Progress with Heat-resisting Steels
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THERE is a regret
table tendency among engineers to confuse stainless steels with specifically heatresisting steels. This is due to the fact that both types of steel possess hi. a measure the prime qualities of corrosion and heatresistance. Whilst stainless steels are, to some extent, heat-resisting, and heatresisting steels are, to a large extent, stainless, they are, nevertheless, to be distinguished by the emphasis which is placed on the specific quality.
At about 1,000 deg. C., the stainless steels proper begin to lose any claim to satisfactory heat-resistance. The heat-resisting steels, however, retain a measure of mechanical strength up to 1,200 deg. C. This fact makes them far superior to the stainless metal for those parts which have to work at temperatures between 1,000 deg. C. and 1,200 deg. C. On the other hand, they cost more, are more difficult to manipulate and cannot be obtained in so many forms as the rust-resisting steels. Nevertheless, they are of great importance.
Standardization Still Distant.
The difficulty in summarizing their progress of recent date is the fact that standardization is far from having been achieved. In no direction are research and experiment so active. So many conflicting claims, so many different analyses, are being put forward by steel makers and chemists that it is difficult to sort out of the consequent confusion the main lines of current progress.
Two main qualities do, however, emerge and may be quoted here as representing fairly stable varieties. The first is a steel containing nickel, chromium, tungsten, copper and silicon. This type of metal has a high heat resistance and is resistant to the scaling caused by oxidation of the metallic surface under heat.
As an example, it may be Mentioned that a piece of this steel would lose, at a temperature of 1,000 deg. C., only .0012 gram. per sq. cm. per hour, as against .0015 for a stainless 842 steel, and .04 for a plain carbon steel. As the temperature rises above 1,000 deg. C., so this loss becomes proportionately less.
In addition to this ability to resist scaling, the steel has what must, in the circumstances, be regarded as a comparatively high mechanical strength at high temperatures. Thus, at BOO deg. C., a maximum stress of 17.4 tons per sq. in. is obtainable with a heat-resisting steel, as against 11.4 with a nickel-chromium stainless steel, 6.1 with a chrome stainless steel of the cutlery type, 5.6 with an ordinary nickel-chrome steel and only 4.8 with a straight carbon steel. It will at once be observed how great is the advantage offered by these steels.
The drawback to their more regular employment for a large variety of engineering purposes was the difficulty of working them. It was almost impossible to forge or work them commercially, and, in consequence, the only form in which they could be obtained was as castings.
Experiment—and Result.
This led to experiment, and eventually the 'second quality of fairly standard character was evolved. In this a reduction in the carbon percentage, combined with a slight modification of the general analysis, enabled a steel to be produced which could be supplied in bar, billet, sheet and plate form. Tubes could not, however, be made in this material, unless cast, and even then a fairly considerable wallthickness was essential, whilst there was a limit to the length of tube that could be cast.
At the same time, this gain in plasticity involved an inevitable reduction in the mechanical strength. As an example, it may be mentioned that whereas at 800 deg. C. the ultimate breaking strain of the cast quality is 24 tons per sq. in., it is 19.8 tons with the second type. In the same way, the reduction of area is 47.2 in the one 'instance and 43.6 in the other. Resistance to oxidation in both steels, however, is alike.
Certain characteristics possessed by these heat-resisting steels call for special mention. Thus, the heat conductivity is much less. than with ordinary carbon steels, being 0.033 calories per sq. cm., as against 0,08 calories. Weight is roughly .29 lb. per cubic in.
Existing reseatch is directed largely towards the discovery of a steel which will unite the joint advantages possessed by both the heat-resisting and the stainless steels. With this end in view, innumerable analyses are being made, none of which can as yet be regarded as standard, the tendency being rather to devote a specific analysis to a specific purpose.
Interesting Alloys.
Such combinations as 60 per cent. of nickel and 20 per cent. molybdenum and 30 per cent, chrome and nickel, 4 per cent. silicon and a small percentage of molybdenum, have been made and utilized in `various directions. (Here, the heat-resisting "irons" are not considered.) Although scarcely a steel, a third development of no mean importance is the material called " nichrome." This has 60 per cent, of nickel with 15 per cent, of chromium, and it is of great value for those parts and purposes requiring electrical resistivity, freedom from scaling, surface carbonization and nitricling, toughness and heat resistance.
Copper, manganese, silicon and carbon are other elements in its composition. It melts at 1,250-1,370 deg. C., and can, therefore, be employed with success up to 1,100 deg. C. It has a tensile strength of 28-30 tons per sq. in. in the form of castings, but when rolled its tensile is practically double this figure, with a yield point rather higher. Nichrome is of special interest to the motor industry, because it constitutes an ideal material for carbonizing boxes, 'vessels for holding molten bearing metals, die-casting moulds, annealing boxes, sodium cyanide baths and the like. The question whether nichrome or a stainless or heat resisting steel should be employed for a particular application is one best solved by the steel maker and the user in collaboration.
Various points have to be considered, such as the degree of heat and sealing likely to be encountered, the form in which the part or piece is required, the corrosive or oxidizing agencies to be resisted„the price that can safely be paid, the amount of work to be done on the part or piece, and so forth.
One interesting feature is the development of a special steel for poppet valves. As a result of extensive tests on materials, one large valve maker in the United States of America has standardized on high nickel chromium heads welded to stems made from steel of a different composition, this construction being used for exhaust valves. A procedure has been instituted for the extrusion of both austentitic and pearlitic nickel chrome steels for either mushroom or tulip-type valves.
On this subject of internal-combustion-engine valves, it may be said that it is not yet possible to find one steel covering all requirements for valve material. The scaling of valves is increased by the use of fuels containing tetraethyl lead.
Austentitic valves with high nickel content are said to be the most resistant to corrosion of this character. The valve steels that give the best results are invariably the most costly, due to high nickel content Some attention has been paid to the design of parts made from the heat-resisting steels. In general, it is recommended that sharp turns, intersecting planes and other shapes impeding the flow of metal in castings should be avoided. Long curves are preferable to sharp angles. Thickness should be uniform.