Preheat and Interpass Temperature

1. Cold Cracking — The Underlying Problem

Cold cracking (also called hydrogen-induced cracking — HIC, or delayed cracking) results from the simultaneous interaction of three factors:

1. Susceptible microstructure — martensite or upper bainite in the HAZ (correlated with CE)
2. Diffusible hydrogen — introduced by the arc during welding (moisture, coatings, oils)
3. Residual stresses — generated by thermal gradients and joint restraint

Fracture can occur hours or days after welding, at temperatures below 200 °C. Preheat acts on all three factors: it reduces the cooling rate (preventing martensite), increases time available for hydrogen diffusion and escape, and reduces residual stresses.

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2. Preheat Calculation — EN 1011-2 Method

2.1 Carbon equivalent CET

$$CET = C + \frac{Mn + Mo}{10} + \frac{Cr + Cu}{20} + \frac{Ni}{40}$$

Different from the IIW CE formula — CET is specific to the Annex A method of EN 1011-2 and better suited for fine-grain and high-strength steels.

2.2 Preheat temperature

$$T_p = 697 \times CET + 160 \times \tanh\left(\frac{t}{35}\right) + 62 \times HD^{0{.}35} + (53 \times CET - 32) \times Q - 328$$

where:

• t = thickness of the thickest joint element (mm)
• HD = diffusible hydrogen content of the consumable (ml/100 g deposited metal)
• Q = heat input (kJ/mm)

2.3 Numerical example

Steel S355 (CET = 0.32); t = 25 mm; HD = 5 ml/100 g (low-hydrogen electrode); Q = 1.2 kJ/mm:

$$T_p \approx 223 + 113 + 99 + (-14{.}9) - 328 \approx \mathbf{92\ °C} \rightarrow \text{round up to } \mathbf{100\ °C}$$

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3. Diffusible Hydrogen — Classification and Control

EN ISO 3690 defines the method for measuring diffusible hydrogen (HD). Consumable classification by HD level:

Practical control measures:

• Dry basic electrodes: 300–350 °C for 1–2 h; keep in holding oven at 150 °C on site
• Clean joint surfaces: remove moisture, oil, rust and paint
• Dry shielding gases: dew point ≤ −40 °C for critical applications

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4. Interpass Temperature (T_ip)

4.1 Minimum interpass temperature

As a rule, equal to the preheat temperature — do not allow the joint to cool below T_p between passes.

4.2 Maximum interpass temperature

4.3 Measurement

• Contact thermometer (thermocouple): most accurate; apply at 75 mm from the fusion line
• Temperature-indicating crayon (Tempilstik): quick on-site check; use the correct grade for each temperature
• Thermal imaging camera: temperature mapping on complex components

EN ISO 13916 defines measurement and documentation requirements for preheat and interpass temperature.

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5. Preheat Maintenance

Preheat must be maintained across the full joint and adjacent zones (minimum 75 mm each side), throughout the welding operation and until PWHT or hydrogen release treatment begins.

For thick or highly restrained joints, a post-weld hydrogen release treatment may be required: hold at 200–300 °C for 2–4 h to allow hydrogen to escape before full cooling. Particularly relevant for steels with CE > 0.45 and thicknesses above 50 mm.

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6. Records and Documentation

The WPS must specify minimum T_p, maximum T_ip, measurement method and maintenance requirements. Temperature records form part of the quality dossier required by EN 1090-2 (steel structure execution) and EN 13445 (pressure vessels).

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References

1. EN 1011-2:2001 — *Welding. Recommendations for welding of metallic materials. Part 2: Arc welding of ferritic steels.* CEN, Brussels.
2. EN ISO 13916:1996 — *Welding. Guidance on the measurement of preheating temperature, interpass temperature and preheat maintenance temperature.* ISO/CEN.
3. EN ISO 3690:2018 — *Welding and allied processes. Determination of hydrogen content in arc weld metal.* ISO/CEN.
4. Lancaster, J.F. — *Metallurgy of Welding.* 6th ed. Abington Publishing, Cambridge, 1999.
5. Chiaverini, V. — *Tecnologia Mecânica: Estrutura e Propriedades das Ligas Metálicas.* Vol. 1. McGraw-Hill, São Paulo.