Alloy Steels

It will also be noted that the above alloys are all low alloys, and this represents an undoubted trend in alloy practice, both in Eng land and America, especially in the smaller sizes of forgings or machined gears where adequate penetration of hardness may be secured by not too drastic quenches. For the massive forgings, steels with higher content of alloys, and long continued heat treat ments to permit the sluggish reactions, are still necessary.

Gear Steels.

As a specific application of the above general principles, the problems of gear manufacture may be considered, for the choice of steel and its treatment depends very intimately on the design of the part and its intended duty. Large gears for rough service are frequently cast direct in gray iron or low carbon steel, but the irregularities in dimensions of wheel and gear teeth are such that speed and power transmitted must be relatively small, life is short and noise is great. An improve ment as to accuracy and wear resistance consists in welding to gether a gear "blank" of appropriate shapes, the rim into which the teeth are to be cut being of medium carbon steel; after machining, the teeth surfaces are individually hardened by passage of an enveloping flame from oxy-acetylene torches, followed closely by quenching jets. In comparison with these, the high duty herringbone gear sets used on turbine-driven propeller shafts are marvels of precision; they are of cast alloy steels heat treated to relieve internal stresses and insure dimensional stabil ity as well as induce as much hardness for wear resistance as is consistent with machinability. This hardness limit is some where about C-40 (Rockwell scale). The steels contain 0.50 to o.6o% carbon, but alloys are necessary to give proper depth of hardening and fine grain size. Examples are 1.25% manganese plus one or more of the elements molybdenum (0.30%), nickel (1%), chromium (o.5%) and vanadium (0.15%). In large sec tions a quenched and tempered chromium-nickel-molybdenum cast steel will have the following physical properties indicating medium strength and hardness with great toughness : Yield point 105,000lb. per sq.in. ; tensile strength 135,0001b. per sq.in. ; elongation 20% in 2in. ; reduction of area 64% ; Rockwell hardness C-32.

In the smaller gears utilized for automobile transmissions and rear axle drives the mass effect or deep hardening effect is not so necessary. Requirements are very diverse and exacting, and these, together with the desire to save weight, and therefore to have gears as small as possible, have preserved this field to alloy steels. The original conception of a gear tooth was that it should have a hard surface to resist wear and a tough centre or core to resist impact. Following this idea, gears were cut from annealed

blanks, forged from low carbon alloy steels (less than 0.20% carbon) ; the machined gears were then carburized by heating in contact with carbonaceous solids or gases until their surfaces had come up to 0.90 or i.00% carbon for a depth of 0.030 to o•o5oin. (about % the thickness of the tooth at the base). The subsequent heat treatment was at times rather intricate, consisting of a high temperature quench to refine the low carbon core, a lower tem perature quench to harden properly the high carbon surface, and a tempering to improve the toughness. Steels widely used in this way included those shown in Table II. Representative properties of the core after an oil quench suitable to harden the carburized case are as shown. Evidently here is a variety of steels suitable for any designer's requirements, where he balances cost (alloy content, machinability, and heat treatment schedule) against strength desired to carry the loads on the gear tooth, and tough ness against impact loadings. Probably the ultimate in properties, disregarding cost, is a so-called "armour plate" analysis with car bon 0-'0%, nickel 4%, and chromium 1.5%, used for aircraft, heavy duty truck, and bus gears. Its heat treatment is quite complicated, including an oil quench from 85o° C. and draw at 650° to make the forgings machinable, 24hr. carburizing of machined gears at 940° F., annealing at 73o° F. and cooling slowly (8ohr.) to soften the case so the teeth can be finish-machined a few thousandths to remove distortion. The gears are then hardened by oil quenching from 76o° C. and tempering at iso° C.

Carburizing all these analyses gives an outer surface containing 0.90 to 1.o% carbon, and since surface hardness of the quenched gear depends on carbon, rather than alloy, content, all of these teeth can be made "file hard"—that is, too hard to be bitten by a testing file whose hardness is Rockwell C-65. Experience has also proven that maximum resistance to abrasion is secured from such high carbon cases on steel containing chromium or molybdenum carbides. One serious objection to the practice out lined above is the cost of heat treatment after carburization, and this has been attacked by the use of the fine-grained steels which may be quenched direct from the carburizing heat and still possess a fine structure and considerable toughness. This practice was fairly common with the 1% chromium, 0.15% vanadium steels, which are inherently fine-grained, before the special fine-grained steels were regularly produced by the steel mills. One much used type is the nickel-molybdenum steel shown in the last line of Table II.