performance of alloy 353 ma (uns s35315) in · pdf filewith improved hot corrosion...

PERFORMANCE OF ALLOY 353 MA (UNS $35315) IN FOSSIL FUEL FIRED APPLICATIONS

J.D. Wilson, T.J. Carney, J.C Kelly Rolled Alloys, Inc.

125 West Stems Road Temperance, MI 48182-9546

B. Ivarsson AvestaPolarit AB

Avesta Research Centre SE-774 80 Avesta

Sweden

ABSTRACT

The use of micro-alloyed (MA) heat resistant alloys in fossil fired systems has been commonplace for nearly 25 years. These materials, alloyed with nitrogen and rare earth elements such as cerium, have been popular upgrades from stainless steels, such as type 309 and 310. Offering the benefits of higher creep-rapture strength with improved hot corrosion resistance, MA grades have been widely utilized for pulverized coal nozzles, fluidized bed combustion systems, tubeshields, etc. This paper provides corrosion data and case histories on the use of this 35Ni-25Cr-1.3Si-Ce-N alloy in more severe combustion environments.

Keywords: Alloy 353 MA, oxidation, sulfidation, chlorination, UNS $35315, UNS 30815, creep rapture, hot abrasion, tube shields, fluidized bed, erosion.

INTRODUCTION

In order to mn more economically, many plants are faring at higher temperatures to increase efficiency and/or burn lower quality fuels. As a result, there has been a demand for high temperature alloys that offer improved resistance to hot corrosion such as sulfidation, oxidation, carburization, and molten salt attack.

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High temperature stainless steels are routinely utilized for coal nozzles, tubeshields, cyclones, etc. for their good resistance to elevated temperatures and relatively low initial cost. The higher chromium austenitic stainless steels such as 253 MA (UNS $30815), type 309 (UNS $30908), and type 310 (UNS $31008) have been popular in coal-fired power plants. As operating temperatures continue to iacrease, some plants are pushing the limits of these stainless steels. Once temperatures exceed 1800 °F, it becomes necessary to look to higher nickel alloys. Unfommately due to the presence of sulfur in coal-fired boilers, sulfidation attack can be a problem if nickel alloys are utilized.

Micro-alloying is a term used to describe the technique of adding small alloying additions, typically less than 0.2% by weight to a material. These minor additions greatly improve the resistance of the material to high temperature environments. Micro-alloying can improve the protection offered by the chromia scale of lean grades to levels similar to nickel based alloys.

UNS $35315 contains 35% nickel, 25% chromium, 1.3% silicon, 0.16% nitrogen, and 0.05% cerium. High chromium along with the silicon and cerium additions form a very thin tenacious oxide scale that is both highly protective and resistant to spallation during thermal cycling. 35% nickel and the addition of silicon provide good resistance to carburization. The nitrogen addition provides excellent strength in the time to creep- rupture at temperatures above 1000°F. This alloy has been successfully applied to more aggressive areas of power boilers, cement plants, and incinerators, where a combination of corrosives such as molten salt, sulfur, etc. are presem. This paper discusses laboratory test data for UNS 35315 in high temperature environments and details field experience with this material in aggressive fossil fired systems.

ENVIRONMENTAL RESISTANCE

Oxidation

UNS $35315 has a very high resistance to oxidation. This is due to the high chromium and silicon content along with rare earth additions. Cerimn, combined with silicon and chromimn, promotes the development of a thin, tightly adherent oxide scale, which resists detachment during thermal cycling. The oxide layer will also reduce the chromium depletion in the matrix and protect the alloy from further oxidation attack. Another beneficial effect of a tightly adherent oxide layer is good hot erosion resistance.

Laboratory testing of UNS $35315 and several other commercially available heat resistant alloys was performed to compare their resistance to oxidation. Cyclic testing was performed in an electrically heated box furnace. Samples were cycled from the test temperature to room temperature weekly, all scale was collected in ceramic crucibles. The samples were weighed and a weight gain/unit area measurement determined.

Results of these tests are shown in Table 2.


Sulfidation

Sulfur and sulfur compounds are otten present in combustion and process gases. The high chromium content and the addition of silicon to UNS $35315 improve its resistance to sulfur attack, particularly under oxidizing conditions. For nickel-base alloys, increasing the nickel generally increases susceptibility to sulfidation attack. 1 Resistance to sulfidation decreases as the nickel content in an alloy is increased, nickel are generally less resistant to sulfidizing environments. As a result, nickel-containing alloys should not be used in reducing sulfidizing environments. The formation of low-melting-point n icke l -~ -compounds will lead to a rapid deterioration of the alloy. In practice, nickel is necessary to ensure stability of the austenitic structure, which in turn is required for good mechanical properties, and a number of nickel alloys have shown excellent performance in sulfur beating environments. Generally, these successful materials are alloyed with adequate amounts of strong oxide formers.

Since UNS $35315 has a higher content of beneficial oxide formers than most other grades with similar or higher nickel contents, it generally has better resistance against sulfidation attack. Service experience has shown UNS $35315 to be more resistant to sulfidation than other 30% to 40% nickel alloys such as UNS N08330 and UNS N08810. Nonetheless, UNS $35315, may get attacked, mainly if the atmosphere is reducing to such an extent that a protective oxide scale hardly can form. Even though many so-called reducing environments are oxidizing for the strong oxide former UNS $35315, it is still advisable, if possible, to keep the atmosphere clearly oxidizing (a few % of free oxygen) if sulfi~ is present in substantial amounts. This is valid for all nickel- containing grades.

Hot Corrosion

Hot corrosion is a type of attack where oxidation processes are affected by a liquid salt (ash) deposit, which may destroy any formed oxide layer. The attack is rarely harmless, and it otten ranges from aggressive to extremely aggressive depending on the exact conditions. The most common elements involved in this type of attack are sulfur and chlorine, usually in combination with the alkali metals sodium and potassium. Hot corrosion occurs most frequently in processes/industries where low-quality fuels are fired, such as waste incineration plants or when coal with high chlorine and sulfur contents is burnt. Occasionally, it may also occur when very clean fuels like propane are fired (as is shown in the applications section).

The most beneficial alloying element is generally chromkma, since this element is needed to slow down the attack of sulfur compounds, but silicon is considered to be advantageous as well. Contrary to what one first might believe (cf. Sulfidation), it seems that moderately high nickel contents are also beneficial in many cases. This is probably due to the fact that nickel is relatively good at slowing down any attack by chlorine and/or chlorides. However, too high of a nickel content may be detrimental, since low melting-point n i c k e l - ~ compounds might form.

The composition of UNS $35315 indicates that it should have a good resistance against hot corrosion. Furthermore, some experience also confirms that the resistance is good under oxidizing conditions. Such a case is discussed in the applications section. Reference the example of testing and application in a recuperator


section of a slab reheat furnace. 2 Hot corrosion is a very complex phenomenon though, and in the most severe cases, for example when municipal waste is fired, no metal is completely resistant.

Hot Erosion and Abrasion

Hot erosive/abrasive conditions can be found in many different industries and applications. Cyclones of fluidized bed boilers and pulverized coal nozzles are common examples found in power generation.

Erosion/abrasion is a very complex phenomenon, in which not only the properties of the construction material but also those of the eroding/abrasive particles are significant. Generally, it can be concluded that for a good resistance to erosion and abrasion at high temperatures, an adherent, tough, and ductile oxide layer is necessary, with an ability to heal rapidly and repeatedly. These are exactly the properties that give UNS $35315 its good high temperature corrosion resistance.

In laboratory tests, UNS $35315 has shown better results than the more highly alloyed UNS N06625 (Figure 2). The reason for this is probably the improved adherence of its oxide layer thanks to the REM additions, but the nitrogen addition might also contribute by making the material harder. This is also supported by the fact that also UNS $30415 and, particularly, UNS $30815 are frequently, and successfully, used for components in erosive/abrasive industries such as the cement industry, and the power generation industry. This quality is therefore a family feature rather than something specific to UNS $35315 alone. 2

Creep-Rupture Strength

The nitrogen addition increases the yield and ultimate tensile strength of UNS $35315 at room temperature and at elevated temperatures.

At temperatures where the time dependent mechanical strength, i.e. the creep strength, is the dimensioning factor (above ~550°C, 1020°F), the stable austenite matrix, solid solution strengthened by nitrogen and carbon, has resulted in a high creep deformation and rapture strength. Intragranular, and to some extent also intergranular, carbide and nitride precipitation also contributes to this high strength. Any effect of the REM addition on the creep properties is probably indirect, i.e. by reducing the amount and modifying the morphology of harmful intermetallic inclusions. It can be noted that above 1800°F there are few other common grades that can compete in strength. A comparison of 10,000 creep-rapture strengths of several commonly used heat resistant grades is provided in Table 3.4

FIELD EXPERIENCE

Boilers/Superheaters

A fluidized-bed is a mixture of fuel and limestone suspended in a flow of hot air. Primary causes of failure in fluidized bed combustion power plants are high temperature corrosion and erosion--either in-bed (low particle velocity/high particle density) or in the freeboard or the cyclone (high velocity/low density). Because the


corrosion attack is enhanced by erosion damage, good erosion resistance is essential for all materials of construction.

A North American electric utility installed tube shields of UNS $35315 to prevent in-bed erosion of the primary superheater tubes in the combustion zone of a CFB boiler s. This unit is fired with high mlfiw, high chlorine coal, making it prone to fouling problems. In addition, the local limestone is itself aggressive because of alkali content. The unit operates at 880°C (1616°F). 11 gage (3 mm) shields were installed May of 1998 to protect the Omega tubes, which are subjected to heavy erosion because of their in-bed location. In March of 1999 additional UNS $35315 tube shields were placed directly in the path of the steam sootblower, an extremely erosive area. As of August 2000, all tube shields were still in service. A photo of one such tube shield is shown in Figure 3.

A power plant in the northeast United States installed UNS $35315 material in several areas. The plant is fueled by anthracite waste coal and petroleum coke. The generating facility uses circulating fluidized-bed (CFB) boilers. 3/4" SCH40 pipe was installed to modify the circulating fluidized bed boiler in June 2003. There are four ash drams at the bottom of the boiler that are about six inches in diameter. Large chunks of ash occasionally block the drains. UNS $35315 was used to make a filter to cover the top of the drain and reduce plugging. Temperatures in this zone are approximately 1700°F. UNS $35315 sheet was also used to manufacture tube shields, which replaced type 309 shields. Type 309 stainless tubeshields required annual replacement. Such frequent replacement has been a costly maintenance item.

Alstom's P-200 pressurized fluidized bed combined cycle (PFBC) pilot power plants were built in Sweden (Vgrtan), the United States (Tidd), and Spain (Escatron). These units were designed to operate in the 800- 900°C range. The tubeshields and tube supports in all three plants utilized UNS $30815 for these components. This alloy performed well at the Tidd and V/irtan sites. There were hot corrosion issues, however, at the Escatron plant. The coal that was burned in the fluid bed at Escatron was higher in sulfur and chlorides than the other two pilot plants. Sulfi~ contents in the coal were as follows: Escatron - 6.8%, T idd- 3.6%, and V/irtan - <1.0%. As a result, a test series of various heat resistant alloys was performed. The total loss in thickness (original thickness - remaining good metal) was only 0.1 mm after 9-10 months' exposure in the bed. UNS $30815 samples lost approximately 1.1 mm during the same time. This shows that under predominantly oxidizing-sulfidizing conditions, the superior oxide properties of UNS $35315 are much more important than the higher nickel content. Based on this testing, UNS $35315 replaced all of the uncooled components of the superheater (Figure 4 ) 6 .

Anti-vibration devices otten called minkle or handcuff bars are being manufactured using UNS $35315. The manufacturer has switched from UNS $30815 and the nickel alloy UNS N08330. In this application, 3/16"plate strips are corrugated, so that two bars when welded together, wrap around a span of several superheater tubes. These assemblies reduce tube vibration. UNS $35315 heavy plate has also been used for header supports. To date, UNS $35315 alloy has been utilized in direct fired boilers, heat recovery steam generators, and waste incinerator boilers. Figure 5 shows examples of how UNS $35315 is used.


Incineration

Refuse is generally burned with excess air, but otherwise the conditions are among the worst possible. A great number of different corrosive substances are present in varying concentrations. Afcer constructional modifications of an incineration plant in Sweden, there was need for lengthening of the dip tube in the cyclone. A decision was made to manufacture it using 63" x 63" cylinder of ½ inch thick alloy UNS $35315 plate (Figure 6). The mean temperature of the cylinder during service was 1832°F (1000°C) with maximum values of up to 2192°F (1200°C). Scanning electron micro-scope investigation of deposits revealed the presence of various oxides, nickel sulfides and sodium and potassium chlorides. 7

In spite of these extremely harsh conditions, the material showed excellent performance. Atter four months' service, the cylinder surface was almost unaffected, an exception being the upper-most 12-18 inches, where the material temperature was lowest 1750°F (955°C) due to cooling from the heat exchanger. This, together with the fact that the gas velocity was lower in this position, led to condensation of corrosive compounds and some minor corrosion attacks~the installation welds could still be discerned. However, after approximately 12 months service the cylinder was so damaged that it was removed to avoid an un-planned breakdown. The excellent behavior of UNS $35315 in this application convinced the plant operator to install a similar, but slightly redesigned cylinder in an identical plant. No results have been reported from that plant (Figure 6).

Cement kilns are often used to incinerate both solid and liquid wastes. Several kilns have been retrofitted to allow scrap tires to be fed into the kilns as fuel. UNS $35315 was used to fabricate a tire drop chute, which was installed in the southwestern United States. The chute requires high creep strength in order to support the whole tires during combustion. Resistance to oxidizing, sulfidizing, and carburizing conditions are also necessary. Areas of the chute that were exposed inside the kiln were made of UNS $35315. Temperatures up to 2000°F (1080°C) are typical. Last inspection of the UNS $35315 chute conducted after several trial burns, showed no degradation of the chute by corrosion or distortion. UNS $35315 has also been used for flapper gates on the tire drop tubes in two other cement plants.

The tubes in the recuperator of a propane fired slab reheat fimaace had a low service life (~ 7 months) due to fuel ash corrosion causing tube wall penetration. The tubes were made of a 19Cr-11Ni-2Si stainless steel (EN 1.4828). The flue gas temperature varied between 800 and 900°C, and the tube surface temperature between 700 and 750°C. Test coupons of a number of high temperature alloys were exposed in the flue gas channel before the recuperator in an attempt to fred a more resistant material. UNS $35315 showed the best performance, together with the cobalt containing alloys UNS R30556 (alloy 556) and UNS N12160. Based on these results, and on material cost and availability, UNS $35315 was chosen as the new tube material. After 3- 4 months and atter 11 months, samples were cut from the tubes for inspection. They showed only a minor, and tmifom~ reduction in wall thickness. No penetration failures have been reported after more than two years of service.


SUMMARY

The austenitic stainless steel UNS $35315 is a member of the MA family of grades, which together cover a variety of high temperature applications and conditions. It is designed primarily for service above 1830°F (1000°C), although it has been used at temperatures as low as l ll0°F (600°C) in more aggressive environments.

UNS $35315 contains 25 percent chromium for oxidation resistance. Moreover, cerium additions and an elevated silicon content improve the oxidation resistance further by enhancing oxide adherence. The resultant scale is highly protective and greatly increases the resistance ofUNS $35315 to sulfidation, carburization, and molten salt attack under oxidizing conditions. A substantial amount of nickel further improves resistance against carbon and nitrogen pick-up, particularly under reducing conditions. In addition, the nickel content also increases the resistance against environments that contain chlorine/chlorides. Nitrogen is added mainly to improve the creep rupture properties at elevated temperatures.

The well-balanced chemistry of UNS $35315 yields a material that can resist a wide variety of high temperature corrosion mechanisms. As a result, UNS $35315 is an excellent candidate for the most severe high temperature conditions in power plants, incinerators, and other industries.


REFERENCES

1. M.A.H. Howes, "High Temperature Corrosion in Coal Gasification Systems," Final Report GRI-871- 0152, Gas Research Institute, Chicago, Aug. 1987

2. M. Willfor, P. Vangeli -AvestaPolarit 353 MA ® - A Problem Solver tSr the Steel and Metals Industry. AISE/2003

, R. Norling, "Oxide Formation and Degradation during Erosion-Corrosion of Iron and Nickel Based Alloys", PhD Thesis, Department of Engineering Metals, Chalmers University of Technology, Gothenburg, Sweden, 2003..

4. "RA Data Sheet- RA 353 MA ® Alloy", Bulletin No. 1354 5/02 2M, Rolled Alloys, 2002.

, "RA Case History - Kvaemer TM Pulping Fabricates RA 353 MA Alloy Tube Shields for a CFB boiler", Bulletin No. 2016 5/00 1M, Rolled Alloys, 2000

6. I. Wright, J. Stringer, J. Wheeldon, "Materials Issues in Bubbling PFBC Systems", MA TERIALS A T

HIGH TEMPERA TURES 2003 Volume 20 Number 2,

o M.Segerb/ick, B.Ivarsson, R.Johansson, and J.C.Kelly, "UNS $35315 - A Material for Very High Temperatures and Harsh Environments", NACE CORROSION/95, Paper No. 472.


TABLE 1

N O M I N A L C O M P O S I T I O N OF HIGH TEMPERATURE ALLOYS (WEIGHT %)

Alloy UNS Ni Cr Si Co Fe Other 3 5 3 M A $35315 35 25 1.2 - 36 Ce, N

2 5 3 M A $30815 11 21 1.7 - 65 Ce, N

310 SS $31008 20 25 0.5 - 52

309 SS $30908 13 23 0.8 - 62

800H N08810 31 21 0.4 - 45 AI, Ti

Alloy 330 N08330 35 19 1.25 - 43

Alloy 556 R30556 21 22 - 18 29 W, Mo, N, La

HR-160 N12160 36 28 2.8 30 2

TABLE 2

CYCLIC OXIDATION TESTING IN AIR 3000 H O U R S - 168 HOUR CYCLES

WEIGHT CHANGE (mg/cm 2) Alloy 1800°F 2000°F 2100°F

UNS $35315 4.4 21 36

UNS $30815 8.7* 41 115

U N S N 0 8 3 3 0 4.2 32 54

UNS $31008 5.2 -- 81"*

UNS N08810 32.0 143 295

UNS $30908 16" 110 --

* Test mn for 1966 hours

**310 SS tested for 1640 hours

TABLE 3

AVERAGE 10,000 HOUR STRESS TO RUPTURE STRENGTH, psi

ALLOY 1200°F 1400°F 1600°F 1800°F 2000°F 2100°F 2200°F

UNS $35315 12.200 5.400 2.600 1.300 680 (450~ (320~

UNS $30815 14.000 5.200 2.500 1.150 680 . . . .

UNS $30908 17.000 4.800 1.600 560 . . . . . .

UNS $31008 14.400 4.500 1.500 660 . . . . . .

UNS N08330 11.000 4.300 1.700 630 (280] . . . .

U N S N 0 8 8 1 0 17.500 7.300 3.500 1.200 . . . . . .


Thickness Loss

~133 . I =

~o2

0.01

0

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0:05 -

0.04

800H

.. .............. ..., .~

.,,:. t

446 353 MA

Figure 1 -Test coupons exposed in a flue gas channel of a recuperator for 5,000 hours. Flue gas temperatures varied between 800 and 900°C (1470 and 1650°F) and contained sulfur and chlorine.

t 40

t 2 0

E 1oo

:~ 80

" 0

:~-~ 60

0 40 IW;

20

:

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~_A t ~ ~ ' ,D , , =wJ~ . ~ ~ " • ,m~'~"- ~ . ~ _ . . . ,,,,,

1 0 0 200 300 400 500 Exposure t ime (hr)

Figure 2 - Results from erosion testing at 550 °C.


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Figure 3 - U N S $35315 tube shield made from 1 lga sheet. Tubeshields installed have lasted in excess of 2

years without replacement in the superheater of a FBC boiler.

Figure 4 - View of UNS $30815 shields in Tidd PFBC superheater. UNS $35315 replaced UNS $30815 in this same area of the Escatron PFBC boiler firing black lignite coal (6.8% sulfur) operating in Spain.


Figure 5 - Photo showing UNS $35315 anti-vibration straps used in an auxiliary steam boiler. This unit was installed at a natural gas fired co-generation plant. UNS $35315 replaced UNS $30815 and UNS N08330 as

the material of choice. UNS $35315 has been used for over 4 years by this boiler OEM.

Figure 6 - Photo taken of UNS $35315 dip tube being installed in a municipal waste incinerator.


performance of alloy 353 ma (uns s35315) in · pdf filewith improved hot corrosion...

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