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Secondary Graphite Formation in Tempered Nodular Cast Iron Weldments

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Secondary Graphite Formation in Tempered Nodular Cast Iron Weldments 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.
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