Roll welding

Along with high mechanical properties, copper improves its conductivity in the range of lower temperatures (at -160°C 400, -190°C 450, and -252°C 1600 kcal/m °C hr). It softens in the temperature range of 200 to 220°C as shown in Figure 3.5. The casting properties of copper are rather fair, but copper can be readily stretched, flattened, rolled, welded and brazed. For chemical plant work, welded or brazed joints have become almost universal. 

The shell of a heat exchanger is commonly made of carbon steel and standard pipes are used for the smaller sizes and rolled welded plate for the larger sizes (say 0.4-1.0 m). 

Apart from the corrosion resistance additionally any limitations on fabrication must be properly considered, i.e. the ability to be machined, rolled, welded, or cast. 

Reboiler heating upon startup should proceed in slow, incremental steps to avoid excessive thermal stresses. The thermal stress problem is most severe when reboiler tubes are roll-welded to the tubesheet (358). 

HOT ROLLED. WELDED ANNEALED. WELDED WELDED. ANNEALfD WELD... 

Welded pipe is made from rolled strips formed into cylinders and seam-welded by various methods. The welds are credited with 60 to 100 percent of the strength of the pipe wall depending on welding and inspec tion procedures. Larger diameters and lower ratios of wall thickness to diameter can be obtained in welded pipe than can be... 

Conical ends vary from machined forgings to roll-formed tubing, and clamps vary from machined forgings to bands to which sever roll-formed channels are attached at their centers by spot welding. A hinge may be inserted in the band as a substitute for one of the draw bolts. Latches may also be substituted for draw bolts. 

Welded Tube Joints When suitable materials of construc tion are used, the tube ends may be welded to the tube sheets. Welded Joints may be seal-welded Tor additional tightness beyond that of tube rolling or may be strength-welded. Strength-welded Joints have been found satisfactoiw in veiy severe seivdces. Welded Joints may or may not be rolled before or after welding. 

Tube-end rolling before welding may leave lubricant from the tube expander in the tube hole. Fouhng during normal operation followed by maintenance operations will leave various impurities in and near the tube ends. Satisfactory welds are rarely possible under such conditions, since tube-end welding requires extreme cleanliness in the area to be welded. 

Tube e] ansion after welding has been found useful for low and moderate pressures. In high-pressure service tube rolling has not been able to prevent leakage after weld failure. 

Any region where two surfaces are loosely joined or come into close proximity qualifies as a crevice site as long as water may enter. Partially exfoliated coatings may produce crevices. Surfaces may be metallic or nonmetallic. Usually, however, at least one surface is metallic. Crevices commonly are present at gaskets, flanges, washers, bolt holes, rolled tube ends, incompletely fused welds, contact points in plate-and-... 

Shell and tube Rolled ends at tube sheet Open welds at tube sheet Beneath deposits Water box gaskets Bolt holes, nuts, washers Baffle openings Disbonded water box linings... 

In general, the higher the residual or applied metal stress, the more severe the corrosion at a given acidic pH. This explains why many heat exchanger tube ends are often attacked so severely (Fig. 7.1). Tube ends that have been rolled or welded often contain high residual stress. Further, crevices are sometimes present in which acidic species may concentrate (see Chap. 2, Crevice Corrosion ). Screens, rolled sheet metal, and other highly worked metals (not stress relieved) are also prone to attack. 

Severe corrosion by turbulent mill coolant was found generally throughout a rolling-oil system. Hose couplings were severely wasted in as little as 8 weeks (Fig. 7.23A and B). Turbulence caused by high-velocity flow through nozzles accelerated attack. Attack at bends, elbows, intrusive welds, and discharge areas was also severe. 

Sites possessing high-residual stresses—such as welded assemblies, rolled-in tube ends—may be susceptible. 

Seams. As defects, seams are distinct from weld seams and may be found in nonwelded (seamless) tubes. They are caused by crevices that have been closed by some rolling process but remain unfused. 

Seams. As a defect, a seam is distinct from the seam resulting from a welding process. Seam defects can be found in nonwelded (seamless) tubes. They can originate from blow holes or nonmetallic inclusions in the ingot and are caused by crevices that have been closed by some rolling process but remain unfused. At times, they will appear in a spiral pattern in tubes. Seams can be very tight and appear as hairlines on the surface. They can cause failure when the component is pressurized. 

The sheet may also be inspected for the quality of welding and checked for pin-holes by a weld-hole detector before it is finally cut into lengths or wound onto rolls. 

Many of the processes used for working metals are energy-intensive. Large amounts of energy are needed to melt metals, to roll them to sections, to machine them or to weld them together. Broadly speaking, the more steps there are between raw metal... 

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