1. Core Objectives of PWHT for Q460NH in Low-Temperature Use
Residual stress: Welding creates uneven thermal expansion/contraction, leaving high residual stress in the weld and heat-affected zone (HAZ). In low temperatures, this stress concentrates at flaws (e.g., microcracks) and triggers brittle fracture.
Brittle microstructure: Rapid cooling post-welding may form hard, brittle martensite in the HAZ. PWHT softens this phase and refines grains to restore toughness.
Hydrogen trapping: Welding absorbs hydrogen (from moisture, flux, etc.), which can cause "delayed cracking" in high-strength steels like Q460NH-especially in low temperatures where hydrogen diffusion slows and accumulates at stress points.
2. When PWHT is Required (Mandatory Scenarios)
Thick-walled components: For Q460NH plates/welds with a thickness ≥25mm (or cumulative weld size ≥20mm), PWHT is mandatory. Thick sections trap more residual stress and cool slower unevenly, increasing HAZ brittleness.
High-load or safety-critical structures: Components bearing dynamic loads (e.g., bridge supports, pressure vessels) or used in extreme low temperatures (≤-30°C) require PWHT to ensure long-term toughness and stress relief.
Welds with high hydrogen content: If low-hydrogen consumables (e.g., E5515-G) were not used, or welding occurred in high-humidity environments (≥80% RH), PWHT is needed to "bake out" trapped hydrogen and prevent delayed cracking.
3. Recommended PWHT Process Parameters for Q460NH
Heating rate: ≤200°C/h (slower for thick sections, e.g., ≤150°C/h for ≥50mm thickness) to prevent thermal shock and uneven expansion.
Hold temperature: 550–620°C (critical range). Below 550°C, residual stress is not fully relieved; above 620°C, the steel's strength decreases (yield strength may drop below 460MPa) and weathering elements (Cu, Cr) may segregate, weakening the protective patina.
Hold time: 1–2 hours (or 1 hour per 25mm of thickness) to ensure uniform heat penetration and microstructure transformation.
Cooling rate: ≤150°C/h until cooling to ≤300°C, then allow natural cooling to room temperature. Rapid cooling after PWHT can reintroduce stress or re-form brittle phases.
4. Cases Where PWHT May Be Exempt (With Strict Precautions)
Thin sections: Welds in plates ≤12mm thick, with single-pass or small multi-pass welds (cumulative size ≤10mm) that cool quickly and trap minimal stress.
Low-risk service: Components used in mild low temperatures (-10°C to -20°C) with static, light loads (e.g., non-load-bearing curtain wall frames).
Optimal welding controls: Use of low-hydrogen consumables, preheating (100–150°C), controlled heat input (20–35kJ/cm), and post-weld slow cooling (≤50°C/h) to minimize stress and HAZ brittleness.
Post-weld inspection validation: Ultrasonic testing (UT) confirms no internal cracks, and low-temperature Charpy tests (at service temp) show weld/HAZ toughness ≥27J.
5. Key Post-PWHT Inspections
Mechanical property testing: Recheck the weld/HAZ's low-temperature impact toughness (Charpy V-notch test) and yield strength to confirm they meet Q460NH's specs (≥460MPa yield, ≥27J toughness at service temp).
Non-destructive testing (NDT): Repeat UT/MT on welds to detect any cracks formed during PWHT (e.g., from improper heating/cooling rates).
