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s LAST month’s column (June, Advertising page 69) the subject of loading chemical equipment to capacity was discussed as an introduction to the subject of the potential possibilities of modern metals. A few generalities about their strength, hardness, machinability, and resistance to heat and atmosphere is worth while to the user of equipment. Most heat-resistant equipment or highly stressed equipment is fabricated from ferrous metals and alloys. Primitive iron was made in furnaces which were really crude. A two-man four-cylinder “blowing engine” produced iron for the ancients of Egypt, Assyria, and China, and a well reduced metal reached Europe, in small quantities a t least, before Caesar. For generations the problem was mainly production, and any kind of material could be sold. Then came knowledge of more exact methods for improving the properties by regulation of carbon, and heat treatment followed. For many years the metallurgical world focused its research on the role of carbon as the basis of all special steels, but it came t o be known almost automatically that many other elements in small quantities could impart valuable properties t o steels. For several generations the best materials for making steel have been steel scrap, and it is used as far as the supply will permit. Scrap steel looks rough and rusty, but actually it is the purest raw material, even better than the best pig iron. The large use of scrap in steel making has resulted in a great deal of unplanned alloying and blending. Most steels today contain an appreciable content of metal which has been returned to the furnace many times. Therefore, eventually a run of steel will contain fractions of a per cent of several alloying agents resulting from the impurities in the scrap; .investigations of the effect of these small additions as well as of carbon on the propertias of steels have been forced with the programs of metallurgical research. For a long time it has been recognized that a small amount, such as 1% of any of the important alloying metals, produces an effect approximately half as great as 5 or 6% of the same element. The possibilities of steels containing a total of 2% of nickel, chromium, molybdenum, tungsten, and manganese, together with the usual carbon, had t o be investigated because as far back as the first World War the scrap had become mixed up. Well known steels of this type are the 8800
and 8700 types; each has remarkable properties, although the total of alloying metals plus carbon is approximately 2%. These steels are evidence that trace metals are even more important than carbon as a basis for the development of physical properties by heat treatment. Early in the century, Kent’s Handbook contained a table labeled “Effect of Carbon on Physical Properties of Steel”, the carbon content listed ranged from 0.05 to 0.85% (pearlite), and the reported ultimate tensile strength of the untreated steel ranged from 45,000 to a maximum of 140,000 pounds per sailare inch, with 27,000 to 78,000 as the corresponding yield points. I n one modern steel of the 8600 type, having 0.4@yo carbon and 2Yo of the fcur alloying metals, the Brinell hardness was developed to 430, the ultimate tensile was 214,000 pounds with an elastic limit of 185,000 pounds per square inch, and 16% elongation in 2 inches produced a reduction in area of 48%. This is a striking example of the value of elements other than carbon in producing a steel of much wider usefulness with good machinability. The yield point of the 8600 type just described is 85% of the ultimate tensile strength of 214,000 pounds per square inch. It would make little difference in this case whether the designer based a factor of safety on the usual “ultimate” or, more logically, on the yield point, and thus avoided the use of any data signifying deformation. I n every-day carbon steel, however, there is a marked difference. The usual steel plate, bars, and shapes have a yield point of 34,500 pounds (at 0.15% carbon) with an ultimate tensile of 57,000 pounds per square inch. The Code requires a factor of safety of 4 (American Petroleum Institute) or 5 (American Society of Mechanical Engineers) applied to the ultimate tensile. The designer may therefore use a stress of 14,250 pounds per square inch. It should be noted, however, that the purchaser of such steel must accept material having a tensile of 51,300 pounds with a yield point d 32,000 pounds per square inch. If the same stress is based on the more scientific yield point, a safety factor of 2.25 is obtained. X a n y designers use a safety factor for ordinary carbon steel of 2.5 based upon the yield point, which is more logical and conservative. There are other factors to be considered when this principle is used with five-element 8600 and 8700 type steels. The yield point, especially (Continued on page 70) 69
with cold working, disappears into the rilt,iniat,e tensile 3trengtli. Even in the hot-worked mat,erial the yield point is w r y high, but, the proportional limit (the stress beyond vvhicli the change in length is no longer proportional to the stress applied) does not folloiv either the high yield point or ultimate t'ensile. A safety factor of 5, based upon 214,OGO pounds would permit a design stress of 42,800 pounds per square inch. A safety factor of 2.5, based on the yield point of 185,000 pounds, would indicate the rather high stress of 74,000 pounds per square inch; biit if the proport,ional limit of 110,000 pounds is substitiit'ed for yield point, then the 2.5 factor of safety gives almost the same design stress of 14,000 pounds per square inch. There are fivtlier important variations in these two physical propcrties. In carbon steels the ultimate tensile strength increases in a straight line proportion to the cnrboii content, up to maximun~pearlite content' corresponding to 0.80?& carbon, which gives an ult~iniatrtensile st rengtlr of 140,000 pounds per square inch. The yield point values do not increase in the same proportioil and at 0.80% carbon reach only 70,000 pounds, a ratio to ultimate of only 50cjC. A safety fact,or of 4, based on the ultimate value, would permit design stresses of 35,000 pounds per square inch which is only a safety factor of 2.0 based on t,he yield point. Of course there are other rules limiting the designer who uses high carbon steels with a reduction in area of 15y0 and an elongation of 10% in 2 inches, such as the change in properties at low and elevated temperatures. The designer should employ physical properties based upon the hot-rolled mill materials and not upon the resuks of tests after cold working, since the yield point is raised by cold working sometimes u p to the tensile strength. After the same steel is annealed, the yield point is restored. The values of t,hese properties for carbon steels which have been cold-worked can be raised as much as 70%; this indicates the dangers of using yield point data which depart too widely from the proportional limit. Not every chemist or chemical engineer is faced with the problem8 of drsigning equipment, but it is somewhat essential to' all users of equipment t o appreciate the approximate values of the chief physical properties of common steels. One often stresses the properties of a special alloy in determining its fitness for a certain problem. The excellent possibilities and high strength of present carbon steels are often overlooked. The use of carbon steel of 75,000 pounds per square inch tensile strength in vessel manufacture will decrease the weight and cost of the completed vessel considerably more than the slight premium required for plates of the higher tensile strength. 70
