Alloy and composite steel tube with corrosion resistance in combustion environment where V, Na, S and Cl are present

An alloy exhibiting corrosion resistance in a combustion environment where V, Na, S and Cl are present comprises, in weight percent, not more than 0.05% C, 0.02-0.5% Si, 0.02-0.5% Mn, 15-35% Cr, 0.5-4% Mo, more than 40% but not more than 60% Co, 5-15% Fe, 0.5-5% W, 0.0003-0.005% Ca and the remainder of Ni at a content of not less than 4% and unavoidable impurities, provided that Cr (%)+0.5Ni (%)+3Mo (%).gtoreq.30 (%). A composite steel tube exhibiting corrosion resistance in a combustion environment where V, Na, S and Cl are present comprises an inner tube constituted of Cr-containing boiler tube and an outer tube constituted of the alloy.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a steel alloy for a tube for use in a combustion 
atmosphere where V, Na, S and Cl are present, e.g. in the combustion 
atmosphere of a boiler burning heavy oil, tar, coal or the like or of a 
garbage incinerator, more particularly to a composite steel tube 
exhibiting hot corrosion resistance and hot erosion resistance in the 
presence of V.sub.2 O.sub.5, Na.sub.2 SO.sub.4 and NaCl formed in such an 
environment. 
2. Description of the Prior Art 
It is well known that the oxides V.sub.2 O.sub.5 and Na.sub.2 SO.sub.4 
which form when crude oil, heavy oil or the like is burned in a boiler or 
other such combustion system build up oxide scale on the systems where 
they form low-melting-point compounds which induce a type of local 
corrosive oxidation known as "vanadium attack". The literature in this 
field points out that alloys of Cr, Ni, Co etc. exhibit a certain amount 
of resistance to these types of corrosion (see Iron and Steel, Vol. 67, 
Page 996, for example). 
A number of processes have been proposed for production of composite steel 
tube. In one of these an alloy cladding material is provisionally attached 
to a carbon or low-alloy steel, the result is subjected to hot rolling to 
obtain a clad steel plate and the clad steel plate is formed into 
composite steel tube by submerged arc welding or the like. There have also 
been proposed methods or directly metal cladding a finished product to 
obtain a composite steel tube. For example, Japanese Patent Public 
Disclosure Sho 61-223106 discloses a method of directly producing a final 
composite steel tube by fixing a high-alloy powder to a metal material by 
hot isostatic pressing. 
In thermal power plants and the like which use the energy released by 
burning a fossil fuel or garbage, when the fuel is tar, coal, heavy oil or 
garbage containing plastic materials, the combustion products of the fuel 
often contain large amounts of V, Na, S and Cl. These elements in turn 
cause low-melting-point compounds containing V.sub.2 O.sub.5, Na.sub.2 
SO.sub.4, NaCl and the like to form on the surfaces of the furnace wall 
tubes, steam superheating tubes etc. of the thermal power or incineration 
facility. As a result, the scale formed on the tube surfaces melts and 
causes hot corrosion. Over long periods of operation this causes breakage 
of the furnace wall tubes, steam superheating tubes etc. 
Moreover, in the case of a dedicated coal-fired boiler or a garbage 
incinerator/power plant of the fluidized bed furnace type, accelerated hot 
corrosion of the furnace wall tubes, the steam superheating tubes etc. is 
caused by the hot erosive effect of the ash or bed sand. 
SUMMARY OF THE INVENTION 
The object of this invention is to provide, at low cost, an alloy which 
exhibits high corrosion resistance in a high-temperature combustion 
environment where V.sub.2 O.sub.5, Na.sub.2 SO.sub.4, NaCl and the like 
are present, an alloy which exhibits steam oxidation resistance in such an 
environment, and composite steel tubes externally clad with these alloys. 
Research conducted by the inventors showed that the corrosion resistance of 
an alloy in an environment in which V.sub.2 O.sub.5, Na.sub.2 SO.sub.4, 
NaCl and the like tend to find their way into an oxide scale formed on the 
alloy surface is dependent not only on the alloy's Cr content but also on 
its combination of Ni, Co, Fe and Mo contents. 
The corrosion resistance of an alloy in a high-temperature oxidative 
environment involving a large amount of O.sub.2 generally increases with 
increasing Cr content. In power plants that burn crude oil, heavy oil, 
tar, coal or the like and in garbage incinerator/power plants, however, 
the O.sub.2 content of the combustion atmosphere is reduced in order to 
decrease the amount of NOx generated. The inventors found that high Cr 
content alloys do not necessarily exhibit corrosion resistance in such an 
environment. 
In an environment in which V.sub.2 O.sub.5, Na.sub.2 SO.sub.4, NaCl and the 
like form in the scale that deposits on the alloy surface, 
low-melting-point compounds (e.g. eutectic compounds such as Na.sub.2 
O.V.sub.2 O.sub.5) form in the scale. 
As a result, the protective scale covering the alloy surface melts away 
locally, enabling corrosion to proceed at an abnormally high rate. 
Moreover, as was mentioned earlier, the corrosion is further accelerated 
by the hot corrosion effect of bed sand, coal ash and the like. 
Through their research, the inventors discovered that the local melting of 
the scale starts when the alloy scale (e.g. Fe.sub.2 O.sub.3) dissolves 
into the molten low-melting-point compounds such as Na.sub.2 O.V.sub.2 
O.sub.54 that form in the scale. From this it was concluded that the 
corrosion-resistance of an alloy used in such an environment can be 
effectively enhanced by ensuring formation of a scale of a composition 
which does not easily dissolve into the aforesaid molten low-melting-point 
compounds. In other words, they concluded that it is necessary to 
establish an alloy composition which forms a scale having such a 
composition. 
The invention defines such alloy compositions for use in steel tube to be 
used in a the 400.degree.-700.degree. C. range. This is a temperature 
range in which various precipitates form. Since the carbides among these 
precipitates act to form low-melting-point compounds, they work to 
increase the amount of corrosion. This is particularly true when they are 
precipitated continuously (at the grain boundaries, for instance). The C 
content of the tube cladding therefore has to be kept low. 
For imparting hot erosion resistance, it is necessary to strengthen the 
matrix and suppress the formation of large precipitates. Growth of 
precipitated intermetallic compounds and carbo-nitrides, either at the 
grain boundaries or inside the grains, degrades the hot erosion resistance 
property. This make it necessary to reduce the content of metals which 
form or promote the formation of these precipitates. What is required is 
an alloy design which, like that to be set out later, is able to satisfy 
both the need for suppressing precipitates and the need for 
high-temperature strength. 
The combustion environment discussed above also includes NaCl. Therefore, 
when the temperature of the environment falls to the new point after the 
facility has been temporarily shut down for maintenance or the like, the 
water vapor of the environment condenses and dissolves NaCl contained in 
the environment, thereby forming a high-concentration NaCl solution 
environment. Invasion of the condensed water into cracks in the scale 
leads to crevice corrosion at the bottom surface of the scale. Since tube 
breakage is just as likely to occur under this condition as duriing 
high-temperature operation, this problem also has to be coped with. 
Since the inner surface of the composite steel tubes according to the 
invention are ordinarily exposed to a steam environment, the inner tube 
material of the composite steel tube is required to possess oxidation 
resistance in a steam environment. 
The tubes ordinarily used for the superheaters etc. of thermal power plants 
are STBA20, STBA26 and other alloy tubes containing 0.5-10% Cr as 
prescribed by JIS G3462, and austenitic steel tubes, such as SUS304TB, 
321TB, 316TB and 374TB, which contain 18% Cr and 9-14% Ni as prescribed by 
JIS G3463. The present invention also uses the alloys prescribed for these 
boiler tubes as the inner tube material. 
In view of the foregoing findings, the present invention achieves its 
object by providing in accordance with a first aspect 
an alloy exhibiting corrosion resistance in a combustion environment where 
V, Na, S and Cl are present comprising, in weight percent, not more than 
0.05% C, 0.02-0.5% Si, 0.02-0.5% Mn, 15-35% Cr, 0.5-4% Mo, more than 40% 
but not more than 60% Co, 5-15% Fe, 0.5-5% W, 0.0003-0.005% Ca and the 
remainder of Ni at a content of not less than 4% and unavoidable 
impurities, provided that Cr (%)+0.5 Ni (%)+3 Mo (%).gtoreq.30 (%), 
in accordance with a second aspect 
a composite steel tube exhibiting corrosion resistance in a combustion 
environment where V, Na, S and Cl are present comprising an inner tube 
constituted of Cr-containing boiler tube and an outer tube constituted of 
the alloy according to the first aspect.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIGS. 1, 2 and 3 indicate the results of corrosion tests conducted in an 
environment established to simulate the environment in which the outer 
tube of the composite steel tube according to the invention is intended 
for use. 
The first tests were carried out by holding a specimen provided on its 
surface with a 1 mm thick simulated low-melting-point scale consisting of 
30% V.sub.2 O.sub.5 +30% Na.sub.2 SO.sub.4 +20% NaCl+20% Fe.sub.2 O.sub.3 
(environment A) or 30% NaCl+20% Na.sub.2 SO.sub.4 +50% FeCl.sub.2 
(environment B) at 700.degree. C. in atmospheric air for 24 hrs. These 
tests made it possible to evaluate resistance to abnormal corrosion caused 
by low-melting-point scale. 
The second test was carried out by immersing overlaid specimens in an 
80.degree. C. solution of 20% NaCl+0.1% FeCl.sub.3 exposed to the 
atmosphere for 200 hrs (environment C). This test made it possible to 
evaluate resistance against crevice corrosion caused by condensed water at 
the bottom surface of the scale. 
From FIG. 1 showing the effect of Cr content on corrosion depth in 
environment A of the test, it can be concluded that the optimum Cr content 
range is 15-35%. 
From FIG. 2 showing the effect of Co content on corrosion depth in 
environment B of the test, it can be concluded that the Co content has to 
be greater that 40% for securing adequate corrosion resistance. 
From FIG. 3 showing the effect of Cr, Ni and Mo content on crevice 
corrosion in environment C, it can be concluded the Cr+0.5 Ni+3 Mo has to 
be not less than 30% for securing adequate crevice corrosion resistance. 
The reason for the limits placed on the constituents of the corrosion 
resistant alloy used for the outer tube will now be explained. 
C: Carbides act as starting points for abnormal corrosion induced by 
low-melting-point scale. It is particularly important to suppress 
continuous precipitation of carbides at the grain boundaries. The C 
content is therefore reduced during production to not more than 0.05%. 
Si: Si is frequently added to alloys for increasing oxidation resistance. 
However, since Si promotes the activity of C in an alloy, which increases 
the amount of carbides precipitated, its content has to be held to a low 
level in this invention. However, some Si must be added to act as a 
deoxidizer at the time of alloy production. As the deoxidizer effect of Si 
is inadequate at contents below 0.02% and saturates at above 0.5%, the 
invention defines the Si content as not less than 0.02% and not more than 
0.5%. 
Mn: Like Si, Mn also has to be added to serve as a deoxidizer during alloy 
production. Since the deoxidizer effect of Mn is too low at contents below 
0.02% and saturates at above 0.5%, the invention defines the Mn content as 
not less than 0.02% and not more than 0.5%. 
Cr: Cr is one of the main elements contributing to the formation of a 
corrosion resistant oxide film that suppresses abnormal corrosion induced 
by the formation of low-melting-point scale. However, Cr is also both a 
ferrite forming element which forms delta-ferrite during alloy production, 
and a strong carbide forming element. As delta-ferrite and carbides are 
causes of abnormal corrosion, excessive addition of Cr tends to degrade 
corrosion resistance rather than improve it. As shown by FIG. 1, the 
optimum Cr content range is 15-35%. On the other hand, Cr is an effective 
element for enhancing resistance against crevice corrosion caused by 
condensed water. As shown by FIG. 3, for ensuring crevice corrosion 
resistance it is necessary to establish a relationship between Cr, Ni and 
Mo such that Cr+0.5 Ni+3 Mo.gtoreq.30. 
Ni: Like Cr and Co, Ni is also one of the main elements contributing to the 
formation of a corrosion resistant oxide film. In this invention Ni is 
added to work together with Co to maintain an austenitic structure. As 
shown by FIG. 3, Ni cooperates with Cr and Mo for ensuring resistance 
against crevice corrosion caused by condensed water. For this purpose it 
has to be present at not less than 4% and in such relationship with Cr and 
Mo that Cr+0.5 Ni+3 Mo.gtoreq.30. 
Mo: As shown by FIG. 3, for ensuring resistance against crevice corrosion 
caused by condensed water, Mo is added together with Cr and Ni so as to 
establish the relationship Cr+0.5 Ni+3Mo.gtoreq.30. However, since 
excessive addition of Mo leads to precipitation of intermetallic compounds 
and thus degrades resistance to abnormal corrosion induced by 
low-melting-point scale, the maximum Mo content is set at 4%. Together 
with Co and W, Mo is also an effective constituent for ensuring hot 
erosion resistance. Since no corrosion resistance or hot erosion 
resistance effect is obtained at a content of less than 0.5%, the lower Mo 
content limit is set at 0.5%. 
Co: Co constitutes an effective element for ensuring corrosion resistance 
and hot erosion resistance against abnormal corrosion induced by 
low-melting-point scale. As shown by FIG. 2, a content in excess of 40% is 
required for achieving adequate corrosion resistance. Since its corrosion 
resistance promoting effect saturates at content levels exceeding 60%, 
however, the optimum range of Co content is defined as more than 40% but 
not more than 60%. 
Fe: While Fe itself does not produce resistance against abnormal corrosion 
induced by low-melting-point scale, it does promote the formation of a 
stable, spinel-type corrosion resistant oxide film. Since it degrades 
corrosion resistance when added to excess, however, its content is defined 
as within the range of 5-15%. 
W: W is added for imparting hot erosion resistance to the alloy according 
to the invention. When present at more than 5% it causes precipitation of 
intermetallic compounds and thus degrades resistance to abnormal corrosion 
induced by low-melting-point scale. When present at less than 0.5% it does 
not enhance resistance to hot erosion. The range of W content is therefore 
defined as 0.5%-5%. 
Ca: As will be explained later, the composite steel tube according to the 
invention can be produced from a composite billet obtained by 
pressure-binding powder of the outer tube alloy according to the invention 
to a billet for the inner tube by hot isostatic pressing. In such cases 
the alloy according to the invention is reduced to a powder after it is 
produced. If Al and/or Ti are used as deoxidizer during alloy production, 
Al oxides, Ti nitrides and the like will precipitate at the molten metal 
ejection nozzle during powder production. Since this would hinder powder 
production, the invention uses Ca together with Si as deoxidizer during 
alloy production. However, since use of a large amount of Ca leads to 
cause formation of Ca sulfides and Ca oxides which degrade resistance 
against crevice corrosion caused by condensed water, the maximum Ca 
content is defined as 0.005%. Since no deoxidizing effect is obtained at 
below 0.0003%, the Ca content range is defined as 0.0003-0.005%. 
The method of producing a composite steel tube according to the invention 
will now be explained. 
A powder of the outer tube alloy according to the invention is attached by 
hot isostatic pressing (HIP) to the surface of an inner tube stainless 
steel billet produced by the ordinary steelmaking and casting processes 
for stainless steel. After being soaked, the resulting composite steel 
tube billet is formed to a prescribed size by hot extrusion. 
Where a plate or tube material is used for the outer tube, alloy powder is 
not attached to the inner tube stainless steel billet by HIP but instead 
the plate or tube having the composition of the outer material is wound 
over or embedded in the surface of the billet and the outer tube material 
and the inner tube billet are welded together. The resulting composite 
steel tube billet is then used in the above-described manner for producing 
a composite steel tube. 
The method of producing the composite steel tube according to the invention 
is not limited to the foregoing and it is alternatively possible to use 
various other methods, including the prior art composite (laminated) steel 
tube production method. 
The present invention also encompasses the case where the alloy according 
to the invention is laminated to obtain tubes or similarly shaped members 
suitable for high-temperature applications (e.g. nozzles for blowing air 
or fuel) by LPPS (low pressure plasma spray) or other such flame spraying 
method. 
EXAMPLE 
Examples of the invention are shown in Table 1 (which show the chemical 
compositions of alloys produced according to the invention and comparative 
examples) and Table 2 (which show the results of tests carried out on the 
invention and comparative example alloys). 
The critical corrosion depth of the alloy according to the invention is 
0.05 mm. 
TABLE 1 
__________________________________________________________________________ 
Chemical composition (wt %) 
No. C Cr Co Fe Mo W Si Mn Ni Ca 
__________________________________________________________________________ 
1 Invention 
0.025 
21 50 7 3.2 
1.2 
0.15 
0.16 
Bal 
0.003 
2 " 0.029 
30 43 6 1.1 
1.8 
0.18 
0.15 
Bal 
0.003 
3 " 0.027 
19 60 5 3.6 
0.7 
0.11 
0.17 
Bal 
0.002 
4 " 0.018 
25 58 8 2.1 
4.3 
0.09 
0.13 
Bal 
0.004 
5 Comparison 
1.381 
20 50 2 0.1 
3.5 
2.1 
0.52 
Bal 
-- 
6 " 0.043 
12 48 25 3.4 
1.3 
1.2 
0.42 
Bal 
-- 
7 " 0.031 
24 60 3 0.3 
1.8 
1.6 
0.48 
Bal 
-- 
8 " 0.035 
30 5 16 1.0 
1.5 
0.52 
0.46 
Bal 
-- 
__________________________________________________________________________ 
TABLE 2 
______________________________________ 
Amount of corrosion 
No. A (mm) B (mm) C (mm/year) 
______________________________________ 
1 Invention 0.042 0.043 0.025 
2 " 0.040 0.038 0.046 
3 " 0.047 0.043 0.043 
4 " 0.030 0.028 0.040 
5 Comparison 0.058 0.061 0.082 
6 " 0.076 0.068 0.120 
7 " 0.041 0.045 0.097 
8 " 0.060 0.098 0.032 
______________________________________ 
A: Simulated scale composition (30% V.sub.2 O.sub.5 + 30% Na.sub.2 
SO.sub.4 + 20% NaCl + 20% Fe.sub.2 O.sub.3) 
B: Simulated scale composition (30% NaCl + 20% Na.sub.2 SO.sub.4 + 50% 
FeCl.sub.2) 
C: Test liquid (80.degree. C. solution of 20% NaCl + 0.1% FeCl.sub.3 
exposed to the atmosphere) 
As will be understood from the foregoing description, the composite steel 
tube provided by the invention consists of an inner tube having corrosion 
resistance against steam oxidation and an outer tube consisting of an 
alloy having superior corrosion resistance against combustion environments 
produced by burning of fuels containing V, Na, S and Cl and against 
garbage and industrial waste incineration environments. The invention is 
therefore able to provide furnace wall tubes, steam superheating tubes and 
the like exhibiting high corrosion resistance when used in such 
environments.