Fabrication and Welding Techniques
Q1: What are the standard procedures for cutting A106B pipe on site?
A1: Standard procedures for cutting A106B pipe on site include: 1. Abrasive Cutting: Using a portable abrasive chop saw for quick, straight cuts. This requires care to avoid burning the material and to ensure a square cut. 2. Cold Cutting: Using a band saw or pipe cutter (for smaller diameters) which produces a clean, cold cut without affecting the material's microstructure. 3. Oxygen-Acetylene Torch (Flame Cutting): This is efficient for large diameters but leaves a hardened heat-affected zone (HAZ) and slag on the cut edge. The HAZ must be ground away entirely (typically 1/8" to 1/4") before welding to remove the brittle layer. Regardless of the method, the cut must be cleaned of debris and checked for squareness.
Q2: What are the essential steps for preparing a A106B beveled end for welding?
A2: Essential steps for preparing a A106B beveled end include: 1. Cleaning: Removing all rust, dirt, paint, moisture, and mill scale from the inside and outside surfaces for at least 1 inch back from the bevel using a wire brush or grinder. 2. Fit-Up: Aligning the pipes within specified tolerances for mismatch (hi-lo) and gap. 3. Bevel Inspection: Verifying the bevel angle (typically 37.5°) and the root face (land) dimension (typically 1/16") conform to the Welding Procedure Specification (WPS). 4. Tack Welding: Installing small tack welds to hold alignment. These tacks must be made by qualified welders using the correct procedure and must be ground smooth at the ends to ensure they are incorporated properly into the root pass.
Q3: Why is preheating often required when welding A106B and how is it determined?
A3: Preheating is often required for welding A106B, especially on thicker walls, to slow the cooling rate of the weld and the heat-affected zone (HAZ). Rapid cooling can cause the formation of hard, brittle martensite, which is susceptible to hydrogen-induced cracking (HIC), also known as cold cracking. The required preheat temperature (typically between 50°C to 200°C / 122°F to 392°F) is determined by the Welding Procedure Specification (WPS), which considers key factors: the pipe's wall thickness (a primary factor, with thicker walls requiring higher preheat), the ambient temperature, the carbon equivalent (CE) of the steel, and the hydrogen potential of the welding consumable.
Q4: What are the common welding processes used for joining A106B pipe?
A4: Common welding processes for joining A106B pipe include: 1. Shielded Metal Arc Welding (SMAW or "Stick"): Versatile and widely used for all positions, especially for installation and repair. Low-hydrogen electrodes (E7018) are standard. 2. Gas Tungsten Arc Welding (GTAW or TIG): Often used for the critical root pass on pipe welds due to its excellent control and high-quality, clean weld deposit. 3. Gas Metal Arc Welding (GMAW or MIG): Used for semi-automatic and automatic welding, offering high deposition rates. 4. Flux-Cored Arc Welding (FCAW): Common in fabrication shops for its high speed and good penetration. The choice depends on application, quality requirements, and available equipment.
Q5: What is involved in Post Weld Heat Treatment (PWHT) for A106B and when is it required?
A5: Post Weld Heat Treatment (PWHT) is a controlled process of heating and cooling a weldment to improve its properties. For A106B, PWHT (or stress relieving) is typically required for thicker wall pipes (as defined by codes like ASME B31.3, often for thicknesses above ~¾ inch or 19mm) or for very rigid assemblies. The process involves heating the welded area to a specific temperature range (typically 595-675°C / 1100-1250°F for A106B), holding it at that temperature for a specified time (usually 1 hour per inch of thickness), and then cooling slowly in still air. PWHT relieves residual stresses, tempers hard martensitic zones, and reduces the risk of brittle fracture.
