3.2 Cracks — Types, Causes, and Significance

The Most Serious Discontinuity

** The reason is mechanical: a crack has an extremely sharp tip, producing the highest stress-concentration factor of any discontinuity, so it can propagate under static, cyclic, or impact loading toward brittle fracture. ** You should also know that cracks are categorized by where they sit relative to the weld — weld-metal cracks lie within the deposited metal, while base-metal cracks lie in the heat-affected zone or unaffected base metal — because the cause and corrective action differ for each. 1 always rejectable.

Classification by Temperature

Classification by Location and Orientation

Hot Cracking — Mechanism

Solidification (centerline) cracking occurs as the weld pool freezes. Elements such as sulfur and phosphorus form low-melting-point eutectic films that remain liquid at grain boundaries after the bulk has solidified. As the weld contracts, these liquid films cannot carry the tensile shrinkage strain and a crack opens. A deep, narrow bead (high depth-to-width ratio) drives impurities to the centerline and promotes this crack. Liquation cracking is the HAZ cousin: partially melted grain boundaries open under strain, common in austenitic stainless and nickel alloys.

Controls: keep S and P low, avoid deep narrow beads, lower heat input, and (for stainless) target a few percent ferrite in the weld metal.

Cold (Hydrogen-Induced) Cracking — Three Factors

HIC requires three conditions simultaneously; remove any one and it cannot occur:

1. Diffusible hydrogen — from moisture in flux/coating, oil, grease, paint, rust, or humid air.
2. Susceptible (hard) microstructure — typically martensite formed by fast cooling of higher-carbon/alloy steel.
3. Tensile stress / restraint — from shrinkage, fixturing, and section thickness, concentrated at notches like the weld toe.

It is delayed — it can appear hours after welding once hydrogen diffuses, which is why critical welds are sometimes held before final inspection. Controls: preheat and control interpass temperature (slows cooling, lets hydrogen diffuse out, avoids martensite), use low-hydrogen electrodes stored in rod ovens, clean the joint, and reduce restraint. Higher-carbon and alloy steels with elevated carbon equivalent (CE) are most prone, because they harden readily on fast cooling; the CE value is a key driver of the required preheat.

The delayed nature is a classic exam point — a weld can pass visual inspection immediately yet crack overnight, so HIC is sometimes called delayed cracking.

Crater Cracks and Lamellar Tearing

Crater cracks form where the welder breaks the arc: the crater cools fast with a high surface-to-volume ratio, and multidirectional shrinkage opens a star crack. Prevent them with proper crater-fill technique (pause, backstep, or use a crater-fill control). Lamellar tearing is a step-like separation in rolled plate loaded in the through-thickness (Z) direction, exploiting elongated sulfide/silicate inclusions parallel to the rolling plane (common in T- and corner joints).

Controls: specify Z-quality (through-thickness tested) steel, use low-sulfur plate, redesign to reduce through-thickness strain, and apply buttering layers.

Exam essentials: Cracks are essentially never acceptable under AWS D1.1. Distinguish hot (solidification, S/P films, high temperature) from cold/HIC (hydrogen + martensite + stress, delayed, underbead/toe). Crater cracks are the most common preventable type; lamellar tearing is a base-metal failure, not a weld-metal failure.

Crack Types the Exam Tests

Beyond the temperature classification, the CWI must place each named crack with its dominant mechanism and location, because cause and corrective action differ:

The Three Conditions for Hydrogen-Induced Cracking

HIC — also called cold cracking or delayed cracking — requires three conditions simultaneously, and removing any one prevents it. This is one of the most heavily tested points in welding metallurgy:

1. Diffusible hydrogen — from moisture in flux or electrode coating, oil, grease, paint, rust, or humid air.
2. Susceptible (hard) microstructure — typically martensite formed when higher-carbon or high-carbon-equivalent steel cools too fast through the transformation range.
3. Tensile stress / restraint — from weld shrinkage, fixturing, and section thickness, concentrated at notches such as the weld toe.

The standard controls each attack one leg of that triangle: preheat and interpass temperature slow cooling so hydrogen diffuses out and martensite is avoided; low-hydrogen electrodes stored in rod ovens and clean joints remove the hydrogen source; and joint redesign reduces restraint. Steels with elevated carbon equivalent (CE) harden readily on fast cooling, so CE is the key driver of the required preheat. The delayed nature is the classic trap: a weld can pass visual inspection immediately and still crack overnight.

Cracks Are Prohibited Regardless of Size

Under AWS D1.1, cracks are essentially never acceptable — any crack of any length, in the weld metal or the HAZ, is a reject regardless of size. There is no aggregate-length or size allowance the way there is for porosity or slag. The mechanical reason is that a crack has the sharpest possible tip and the highest stress-concentration factor of any discontinuity, so it can propagate under static, cyclic, or impact loading toward brittle fracture. That is why crack-prevention controls — preheat, low-hydrogen practice, crater fill, and Z-quality steel for through-thickness loading — are emphasized so heavily in code work.