6.2 Hot & Solidification Cracking

6.2 Hot & Solidification Cracking

Quick Answer: Hot cracks form at high temperature while the weld is solidifying or while HAZ grain boundaries are liquated—not hours later at ambient temperature. Solidification cracking hits the weld metal; liquation cracking hits the HAZ. Contrast with hydrogen cold cracking, which is delayed after cooling.

WT2.5 groups several high-temperature crack mechanisms. For Standard-level inspection, master two names, their locations, the main risk factors, and how procedure and chemistry prevent them. You do not need full solidification-modelling depth (IWI-C / metallurgist territory), but you must recognise centreline cracks, crater cracks, and HAZ liquation-type defects as process signals.

Hot Cracking vs Cold Cracking — Timing Contrast

This table is high-yield for WTE/WIE: if the crack formed while the pool was freezing, think hot; if it appeared overnight after a hard HAZ cooled, think cold.

Solidification Cracking (Weld Metal)

Solidification cracking (hot cracking in the fusion zone) occurs when the last liquid films between solidifying dendrites cannot support the tensile strain imposed by solidification shrinkage and restraint. The crack follows the solidification structure—often the weld centreline in single-pass or last-pass beads, or interdendritic paths.

Why chemistry matters

Alloys with a wide freezing range (large gap between liquidus and solidus) spend longer with liquid films present. Impurity elements that segregate to last-freezing liquid—especially sulphur (S) and phosphorus (P) in steels—lower the melting point of residual films and wet grain/dendrite boundaries. High carbon and some alloy combinations can worsen segregation behaviour depending on the system.

In stainless and nickel alloys, solidification mode and impurity/segregant control remain critical; the inspector’s general rule is the same: wrong filler or high dilution of dirty parent metal can put the weld metal into a crack-sensitive composition.

Dilution and parent-metal contribution

Dilution is the fraction of weld metal that came from melted parent plate rather than filler. High dilution on contaminated, high-S free-cutting steels, or on castings with residual elements, can push weld-metal chemistry into a sensitive range even when the filler wire itself is clean. Procedure controls that limit unnecessary penetration into questionable base metal, or that use buttering layers with a tolerant filler, are engineering responses—inspectors verify they are followed when specified.

Bead shape and heat input pattern

Deep, narrow beads (high depth-to-width ratio) concentrate centreline segregation and tensile strain at the mid-plane of the bead—classic solidification-crack geometry. Excessively high travel speed with low energy can produce tear-drop pools that also favour centreline cracking. Conversely, overly wide, thin beads may create other problems; the WPS heat-input and technique windows balance fusion against crack morphology.

Crater cracks at weld stops are a common solidification-crack variant: the crater freezes under high shrinkage strain with a concave free surface. Prevention includes proper crater fill, back-step, or crater-fill current decay as trained and specified.

Liquation Cracking (HAZ)

Liquation cracking occurs in the partially melted zone / HAZ when grain-boundary regions of low melting temperature remelt (liquate) during the weld thermal cycle. On cooling, those liquid films open under residual tensile strain, producing cracks along HAZ grain boundaries adjacent to the fusion line.

Susceptible situations include:

• Certain aluminium alloys and some nickel alloys (highly examinable in advanced modules)
• Steels with grain-boundary segregants or previous weld metal reheated by multipass thermal cycles
• High heat input that widens the liquation-prone zone combined with high restraint

For IWI-S steel fabrication work, treat liquation as the HAZ hot-crack counterpart to weld-metal solidification cracking: same temperature regime family, different location and metallurgical detail.

Risk Factors Checklist

1. Wide freezing range of weld metal or liquated boundary films
2. S and P (and related impurities) above levels the alloy system can tolerate
3. High dilution of crack-sensitive or contaminated parent metal
4. Deep, narrow bead geometry and unfavourable pool shape
5. High restraint and high tensile strain during the last stages of freezing
6. Poor crater technique at terminations
7. Wrong filler for the alloy (e.g. filler that solidifies in a sensitive mode)

Prevention — Chemistry, Procedure, Bead Shape

Chemistry control

• Select parent materials and consumables with controlled impurity limits for the service and process
• Match filler solidification behaviour to the alloy (especially stainless/nickel—follow qualified WPS)
• Avoid welding free-machining high-S steels with procedures meant for clean structural grades without metallurgical review

Procedure and technique

• Stay inside WPS heat-input and travel-speed windows that produce acceptable bead shape
• Prefer bead shapes that are not excessively deep and narrow for the joint
• Use stringer or weave as qualified—do not improvise a “digging” technique that creates a knife-like profile
• Fill craters; avoid leaving concave, unfilled stops on restrained joints

Restraint and sequence

• Improve fit-up to reduce forced gap and forced alignment stress where practical
• Use welding sequences that balance shrinkage
• For critical alloys, follow any preheat or interpass rules that the procedure uses for hot-crack control (note: preheat logic differs from pure HICC control—always follow the WPS, not a generic rule of thumb)

Inspector Recognition and Actions

What you may see

• Centreline cracks on the bead surface or revealed by NDT
• Crater cracks at stop positions
• Fine HAZ cracks near the fusion line after multipass welding (liquation-type)
• Cracks present immediately after the pass solidifies—not only after overnight delay

What you check

1. Consumable identity vs WPS (wrong filler is a chemistry problem).
2. Parent material grade and any free-machining or residual-element flags on material certs.
3. Parameters affecting pool shape (current, voltage, speed, technique).
4. Crater practice and stop/start locations relative to high-stress details.
5. Whether repairs repeatedly remelt the same crack-sensitive zone without procedure change.

When a hot crack is found, do not assume the same repair recipe as for a hydrogen crack. Root cause may be chemistry or bead shape; simply “preheating more” without addressing solidification behaviour can fail.

Multipass Reheat and Hot Cracking Awareness

In multipass welds, earlier passes can be reheated into temperature ranges that liquate sensitive boundaries or redistribute residual stress. Hot cracks can therefore appear not only in the final bead but in reheated weld metal or HAZ of previous passes. This is another reason procedures specify pass sequence, heat input, and sometimes interpass temperature limits for crack-sensitive alloys.

Summary for Exam and Site

• Solidification crack → weld metal, during freezing, centreline/interdendritic/crater.
• Liquation crack → HAZ, liquid films on grain boundaries, high temperature.
• Cold hydrogen crack → after cooling, delayed, four-condition model.

Keep those three stories separate and you will handle most WT2.5 cracking questions and most shop-floor first diagnoses correctly.