What Is Hot Cracking?

Hot cracking is a weld defect that occurs at elevated temperatures during or immediately after solidification of the weld metal. It forms when tensile stresses (from shrinkage) act on the weld while thin liquid films still exist between solidifying grains, tearing the grains apart before they can bond. Hot cracking is also called solidification cracking when it occurs in the weld metal, or liquation cracking when it occurs in the heat-affected zone (HAZ).

Type	Location	Mechanism
Solidification cracking	Weld metal centerline or between columnar grains	Low-melting-point phases (sulfides, phosphides) form liquid films at grain boundaries during solidification
Liquation cracking	HAZ, immediately adjacent to the fusion line	Grain boundary phases in the base metal partially melt from welding heat, then crack under shrinkage stress
Ductility-dip cracking	Weld metal (multi-pass)	Loss of ductility at intermediate temperatures (700-900 deg C) in certain nickel alloys

Key Factors

Factor	Effect on Hot Cracking
Sulfur and phosphorus	Form low-melting-point eutectics that remain liquid between grains; S >0.015% and P >0.015% increase risk significantly
High depth-to-width ratio	Deep, narrow weld beads concentrate shrinkage stress at the centerline
High restraint	Increases tensile stress on the solidifying weld
High welding speed	Creates a teardrop-shaped weld pool that promotes centerline segregation
Alloy composition	Austenitic stainless steels, nickel alloys, and aluminum alloys are more susceptible than ferritic steels
Primary solidification mode	Austenitic (A mode) is more susceptible than ferritic (FA mode) in stainless steels

Prevention Methods

Strategy	How It Helps
Control S and P	Specify base metal and filler with S <0.010% and P <0.015%
Select proper filler metal	Use filler that solidifies in FA mode (ferritic-austenitic) for austenitic stainless steels; e.g., 308L/309L with adequate ferrite
Reduce depth-to-width ratio	Use wider, shallower weld passes; limit current and increase voltage slightly
Reduce restraint	Use balanced welding sequences, smaller tack welds, and allow contraction freedom
Reduce travel speed	Creates an elliptical pool shape that pushes impurities to the surface instead of the centerline
Convex bead profile	A slightly convex cap resists centerline cracking better than a concave one

Pro Tip

For austenitic stainless steel welds, verify that the filler metal produces 3-10 FN (Ferrite Number) in the as-deposited weld metal. A small amount of delta ferrite breaks up the continuous austenite grain boundary network and dramatically reduces solidification cracking susceptibility.

Detection

Hot cracks typically form along the weld centerline or at crater ends and are often visible during visual inspection. For subsurface or tight cracks, radiographic or ultrasonic testing is used. Surface-breaking hot cracks are confirmed by liquid penetrant testing (PT), which is the preferred surface NDE method for stainless steel (MT is not effective on non-magnetic austenitic grades).

Hot cracking is repaired by grinding out the crack completely and re-welding with adjusted parameters or a different filler metal.

Read the full guide to fillet welds