This section is from the book "Welding And Cutting Metals By Aid Of Gases Or Electricity", by L. A. Groth. Also available from Amazon: Welding and cutting metals by aid of gases or electricity.
General - Welding Instead of Riveting - Advantages and Disadvantages of Welded Boiler Joints - Difficulties of Riveted Joints - Repairs on Steam Boilers : Cracks - Locating the Cracks - Strength of Riveted Joints - Cost of Riveted Seams - Autogenous Welding - Electric Welding - Hydrogen Welding - Acetylene - Dissous Welding - Repairs on Marine Boilers : Outside Corrosion - Internal Corrosion - Cracks - Autogenous Welding - Electric Welding - German Steam Users Association - British Steam Users Association - Application of Compressed Gases to Welding - Defects of Welds - Welding of Tanks - Relative Cost of Riveting and Acetylene Welding - Keeping Down the Heap.
In 1863 an article appeared in the Mechanics' Magazine urging that boiler seams should be welded instead of riveted, and stating that almost the only difference between the seam then in vogue and that of a hundred years before was the use of a contrivance for finishing the rivet head.
The rings which form the cylindrical shell of the boiler are curved from flat plates, and must be joined at the edges and at their ends. The requisites of such joints are : -
1. Strength to resist the strain from internal pressure.
2. Tightness against leakage of water or steam, with a construction which shall not be too costly.
3. Ability to withstand heat.
4. Ability to undergo changes of shape from expansions and contractions without injury to the metal.
The two edges of the plate which are to be joined are arranged so as to lap over each other to be secured together, and this attaching can be done by welding.
Bolting with a thread and nut will not meet the requirement of tightness against leakage unless the joint surfaces are planed and finished, and the boltholes seamed, and the bolts turned. This is prohibitory from its cost, and even if this were not a barrier, the friction of the nut so reduces the clamping power of the screw bolt that it would make a much weaker joint than is secured by the other plans.
Welding of boiler plate to make the joint with itself or other parts of the shell offers many advantages.
The welding property of wrought iron and ductile steel enables them to unite at clean surfaces when pressed together with sufficient force in a state of sufficient plasticity from heat. The presence of oxide of iron, or dirt, or cinder between the contact surfaces or inadequate pressure to unite the surfaces together will prevent a satisfactory weld. When satisfactory the weld may be expected to be as strong as the rest of the metal.
Welding of plate is done by lapping the two edges over for two or three inches, heating the lap to a welding heat on both sides by a flame or jet of gas free from sulphur or other oxidising tendencies, and then bringing the lapped surfaces together either by the force of percussive hammer or sledge blows or by steady pressure of cams or roller presses. Some fluxing material, like borax, which will make a fluid glass with oxide of iron, may be used as a protection for the contact surfaces, so as to prevent oxidation from exposure to air, with the expectation that it will be expelled from the joint by the welding pressure, and carry with it everything which would interfere with good welding.
Advantages of welded boiler joints : -
1. It makes the joint as strong as the rest of the plate, or nearly so.
2. The plate is no thicker at the joints than elsewhere. This avoidance of a lap keeps the tensile strain from internal pressure always in the axis of the plate and without a tendency to flux at the lap or joint.
3. Double or extra thickness is avoided at laps or joints. The plate gets unnecessarily hot at multiple thicknesses, and oxidation is more rapid there.
4. No rivets are required, which makes the boiler lighter and less liable to leak.
5. A good welded seam is watertight, and requires no caulking.
Disadvantages of welded seam in boilers : -
1. It cannot be inspected for its satisfactory quality, unless it is so bad as to allow water to leak through it under pressure. But it may be watertight and yet be far from having full strength. While a test by hammer taps to observe the resonance of the metal at the joint will reveal much to the practised ear, it lacks the convincing force of an inspection of each single rivet in a riveted seam.
2. Welded joints in large shells can only be obtained from a few firms with facilities and experience for such work. This has some effect upon the cost of such joints. But when a satisfactorily welded seam can be obtained it makes an ideal joint.
In cylinders with closed ends the last seam must be riveted even if the others are welded. The exception is where the head is flanged outward, or is convex inward so as to bring the closing joint outside the shell.
Riveted joints, with their consequent double thickness of metal, in parts exposed to the fire, give rise to serious difficulties. Being the weakest parts of the structure, they concentrate upon themselves all strains of unequal expansion, giving rise to frequent leaks, and not rarely to actual rupture.
The joints between tubes and tube sheets also give much trouble when exposed to the direct fire, as in locomotive and tubular boilers. This difficulty is partly overcome by welding.