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The Effects of Minor Elements on the Welding Characteristics of Stainless Steel

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The Effects of Minor Elements on the Welding Characteristics of Stainless Steel The Effects of Minor Elements on the Welding Characteristics of Stainless Steel Sulfur, oxygen and deoxidant contents determine penetration, while the silicon-to-aluminum ratio controls slagging characteristics BY B. POLLARD ABSTRACT. The effects of systematic inclusions in the steel because it tended who supply stainless steel strip to welded variations in the aluminum, sulfur, silicon, to approach an equilibrium correspond­ tubing manufacturers. Welded stainless manganese, phosphorus, molybdenum ing to temperatures near the surface of steel tubing is made by continuously roll- and copper contents on the penetration the weld pool, which were much higher forming a strip of steel into a tube and and slagging characteristics of laboratory than those encountered in steelmaking. welding the butt joint of the strip togeth­ heats of Type 304 stainless steel have Provided that sufficient amounts of the er. For wall thicknesses up to 0.125 in. been investigated. The effects of alumi­ appropriate deoxidant was present, (3.18 mm), the welding process most num and sulfur on penetration in tung­ these high temperatures favored the for­ commonly used is gas tungsten arc sten inert gas welding can be satisfactorily mation of slags that were richer in the (GTA). Since no filler wire is used, welding explained in terms of their effects on the high-stability oxides, such as those of characteristics of the steel are deter­ temperature coefficient of surface ten­ calcium, aluminum, titanium and silicon, mined completely by its composition. sion and the theory of surface tension and poorer in the low-stability oxides, The two welding characteristics of partic­ driven fluid flow. However, the results such as those of manganese and chromi­ ular concern to tubemakers are penetra­ for silicon and manganese are more com­ um, than the inclusions. In general, the tion and slagging. Poor penetration char­ plex and, while not inconsistent with the slagging characteristics of stainless steel acteristics limit welding speed and, there­ surface tension driven fluid flow model, were found to be determined primarily fore, have an adverse effect on the indicate that other factors besides the by the silicon-to-aluminum ratio of the economics of the tubemaking operation. temperature coefficient of surface ten­ steel. Patch type slags, which caused an In extreme cases, they can result in mate­ sion must be taken into account. In the uneven weld bead and irregular penetra­ rial that is unweldable because there is case of silicon, its influence on the viscos­ tion, were formed when the silicon-to- essentially no difference between the ity of the molten metal appears to have a aluminum ratio was less than 50 and the current required for full penetration and significant effect on penetration. As less troublesome globular slags formed that which causes excessive melt expected, phosphorus, molybdenum and when it was greater than 50. through. The second problem, slag for­ copper had no effect on penetration mation, is covered in Part II. Compositional limits for stainless steel since they are not surface active ele­ with welding characteristics suitable for ments, or likely to interact with surface The problem of heat-to-heat variations the production of welded tubing are active elements and, therefore, have no in the penetration characteristics of stain­ given. effect on the temperature coefficient of less steel was first reported in the litera­ surface tension. ture by Oyler, et al. (Ref. 1), in 1967. Part I—Penetration Characteristics However, it was not until 1975 that it Penetration values for production became a major problem for steelmak­ heats of Types 304 and 316 stainless Introduction ers. At that time, steelmakers noticed a steels showed a strong dependence on The effects of minor elements on the significant increase in the number of com­ sulfur content, but were lower for the welding characteristics of stainless steel plaints from tube-making customers laboratory heats, which was attributed to are of particular interest to steelmakers regarding the poor penetration charac­ their lower oxygen contents. teristics of 300 Series stainless steels. Slag formation during the GTA welding There were two reasons for these com­ of stainless steels was found to be caused plaints: first, many tube mills had installed primarily by the oxide inclusions in the new high-speed welding lines, which steel. However, EDS analyses indicated KEY WORDS required steel with improved penetration that the composition of the slag was not characteristics in order to exploit the always equivalent to a mixture of all the GTA Welding higher welding speeds which the equip­ Stainless Steels ment was capable of; and second, the Minor Element Effect argon-oxygen decarburization (AOD) Weld Characteristics process was rapidly replacing the electric B. POLLARD is with the Research Center, Weld Penetration Armco, Inc., Middletown, Ohio. The paper furnace (EF) for refining stainless steel. was prepared while the author was with I TV Weld Slagging The most obvious difference between Steel Co., Independence, Ohio. Welded Tubing AOD and EF steel is the oxygen content, Si-to-AI Ratio which typically averages about 60 ppm Paper presented at a conference on "Progress Sulfur-02 Deoxidants for AOD steel versus 120 ppm for EF in Resolving Undesirable Trace Element Effects Weld Bead Geometry steel. Therefore, poor penetration was on the Weldability of Steels," held November originally attributed to the low oxygen 21, 1985, in Charlotte, N.C. 202-s | SEPTEMBER 1988 content of AOD steel (Ref. 2). Evidence low-sulfur heats, which were solved by going to lower sulfur contents if this for the beneficial effect of oxygen was increasing the sulfur content to 0.015%. could be done without sacrificing weld­ provided by the results of other investi­ The adverse effect of aluminum and the ability. gators who showed that oxygen present beneficial effect of sulfur on weld pene­ In order to determine if it were possi­ as an oxide film improved penetration tration have been confirmed by more ble to produce low-sulfur steel with good (Refs. 3, 4). However, laboratory welding recent studies (Refs. 7-9). Unfortunately, penetration characteristics, a better tests and customer experience revealed although high-sulfur contents result in understanding of the interaction be­ that, although on average EF steel exhib­ good penetration characteristics, they tween the surface active elements oxy­ ited slightly better penetration than AOD have an adverse effect on the hot work­ gen and sulfur, and the deoxidants alumi­ steel, there were wide variations in pene­ ability, cold formability and pitting corro­ num, silicon and manganese was tration for steel produced by either pro­ sion resistance of stainless steel. There­ required. Optimum manganese and sili­ cess. Low penetration was found to be fore, the present practice at LTV Steel con contents were suggested by Oyler, associated with the presence of strong Specialty Products Co.1 for common et al. (Ref. 1), but no data were provided deoxidants, specifically aluminum, mag­ grades of stainless steels, such as Types to support their recommendations. A nesium, cerium and calcium (Refs. 5, 6). If 304 and 316 and their low-carbon equiv­ systematic study was, therefore, made of oxygen increases penetration, then it is to alents, is to produce both a regular quali­ the effects of aluminum, sulfur, silicon be expected that strong deoxidants will ty grade, which may be low-sulfur if and manganese on the welding charac­ reduce penetration. Of the foremen- desired, and a tubing quality grade in teristics of Type 304 stainless steel using tioned deoxidants, only aluminum has which maximum aluminum and minimum laboratory heats in which one element at been widely used in stainless steel. It is sulfur contents are specified so as to a time was varied. The effects of phos­ sometimes added to the desulfurization ensure good welding characteristics. phorus, molybdenum and copper addi­ mix to facilitate the removal of sulfur by Recently, questions about the effect of tions in amounts typical of those found in the reaction composition on the welding characteris­ production heats were also investigated. 3 (CaO)s 4- 2 [AI]M + 3 [5]*,— tics of stainless steel have again been The results of this study were then com­ 3 (CaS)s + (Al203)s (1) raised. The reasons for this resurgence in pared with penetration data for produc­ Low penetration was found to be interest are primarily economic. First, a tion heats of Types 304 and 316 stainless associated with high-aluminum residuals, significant reduction in inventory cost steel. i.e., aluminum contents up to 0.030%. To could be achieved by combining regular prevent high-aluminum levels in steel for and tubing grades; and second, an tubing applications, aluminum additions improvement in the hot workability of Experimental Procedure were prohibited and penetration was difficult-to-roll grades such as Type 316, Material generally found to be satisfactory when which would reduce surface defects and the aluminum content did not exceed increase yields, could be obtained by tion problems were still e xperienced with 7. Now l&L Specialty Products Co ingots were made with the aluminum, Table 1--Compositions and Penetration Ratios for Laboratory Heats 'ercent Heat . No. C Mn Si S P Cr Ni Mo Cu Al N O, ppm D/W V154 0.040 0.050 0.50 0.014 0.016 18.79 8.50 0.01 0.05 0.001 0.024 194 0.472 1329 0.034 0.12 0.47 0.009 0.012 18.92 8.39 0.01 0.04 0.005 0.018 178 0.493 1330 0.044
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