New Welding Procedure Specification (WPS) for Joining Dissimilar Metals (e.g., Stainless to Carbon Steel) in Piping Systems

New Welding Procedure Specification (WPS) for Joining Dissimilar Metals in Piping Systems

1.0 Scope and Application

This Welding Procedure Specification (WPS) establishes qualified parameters for joining austenitic stainless steel (e.g., 304/316/L) to carbon steel (e.g., A106 Gr.B, A516 Gr.70) in piping systems. The procedure addresses the technical challenges associated with dissimilar metal welding (DMW), including differential thermal expansion, carbon migration, and residual stress management.

2.0 Base Materials

2.1 Material Combinations Qualified

2.2 Material Thickness Range

• Pipe Diameter: ½" NPS to 48" NPS

• Wall Thickness: 1.6mm to 50mm

3.0 Filler Metal Selection

3.1 Recommended Filler Metals

Table: Filler Metal Selection Based on Service Conditions

3.2 Filler Metal Considerations

• Carbon migration control: High chromium-nickel fillers prevent carbon diffusion

• Dilution management: Minimum 30% dilution into carbon steel side required

• Ferrite control: 5-12 FN (Ferrite Number) to prevent solidification cracking

4.0 Joint Design and Preparation

4.1 Standard Joint Configuration

Single-V Groove with 37.5° bevel angle, 1.6mm root face, and 3.2mm root opening

4.2 Preparation Requirements

• Stainless steel side: Mechanical cleaning only, no carbon steel contamination

• Carbon steel side: Remove all scale, rust, and paint 25mm from joint

• Fit-up: Maximum misalignment 1.6mm or 10% of wall thickness, whichever is less

5.0 Welding Technique Parameters

5.1 Welding Process: GTAW (Root) + SMAW (Fill/Cap)

Table: Welding Parameters by Position

5.2 Critical Technique Requirements

• Arc direction: Always direct arc toward carbon steel side

• Bead placement: Place weld metal primarily on carbon steel side

• Interpass temperature: 150°C maximum, 16°C minimum

• Peening: Light peening of intermediate passes permitted

6.0 Preheat and Interpass Temperature

6.1 Preheat Requirements

6.2 Interpass Temperature Control

• Maximum: 150°C for all thicknesses

• Minimum: 10°C above preheat temperature

• Monitoring: Continuous monitoring required for thick sections

7.0 Post-Weld Heat Treatment (PWHT)

7.1 PWHT Requirements

Table: PWHT Parameters Based on Material Combination

7.2 PWHT Considerations

• Avoid sensitization: Keep below 425°C for 300-series stainless

• Carbon migration: PWHT accelerates diffusion - minimize time at temperature

• Fixture removal: Perform PWHT before removing alignment fixtures

8.0 Non-Destructive Examination (NDE)

8.1 Required Examination Methods

• 100% Visual Examination: VT per AWS D1.1

• Radiographic Examination: RT per ASME Sec V Art 2

• Liquid Penetrant Examination: PT on accessible root pass

8.2 Acceptance Criteria

• Porosity: Maximum 3.2mm diameter, 6mm between indications

• Cracks: None permitted

• Incomplete fusion: None permitted

• Undercut: Maximum 0.8mm depth, 0.5mm for cyclic service

9.0 Procedure Qualification Records (PQR)

9.1 Required Tests

• Transverse tension test: Minimum strength equal to weaker base metal

• Root and face bend tests: 4 specimens minimum

• Macro examination: Etched cross-section

• Hardness survey: Vickers hardness across weldment

9.2 Hardness Limits

• Weld metal: ≤ 225 HV

• HAZ carbon steel: ≤ 240 HV

• HAZ stainless: ≤ 220 HV

10.0 Safety and Environmental Considerations

10.1 Ventilation Requirements

• Local exhaust: Required for all welding operations

• Fume extraction: Mandatory for SMAW operations

• Air monitoring: Required for confined space welding

10.2 Material Handling

• Stainless segregation: Prevent contamination from carbon steel tools

• Consumable storage: ER309L must be stored in original packaging until use

• Cleanliness: Alcohol cleaning required for stainless surfaces

11.0 Procedure Limitations and Restrictions

11.1 Process Restrictions

• No oxyfuel cutting on prepared edges

• No carbon arc gouging on stainless side

• Minimum 3 passes for wall thickness > 12mm

11.2 Service Restrictions

• Not qualified for lethal service without additional testing

• Temperature limitation: -29°C to 425°C

• pH limitation: Not for caustic service above 50°C

12.0 Troubleshooting Guide

12.1 Common Issues and Solutions

13.0 Records and Documentation

13.1 Required Documentation

• WPS: This specification

• PQR: Supporting procedure qualification record

• WPQ: Welder performance qualifications

• NDE reports: All examination results

• Heat treatment charts: Continuous recording for PWHT

13.2 Retention Period

• Minimum 5 years for all records

• Life of facility for safety-critical applications