D1.5 Bridge Preheat Calculator

Non-fracture-critical (NFC) preheat is simpler — D1.5 Table 6.3 groups steels into two categories by strength and assigns preheat by thickness only. Fracture-critical (FC) preheat from Clause 12 (Tables 12.4–12.8) adds two more variables: diffusible hydrogen level (H4, H8, or H16) and heat input (kJ/mm or kJ/in). FC preheat temperatures are generally higher because fracture-critical members have no redundant load path — a single crack can cause structural failure.

Yes. D1.5 Clause 6 requires low-hydrogen electrodes for all SMAW on bridge steels — non-low-hydrogen SMAW is not permitted. This is stricter than D1.1, which allows non-low-hydrogen SMAW for Category A steels. For fracture-critical members, D1.5 Clause 12 further requires hydrogen control verified by H4, H8, or H16 designators per AWS A4.3.

Check your electrode or flux packaging for the H-designator: H4 (≤4 mL/100g), H8 (≤8 mL/100g), or H16 (≤16 mL/100g). Lower hydrogen means lower preheat — H4 electrodes allow the lowest FC preheat temperatures. If you cannot verify the H-designator, use H16 (highest preheat) as a conservative default. HPS690W (Table 12.8) requires H4 or H8 only — H16 is not permitted.

D1.5 fracture-critical preheat tables use heat input in kJ/mm (metric) or kJ/in (US customary). For Tables 12.4–12.7, three bands apply: low (1.2–2.0 kJ/mm), medium (2.0–2.8 kJ/mm), and high (>2.8 kJ/mm). Table 12.8 (HPS690W) uses five narrower bands from 1.2 to >3.6 kJ/mm. Calculate heat input as: (Voltage × Amperage × 60) ÷ (Travel Speed in mm/min × 1000).

HPS690W (HPS100W) is a very high-strength bridge steel that is highly susceptible to hydrogen-induced cracking. Table 12.8 prohibits combinations where low heat input meets thick material or high heat input meets thin material — both create unfavorable thermal conditions. Low heat input on thick plate causes rapid cooling (high cracking risk), while high heat input on thin plate causes excessive softening of the heat-affected zone.