Alloy Steel Valves for Subzero Temperatures GEORGE I?. SCHERER Merco Nordstrom Valve Company, Oakland, Calif.
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HE field of chemical process technology frequently presents problems requiring the use of materials of construction suitable for handling high temperatures and high pressures, but with a few exceptions the field of subzero temperatures has received scant attention. For the limited number of processe% where extremely low temperatures were a necessary critical condition, usually the small size of plant required was such that high-cost alloys could be used with impunity and without reflecting a disproportionate amount of plant investment on the cost of the product manuf actured. However, the introduction of various dewaxing processes for the solvent refining of lubricating oils in the petroleum industry during the past several years has focused an increasing degree of interest on metals suitable for use at subzero temperatures. In most of these processes temperatures of -50" to -75" F. and pressures up to 300 pounds per square inch are required for processing of the oil to remove wax and other organic material a t subzero temperature, with a consequent, need for metals possessing suitable physical characteristics and lending themselves readily to fabrication into forms such as valves, meters, pipe fittings, etc., preferably a t a minimum cost.
Shock Resistance Although some of the physical characteristics of metals change favorably with decrease in temperature-for example, tensile strength, yield point, modulus of elasticity, and hardness-another important characteristic, shock resistance, is affected adversely with a decline in temperature. Valve metals which may be required for subzero service include both cast and forged or rolled materials to provide bodies, covers, bolting material, stems, and other trim. The resistance to shock, as measured by the energy absorption of a test specimen when subjected to a sudden blow in an impact measuring apparatus such as a Charpy testing machine, is the chief qualification t o be met in the selection of metals for subzero service. However, besides satisfactory shock resistance the metals must possess satisfactory tensile strength, workability from a fabrication standpoint, and corrosion resistance, and must be obtainable at a cost compatible with practical commercial application. The impact stresses to which such chemical plant equipment may be subjected result not only from varying internal pressure but also from sudden shocks resulting from water hammer, pump vibration, or accidentally applied mechanical forces.
WRENCH-OPERATED LUBRICATED PLUG VALVEWITH ExTENDED COVERAND STEM
There is no means of determining in advance the magnitude of all of these stresses, but the shock-resisting characteristics of the metals used for construction must be empirically set by the designers as a result of experience and a study of the particular characteristics of the process involved. To provide satisfactory shock resistance, most specifications call for a minimum Charpy test of 15 foot-pounds of absorbed energy, made on a standard 10 X 10 X 50 mm. specimen with keyhole notch, a t the minimum temperature a t which the metal is to be stressed. The measurement of other tensile characteristics, such as tensile strength, yield point, per cent elongation, and reduction of area, are made a t room temperature. An attempt to correlate the results of various investigations of the impact values of metals a t subzero temperatures shows a wide diversity of results in most cases. This diversity probably results from the large number of indeterminate variables which affect the tests and the sensitive nature of the Charpy test, Certain factors, however, which are generally agreed upon as controlling the degree of shock resistance are as follows: 1. The chemical composition of the metal itself 2. The heat treatment to which it is subjected 3. *Grainsize, particularly after heat treatment 4. Whether the metal is cast or forged and what particular portion of the castings or forging is being tested 5. The foundry practice followed in producing the metal
It is not the purpose of this article to go into a detailed exposition of the effect of these various factors on the shock resistance of metals which might be used in such equipment, 1.2!20
NOVEMBER, 1938
INDUSTRIAL AND ENGINEERING CHEMISTRY
but only to point out the characteristics of several cast and forged alloys which have been successfully used in fabricating valves and pipe fittings for chemical processes involving subzero temperatures. Also the characteristics of these alloys are correlated with cost, availability, and adaptability to fabrication into valves.
Metal Specifications Among those well-known metals, which for a long time have been known to possess high impact resistance a t subzero temperatures, are the high nickel-copper alloys and the austenitic chrome-nickel-iron alloys, of which the well-known stainless steels, designated as KA2 or KA2S, are typical. These alloys possess a satisfactory shock resistance combined with high tensile strength for most services, and their corrosion-resisting qualities are also much in their favor for chemical equipment. However, both types are relatively high in cost and, when produced in the form of castings, require special foundry technic which the average steel foundry is not prepared to perform. When used in small quantities, the cost is not so important, but, where large tonnage is involved, it becomes necessary to fit the lowest cost materials to the process which will adequately handle the job, taking all factors into consideration. A typical subzero process which has attained wide vogue today is that in which lubricating oil is treated with selective solvents a t low temperatures in order to remove paraffin wax and certain gums and asphaltic materials from the oil. The temperatures employed range from -50" to -75" F. and the pressures vary up to 300 pounds per square inch. To provide
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a suitable metal for valve castings it has become quite general to use an alloy steel containing from 2.5 to 3 per cent of nickel. When properly heat-treated, such a steel may be produced in the form of castings with suitable physical characteristics for service a t this temperature and a t a relatively low cost. A typical steel of this type will have the following approximate compositidn and physical characteristics : Chemical Compn., 70 Carbon 0.25 Manganese 0.60 Silicon 0.25 Phosphorus 0.046 Sulfur 0.045 Nickel 2.92
Physical Characteristics Tensile strength, lb./sq. in. Yield point, lb./sq. in. Elongation, yo Reduction of area, 70 Brinell hardness Charpy impact resistance at -50" F., ft-lb.
80,000 57,000 28.5 48.0 138 19.4
Heat Treatment Heat to 1700-1750° F. Quench in air Reheat t o 1550" F. Quench in air Draw to 1200' F.
For a satisfactory material for studs or bolts or forged parts the following bar stock, corresponding to an 8. A. E. 4140 steel, may well be used: Chemical Compn., % Carbon 0.40 Manganese 0.89 Silicon 0.21 Phosphorus 0.016 Sulfur 0.018 Chromium 0.99 Molybdenum 0.23
Physical Characteristics Tensile strength, lb./sq. in. 131,000 Yield point, lb./sq. in. 113,200 Elongation, yo 22 Reduction of area, yo 64 Brinell hardness 269 Charpy impact resistance at -50" F., ft-lb. 39.8 Heat Treatment Heat to 1 4 7 5 O F. Quench in oil Draw to 1260' F.
WORM-GEAROPERATEDLUBRICATED PLUQ VALVE WITH EXTENDED COVERAND STEM
WRENCH-OPERATED LUBRICAPED PLUQ VALVEWITH EXTENDBD BODYAND STEM
Both the above alloys lend themselves well to fabrication and the ordinary machining and forming operations. When welded connections are necessary or desired, the technic followed must provide for suitable heat treatment of the parts after welding and also i t should be remembered that any absorption of carbon into the metal will adversely affect its impact resistance. Another typical example of a modern chemical process involving the use of extremely low temperatures is a process for the manufacture of ethyl chloride, in which temperatures of -150" F. with pressures of 600 pounds per square inch are utilized. The portions of the equipment which are subjected to this temperature are required to show an impact resistance of 15 foot-pounds on the standard Charpy test specimens
when ruptured at -150" F. This applies to both cast and forged or rolled parts. A steel of the following type has been found suitable for making valve castings which, when heat-treated in accordance with the outlined procedure, will give Charpy values averaging around 20 foot-pounds and excellent tensile values as well. It can be cast into intricate forms, such as valve bodies and bonnets, without great difficulty, and it compares in cost favorably with ordinary alloy steels of high quality. A typical chemical analysis of this steel and its physical characteristics follows: Chemical Compn., Carbon Manganese Silicon Phosphorus Sulfur Nickel Vanadium Molybdenum
yo
0.08 0.29 0.12 0.015 0.018 4.06 0.16 0.50
Physical Characteristics Tensile strength, lb./sq. in. 1L07,OOO Yield point, lb./sq. in, 88,000 Elongation, % 22.5 Reduction of area, yo 61 Brinell hardness 200 Charpy impact resistance a t - 150' F., ft-lh. 20.6 Heat Treatment Heat t o 1800-1850" F Cool in air Reheat to 1550' F. Quench in oil Draw to 1250' F.
Carbon Manganese Silicon Chromium Molybdenum Phosphorus Sulfur
0.40 0.70 0.22 0.95 0.20 0.016 0.018
Physical Characteristics Tensile strength, lb./sq. in. Yield point, Ib./sq. in. Elongation, yo Reduction of area, yo Brinell hardness Charpy impact resistance a t -- 150' F., ft-lb.
Mannitol and
Sorbitol R. MAX GOEPP, J R . , AND ~ K. R. BROWN Atlas Research Laboratory, Tamaqua, Pa.
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For bolts, studs, and other trim used in the construction of valves, a bar stock material such as S. A. E. 4140 steel with the following approximate specifications will give satisfactory results : Chemical Compn., yo
Resins from
130,000 105,000 21 58 238 18.3
Heat Treatment Heat to 1575' F. Quench in oil Draw to 1275' F.
Sometimes it may be desirable to surface-harden parts of the equipment in order to reduce the chances of galling where such parts are in sliding contact under high pressure with other parts of the same metal. In this case such parts may be surface-hardened by a carburizing treatment. Numerous tests have been made to determine whether there would be a tendency to spalling off of the hardened surfaces a t subzero temperature. These tests indicated that this would be extremely unlikely if the thickness of the case is not excessive. Where plug valves of the pressure-lubricated type are installed, suitable lubricants must also be used. They are available in several varieties with satisfactory characteristics a t low temperatures and are suitable for resistance against various solvents. They are generally applied by means of pressure guns and lubricant nipples. These lubricants offer a valuable means of preventing galling and seizing of the soft metal valve surfaces, and they are also extremely effective in sealing the valves against leakage of the high vapor-pressure fluids entering into these subzero processes. I n view of the likelihood of the extension of processes involving the use of extremely low temperatures and high pressures, it would appear that alloy steels of high impact resistance a t these temperatures will come into more frequent use. RECEIVFOD July 28, 1938.
HE hexahydric alcohols, mannitol and sorbitol, as polyfunctional bodies are capable of forming resins with polybasic acids or with resinous monobasic acids. Such resins derivable from the hexitols include the alkyd or phthalate, the succinate, maleate, adipate, the citrate mentioned by Hovey (6),the oil-acid modified alkyd, and the ester gum from rosin. Previously it has been generally assumed that all 6 hydroxyl groups of mannitol and sorbitol are reactable in resinous esterifications (3,6 ) . In 1930 Kienle ( 6 ) published data on a mannitol-phthalate resin made with 6 acid equivalents, and showed that the product had a higher flow point and poorer moisture resistance than the corresponding glycerol resin; he drew the inference that "the cause of the decrease in water resistance is undoubtedly due to the presence of uncombined hydroxyl groups which are necessarily more numerous the greater the reactivity of the alcohol molecule, as earlier gelation blocks their combination." A few preliminary experiments on modified alkyd resins soon demonstrated that the ordinary glycerol technic was not suitable, and that the course of the reaction was apparently different, since, using acid equivalents equal to about threefourths of theory, the hexitol resins had acid numbers of 106 to 138, compared to glycerol resins a t 20 to 30. It was suspected that inner ether formation, according to the equation, CsH*(oH)e-----f CsHsO(0H)r
was taking place, and that in effect, only 4 instead of 6 hy-
droxyls were available. Accordingly, the acid ratio was cut in half and the acid numbers of the hexitol resin were brought down to 50. The inner ethers of the hexitols are the tetrahydric or dihydric, mono- or dianhydro products, having the empirical formulas CBHsO(0H)d and CsHsO,(OH),, obtainable from the hexitols by intramolecular loss of one or two moles of water under various conditions (4,7). Later a more extensive investigation of hexitol resins showed that, with ester gums, the optimum ratio of rosin to hexitol was 5 to 1 by weight, or about 2.76 acid equivalents. This would be (2.76/4.00=) 0.69 of theory for the inferred tetrahydric inner ether. Since the observed effect might have been due to steric hindrance or diminished reactivity of hexitols compared to glycerol, the inner ether hypothesis was further tested by substituting a mixture of preformed sorbitol inner ethers, or sorbitans, for the sorbitol a t the same acid ratio. The sorbitan 1
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+ H,O
Present address, Atlas Powder Co., Wilmington, Del.
