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WHEELER—A HIGH-TEMPERATURE BOLTING MATERIAL 663D i s c u s s i o nA. J. H e r z i g 4 a n d R. L. W i l s o n . 6 The author has directedattention to several aspects of the selection of bolting steels forhigh-temperature service which are not generally appreciated.Certainly there is slight agreement as to what constitutes anacceptable standard for judging the merits of a high-temperaturebolting steel, but there is a growing realization of the many compromiseswhich may have to be made in the choice of a materialfor a particular application.The problem in searching for a good high-temperature boltingsteel is to find a material having high strength at elevated temperaturescombined with high room-temperature elastic strength,stability on heating, satisfactory notch toughness, and goodmachinability. It is also important to obtain these desirableproperties in the heat-treatment of sizes ranging from Vz to 4 in.or more with consistent uniformity both in the section treatedand from lot to lot. High temperature strength would meaneither the reluctance to relaxation of stress for a fixed strainor would be measured by the creep strain under constantstress.From the rather meager data available it would seem that theconstant-stress tests show somewhat higher relative strength valuesfor air-treated as against quenched steels than are reportedin the relaxation tests. This may be due to a persistent effectof a high initial rate of straining in the down-step test. At anyrate there is ample evidence, supported by this paper, to indicatea preference for air-treated bolting steels to obtain best hightemperaturestrength were it not for the variation of room-temperaturemechanical properties when the same heat-treatmentis applied to a range of sizes.We are now aware that seemingly small changes in microstructurecan cause significant differences in mechanical properties,particularly the notch toughness and creep of steels. Themicrostructure and related properties will thus be changed byvariations in chemical composition of the steel and by the rateof cooling in different sizes and media. For any preferred microstructure,the problem thus becomes one of hardenability of thesteel. The hardenability of the steel can be changed by suitableadjustments in the chemical composition to produce the desiredmicrostructure and associated properties by any kind of heattreatment.Since the normalizing and tempering treatment gives the higheststrength at elevated temperatures, bolting materials shouldpreferably be heat-treated in this manner by adjusting the compositionto give a good compromise of room-temperature elasticstrength and notch toughness, depending upon the sizes involved.Best all-round results will be achieved by using a normalizingtemperature below the coarsening range, and increasing the hardeningelements in the steel as section size increases. This mightbe handled commercially by selective application of steels toseveral size ranges.Elem entPer centC arbon.............................................................. ... 0.35 to 0.50M anganese....................................................... ...0.40 to 0.70Phosphorus.......................................................... 0.04 MaximumSulphur.................................................................0.05 MaximumSilicon...................................................................0.15 to 0.30C hrom ium ........................................................ ...0.80 to 1.10M olybdenum ................................................... ...0.30 to 0.40V anadium ............................................................0.20 to 0.30Minimum tensile requirements established for normalizingheat-treatments for sizes up to in. diam and various drawtemperatures are given in Table 6.TABLE 6 MINIMUM TENSILE REQUIREMENTS FOR NORMAL­IZING AND VARIOUS DRAW TEMPERATURESM inim um Tensile Yield E longation Reductiondraw tem p, strength, strength, in 2 in., of area,F psi psi per cent per cent1000 145000 120000 14 451100 135000 115000 IS 451200 125000 105000 16 50The properties obtained upon this steel, utilizing an air quenchor normalize followed by a draw at 1200 F, are particularly notableas representing the class C physicals of A.S.T.M. SpecificationA96-39, which have long been recognized as desirable in high-strength bolting materials. This steel B14 was introduced intogeneral high-temperature use upon the discovery that it respondedto the normalizing treatment in such a manner as toattain exceptional creep and relaxation resistance at high temperaturesand yet possess the high elastic strength implied forclass C A96-39, i.e., 105,000 psi minimum yield strength.Numerous alloy bolting steels were found which developed goodcreep strength upon normalizing and drawing at 1200 F (thelowest draw permissible for 1100 F service according to Spec.A193), but which failed to attain the elastic strength so essentialto a good bolting steel. The chromium-molybdenum-vanadiumcomposition was found to respond to tempering after normalizingin an entirely different manner from a chromium-molybdenumsteel of equivalent composition but without a vanadium content.The B14 composition was found actually to increase in hardnessJ. J. K a n t eh.6 The bolting steel upon which the author reportshis extensive and valuable data conforms to a compositionwhich has been known since 1936, as grade B14.7 The chemicalrequirements for B14 steel as established in A.S.T.M. Specificstion A193-40T are as follows:4 Climax Molybdenum Company, Canton, Ohio.s M etals Engineer, Climax Molybdenum Company, Canton, Ohio.Mem. A.S.M.E.* Research Laboratories, Crane Company, Chicago, 111. Mem.A.S.M.E.7 ‘‘Tentative Specifications for Alloy-Steel Bolting M aterials forHigli-Temperature Service From 750 to 1100 F M etals Tem perature,”A193-40T, American Society for Testing Materials, 1936.F i g . 27 D i a g b a m R e p r e s e n t i n g S u p e r p o s e d P h e n o m e n a W h i c hO c c u r o n T e m p e r i n g V a n a d i u m S t e e l Q u e n c h e d F r o m H i g hT e m p e r a t u r e 8upon drawing at 1200 F. Whereas, its “as normalized” hardnessmight be about 280 Brinell upon tempering at 1200 F, this increasedto about 300 Brinell. This effect has been explained forvanadium steels by Houdremont, Bennek, and Schrader8 as beingdue to precipitation hardening caused by separation of specialcarbides. Usually an air-hardening steel progressively softens8 “ Hardening and Tempering of Steels Containing Carbides of LowSolubility, Especially Vanadium Steels,” by E. Houdrem ont. H.Bennek, and H. Schrader, A .I.M .E. Technical Publication No. 585,Class C, Iron and Steel Division, 1934.