Weld overlays

Ferrous-aUoy-clad tube sheets are generally prepared by a weld overlay process in which the alloy material is deposited by welding upon the face of the tube sheet. Precautions are required to produce a weld deposit free of defects, since these may permit the process fluid to attack the base metal below the alloy. Copper-aUoy-clad tube sheets are prepared by brazing the alloy to the carbon steel backing material. 

In a third process, a loose liner is fastened to a carbon steel shell by welds spaced so as to prevent collapse of the hner. A fourth method is weld overlay, which involves depositing multiple layers of alloy weld metal to cover the steel surface. 

Weld overlays of stainless steel or cobalt-based wear-resistant and hard-facing alloys such as Stellite may salvage damaged equipment. In addition, weld overlays incorporated into susceptible zones of new equipment may provide cost-effective resistance to cavitation damage. 

The next type of joint is weld overlays, which are often utilized to eliminate the need for costly thrust blocks [Fig. 4-2(p)]. In designing the pipe an analysis was made to ensure that it possessed sufficient longitudinal strength. It makes sense, then, to make the weld joints be at least as strong as the longitudinal strength of the pipe rather than just as an internal pressure-seal pipe. 

The amount of hydrogen partial pressure reduction depends upon the materials and the relative thickness of the cladding/ weld overlay and the base metal—the thicker the stainless barrier is relative to the base metal the better.32 Archakov and Grebeshkova33 mathematically considered how stainless steel corrosion barrier layers increase resistance of carbon and low alloy steels to high temperature hydrogen attack. 

EFFECT OF STAINLESS STEEL CLADDING OR WELD OVERLAY... 

Effect of Stainless Steel Cladding or Weld Overlay. 10... 

Report on the Effect of Stainless Steel Weld Overlay or Cladding on Hydrogen Attack of Underlying Steel, Materials Properties Council, New York, September 1984. 

In weld-overlay cladding, the solidifying melt forms columnar crystals perpendicular to the wall surface. Along the grain boundary of these crystals, the corrosion resistance is lower than in the rolled material. In the case of heavy corrosion, at least a two-layer weld-overlay cladding should therefore be selected. 

The wrought version of Haynes 6B alloy has higher corrosion resistance in aqueous media than the cast and weld overlay version (Stellite 6) because of hot forging and rolling breakdown of the interconnected carbides into discrete particles. Haynes 6B can be used in sulfuric acid in the concentration range 2-77 wt % at 24°C and in dilute solutions at higher temperatures (see Table 4.50). 

Figure 7.40 Surface appearance of the Unifuse composite tubes with alloy 625 weld overlay (Reproduced from COM 1997 with permission from the metallurgy Society of ClM)... 

Figure 7.41 309 Unifuse overlay on 1-1 /4Cr-l /2Mo boiler tube in the wall blower region after service for about 7 yr in a supercritical unit of the coal-fired boiler (a) surface appearance of the weld overlay, weld beads are still clearly visible (b) cross-section of the weld overlay showing no corrosion and no cracking on the weld overlay at the fusion line (Reproduced from COM 1997 with permission from the metallurgy Society of CIM)...

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