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Weldability and Solidification Phenomena of Cast Stainless Steel

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Weldability and Solidification Phenomena of Cast Stainless Steel Weldability and Solidification Phenomena of Cast Stainless Steel The weldability of cast alloy CF-8M is found to be largely dependent on its initial solidification mode, with primary ferrite heats exhibiting greater weldability BY M. J. CIESLAK AND W. F. SAVAGE Introduction tion established during solidification vations has been generated and of weld metal is far better preserved. reported by other investiga­ 4 5 15 1 29 30 31 34 35 41 Stainless steel casting alloys have Therefore, a more accurate interpreta­ tors - - - "- - - - . - been employed successfully for many tion of the mechanics of solidification Several theories have been sug­ years in the severe environments can be obtained from examination of gested to explain weld metal hot encountered in the chemical, petro­ the microstructure and the pattern of cracking. Early work by Bochvar,6 leum, and nuclear industries. An entire microsegregation in weld metal at Pumphrey and Jennings,7 Pumphrey class of alloys has been developed for room temperature. and Lyons,8 Medovar,'1 and Toropov10 use in corrosive environments at work­ The redistribution of solute during resulted in the formulation of the ing temperatures below 1200°F solidification has a strong influence on Shrinkage-Brittleness Theory. This the­ (649°C). Unfortunately, in many cases both the weldability and the castabili­ ory postulates a semi-rigid, "coher­ where acceptable mechanical proper­ ty of many stainless steels. "Hot tears" ent," dendritic network which is sus­ ties and corrosion resistance have in castings and "hot cracks" in welds ceptible to hot cracking if shrinkage been achieved, both castability and appear to occur by similar mechanisms strains exceed some critical level. The weldability have been poor. Since as a direct result of the microsegrega­ theory also states that, since diffusion there is often a correlation between tion of certain solute elements. is limited in welding, microsegregation castability and weldability, a better is augmented and the resulting low melting constituents aggravate the hot understanding of the solidification Hot Cracking Studies mechanics of these materials should cracking problem. This theory also make the design of improved alloys Studies of the solidification of aus­ recognizes the possibility of "healing" or "backfilling" of cracks with low possible. tenitic stainless steels have been con­ melting point constituents, such as Solidification research based on ducted for many years. It has been 1 eutectic liquids, if they are present in commercial alloy systems is difficult. observed since the early 1940's that large enough quantities. This is both because of the complexity hot cracking in austenitic stainless of the alloys and because the high steel weldments is reduced or pre­ A second theory—the Strain Theory temperatures involved make direct vented when a small amount of delta of Hot Cracking—offered by Pellini11 observation of the solidification pro­ ferrite is present. Early studies by Bor­ and Apblett1- proposes that a contin­ 2 3 cess difficult. As a result, most infor­ land and Hull supported the fact that uous, thin liquid film present at some mation on the solidification mechan­ 5-10 vol-% of delta ferrite in weld­ stage of the solidification process can­ ics of metal systems has been based ments and castings afforded the best not support the strains induced in the upon room temperature metallograph­ resistance to hot cracking and hot weldment or casting without hot ic examination of the solidification tearing. Since that time, a voluminous cracking. substructures of castings and weld­ amount of data which is in general The Generalized Theory of Super- ments and upon mathematical theo­ agreement with these initial obser- Solidus Cracking as proposed by Bor­ ries. land13 is probably the most widely In castings, the cooling times are accepted theory of hot cracking and normally long enough for diffusion- hot tearing today. Borland postulates Paper presented at the AWS 61st Annual that a semi-continuous liquid film controlled processes to modify both Meeting held in Los Angeles, California, exists in the weldment or casting sepa­ the microstructure and pattern of during April 14-18, 1980. microsegregation during cooling from rated by bridges of solidified material. M. j. CIESLAK is the Steel Founders' Society the solidus to room temperature. Con­ The solid bridges, when subjected to a of America Graduate Fellow, and W. F. critical level of induced strain, will versely, the cooling times for most SAVAGE is Professor of Metallurgical Engi­ weldments are so short as to suppress fracture, forming hot cracks. Borland neering and Director of Welding Research, uses a composition-dependent surface most diffusion-controlled reactions in School of Engineering, Materials Division, energy argument in conjunction with the solid state. Thus, the microstruc­ Rensselaer Polytechnic Institute, Troy, New ture and the pattern of microsegrega­ York. rapid solidification rates to describe 136-sl MAY 1980 Figure 1 consists of a projection of the solidus and liquidus curves on a plane of the Fe-Cr-Ni ternary dia­ gram.22 According to Fig. 1, composi­ tions on the nickel-rich side of the dashed liquidus curve should solidify as primary austenite while composi­ tions on the chromium-rich side should solidify as primary delta ferrite. Masumoto et al-3 reported that welds solidifying on the nickel-rich side of the liquidus were extremely suscepti­ ble to hot cracking whereas welds solidifying on the chromium side did not hot crack. The DeLong24 and Schaeffler' dia grams attempt to predict room tem­ perature ferrite contents from the compositions of the alloys. Predicting weldability based on these diagrams has met with mixed success. Masumo­ to21 found no correlation between weldability and ferrite content based on these diagrams. Recent work by Lippold' has shown that the form and volume of room temperature ferrite is a function of composition, the shape Fig. 1— Solidus and liquidus curves oi the Fe-Cr-Ni system of the Fe-Ci-Ni phase diagram, and the cooling rate experienced by the mate­ rial. the form and composition of the last- jected to the liquid during primary to-solidify material. delta ferrite solidification. Elements Materials and Procedures In addition to theories postulating a such as silicon (Si), molybdenum liquid phase as a prerequisite for hot (Mo), sulfur (S), and phosphorus (P), Materials for this investigation were cracking, a solid-state theory of hot which are more soluble in delta ferrite, supplied by member foundries of the cracking has been proposed by Mov- are rejected to the liquid during pri­ Steel Founder's Society of America. chan.14 mary austenite solidification. Elements Seven heats of CF-8M containing vary­ Several tests have been developed such as carbon (C) and manganese ing amounts of delta ferrite were to rate the relative hot cracking ten­ (Mn) are more soluble in austenite and received. All heats were annealed for dencies of commercial austenitic are thus rejected to the liquid during one hour at 2050°F (1121°C) and water stainless steels. These include the Fis­ primary delta ferrite solidification. quenched. The chemical compositions sure Bend Test,18 the Cast Pin Tear Sulfur and phosphorus form low of the heats are given in Table 1. Th Test,3-19 the Varestraint Test,5-*1*-0-21 and melting eutectics between sulfides compositions shown were determined many others. These tests generally and phosphides, and silicon tends to by wet analysis and are the average of conclude that austenitic stainless form glassy silicate films. In alloys sol­ three analyses. It should be noted that steels containing a few volume-per­ idifying as primary austenite, sulfur, the carbon content of heat 6 and the cent delta ferrite offer a much higher phosphorus, and silicon are rejected to nickel content of heat 3 were greater resistance to hot cracking than do the liquid, form low melting constit­ than allowed by CF-8M specifica­ wholly austenitic stainless steels. uents in the interdendritic regions dur­ tions. All fusion welding processes involve ing the terminal transient period, and Ferrite contents as measured by microsegregation of the component thus cause hot cracking. Alloys solid­ point counts taken of the as-cast alloying elements. In stainless steel ifying as primary delta ferrite reject microstructure in accordance with weldments, chromium (Cr) is rejected fewer of these elements to the liquid ASTM E-562 are listed in Table 1. to the liquid during primary austenite and consequently are more resistant to Magne-gage readings for both the as- solidification and nickel (Ni) is re­ hot cracking. cast and weld microstructures are also Table 1-Chemical Compositions and Ferrite Contents of Seven CF-8M Heats Phase anal /SIS Magne- gage ferrite no. Heat . Vol-% ferri e by no. C Mn Si Cr Ni Mo S P N point count in castir g Casting Weld 1 .08 .60 1.05 18.32 13.20 2.26 .016 .035 .041 0.2 0.2 0.2 2 .05 .26 .83 17.65 12.03 2.05 .017 .021 .06 1.5 1.2 2.0 3 .04 .21 .38 19.55 15.38 2.88 .025 .036 .04 2.0 2.6 2.6 4 .06 1.17 .60 18.18 12.08 2.48 .016 .027 .05 4.2 3.0 5.8 5 .07 .57 .99 18.21 9.57 2.39 .019 .024 .06 7.6 5.0 8.7 6 .10 .30 .69 20.31 10.62 2.34 .032 .046 .05 14.2 14 17 7 .06 .44 1.08 21.10 9.55 2.52 .008 .021 .048 22.8 > 25 >25 WELDING RESEARCH SUPPLEME NT I 137-s I-IV- \— l"— I Fig. 3—Perspective view of position of Var­ estraint specimen within the leg of the casting keel block mm) wide, voltages and currents were adjusted as shown in Table 3 for each heat.
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