This document discusses phase transformations that occur during welding of different materials. It covers topics like weld CCT diagrams, carbon equivalent calculations for preheating requirements of steels, welding metallurgy of stainless steels, and Schaeffler and DeLong diagrams. The objectives are to understand weld metal microstructure development, factors affecting weldability, and predicting weld metal phase constitution. Keywords discussed include CCT diagrams, carbon equivalent values, Schaeffler and DeLong diagrams, and microstructures like grain boundary ferrite and Widmanstatten ferrite.
1. Design and Metallurgy of Weld Joints
(MEM-510)
1 - 1
Phase
Transformations
In Welding
Dr. Chaitanya Sharma
2. Phase Transformations
Lesson Objectives
In 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:
3. 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
4. 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
5. 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, lower
bainite.
8. 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%.
1 - 8
9. 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
10. 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.
The solidification occurs when a single phase (I.e. liquid) solidifies into two phases (I.e. solid plus liquid). As we learned previously in solid transformations between a single phase to two solid phases, there was a redistribution of solute (in the previous case, the diffusion of carbon as austenite transformed to ferrite plus austenite). So it is with the solidification of a material. In the figure above, the liquid at composition Co produces solid material which forms at the start of the solidification process with a composition kCo, where k is called the distribution coefficient. Since the solid has slightly less %B than the liquid, the liquid immediately in front of the advancing solid-liquid interface get slightly enriched in %B. This continues until a solute spike is produced, the peak composition being Co/k. Thereafter the solute spike gets pushed ahead of the solid-liquid interface until solidification is completed.
The distribution coefficient k in the previous case represented a phase diagram as illustrated in the lower right portion of this figure where the liquidus line as illustrated has a negative slope. The solute spike is seen in the upper figure, and the resulting effective liquidus curve corresponding to the composition of the solute spike over distance is in the lower left curve. Note that over a region called the region of constitutional supercooling, the actual temperature of the liquid is lower than the effective liquidus. This means that material of this composition over this constitutional supercooled region wants to instantaneously solidify. A condition like this make the dendrites immediately jump to a solidified distance “y” as illustrated on the lower left diagram.
For a phase diagram where the liquidus has a positive slope a similar spike, however in the opposite direction, as illustrated in figures b & d results, but the constitutional supercooled region remains the same. (Try it out by redrawing the previous slide with a positive liquidus and the depressed spike as in illustration d.
When the extend of the supercooled region gets larger, the dendrite morphology transforms more from the cells as illustrated on the left toward multiple branched dendritic represented by the figure on the right. Because solute is redistributed between the core of the dendrites and the boundaries where the last liquid to solidify resides, there is also a solute distribution between the solid core and the mostly liquid boundaries. Note that for the negative sloped liquidus, this results in a spike at the cell and dendrite boundaries (higher in the dendrites). Reinvestigating the phase diagram, these spikes result in liquid with lower effective melting temperatures. That means that liquid tends to remain at the dendrite boundaries with the core of the dendrites being solid. Solid material can support tensile stresses do to solidification shrinkage mentioned previously, but liquid interdendritic films can not support tensile stresses. The result is hot tears occurring along the dendrite boundaries.
When the solidifying interface reaches the middle of the weld, it meets the approaching interface from the other side with its solute spike in advance. The result is a combination of the two solute spikes, and an even more lowering of the effective liquidus temperature of this last to solidify material located in the final interface boundary.