The strength of an ordinary weld in wrought iron varies from 10 to almost 100 per cent, of the strength of an equivalent cross-section of the metal. In general, however, a weld made under proper conditions runs between 50 and 70 per cent, for high-carbon iron and between 60 and 80 per cent, for low-carbon iron. The strength of a thermit-weld is subject to quite as great variance, for the reason that thermit steel is a definite compound and may be of quite different composition from the parts welded by it. Also it is well to bear in mind that the initial strength of thermit steel itself is subject to variations due to the amount of included slag, air holes, and to the rapidity of cooling; also, the chemical composition can be varied by the addition of alloy formers, such as nickel, chromium, and manganese; and the addition of titanium and manganese in small quantities, which are purifiers.

A number of tests of different character have been made by the company and by railroad and repair shops, some of the results of which are given as follows:

Test No. 1

"At the St. Louis and San Francisco Railroad shops, Springfield, Mo., recently the following test of a thermit-weld was made:

"A section of a cast-steel frame, 4 by 5 1/2 inches, was welded by the thermit process. In making the weld 75 pounds of thermit, 12 pounds of punchings, and 11/2 pounds of manganese were used. For molds, fire-brick was used, cut to shape.

"After the weld was cold, the collar on the bottom and one side was planed off 1/4 of an inch below the original surface of the casting, in order to show the place where the two metals had joined. The riser also was cut off, leaving the collar, however. The weld was absolutely solid, not a single blow hole appearing anywhere - not even the riser.

1 Reactions, Vol. I, 1908, published by Goldschmidt Thermit Co.

"The welded section (now 3 3/4 x 5 1/4 inches), with collar 1 inch thick on top and on one side, was then placed in wheel press on supports 14 3/4 inches apart and a piece of hardened steel, 1 inch square, placed as shown in figure 87.

"A pressure of 170 tons was applied before breaking. The fracture started at the bottom outside welded section, extending into the center of the weld at the top. The fracture showed that perfect amalgamation of the metals had taken place.

"In comparing the strength of this weld with original stock, assuming a maximum stress in the outer fiber for cast steel of 60,000 pounds to the square inch, a section 3 3/4 x 5 1/4 inches tested in the same way would break at 100 tons."

Fig. 87. - Arrangement of test piece for test No. 1.

In this test No. 1 it is presumed that the 12 pounds of punchings were mild steel. The manganese was used to freshen the iron, and most of it probably slagged as manganese oxid and came to the surface.

Test No. 2

"Two test bars taken from the upper part of a previously, but unsuccessfully, poured casting gave, on an average, 66,000 pounds per square inch tensile strength and 9. 5 per cent, elongation on a measured length of 2 inches. This casting showed in all the sections a clean, non-porous, dense grain. It appears possible, therefore, to produce steel castings of thermit and, in a case of necessity, the higher price would not be of importance."

1 Iran Age, April 26, 1906.

Test No. 3

The thermit process has been used by the Fore Shipbuilding Co./ of Quincy, Mass., who have made a number of tests of the physical properties of thermit metal. Bars of rolled steel, of section 2x4 1/2 inches were drilled, broken, and welded with thermit. Standard test bar were cut from the centre of the welded bar, and were submitted to the ordinary tests. As the test pieces were of uniform size, both in the stock and the welded section, the result is worth recording:

It is to be noticed that the tensile strength is 12.7 per cent, less in the weld than in the stock, and the tensile strength 2.5 per cent, less - a fair showing.

Test No. 4

By the Illinois Steel Co., Chicago.

Test No. 5

By the Pennsylvania Railroad, Altoona, Pa.3

1 Journal United States Artillery, Gustav Reiniger, July-August, 1907.

2 Transactions of the Society for Testing Materials, E. Stutz, 1905.

3 Transactions of the Society for Testing Materials, E. Stutz, 1905.

Test No. 6

It has been suggested that the thermit-weld may be strong in itself, but that it weakens the adjacent iron. To find if this is so, a section of welded rail was subjected to equal blows by a steam hammer, both on the unaffected rail and on the metal nearest the weld. The die used was a blunt tool, 1/4 inch in diameter. Measurement with a micrometer showed a depression of o. 1432 inch in the rail nearest the weld and o. 1596 inch 3 feet from the weld.

Tests, under varying conditions without number, might be multiplied. But for the practical man, those already given show that the ultimate strength of the thermit steel in practice can be estimated as over 30 tons to the inch section. By practice, I mean the thermit steel produced for repair work, according to directions: thermit, about 51 per cent, mild steel punching, and about 22 per cent, manganese for purifier.

Annealing for 3 hours brings the elongation well over 10 per cent.

Addition of about 32 percent, nickel raises the ultimate strength about 5 tons without decreasing the elastic limit. Further addition of about 22 per cent, of chromium with the nickel brought the elastic limit to about 47 tons - as high as can be wished. Addition of 1 per cent, titanium raises the tensile strength. Tests have also been made of thermit steel that has been toned up with molybdenum, ferro-silicon, etc.