Calculate minimum preheat temperature, carbon equivalent, and interpass requirements for welding steel — to BS EN 1011, AWS D1.1, and ASME Section IX.
Hydrogen-induced cracking (HIC), also called cold cracking or delayed cracking, is one of the most serious weld defects in structural and pressure vessel fabrication. It occurs when three conditions coincide: sufficient hydrogen (from moisture in consumables, base metal surface, or the arc atmosphere), a susceptible microstructure (martensite or bainite in the heat-affected zone), and tensile stress. Preheating the base metal before welding slows the cooling rate, reduces the amount of hard martensite formed in the HAZ, and gives hydrogen more time to diffuse out of the weld zone before it can cause damage.
The minimum preheat temperature required depends on the steel's hardenability (expressed through carbon equivalent), plate thickness (which determines the heat input per unit volume and the cooling rate), heat input from the welding procedure, and the hydrogen content of the consumable. BS EN 1011-2 provides two methods: Method A uses CE IIW and a series of nomographs; Method B uses the CET formula and gives a more precisely calculated minimum temperature. Our calculator implements both and highlights where the two methods diverge significantly.
Post-weld heat treatment (PWHT) is a separate requirement from preheat, governed by the applicable construction code (PD 5500, ASME VIII, EN 13480). The calculator flags when PWHT is mandatory based on material group, thickness, and hardness limits — useful for welding procedure qualification documents (WPQ/WPAR) and weld maps.
Carbon equivalent is a single number that represents the combined hardenability contribution of all alloying elements in a steel, referenced to carbon. The IIW formula (CE IIW) is: CE = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15. A steel with CE below 0.40 is generally considered readily weldable without preheat in moderate thicknesses; CE above 0.45 requires careful preheat management. The CET formula (used in BS EN 1011-2 Method B) is similar but uses different divisors and is considered more accurate for modern microalloyed steels: CET = C + (Mn+Mo)/10 + (Cr+Cu)/20 + Ni/40.
The H designators refer to the maximum diffusible hydrogen content in the deposited weld metal: H5 = max 5 ml/100g, H10 = max 10 ml/100g, H15 = max 15 ml/100g. Low hydrogen electrodes (H5, H10) allow lower or no preheat compared to basic electrodes at H15. Cellulosic (E6010/E6011) electrodes are inherently high hydrogen (>15 ml/100g) and require more preheat. Submerged arc welding with properly dried flux achieves H5 or better. When in doubt, assume H10 for arc processes with controlled consumables.
S355 typically has a CE IIW of 0.39–0.47 depending on the sub-grade and plate thickness. For thin plate (under 25 mm) with a low hydrogen process (H5–H10), preheat is often not required. For thicker sections (50 mm+) or high restraint joints, a preheat of 75–125 °C is commonly specified. S355J2+N and S355NL grades have tighter carbon limits than S355JR and are generally more weldable. Always verify against the actual mill certificate rather than relying on the grade designation alone — the calculator accepts direct CE input from the material test certificate.
PWHT requirements depend on the applicable code, material, and thickness. Under PD 5500 (unfired pressure vessels), PWHT is mandatory for carbon and carbon-manganese steels above 35 mm nominal thickness, and for Cr-Mo steels (P11, P22, P91) at any thickness. Under ASME VIII Div 1, P-No.1 steels require PWHT above 38 mm (1.5 inches). PWHT is also used to reduce residual stresses in highly restrained structures, improve toughness in the HAZ, and meet hardness limits (typically max 250–350 HV10 depending on service). The calculator flags mandatory PWHT thresholds and suggests typical soak temperatures and hold times.
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