Secondary Graphite Formation in Tempered Nodular Cast Iron Weldments

Graphitization of martensite and carbides during postheat treatment produces secondary graphite nodules in the heat-affected area; preheating can control the amount and morphology of these nodules

BY D. R. ASKELAND AND N. BIRER

ABSTRACT. Postheat treatments below lar iron weldment is postheated at very the critical temperature are effective in high temperatures, the carbides will reducing the hardness of nodular cast graphitize, just as white cast iron is iron weldments by eliminating car­ converted to malleable cast iron, bides and martensite. However, be­ producing small graphite nodules from cause of the precipitation of secondary the carbides. These nodules may often graphite nodules, such postheat treat­ be arrayed in the form of long chains ments may not completely restore of graphite particles.3 ductility to the weldment. High pre­ The second source of secondary heat temperatures minimize martens­ graphite is high carbon martensite ite and the amount of secondary located in the partial fusion zone and graphite that forms during tempering; transformation zone of the weldment. they also increase the amount and Large amounts of carbon can be continuity of the carbides, resulting in Fig. 1—Round secondary graphite nodules dissolved in austenite during the weld­ elongated and chain-like graphite par­ and uncoalesced carbon clusters in the ing cycle by solution of carbon from ticles. heat-affected region of a nodular iron weld­ the original primary graphite nodules. Full annealing of the weldments ment after tempering at 1250 F (677 C). X500 On rapid cooling, high carbon mar­ prevents the formation of chain-like (reduced 50% on reproduction) tensite will form. Initially, clusters of graphite but does not completely tiny carbon particles form from the prevent the formation of secondary ment. martensite during tempering; the clus­ graphite nodules. Best results might be In nodular iron weldments, the ters eventually coalesce into distinct obtained by combining a suitable secondary graphite particles originate secondary graphite nodules. from two sources. First, iron carbide preheat temperature to prevent mar­ Figure 1 shows an example of both forms in the fused and diluted base tensite formation with proper selec­ uncoalesced carbon clusters and sec­ metal adjacent to the fusion zone due tion of base metal microstructure and ondary graphite nodules in the heat- to fast cooling rates and the absence of welding process and parameters to affected region of a tempered weld. effective nuclei for graphite during minimize carbide formation. A full These secondary graphite particles solidification. The iron carbide is diffi­ anneal gives better microstructure originate at lattice defects, particularly cult to prevent except by using control than a subcritical heat treat­ 1 at martensite plate boundaries or extremely high preheat temperatures, 3 ment. intersections'- ; consequently, their although the continuity of the car­ number is at least partly determined by bides can be disrupted by control over the structure and coarseness of the Introduction the initial microstructure and the martensite. The secondary graphite is welding parameters.3 When the nodu- The microstructure of nodular cast only observed after tempering at or iron weldments is characterized by the above about 1000 F (538 C) and after formation of hard, brittle iron carbides the martensite has at least partially D. R. ASKELAND is Associate Professor of 3 and high carbon martensite in the Metallurgical Engineering, and N. BIRER is a ferritized."" The amount and morphol­ heat-affected area. Although these Graduate Student in Metallurgical Engi­ ogy of the secondary graphite depends constituents can subsequently be neering, University of Missouri—Rolla, Rol­ on the amount of carbon in the eliminated by a lengthy heat treat­ la, Missouri. martensite, or on the peak tempera­ ment, secondary graphite particles are ture and time to which the austenite Paper presented at the AWS 60th Annual was exposed. produced which still impair the ductil­ Meeting, held in Detroit, Michigan, during ity and impact properties of the weld- April 2-6, 1979. Higher austenite temperatures or

WELDING RESEARCH SUPPLEMENT I 337-s longer times at elevated temperatures content helped assure that large while the remainder of the matrix increase the amount of carbon dis­ amounts of martensite would form in transformed to austenite. Both the solved in the martensite. For low the heat-affected area during cooling liquid and the austenite were enriched carbon martensite, no secondary of the weld, both by promoting pear­ in carbon by partial solution of the graphite forms; for intermediate car­ lite in the base metal of the casting graphite nodules. On cooling, the bon levels the secondary graphite is and by increasing the hardenability of liquid solidified as white iron, contain­ round and randomly distributed, while the austenite. ing massive iron carbides, while the at high carbon contents elongated Castings were preheated in an elec­ high carbon austenite transformed to nodules or chains of secondary graph­ tric muffle furnace; immediately after coarse martensite. ite are produced.8 Elongated ferrite removal from the furnace, a bead- 4. Transformation Zone (TZ): This grains are also normally associated on-plate weld was produced using the portion of the heat-affected area was with the high carbon contents. shielded metal arc welding process heated above the critical temperature with 0.125 in. (0.3175 cm) Cl-Ni elec­ so that austenite formed during the Procedure trodes. The heat input varied from weld pass; however, no melting oc­ 27,000 to 44,000 joules/in. (2540 to curred. The amount of carbon dis­ Nodular iron plates 0.5 in. (1.27 cm) 4140 cal/cm). Samples were cut from solved in the austenite depended on thick were poured in green sand molds the welded plate and heat treated in the peak temperature reached during after the iron was melted in a 50 pound an electric muffle furnace for up to the weld pass and increased towards (22.8 kg) capacity magnesia-lined high sixteen hours. Metallographic speci­ the fusion zone. On cooling, a grada­ frequency induction furnace. The iron mens were prepared from the welds tion of transformation products was was nodulized at 2710 F (1488 C), and etched in 5% nital. obtained, ranging from coarse, high inoculated, and poured at 2550 F (1400 carbon martensite near the fusion C). The composition of the cast plates Results and Discussion zone to fine, lower carbon martensite was 3.6% C, 2.14% Si, 0.39% Ni, 0.038% and eventually to fine pearlite. Mg, and 0.013% S. The high nickel Microstructure of Weldments 5. Base Metal (BM): The original structure of the base metal contained Figure 2 shows a typical microstruc­ spheroidal graphite nodules in a ture of an as-welded nodular cast iron predominantly pearlite matrix due to plate, with no preheat. Five regions in the high nickel content. A small ferrite the weld were identified to simplify ring was observed around most of the subsequent analysis: nodules. As a result of preheating and 1. Fusion Zone (FZ) The weld postheating, spheroidization of the deposit contained tiny graphite nod­ base metal matrix occurred. ules in an austenitic matrix, since an iron-nickel electrode was used to Width of the Heat-Affected Regions produce the weld. 2. Carbide Zone (CZ): This portion The average widths of each of the of the heat-affected zone is composed three zones in the heat-affected area of partly diluted base metal that were measured using a Filar eyepiece melted during the weld pass and solid­ on the microscope. Figure 3 shows the Fig. 2—Typical microstructure of a nodular ified as white iron containing massive effect of preheat temperature on the iron weldment with no preheat, showing width of each of the zones as well as the carbide, partial fusion, and transforma­ and often continuous iron carbide. on the total width of the heat-affected tion zones. X250 (reduced 50% on repro­ 3. Partial Fusion Zone (PFZ): In this duction) region, the portion of the matrix of the area. The carbide zone was very small base metal near the primary graphite except at very high preheat tempera­ nodules melted during the weld pass, tures. At high preheat temperatures,

I s

_ 5 x 5 CARBIDE ZONE 0 200 400 600 PRE HEAT TEMPERATURE (C) Fig. 3—The effect of preheat temperature on Distance From Fusion Zone the width of the zones in the heat-affected Fig. 4-A schematic diagram showing a peak temperature distribution in the area of nodular iron weldments heat-affected region of nodular iron weldments at two preheat temperatures

338-s I NOVEMBER 1979 mm Fig. 6.—Microstructure in the partial fusion zone of nodular iron weldments after welding; fusion zone is at top ol photomicrograph: A—no preheat; B—930 F (500 C) preheat; C-1200 F (650 C) preheat. X250 (reduced 53% on reproduction)

be higher than the equilibrium liqui-. of secondary graphite during temper­ dus. ing of martensite depend on the DISTANCE (mm) At high preheat temperatures, the coarseness and carbon content of the Fig. 5—The effect of preheat temperature on rate of solution and liquation is faster martensite which is produced. One the hardness in the transformation zone of than at low preheat temperatures, thus indication of the carbon content in the as-welded nodular iron weldments reducing the effective liquidus and martensite is its hardness. Microhard­ permitting smaller fusion zones, larger ness traverses were made through the the carbide region became more transformation zones, but an overall transformation zone in order to com­ extensive and continuous, as has been smaller total heat-affected area. In Fig. pare the tendency of the martensite to reported previously in the available 4, the peak temperature profile for the temper and produce secondary graph­ literature." Carbides also precipitated low preheat temperature is depicted as ite. The effect of preheat temperature more frequently at grain boundaries in being above that for the higher on the diamond pyramid hardness in the partial fusion and transformation preheat temperature; further from the as-welded nodular iron is shown in zones at high preheat temperatures. weld, the profile for the low preheat Fig. 5. The transformation zone increased temperature should fall below that for When the preheat temperature is continuously as the preheat tempera­ the high preheat temperature. 482 F (250 C) or less, a significant ture increased, as expected, while the The actual dependence of these amount of high carbon martensite is partial fusion zone decreased in size, widths on the preheat temperature produced in the transformation zone. causing the overall width of the heat- may differ depending on welding The hardness exceeds Rockwell C 60 in affected area to also decrease. conditions, base metal thickness, and these regions, consistent with a large The schematic diagram showing the base metal microstructure. Because fraction of high carbon martensite effect of preheat temperature on the there was an increasing amount of produced by rapid cooling rates peak temperature curve (Fig. 4) may ferrite in the base metal as the preheat achieved after welding. At higher help explain these observations. Melt­ temperature increased, and because preheat temperatures, the cooling ing of the iron is not instantaneous ferritic microstructures are known to rates were slower after welding, giving upon heating to the liquidus tempera­ reduce transformation rates during less martensite, more fine pearlite, and ture, since time is required for carbon heating, the total heat-affected zone consequently lower hardnesses in the to dissolve and diffuse from the might be expected to be smaller at transformation zone. primary graphite nodules and even­ high preheat temperatures. The as-welded microstructure in the tually combine with the iron to partial fusion zone and in the transfor­ produce a liquid. Because of this mation zone near the weld is shown in nonequilibrium effect, the effective Microhardness of the Transformation Fig. 6 after different preheat tempera­ Zone liquidus temperature on heating, tures. The martensite is very coarse above which melting is complete, may The characteristics of the formation and abundant at low preheat tempera-

Fig. 7-Microstructure in the partial fusion and transformation zones before and after tempering at 1250 E (677 C), showing the effect of initial matrix structure on formation of secondary graphite: A-PFZ (as-welded); B—PFZ (tempered); C-TZ (as-welded); D—TZ (tempered). X250 (reduced 52% on reproduction)

WELDING RESEARCH SUPPLEMENT | 339-s tures and disappears at the highest 800 preheat temperature. Consequently, little secondary graphite caused by 60 " A graphitization of the matrix would be expected during tempering after high *%•«*--* >M preheat temperatures; practically no *»£•* martensite is present, while secondary 35 ,• ,••.. U ,•»»! 3* ., graphite does not form during the 50 Z y^ ferritization of pearlite. s f as However, greater quantities of more X continuous iron carbides, including 40 U grain boundary carbides, are observed in Fig. 6 as the preheat temperature increases above 932 F (500 C). After 30 Fig. 9—Microhardness indentations overlap tempering, the appearance of chain­ O secondary graphite nodules, reducing hard­ like secondary graphite nodules from 2 ness values for the matrix. X1000 (reduced graphitization of the carbides would 50% on reproduction) be expected in welds produced at high preheat temperatures. I 2 The amount, hardness, and coarse­ DISTANCE (mm) ness of the martensite also decreased - TEMPERED 1 HOUR Fig. 8—The effect of postheat treatment on - TEMPERED 4 HOURS with increasing distance from the hardness in the transformation zone of fusion zone, producing a change in the nodular iron weldments. No preheat incidence and morphology of second­ E. ary graphite nodules in each weld. Figure 7 shows the original as-welded cementite is redissolved and a very microstructure and the tempered mi­ fine dispersion of carbon particles, or crostructure in a plate welded with no clusters, is produced during the tem­ z preheat, revealing the difference in pering of quenched nodular iron. 6 matrix structure on secondary graphite These carbon particles eventually co­ formation. At distances more remote alesce and produce distinct secondary from the fusion zone, the peak graphite nodules in a ferritic matrix on temperature was lower, less carbon tempering at 1000 F (538 C) or above. 1 dissolved in the austenite, less austen­ The number of secondary graphite ite transformed to martensite, and nodules depends on the tempering consequently fewer secondary graph­ temperature and time, on the martens­ ite nodules formed during tempering. ite coarseness and carbon content, and Postheat treatment reduced the on the composition of the iron. For hardness in the heat-affected area due instance, silicon increases the number to tempering of the martensite, graphi­ of secondary graphite nodules, while PRE HFAT TEMPERATURE (C) tization of carbides, and ferritization copper and nickel decrease the num­ ber.' Similar behavior should be Fig. 10—The effect of preheat temperature of the base metal matrix. Figure 8 and tempering time on the number of expected in nodular iron weldments, shows the hardness profile in the heat- secondary graphite nodules produced dur­ affected area of a weld produced with where martensites are produced hav­ ing tempering at 1250 F (677 C) at three no preheat both before and after ing a wide range of previous thermal locations in the heat-affected region tempering. The hardness in the trans­ histories. formation zone is relatively uniform Figure 10 shows the number of after tempering and decreases as the secondary graphite nodules produced tempering temperature increases. at three locations in the heat-affected less carbon and is less likely to form, Ferritization of both the heat- area as a function of the preheat again reducing the number of sites at affected area and the base metal was temperature after tempering 1 hour (h) which graphite can nucleate as well as slow in nodular iron due to the pearl­ and 4 h at 1250 F (677 C). These areas reducing the amount of carbon avail­ ite stabilization caused by the high include locations near the boundaries able in the martensite to form nickel content of the iron. The hard­ between the carbide and partial fusion graphite. There are some secondary ness readings do not necessarily reflect zones, between the partial fusion and graphite nodules even near the base the true hardness of the matrix, since it transformation zones, and between metal; there apparently is some base was almost impossible to completely the transformation zone and the base line number of nodules, for which avoid secondary graphite nodules dur­ metal. The number of secondary nuclei were produced during the ing testing. Figure 9 shows microhard­ graphite nodules decreases as the transformation processes occurring ness indentations which overlap sev­ preheat temperature increases; less during welding and heat treatment. martensite forms in the transformation eral nodules, resulting in a lower hard­ Finally, the number of secondary zone due to slower cooling rates. With ness value. In fact, the hardness in the graphite nodules is reduced by use of less martensite, and consequently few­ base metal was often higher than in longer tempering times. Some of the er martensite plate boundaries and the transformation zone due to the less favored nodules dissolve and the intersections, the number of nuclea­ presence of the secondary graphite. carbon re-precipitates on other sec­ tion sites for secondary graphite, are ondary graphite particles, causing their reduced. growth. This appears to occur more Secondary Graphite The number of secondary graphite rapidly when large numbers of nod­ When martensite in nodular iron is nodules also decreases as the distance ules are present, as near the fusion heated, the initial stages of tempering from the fusion zone increases. Due to zone. resemble that of steel, where cement­ slower cooling rates and lower peak The relationship between nodule ite is the eventual product. However, temperatures, the martensite contains number and nodule size is shown in

340-s I NOVEMBER 1979 ouuu 21 ^^ Xl 250

CN 21 | 4000 250^\ \fa 500 on LU 500°^ ^O 650

CO LU 650 100 200 300 400 500 600 3 2000 • PRE HEAT TEMPERATURE Q (c) O TEMPERED HOUR Fig. 12—The-The effecteffect of preheat temperaturetemperature on the amount of o D TEMPERED 4 HOURS secondary graphite in the partial fusion zone while tempering at 2 1250 F (677 C) for I h

1 1 2 3 4 Fig. 11—Relationship between the number and size of secondary graphite nodules formed during tempering at 1250 F (677 C). NODULE DIAMETER MICRONS Numbers refer to preheat temperature

from the average number and diame­ ter of the nodules. Although large * numbers of secondary graphite nod­ ^V. ules form at low preheat temperatures, the time at the peak temperatures is short and the martensite tends to be relatively low in carbon, since there is insufficient time to dissolve large i * amounts of carbon from primary graphite nodules in the austenite. rf . At higher preheat temperatures, » more carbon is dissolved in the aus­ tenite. Although fewer nucleation sites Fig. 13—Elongated secondary graphite nod­ ules produced while tempering following for secondary graphite nodules are high preheat temperatures. X500 (reduced produced, there is a greater amount of 50% on reproduction) carbon available to form the nodules. At very high preheat temperatures, the total percent of secondary graphite Fig. 11. Both increasing the original begins to decrease again, even though Fig. 14-Chains of graphite nodules in the preheat temperature (shown as num­ there is a large amount of available carbide zone formed during tempering bers on the graph) and tempering time carbon, because there are too few following high preheat temperatures due to reduced the number of secondary nuclei. Most of the austenite trans­ decomposition ol iron carbides. X1000 (reduced 38% on reproduction) graphite nodules and increased their forms to pearlite rather than martens­ size. Long tempering times permitted ite. As the pearlite is ferritized, carbon in the regions that contain high carbon coalescence and growth of favored re-precipitates on primary nodules secondary graphite nodules, as ex­ martensite. These regions produce a rather than as secondary graphite large number of secondary graphite, plained previously. High preheat tem­ nodules. peratures, on the other hand, reduced often elongated. The nodules effec­ the amount of martensite, and thus the tively pin the ferrite grain boundaries, number of nucleation sites at which Morphology of Secondary Graphite which are initially acicular in shape, outlining the former martensite plates, the nodules could form. However, due The shape of the secondary graphite thus preventing the grains from grow­ to the higher peak temperatures that nodules is significantly affected by the ing into the equiaxed morphology would be expected at high preheat carbon content of the martensite; which is more desirable. When fewer temperatures, any martensite that does secondary graphite may be round and secondary graphite nodules are pres­ eventually form should have a high randomly distributed, as in Fig. 1, or ent, as when high preheat tempera­ carbon content and should permit may be elongated, as in Fig. 13. Gener­ tures are used or at distances further large secondary graphite nodules to ally, the higher carbon martensites from the fusion zone, the ferrite grains form. Thus, only a few, large secondary encourage elongated secondary graphite nodules should precipitate graphite; consequently, this morphol­ will more likely grow and become when tempering the weld after high ogy is found in the partial fusion zone equiaxed during tempering. preheat temperatures were em­ and in the transformation zone nearest ployed. the fusion zone. Round secondary Graphitization of Carbides Figure 12 also shows that high graphite is observed in the transforma­ A second source of secondary preheat temperatures will cause a tion zone nearer the base metal, where graphite nodules is the iron carbide large amount of carbon to be made the peak temperature was lower and a present in the carbide zone. The available for secondary graphite for­ lower carbon martensite was pro­ carbides form during solidification of mation. The amount of secondary duced. the molten cast iron. Only very slow graphite, in area 1%, was estimated The ferrite grains are also elongated cooling rates can prevent their occur-

WELDING RESEARCH SUPPLEMENT I 341-s the hardness of martensite without causing secondary graphite formation, T higher postheats are required to graphitize the carbides; consequently, secondary graphite nodules will form. The number of secondary graphite f& v*^- nodules will diminish slightly with increasing postheat times. Fully annealing the weld appears to most effectively minimize the second­ ary graphite nodules; however, it is not successful in completely preventing their formation. The most effective way to prevent or minimize secondary *^g • <*r, "©.»•* graphite nodules is to prevent the formation of carbides and martensite Fig. 15—Microstructure of two nodular iron weldments after annealing at 1650 F (900 C) lor 2 during welding. This requires careful h. Welds were preheated at three different temperatures: A—no preheat; B—480 E (250 C) control of the base metal composition preheat; C—1200 F (650 C) preheat. X250 (reduced 53% on reproduction) and microstructure, the preheat tem­ perature, and other welding parame­ rence; the typical preheat tempera­ ples of the annealed microstructures ters. tures often tend to increase the are shown in Fig. 15; complete elimi­ amount and continuity of the carbides nation of the pearlite was not obtained Acknowledgments rather than prevent their formation. due to the presence of nickel in the The authors gratefully acknowledge During tempering at high tempera­ base metal. the generous assistance of the Weld­ tures, the iron carbides graphitize, ing Research Council for its partial with the graphite particles often Summary preferentially nucleating at the inter­ support of this research and of Wagner face between the fusion zone and the In order to reduce the hardness and Casting Company for providing chemi­ heat-affected area. Long chains of brittleness in the heat-affected area of cal analysis. discrete secondary graphite nodules nodular iron weldments, the forma­ are formed, producing an almost tion of martensite and carbides must References be prevented or these constituents continuous film of graphite—Fig. 14. 1. Nippes, E. F., Savage, W. F., and The severity of the chain-like nodules must be eliminated by heat treatment. Owczarski, W. A., "The Heat Affected Zone worsens at the higher preheat temper­ However, any heat treatment must be of Arc-Welded Ductile Iron," Welding lour­ atures, where problems with martens­ carefully controlled to minimize sec­ nal, 39 (11), Nov. 1960, Research Suppl., pp ite and the secondary graphite from ondary graphite nodules and to max­ 465-s to 472-s. the martensite are least pronounced. imize the ductility in the weld. 2. Hirota, Y., "The Effect of Welding and Consequently, it is difficult to select a Increasing the preheat temperature Microstructure Variables on Carbides and preheat temperature that will prevent produces a number of desirable Impact Properties in Ductile Cast Iron Weldments," M.S. Thesis, 1977, University the carbides, the martensite, and the effects. First, high preheat tempera­ of Missouri-Rolla, Rolla, Mo. secondary graphite nodules that are tures, by reducing the cooling rates, 3. Pease, G. R., "The Welding of Ductile produced from both. minimize the formation of martensite Iron," Welding lournal, 39 (1), )an. 1960, in the transformation zone. In addi­ Research Suppl., pp 1-s to 9-s. tion, the size of the partial fusion zone, 4. Farinez, F., "The Effect of Alloying which contains high carbon martens­ Additions on the Formation of Secondary Annealing ite as well as carbides, is reduced. Graphite in Quenched and Tempered Nodular Cast Iron," M.S. Thesis, 1976, Specimens from the nodular iron Finally, due to the formation of coarse University of Missouri-Rolla, Mo. welds were also annealed at 1650 F martensite, relatively few secondary graphite particles will grow during 5. Rys, P., "Graphitization of Nodular (900 C) for 2 h and furnace cooled. Iron During Tempering," 26th International Problems with secondary graphite for­ tempering; in fact tempering may not be necessary if the preheat was suffi­ Foundry Congress, Madrid, October, 1959. mation were largely reduced com­ 6. Danko, |. C, and Libsch, |. R., "Second­ cient to reduce the amount and pared to the results obtained from ary Graphitization of Quenched and Tem­ subcritical tempering. At intermediate continuity of the martensite. pered Ductile Cast Iron," ASM Trans., 47, preheat temperatures, very few sec­ Unfortunately, high preheat temper­ 1955, pp. 853 to 863. ondary nodules were observed. A atures may increase the size and 7. Gilbert, G. M. and White, D. O, "Me­ somewhat greater incidence of nod­ continuity of the carbide zone and the chanical Properties of Nodular Irons Heat ules was observed at the highest carbon content in the martensite. Treated to Obtain Ferrite and Tempered Martensitic Structures," BCIRA /., 11, 1963, preheat temperature; these nodules Unless the carbides are rendered pp. 199 to 222. probably formed from graphitization discontinuous by proper control of the 8. Desai, M. )., "Metallographic Study of of the carbides during annealing. welding parameters and the base Phase Changes in Heat Treated Ductile However, the secondary graphite nod­ metal microstructure, postheat treat­ Iron," M.S. Thesis, 1970, University of ules, although closely spaced, did not ment will be necessary and elongated Missouri-Rolla, Rolla, Mo. have the chain-like morphology. secondary graphite and chains of 9. Hucke, E. E., and Udin, H., "Welding The greatest number of secondary secondary graphite may be produced Metallurgy of Nodular Cast Iron," Welding lournal, 32 (8), Aug. 1953, Research Suppl., graphite nodules was observed when if a subcritical postheat temperature is pp. 378-s to 385-s. no preheat was used; these nodules used. 10. Flannery, J. W., "Welding Ductile probably formed from nuclei pro­ Postheating is necessary in most Iron-Part Two," Welding Engineer, 53 (12), duced by the original martensite. cases in order to eliminate the carbides Dec. 1968, pp. 50 to 53. Previous work has shown that these and martensite and thus reduce hard­ 11. Eckel, E. )., "A Study of the Ferritiza­ secondary graphite nuclei are not ness and brittleness. Although low tion of Nodular Iron," AFS Trans., 66, 1958, eliminated by re-austenitizing.8 Exam­ tempering temperatures can reduce pp. 151 to 165.

342-s I NOVEMBER 1979