The inner and outer surface cracking of the recovery boiler floor tubes.

Application: Examining the cracks in AISI 304/304L composite tubes in recovery boilers can offer some clues on how well the material is holding up and whether or not the tubes need to be replaced.

Cracks were found in the 304L composite floor tubes of a recovery boiler. Based on previous experience, mill personnel also suspected that cracks would be found on the inner surfaces of the tubes. The floor tubes were removed, and in fact, cracks were found on the inside.

A study was undertaken to gain more information about composite floor tube cracking and to see if this information could help in finding a safer way to use 304L composite tubes. In other words, could the appearance of cracks on a given tube give any insights as to whether it would need to be repaired or replaced?

Transverse vs. branching cracks

There are two basic types of cracking--transverse and branching. Both types were found in tubes examined in the comprehensive investigation.

The branching type is thought to be caused by stress corrosion cracking. Branching cracks propagate through the stainless steel cladding, and they either stop where the cladding meets the base material or change their direction of propagation along the interface.

Transverse cracking is thought to be caused by thermal fatigue. Transverse cracks propagate through the interface into the base material.

Crack categorization

To facilitate the examination, we decided to categorize the cracks. For that, the tubes were examined with an optical stereo microscope. The cracks in the tubes were categorized into five classes. Types 1-3 were transverse cracks. Type 1 consisted of relatively small cracks, located mostly near the tube crown. Type 2 cracks were longer than Type 1 cracks, and Type 3 cracks were more open on the surface than were the Type 2 cracks.

Types 4 and 5 were branching types. The fourth class consisted of cracks in a snake skin pattern. These cracks were located near the tube crown separating them from the cracks of Type 5, which were situated near the fins. The pattern of line Type 5 cracks was also a little different--more regular, resembling a mosaic pattern.

Testing and measurements

We used optical microscopy, electron microscopy, X-ray diffraction, and EPR (electrochemical potentiokinetic reactivation) to make tests and measurements on tube samples. We also took hardness measurements.

Speculation on crack propagation

If cracks were to be classified according to how fast they cause a final rupture of a composite tube, then Type 3 cracks, shown in Fig. 1, may be the worst kind. A crack of this type invades the tube in a carrot shape, growing across the interface and penetrating the carbon steel.

[FIGURE 1 OMITTED]

Summary and conclusions

Composite tubes in recovery boiler floors contain two different types of cracks: transverse cracks in the tube crown and branching cracks in the tribe crown and on the sides of the tube.

If transverse cracks appear in the tube crown, then cracks may have also developed on the inner surface of the tube. The larger the transverse cracks, the more likely it is that transverse cracks will be found in the tubes and the more important it is for the tubes to be replaced. Branching cracks propagate through the cladding but stop at the interface or continue along the interface.

Saarinen and Hanninen are at Helsinki University of Technology, Laboratory of Engineering Materials, P.O. Box 4200, FIN-02015 HUT, Finland, Tuiremo and Salmi are at Kvaerner Pulping Oy, P.O. Box 109, FIN-33101, Tampere, Finland. Address correspondence to Saarinen by email at [e-mail].