Welding Procedure Specifications — Preheat, WPS & Qualification

AWS D1.1 prequalified vs qualified-by-test WPS, preheat temperature requirements, carbon equivalent (CE), heat input control, welder qualification, and post-weld heat treatment (PWHT).

What is a WPS?

A Welding Procedure Specification (WPS) is a written document that defines all essential variables for producing a sound weld: base metal type, filler metal, joint design, position, preheat, interpass temperature, current, voltage, travel speed, shielding gas, and technique. Every structural weld must be made in accordance with a qualified WPS.

AWS D1.1 provides two paths to WPS qualification:

• Prequalified WPS (Clause 3) — for common combinations of base metal, filler metal, joint design, and process. No testing required. The engineer simply documents that all prequalified variables are satisfied. This covers the vast majority of building connections using SMAW, GMAW, FCAW, and SAW processes with standard steels (A36, A572 Gr. 50, A992).
• Qualified-by-test WPS (Clause 4) — required when any essential variable falls outside the prequalified range. The fabricator must perform PQR (Procedure Qualification Record) testing: weld test plates, perform mechanical tests (tensile, bend, CVN), and document results. Common triggers include non-prequalified steels, non-standard joint designs, and electroslag welding.

Preheat requirements

Preheat slows the cooling rate of the weld and heat-affected zone (HAZ), reducing the risk of hydrogen-induced cracking (cold cracking). AWS D1.1 Table 3.3 specifies minimum preheat based on:

• Steel category — grouped by carbon equivalent and thickness.
• Thickness — thicker material requires higher preheat because it acts as a larger heat sink.
• Hydrogen level — low-hydrogen processes (E70XX-H8, FCAW-G) require less preheat than high-hydrogen processes.

These values assume low-hydrogen electrodes (H8 or H16 designation). Using non-low-hydrogen electrodes (e.g., E6010, E6011) increases the preheat requirement significantly — often to 150-200 deg C for Group II steels over 25 mm thick.

Carbon equivalent

The carbon equivalent (CE) predicts a steel's susceptibility to hydrogen-induced cracking. Higher CE means harder HAZ, greater cracking risk, and higher preheat requirements.

CE(IIW) = C + Mn/6 + (Cr + Mo + V)/5 + (Cu + Ni)/15

For A992 steel (typical mill cert): C = 0.08, Mn = 1.0, Cr = 0.05, Mo = 0.02, V = 0.03, Cu = 0.20, Ni = 0.05. CE = 0.08 + 1.0/6 + 0.10/5 + 0.25/15 = 0.08 + 0.167 + 0.02 + 0.017 = 0.28.

CE < 0.40 indicates low cracking risk. CE = 0.40-0.50 is moderate. CE > 0.50 is high risk requiring careful preheat and hydrogen control.

The Pcm formula is preferred for modern low-carbon steels: Pcm = C + Si/30 + (Mn + Cu + Cr)/20 + Ni/60 + Mo/15 + V/10 + 5B. For the same A992 steel: Pcm = 0.08 + 0.25/30 + (1.0+0.20+0.05)/20 + 0.05/60 + 0.02/15 + 0.03/10 + 0 = 0.08 + 0.008 + 0.063 + 0.001 + 0.001 + 0.003 = 0.156. Pcm < 0.20 confirms low cracking susceptibility.

Heat input

Heat input controls the weld cooling rate and affects grain structure, toughness, and distortion. It is calculated as:

H = (V x I x 60) / (S x 1000) [kJ/mm]

where V = voltage (V), I = current (A), S = travel speed (mm/min).

Typical heat input ranges:

• SMAW: 1.0-3.0 kJ/mm
• GMAW (spray): 0.8-2.5 kJ/mm
• FCAW: 1.0-3.5 kJ/mm
• SAW: 2.0-6.0 kJ/mm

Excessive heat input (> 3.5 kJ/mm for most structural steels) causes grain coarsening in the HAZ, reducing toughness. This is critical for steels requiring Charpy V-notch (CVN) toughness, such as those specified for seismic or cold-temperature service.

Worked example — preheat determination

Joint: CJP groove weld, beam flange to column flange. Beam: W24x76 (A992, tf = 15.9 mm). Column: W14x283 (A992, tf = 44.3 mm). Process: FCAW-G with E71T-1C H8 electrode.

Governing thickness = column flange = 44.3 mm (the thicker part controls preheat). Steel group II (A992). From AWS D1.1 Table 3.3 for Group II, 38-64 mm thickness, low-hydrogen (H8): minimum preheat = 66 deg C (150 deg F).

In practice, many fabricators use 100 deg C (212 deg F) for thick moment-frame joints regardless of the minimum, because the additional margin virtually eliminates hydrogen cracking risk.

Interpass temperature (maximum): AWS D1.1 does not specify a maximum interpass temperature for Group I and II steels, but AISC 341 and most seismic specifications cap it at 260 deg C (500 deg F) to avoid excessive grain growth. For quenched-and-tempered steels (A514), maximum interpass is typically 200 deg C (400 deg F).

International preheat standards

EN 1011-2 uses a more sophisticated approach than AWS D1.1, combining carbon equivalent, plate thickness, heat input, and hydrogen content in nomograms to determine preheat. This generally produces lower preheat values than AWS D1.1 tables for low-CE modern steels.

Common pitfalls

• Skipping preheat on thick joints in warm weather. A 50 mm column flange at 25 deg C ambient still requires 66 deg C preheat per AWS D1.1. Warm weather does not eliminate the need for preheat — the steel must be heated above the minimum before welding starts.
• Using non-low-hydrogen electrodes for critical connections. E6010 and E6011 electrodes deposit hydrogen-rich weld metal. Using them on A992 steel thicker than 25 mm without increased preheat causes cold cracking, often appearing 24-48 hours after welding.
• Not monitoring interpass temperature. If a multi-pass weld cools below the preheat temperature between passes, the faster cooling rate can cause hydrogen cracking in the previously deposited passes. The minimum interpass temperature equals the preheat temperature.
• Confusing prequalified WPS with no WPS at all. A prequalified WPS still must be documented in writing, listing all essential variables. "We always do it this way" is not a WPS. AWS D1.1 Clause 3.6 requires the written document to be available at the welding station.

Run this calculation

• Welded Connections Calculator
• Section Properties Database

Related references

• Minimum Weld Size
• Weld Electrodes
• Weld Joint Types
• Weld Inspection
• Connection Design Workflow
• Steel Material Properties
• How to Verify Calculations

Disclaimer

This page is for educational and reference use only. It does not constitute professional engineering advice. All design values must be verified against the applicable standard and project specification before use. The site operator disclaims liability for any loss arising from the use of this information.