Fig. 1 — Etched cross sections from aluminum-free (top) and aluminum-containing (bottom) 26%Cr, 6%Ni, bal. Fe welds made under identical conditions using the gas metal-arc welding process. Note the columnar structure in the upper weld and fine grained structure in the lower weld. Lepito's etchant; X2, reduced 50%

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Fine Grained Weld Structures Superp/astic welds can be obtained in certain Cr-Ni iron base alloys using filler metals of controlled Al-N contents and conventional welding procedures BY W. A. PETERSEN

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Fig. 2 — This photomicrograph shows the structure of the base alloy at the left, grain coarsened heat-affected zone, and coarse grained weld deposit of the aluminumfree gas metal-arc weld. The arrows indicate the junction of the weld and heataffected zone. X50, reduced 50%; electrolytic 10% oxalic acid etchant

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ABSTRACT. Grain refinement in the weld pool of a microduplex stainless steel was attempted by inoculating w i t h stable oxides, nitrides and carbides. Fine grained welds were made w i t h small additions of aluminum to alloys containing controlled amounts of nitrogen. Initial weld solidification was epitaxial and planar; however, it then changed abruptly to an equiaxed fine grained structure. The equiaxed structure resulted from the solidification process and not transformation. Superplastic behavior was observed in elevated temperature tensile tests of these welds.

Fig. 3 — The addition of 0.05% aluminum to the filler metal used to produce the weld shown in Fig. 2 resulted in initial growth by a planar mechanism which ended abruptly with the appearance of an extremely fine grained structure. The arrows indicate the edge of the heat-affected zone and the weld metal is located at the right. X50, reduced 50%; electrolytic 10% oxalic acid etchant

W. A. PETERSEN is associated with The International Nickel Co., Inc., Paul D. Merica Research Laboratory, Sterling Forest, Suffern, N. Y. Paper presented at the 53rd AWS Annual Meeting held in Detroit, Mich., during April 10-14, 1972.

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/X#;^m^ Fig. 7 — Appearance of dendrites in the crater of a gas tungstenarc weld in an aluminum-free 32%Cr, 14%Ni, bal. Fe alloy. X100, reduced 15%; unpolished and unetched

to scale-up heat size w e r e unsuccessful due to extreme hardness and brittleness w h i c h precluded ingot preparation prior to hot working. This condition was attributed to the slower cooling rate of the heavier sections and the resultant formation of sigma phase. The fine grained effect was not observed in alloys w i t h chromium contents below 2 0 % although a fine grained effect has been reported for the nickel-free, 17% chromium steel. 11 Superplasticity in Fine Grained Welds Tensile tests of welded joints showed that the coarse grained w e l d metal did not behave superplastically, showing only 4 0 to 6 0 % elongation. 9 Because the 0.03 to 0.06 mm grain size in the equiaxed welds was w i t h i n the grain size range k n o w n to behave superplastically, the possibility of obtaining similar behavior in welded structures w a s examined. Figure 10 shows the appearance of an all-weld-metal tensile coupon prior to testing and aluminum-free and aluminum-containing coupons after testing at 1 700 F at a strain rate of 0.05 i n . / i n . / m i n . The tensile coupon from the aluminum-containing weld pictured, exhibited an extension of 173% as compared to the 6 1 % exhibited by the normal or coarse grained alloy. A wrought microduplex alloy of this composition would be expected to show 2 0 0 to 6 0 0 % elonga-

F/g. S — Appearance of dendrites in the crater of a gas tungstenarc weld in a 0.05%AI, 32%Cr, 14%Ni, bal. Fe alloy. X100, reduced 15%; unpolished and unetched.

tion under these test conditions. 9 A number of all-weld-metal tensile coupons were prepared and tested using a variety of compositions w i t h i n the fine grained region s h o w n previously on the ternary diagram. The tensile data revealed that the fine grained behavior was related to not only the aluminum but also the nitrogen content of the weld deposits. The aluminum content of these welds ranged from 0.01 to 0.20% and the nitrogen levels varied from 0.002 to 0.06%. Figure 11 shows that the maximum superplastic elongation was obtained at an aluminum-tonitrogen ratio of about 1.8 to 1. S i m ilarly, the finest grain size was found to correspond to the highest elongation value (Fig. 1 2). Process Variation Fine grained welds have been produced w i t h the common welding processes using the conditions s h o w n in Table 2. Manual and automatic inert gas-shielded processes were examined in greatest detail. Fine grained welds could be produced w i t h covered electrodes by making generous aluminum additions to the flux covering (compare welds 6 and 7). As in the case of conventional w e l d deposits, 12 - 13 energy input was found to influence grain size. Lowering the energy input led to some reduction in grain size and greater elongation in elevated temperature tensile tests

(compare weids 4 and 5 in Table 2).

Discussion Significance of Observations Fine grained structures were observed in welds made w i t h the gas tungsten-arc, gas metal-arc, and shielded metal-arc welding processes using normal conditions w i t h no special controls or equipment. A useful fine grained effect was observed in iron-base alloys containing about 20 to 35% chromium, and 5 to 15% nickel. This was principally related to the presence of aluminum and nitrogen in certain ratios. The fine grain size appeared to be due to the spontaneous nucleation of new grains w h i c h formed during a very early stage of solidification since, the normal epitaxial growth only proceeded a short distance (e.g., about 0.07 mm) before the solidification mode changed to one of fine equiaxed grains.

Mechanism for Formation A l u m i n u m nitride, or other nitrides, carbides, etc., of appropriate size can exist as stable nucleii in liquid metal. 1 4 These may act as sites for the formation of small, individual dendrites during cooling of weld metal. As noted earlier, microprobe examination of fine grained weld samples

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change was found during microstructural examination. Due to the presence of a great number of nucleation sites in the weld pool and the other factors enumerated above, it appears that solidification from these nucleation sites overrides the normal epitaxial g r o w t h mechanism 1 8 and dominates the solidification reaction. The work of Matsuda, er al. 19 w i t h thin sheet lends support to this hypothesis in that they found that under certain welding conditions, equiaxed grains began to prevail over the columnar growth form.

GRAIHED

O - CO ARSE GRAINED Cr-CRACKIHG III HELD

Areas Where Fine Grained Weld Structures May be Applied

Fig. 9 — Compositions of alloys exhibiting fine and coarse grain sizes in welds plotted on a section of the Ni-Cr-Fe equilibrium diagram showing phases just below the solidus

showed that many of the particles w i t h i n the center of the grains were high in aluminum content. These particles apparently promote the fine grain effect but do not fully account for the observed refinement since it occurs over only a limited range of chromium and nickel contents. In addition to the presence of these particles, it is felt that several other factors may contribute. Reference to the iron-nickelchromium phase diagram 1 5 shows that the difference between solidus and liquidus surfaces is about 15 C for the alloys of interest. This very narrow solidification range would promote rapid freezing and limit grain g r o w t h in the w e l d deposit. Aborn and Bain had originally suggested that a peritectic reaction w a s involved in alloys of the type studied in this investigation. 1 6 Such a reaction may provide an additional source of grain refinement as indicated by the recent work of Delamore and Smith on aluminum-titanium castings. 1 7 More importantly though, the general shape perceived for such a binary iron-nickel diagram at a constant chromium level shows that the first l i quid to freeze is solute depleted ferrite and the last liquid to solidify is 78-s I F E B R U A R Y

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solute enriched and consequently austenitic. This is considered important because the austenitic areas serve to block grain g r o w t h at the welding temperature as w e l l as during reheating by later w e l d passes. The stability of the austenite in the welded structure was confirmed by heating a weld to 2300 F for 1 hr. No grain g r o w t h or other structural

Since the welds and base metal now both exhibit superplasticity, this property might be used to advantage in welded articles to increase the utility of pressure forming or other elevated temperature forming processes since the weld areas can now be deformed the same amount as the surrounding base metal rather than remaining rigidly fixed in place. Also, it is conceivable that an entirely w e l d fabricated structure could be produced, lightly machined to provide uniform surfaces, and then formed w i t h i n a mold using a pressure f o r m ing process. Other applications involve such areas as the surfacing of low alloy steel to provide a corrosion resistant surface layer. Such a fine grained structure should provide more corrosion resistance, greater toughness, and better fatigue properties than a similarly used coarse grained deposit. A fine grained structure should be more resistant to weld cracking than a similar coarse grained structure since segregated low melting point impurities w i l l be more widely dispersed and consequently less effec-

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Fig. 10 — An all-weld-metal tensile specimen is shown at the top of the photograph prior to testing. The middle coupon was made with an aluminum-free filler and exhibited 61% elongation in the tensile test at 1 700 F. The lower coupon was prepared from a weld made with an aluminum-containing filler and exhibited 173% elongation. Full scale, reduced 17%

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Fig. 11 - Plot showing the relationship between percent elongation in 1700 F tensile tests and the ratio of aluminum to nitrogen in all-weld-metal tensile specimens

Conclusions 1. Fine grained welds can be attained in alloys similar to the 26 % Cr, 6.5 % N i stainless steel using conventional welding equipment and conditions. 2. The fine grained structure is due to the solidification process and not to a transformation effect. 3. The phenomenon occurs over a limited range of chromium and nickel contents in iron-base alloys and is dependent upon the presence of aluminum and nitrogen in critical ratios.

.120

160

GRAIN SIZE,mm

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tive in forming continuous liquid films w h i c h are the eventual cause of some w e l d cracks. Similarly, the fine structure should be more resistant to the "ductility-dip" form of weld cracking since large grain size is k n o w n to have an adverse effect on this phenomenon. 2 0 In certain alloys the finer grain size could also reduce susceptibility to post-weld heat treatment cracking. 2 1 The fine structure of the aluminum/nitrogen containing welds could be advantageously used in parts that require a cold forming operation after welding. Bend tests have s h o w n that the deformed surface of such welds are not prone to the columnargrain-marked deformation normally associated w i t h coarse grained w e l d deposits after bending. This benefit would help to reduce the finishing costs in articles w h e r e cosmetic appearance is important.

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Fig. 12 — Plot showing the relationship between percent elongation in 1700 F tensile test and the grain size of all-weld-metal tensile specimens

4. Welds of the preferred composition exhibited superplastic elongation in elevated temperature tensile tests. W e believe that this represents the first demonstration of superplastic behavior in weld metal.

References 1. Hall, E. O., "The Deformation and Aging of Mild Steel," Proc. Phys. Soc, 64B(9), 747(1951). 2. Garstone, J., Johnson, F. A., "Impact Properties of Mild Steel Weld Metals," Brit. Weld. Jnf, 10(5), 224 (1 963). 3. Hayden, H. W„ Floreen, S., "The Fatigue Behavior of Fine Grained Two-Phase Alloys," to be published in Met. Trans. 4. Weiser, P. F„ Church, N., Wallace, J. F., "Grain Refinement of Steel Castings," J. of Metals, 1 8(6), 44 (1 967). 5. Weiser, P. F„ Wallace, J. F„ "The Effect of Solidification Time and NonMetallics on the Ductility of High Strength Steel Castings," Mod. Cast., 55(2), 22 (1969). 6. DAntonio, C, Vecchio, A. J., "Effect of Ultrasonic Agitation on Grain Size in Welds," Technical Note, Weld. Jnf, 41(4), Research Suppl., 166-s (1962). 7. Brown, D. C, Crossley, F. A., Rudy, J. F., Schwartzbart, H., "The Effect of Electromagnetic Stirring and Mechanical Vibration of Arc Welds," Weld. Jnf, 41(6), Research Suppl., 241 -s (1962). 8. Hayden, H. W., Gibson, R. C, Merrick, H. F., Brophy, J. H., "Superplasticity in the Ni-Fe-Cr System," ASM Trans. Quart, 60(3), 3(1967). 9. Gibson, R. C, Hayden, H. W., Brophy, J. H., "Properties of Stainless Steels With a Microduplex Structure," Trans. ASM, 61(1),85(1968).

10. Petersen, W. A., Lang, F. H., "Welding of a High Strength Stainless Steel," Weld. Jnf, 49(6), Research Suppl., 267-s (1970). 11. Nishio, Y., Ohmae, T„ Yoshida, Y., Miura, Y., "Weld Cracking and Mechanical Properties of 17% Chromium Steel Weldment," Weld. Jnf, 50(1), Research Suppl., 9-s(1971). 12. Irvine, K. J., Pickering, F. B., "Relationship Between Microstructure and Mechanical Properties of Mild Steel Weld Deposits," Brit. Weld. Jnf. 7(5), 353 (1960). 13. Tremlett, H. F., Baker, R. G., Wheatley, J. M., "Mechanical Properties and Metallurgical Features of Mild Steel Weld Metals,"Brit. Weld. Jnf, 8(9), 437(1 961). 14. Chalmers, B., Principles of Solidification, John Wiley and Sons, Inc., New York (1964). 15. Metals Handbook, ASM (1948). 16. Camp, J. M., Francis, C. B„ The Making, Shaping and Treating of Steels, U.S. Steel Corp., 6th Ed., 131 7 (1951). 17. Delamore, G. W., Smith, R. W., "The Mechanisms of Grain Refinement in Dilute Aluminum Alloys," Met. Trans., 2(6), 1733(1971). 18. Savage, W. F., Lundin, C. D., Aronson, A. H., "Weld Metal Solidification Mechanics," Weld. Jnf, 44(4), Research Suppl., 1 75-s (1965). 19. Matsuda, F., Hashimoto, T., Senda, T., "Fundamental Investigations on Solidification Structure in Weld Metal," Trans. Nat. Res. Inst. for Met (Japan), 11(1), 43(1969). 20. Yeniscavich, W., "A Correlation of Ni-Cr-Fe Alloy Weld Metal Fissuring With Hot Ductility Behavior," Weld. Jnf., 48(8), Research Suppl., 344-s (1 966). 21. Duvall, D. S., Owczarski, W. A., "Studies of Postweld Heat Treatment Cracking in Nickel-Base Alloys," Weld Jnf, 48(1). Research Suppl., 10-s(1969).

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WRC Bulletin No. 176 Sept. 1972

"Long-Range Plan for Pressure-Vessel Research—Third Edition" By the Pressure Vessel Research Committee

A suitable group to carry out the research p l a n n i n g for P V R C w a s created when the PVRC P r o g r a m E v a l u a t i o n Committee (now designated the E v a l u a t i o n and P l a n n i n g Committee) was formed in 1961. T h i s group w a s originally charged with the responsibility of e v a l u a t i n g the research work done by P V R C a n d others, and to prepare a "PVRC Interpretive Report of Pressure Vessel R e s e a r c h " to make the results directly useable to the designer a n d Code-making bodies. During the review a n d evaluation of available information, voids in the s t a t e of knowledge a n d the need for further research became a p p a r e n t . Although these items were mentioned in the report, they needed to be organized into a consistent plan. Thus, the 18 research topics submitted to P V R C by ASME in 1959 were combined with the research problems uncovered by the P V R C Interpretive Report and published as the "PVRC Long-Range P l a n for Pressure-Vessel R e s e a r c h " in WRC Bulletin 116, September 1966. The P V R C "long-range p l a n " w a s distributed as widely a s possible for review and comment. Since then, a n u m b e r of additional problem a r e a s h a v e been suggested by the ASME BPVC a s well a s by other organizations a n d by individuals within PVRC. Therefore, to keep t h e long-range p l a n timely a n d u p to date, the Evaluation and P l a n n i n g Committee agreed t h a t it should be re-issued every three years. In accordance with this decision, the Second Edition of the longrange plan w a s issued in September 1969, in WRC Bulletin 144, a n d the Third Edition in September 1972, in WRC Bulletin 176. Some of the problems in the Second Edition were dropped a n d a n u m b e r of new problems were added in the Third Edition. The list of " P V R C Research P r o b l e m s " is comprised of 42 research topics, divided into three groups relating to the three divisions of P V R C , i.e., Materials, Design and Fabrication. E a c h project is outlined briefly in a project description giving the: (a) Title; (b) S t a t e m e n t of Problem a n d Objectives; (c) C u r r e n t S t a t u s ; and (d) Action Proposed. The price of WRC Bulletin 176 is $3.00 per copy. Orders for single copies should be sent to the American Welding Society, 2501 N.W. 7th Street, Miami, Fla. 33125. Orders for bulk lots, 10 or more copies, should be sent to the Welding Research Council, 345 E a s t 47th Street, New York, N.Y. 10017.

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