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Abstract
In order to improve the quality of welds for cryogenic applications, the factors most influencing the quality of the weld first must be identified. To this end, a designed experiment using a fractional factorial test plan was developed to evaluate the effects of six independent variables upon the weld quality. These independent variables were the bevel preparation method, the welding process, the material thickness, the position of the weld, the heat input, and the joint design. Two levels of each variable were selected resulting in a 26 experiment; a 26-2 fractional test plan was run. A553 Gr. I low alloy steel was the base material studied; commonly known as 9% nickel steel, this material has wide application in cryogenic field storage tanks.
The weld quality was evaluated by four response variables: the width of the heat affected zone (HAZ) at the comers of the weld, the HAZ width at the midpoint of the plate, the HAZ hardness readings, and the HAZ impact values. The results of this study indicated a number of the independent variables affect the quality of welds significantly. A reduction in the heat input was shown to minimize the HAZ width in the two areas reviewed. An increase in heat input led to a decrease in the HAZ hardness. An increase in the thickness resulted in a narrower HAZ at the midpoint of the plate and reduction in the HAZ hardnesses. The use of the horizontal position increased the HAZ impact strength while the hardness of the HAZ was reduced significantly.
Because of the fractional factorials, the second order interactions possessed aliases indistinguishable from their counterparts; therefore, only opinions could be expressed as to which of the aliases were probably valid. An interaction between the bevel preparation method and the joint design, aliased by an interaction between position and heat input, was significant for three of the four response variables. The former interaction, if true, would be of particular interest to industry considering the design and cost implications. It was concluded that further study was imperative in order to identify positively these second order effects.
