Steel can be welded to cast iron if the work pieces are free to align themselves after contraction strains which result during cooling. Studding can be done when the weld is in a position in which the parts are not free to align themselves. A steel weld made to cast iron appears acceptable until just about the time it is being most admired during cooling, when it invariably cracks along the edge of the weld. This is easily explained when we consider that steel has approximately four times the strength of cast iron. The definite location of the break comes about because the cast iron adjacent to the weld has become chilled cast iron, no matter whether it was originally so or not, and a sharp demarcation of structure, together with a possible layer of weaker cast iron, invariably causes the break to take place in this layer of cementite. Studding

is recommended for large castings where strength is required.

Properly aligned and spaced holes are drilled carefully so that they are not drilled all the way through the casting. The holes are tapped for the correct threading and the headless stud bolts are screwed into these holes; they should project from 3.2 mm to 6 mm (1/8 to 1/4 in.) above the surface of the casting.

Studs can be ordinary steel headless stud bolts, and are welded with a low-carbon steel electrode. While shielded metal arc welding is usually chosen for the studding procedure, any process which uses this electrode can be used. The thickness and spacing of the studs should be proportioned so that the studs will have at least the full strength of the cast iron section. The space between the studs should be about 2-1/2 times the diameter of the stud, and they should be staggered. They should also have a reasonably fine thread.

Strength

Since steel has four times the strength of cast iron, the problem is to proportion the studs so that the ratio of the cross section of the steel to the cross section of the remaining cast iron is something less than four to one at any one section that will be subjected to cooling strains at the same time. Simply stated, this means large studs for large sections and smaller studs for smaller sections. Steam-tight joints on cast iron can be made with the efficient use of studs.

Another good application of studding is the use of large studs as an anchor or nucleus for breaks in castings such as gear teeth. One to three of these large anchor studs, together with some small regular studs appropriately distributed, will provide the necessary weld strength to the cast iron, as well as strength to withstand some shear and thrust strains. The weld, built up to size and machined off, finishes into a perfect wearing or bearing surface. This method of studding is also applied to cast steel, in cases where the original section has not been adequate to withstand the

strains.

Studding Methods

Following are procedures to produce studded welds to repair breaks in cast iron:

(1) Grind or chip the crack to form a V from one surface, if only one surface is accessible for welding, or half way from both faces if both are accessible, saving enough pieces of the original assembly to keep the piece securely clamped in alignment. Before any of the welding is done, the stud holes are to be drilled and tapped with a bottoming tap, and the studs screwed in

tightly, completely filling the hole for a depth of at least four times the diameter of the studs. A convenient way of doing this is to have rods threaded for their entire length, screw these in tightly, and saw them off 3.2 to 6 mm (1/8 to 114 to in.) above the surface of the work, depending on accessibility. A narrow V should have the studs closer to the work than a more open one.

(2) Weld around the studs so that they are part of the cast iron; then weld between the studs crosswise or diagonally until the entire surface of the V and the adjacent surface forming the underside of the pad is completely covered before proceeding with the main filling in of the weld. Add metal one bead at a time so that no large section of the weld solidifies from the molten state at any one time.

Welding the stud to the cast iron makes the stud an integral part of the casting. Welding from stud to stud diagonally or crosswise draws the fractured surfaces together to their original contact if a small amount of the original break has been left intact at the bottom of the V. This can be done in most instances, but not if pieces are broken out and lost, in which case they must be replaced by a casting or forging.

In this steel studding process for cast iron, especially for cylinders and similar work, machining is often necessary after welding. The hard stratum of metal directly under the weld prevents ordinary cut- ting operations, so it must be ground. There are other means of taking care of this hardened layer for machining on the surface; one of the methods is to finish the machined part with a nickel-copper electrode, which has the property of merging with the cast iron so

that no hard layer of cementite is formed. This nickel-copper electrode was developed for repairing scored cylinders and similar applications.

 

X