Assessment of high temperature performance of induction pressure welded 2.25Cr-1Mo steel in corrosive environment

Scripta METALLURGICA et MATERIALIA

Vol. 28, pp. 1223-1228, 1993 Printed in the U.S.A.

Pergamon Press Ltd. All rights reserved

ASSESSMENT OF HIGH TEMPERATURE PERFORMANCE OF INDUCTION PRESSURE WELDED 2.25Cr-lMo STEEL IN CORROSIVE ENVIRONMENT S. Ahila, S.Ramakrishna lyer and V.M.Radhakrishnan Department of Metallurgical Engineering Indian Institute of Technology Madras 600 036, INDIA. (Received December 29, 1992) (Revised March 2, 1993)

Introduction

Creep is the slow deformation of a material under stress that results in a permanent change in shape. Although creep can occur at any temperature, only at temperatures exceeding about 0.4 Tin, where T m is the melting point of the material, are the full range of effects visible. The optimum conditions for a creep rupture are stress and temperature. In real structures, as in power plant components exposed to hot reactive gases that cause corrosion, the effect due to environment is also added to stress and temperature. Reaction between the environment and the material at high temperatures play an important role in deciding the useful life of the component. It is always the fact that the environment induces early failure of the component. It is now widely accepted that accelerated corrosion of alloys used in gas turbines are caused by the deposition of sulphates of alkali metals resulting from the ingestion of salt in engines and sulphur from the combustion of fuel (1). When operating temperatures go above the melting point of the deposited salt, hot corrosion due to the formation of a liquid phase occurs (2). In addition to sulphates of sodium and potassium, sodium chloride through ingressed air also enters the system when the plant is operated in marine environments. Sodium chloride together with sodium sulphate can easily form a low melting mixture at a temperature lower than the melting point of both the salts (3). In such a case, corrosion is accelerated even at lower temperatures where the liquid phase starts forming. So the mixture of salts is always more deleterious. The boiler tubes (superheaters and reheaters) are commonly made of 2.25Cr- 1Mo steel. The external surface of these tubes is found to be corroded heavily. So it is necessary at this stage to study the effect of corrosion on the creep rupture of 2.25Cr-lMo steel. In this article, an attempt has been made to evaluate the effect of hot corrosion due to potassium sulphate and sodium chloride mixture on creep rupture of induction pressure welded (IPW) 2.25Cr- 1Mo steel. 1223 0956-716X/93 $6.00 + .00 Copyright (c) 1993 Pergamon Press Ltd.

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Experimental Method

2.25Cr-lMo steel samples of 40 mm gauge length, 4 mm width and 2 mm thick with induction pressure welding at the center of the gauge length were used for the present study. The mixture containing 60 % by weight of potassium sulphate and 40 % by weight of sodium chloride was dissolved in water and sprayed on heated specimens at 200 o C to form a uniform coating on the surface of the sample . This particular ratio of salts corresponds to the lowest melting composition of the system containing potassium sulphate and sodium chloride. Creep tests were done on coated samples at temperatures of 550 o and 600 o C and at stress levels of 112.5 MPa and 135 MPa. The same tests were repeated for uncoated samples in air for comparison purposes. To study the effect of hot corrosion on material degradation, coupons of 2 sq.cm were coated and kept in a furnace for 100 hours at 550 and 600 degrees C. After the experiment, the coupons were polished and etched in 2 % nital and analysed for the change in structure using optical microscope. Results and Discussion

Figures 1 and 2 show the creep curves of induction pressure welded, uncoated and coated samples, at two stresses and two temperatures. Coated samples showed reduced rupture life and ductility. At 550 degrees C, the rupture time is not indicated in the figures since the samples did not fail even after 4000 hours. All samples were found to fail at the heat affected zone (HAZ), proving thereby that the H A Z becomes unstable during the creep test. In the case of coated samples, two important factors were found to contribute to reduced rupture life: 1. The surface layers were attacked to a thickness of 0.75 mm and the loosely held layers spalled off during creep testing thus reducing the available cross section of the sample. This resulted in increased stress and early failure. 2. During creep testing of 2.25Cr-lMo steel, carbide precipitation is reported to occur throughout the gauge length (4). This resulted in reduction in solid solution strengthening of the material. Chrome - moly steel forms on the surface protective oxide layer. This oxide film is attacked first due to corrosive salt coating. That is, the salt destroys the protective nature of the oxide film at localised spots (Fig.3), thus exposing the base metal to the corrosive environment through the surface cracks in the oxide layer. Along these cracks and also through grain boundaries, these corrodents (salt mixture and oxygen) can have an easy flow to the base metal (5). This results in enhanced attack. A possible example of corrosion reaction could be 3SO~4-+ 7 C r ( i n alloy)+ 3/202 . . . .

3CtO 2- + Cr2S3 + Cr203

Thus when coating was present on the surface, it resulted in accelerated oxidation and sulphidation forming oxide and sulphide of chromium on the surface. The presence of these products

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was indicated by x-ray diffraction studies. Once the sulphate penetrates the oxide layer through cracks in the scale and forms chromium sulphide and oxide by the above reaction, the alloy was depleted of its chromium content and rendered more susceptible to attack. Hence the hot corrosion reaction is an autocatalytic process. A notable characteristic of severe hot corrosion attack was very high internal oxidation and exfoliation of the surface layers due to the increase in volume of oxides formed with time and the consequent stress developed in the oxide layer. The oxygen potential was reduced at the metal scale interface forming the conditions favorable for sulphide formation. Once the sulphides are formed, this can degrade the material faster than the oxide by flowing along the grain boundaries as in Figure.4 The cross section of the sample showed the path followed by the corrodents as in Figure 4. The grain boundaries were attacked in preference to the grains. As the subsurface attack proceeded through the grain boundaries, the corrosion products formed built up stress under the outer scale leading to spalling of the scale from base metal. Now fresh metal is ready for further attack. This indicated that the grain boundaries were weakened by the coating. Figure 5 shows the microstructure of the uncoated sample at 550 degrees C. The attack was not severe in uncoated samples except for the fact that the carbide precipitation occured (4). Carbide formation was seen in coated samples also. Thus the breakdown of protective properties of the oxide scale by the sulphate in the salt mixture led to enhanced attack. In our salt coating, the simultaneous presence of chloride has an effect of increasing the rate of attack by forming volatile chlorides of alloying elements which escaped from the system. This also led to depletion of alloying elements like Cr resulting in accelerated attack

(6) Conclusions

Based on the results obtained form creep and hot corrosion tests at 550 and 600 degrees C, the following conclusions are drawn: * The coating of 40% K2SO4 - 60% NaCI led to lowering of rupture life of the material. Coated samples showed localised oxidation and sulphidation along the grain boundary thereby weakening it. The enhanced oxidation due to the coating was attributed to continuous supply of oxygen to the metal through the liquid phase of salt coating. * Scale failure occurs due to the development of stress on the scale on of salt coating with the base metal. * Once the scale spalled off, fresh metal is exposed for further attack failure.

continuous reaction

leading to early

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Acknowledgements The first author wishes to thank Steel Authority of India Limited for their financial assistance. The authors thank Professor K.J.L. Iyer for his useful suggestions.

References .

J.A. Goebel, F.S. Pettit and G.W. Goward, 4, 261 (1973)

2.

S. Ahila and S. Ramakrishna Iyer, J. Mater. Sci. Lett., in press.

3.

V. Suriyanarayanan, Ph.D. Thesis, p.44,IndianInstituteof Technology, Madras, INDIA, (1991)

4.

M.C. Murphy and G.D. Branch, J Iron and Steel Inst, 209, 546 (1971).

5.

V. Suriyanarayanan, K.J.L. Iyer and V.M.Radhakrishnan, High Temp. Tech., 7, 33 (1989).

6.

H. Picketing, F. Beck and M. Fontana, Met. Trans. ASM, 53,793 (1961).

IPW

Uncoated

o 112.5 MPO A 135"0 MF~

6

..5. = .o

U

.42

6oo°c

~f,so'c I

I

I

1000

20(313

3000

Z.O00

Ti me (h}

FIG.1. Creep curves of Induction Pressure Welded (IPW) specimens in uncoated condition at different stress levels and temperatures.

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IPW

ew

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Coated

•

112.5 M P o

•

135.0 M Po

'600"C

t_)

I

I

200

/.00

600

Time (h)

FIG.2. Creep curves of Induction Pressure Welded (IPW) specimens coated with 40°~ K2SO 4 - 60% NaC1 mixture tested at different stress levels and temperatures.

FIG.3. Surface degradation of 2.25Cr-1Mo steel due to salt coating.

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FIG.4. Hot corrosion attack proceeding along grain boundaries at 550°C.

FIG.5. Microstructure of uncoated sample at 550°C

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