Macro vs Micro Tests in Welding — Complete Guide to Macroscopic and Microscopic Examination

Macro and micro tests are two of the most important destructive examination techniques used in weld qualification and quality assurance. Together, they form a complementary pair: macroscopic examination assesses the overall geometry, fusion quality, and large-scale discontinuities of a weld cross-section, while microscopic examination probes the metallurgical microstructure of the weld metal, fusion zone, and heat-affected zone at grain level. Neither test alone tells the complete story — a weld that passes macro examination can still harbour microstructural features that compromise toughness, creep resistance, or susceptibility to hydrogen embrittlement in service.

In weld procedure qualification (PQR) testing under ASME Section IX, AWS D1.1, and ISO 15614, macro examination is a standard deliverable in virtually every qualification package. Micro examination is required selectively — for high-alloy materials, nuclear and cryogenic applications, sour service welds, and wherever the construction code or client specification demands metallographic verification of phase content or grain structure. Understanding when each test is required, how specimens are prepared, what each test can and cannot reveal, and how results are interpreted are core competencies for any welding engineer, CWI, or quality manager.

This guide covers the full scope of both examination methods: the science behind them, specimen preparation procedures, etching reagents, acceptance criteria under major welding codes, industry-specific requirements, and the interaction between macro/micro findings and weld procedure qualification records.

Macro Test — Macroscopic Weld Examination

Macroscopic examination of welds is the process of visually inspecting a transverse cross-section of a welded joint, after appropriate surface preparation and chemical etching, to assess the overall soundness and geometry of the weld. The examination is carried out with the naked eye or at magnifications up to 10x using a hand lens or low-power stereo microscope. Because it requires no specialised laboratory equipment and can be completed in under an hour, macro examination is one of the most cost-effective and universally applied tests in weld quality assurance.

What Macro Examination Reveals

After etching, the weld metal, fusion line, HAZ, and base metal are differentiated by their distinct contrast under illumination. The examiner can assess:

• Weld profile and geometry — bead width, reinforcement height, root penetration, weld toe angle
• Fusion at the root — whether the root pass has fully fused the joint root without lack of penetration (LOP) or lack of fusion (LOF)
• Inter-run fusion — fusion between weld passes in multi-run welds; poor inter-run fusion appears as distinct boundaries between runs without proper fusion
• HAZ extent and symmetry — the width of the heat-affected zone on each side of the weld, which indicates heat input and thermal cycle characteristics
• Large-scale defects — porosity clusters, slag inclusions, solidification cracks, hot cracks, and cold cracks visible at macro scale
• Number and sequence of weld passes — which can be compared against the WPS to confirm procedure compliance
• Dilution and weld metal geometry — relevant for overlay and cladding applications

Figure 1. Etched macro cross-section of a tube-to-tubesheet weld, showing the weld profile, fusion line, and HAZ extent. Macro examination is mandatory for tube-to-tubesheet PQR qualification under ASME Section IX QW-193.

Specimen Preparation for Macro Examination

The quality of the macro result is entirely dependent on the quality of specimen preparation. A poorly ground or inadequately etched specimen can miss genuine defects or create artefacts that are mistaken for flaws. The procedure below is applicable to carbon and low-alloy steel weld macro specimens.

1. Sectioning: Cut a transverse section perpendicular to the weld axis using an abrasive cut-off saw or band saw with continuous water cooling. The section should be at least 6–12 mm thick to provide a stable, flat grinding surface. Avoid thermal cutting, which introduces a HAZ into the section face.
2. Coarse grinding: Grind the cut face on a bench grinder or belt grinder using 80–120 grit abrasive to remove the saw cut damage layer and establish a flat surface. Keep the face as flat and perpendicular to the weld axis as possible.
3. Progressive abrasive grinding: Progress through 120, 240, 400, and 600 grit silicon carbide abrasive papers on a flat platen (either manual or motorised). At each grade, rotate the specimen 90 degrees to the previous direction and grind until all scratches from the previous grade are eliminated. Rinse thoroughly between grades.
4. Cleaning and drying: Rinse with clean water, degrease with acetone or ethanol, and dry completely with warm air or a clean cloth before etching. Any residual contamination will inhibit the etch.
5. Etching: Apply the appropriate etchant to the ground surface by swabbing with a cotton ball or gauze pad. For carbon steels in production testing, 10% ammonium persulfate aqueous solution is most common (swab 30–60 seconds). For nital (2–5% nitric acid in ethanol), swab for 5–20 seconds.
6. Rinse, dry, and examine: Immediately rinse with clean water (and neutralise with 10% sodium hydroxide if using nital), dry, and examine under good illumination. Photograph at appropriate magnification for the record.

Micro Test — Microscopic Weld Examination

Microscopic examination of welds — weld metallography — involves preparing a section from the weld to a mirror-quality polished finish, etching to reveal microstructural features, and examining using an optical microscope at magnifications from 50x to 1000x. The technique provides information that cannot be obtained by any other non-destructive or macro method: the grain structure, phase constitution, and sub-micron features of the weld metal, fusion zone, and heat-affected zone are all revealed at this scale.

Micro examination is essential when the performance of the weld depends on microstructural control — which is the case for all high-strength, high-alloy, and creep-resistant steels used in power generation, nuclear, sour service oil and gas, and cryogenic applications. For a P91 chrome-moly weld, for example, micro examination is the only way to confirm that the tempered martensite lath structure required for creep strength has been achieved throughout the weld metal and HAZ — a requirement that cannot be inferred from macro appearance or even hardness testing alone.

Figure 2. Optical micrograph of a P91 (Grade 91) weld HAZ showing the tempered martensite lath structure characteristic of correctly processed 9Cr-1Mo-V steel. Micro examination is essential for verifying microstructural integrity in creep-resistant alloys.

What Micro Examination Reveals

• Grain size and grain growth — particularly in the CGHAZ (coarse-grained HAZ) adjacent to the fusion line, where excessive grain growth reduces toughness and fatigue resistance
• Phase constituents — ferrite, pearlite, bainite, martensite, Widmanstatten ferrite, and retained austenite in carbon and low-alloy steels; austenite, delta ferrite, and sigma phase in stainless steels
• Microcracks — solidification cracks in weld metal, liquation cracks in the CGHAZ (particularly in nickel alloys), and stress-relief cracks in the HAZ of chromium-containing steels
• Precipitate distribution — carbide precipitation, sensitisation in stainless steels (see stainless steel weld decay), and creep-damage precipitates in service-exposed welds
• Hardness variation across weld zones — though hardness is more precisely measured by microhardness traverses, micro examination gives a qualitative indication of hard phases
• Inclusions and second-phase particles — oxide and sulphide inclusions that influence toughness and fatigue crack initiation
• Temper bead effectiveness — in temper bead welding of P-No. 4 and 5 materials without PWHT, micro examination confirms that the CGHAZ has been tempered by subsequent passes

Zones Examined in a Weld Micro Section

Specimen Preparation for Micro Examination

Micro specimen preparation follows the same early stages as macro preparation but extends to a much finer final finish. The surface must be free of all preparation artefacts — scratches, smearing, pitting, and pull-out of inclusions — that could be mistaken for microstructural features. This level of preparation is more time-consuming and typically requires a motorised polishing system with precise control of load and time.

1. Sectioning and mounting: Section a smaller specimen (typically 15–25 mm diameter for mounting) and mount in thermosetting or cold-setting resin to facilitate handling and edge retention. Edge retention additives in the mounting medium help preserve the specimen edge near the weld surface.
2. Grinding through SiC grades: Progress through 120, 240, 400, 600, 800, and 1200 grit silicon carbide papers, rotating 90° between grades and removing all previous-grade scratches before advancing.
3. Diamond polishing: Polish on a nap cloth with 6 µm, then 3 µm diamond suspension. Each step takes 2–5 minutes at moderate pressure on a motorised polisher.
4. Final polishing: Finish with 1 µm diamond or 0.05 µm colloidal silica (OP-S) on a short-nap cloth. The surface should be scratch-free and reflect a clear mirror image.
5. Ultrasonic cleaning: Clean ultrasonically in ethanol for 1–2 minutes to remove any polishing media from the surface, then dry with warm air.
6. Etching and examination: Etch with the appropriate reagent (nital for carbon steels, Kalling’s for stainless steels), rinse, dry, and examine immediately on the optical microscope at 50x–500x. Capture images at representative locations across all weld zones.

Macro vs Micro Examination — Detailed Comparison

Requirements Under Welding Codes and Standards

ASME Section IX

Under ASME Section IX, macro examination is explicitly required for fillet weld procedure qualification (QW-183), where two macro sections from the test coupon must be examined and show complete fusion and freedom from cracks. For tube-to-tubesheet weld qualification under QW-193, macro examination is required on sections taken at multiple angular positions around the tube circumference to verify full fusion and groove geometry. Macro examination also forms part of the supporting evidence in groove weld PQRs to confirm the joint geometry and weld profile are consistent with the WPS.

ASME Section IX does not list micro examination as a mandatory standard test for most PQRs. However, supplementary essential variables, material-specific annexures (such as those for P-No. 15E group 1 material — P91/Grade 91), and requirements in the applicable construction code (ASME Section III, Section VIII Division 2, etc.) can impose micro examination as a qualification requirement.

AWS D1.1 Structural Welding Code

AWS D1.1 requires macro examination for prequalified and qualified procedure verification, particularly for fillet weld qualification tests and for verifying weld geometry in fatigue-critical applications. For fracture-critical bridge members, additional mechanical testing and metallographic examination requirements may be imposed by the bridge owner or engineer of record.

ISO 15614-1

ISO 15614-1 (Specification and qualification of welding procedures for metallic materials) requires macroscopic examination of the weld cross-section as part of the standard test suite. It also explicitly requires microscopic examination for ferritic steels when microstructural assessment of the HAZ is relevant to the application — particularly for materials with a nominal yield strength exceeding 460 MPa, for P91 and other creep-resistant steels, and for any material where the code or product standard specifies it.

Acceptance Criteria

Acceptance criteria for macro examination are generally stated in terms of: absence of cracks, complete fusion at the root and between weld passes, no slag inclusions exceeding specified size limits, porosity within the limits of the applicable table, and weld geometry (penetration, reinforcement, throat) within the WPS tolerances. For micro examination, acceptance criteria are more complex and typically described in terms of: absence of microcracks, absence of sensitisation in the HAZ of stainless steels, specified maximum grain size in the CGHAZ, absence of untempered martensite where the WPS requires a specified preheat and interpass temperature regime, and phase proportions within specified limits for duplex stainless steels.

Industry Applications and When Each Test Is Used

Interaction with WPS, PQR, and Essential Variables

It is important to understand that macro and micro tests are performed on the test coupon used to qualify the welding procedure — they are PQR tests, not WPS requirements. The Welding Procedure Specification (WPS) documents the variables within which the welder will operate; the PQR is the objective test evidence that the procedure, as applied, produces a weld of the required quality. Macro and micro examination are part of that test evidence.

When an essential variable changes — a different base material P-number group, a different filler metal F-number, a change in heat input beyond the qualified range, or a change in the post-weld heat treatment regime — the existing PQR is invalidated for that combination and a new test coupon must be welded and tested, including new macro (and micro, if required) examination. The P-number and material grouping system under ASME Section IX directly governs which material combinations can be qualified by a single PQR, and therefore how many macro/micro tests are needed across a project’s range of materials.

Micro Examination as a Quality Audit Tool

Beyond PQR qualification, micro examination is used as a quality audit tool in several important situations. Post-weld heat treatment (PWHT) verification for P91 and other creep-resistant steels routinely employs micro examination to confirm that the required tempering has occurred and that no untempered martensite or deleterious precipitate phases are present after the thermal cycle. In failure investigation and fitness-for-service assessments, micro examination of cross-sections from the failed weld or adjacent HAZ is often the most direct path to establishing the root cause of a weld failure — revealing whether the failure was initiated by a pre-existing crack, a microstructural anomaly, or a service-induced degradation mechanism.

Recommended Resources for Weld Metallography and Inspection

Frequently Asked Questions

Conclusion

Macro and micro examination are complementary tools in the welding quality engineer’s arsenal. Macro examination provides a rapid, cost-effective assessment of weld geometry, fusion quality, and large-scale soundness that is sufficient for the vast majority of production and procedure qualification testing. Micro examination adds the metallurgical depth that is essential for high-alloy, high-integrity, and critical-service welds where phase constitution, grain structure, and the character of the HAZ directly govern long-term performance.

Understanding both tests — their preparation procedures, their detection capabilities and limitations, the codes that require them, and the features that constitute acceptance or rejection — is fundamental knowledge for welding engineers, CWIs, metallurgists, and quality managers working in any sector where weld integrity carries safety or reliability consequences. Used together, and supported by the full suite of mechanical tests in weld qualification, they form a rigorous quality assurance framework that underpins confidence in welded structures across all critical industries.

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