ASTM A36 Steel Welding Methods and Compatible Metals

ASTM A36 Steel: Welding Methods and Compatible Metals

ASTM A36 is a standard specification for carbon structural steel, widely used in construction, bridges, buildings, and general structural applications due to its good strength, formability, and weldability. It is a low-carbon steel with a minimum yield strength of 36 ksi (250 MPa) and a tensile strength ranging from 58 to 80 ksi (400-550 MPa). The chemical composition typically includes carbon (up to 0.26%), manganese (0.60-1.20%), phosphorus (up to 0.04%), sulfur (up to 0.05%), and silicon (up to 0.40%), which contributes to its excellent weldability with minimal risk of cracking or brittleness when proper procedures are followed.

A36 steel was developed to improve upon earlier structural steels by controlling elements like carbon, sulfur, and phosphorus, which can cause welding issues in higher concentrations. It replaced older specifications like ASTM A7 in 1967 and is considered highly weldable using conventional methods, as long as the carbon equivalent (CE) is considered to assess hardenability—typically low for A36, reducing the need for extensive preheating.

Weldability of ASTM A36 Steel

A36 is classified as a Group I base metal in AWS D1.1 (Structural Welding Code - Steel), indicating it is easily weldable with standard processes. Its low carbon content minimizes hydrogen-induced cracking, and it does not typically require post-weld heat treatment (PWHT) unless specified for thick sections or high-restraint joints. However, for prequalified welding procedure specifications (WPS), filler metals must match the base metal's strength and be compatible per AWS guidelines.

Key factors influencing weldability include:

• Thickness: For plates over 1 inch (25 mm), preheating to 100°F (38°C) may be recommended if non-low-hydrogen electrodes are used to prevent cracking.
• Joint Design: Groove, fillet, or butt joints are common; proper preparation (cleaning to remove mill scale, rust, or contaminants) is essential.
• Preheat and Interpass Temperature: Minimal for thin sections (room temperature suffices); up to 150°F (65°C) for thicker materials or when using low-hydrogen processes to control cooling rates.
• Shielding Gas (for GMAW/GTAW): Typically 75-90% Argon/10-25% CO₂ for GMAW to ensure good arc stability and minimal spatter.
• Mechanical Properties of Welds: Weld metal should achieve at least 58 ksi tensile strength to match A36, with no inherent notch toughness requirements unless specified for impact-loaded applications.

Weld qualification often follows AWS D1.1, involving bend tests, tensile tests (per ASTM A370), and visual inspections for defects like cracks over 1/8 inch (3 mm). Qualification positions include flat (1G) and vertical (3G) for processes like GTAW and GMAW.

Recommended Welding Methods

Several arc welding processes are suitable for A36 steel, with selection based on application, equipment availability, position, and desired productivity. Common methods include:

1. Shielded Metal Arc Welding (SMAW, or Stick Welding):
   • Versatile for field or shop work, especially on thicker sections.
   • Recommended electrodes: E6010 or E6011 for all positions with good penetration; E7018 (low-hydrogen) for higher quality welds to reduce cracking risk.
   • Current: DC electrode positive (DCEP) for E7018; amperage ranges 70-200 A depending on electrode diameter (e.g., 1/8 inch at 90-140 A).
   • Advantages: Low cost, portable; suitable for outdoor use.
   • Precautions: Use low-hydrogen electrodes for critical joints; store electrodes dry to avoid moisture pickup.
2. Gas Metal Arc Welding (GMAW, or MIG Welding):
   • High productivity for semi-automatic or automatic welding on clean surfaces.
   • Filler wires: ER70S-3, ER70S-6, or ER70S-7 (70 ksi tensile strength), with wire diameters 0.035-0.045 inch.
   • Shielding: 90% Argon/10% CO₂ or similar mixes.
   • Parameters: Voltage 18-28 V, current 150-300 A, travel speed 10-20 in/min.
   • Advantages: Clean welds with minimal slag; good for thin to medium thicknesses.
   • Precautions: Avoid short-circuit transfer mode on thick plates to prevent lack of fusion.
3. Gas Tungsten Arc Welding (GTAW, or TIG Welding):
   • Ideal for precise, high-quality welds on thinner sections or root passes.
   • Filler rods: ER70S-2, ER70S-3, or ER70S-6.
   • Shielding: 100% Argon.
   • Parameters: DC electrode negative (DCEN), amperage 100-200 A.
   • Advantages: Excellent control, low distortion; suitable for all positions.
   • Precautions: Slower process; requires skilled operators.
4. Flux-Cored Arc Welding (FCAW):
   • Similar to GMAW but with flux in the wire for better penetration on dirty surfaces.
   • Wires: E70T-1 or E71T-1 (self-shielded or gas-shielded).
   • Advantages: High deposition rates; good for outdoor welding without gas.
   • Precautions: Remove slag between passes; use for thicknesses over 1/4 inch.

Other processes like Submerged Arc Welding (SAW) can be used for high-volume production on thick plates, with neutral flux and ER70S wires.

Filler Metals

Filler metals should "match" A36's properties, meaning a minimum tensile strength of 60-70 ksi to avoid undermatching. Common classifications per AWS A5 specifications:

• SMAW: E60XX (e.g., E6013 for general use) or E70XX (e.g., E7018 for structural).
• GMAW/GTAW: ER70S series (e.g., ER70S-6 provides good deoxidation for rusty surfaces).
• FCAW: E70T or E71T series.

For low-alloy additions, if needed for corrosion resistance, consult specific codes. Low-hydrogen fillers (H4 or H8 designation) are preferred to minimize diffusible hydrogen.

Precautions and Best Practices

• Clean base metal thoroughly to remove oil, paint, or scale.
• Control heat input to avoid excessive grain growth or distortion (typically 20-40 kJ/in).
• For impact or seismic applications, ensure weld metal meets Charpy V-notch (CVN) requirements if specified, though A36 itself has none.
• Avoid welding in cold weather without preheat; monitor interpass temperatures below 500°F (260°C) to prevent brittleness.
• Test welds: Visual inspection, bend tests, and tensile tests ensure no defects like porosity, inclusions, or lack of fusion.

Welding A36 to Compatible Metals

A36 can be welded to a variety of similar or dissimilar metals, but compatibility depends on matching chemistries, strengths, and service conditions. Always select filler based on the lower-strength metal and preheat based on the higher-strength one to minimize cracking. Common combinations include:

• To Other Carbon Steels:
  • A36 to A36: Standard procedures; no special issues.
  • A36 to A572 Grade 50: Common in structural work; use 70 ksi fillers like E7018 or ER70S-6. Mean yield strengths are similar (48 ksi for A36, 57 ksi for A572 Gr. 50).
  • A36 to 1018/1040: Possible, but higher-carbon steels like 1040 may require preheat to avoid warping.
• To Low-Alloy Steels:
  • A36 to AISI 4130: Frequent in heavy equipment; use 70 ksi low-hydrogen fillers (e.g., E7018). Preheat 4130 to 300-400°F if thick.
  • A36 to A514: Structural applications; filler matches A36 (70 ksi), but preheat and interpass follow A514 (up to 400°F) to prevent HAZ cracking.
  • A36 to A387 (Chrome-Moly): For high-temperature service; use low-alloy fillers (e.g., 1.25% Cr-0.5% Mo). PWHT may be needed for A387; match filler to A36 but consider stress relief.
• To High-Strength or Wear-Resistant Steels:
  • A36 to AR Plate (e.g., abrasion-resistant): Use small fillet welds to reduce heat input and residual stress; low-hydrogen 70 ksi fillers.
  • A36 to HPS70W (High-Performance Steel): Bridge applications; special procedures per guidelines like Appendix F for bridges.
• To Weathering Steels:
  • A36 to A588 (Cor-Ten): Minimal dissimilar issues; paint welds if galvanized concerns arise, but no major galvanic corrosion.
• To Other Materials:
  • A36 to Cast Iron: Challenging due to thermal expansion; use nickel-based fillers (e.g., ENiFe-CI) with preheat, but avoid if possible for cyclic loading.
  • Avoid welding to high-carbon tool steels (e.g., AISI A7) without extensive preheat/PWHT due to cracking risks.

For dissimilar welds, calculate carbon equivalent for both metals; if high, use low-hydrogen processes and PWHT. Consult AWS D1.1 or specific codes for WPS qualification. In marine or corrosive environments, consider coatings like zinc-rich primers, but avoid welding through them due to fumes.