Design and Metallurgy of Weld Joints

(MEM-510)

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Phase Transformations

In Welding

Dr. Chaitanya Sharma

Phase Transformations

Lesson ObjectivesIn this chapter we shall discuss the following:

1. Weld CCT diagrams

2. Carbon equivalent-preheating and post

heating weldability of low alloy steels

3. Welding of stainless steels.

4. Schaffler and Delong diagrams;

5. welding of cast irons, Cu; Al; Ti and Ni

alloys.

6. Processes- difficulties;

7. Microstructures; defects and remedial

measures.

Learning Activities

1. Look up Keywords

2. View Slides;

3. Read Notes,

4. Listen to lecture

Keywords:

CCT Diagram for Weld

• Continuous-cooling transformation (CCT) diagrams explain

development of weld metal microstructure of low-carbon, low-alloy

steels .

• The hexagons represent the transverse cross sections of columnar

austenite grains in the weld metal. As austenite (g) is cooled down

from high temperature, ferrite (a) nucleates at the grain boundary

and grows inward.

• The grain boundary ferrite is

also called “allotriomorphic”

ferrite, meaning that it is a

ferrite without a regular

faceted shape reflecting its

internal crystalline structure.

Fig: CCT diagram for weld metal of low carbon steel

CCT Diagram for Weld

continued…• At lower temperatures mobility of the planar growth front of the grain

boundary ferrite decreases and Widmanstatten ferrite, also called side-

plate ferrite, forms instead.

• These side plates can grow faster because carbon, instead of piling up at

planar growth front, is pushed to sides of the growing tips. Substitutional

atoms do not diffuse during the growth of Widmanstatten ferrite.• At even lower temperatures it is too

slow for Widmanstatten ferrite to

grow to the grain interior and it is

faster if new ferrite nucleates

ahead of the growing ferrite.

• This new ferrite, that is, acicular

ferrite, nucleates at inclusion

particles and has randomly oriented

short ferrite needles with a basket

weave feature. Fig: CCT diagram for weld metal of low carbon steel

Microstructure of Weld Metal

Fig: Micrographs showing typical weld metal microstructures in low-carbon steels: A, grain boundary ferrite; B, polygonal ferrite; C,Widmanstatten ferrite; D, acicular ferrite; E, upper bainite; F, lowerbainite.

Weldability

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Weldability of steel

• Weldability of steel is closely related to the amount of carbon in

steel.

• Weldability is also affected by the presence of other elements.

• The combined effect of carbon and other alloying elements on the

weldability is given by “carbon equivalent value (Ceq)”, which is

given by

Ceq =%C + % Mn/6 + (% Cr + % Mo + % V)/5+(% Ni + % Cu)/15

• The steel is considered to be weldable without preheating, if Ceq

< 0.42%.

• However, if carbon is less than 0.12% then Ceq can be tolerated

upto 0.45%.

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Schaeffler Diagram

• Schaeffler constitution diagram (shown in fig), provide

quantitative relationship between the composition and ferrite

content of the weld metal.

• It also helps in predicting solidification mode.

• The chromium equivalent of

a given alloy is determined

from the concentrations of

ferrite formers Cr, Mo, Si,

and Cb,

• The austenite equivalent is

determined from the

concentrations of austenite

formers Ni, C, and Mn.

Fig: Schaeffler diagram for predicting weld ferrite contents and solidification mode

DeLong’s Diagram

• DeLong refined Schaeffler’s diagram to include nitrogen, a

strong austenite former.

Fig: Delong diagram for predicting weld ferrite contents and solidification mode

• Also, the ferrite

content is expressed

in terms of the ferrite

number, which is

more reproducible

than the ferrite

percentage and can

be determined

nondestructively by

magnetic means.

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TTT Diagram Carbon Steel

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