DECEMBER, 1936
INDUSTRIAL AND ENGINEERING CHEMISTRY
chemical industry has found t h a t its processes for production of new products must be tried out not only in the laboratory but on a pilot-plant scale, so also must the producer of metals have facilities to produce new alloys and metals in commercially small quantities. H e will then have material in such quant,ity and form as t o enable him to test it practically under service conditions. This will give him knowledge of the fabricating and welding properties of the metal on commercial sizes and sections and suggest the possibilities of the application of the new product to various other industries. Research in metallurgy from the scientific and theoretical standpoint has been and is carried out on a large scale. Methods and means of testing metals have been developed from various angles. Considerable work is being done to improve fabricating plant practice, but there still seems to be room for intensive work in applied metallurgy. New alloys can be created only by knowledge of the specific need of a metal of definite characteristics for a new engineering requirement. The application of existing alloys into newer fields
1375
usually demands change in design of existing structures. After a new alloy is introduced, its performance must be closely followed because various initial deficiencies of the alloy may have to be remedied even to the extent of changing the composition. I n spite of the wide use of metals in all fields of engineering, there are still applications where metals could be used in place of nonmetallic substances. The competition lies no longer within the metallurgical camp itself but is facing outside adversaries, such as glass, paper, plastics, etc. Not only engineering aspects but also those of daily life are changing, such as our system of housing, house furnishing, and preservation and transportation of food supplies. The metallurgical industry has to be cognizant of these changes and prepare itself to meet the demand of a new era. As in all activities, so in metallurgical industry cooperation between consumer and supplier and coordination b y the manufacturer of all factors will undoubtedly lead to success, furthering the cause of metallurgy as well as of chemical engineering. RECEIVED September 9, 1936.
*...
Discussion CLYDE E. WILLIAMS Battelle Memorial Institute, Columbus,Ohio
I
N CONNECTION with Saklatwalla's paper, i t is of interest to note that in the development of some of the newer steela the two elements copper and phosphorus have been used, whereas they were previously considered as nuisance elements. Their presence in most steels was merely adventitious, the phosphorus coming largely from ore and the copper either from ores or steel scrap. Copper, in amounts up to about 0.25 per cent, has been used to increase the atmospheric corrosion resistance of steel; phosphorus in amounts up to 0.07 or 0.08 per cent has been used in sheet to be pack-rolled to thin gages. Aluminum is another element that has been in common use by the steel makers whose value in making better steel is becoming increasingly recognized. Saklatwalla's reference to the unsuitability of laboratory appraisal of corrosion resistance of metals shows the need for more fundamental work on the causes of corrosion and on the development of laboratory and accelerated test methods that will give results more closely simulating those obtained when the metals are used commercially. Similarly, in the development of steels for service a t elevated temperatures, more knowledge is needed as to the effect of variables such as melting practice, composition, heat treatment, structure, grain size, and stability, on resistance to deformation under sustained loads a t elevated temperatures. Short-time testa are not generally acceptable, although they are useful in making a preliminary survey of a field of alloys. Now creep tests are ordinarily run for 1000 or 2000 hours, and even longer times have been reported and advocated. As in the case of ordinary corrosion the character of the scale developed at elevated temperatures, whether by products being treated or by the combustion gases, further determines the quality of the metal. I n addition, resistance to creep and to embrittlement are essential. The high chromium content in ferrous alloys is not only beneficial in resisting scaling, but i t also greatly aids resistance to creep. Thus chromium, usually with nickel, is used in alloys for extreme service conditions. The well-known 18-8 chromiumnickel steel modified to resist intergranular corrosion by titanium or columbium has come into common use and has good creep properties up to 1250" F. (678' C.). For higher temperatures (up to 1800" F. or 983" C,), 25 per cent chromium and 12 per
cent nickel compositions are used and, where corrosion by sulfur dioxide must be combated, 29-9 chromium-nickel is preferred. For still higher temperatures and when resistance to sulfur dioxide is not required, 35-16 nickel-chromium iron alloys are used. Scaling by oxidation is satisfactorily combated in alloys containing chromium contents above 24 per cent. For less severe conditions the cheaper 4 to 6 per cent chromium steel, containing about 0.5 per cent molybdenum or tungsten is used. This steel retains high strength and good creep resistance up to 1100" F. (594" (2,). A still cheaper product, but one with considerably better properties than plain carbon steel, contains 1.25 per cent chromium, 0.75 per cent silicon, and 0.5 per cent molybdenum. This type of steel is generally used where corrosion conditions are less severe than those requiring the 4 to 6 per cent chromium alloy. In these low-alloy steels, molybdenum is the effective agent in enhancing creep resistance, but ita effect is greatly increased by the presence of chromium. Recent research work which has not yet been checked in commercial installations indicates that phosphorus is likely to become a useful alloying element in steels for high temperature service. These tests show that phosphorus in amounts up to 0.20-0.30 per cent increase both short-time tensile properties and resistance to creep at elevated temperatures. Thus, the effect of phosphorus is similar to that imparted by molybdenum and tungsten. Also, the effect of phosphorus is increased by the presence of chromium and other elements which also serve to maintain a satisfactory toughness. Preliminary creep tests indicate that in low-carbon (0.15 per cent) 1 per cent chromium steels, phosphorus may be substituted for a portion of the molybdenum (0.5 per cent) sometimes used, with little change in properties. I n recent years much progress has been made in the improvement of the serviceability of cast iron a t elevated temperatures. Cast iron is useful as a structural material because, owing to ita easy castability and machinability, it can be made into varioua and intricate shapes. Its disadvantage of being subject to growth and consequent warping and cracking when held a t elevated temperatures can now be greatly reduced. This improvement is achieved largely by producing a more uniform structure in which the graphite is finely divided and which has higher strength. It can be effected either by holding the silicon content to B point where extreme graphitization does not take place, by melting
INDUSTRIAL AND ENGINEERING CHEMISTRY
1374
a t higher temperatures, by treating the metal before casting (for example, with calcium silicide), or by the m e of alloys. Any reduction in the growth of cast iron is also frequently accompanied by less scaling and corrosion. The most effective method is to use alloys. Well-known compositions contain from 1.5 to 2 per cent nickel and about 0.5 per cent chromium. Other effective compositions contain 1 or 2 per cent chromium and 0.5 to 0.8 per cent molybdenum, the chromium improving resistance to scaling and the molybdenum aiding the resistance to deformation at elevated temperatures. Cast iron of exceptionally high tensile strength can now be made without the use of alloys by holding the carbon content to 2.60-2.95 per cent. Nickel, chromium, copper, and molybdenum, either singly or in combination, are effectively used t o increase the strength of the ordinary higher carbon iron or the low-carbon high-test cast iron.
VOL. 28, NO. 12
Use of the austenitic cast iron containing about 15 per cent nickel, 6 per cent copper, 2 per cent chromium, 1 per cent manganese, 1.5 per cent silicon, and 2.75-3.1 per cent carbon, developed several years ago, has now become common and has proved to be a most advantageous material for use at high temperatures and for extremely corrosive conditions. In the nonferrous field, one fairly recent development deserves notice. Advantage of the high thermal conductivity of copper, which is about nine times that of iron or steel, has not normally been taken because of the relatively poor strength properties of copper. A copper alloy containing about 2 per cent chromium. heat-treated, to cause precipitation hardening by the chromium, can now be produced which has highly increased strength and hardness, and yet retains 80 to 90 per cent of the thermal conductivity of pure copper. RECEIVED October 9 , 1936.
STEELS RESISTANT TO SCALING AND CORROSION FLORENCE FENWICK AND JOHN JOHNSTON
United States Steel Corporation, Kearny, New Jersey
T
HE newer f e r r o u s alloys developed in recent years have, in general, been characteiied by both increased strength and enhanced resistance t o corrosion. These properties are not necessarily related, yet for many purposes increased strength is of little practical advantage unless the metal is more resistant to corrosion than plain carbon steels. For in service the same depth of corrosive attack would endanger the thinner sections made possible b y greater strength more than the thicker sections of ordinary steel used for the same purpose, because i t would cause a proportionately greater lessening of the safe load which the member could support. Consequently corrosion resistance is, in perhaps the majority of everyday applications, almost the controlling factor, even when higher strength is the primary consideration; this fact, together with the circumstance that enhanced resistance to corrosion is harder to secure at a reasonable cost than is increased strength, directs our attention at this time to some of the many questions involved in the scaling
Courtesy, U.S. Steel Corporation Subeidiaries Uary Sheet Mills, and Chicago-Illinois Steel Corpirotion
CHARQINQ STEELSHEETSINTO
THE
HEAT-TREATING FURNACE
This treatment at 1300' F., usually for 50 hours releases the strains set up in the steel during ooldworking, and i m p a d s ductility and softness.
