How do you prepare a pad to measure delta ferrite content in duplex stainless-steel?

Duplex stainless steel is known for its exceptional properties, including corrosion resistance and high mechanical strength. To accurately measure the delta ferrite content in duplex stainless steel weld seams, a well-prepared pad is essential. Follow this comprehensive guide, incorporating plate thickness, ray diffraction, optical microscopy, and ferrite number measurement to ensure high-quality results and prevent stress corrosion cracking.

Step One: Set Up the Weld Pad

Optional: Begin by placing two copper bars parallel to each other on the base plate. This setup will form the foundation for the weld pad. Carefully deposit single weld bead layers between the copper bars, ensuring the height reaches a minimum of 13 mm. Plate thickness plays a crucial role in maintaining the weld pad's integrity during the welding process, so adhering to specified requirements.

Step Two: Select the Right Material

The choice of base plate material is vital for accurate ferrite content measurement. Opt for Type 301, 302, or 304 austenitic stainless steel, which complies with ASTM Specification A167 or A240. Alternatively, carbon steel may be used, but ensure the weld pad is built up to a minimum height of 18 mm to avoid adversely affecting the results.

Step Three: Determine Welding Parameters

Before proceeding with the welding process, identify the appropriate welding current based on the electrode's size. Ensure a short arc length for precise weld seams and uniform distribution of the weld metal. Ray diffraction techniques, coupled with optical microscopy, aid in evaluating the weld structure and ferrite content.

Step Four: Implement Welding Procedure

To maintain homogeneity, alternate the welding direction with each pass. Position the weld stops and starts at the ends of the weld build-up to create consistent weld seams. Proper welding parameters and technique are crucial to achieving a high-quality weld with the desired ferrite content.

Step Five: Control Interpass Cooling

Proper interpass cooling prevents stress corrosion cracking and maintains the weld's mechanical properties. Clean each pass before depositing the next weld bead, and limit the maximum interpass temperature to 95°C. Cooling the weld pad between passes can be achieved through quenching in water no sooner than 20s, ensuring no adverse effects on the ferrite number measurement. Each pass of the last layer shall be air cooled to a temperature below 425°C before water quenching.

Step Six: Surface Preparation for Accurate Ferrite Measurement

When the anticipated ferrite content is 30 FN or less, utilize a 360 mm mill bastard file to draw file the surface. Achieving a smooth finish with no traces of weld ripple is vital for accurate optical microscopy and ferrite number readings. This step ensures precise ferrite content measurement.

Step Seven: Employing Optical Microscopy

Using optical microscopy, examine the prepared surface to evaluate the weld's microstructure and ferrite content. This analysis is essential for confirming the weld's integrity and high-quality ferrite distribution.

Step Eight: Measure Ferrite Number

Determine the ferrite number using appropriate methods such as ASTM E562. Accurate ferrite number measurement aids in assessing corrosion resistance and mechanical properties of the duplex stainless steel weld metal.

By following this step-by-step guide and incorporating plate thickness considerations, ray diffraction techniques, optical microscopy, and ferrite number measurement, welders can ensure high-quality welds with desirable ferrite content in duplex stainless steel. Properly prepared weld pads prevent stress corrosion cracking, contributing to the longevity and performance of welded components.