughness could be achieved by PWHT. Grain refining thermal cycles do not improve toughness per se if the transformation product is untempered bainite. The heat flow associated with and peak temperatures reached by additional passes are considered necessary for optimum HAZ toughness.

It is recommended that during welding of SA 533 and SA 508, weld energy input should be controlled to obtain a cooling rate faster than the critical cooling rate for bainite, and thus form a primarily martensitic structure. PWHT of this structure is recommended after welding in order to temper the martensite and optimize the HAZ behavior.

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