Preheat is where many WPS documentation problems begin. The code table looks simple — a grid of steel types, thicknesses, and temperatures — but applying it correctly to a specific joint requires four separate decisions that the table alone does not make for you. Get any one of them wrong and your preheat documentation is either non-conservative (a quality risk) or over-conservative (a productivity cost).
What Table 6.3 Actually Controls
AWS D1.1:2025 Table 6.3 specifies minimum preheat and interpass temperatures. It applies to prequalified WPS and serves as the default starting point for qualified WPS — though a WPS qualified by test can document lower preheat if the PQR demonstrates acceptable mechanical properties at the reduced temperature.
The table is organized by:
- Base metal category (from Table 6.9 groupings — broadly Group I through Group V)
- Thickness of the thickest member at the joint
- Process heat input category (high or low)
The intersection gives you the minimum temperature in °F [°C]. Anything below that value before striking the arc is a prequalification violation or a WPS nonconformance.
Rule library based on AWS D1.1:2025; verify against your governing edition (the AHJ or contract may specify 2020 or earlier).
Step 1 — Identify the Correct Steel Category
Table 6.9 lists structural steels by ASTM designation and assigns each to a group (I through V). Most common structural steels — A36, A572 Grade 50, A992, A500 — fall in Group I or Group II and carry the lowest preheat requirements. High-strength steels such as A514 and A517 fall in Group IV and require significantly higher preheat.
The critical mistake is using the wrong group. A joint between an A36 web and an A572 Grade 65 flange is not all Group I — the preheat should be based on the higher-group material (A572 Grade 65 = Group II) and the thicker member. When in doubt, use the more conservative group.
Watch the dual-certification trap: A36/A572-50 dual-certified plates are common. For preheat purposes under Table 6.9, a plate certified to both A36 and A572 Grade 50 may be classified as Group I (A36 base) by some interpretations and Group II (A572-50 base) by others. AWS D1.1:2025 Annex B and the commentary address this; the safest practice is to use the higher-strength certified designation — Group II — unless the contract explicitly allows Group I for dual-certified material.
Step 2 — Measure the Governing Thickness
Table 6.3 thresholds are 3/4 in, 1-1/2 in, and 2-1/2 in [19 mm, 38 mm, 64 mm]. The governing dimension is the thickness of the thickest member at the joint, not the thinner member.
For a fillet weld attaching a 3/8 in web to a 1-3/8 in flange, the governing thickness is 1-3/8 in (over 3/4 in, under 1-1/2 in). Using the web thickness would understate the preheat need.
For T-joints and corner joints, use the thicker of the two pieces. For CJP groove butt joints, use the plate thickness (both sides should be the same or close; use the thicker). For repair welds over an existing weld, use the full plate thickness of the piece being repaired.
Step 3 — Determine the Heat Input Category
Table 6.3 has two sub-columns for many combinations: high and low heat input. This is the column selection most often skipped in WPS documentation.
High heat input processes in the context of Table 6.3 include:
- SMAW with 5/32 in [4 mm] and larger electrodes
- SAW (inherently high heat input)
- GMAW spray transfer (higher energy input per unit length)
Low heat input processes include:
- SMAW with 1/8 in [3 mm] and smaller electrodes
- GMAW short-circuit transfer (low voltage, low wire feed)
- FCAW at low parametric settings
- GTAW (almost always low heat input)
The distinction matters because lower heat input deposits less thermal energy into the joint per unit length, which means hydrogen has less time to diffuse out before the weld cools — hence the higher required preheat for low-heat-input welds at the same steel thickness.
Your WPS documents heat input as a calculated range (Arc Voltage × Amperage × 60 / Travel Speed). When you document the heat input range, Table 6.3 can be applied correctly. A WPS that lists only the minimum amperage without heat input calculation makes column selection ambiguous — a common audit finding.
Step 4 — Record the Preheat in the WPS Correctly
AWS D1.1:2025 Annex M (the standard WPS form) has a dedicated Preheat and Interpass Temperature section. Fill in three values:
- Minimum preheat temperature — from Table 6.3, or higher if required by a qualified PQR or by the project specification
- Maximum interpass temperature — the code does not set a universal maximum for most structural steels, but the WPS must document one; common practice is 400–600 °F [200–315 °C] for carbon steel, 400 °F [200 °C] for quenched-and-tempered steels
- Preheat maintenance method — torch heating, induction, resistance blanket, or combination; document the method that will be used in the field
A WPS that says only "preheat per AWS D1.1 Table 6.3" without stating the actual temperature value is not wrong per se, but it is incomplete and will fail most third-party audits. State the number.
How the CWI Verifies Preheat
The inspector's sequence before the arc strikes is:
- Confirm the steel heat number matches the MTR and the WPS base metal group.
- Measure joint thickness — confirm the right row in Table 6.3.
- Note the process and electrode size — confirm the right heat input column.
- Check actual temperature at least 3 in [75 mm] from the weld centerline, on the thicker member.
- Record on the weld inspection report. Hold the arc until temperature is confirmed.
Temperature-indicating crayons are common but have a key limitation: they melt at a specific temperature, but do not tell you whether the heat has penetrated through the section depth. On heavy plate (over 2 in [51 mm]), contact thermometers or digital infrared thermometers should be used at multiple points on the cross-section to confirm through-thickness preheat — not just the surface.
What Happens If You Underpreheat
An underpreheat condition does not immediately produce a visible weld defect. Hydrogen cracking (also called cold cracking or delayed cracking) can occur 24–48 hours after welding, sometimes not until the structure is put in service. The crack typically forms in the HAZ parallel to the fusion line and may propagate into the base metal.
If underpreheat is discovered during or after welding, the WPS provisions apply: the fabricator documents the nonconformance, the EOR is notified, NDE is performed on the affected welds, and a corrective action plan is issued. AWS D1.1:2025 Clause 9 and Clause 10 cover weld rejection and repair; the preheat nonconformance does not automatically require weld removal, but it does require a documented engineering evaluation.
Common Table 6.3 Mistakes and How to Avoid Them
| Mistake | Prevention |
|---|---|
| Using thinner member thickness | Always use thickest member at the joint |
| Wrong steel group (dual-certified) | Default to higher group unless explicitly permitted otherwise |
| Omitting heat input column choice from WPS | Document calculated heat input range; specify high/low designation |
| Preheat not re-checked after interruption | Require re-check on welding log after any stop exceeding 15 min |
| Preheat recorded but not witnessed by CWI | CWI initials on weld log before arc start for high-risk joints |
| Maximum interpass temperature not documented | Always document max interpass, especially for Q&T steels |
Summary
Reading AWS D1.1:2025 Table 6.3 correctly is four sequential decisions: steel group, governing thickness, heat input category, and then the temperature floor from the table. None of those decisions can be made by looking at the table alone — they require reading the WPS, the MTR, and the joint drawing together.
WPS software that auto-populates Table 6.3 requirements based on material and process data eliminates manual lookup errors and ensures audit-ready documentation. See how the WPS Welding platform handles preheat documentation at /pricing.