In making cost estimates for alloy-steel bridges, the following unit prices have been assumed. They are based upon the market conditions governing in March, 1920.

Ordinary carbon-steel work, erected........8 ¢ per lb.

Nickel....................................................40 ¢ per lb.

Chromium..............................................22 ¢ per lb.

Vanadium residue in steel (to include

wastage).......................................$10.00 per lb.

Molybdenum.........................................$2.50 per lb.

Heat-treatment of eye-bars........................1 ¢ per lb.

There will first be determined from the various diagrams in "Molybdenum Commercial Steels" the best drawing temperature for the heat treatment of alloy steels to be used in bridgework. In modern alloy-steel practice there has come into vogue the term "Quality Number," meaning the product of the elastic limit in pounds per square inch by the reduction of area expressed in ratio to unity. This is considered the criterion of excellence, because the maximum values of these two quantities are desirable. As the drawing temperature is increased, the elastic limit drops and the reduction of area augments, consequently that temperature which makes the product of the two values a maximum probably gives the best result for combined strength and toughness.

An analysis of fifteen diagrams in "Molybdenum Commercial Steels" shows that the average best drawing temperature is 800° F., it being a trifle greater for oil quenching than for water quenching. A higher drawing temperature than this seems to be preferable for automobile steel, in order to secure great toughness for resisting shock; but it gives very good results with steel having main characteristics suitable for bridge building, the average percentage of elongation for small test pieces being 15.5. As the principal standard bridge specifications call for an elongation of 15 per cent in specimen tests of nickel steel, it may be concluded that the molybdenum-steel alloys above considered are satisfactory in respect to this criterion.

There will next be determined, as accurately as the rather meagre data available will permit, for all of the usual kinds of alloy steels, the economic benefit to be derived by adding molybdenum to the other ingredients of the steel.

Carmol Steel versus Carbon Steel

From a reliable source the author has learned that the heat-treatment of carbon-steel eye-bars raises the elastic limit from 30,000 lbs. per square inch to at least 50,000 lbs. per square inch, and that the present-day cost of the heat-treatment is about one cent per pound.

Let us assume that in a pin-connected bridge the average value in place