Carbon Equivalent and Preheat: Preventing Hydrogen-Induced Cracking

Why Hydrogen-Induced Cracking Matters

Hydrogen-induced cracking (HIC)—also called cold cracking, delayed cracking, or hydrogen-assisted cracking—is one of the most dangerous weld defects in structural and pipe welding because it can appear hours or even days after welding is complete. A weld that looks clean at the end of a shift may crack overnight as hydrogen diffuses through the heat-affected zone (HAZ) and residual stresses act on hard, brittle microstructure. In structural steel and pressure-containing pipe work, an undetected crack of this kind can lead to catastrophic failure. Understanding carbon equivalent and preheat is your first line of defense.

What Makes Steel Crack?

Three conditions must exist together for HIC to occur:

1. A susceptible microstructure — high-hardness martensite in the HAZ, driven by carbon and alloy content

2. Hydrogen present in the weld area — sourced from moisture on electrodes, base metal, or the atmosphere

3. Tensile stress — residual stress from weld shrinkage, or applied load

Eliminate any one of these and HIC cannot occur. Preheat addresses all three: it slows the cooling rate (reducing martensite), allows hydrogen to diffuse out before it can do damage, and reduces thermal stress gradients.

Carbon Equivalent (CE): Measuring Susceptibility

The International Institute of Welding (IIW) formula gives a single number that combines the hardening effect of carbon and alloying elements:

CE = C + Mn/6 + (Cr + Mo + V)/5 + (Cu + Ni)/15

All values are in weight percent, taken directly from the Material Test Report (MTR) that accompanies each heat of steel. The MTR is the certified chemical analysis of the actual steel you are welding—not a handbook average. Always use MTR data, not nominal specification limits, when calculating CE for a real production weld.

Interpreting CE:

• CE below approximately 0.40: Generally low susceptibility; preheat may not be required for thinner sections
• CE 0.40–0.60: Moderate susceptibility; preheat is typically required
• CE above 0.60: High susceptibility; preheat is essential and must be carefully controlled

These are general guidelines. The actual minimum preheat temperature is determined by consulting the preheat table in AWS D1.1 (or ASME Section IX for pipe), which accounts for both CE and the combined plate thickness at the joint.

Reading an MTR

An MTR lists each element as a percentage by weight. You will see columns for C, Mn, P, S, Si, Cr, Mo, V, Cu, Ni, and others depending on the grade. Not every element appears in every steel—for example, A36 mild steel typically contains negligible chromium, molybdenum, vanadium, copper, and nickel, so those terms in the CE formula contribute nearly zero. A514 high-strength quenched-and-tempered plate and 4130 chrome-moly pipe contain significant Cr and Mo, making their CE values substantially higher.

Preheat and Interpass Temperature

Preheat temperature is the minimum temperature of the base metal in the joint area immediately before the arc is struck. AWS D1.1 provides a preheat and interpass temperature table that groups steels by minimum specified yield strength and lists required minimum preheat based on thickness. Thicker sections require higher preheat because more mass conducts heat away faster, driving a faster cooling rate.

Interpass temperature is the temperature of the weld and base metal between passes in a multi-pass weld. AWS D1.1 sets both a minimum (same as preheat, to keep hydrogen mobile and microstructure controlled) and, for some steels, a maximum (to prevent grain growth or loss of toughness).

Measuring preheat: Use a calibrated contact thermometer or temperature-indicating sticks (such as Tempilstik). Measure at least 3 inches from the weld joint on both sides. Temperature-indicating sticks melt at their rated temperature; if the mark smears rather than stays solid, the surface is at or above that temperature.

Setting Up Preheat Safely

Preheat is applied with a propane rosebud torch or an induction preheat blanket. Both methods require strict safety discipline:

• PPE required at all times: Wear leather welding gloves, a welding jacket, and a face shield when operating a torch. Induction blankets eliminate open flame but the metal becomes hot enough to cause severe burns.
• Fire hazard: Clear the area of combustibles before lighting a torch. Never leave an open flame unattended.
• Ventilation: Propane combustion consumes oxygen and produces carbon monoxide. Ensure adequate ventilation in the work area.
• Lockout/tagout (LOTO): If an induction preheat unit is connected to facility power, follow LOTO procedures when connecting or disconnecting.
• Hot metal marking: After preheating, mark the plate and the surrounding area as HOT. Heated steel looks identical to cold steel.
• Instructor sign-off is required before any arc is struck on a preheated coupon. This is a safety gate, not a formality.

From Calculation to Code Compliance

On a real job site, the welding procedure specification (WPS) lists the required preheat for each material and thickness. That WPS was qualified under AWS D1.1 or ASME Section IX using exactly the kind of CE analysis and preheat table lookup you will practice today. By learning to do this calculation yourself, you understand why the WPS says what it says—and you are prepared to recognize when a procedure has been written correctly and when something does not look right.