Corrosion-Fatigue Failure of Finned Stainless Steel Economizer Tubes Due to Differential Thermal Expansion

From: A.G. Glover, D. Hauser, and E.A. Metzbower, Failures of Weldments, Failure Analysis and Prevention, Vol 11, ASM Handbook, ASM International, 1986, p 411–449

Abstract: After 10 years of satisfactory operation, economizer-tube failures occurred in a large black liquor recovery boiler for a paper mill. The economizer contained 1320 finned tubes. Two fins ran longitudinally for most of the tube length and were attached by fillet welding on one side. The economizer tube leaks occurred at the end of the fin near the bottom of the economizer. A sample from a tube that had not failed showed heavy pitting attack on the inside of the tube, probably due to excess oxygen in the feedwater. Penetrant testing revealed numerous longitudinal cracks on the inside in the area of the fin tip. Cracking at the end of the fin-to-tube fillet weld was noted. The results indicate the failures were due to corrosion fatigue whose stresses were primarily thermally induced. A temporary solution included inspecting all tubes with shear-wave ultrasonics. Tubes with the most severe cracking were ground and repair welded. The square corners of the fins were trimmed back with a gradual taper so that expansion strains would be more gradually transferred to the tube surface. Water chemistry was closely evaluated and monitored, especially with regard to oxygen content.

Keywords: Boiler tubes; Liquids; Stress concentration; Water chemistry

Material: Stainless steel (Stainless steel, general)

Failure type: Corrosion fatigue


After 10 years of satisfactory operation, several economizer-tube failures occurred in a large black liquor recovery boiler for a paper mill. Several other areas of the boiler had experienced problems, but this was the first unscheduled outage due to economizer problems.
The economizer contained 1320 tinned tubes approximately 13.7 m (45 ft) long. These tubes were specified to be 50 mm (2 in.) OD × 4.2 mm (0.165 in.) minimum wall thickness. Two fins ran longitudinally 180° apart for most of the tube length and were attached by fillet welding on one side. The fins were about 6.4 mm ($\genfrac{}{}{0.1ex}{}{1}{4}$ in.) thick and 50 mm (2 in.) high. The weld was started and stopped by running up on the fin for the last 13 mm (0.5 in.). This technique left the ends of fins unwelded (Fig. 1). This fin/tube configuration is commonly used in black liquor recovery boilery.
Figure 1

Fig. 1  Section of stainless steel tube/fin assembly removed from economizer. Note the weld geometry and fin weld locations.

The economizer began experiencing tube leaks that resulted in unscheduled outages. These leaks occurred at the end of the fin near the bottom of the economizer. After the initial failure, the frequency of failures increased, the time between failures decreased, and a failure analysis was eventually initiated.
Investigation. During an unscheduled outage, a sample was removed from a tube that had not failed. The inside of the tube showed heavy pitting attack, probably due to excess oxygen in the feedwater. After cathodic cleaning, the tube was penetrant tested, and numerous longitudinal cracks were noted on the inside in the area of the fin tip (Fig. 2). Cracking at the end of the fin-to-tube fillet weld was noted during visual examination of the outside surface.
Figure 2

Fig. 2  Interior surface of economizer tube after penetrant testing. Note indications of cracking (arrows).

Metallographic examination of the section near the end of the fin showed cracking on the inside under the fin and on the outside at the root and toe of the weld (Fig. 3a). A cross section under the fin tip showed one large straight crack on the inside-diameter surface that penetrated approximately one-third of the wall; heavy pitting was also visible (Fig. 3b). Examination of the inside-diameter crack at higher magnification revealed numerous small cracks adjacent to it that initiated at the inside-diameter surface (Fig. 3c). All of these cracks were transgranular, straight, unbranched, and contained oxide typical of corrosion fatigue.
Figure 3

Fig. 3  Inside-diameter cracking in stainless steel economizer tube. (a) Cross section of tube near the end of fin fillet weld. Note cracking at the root and toe of the weld. (b) Cross section of tube under unwelded fin end showing crack below fin and pitting on inside-diameter surface. (c) Detail of inside-diameter cracking showing one large oxide-filled crack and two smaller cracks. 45×. All as-polished

Discussion. A general review of the stresses in the tube fin area indicated that failure was most probably due to thermal stress from differential expansion of the fin and tube. Specifically, the outside of the fin approaches the temperature of the flue gas, while the tube is nearer to the temperature of the cooler inlet water.
This temperature difference results in more expansion of the fin than the tube. Furthermore, the outside of the fin expands more than the portion welded to the tube due to the temperature gradient in the fin. These actions cause the fin to bow such that the outside surface becomes convex. This distortion forces the unrestrained fin end to press down on the outside surface of the tube. At the fin tip, this pressing subjects the tube to a localized bending stress in which the outside is in compression and the inside in tension.
Conclusions. The results indicate the failures were due to corrosion fatigue whose stresses were primarily thermally induced. Corrosion, probably due to a high oxygen content in the feedwater, caused pitting, which resulted in localized stress concentration. Corrosion also lowered the threshold for fatigue. The fatigue cycles were associated with start-ups and shutdowns. Problems in other areas of the boiler resulted in numerous shutdowns over the 10 years of operation. Increased cycling of the boiler for repairs caused accelerated crack propagation in the tubes. This increased the frequency of failure and shortened the time interval between failures. Accelerated shutdown procedures associated with the unscheduled outages caused severe thermal gradients, which further raised the stress level.
Recommendations. A temporary solution included inspecting all 1320 tubes with shear-wave ultrasonics to determine the extent of damage (many tubes contained cracks). Tubes with the most severe cracking were ground and repair welded. The square corners of the fins were trimmed back with a gradual taper so that expansion strains would be more gradually transferred to the tube surface. Water chemistry was closely evaluated and monitored, especially with regard to oxygen content. After the inspection, repair, and fin rework was completed, the boiler ran for approximately 1$\genfrac{}{}{0.1ex}{}{1}{2}$ years without any additional failures. The economizer was then replaced due to efficiency and other considerations.

Related Information

Fatigue Failures, Failure Analysis and Prevention, Vol 11, ASM Handbook, ASM International, 2002, p 700–727