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D A V I D N . F R E N C H , I N C . , M E T A L L U R G I S T SO N E L A N C A S T E R R O A D

N O R T H B O R O U G H . M A S S A C H U S E TT S 01532

VOL. VIII, NO.

A VIEW FROM THE PENTHOUBE; UBEFUL INFORMATION FOR THE WORLD OF BOILERSFECTED ZONE8

This issue of the newsletter isa series of articles onfeatures of welds.al characteristics andf the base metal to

g. The heating and cooling of theal during welding willin different ways. We begin

heat-affected zone (HAZ).

Figu re 1mil' *F

0 - CENTEWLIWE OF VELD1 - EDGE OF FUSION ZONE2 - 16W'F POSITION

BEXEEN 1-2 MICROSTRUCTURE IS ALL AUSTFNITEDURING WELDING

3 - 1350F POSITIONBEWEEN 2-3 MICROSTRUCTURE IS MIXTURE OF

AUSTEWITE AHD FERRITEFQSITIONS 1-3 ARE HEAT AFFECTED ZOSE

Figure 1 shows schematicallyn of the HAZ in ferrit-profile from the center-of the weld into the baseal. For simplicity, Figure 1the temperature profile on oneonly. Heat flows away from thed in two directions. In a thick

the weld is a semicylinder,

and heat flows away in three direc-tions. The temperature at the edgeof the fusion zone is the meltingpoint of the steel. The peak tem-perature decreases as we move awayfrom the edge of the weld as shownin Figure 1.For ferritic steels similar tocarbon steel, T-11 or T-22, there ia transformation of the normal roomtemperature microstructure as thetemperature is increased. Ferriteand pearlite transform first to ferrite and austenite and then at astill higher temperature to all austenite. The first or lower trans-formation temperature from ferriteand pearlite to ferrite and austen-ite is called the lower-criticaltransformation temperature. Theexact temperature depends on thecomposition but is about 1340F forplain carbon steels. The additionof alloying elements changes thislower-critical transformation tem-perature. For T-11 it is approxi-mately 1430F, and for T-22 it isapproximately 1480F.The transformation from ferritand austenite to all austenite iscalled the upper-critical transformation temperature and this toodepends on the composition. Forexample, in plain carbon steelssimilar to SA178A and SA210 A-1, thupper-critical transformation tem-perature changes with carboncontent. At 0.1% carbon (SA178A),the temperature is about 1580F, at0.25% carbon (SA210 A-1), the tem-perature is about 1490F.The heat-affected zone (HAZ) iferritic steels is defined as thatregion of the base metal near theedge of the fusion zone, or weldmetal, that has a peak temperatureduring welding high enough to affeca transformation from ferrite and

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pearlite to ferrite and austenite orall austenite.For the austenitic stainlesssteels, there is no transformationfrom ferrite to austenite. Austen-itic stainless steels, similar to304, 321, or 347, are austenite overthe entire temperature range. Thereare, however, other changes that dooccur at temperatures near themelting point.The microstructure of the HAZof austenitic stainless steelsdepends on the amount of cold work.For solution-annealed material, theusual case for boiler tubes, the HAZill show only grain growth. Thisincrease in size of the austenitegrains begins at a metal temperature

of about 1800F or so. The basemetal from the edge of the fusionzone to the peak-temperature iso-therm of 1800F will have enlargedgrains. In heavily cold-workedstainless, as in support structures,the HAZ will first show some relaxa-tion of the cold-worked microstruc-ture at a temperature of about1600F and then the grain growth.Cold-worked structures trans-form from a distorted microstructureto an annealed microstructure in twosteps. Cold-worked grains recrys-tallize into strain-free but verysmall or fine grain sizes. The finegrain size changes to a coarse grainsize by grain growth. Thus the HAZof these cold worked materials willshow grain growth next to the fusionline and a fine grain size at the1600F isotherm.The microstructure of the HAZof ferritic steels depends not onlyon the peak temperature, but also

the cooling rate and alloy contenRapid cooling or quenching of theaustenite forms martensite, a hardbrittle material that can cause HAcracking. Slower cooling forms mitures of ferrite and bainite orpearlite. Higher alloying materi-als, for example T-22, will moreeasily form martensite than theplain carbon steels. A cooling rathat forms ferrite in a low carbonsteel, SA178A, will form all martesite in a higher alloy material, T22. The same type of weld made inT-22 as made in SA178 will have avery different HAZ microstructure.In order to prevent the formation martensite within the HAZ, slowercooling rates for these alloy steeare required. The most convenientway to accomplish this is by pre-heating.The ASME Boiler & PressureVessel Code has non-mandatoryguidelines for the amount of preherecommended, and these are given iTable I.

TABLE I. SUGGESTED PREHEATMATERIAL THICKNESS PREHEAT TEMP.Carbon steel lw* 175FT-11 +I1 250FT-22 + t 400F*~lso 0.30%C.

Next time we will presentseveral representative microstruc-tures of both ferritic and austen-itic stainless steels.OTHERS WHO MIGHT LIKE TO RECEIVE OUR NEWSLETTER:NAME : COMPANY :DDRESS :IDEAS FOR TOPICS:

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