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Design and Metallurgy of Weld Joints (MEM-510)1 - 1

Phase Transformations In WeldingDr. Chaitanya SharmaPhase Transformations

Lesson ObjectivesIn this chapter we shall discuss the following:Weld CCT diagramsCarbon equivalent-preheating and post heating weldability of low alloy steelsWelding of stainless steels. Schaffler and Delong diagrams; welding of cast irons, Cu; Al; Ti and Ni alloys. Processes- difficulties; Microstructures; defects and remedial measures.Learning ActivitiesLook up KeywordsView Slides; Read Notes, Listen to lectureKeywords:

CCT Diagram for WeldContinuous-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 steelCCT Diagram for Weld continuedAt 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 steelMicrostructure 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.

Weldability

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Weldability of steelWeldability 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 byCeq =%C + % Mn/6 + (% Cr + % Mo + % V)/5+(% Ni + % Cu)/15The 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 - 8Schaeffler DiagramSchaeffler 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 modeDeLongs DiagramDeLong refined Schaefflers diagram to include nitrogen, a strong austenite former.

Fig: Delong diagram for predicting weld ferrite contents and solidification modeAlso, 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.1 - 11

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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.TTT Diagram Carbon Steel

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