What are the welding procedures and filler metal selection for A106 A106M steel pipe?

ASTM A106/A106M seamless carbon steel pipe is one of the most widely specified materials for high-temperature and high-pressure service. It is commonly found in power plant steam lines, refinery process piping, and boiler feed systems. Because the pipe is always joined to other components in the field, the welding procedure and the choice of filler metal determine whether the pipeline will deliver safe, long-term performance. This guide explains how to ｓｅｌｅｃｔ the right procedure and filler metal for A106/A106M pipe across Grades A, B, and C, and how the choices change with wall thickness and service conditions.

Why A106/A106M Welding Needs Careful Planning

A106/A106M is a seamless product, so there is no longitudinal weld seam to consider in the pipe body itself. The challenge is the field joint between pipe-to-pipe, pipe-to-fitting, or pipe-to-flange. The carbon content of A106 increases from Grade A to Grade C, which means the higher-strength grades are also more sensitive to hydrogen-assisted cracking and require tighter thermal control. Selecting the wrong filler metal, skipping preheat, or ignoring interpass temperature can lead to hard heat-affected zones, lack of fusion, or cold laps. For a consistent result, every A106 weld should be supported by a qualified Welding Procedure Specification (WPS) and a Procedure Qualification Record (PQR).

Material Properties That Drive Filler Metal Selection

Before selecting a filler metal, review the actual chemical composition and mechanical properties of the pipe being welded. The three grades covered by ASTM A106/A106M seamless steel pipe differ mainly in tensile strength:

• Grade A: minimum tensile strength 330 MPa (48 ksi) — easiest to weld, typically chosen for lower-pressure service.
• Grade B: minimum tensile strength 415 MPa (60 ksi) — the most common grade for refineries, power plants, and process plants.
• Grade C: minimum tensile strength 485 MPa (70 ksi) — used in higher-temperature piping where extra strength is required.

Carbon equivalent (CE) calculated from the heat analysis is a practical indicator of weldability. When CE rises above approximately 0.45, low-hydrogen practices and preheat become mandatory. For Grade C with thicker walls, both preheat and controlled interpass are required regardless of the welding process.

Pre-Weld Preparation

Sound welds start before the arc is struck. The following points are essential for any A106 joint:

• Bevel and gap: machine a clean 30°–37.5° single-V or double-V groove, with a root face of 1.6–3.2 mm and a root gap of 1.6–3.2 mm, per ASME B31.1 or B31.3.
• Cleanliness: remove oil, grease, paint, rust, and moisture from the groove and adjacent surfaces for at least 20 mm on each side.
• Fitting-up: align the pipes so that the internal diameter mismatch does not exceed 1.6 mm, then tack-weld with the same filler metal as the root pass.
• Electrode handling: keep low-hydrogen electrodes in their original packaging, store them in a holding oven at 120–150 °C, and re-dry if exposed.

Common Welding Processes for A106/A106M

Several processes are qualified for A106/A106M service. The choice depends on position, wall thickness, and project code.

Shielded Metal Arc Welding (SMAW)

SMAW is the most common field process for A106 because it tolerates outdoor conditions and works in all positions. It is typically used for root, fill, and cap passes in shop fabrication and on site. SMAW relies on stick electrodes, which makes it flexible but slower than wire processes.

Gas Tungsten Arc Welding (GTAW / TIG)

GTAW is preferred for the root pass on critical service lines, especially thin-wall pipe and high-purity process systems. The arc is stable, the heat input is easy to control, and there is no slag to trap in the root. Most specifications require a GTAW root followed by SMAW or FCAW fill and cap.

Gas Metal Arc Welding (GMAW / MIG) and Flux-Cored Arc Welding (FCAW)

GMAW and FCAW deliver high deposition rates and are widely used for fill and cap passes on pressure tubes and long pipeline runs. They are commonly paired with a GTAW root. FCAW with a gas shield is favored for outdoor structural and process piping where wind protection is provided.

Filler Metal Selection by Grade and Process

The general rule is to match the filler metal's tensile strength to the base metal grade while keeping the weld metal's low-temperature toughness equal to or better than the base metal. For A106/A106M, the following filler metal families are widely accepted in ASME B31.1 and B31.3 work:

• SMAW electrodes: E6010 for root pass on cellulosic-coated welding of pipe; E7010-P1 and E7018 low-hydrogen electrodes for fill and cap passes. E7018 is the most common choice for Grade B and Grade C because of its low hydrogen content and stable arc.
• GTAW wire: ER70S-2 or ER70S-3 for root passes on Grade A and Grade B. For Grade C, use ER70S-3 with controlled heat input.
• GMAW wire: ER70S-6 solid wire with 75–80% Ar / balance CO₂ shielding gas. Suitable for fill and cap on Grades A, B, and C when low-hydrogen performance is needed.
• FCAW wire: E71T-1 (gas-shielded) or E71T-8 (self-shielded) for high-deposition fill and cap. ｓｅｌｅｃｔ low-hydrogen variants for thick-wall Grade C pipe.

All filler metals should be supplied with a valid Certificate of Conformance and, where required, a CE marking or equivalent traceability document. For sour service (NACE MR0175) or nuclear-adjacent service, additional hardness and SSC testing apply.

Preheat and Interpass Temperature

Preheat reduces the cooling rate of the weld and helps drive off surface moisture. For A106/A106M, typical practice is:

• Grade A, wall thickness under 20 mm: preheat to 50–100 °C, interpass 150–250 °C.
• Grade B, wall thickness 20–40 mm: preheat to 100–150 °C, interpass 150–260 °C.
• Grade C, wall thickness over 40 mm: preheat to 150–200 °C, interpass controlled at 200–300 °C.

Use temperature-indicating crayons or a calibrated contact pyrometer on the groove face, not on the heat-affected zone, to avoid under-reading. Avoid exceeding the upper interpass limit, which can degrade the heat-affected zone toughness.

Post-Weld Heat Treatment (PWHT)

PWHT is not always required, but it is commonly specified to relieve residual stress, temper hard microstructures, and improve dimensional stability. The applicable construction code (ASME B31.1, B31.3, B31.4, or the customer's project specification) defines when PWHT is mandatory. For A106/A106M:

• Carbon-steel carbon & carbon alloy steel piping with wall thickness above the code threshold (commonly 19 mm for B31.1 and 19 mm for B31.3 in certain services) usually requires PWHT at 595–705 °C for at least one hour per 25 mm of thickness.
• For sour service, PWHT may be required at a lower temperature to keep hardness below 22 HRC.
• Heat treatment must be recorded on a time-temperature chart, and thermocouples must remain attached throughout the soak.

Inspection and Common Defects

After welding, every joint should be visually inspected and then examined using the methods required by the project specification. For A106/A106M in pressure service, this typically includes 100% radiographic testing (RT) or ultrasonic testing (UT) of butt welds, magnetic particle testing (MT) or liquid penetrant testing (PT) of fillet welds, and hydrostatic testing of the completed line. The most common defects in A106 welds are:

• Lack of fusion and incomplete penetration at the root, often caused by low current or improper gap.
• Porosity and slag inclusions from contaminated grooves or damp electrodes.
• Undercut and overlap from excessive current or wrong torch angle.
• Hydrogen cracking in thick-wall Grade C joints when preheat is missed.

Practical Recommendations for Project Teams

For most A106/A106M Grade B shop and field work, a robust default is a GTAW root with ER70S-2 or ER70S-3, followed by SMAW fill and cap using E7018 low-hydrogen electrodes, with 100 °C preheat and 150–250 °C interpass. For Grade C or thick-wall service, switch to 150 °C preheat, control the interpass to the upper end of the allowed range, and apply PWHT per code. Match the WPS to the actual heat number, the actual wall thickness, and the actual position, and have it qualified before production welding begins.

A consistent procedure and the right filler metal turn an A106/A106M joint into a reliable part of the piping system. For supply of ASTM A106/A106M seamless pipe in Grades A, B, and C — together with matching butt-weld fittings, flanges, and gaskets for a complete pressure piping package — see the EZ Steel Industrial product range.