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DAVID N . FRENC H, INC. , METALLURGISTSO N E L A N C A S T E R R O A D

1993N O R T H B O R O U G H , M A S S A C H U S E T T S 01532

T E L: ( 5 0 6 ) 3 9 3 - 3 6 3 5 VOL* IX, No* F A X : (508) 3 9 3 - 3 9 1 4

A VIEW FROM THE PENTHOUSE: USEFUL INFORMATION FOR THE WORLD OF BOILER8UE, THERMAL FATIGUE. CORROSION shaft from a coal pulverizer.Fatigue failures have characteristiNclam-shellN markings that point baFatigue may be defined as the to or surround the origin. There mof a material to a variable be more than one fracture-initiatioor strain. The most obvious site, as evidenced by steps in thebe a rotating shaft in a pump perimeter of the shaft.a fan. For steel, no failuree expected below some stressvel. This minimum stress is - --imes called the vvendurance.vv Failures occur when thetional stress exceeds the

because oftensification factors: suchkey-way, a cut thread,external damage. Failures occurnear ambient temperatures.Thermal fatigue occurs atted temperatures where theand steel formoxide. Under a variable stresse said to be

ess orin aue. Under ambient conditions,f an endurance limitto a safe design stress, under

no similar enduranceit exists. Thus, under theseailure can beted regardless of the stress.All three of these fatigue-

failures. By brittle ismacroscopicthe thickness of theof failure.w, with no warning of any

Figure 1 shows the failure of a

Figure 1.Cracks that initiate from themultiple surface origins join in asingle crack until the cross-sectional area of sound metal is tosmall for the applied stress, andfailure occurs by tensile overload.On a microscopic scale the fracturesurface contains striations thatdevelop one per cycle.Within the boiler environment,corrosion fatigue can occur on thewaterside of a tube, usually at anattachment weld. In cross section,corrosion-fatigue cracks aretransgranular and somewhat bulbous cross section, as shown in Figure 2.The corrosive environment attacks tfresh surface developed by thefatigue-crack growth, leaving behinthe shape shown in Figure 2.

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Figure 2.Coal-ash corrosion of waterwallsgrooving. Insuperheaters th e appearance issometimes called "alligator hidew.too is a corrosion fatigue. A

liquid phase forms from th e ash onsurface of th e tube. Thatliquid can support only a smallof fuel ash. When theess is too great, t he slag ished from the tube at the liquidlayer. The insulating effect islost, and the tube temperatures. This temperature cycling ass and sheds leadsstrain within the tubeexpands and contracts with th ein surface temperature. Thisatigue, in connection with theo corrosion-fatigue. In cross section they

nto the tube surface a s shown in3.

Figure 3.~her mal- fati gue cracks occur ins at superheater header

lugs. For example, differences inexpansion between the roof tubes ana superheater outlet header' ause abending of th e superheater tubes,which, in turn, leads to thermal-fatigue cracks at th e weld.Refractory-retaining lugs are alsoalternately heated and cooled as asor refractory forms and erodes. Incross section these thermal-fatiguecracks are, again, transgranular andagger-like in appearance, as shownin Figure 4.

Figure 4.Figure 4 is from a refractory-retaining lug.In review, th e differences betwefatigue, corrosion-fatigue, andthermal fatigue are one of atmospheor environment only. Fatigue occurunder ambient temperature conditionthermal fatigue occurs at elevatedtemperatures where iron oxide isessentially the only corrosionproduct, and corrosion fatigue occuin corrosive environments. All havvariable stress or strain applied apart of the failure mechanism.OTHERS WHO MIGHT LIKE TO RECEIVE OUNEWSLETTERNAME :COMPANY :ADDRESS :

METALLURGICAL PROBLEMS?CALL DR. FRENCH AT (508) 393-3635