1. Welding Metallurgy
This module will build upon the previous metallurgy modules and will start to look at the metallurgical factors that are effected by the presence of a weld in parts.

2. When you finish this lesson you will understand:
Learning Activities
Read Handbook pp
View Slides;
Read Notes,
Listen to lecture
Do on-line workbook
Do homework
Lesson Objectives
When you finish this lesson you will understand:
The various region of the weld where liquid forms
Mechanisms of cracking associated with these regions
Keywords
Composite Zone, Hot Cracking, Constitutional Supercooling, Unmixed Zone, Partially Melted Zone, Constitutional Liquation,

3. Introduction
Materials Behavior
Weldability is the capacity of a material to be welded under the fabrication conditions imposed, into a specific, suitably designed structure, and to perform satisfactorily in the intended service.
Materials compatibility
Process
Response to stress and strain during welding
A good example of weldability is seen in the task of joining copper to steel. These materials cannot be arc welded together; there is a basic material incompatibility between the two. However, these two materials can be joined together easily by friction welding.
Weldability is also an issue in the welding of newer materials such as intermetallics or even in high-strength, low-alloy (HSLA) steels. The ordered arrangement of the atoms in the intermetallics is disrupted by welding. In HSLA steels, the precipitate structure that strengthens the steel is also disrupted by welding. T

4. Basic Regions of a Weld Fusion Zone - area that is completely melted
Introduction
Basic Regions of a Weld
Fusion Zone - area that is completely melted
Heat-Affected Zone - portion of the base metal not melted but whose mechanical properties and microstructure were affected by the heat of the joining process
Base Metal
Fusion zone
Base metal
Heat-affected zone
The fusion and heat-affected zones of welded joints can exhibit very different mechanical properties from the base metal. In steels, the heat-affected zone (HAZ) near the fusion line can be brittle after welding. Preheat and post-weld heat treatments are often employed to mitigate this problem. Aluminum alloys can exhibit softening of the HAZ after welding. T

5. Composite Zone Concerns
The three regions of the weld can be more finely divided and the distinctions of where one region ends and another begins is somewhat blurred as indicated above. The composite region is where complete mixing of the molten base metal and molten filler metals occur. The unmixed region, although small in most cases is where the base metal melts but the turbulent stirring in the weld metal is insufficient to effect complete mixing of the two materials. The partially melted region is between the fully molten zones and the heat affected zone and consists of intermittent liquid and solid. The true heat affected zone is the region whose properties or structure has been effected by the heat of the weld. We will be looking at factors which effect structure and properties in each of these regions, starting with the composite liquid metal region.

6. Solidification (Hot) Cracking
Cracking in Welds
Solidification (Hot) Cracking
Solidification (hot) cracking requires
Low ductility material
High tensile contraction stress
Solidification occurs over a range of temperature
Low melting point intergranular films
Sulfur, phosphorus, boron
Prevention by
Low C, S, P levels
Increased Mn
The first concern we will consider that occurs in the composite region is solidification cracking or sometimes called hot cracking. Weld geometry and impurity elements can contribute to solidification cracking. Welds with a depth-to-width ratio greater than 2:1 are susceptible to solidification cracking due to the buildup of excessive transverse stress. This is especially noted in submerged arc welding, which exhibits deep penetration.
Excess sulfur and phosphorus drastically lower the solidification temperature of steel. Thus, complete solidification occurs at a much lower temperature along the weld centerline, where sulfur and phosphorus tend to segregate. Residual tensile stress develops as the weld cools; this stress can lead to centerline cracking, because the high sulfur and phosphorus areas are weak and may not have completely solidified.
The cracked region must be cut out beyond the visible end of the crack and re-welded. T

7. As noted earlier, in the process of making a weld, a thermal distribution is set up around the traveling weld. Since material expands and contracts under the action of moving thermal gradients, we saw previously that a distribution of stress also occurs. This stress profile tends to be compressive in front of the moving weld, but it tends to be tensile behind the weld where solidification is simultaneously occurring. The speed of welding, the overall heat input and preheat, and the type of material welded can all have an effect on this temperature and stress profiles. The tensile stresses in the region of solidification are significant since it is tensile stress which tend to pull material apart and this would be the case in the hot and weakly solidified areas.

8. Introductory Welding Metallurgy,
Before proceeding we need to examine a little more closely the process of weld solidification as illustrated here. The welding process is a bit unusual in solidification in that the liquid weld metal is contained within the base metal, sort of in its own imaginary very hot mold which is made of the same material (or nearly the same material) as the composite weld metal. Solidification starts at the fusion line and the nucleation site for the start of solidification is the identical or nearly identical grains from the heat affected zone of the base metal. This type of nucleation is called epitaxial nucleation. In epitaxial nucleation, the crystal structure present in the hot solid heat affected zone is transferred to the initial dendritic crystals which begin to extend into the liquid weld metal (the lattice structure is identical across the fusion line but not necessarily from dendrite to dendrite. After nucleation, the dendrite grains continue to grow into the liquid metal, those which are more favorably oriented grow faster and pinch off the less favorable oriented grains until the entire liquid is consumed.
Introductory Welding Metallurgy,
AWS, 1979

9. Phase Diagram When Interstitial Carbon Alloys with Iron
There is, however, a limit to how much of this second element can be absorbed into the first. This limit is dependent on both the temperature and the crystal structure present at that temperature. The diagram above shows this limit of solubility for carbon in iron and is called the iron-carbon phase diagram. This will become critically important in the remainder of our discussions on weldability of iron base alloys.
Note that the delta ferrite can absorb about 0.1% carbon at the temperature where maximum carbon content is allowed. The lower temperature ferrite can only absorb 0.02% carbon into its lattice structure. But the austenite can absorb more than 1.6& carbon into its lattice structure if the temperature is about 2000F.
Also note that if more carbon is present than can be absorbed in these single phase crystal lattices, they are forced to go into two phases. In other words, between every single phase region on this diagram lies a two phase region. More will be said about the transition between phases later.

10. The solidification occurs when a single phase (I. e
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.

11. 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.

12. 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.

13. 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.

14. 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.

15. When this last to solidify material is present along a plane of weakness as illustrated in the top weld bead, centerline cracking occurs. The top weld represents the shape of the weld puddle under rapid travel speed conditions. The lower puddle shape is for slower travel speeds. Note the absence of the clearly defined plane of centerline weakness in the slower weld. Thus one method to reduce hot cracking is to adjust the weld travel speed to slower rates. This can lead to losses in weld economic efficiency, so other options must also be explored.

16. Perils of Welding Free-Machining Steels
Solidification cracking due to impurity elements
Sulfur, phosphorus, boron
Lead doesn’t seem to cause a problem, e.g. 12L14
Impurity segregation at weld centerline creates low ductility area
Combines with shrinkage stress to cause cracking
Excess sulfur and phosphorus drastically lower the solidification temperature of steel. Thus, complete solidification occurs at a much lower temperature along the weld centerline, where sulfur and phosphorus tend to segregate. Also, sulfur and other impurity elements, when segregated to the weld centerline, lower the mechanical properties of this region. During solidification and subsequent cooling of the weld, tensile shrinkage stress develops., Solidification cracking occurs 1) because the unsolidified, low-melting-point centerline region has no strength, or 2) the magnitude of the shrinkage stress overcomes the low strength of the centerline region, which has a high impurity content. The cracked region must be cut out beyond the end of the crack and rewelded. T

17. Manganese Can Prevent Solidification Cracking
Steel
Manganese Can Prevent Solidification Cracking
Manganese combines with sulfur to form MnS particles
Use a filler metal with higher manganese to absorb sulfur
Filler metals or flux-wire combinations that result in higher manganese contents in the weld metal are noted for reducing solidification cracking. This occurs through two mechanisms:
1) Manganese combines with sulfur to form manganese sulfide particles. This process effectively reduces the amount of free sulfur available to segregate to the centerline where it would otherwise lower the solidification temperature and result in a low-strength region.
2) Manganese atoms have a strengthening effect on steel as they substitute onto the iron crystal structure. This results in weld metal with a somewhat higher strength during the cooling process.

18. Questions?
Turn to the person sitting next to you and discuss (1 min.):
Constitutional supercooling works for alloys with K values less than 1.0 but what happens for alloys with K greater than 1.0 (I.e. rising liguidus with increasing temperature)? Can you draw the three corves?

19. Unmixed Zone Concerns
The unmixed zone results because the liquid material immediately adjacent to the fusion line does not get caught in the turbulent action of the remainder of the weld metal as the electrode melts and mixes with the molten base metal. This region can often be seen in micro etched cross sections of welds, but is rarely associated with any defects of substantive nature.

20. Sometimes when cracks occur in the heat effected zone and open up into the unmixed region, this liquid gets sucked back into the cracks repairing the cracked region but again etching slightly different than the surrounding metal because of its varying composition. Some false crack indications may be noted by certain non-destructive testing techniques.

21. Questions?

22. Partially Melted Zone Concerns
The partially melted zone is the hottest region of the heat effected zone where mixtures of solid and liquid coexist.

23. In any region of material there can be found slight localized variations in chemistry due to various inhomogeneous reactions and processing routes which materials have seen in past processing. We have already seen that variations in chemistry lead to variations in effective liquidus temperature, that is that melting can occur in some regions where the effective liquidus is lower than the instantaneous temperature at that point. This is represented in the diagram above. Unfortunately, in this same hottest region of the heat affected zone is the pla ce where grain growth is also accentuated. When these growing grains intersect pools of liquid, the liquid tends to penetrate the grain boundaries and under the action of solidification shrinkage stresses, the liquefied boundary can open up into networks of microcracks. Thus the partially melted region can cause some weldability concern.

24. Another mechanism for melting in the partially melted region has also been identified and it is called “constitutional liquation”. In this mechanism, specific particles which can dissociate and part of their elemental components diffuse into the matrix may under certain circumstances produce a composition with low meting eutectic compositions. When this happens, a liquid puddle forms surrounding the particle. This liquid will remain until the entire particle is dissolved. If in the meantime, an advancing grain boundary intersects the partially melted particle, once again liquid can penetrate the boundary and cracking occur.

25. Questions?
Turn to the person sitting next to you and discuss (1 min.):
What sequence of event needs to take place for constitutional liquation to result in liquid films? Can we use a phase diagram to predict if this will happen or not?