Success with hand weld overlays of Type 310 stainless to repair corrosion damage in bottom cones of Kraft digesters at Longview, Wash., induced Weyerhauser to use Type 310 stainless steel to repair complete digester shells of five units in 1956. Automatic welding guns were used with argon gas. At Everett, seven more digesters were overlaid in much the same manner. Type 316 was used for work at Everett because, by the time work was begun, some failures had been noted in the upper parts of the units at Longview.
Analysis showed that the original weld material was being diluted by the base metal in the ratio of 1 to 2.
In an effort to reduce the amount of dilution experienced, alterations were made in the method of deposit on the upper shell of the final unit at Everett. The gun was tilted 20 degrees to normal to effect greater impingement on preceding beads. Weight of deposit was increased, and rate of feed of argon ^ gas also was increased to effect better shielding. The top four feet of the shell was given a double overlay. Tilting the gun did not materially improve the quality of the deposit. Double overlay produced a surface with an analysis close to that of the overlay wire.
Reports of inspections in several zones of the digesters are given. Significance of pits in overlaid material of this kind is emphasized.
Tests are being conducted using various types of weld overlay to find one more resistant. Some design changes were made in the digester. Impingement plates were installed to absorb erosive force of liquor and to provide cathodic protection to the overlay.
3.7.3, 9.6.6, 8.5.3, 6.2.5.