Weld Metal Solidification: Nucleation, Grain Growth and Grain Boundaries

Weld metal solidification is the foundational metallurgical event that determines the microstructure — and therefore the mechanical properties — of every completed weld. The transition of the weld pool from liquid metal back to a solid crystalline structure is not an instantaneous event; it is a precisely ordered sequence of nucleation, grain growth, solute redistribution, and grain boundary formation, all occurring within seconds as the arc moves on. For the welding engineer, inspector, or metallurgist, a thorough understanding of this solidification sequence is the essential starting point for diagnosing weld defects, specifying heat input limits, designing post-weld heat treatments, and selecting consumables that promote the grain morphology best suited to the service condition.

The microstructure that crystallises from the weld pool is directly controlled by two variables that the welder and welding engineer can influence: the thermal gradient (how steeply temperature falls away from the pool) and the solidification rate (how fast the solidification front advances). Together these two parameters determine whether the weld metal develops coarse columnar grains, fine equiaxed grains, or the acicular ferrite microstructure that delivers the highest combination of strength and toughness in steel welds. The heat-affected zone (HAZ) alongside the weld pool undergoes its own equally critical microstructural changes — particularly grain coarsening in the region immediately adjacent to the fusion line — which directly determine the Charpy toughness of the completed weldment.

The Weld Pool and the Solidification Front

To understand solidification it is first necessary to understand the thermal environment within which it occurs. The weld pool is a small volume of liquid metal — typically only a few millimetres deep and a few centimetres long depending on the process and parameters — maintained in the liquid state by the continuous energy input of the welding arc. The temperature at the centre of the pool is significantly above the liquidus temperature of the alloy. At the periphery of the pool, temperature falls sharply to the liquidus and then to the solidus, below which solid metal exists.

As the arc advances, the leading edge of the pool continuously melts new base metal, while the trailing edge continuously solidifies. The solidification front — the interface between liquid and solid — advances from the fusion boundary inward toward the pool centreline at a rate directly coupled to the welding travel speed. The geometry of this front, and the temperature gradient across it, govern everything about the resulting grain structure.

The Two Critical Solidification Parameters

Every solidification phenomenon — nucleation mode, grain morphology, grain size, segregation — is ultimately controlled by two variables at any point on the solidification front:

Nucleation Mechanisms in Weld Metal

Solidification cannot begin until a stable nucleus — a small cluster of atoms in the solid crystalline arrangement — forms within the liquid. Nucleation requires that atoms overcome an energy barrier related to the free energy cost of creating a new solid-liquid interface. There are two principal nucleation mechanisms: homogeneous and heterogeneous.

Homogeneous Nucleation

Homogeneous nucleation occurs when a nucleus forms spontaneously within the bulk liquid without the aid of any substrate or pre-existing surface. It requires very large degrees of supercooling — typically tens to hundreds of degrees below the melting point — because all the atoms forming the nucleus must overcome the full interfacial energy barrier without any substrate assistance. In practice, homogeneous nucleation is extremely rare in weld metal. The weld pool contains abundant impurities and inclusions, and its entire boundary is in contact with solid metal, so nucleation always occurs heterogeneously.

Heterogeneous Nucleation

Heterogeneous nucleation occurs at pre-existing surfaces, where the solid-liquid interfacial energy is reduced because the substrate provides a partial template for atomic arrangement. In the weld pool, heterogeneous nucleation occurs at:

• Non-metallic inclusions — oxide and silicate particles within the weld pool serve as nucleation sites. This mechanism is exploited in the production of acicular ferrite (see below).
• The fusion boundary — the solid HAZ grains at the pool perimeter provide the lowest-energy nucleation site of all, because the atomic arrangement of the substrate is crystallographically identical to the phase that is forming. This leads to epitaxial nucleation.
• Inoculants — certain filler metal additions or shielding gas compositions can introduce particles that act as nucleation sites, refining grain size. This is one mechanism by which consumable design influences weld microstructure.

Epitaxial Nucleation: The Primary Mechanism

Epitaxial nucleation is by far the most important nucleation mechanism in fusion welding. At the fusion boundary, the pre-existing solid HAZ grain acts as a perfect substrate — the solidifying weld metal atom adopts exactly the same crystallographic orientation as the grain beneath it. No nucleation energy barrier needs to be overcome because no new interface is created; the crystal simply continues growing from where the parent HAZ grain ended.

This means that the first solid to form in the weld metal is crystallographically continuous with the HAZ parent grain. In metallographic sections, this is observed as weld metal grains that appear to be a direct continuation of the HAZ grains across the fusion boundary — there is no sharp microstructural discontinuity at the fusion line in terms of crystallographic orientation. This epitaxial relationship also means that the grain structure of the HAZ directly seeds the grain structure that develops in the weld metal.

Grain Growth: Columnar and Equiaxed Morphologies

Once nucleation has occurred — primarily epitaxially at the fusion boundary — grains begin to grow by the continuous addition of atoms from the liquid to the solid front. The direction and morphology of this growth is governed by two competing factors: the crystallographic preference of each grain (which direction its atoms prefer to add along) and the thermal gradient in the surrounding liquid (which direction offers the lowest undercooling and therefore the highest driving force for growth).

Columnar Grain Formation

Grains whose fastest-growth crystallographic direction (the <100> direction in cubic metals such as steel) is closely aligned with the local direction of maximum heat extraction grow preferentially and rapidly. Grains with unfavourable orientations grow more slowly and are eventually pinched off and consumed by their faster-growing neighbours. The result is a process of competitive grain growth that produces long, column-shaped grains — the columnar grain zone — that extend from the fusion boundary inward toward the pool centreline, with their long axes approximately perpendicular to the fusion boundary (and thus parallel to the dominant heat flow direction).

In multi-pass welds, successive passes can partially remelt and partially re-epitaxially seed the columnar grains of the previous pass. If columnar grains from multiple passes align, their elongated boundaries can create a continuous preferential crack path known as a continuous columnar grain boundary — a condition that increases the susceptibility to hot cracking and also provides a path for crack propagation during service loading.

Equiaxed Grain Formation

Equiaxed grains — roughly equal-sized grains without a preferred growth direction — form when the thermal gradient ahead of the advancing solidification front decreases to the point where constitutional supercooling occurs over a substantial volume of liquid ahead of the front. Constitutional supercooling develops when solute elements are rejected ahead of the solidification front, locally depressing the liquidus temperature of the remaining liquid. If the actual temperature of this solute-enriched liquid falls below its locally depressed liquidus temperature, the liquid is supercooled — creating a driving force for nucleation of new solid grains anywhere in that volume, not just at the existing solid front.

Equiaxed grains generally appear at the centre of the weld pool, where the thermal gradient is lowest and constitutional supercooling is greatest. Their formation at the centreline is highly beneficial because they interrupt the continuous columnar grain boundaries that would otherwise extend all the way to the centreline and concentrate segregated impurities there.

Acicular Ferrite: The Ideal Steel Weld Microstructure

In carbon and low-alloy steel weld metals, the solidification microstructure is not the final microstructure — the weld metal undergoes a further solid-state transformation from austenite (FCC) to ferrite (BCC) as it cools through the transformation range. The ferrite morphology that forms during this transformation determines the final toughness of the weld metal far more than the original solidification grain morphology.

Acicular ferrite is a fine, randomly oriented, interlocking needle-like ferrite that nucleates intragranularly at fine non-metallic oxide inclusions within the prior austenite grains of the weld metal. Because it forms at many nucleation sites simultaneously and grows in many directions, acicular ferrite divides the large prior austenite grains into many small, randomly oriented crystals. The interlocking nature of the acicular ferrite needles also provides a tortuous crack path that absorbs energy very efficiently, giving excellent Charpy impact values.

Solute Redistribution and Hot Cracking

During solidification, solute elements (carbon, manganese, sulphur, phosphorus, silicon, and others) are distributed unequally between the liquid and solid phases. The ratio of the concentration in the solid to the concentration in the liquid at a given temperature is the partition coefficient (k). For most alloying elements in steel, k is less than 1 — the growing solid rejects solute into the adjacent liquid, progressively enriching the remaining liquid ahead of the solidification front.

This enrichment of the liquid in rejected solute elements (particularly sulphur, phosphorus, and carbon) has two important consequences:

Constitutional Supercooling

As noted above, the solute-enriched liquid ahead of the front has a locally lower liquidus temperature than the bulk liquid. If actual temperature falls below this depressed liquidus, constitutional supercooling promotes equiaxed nucleation and grain refinement. This is a beneficial effect of segregation.

Hot Cracking and Centreline Segregation

As solidification progresses inward from both sides of the weld, columnar grains converge at the weld centreline. The last liquid to solidify in a weld — enriched in rejected solutes and low-melting-point impurities — is trapped at the centreline grain boundaries. At this stage the solid skeleton is almost fully solidified and the residual liquid exists only as a thin, continuous film along grain boundaries. Welding-induced tensile stresses, which are significant even during the solidification process, can cause this liquid film to rupture, opening a hot crack along the centreline.

Grain Boundaries: Formation, Properties and Significance

A grain boundary is the planar defect that forms at the interface between two adjacent grains of different crystallographic orientation. When two growing grains meet, neither can continue to grow without disrupting the crystal structure of the other. The meeting zone — typically only 2 to 5 atomic diameters wide — is a region of atomic mismatch and high internal energy: the atoms in this narrow zone cannot adopt the low-energy ordered arrangement of either grain.

Grain boundaries have several metallurgically important properties that are directly relevant to weld behaviour:

• Higher atomic energy: The misfit strain at grain boundaries makes them more reactive sites for corrosion, carbide precipitation, and diffusion.
• Dislocation barriers: At room temperature, grain boundaries impede dislocation movement, which is the Hall-Petch strengthening mechanism.
• Preferential diffusion paths: Hydrogen, carbon, and other interstitial species diffuse significantly faster along grain boundaries than through the grain interior — grain boundary diffusion is often several orders of magnitude faster than lattice diffusion at low temperatures.
• Sites for precipitation: Chromium carbide precipitation along austenite grain boundaries during sensitisation of stainless steel, and martensite nucleation at prior austenite grain boundaries during cooling of steel — both occur preferentially at grain boundaries. For more on sensitisation, see the stainless steel weld decay guide.
• Creep weakness at high temperature: At elevated temperatures, grain boundaries become mobile and can slide, which is the primary mechanism of creep deformation. This is why coarser-grained structures are preferred for high-temperature service such as in power plant components made from P91 chromium-molybdenum steel.

The Hall-Petch Relationship: Grain Size and Mechanical Properties

The quantitative relationship between grain size and yield strength in metals is described by the Hall-Petch equation, one of the most important empirical relationships in physical metallurgy:

The Hall-Petch relationship shows that grain size control is not merely a theoretical nicety — it has quantitatively large effects on strength and toughness. In the context of weld procedure qualification, mechanical testing is required precisely to verify that the grain structure produced by the welding procedure delivers the required properties, even when the actual grain size is not directly measured.

Grain Size and Charpy Toughness

The relationship between grain size and Charpy impact energy is even more pronounced than the relationship with yield strength. Coarser grains increase the length of individual cleavage facets, requiring less energy to propagate brittle fracture. They also raise the ductile-to-brittle transition temperature (DBTT) — a coarse-grained HAZ will exhibit brittle fracture at temperatures where a fine-grained structure of the same composition would still be ductile and tough. For more detail on Charpy impact testing requirements under ASME codes, see the UG-84 Charpy impact test requirements guide.

Grain Growth in the Heat-Affected Zone

While the weld metal solidifies and develops its new grain structure, the heat-affected zone (HAZ) — the solid base metal adjacent to the fusion line that did not melt — undergoes significant microstructural changes driven by thermal energy alone. The most severe of these changes is grain coarsening in the coarse-grained HAZ (CGHAZ).

Mechanism of Grain Growth

Grain growth occurs when larger grains consume smaller neighbours by migrating grain boundaries. The driving force is the reduction in total grain boundary surface energy (which is proportional to the total grain boundary area). At high temperatures, grain boundary atoms have sufficient thermal energy to detach from the smaller grain and reattach to the larger one — the boundary migrates, the smaller grain shrinks and eventually disappears, and the larger grain grows. The rate of grain boundary migration increases exponentially with temperature.

In steel, grain growth is normally limited at lower temperatures by grain boundary pinning particles — fine carbides, nitrides (particularly AlN in Al-killed steel), and carbonitrides that sit on grain boundaries and physically obstruct their movement. When the HAZ temperature exceeds the dissolution temperature of these pinning precipitates (typically above approximately 1000°C to 1100°C depending on the steel composition), the pinning effect is lost and grain growth becomes very rapid.

The Four Characteristic HAZ Sub-Zones

The HAZ is not a uniform zone — it comprises several distinct sub-zones that experienced different peak temperatures during the weld thermal cycle and therefore have different microstructures and properties:

Controlling Grain Size: Practical Levers for the Fabricator and Inspector

A thorough understanding of solidification and grain growth is only useful if it translates into practical controls that the fabricator and inspector can apply and verify. The following are the principal controls available, along with their mechanism of action and the inspection variables that correspond to each.

1. Heat Input Control

Heat input is the single most important variable controlling grain size in both the weld metal and the CGHAZ. Lower heat input produces steeper thermal gradients, faster solidification, faster cooling through the grain growth temperature range, and therefore finer grain structures. Welding procedure specifications (WPS) should specify maximum heat input limits, not merely voltage, current, and travel speed independently.

ASME Section IX does not directly limit heat input in its qualification requirements, but WPS essential variables include changes in heat input above or below qualification limits for impact-tested applications. AWS D1.1 Clause 6.2 sets heat input verification requirements for prequalified and engineer-qualified weld procedures. For submerged arc welding (SAW), heat input management is particularly important because the process inherently delivers high heat input and proportionally larger CGHAZ widths.

2. Interpass Temperature Control

In multi-pass welding, the temperature of the previously deposited weld and adjacent HAZ at the time the next pass is deposited — the interpass temperature — acts as an additional preheat. A high interpass temperature extends the time spent in the grain growth temperature range, promotes grain coarsening, and slows the cooling rate through the transformation range. Maximum interpass temperatures must be controlled and measured by calibrated contact thermometers or temperature-indicating crayons, particularly for impact-tested applications. For hardenable steels such as P91, the interpass temperature window is also bounded from above to limit grain growth and from below to prevent cold cracking.

3. Multi-Pass Welding Strategy

Each subsequent weld pass reheats the HAZ of the previous pass. If the inter-pass peak temperature falls in the FGHAZ range for the previous pass CGHAZ, the coarse grains of the previous CGHAZ are refined by re-austenitisation and transformation to fine ferrite. This thermally-induced grain refinement by subsequent passes is a significant advantage of multi-pass procedures over single-pass high heat input deposits, and is why narrow groove, multi-pass procedures are often specified for toughness-critical applications.

4. Post-Weld Heat Treatment (PWHT and Normalising)

Standard PWHT at subcritical temperatures (below Ac1) tempers martensite, relieves residual stress, and reduces hardness, but does not refine grain size. Full grain refinement requires normalising — heating above Ac3 (full austenitisation) followed by air cooling — which produces an entirely new fine-grained microstructure throughout the weldment. Normalising is specified in ASME Section VIII Division 1 for certain material grades and thicknesses, and in ASME B31.1 for some service conditions. Note that for P91 and similar creep-resistant materials, the PWHT protocol is a mandatory tempering treatment (not normalising) to achieve the specific tempered martensite microstructure required for creep resistance. Full details are in the heat treatment guide.

5. Grain-Refining Alloying Additions

Fine carbide and nitride precipitates — particularly aluminium nitride (AlN), niobium carbonitride (Nb(C,N)), titanium nitride (TiN), and vanadium carbonitride (V(C,N)) in microalloyed HSLA steels — pin grain boundaries and prevent grain growth during welding. TiN is particularly effective because its dissolution temperature (~1400°C) is significantly higher than that of most other carbides and nitrides, allowing it to maintain a grain-pinning effect even in the near-fusion-line portion of the CGHAZ. This is why titanium additions at the 20 to 100 ppm level are incorporated into many modern high-toughness structural steels. The carbon equivalent calculator accounts for the niobium, vanadium, and titanium content when assessing hardenability.

Recommended Books on Weld Solidification and Metallurgy

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