anil vaghamshi ppt

8/3/2019 Anil Vaghamshi Ppt

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INTERGRNULAR

CORROSION OF

AUSTENITIC STAINLESS

STEELPrepared by: Anil N. VAGHAMSHI

Roll No: 923(BE-IV)

Metallurgical & Material Science Department

The M.S. University of Baroda.


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CONTENTS

INTRODUCTION

REASONS FOR IGC OCCURANCE

MECHANISM OF GRAIN BOUNDARY

IGC BY SENSITIZATION OF AUSTENITIC S.SREAL CASE OF IGC

IGC IN WELD

HOW TO CONTROL IGCREFRENCES


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INTERGRANULAR CORROSION

Intergranular corrsion occurs at grainboundries.Rate of corrosion of grain boundary ishigher than grains.

In some cases, grain boundaries are moresusceptible to localize attack than rest of thematerial.

It is most often observed in stainless steel.


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REASONS

Phase precipitation at grain boundries anddeplete the matrix that decreases the corrosionresistance.

Grain boundary phase may be more reactivethan matrix .

Segration of various solute atoms at the grain.

Some working conditions in which given metal

do not have resistaance to IGC.


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MECHANISM

Metals are made up of polycrystallinegrains,These grains are separated by grainboundary. These grain boundaries have higherdisorder than grains thats why their energy ishigher than grains. These grain boundaries areof two types:

Low Angle Grain Boundary

High Angle Grain Boundry


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The order of grain boundaries according to their energy is given below:

High angle grain boundary > Incoherent > Twin boundary >Coherent grain boundary.


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SENSITIZATION OF AUSTENITICS.S

The main effect that causes IGC is sensitization anddepletion. High angle grain boundaries are more proneto sensitization and depletion of element.

FIG:Electron photomicrograph of carbides

isolated from sensitized S.S.


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It is a process ofprecipitation of new

phase at grainboundaries.Whenever stainless steelis heated to 450 to 850C.the carbon present

diffuses to the grainboundary. This givesprecipitation of carbide bycombination with Cr. Such

as precipitation of Cr23C6which shows IGC whenexposed to atmosphere.

FIG: Sensitization of austenitic stainless steel


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C content in austenitic s.s usually is 0.06 to0.08% so excess carbon is available to combine

with Cr.Thus region forms with very low Crshows very less resistance to corrosion anddisolves in any corroding atmosphere such asHCL,HNO3 etc.

Another mechanism of IGC is segregation ofelements at grain boundary. Such as Cr in S.S,

This Chromium carbide precipitation depends on

many factors such as temperature, exposuretime, %of carbon. Temp. and carbon content arediectly proportional to sensitiztion.


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REAL CASE OF IGCFAILURE OF PREHEATING TUBE

FIG: FAILURE OF PREHEATING

TUBE

FIG: MICROSTRUCTURE OFFAILURE PART


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A cracked 316Ti stainless steel Preheater tube was

analyzed to determine the failure mechanism. A secondnon-cracked tube section from a different elevation wasalso analyzed to determine the condition of the tubematerial. The interior of the tube carries process gas

that reportedly contains hydrogen, carbon dioxide andmonoxide, water and water vapor, methane, nitrogenand traces of hydrogen sulfide. The tube exterior isheated with flue gas.

Uniform intergranular corrosion was observed on the IDsurface. The intergranular corrosion was attributed to asensitized microstructure (chromium carbideprecipitation along grain boundaries when held at 932-1562o F or 500 to 850o C) resulting from the high flue

gas temperatures.


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Failure of a Stainless steel Tank Used forStorage of Heavy Water/Helium

The dished ends of aheavy water/heliumstorage tankmanufactured from 8 mm

(0.3 in.) thick type 304stainless plate leakedduring hydro testing.Repeated attempts at

repair welding did notalleviate the problem.Examination of samplesfrom one dished end


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the cracking was confined to the heat affected zone(HAZ) surrounding circumferential welds and, to a lesserextent, radial welds that were part of the original

construction. Most of the cracks initiated and propagatedfrom the inside surface of the dished ends.

Microstructures of the base metal, HAZ, and weld metalindicated severe sensitization in the HAZ due to high heat

input during welding. An intergranular corrosion testconfirmed the observations.

FIG: Macrograph ofthe specimen after

intergranularcorrosion testing


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The severe sensitization was coupled with residualstresses and exposure of the assembly to a coastalatmosphere during storage prior to installation. This

combination of factors resulted in failure by stress-corrosion cracking.

FIG: Macrograph of the etched cross section on the weld metal.


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IGC IN WELD

In weld of austenetic s.s sensitization observedin Heat Affected Zone.In HAZ area shoescombination of temperature and composition

that result in formation of chromium depletedzone at grain boundary.

Welded structures and materials attackedintergranularly are called Weld Decay.

It is usually a band in the parent plate somewhatremoved from the weld.


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Microstructure also effects the IGC of austenitics.s. Large size grain shows high susceptibility tointergranular corrosion. It is bacause small graingives large grain boundary area and lesscontinous network of carbide.

WELD

INTERGRANULAR CORROSION


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CONTROL OF IGC INAUSTENITIC S.S.

Employing high temperature solution heattreatment commonly termed as Quench

temperin or solution annealing.

Adding elements that are strong carbide former(called stabilizers).

Lowering the carbon content to below 0.03%.


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REFRENCES:

Corrosion Engg. By M G Fontana.

Introduction to Physical Metallurgy By Sidney H.Avenor.

Corrosion Engg. By Philip A. Schweitzer.

Internet Research.


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anil vaghamshi ppt

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