welding of stainless steel -sndo kou

8/13/2019 Welding of Stainless Steel -Sndo Kou

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Welding of Stainless Steel

Submitted by A R Dhamodaran


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Weld Decay

Definition

The HAZ of ASS with

more than .05% Carbon

content is susceptibleto Intergranular

Corrosion known as

Weld Decay

C


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Cause for Weld Decay

Precipitation of Cr carbide at the grain

boundaries (known as Sensitization)

Cr carbide is Cr enriched M23

C6 ,

where M

represents Cr and some amount of Fe

Within the Sensitization temperature range,

C rapidly diffuses freely to the grainboundaries and combine with Cr to form Cr

carbides


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TEM micrograph of Cr carbide particles along

a grain boundary of a 316 stainless steel that

has been sensitized at 750C for 1 hour

63-71% wt Cr along grain boundaries


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Interference

Above slide shows, because of Cr carbide

precipitation at the grain boundary, the areasadjacent to the grain boundary are depleted

of Cr and this is explained in below figure,

These areas become anodic to the

rest of the grain and hence are

preferentially attacked in corrosive

media, resulting in intergranularcorrosion


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Isothermal precipitation curve for

Cr23C6in 304 stainless steel

Sensitization Temperature600-850 C


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Solvus curves for Cr23C6 and TiC in

304 stainless steel.

Sensitization temperature for Tic

850-1100 C

Sensitization temperature for Cr23C6

600-850C


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Sensitization in austenitic stainless steel

Phase Diagram Thermal Cycle Precipitation Curve

Microstructure


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The sensitizedtemperature

reached is held for

longer time

Thus more Cr23

C6

gets precipitated

Peak temperature

is too low to

obtain Cr carbideprecipitation i.e.,

T


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Thermal cycles and weld decay in 304

stainless steel weld


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Effect of carbon content on isothermal precipitation of

Cr23C6in 304 stainless steel


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Effect of Heat Input


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Remedies

Post Weld Heat Treatment Reduction of Carbon Content

Addition of Strong Carbide Formers


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Post Weld Heat Treatment

The weldment can be heat treated at10001100C followed by quenching.

The high-temperature heat treatment

dissolves the Cr carbide that has precipitatedduring welding, and quenching prevents its re-formation.

This treatment is not always possible, howeverbecause of the size and/or the quenching-induced distortion of the weldment.


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Reduction of Carbon Content

Low-carbon grades such as 304L and 316L

stainless steels can be used.

These stainless steels are designed to have

less than 0.035% C to reduce susceptibility to

weld decay


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Addition of Strong Carbide Formers

Elements such as titanium (Ti) and niobium

(Nb) have a higher affinity for C than Cr and

thus form carbides more easily than Cr.

Types 321 and 347 stainless steels are

essentially identical to 304 stainless steel

except for the addition of Ti and Nb,

respectively.

welding of stainless steel -sndo kou

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