Complete Workflow for Metallographic and Hardness Evaluation of Weld Joints

Applicable Samples and Objectives

This workflow applies to the evaluation of macroscopic morphology, zonal microstructure and hardness distribution of fusion-welded (arc, laser, electron-beam, etc.) butt and fillet joints. The objective is to complete, from a single preparation, the weld-profile examination, zonal microscopy of base metal/heat-affected zone/fusion line/weld metal, and a hardness line scan across the joint, comprehensively assessing the soundness of the welding procedure and locating weak zones.

Complete Workflow

1. Sampling and Cutting

   Cut a cross-section perpendicular to the weld axis; the section must fully contain the base metal, the heat-affected zones on both sides and all weld metal. Section large joints with a precision cut-off machine under ample coolant to prevent structural change.

2. Mounting

   Mount small sections as a whole; clamp thin-sheet joints along the thickness direction for reinforcement. Leave grinding allowance during mounting so that the final examination plane lies at the center of the weld cross-section.

3. Grinding and Polishing

   Grind the macro-examination surface to P600~P1200; continue the microscopic surface to P2000 and polish with 3 μm and 1 μm diamond. For multi-pass welds, ensure no residual scratches at the interfaces between passes.

4. Macro-Etching

   Select the etchant by material (10% nital or ammonium persulfate solution for carbon steels; electrolytic etching for stainless steels). Swab or immerse until the bead profile, fusion line and HAZ boundaries are clearly revealed, for measuring penetration, width and reinforcement, and for detecting macroscopic imperfections such as lack of fusion, slag inclusions, porosity and cracks.

5. Zonal Microscopy

   After re-polishing, etch with a suitable reagent (4% nital for carbon steels) and examine at 100×~500× by zone: base metal, heat-affected zone (coarse-grained/fine-grained regions), fusion line and weld metal, recording the structure type and grain condition of each zone.

6. Microhardness Line Scan

   On the polished surface (hardness may be tested before etching), scan with a microhardness tester perpendicular to the fusion line, usually at 1 kgf or 0.5 kgf, with indent spacing of 0.5~1 mm (refined to 0.1~0.2 mm where the HAZ is narrow). Scan lines are generally placed about 2 mm below the surface and at root level, covering the full path of base metal–HAZ–weld metal.

7. Data Processing and Determination

   Plot the hardness distribution curve, mark the hardness range of each zone, check for hardening peaks or softened bands in the HAZ, and judge against the hardness limits specified in the material and welding procedure specifications.

8. Recording and Reporting

   The report includes the macrograph (with penetration and width annotated), zonal micrographs, the hardness curve and the conclusion, and states the welding process, material grades and etchants used.

Zonal observation highlights:

Key Control Points

• The section should avoid abnormal arc-start/arc-stop regions, or be sampled deliberately for that purpose and noted.
• Macro-etching and microscopy require different surface finishes; perform macro examination first, then re-polish for microscopy.
• Keep etching time as short as practical; over-etching blurs the fusion-line boundary and affects penetration measurement.
• The hardness scan must cross the fusion line perpendicularly; for narrow HAZ (e.g. laser welds), refine the step and reduce the load.
• Indents must not straddle the fusion line or fall on imperfections; abnormal points should be annotated, not simply discarded.
• Stainless and dissimilar-metal joints require separate etchant selection to avoid compromising one side for the other.

Common Deviations and Remedies

• Unclear macro profile: re-grind and re-polish, change the etchant or switch to electrolytic etching.
• Missed hardness peak in the coarse-grained HAZ: densify indents on both sides of the fusion line, reducing the step to 0.1 mm.
• Zones hard to distinguish after etching: locate at low magnification first, then examine each zone at higher magnification.
• Large scatter in the hardness curve: check surface flatness and indent spacing; add parallel scan lines and average.
• Grinding through thin-sheet joints: reduce stock removal, clamp multiple sheets and start from finer grits.

Reference Standards

• ISO 17639 Destructive tests on welds in metallic materials — Macroscopic and microscopic examination of welds
• GB/T 26955 Destructive tests on welds in metallic materials — Etchants for macroscopic and microscopic examination
• GB/T 2654 Hardness test methods on welded joints
• GB/T 4340.1 Metallic materials — Vickers hardness test — Part 1: Test method
• GB/T 13298 Metallographic examination methods for metals

The above is a methodological example for technical communication and process discussion, not actual measured data from a specific customer; the actual test plan, sampling locations and acceptance limits are subject to sample evaluation and the procedure confirmed by both parties.