development of a new weld overlay for pot hardware in ... - Pyrotek

DEVELOPMENT OF A NEW WELD OVERLAY FOR POT HARDWARE IN
CONTINUOUS GALVANIZING LINES
Xingbo Liu 1 , Vinod K. Sikka 2 , Mark Bright 3 , Scott Sexton 4 , Ever Barbero 1 , James W. Hales 5
1. Mechanical & Aerospace Engineering Department, West Virginia University,
Morgantown, WV 26506, USA
2. Materials Science and Technology Division, Oak Ridge National Laboratory
Oak Ridge, TN 37831, USA
3. Metaullics Systems (Division of Pyrotek Incorporated)
Solon, OH 44139, USA
4. Nucor Steel – Indiana
Crawfordsville, IN 37933, USA
5. Specialty Welding & Machining, Inc.
Harrison, TN 37341, USA
ABSTRACT
Molten-metal corrosion and dross buildup on the rolls are the primary reasons for the shutdown of continuous
galvanizing lines. Recently, a new weld overlay of iron-based super alloy on the 316L stainless steel substrate has been
developed to reduce molten-metal corrosion and dross buildup on the stabilizer and sink rolls. Both single- and multiplelayer
welds have been developed. Both laboratory-scale static and dynamic corrosion tests have been carried out. The
laboratory-scale results show that this newly developed overlay has strong resistance to dross buildup; there is almost no
buildup on the surface after a 15-day test. The line-scan and X-ray mapping micrographs show that there is a
reaction/dissolution layer next to the overlay. Cobalt and iron are dissolved into the bath and aluminum from the bath
diffuses into this layer. In comparison, 316L forms a continuous layer of dross after a 6-day test. The laboratory-scale
results have been validated through the exposure of the weld-overlay material specimens for over 150 days in an industrial
setting. The industrial tests show that the new overlay has at least 3 times - better resistance to dross buildup than 316L.
Keywords: galvanizing, MSA 2020 overlay, dross buildup
1 INTRODUCTION
The coating of sheet steel by continuous hot dipping in a molten metal bath of zinc or in a Zn/Al melt is the most
efficient and economical method of providing corrosion protection to most steel sheet compositions. However, the
performance of galvanizing and Zn/Al molten metal bath hardware can strongly influence both the downtimes experienced
by a line and coating quality. Typical coating lines operate for an average of two weeks prior to required downtime for
maintenance of hardware.
Corrosion by molten Zn/Al alloys and dross buildup on the sink roll and stabilizer rolls are the most important reasons
to cause the downtime of the galvanizing lines. Thus then, corrosion and dross buildup resistance become the primary
criteria for the selection and development of pot hardware materials. Corrosion behaviors of the pot hardware materials and
coating have been extensively studied [1-5] and indeed, many investigators show contrasting liquid zinc immersion results
for the same alloys [3-5]. 316L stainless steel is the most popular material for stabilizer and sink rolls in the industry, but in
recent years, WC-Co thermal spray coating has become more and more popular as the roll coating to resist corrosion and
dross buildup.
Since 2004, a fundamental approach on the improvement of molten metal handling materials and designs, aimed at
creating new classes of molten metal handling materials, has been underway in a cooperative program co-funded by the US
Department of Energy, in cooperating with 24 industrial partners include US steel, aluminum, metal casting, and refractory
partners, as well as hardware suppliers and research organizations. During this project, extensive efforts have been devoted
to (1) investigate fundamental mechanisms of molten metal corrosion and dross buildup in hot-dipping industries, and (2)
develop new materials and coatings with significant improvement on corrosion and dross buildup performance over the
current materials, and thus extend the service life of pot hardware.
In this paper we described our work on a comparative effort to develop a weld overlay of MSA 2020 on the 316L
substrate, in the aim of improving dross buildup resistance of sink roll and stabilizer rolls in hot-dipping process.
1

2 EXPERIMENTAL
2.1 Materials Development
316L stainless steel and MSA 2020, an iron-based superalloy, which contains a continuous microscopic network of
interpenetrating microscopic intermetallic phases and solid solution metallic phases, was employed in this research. The
presence of metallic phases in MSA 2020 provides significant improvement in toughness and damage tolerance, while the
intermetallic phases would lead to high hardness and improved performance at elevated temperatures. However, to employ
this alloy in the applications of sink and stabilizer rolls, a special metal inert gas (MIG) welding process was developed to
apply this alloy as a coating on 316L substrate.
2.2 Corrosion and Dross Buildup Testing
There were totally three kinds of tests carried out during this research: lab-scale static corrosion test, lab-scale dynamic
dross buildup test, and corrosion/dross buildup test in industrial galvanizing baths.
Static corrosion/dross buildup tests were conducted in a lab-scale zinc bath containing aluminum with the percentage
ranging from 0.19% to 0.22% (GI) at temperatures of around 460 ± 5ºC. The GI alloy ingots were provided by Wheeling
Nisshin Co. at Follansbee, WV. The chemical compositions of the ingots are: 99.98Zn, 0.011Si, 0.003 Al, 0.002Fe, 0.002Pb.
Dynamic dross buildup tests were conducted by rotating samples, 316L and MSA 2020 overlay, in a lab-scale
galvanizing bath with 80-lbs molten Zn-Al alloy, for up to 15 days. Similar to the static tests, Al contents were around
0.20% and temperature was set at 460ºC. The alloy ingots were saturated with Fe.
Industrial tests were conducted by dipping the samples in four galvanizing lines for up to 150 days.
After each test, cross-sectional metallographic samples were prepared following a standard procedure. The identification
of some reaction products was conducted using a JOEL 8200 Electron Probe Micro Analyzer (EPMA). Details of the
reaction products in the samples were examined using a HITACHI 4700 Scanning Electron Microscope (SEM) equipped
with an EDS unit. The depths of reaction layers were then measured using image analysis software (Image-Pro Plus 4.0).
3 RESULTS and DISCUSSIONS
3.1 MSA 2020 Overlay
In order to carry out the weld overlays of 2020, its wire was produced by Stoody Co. by the powder core approach.
This is the first time such a filler wire was ever produced. Multiple pass welds were prepared using this wire on type 316L
plate. Two weld samples were prepared, each using a metal inert gas (MIG) process. The detailed microstructural analysis
of one of the multiple pass welds of 2020 on type 316L is shown in Figure2. Prior to making multiple pass welds, a single
pass weld was also prepared. The microhardness data for the single and multiple pass welds were measured and it can be
found from hardness measurement that there are four regions in the overlay: weld top (MSA 2020), intermix region,
interface, and substrate (type 316L). In the multiple pass welds, surface hardness was maximum and reached average
values of ~ 575 Vicker’s. In the single pass weld where the weld surface is diluted, its hardness was ~ 480 Vicker’s.
3.2 Corrosion and Dross Buildup Resistance
Static Corrosion
Figure 2: Top of weld overlay specimen.
2

Figure 3 shows the lab-scale static corrosion of MSA 2020 in galvanizing bath, as compared to 316L stain steel. It is
indicated in the figure that the average thickness lost of the overlay is 2mm after submerged in the bath for 895 hours, while
the stainless steel lost 8mm during the same testing period.
Corrsion (in)
0.035
0.03
0.025
0.02
0.015
0.01
0.005
0
143h
Steel
895h
143h
Alloy 2020 Bead #
Sample 1
143h
Alloy 2020 Bead # 2
Figure 3: Lab-scale static corrosion of 316L and MSA 2020 overlay.
Dynamic Dross Buildup
Figure 5 shows the SEM micrographs of 316L stainless steel samples after lab-scale dynamic test in Zn-0.22%Al bath
for 2 days and 6 days. It is indicated in figure 5(a) that after 2 days dynamic test, there is a continuous internal reaction layer
with the thickness around 5 micron, which provides the favorable position for the buildup of big dross particles from the
bath, but the dross buildup layer is semi-continuous. By comparison, after 6 days of dynamic rotating test, there is a dense
continuous dross buildup layer on the surface of the steel. In addition, by comparing dynamic results with static ones, it can
be concluded that (1) the reaction layer after dynamic test is much thicker than that after static test; (2) the size of dross
particles is bigger than that after static test; and (3) the agglomeration of dross particles is faster than that after static test.
(a) 2 days (b) 6 days
Figure 5 Dynamic dross buildup tests on 316L stainless in GI bath at 465ºC (rotating rate 60 rpm).
Figure 6 shows the line-scan of MSA 2020 overlay after lab-scale dynamic test for 15 days. It is indicated that this
newly developed overlay has strong resistance to the dross buildup and there is very few Fe-Al(Zn) dross particles attached
to the surface. Figure 7 shows the X-day maps of the reaction layer on the top of the overlay. The line-scan (Figure 6) and
X-ray mapping (Figure 7) micrographs show that there is a reaction/dissolution layer next to the overlay, within which Co
and Fe dissolved into the bath and Al from bath diffuses into this layer, within which cobalt and iron are dissolved into the
bath and aluminum from the bath diffuses into this layer.
895h
895h
3

Wt.%
80
70
60
50
40
30
20
10
0
0 10 20 30 40 50 60 70
Microns
Figure 6: Line-scan of MSA 2020 overlay after 15-day dynamic test.
(a) BSI (b) Co
(c) Al (d) Fe
Figure 7: BSI image and X-ray mapping of Co, Al & Fe of the reaction area.
3.3 Testing Results in Industrial Baths
Figure 8 shows the 316L stainless steel and MSA 2020 overlay after dipped in Nucor-Crawfordsville for 81 days. A
detailed microstructural analysis of the sections taken from 316L and 2020 weld overlay was carried out, which included
optical microstructure, microhardness, and microprobe analysis. From this figure we made the following observations:
a) 316L surface had a thick dross deposit and a narrow reacted layer underneath. The interface between the dross and
316L surface was rough, indicating a dissolution type of mechanism occurring at the surface.
b) For 2020 weld overlay, there was a very thin layer of dross and a larger reaction layer underneath. Furthermore,
the interface underneath the dross was smooth, indicating the absence of any dissolution type of mechanism at the interface.
O
Ni
Al
Si
W
Zn
Mo
Fe
Cr
4

Both 33- and 81-day exposed samples were also subjected to microprobe analysis. The results of the microprobe scans
after 81 days are compared for 316L and 2020 weld overlay. The elemental scans show the variation of different alloying
elements in the reaction layer. A comparison of the two scans shows that the reaction layers for both 316L and 2020 consist
of Al, Fe, Zn, O, and small amounts of other alloying elements. The reaction layer for 316L is richer in Al and Fe as
opposed to 2020 weld overlay. The reaction layer for 2020 contained less amounts of Al and Fe but contained Co, Cr, and
Si. It is not clear at this time how the reaction layer composition in 2020 weld overlay, which is nearly the same after 81
days, retards the dross build-up as opposed to 316L. It is suggested that there are initial differences in reaction layer
composition between 316L and 316L/2020. However, after 81 days, reaction layer for 316L and 316L/2020 consists
essentially of the same composition as Al5Fe2Zn1. Future work on wettability studies and other in-plant tests may help to
understand the noted differences more clearly.
Dross
444.5 µm
Dross
Reaction Layer
316L 316L + 2020
Reaction Layer
Figure 8: 316L and 2020 weld overlay tubes after 81 days of GI-bath exposure at Nucor Steel-Crawfordsville plant.
Table I: Dross and Reaction Data on All In-plant Trial Samples of 316L and 316L/2020
Microstructural analysis of specimens exposed for 30, 90, 150, and 174 days was carried out. The dross and reaction
data for all of the specimens analyzed to date are summarized in Table I. These data show that the reaction depth can be
given by:
Reaction depth (R) = Ae Bt , (1)
where A and B = constants,
t = bath exposure time in days.
The values of constant B for 316L and 316L/2020 were 0.012 and 0.016, respectively, and the values of constant A for
316L and 316L/2020 were 18.2 and 24.7, respectively. This suggests that at time zero or initial contact with molten zinc, the
63.5 µm
Dross
5

2020 surface reacted quickly and then it grew at a rate very similar to that of 316L. The rapid formation of the initial
reaction may be responsible for less dross formation on 316L/2020.
A general equation for dross, D, can be given as:
D = A′e -0.005t , (2)
where A′ = the constant and its values are 387 and 185, respectively, for 316L and 316L/2020.
t = time in days.
Equations (1) and (2) and the respective constants were used to calculate the dross plus reaction build-up on 316L and
316L/2020. It can be found that if 7 mil of the weld-overlay surface is removed every 60 days, the 316L/2020 will continue
to outperform 316L. Furthermore, at a removal rate of 7 mil/60 days, a 3.5-mm (140-mil) –thick weld overlay will last over
three years with significantly improved performance over 316L
5 CONCLUSIONS
In this research, a new weld overlay of MSA 2020, an iron-based super alloy, on the 316L stainless steel substrate has
been developed to reduce molten-metal corrosion and dross buildup on the stabilizer and sink rolls. Both single- and multilayer
welds have been developed. Both laboratory-scale static and dynamic corrosion tests have been carried out. The
following conclusions can be drawn:
(1) This newly developed overlay has strong resistance to dross buildup; there is almost no buildup on the surface
after a 15-day test. In comparison, 316L forms a continuous layer of dross after a 6-day test.
(2) The line-scan and X-ray mapping micrographs show that there is a reaction/dissolution layer next to the
overlay. Cobalt and iron are dissolved into the bath and aluminum from the bath diffuses into this layer.
(3) Industrial tests in various galvanizing lines show that this overlay is a candidate for next generation coating on
sink and stabilizer rolls in hot-dipping processes.
ACKNOWLEDGEMENTS
This research was sponsored by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy,
Industrial Technologies Program, under contract DE-FC36-04GO14038. The authors would like to thank Dr. Frank
Goodwin from International Lead Zinc Research Organization for his technical contribution and continuous encouragement.
Thanks are due to Frank Mollica of Wheeling Nisshin Inc. who provided galvanizing ingots. Ms. Jing Xu conducted labscale
static and dynamic tests of 316L and MSA 2020 overlay, and the subsequencial microstructural analysis.
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