3.3 Prequalified Minimum Fillet Sizes & Preheat/Interpass Temperatures
Key Takeaways
- Table 7.7 establishes minimum fillet weld sizes based entirely on the thickness of the thicker base metal joined, safeguarding against rapid cooling and hydrogen-induced cracking.
- The fillet weld size need not exceed the thickness of the thinner part joined, provided calculated design stress is satisfied and minimum preheat for the thicker plate is maintained.
- Table 5.11 governs prequalified minimum preheat and interpass temperatures, organized into preheat categories (A through D for common structural steels, plus Categories F and G added in the 2025 edition for ASTM A913 Grades 70 and 80) based on base metal composition and consumable hydrogen control.
- Under Clause 7.6.2 of the 2025 edition, preheat must extend through the full thickness and at least twice the base metal thickness in all directions for material under 1-1/2 in.; for 1-1/2 in. and thicker, at least the base metal thickness but never less than 3 in. (75 mm).
- Any deviation outside the parameters of Clause 5 strips the WPS of prequalified status, requiring complete mechanical qualification testing under Clause 6.
Minimum Fillet Weld Sizes (Clause 7.13 & Table 7.7) and The Heat Sink Effect
In structural welding, specifying a fillet weld that is too small for a given plate thickness creates a severe metallurgical hazard known as the heat sink effect. Thick base metal sections absorb heat from the welding arc at an extremely rapid rate. If a tiny weld bead (low heat input) is deposited onto thick steel, the molten weld pool and the surrounding Heat-Affected Zone (HAZ) quench rapidly.
This rapid cooling rate transforms austenite directly into brittle untempered martensite. When combined with dissolved hydrogen and high joint restraint, this brittle microstructure results in hydrogen-induced cold cracking (underbead cracking).
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| TABLE 7.7 (CLAUSE 7.13): MINIMUM FILLET WELD SIZES |
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| |
| Base Metal Thickness of Thicker Part Joined (T) Minimum Size of Fillet Weld |
| ================================================================================ |
| T <= 1/4 in. (6 mm) 1/8 in. (3 mm) |
| 1/4 in. < T <= 1/2 in. (12 mm) 3/16 in. (5 mm) |
| 1/2 in. < T <= 3/4 in. (20 mm) 1/4 in. (6 mm) |
| T > 3/4 in. (20 mm) 5/16 in. (8 mm) |
| |
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Key Rules Governing Table 7.7 Application
- Governed by Thicker Part: The minimum fillet weld size is strictly determined by the thickness of the thicker part joined, NOT the thinner part. For example, if a $3/8\text{ in.}$ gusset plate is welded to a $1\text{ in.}$ column flange, the thicker part ($1\text{ in.}$) dictates a minimum fillet weld leg size of $5/16\text{ in.}$.
- The "Thinner Part" Exception: Footnote 'b' of Table 7.7 states that the fillet weld size need not exceed the thickness of the thinner part joined, provided that:
- The design stress calculations demonstrate that the smaller weld size is structurally adequate.
- The minimum preheat and interpass temperature required by Table 5.11 for the thicker part is strictly maintained.
- Single-Pass Deposit: For prequalified status, minimum fillet weld sizes should generally be deposited as a single pass unless multi-pass technique is qualified and specified in the WPS.
Prequalified Minimum Preheat and Interpass Temperatures (Table 5.11)
Preheating slows the cooling rate of both the weld metal and the base metal HAZ, allowing dissolved hydrogen to diffuse safely out of the crystalline lattice before martensitic transformation can trigger cracking. Table 5.11 assigns each prequalified base metal to a Preheat Category. Categories A through D cover the great majority of common structural steels; the 2025 edition also adds Categories F and G for ASTM A913 Grades 70 and 80, and brings cast steels (ASTM A216) into Categories A and B.
The Principal Preheat Categories
- Category A: Non-low-hydrogen processes on Group I steels (e.g., SMAW with E6010 on ASTM A36).
- Category B: Low-hydrogen processes on Group I and Group II steels (e.g., SMAW with E7018, FCAW, GMAW, SAW on ASTM A36, A572 Gr 50, A992).
- Category C: Low-hydrogen processes on Group III steels (e.g., ASTM A572 Grades 60 and 65).
- Category D: Quenched and tempered alloy steels (Group IV: ASTM A514, A517, ASTM A709 Grades 100 and 100W).
Master Preheat and Interpass Temperature Matrix (Table 5.11)
| Thickness of Thicker Part Joined ($T$) | Category A (Non-Low Hydrogen) | Category B (Low Hydrogen) | Category C (Group III) | Category D (Group IV / A514) |
|---|---|---|---|---|
| $T \le 3/4\text{ in.} \ (19\text{ mm})$ | $32^\circ\text{F}^* \ (0^\circ\text{C})$ | $32^\circ\text{F}^* \ (0^\circ\text{C})$ | $50^\circ\text{F} \ (10^\circ\text{C})$ | $50^\circ\text{F} \ (10^\circ\text{C})$ |
| $3/4\text{ in.} < T \le 1-1/2\text{ in.} \ (38\text{ mm})$ | $150^\circ\text{F} \ (65^\circ\text{C})$ | $50^\circ\text{F} \ (10^\circ\text{C})$ | $150^\circ\text{F} \ (65^\circ\text{C})$ | $125^\circ\text{F} \ (50^\circ\text{C})$ |
| $1-1/2\text{ in.} < T \le 2-1/2\text{ in.} \ (65\text{ mm})$ | $225^\circ\text{F} \ (110^\circ\text{C})$ | $150^\circ\text{F} \ (65^\circ\text{C})$ | $225^\circ\text{F} \ (110^\circ\text{C})$ | $175^\circ\text{F} \ (80^\circ\text{C})$ |
| $T > 2-1/2\text{ in.} \ (65\text{ mm})$ | $300^\circ\text{F} \ (150^\circ\text{C})$ | $225^\circ\text{F} \ (110^\circ\text{C})$ | $300^\circ\text{F} \ (150^\circ\text{C})$ | $225^\circ\text{F} \ (110^\circ\text{C})$ |
[!NOTE]
- The $32^\circ\text{F} \ (0^\circ\text{C})$ Freezing Rule: When the base metal temperature is below $32^\circ\text{F} \ (0^\circ\text{C})$, the base metal must be preheated to a minimum of $70^\circ\text{F} \ (20^\circ\text{C})$ before welding begins. Welding on frozen steel is strictly prohibited under AWS D1.1.
The Low-Hydrogen Benefit
Comparing Category A and Category B highlights the dramatic engineering benefit of low-hydrogen consumables. For a $1-1/4\text{ in.}$ ASTM A36 plate, using an E6010 non-low-hydrogen electrode requires a preheat of $150^\circ\text{F}$ (Category A). Switching to an E7018 low-hydrogen electrode reduces the mandatory preheat to only $50^\circ\text{F}$ (Category B), saving enormous heating fuel, labor, and time on the jobsite.
Preheat Measurement Zone and Interpass Temperature Control
AWS D1.1 Clause 5.7 (prequalified WPSs) and Clause 7.6 (production welding) establish precise rules for how and where preheat and interpass temperatures must be measured during fabrication.
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| AWS D1.1 PREHEAT MEASUREMENT ZONE (CLAUSE 7.6.2, 2025 EDITION) |
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| |
| <--- heated distance per 7.6.2 ---> |
| +-----------------------------------+ |
| | | |
| | Preheat Zone | |
| | (Full Plate Thickness) | |
| | | |
| | ( ARC ) | |
| | Point of | |
| | Welding | |
| | | |
| | | |
| +-----------------------------------+ |
| <--- heated distance per 7.6.2 ---> |
| |
| t < 1-1/2 in. -> heat at least 2 x t in all directions |
| t >= 1-1/2 in. -> heat at least t, but never less than 3 in. (75 mm) |
| |
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Extent of Preheat and Through-Thickness Heating (2025 change)
- Heated Distance: Clause 7.6.2 of the 2025 edition reduced the heated distance for thinner material. For base metal less than 1-1/2 in. thick, the distance heated in all directions from the point of welding shall be at least twice the base metal thickness. For base metal 1-1/2 in. thick and greater, the distance heated shall be at least equal to the base metal thickness, but not less than 3 in. (75 mm). The blanket "3 in. in all directions" rule printed in the 2020 edition no longer applies to thin material.
- Through-Thickness Soaking: Heating the top surface skin with a torch immediately before striking the arc is unacceptable. The preheat temperature must penetrate through the entire cross-sectional thickness of the joint.
- Measurement Tools: Certified temperature-indicating crayons (Tempilstik), calibrated contact thermocouples, or optical infrared pyrometers. When using temperature crayons, the crayon should not be applied directly in the weld joint groove where melted wax can contaminate the weld pool.
Interpass Temperature and Maximum Limits
- Minimum Interpass Temperature: Must never drop below the minimum preheat temperature required by Table 5.11 throughout the entire welding operation. Clause 6.8.4 of the 2025 edition additionally defines minimum and maximum interpass temperatures as qualification requirements.
- Maximum Interpass Temperature Limits: For conventional carbon and HSLA steels (Groups I-III), maximum interpass temperature is typically specified by engineering design (commonly $450^\circ\text{F}$ to $550^\circ\text{F}$). However, for Group IV quenched and tempered steels (ASTM A514/A517), strict control of maximum interpass temperature (typically $\le 400^\circ\text{F}-450^\circ\text{F}$) is mandatory. Excessive interpass heat slows the cooling rate too much, destroying the quenched-and-tempered martensitic/bainitic grain structure and causing drastic loss of tensile strength and CVN notch toughness in the HAZ.
Summary of Clause 5 Prequalification Boundaries
To ensure full compliance and avoid costly jobsite rejections, the welding inspector and procedure author must verify that every single attribute of the WPS satisfies Clause 5:
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| PREQUALIFICATION VERIFICATION CHECKLIST |
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| [ ] Prequalified Process: SMAW, SAW, GMAW (non-short circuit), or FCAW only |
| [ ] Approved Base Metal: Listed in Table 5.6 (Groups I through V) |
| [ ] Matching / Approved Consumable: Table 5.7 classification and hydrogen limit |
| [ ] Prequalified Joint Geometry: Figures 5.1 (CJP) or 5.2 (PJP) standard detail |
| [ ] Dimensions & Tolerances: Root face, opening, and groove angle within Clause 5.4 |
| [ ] Backing Thickness: Meets 3/16 in. (SMAW), 1/4 in. (GMAW/FCAW), or 3/8 in. (SAW) |
| [ ] Minimum Fillet Weld Size: Satisfies Table 7.7 for the thicker part joined |
| [ ] Minimum Preheat & Interpass: Satisfies the Table 5.11 category for the base metal |
| [ ] Extent of Preheat: Meets Clause 7.6.2 (2 x t, or t but >= 3 in. for t >= 1-1/2 in.) |
| [ ] Electrical Variables: Passes within prequalified amperage, voltage, and travel speed|
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If any single check box cannot be verified, the WPS cannot be stamped as prequalified. The fabricator must prepare test coupons, record actual welding parameters on a PQR, perform nondestructive and destructive mechanical tests per Clause 6, and qualify the WPS through testing.
A structural detail calls for joining a 3/8 in. (10 mm) web plate to a 1-1/4 in. (32 mm) base plate using a prequalified fillet weld. According to AWS D1.1 Table 7.7, what is the minimum prequalified fillet weld size required for this connection?
According to AWS D1.1 Table 5.11, what is the minimum prequalified preheat and interpass temperature when welding a 2-inch (50 mm) thick plate of ASTM A992 steel using the FCAW process with low-hydrogen filler metal (Category B)?
Under Clause 7.6.2 of AWS D1.1/D1.1M:2025, how far from the point of welding must preheat be established through the full plate thickness when the base metal is 2 in. [50 mm] thick?