3.1 Prequalified Processes, Base Metal Groups & Matching Filler Metals

The Concept and Philosophy of Prequalification

Under AWS D1.1/D1.1M:2025, welding procedure qualification is generally required to demonstrate that a proposed welding procedure will produce joints with the required mechanical properties, soundness, and structural integrity. However, Clause 5 (Prequalification of WPSs) provides a unique, highly standardized pathway: exemption from destructive procedure qualification testing under Clause 6.

+-----------------------------------------------------------------------------------------+
|                              AWS D1.1 WPS QUALIFICATION PATHWAYS                        |
+-----------------------------------------------------------------------------------------+
|                                                                                         |
|   +---------------------------------------+   +-------------------------------------+   |
|   |         CLAUSE 5: PREQUALIFIED        |   |         CLAUSE 6: QUALIFIED         |   |
|   |                 WPS                   |   |            BY TESTING (PQR)         |   |
|   +---------------------------------------+   +-------------------------------------+   |
|   | * Zero destructive testing required   |   | * Plate/pipe coupon welded          |   |
|   | * Strict compliance with Clause 5     |   | * Visual, tension, bend, macroetch  |   |
|   | * Limited processes (SMAW/SAW/GMAW/   |   | * Mandatory for GTAW, GMAW-S, ESW,  |   |
|   |   FCAW)                               |   |   EGW, unlisted steels, non-code    |   |
|   | * Approved base metals (Groups I-IV)  |   |   joint geometries                  |   |
|   | * Standardized joint geometries       |   | * Supported by Procedure            |   |
|   | * Mandatory preheat / interpass       |   |   Qualification Record (PQR)        |   |
|   +---------------------------------------+   +-------------------------------------+   |
+-----------------------------------------------------------------------------------------+

Prequalification is founded on decades of empirical data, extensive metallurgical research, and established structural welding history. The American Welding Society recognizes that when specific, thoroughly understood combinations of base metals, filler metals, joint geometries, welding processes, and preheat/interpass temperatures are utilized within defined parameter envelopes, the resulting welded joints consistently exhibit acceptable strength, ductility, and toughness.

[!IMPORTANT] Prequalification is not an exemption from writing a WPS. A written WPS is always required on the shop floor and in the field. Prequalification is solely an exemption from Procedure Qualification Record (PQR) coupon preparation and mechanical testing.

The "All-or-Nothing" Rule of Clause 5

Prequalification operates on a strict all-or-nothing standard. For a WPS to be prequalified, every single variable and condition must comply with all applicable provisions of Clause 5:

1. The welding process must be prequalified (Clause 5.5).
2. The base metal must be an approved steel listed in Table 5.6.
3. The filler metal must match the base metal group per Table 5.7.
4. The joint geometry must match the standard details in Figures 5.1 or 5.2 within codified tolerances.
5. The minimum preheat and interpass temperatures must satisfy Table 5.11.
6. The welding parameters (current, voltage, travel speed, layer thickness, heat input) must remain within the prequalified limits of Clause 5.

If even a single parameter deviates from Clause 5—such as modifying a groove angle by $2^\circ$ beyond tolerance or using an unlisted steel—the entire WPS loses its prequalified status and must be qualified by destructive testing under Clause 6.

Prequalified vs. Non-Prequalified Welding Processes

AWS D1.1 Clause 5.5 explicitly restricts prequalification to four specific arc welding processes. All other welding processes are non-prequalified and require formal qualification testing.

Why GMAW-S is Excluded from Prequalification

A frequent exam topic on the AWS D1.1 endorsement is the status of GMAW-S (Short Circuiting Metal Transfer). In GMAW-S, the consumable electrode touches the molten weld pool at frequencies of 20 to 200 times per second, extinguishing the arc during each short circuit. While this produces low heat input and minimal distortion—ideal for thin sheet metal—it frequently fails to achieve adequate base metal penetration on structural steel plates ($\ge 1/8\text{ in.}$ or $3\text{ mm}$).

This lack of penetration results in incomplete sidewall fusion and cold lapping (molten filler metal rolling over cold base metal without fusing), which cannot be reliably detected by visual examination. Therefore, AWS D1.1 strictly mandates that any WPS utilizing GMAW-S must be qualified by mechanical testing (guided bend and tension tests) per Clause 6.

Base Metal Groupings (Table 5.6)

AWS D1.1 Table 5.6 categorizes approved structural steels into five Base Metal Groups based on chemical composition, carbon equivalent, yield strength ($F_y$), tensile strength ($F_u$), and weldability.

+-----------------------------------------------------------------------------------------+
|                        AWS D1.1 BASE METAL STRENGTH GROUPINGS (TABLE 5.6)               |
+-----------------------------------------------------------------------------------------+
|                                                                                         |
|  GROUP I        Carbon Steels (Fy <= 36-42 ksi / 250-290 MPa)                           |
|                 * ASTM A36, ASTM A53 Gr B (pipe), ASTM A500 Gr A/B (HSS),               |
|                   ASTM A501, ASTM A709 Gr 36                                            |
|                                                                                         |
|  GROUP II       High-Strength Low-Alloy (HSLA) Steels (Fy = 50-55 ksi / 345-380 MPa)    |
|                 * ASTM A572 Gr 50, ASTM A992 (Wide-Flange Shapes), ASTM A588,          |
|                   ASTM A709 Gr 50/50W, ASTM A500 Gr C                                   |
|                                                                                         |
|  GROUP III      Intermediate High-Strength Steels (Fy = 60-65 ksi / 415-450 MPa)        |
|                 * ASTM A572 Gr 60 and Gr 65, ASTM A709 Gr 65                           |
|                                                                                         |
|  GROUP IV       Quenched and Tempered (Q&T) Alloy Steels (Fy = 90-100 ksi / 620-690 MPa)|
|                 * ASTM A709 Gr 100 / 100W, ASTM A514, ASTM A517                         |
|                                                                                         |
|  GROUP V        Quenched and Self-Tempered (QST) Shapes (new in the 2025 edition)        |
|                 * ASTM A913 Grade 80                                                    |
|                                                                                         |
+-----------------------------------------------------------------------------------------+

Group Characteristics and Typical Applications

• Group I Steels: Minimum yield strengths ranging from $36\text{ ksi}$ to $42\text{ ksi}$. These are traditional carbon-manganese structural steels. ASTM A36 is the historical standard for structural plates and shapes, while ASTM A500 Grades A and B are common hollow structural sections (HSS). Non-low-hydrogen electrodes (e.g., E6010, E6011) are permitted for prequalified WPSs on Group I steels only up to certain thickness limits.
• Group II Steels: Minimum yield strengths of $50\text{ ksi}$ to $55\text{ ksi}$. ASTM A992 is the premier specification for structural wide-flange shapes (W-shapes), featuring controlled carbon equivalent and yield-to-tensile ratios to enhance seismic performance and weldability. ASTM A588 is atmospheric corrosion-resistant (weathering) steel. Low-hydrogen consumables (e.g., E7018) are mandatory for all Group II steels.
• Group III Steels: Minimum yield strengths of $60\text{ ksi}$ to $65\text{ ksi}$. These include high-strength columbium-vanadium structural steels like ASTM A572 Grades 60 and 65. They require elevated preheat temperatures and strict low-hydrogen practice.
• Group IV Steels: High-yield quenched-and-tempered (Q&T) alloy steels with minimum yield strengths of $90\text{ ksi}$ to $100\text{ ksi}$ (e.g., ASTM A514 and ASTM A709 Gr 100/100W). These steels are highly sensitive to heat input; excessive heat input or improper preheat degrades the heat-affected zone (HAZ) toughness and tensile properties.

Unlisted Base Metals

Any steel not explicitly tabulated in Table 5.6 is classified as an unlisted base metal. Common unlisted steels encountered in industry include AISI 1018/1020 cold-rolled bar, AISI 4140 chrome-moly steel, ASTM A516 pressure vessel plate, and foreign specifications (e.g., EN 10025 S355) that do not carry dual ASTM certification. An unlisted base metal can NEVER be used on a prequalified WPS. To weld an unlisted steel, the fabricator must qualify the WPS by testing in accordance with Clause 6.

Matching, Undermatching, and Overmatching Filler Metals (Table 5.7)

AWS D1.1 Table 5.7 establishes the required matching filler metal classifications for each base metal group across all four prequalified processes.

Matching vs. Undermatching vs. Overmatching Principles

• Matching Filler Metal: The deposited weld metal has a minimum specified tensile strength equal to or slightly greater than the nominal tensile strength of the base metal (e.g., using an E7018 electrode with $70\text{ ksi}$ tensile strength on ASTM A36 or ASTM A992 steel). Matching filler metal is mandatory for all prequalified Complete Joint Penetration (CJP) groove welds subjected to tension stresses normal to the effective area.
• Undermatching Filler Metal: The deposited weld metal has a lower tensile strength than the base metal (e.g., depositing E7018 weld metal on ASTM A514 Group IV steel). Table 4.3 and Clause 5.6 permit undermatching filler metals for fillet welds and Partial Joint Penetration (PJP) groove welds. Undermatching is often engineered deliberately to provide lower residual stress, increased joint ductility, and superior resistance to hydrogen-induced cracking.
• Overmatching Filler Metal: The filler metal strength exceeds the base metal strength significantly (e.g., using E9018 on ASTM A36). Overmatching is generally discouraged in structural seismic design because it forces plastic yielding into the base metal HAZ rather than allowing uniform deformation across the joint.

Rules for Joining Dissimilar Base Metal Groups

When structural connections join base metals from two different strength groups (for example, welding a Group I ASTM A36 gusset plate to a Group II ASTM A992 column flange):

1. Filler Metal Selection: The prequalified filler metal may be selected to match either the higher strength group or the lower strength group per Table 5.7.
2. Allowable Stress: The allowable stress of the welded joint and the structural design capacity must be based on the lower strength base metal (Group I).
3. Preheat Requirements: The minimum preheat and interpass temperature must satisfy the requirements of Table 5.11 for the higher preheat group / thicker member.