Question 1

What is welding heat input and why does it matter?

Accepted Answer

Welding heat input (HI) is the amount of heat energy deposited per unit length of weld: HI = (V × I × η) / v, where V = arc voltage (V), I = welding current (A), η = thermal efficiency of the process, v = travel speed (mm/s or mm/min with appropriate conversion). Units: kJ/mm or kJ/m. Heat input matters because: Too high → excessive grain growth, HAZ softening, distortion, reduced toughness, sensitisation of stainless steel. Too low → poor fusion, hydrogen cracking (for hardenable steels), incomplete penetration. WPS (Welding Procedure Specification) qualification always specifies min/max heat input range per AWS D1.1, SANS 10287, or ISO 15609.

Question 2

What is thermal efficiency η for different welding processes?

Accepted Answer

Thermal efficiency (η) accounts for heat lost to radiation, spatter, and arc instability: GTAW (TIG): η = 0.60. SMAW (Stick): η = 0.80. GMAW (MIG/MAG): η = 0.85. FCAW: η = 0.85. SAW (Submerged Arc): η = 1.00 (fully submerged arc, highest efficiency). These values are specified in ISO 3834 / BS EN 1011 / AWS D1.1. Using the wrong η will give an inaccurate HI and may lead to failed WPS qualification or unexpected HAZ properties.

Question 3

How do I calculate the minimum preheat temperature?

Accepted Answer

Preheat prevents hydrogen-induced cracking (HIC/HAC) by slowing the cooling rate and allowing hydrogen to diffuse out. The Dearden-O'Neill Carbon Equivalent formula: CE = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15. Preheat guideline per CE: CE < 0.35: No preheat needed. CE 0.35–0.45: Preheat to 50–100°C. CE 0.45–0.60: Preheat to 100–200°C. CE > 0.60: Preheat to 200–300°C. South African practice: SANS 10287 also considers heat input and diffusible hydrogen content (HD). Always verify preheat requirement against the WPS and material certificate.

Question 4

What are common heat input limits for different steels in South Africa?

Accepted Answer

Heat input limits depend on material and application standard: Structural steel (SANS 10287): 0.5–3.5 kJ/mm. Fine grain structural steel (S355J2): max 2.5 kJ/mm. Q&T wear plate (Hardox, Abratech): max 1.0–1.5 kJ/mm. Duplex stainless (2205): 0.5–2.5 kJ/mm to maintain ferrite/austenite balance. Austenitic SS: max 1.5 kJ/mm to prevent sensitisation and sigma phase. Chrome-moly (P91, 2.25Cr1Mo): 1.0–3.0 kJ/mm; PWHT essential. South African mining projects often specify ASME IX or AWS D1.1 with local addenda for extreme environment conditions.

Question 5

How does travel speed affect weld quality and heat input?

Accepted Answer

Travel speed directly affects heat input — doubling travel speed halves heat input: HI ∝ 1/v. Slow travel speed: Higher heat input → wider HAZ, more distortion, risk of burn-through on thin plate. Fast travel speed: Lower heat input → narrower HAZ, but risk of lack of fusion, porosity, and undercut. Optimal speed for GMAW: 200–600 mm/min for most structural work. For SAW: 400–800 mm/min. Automated/robotic welding controls speed precisely — this is a key advantage for reproducible heat input in WPS-qualified procedures.

Question 6

What is deposition rate and how is it calculated?

Accepted Answer

Deposition rate (DR) = mass of weld metal deposited per unit time: DR = Wire feed speed (m/min) × Wire cross-section area (m²) × Wire density (kg/m³). In practical units: DR (kg/h) = WFS (m/min) × (π/4) × D² (m²) × ρ (kg/m³) × 60. Example: 1.2mm wire, 9 m/min WFS, steel density 7.85 g/cm³: Area = π/4 × (0.0012)² = 1.131 × 10⁻⁶ m². DR = 9 × 1.131 × 10⁻⁶ × 7850 × 60 = 4.79 kg/h. Deposition efficiency also considers spatter loss: GMAW: 90–98%, SMAW: 60–75%, SAW: 100%.

Question 7

What welding standards apply in South Africa?

Accepted Answer

Key South African and international welding standards: SANS 287 (ISO 5817 equivalent): Weld quality levels B, C, D for fusion welds. SANS 10287: Welding of structures. AWS D1.1: Structural welding — steel (widely used in mining and industrial). ASME IX: Boilers and pressure vessels (petrochemical, power generation). ISO 9606-1: Welder qualification. ISO 15614-1: WPS qualification for steel. ISO 3834: Quality requirements for fusion welding. NDT: SANS 10197 (radiography methods) and SANS 462 (UT). South African Welding Academy (SAIW) is the local body for welder and inspector certification.

Question 8

How do I control heat input when welding wear-resistant plate?

Accepted Answer

Wear-resistant steels (Hardox, Abratech, Everhard): Have high CE (0.4–0.7+). High heat input causes HAZ softening — the hardened zone loses its wear resistance. Maximum heat input: typically 1.0–1.5 kJ/mm. Essential precautions: Use low-hydrogen electrodes (E7018, E7016). Preheat 75–150°C depending on CE and thickness. Maximum interpass temperature 150–200°C. Cool slowly (add insulating blanket after welding). For weld overlays: use the minimum passes possible with optimal heat input to balance deposition rate and metallurgical requirements.

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