Solidification Behavior and Weldability of Dissimilar Welds Between a Cr-Free, NiCu Welding Consumable and Type 304L Austenitic Stainless Steel Jeffrey W. Sowards, Dong Liang, Boian T. Alexandrov, Gerald S. Frankel & John C. Lippold Metallurgical and Materials Transactions A ISSN 1073-5623 Volume 43 Number 4 Metall and Mat Trans A (2012) 43:1209-1222 DOI 10.1007/s11661-011-0961-z
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Author's personal copy Solidification Behavior and Weldability of Dissimilar Welds Between a Cr-Free, Ni-Cu Welding Consumable and Type 304L Austenitic Stainless Steel JEFFREY W. SOWARDS, DONG LIANG, BOIAN T. ALEXANDROV, GERALD S. FRANKEL, and JOHN C. LIPPOLD The solidification behavior of a Cr-free welding consumable based on the Ni-Cu system was evaluated in conjunction with Type 304L stainless steel. The weld metal microstructure evolution was evaluated with optical and secondary electron microscopy, energy dispersive spectroscopy, X-ray diffraction, button melting, and thermodynamic (CALPHAD-based) modeling. Solidification partitioning patterns showed that higher dilutions of the filler metal by Type 304L increased segregation of Ti, Cu, and Si to interdendritic regions. Button melting experiments showed a widening of the solidification temperature range with increasing dilution because of the expansion of the austenite solidification range and formation of Ti(C,N) via a eutectic reaction. The model predictions showed good correlation with button melting experiments and were used to evaluate the nature of the Ti(C,N) precipitation reaction. Solidification cracking susceptibility of the weld metal was shown to increase with dilution of 304L stainless steel based on testing conducted with the cast pin tear test. The increase in cracking susceptibility is associated with expansion of the solidification temperature range and the presence of eutectic liquid at the end of solidification that wets solidification grain boundaries. DOI: 10.1007/s11661-011-0961-z The Minerals, Metals & Materials Society and ASM International 2011
I.
INTRODUCTION
GOVERNMENT regulations in the United States and elsewhere are decreasing the allowable exposure levels of hexavalent chromium (Cr VI) to weldingrelated personnel. Achieving these reduced exposure levels may not be practical from an engineering controls standpoint during stainless steel component fabrication in tightly enclosed locations. Such applications include the inside of ship hulls and pressure vessels. One method of addressing this problem is the implementation of a chromium-free welding consumable that provides equivalent mechanical performance and corrosion characteristics to those exhibited by current stainless steel welding consumables. Consumable development research has shown that a Ni-Cu welding consumable, with noble alloying additions of either Pd or Ru, may be suitable for this purpose.[1–6] The corrosion of dissimilar welds is typically controlled by galvanic interactions. The current study was part of a larger effort investigating alloys JEFFREY W. SOWARDS, formerly Graduate Research Assistant, with the Welding & Joining Metallurgy Group, The Ohio State University, Columbus, OH 43210, is now Metallurgist, with NIST, Boulder, CO 80305. Contact e-mail: jeff
[email protected] DONG LIANG, formerly Graduate Research Assistant, with the Fontana Corrosion Center, The Ohio State University, is now Project Engineer, with DNV, Columbus, OH. BOIAN T. ALEXANDROV, Research Scientist, and JOHN C. LIPPOLD, Professor, are with the Welding & Joining Metallurgy Group, The Ohio State University. GERALD S. FRANKEL, Professor, is with the Fontana Corrosion Center, The Ohio State University. Manuscript submitted March 9, 2010 Article published online October 26, 2011 METALLURGICAL AND MATERIALS TRANSACTIONS A
from the Ni-Cu metallurgical system for use as a welding consumable because they are galvanically compatible with Type 304 stainless steel in seawater. However, when switching from a commonly used welding consumable that solidifies under primary ferrite solidification mode (such as the filler metal E308L) to one that solidifies as austenite, weldability may be affected adversely, particularly with respect to weld solidification cracking.[7] The weld solidification cracking susceptibility of Ni-base alloys is typically higher than Type 304L/308L stainless steel as a result of the fully austenitic solidification mode.[7–9] Previous research has shown that MONEL* consum*MONEL is a registered trademark of the Special Metals Corporation, Huntington, WV.
ables (nominally 70Ni-30Cu; Special Metals Corporation, Huntington, WV) may be used to weld stainless steel without any adverse weldability issues as long as the proper weld practice is followed.[10–13] Based on the reviewed literature regarding dissimilar metal welding of Ni-Cu consumables with stainless steel, composition is the most important factor that must be considered in evaluation of the weldability performance. Dilution of the welding consumable with the base metal can alter the weldability with regard to solidification and liquation cracking, porosity formation, weld integrity, and corrosion resistance. The composition of weld metal is affected by the pickup of fluxing agents or dilution by the base metal.[14] VOLUME 43A, APRIL 2012—1209
Author's personal copy Dilution plays an important role on solidification behavior of dissimilar metal welds between the Ni-Cu system and ferrous alloys such as low alloy and stainless steel. Welds containing a high Cu content (such as in Monel electrodes) seem to have a low tolerance for dilution by Fe, and this element seems to be the most important with regard to weldability of Monel-steel dissimilar weld combinations. Iron can affect the weldability adversely with regard to solidification cracking,[10,15–17] liquation cracking,[12] and fluid flow properties.[18] Also, iron can potentially embrittle[10,18,19] the transition region and bulk weld metal. Al and Ti are necessary additions to many Ni-base consumables to deoxidize the weld metal,[15,17] although they have been shown to increase the pickup of Si,[15] which can exacerbate weld solidification cracking.[15] Si can promote weld solidification cracking at levels greater than 0.7 wt pct[15] and reduce weld ductility at levels greater than 1.5 wt pct.[10] Al and Ti additions also have been found to increase the solidification temperature range of Monel weld metal,[20] which in itself may increase solidification cracking susceptibility. Cr should be limited in the weld metal as it also has some detrimental effect on solidification cracking susceptibility.[10,16,21] Mg[20] and Mn[17] seem to be beneficial alloying additions to the weld metal to minimize porosity and improve welding characteristics, respectively. The referenced studies are generally outside the composition range suggested[5] for a new welding consumable for joining stainless steel, which was Ni-(5-10)Cu. The following work was performed to evaluate the effects of dilution of the new welding consumable (Ni-7.5Cu) by 304L stainless steel on solidification and weldability.
II.
EXPERIMENTAL PROCEDURES
A. Materials A hot rolled, annealed, and pickled type 304L stainless steel plate was acquired in 6.4 mm thickness. Two alloys with target compositions (wt pct) Ni-7.5Cu1Ru-0.5Al-0.5Ti-0.02C and Ni-7.5Cu-1Ru-0.5Al-4Ti0.02C were produced in 11.3-kg ingots by Haynes International (Kokomo, IN) for the production of welding consumable core wire. Higher Ti levels were alloyed in the second heat so that weld pool deoxidation would be adequate during shielded metal arc welding (SMAW). The compositions of base material and both heats of welding wire are provided in Table I. Wire was drawn from the ingots to 1.1-mm diameter wire for welding trials with the GTAW process and to 3.2 mm wire for SMAW. The 3.2-mm wire was provided to Euroweld, Ltd. (Chattanooga, TN) for the extrusion of flux coatings over the wire. Three layers of weld metal were deposited with the SMAW electrodes in the direct current electrode positive configuration on 304L base metal as depicted in Figure 1. This configuration was used to achieve several dilution levels, thus representing compositions achieved in different weld joint configurations. The current was set to 90 A and the weld travel speed was approximately 1210—VOLUME 43A, APRIL 2012
Table I. Composition of Type 304L Base Material, gas tungsten arc welding (GTAW), SMAW Welding Wire Compositions (Values in Weight Percent) Element C Cu Fe Mn Mo Ni Cr Al Ti Ru Si N P S
304L (Heat 9JA7)
GTAW Wire
SMAW Wire
0.02
0.014 8.2 — — — balance — 0.56 0.53 1.36 —
