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DAVID N. FRENCH , INC., METALLURGISTSONE LANCASTER ROAD

NORTHBOROUGH, MASSACHU SElTS 01532

WINTER 1995TEL: (508) 393-3635FAX: (508) 393-3914 VOL. XII , No.

A VIEW FROM THE PENTHOUSE: USEFUL INFORMATION FOR THE WORLD OF BOILERS

THERMAL FATIGUE thermal-fatigue crack formation areat superheater header connections andThe operation of sootblowers may refractory-retaining lugs.

lead to both erosion wastage ofneighboring tubes and the developmentof thermal-fatigue cracks. Thelatter are particularly likely whenthe first fluid is liquid condensatefrom a steam system. The cold waterquenches, or thermally shocks, thesteel. Rapid cooling over many soot-blowing cycles results incircumferential cracks and leaks.

~hermal-fatigue ccurs underconditions that combine a variablestress and elevated temperatures;those temperatures where the reactionof oxygen and steel forms iron oxide.Variable stress, or more accuratelyvariable strain, leads to thethermal-fatigue crack formation. Thesequence of events is something like:a dense, tightly bonded iron oxideforms at the operating temperature.These scales are necessary foroxidation resistance and protect the

steel from further wastage, arebrittle and exhibit virtually noductility prior to crack formation.The strain fractures the protectiveoxide scale. This crack or fracturettshort-circuitsw he path between theair and the bare steel. As the scalere-forms at the root of the crack, asmall cusp forms as the break in thescale is Ithealedw y the formation ofnew oxide. Continued cyclic strainwill then re-crack the new scale, andthe cycle continues. The resultantcrack is transgranular, dagger-shaped, and oxide-filled, with acentral crack down the oxide wedge;see for example Figure 1. When thereare reducing conditions or unburnedcoal particles within the ashdeposit, these scales may alsocontain iron sulfide. The presenceof metallic sulfides is easilydetermined with a sulfur print.

The most common locations for

Figure 1

The differences in expansion betweenthe roof tubes, for example, and thesuperheater outlet header cause abending of the superheater tubeswhich, in turn, leads to thermal-fatigue cracks at the socket-weldconnections. The operatingtemperature of the superheater outletheader is, for example, 1005F; and

the saturation steam temperaturewithin the waterwall or roof tubes is650F. This 350F temperaturedifference means the superheateroutlet header will expand more thanthe waterwall tubes, the differencein expansion "bendstt he tubeconnectors and leads to cracking.

Refractory-retaining lugs arealternately heated and cooled as theash or refractory forms and erodes.Regardless of the location, thesethermal-fatigue cracks aretransgranular and dagger-like inappearance.

BOOTBLOWER EROSION

The frequency of operation ofsoot-blowers is somewhat variable,obviously, depending on theparticular conditions within anindividual boiler. Whether the soot-blowing cycle is once per shift or

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once per day, the real problemdevelops when condensate rather thansteam is the first soot-blowingmedium to strike the tubes. Thequenching action of water on thesuperheater tubes will quickly lead

to the formation of circumferential,thermal-fatigue cracks, as shown inFigure 2.

Figure 2

The shape of the cracks issimilar, but the frequency (that is,the number of cracks per linear inchof tube) differs. This frequency isa function of the severity of thethermal strain developed by thequenchinq of the hot superheater orGeheater -tube by the liquidcondensate.

Figure 3 shows a cross sectionthrough the circumferential crackingshown in Figure 2, and the generalshape or appearance is similar tothat shown in Figure 1.

The wastage on the superheater or

reheater tube as a result ofsootblower use, again, is somewhatvariable. The wastage on the tubeshown in Figure 2 was about 10% ofthe wall thickness. If there isliquid coal- or oil-ash corrosionpresent, the molten ash constituentswill be easily removed; and any soot-blower erosion will be exacerbated bythe presence of rapid coal/oil-ashcorrosion. Figure 4 is one suchexample.

Figure 4

In this case, any thermal-fatiguecracks that would form were removedby the rapid coal-ash corrosion.

In summary, the morphology of allthermal-fatigue cracks is similar,regardless of the root cause. Thesecracks are dagger-shaped,transgranular, oxide- (or perhaps amixture of oxide and sulfide) filled,and lead to thick-lipped, limited-ductility, or brittle-appearingfailures. Well-developed cracks areeasily identified by either ultra-sonic techniques or liquid-penetrant

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