BME L1

Macroscopic Investigation: Principles & Goals

1.1 Definition

  • Visual examination of a metal’s surface or specifically prepared cross-section performed under the naked eye or low magnification (up to 20×).

  • Reveals large-scale features ranging from fractions of millimeters to centimeters.

1.2 Primary Goals

a) Characterize the Internal Structure

  • Casting defects:

    • Shrinkage cavities.

    • Gas porosity.

    • Cold shuts.

  • Hot- and cold-worked structures:

    • Columnar grains.

    • Fibrous flow lines.

  • Other characteristics:

    • Grain size.

    • Phase distribution.

    • Non-metallic inclusions (e.g., oxides, sulfides).

    • Heat-Affected Zones (weld HAZ): width, grain growth.

b) Evaluate Material Quality and Processing History

  • Assessment questions:

    • Was the casting sound?

    • Was the forging/rolling adequate?

    • Was the heat-treatment appropriate?

  • Measurement of surface-hardened or carburized layer thickness.

c) Failure Analysis

  • Evaluation of crack origin, propagation direction, and fracture mode (brittle vs. ductile):

    • Approximations of loading type (e.g., tensile, shear, fatigue).

  • Estimation of service temperature at failure:

    • Example: “quench cracks” in hardened steel indicate high-stress thermal events.

Sample Preparation & Etching Techniques

2.1 Cutting and Sectioning

  • Equipment: Precision saw (abrasive or band saw) used with liquid coolant to prevent thermal-microstructural alterations.

  • Orientation: Cut planes should be aligned to intersect suspected defect paths or the weld centerline.

2.2 Grinding and Polishing Steps

  1. Coarse grinding: Use SiC papers P240–P400 under water.

  2. Medium grinding: Use P600–P800 to remove deep scratches.

  3. Fine grinding: Use P1200–P2500 for achieving a mirror finish.

  4. Polishing: Diamond suspension (6 μm → 3 μm → 1 μm) or colloidal silica for the final surface finish.

  • Key considerations: Maintain uniform speed, apply light pressure, and use intermittent cleaning to avoid embedded abrasives or heat build-up.

2.3 Chemical Etching

  • Process: Selective etchants attack grain boundaries, different phases, or carbides, creating contrast through relief or color change.

  • Common Steel Etchants:

    • 2% Nital (2% HNO₃ in ethanol): General etchant for ferrite/pearlite contrast.

    • 5–10% Picral (picric acid in ethanol): Better for revealing carbide networks.

    • Glyceregia (HCl + glycerol + HNO₃): Used to reveal martensite laths.

  • Procedure:

    1. Immerse or swab etchant on the polished sample for a controlled time (5–30 s).

    2. Rinse immediately in water, followed by alcohol, and then air dry.

    3. Examine under raking light to accentuate relief.

  • Interpretation of Etch Results:

    • Large equiaxed grains: Indicate coarse-grained structure and slow cooling.

    • Fine grains: Suggest rapid solidification or grain-refinement treatment.

    • Banded structure: Represents segregation of alloying elements and poor mixing.

    • Martensitic laths: Needle-like structures indicating quenching.

Failure Modes & Fractographic Features

3.1 Brittle Fracture

  • Characteristics:

    • Cleavage facets: Flat, shiny surfaces oriented along specific crystallographic planes.

    • River patterns: Step lines showing local crack-front advance.

    • Granular appearance: Occurs in inter-granular brittle fractures if grain boundaries are weakened by segregation or corrosion.

  • Sequence of events:

    1. Flaw initiation at stress concentrator (e.g., inclusion, sharp notch).

    2. Subcritical crack growth occurs under cyclic/static loads; crack advances stably.

    3. Instability occurs when $KI \, > \, K{IC}$ (fracture toughness), leading to sudden failure.

3.2 Ductile Fracture

  • Characteristics:

    • Micro-void coalescence indicates ductility, observed as dimples on the fracture surface.

    • Cup-and-cone structure: Featuring a 45° shear lip surrounding a central cup in tensile specimens.

    • Necking observed: Localized reduction in cross-section before final break.

3.3 Fatigue Fracture

  • Features:

    • Beach marks or striations: Progressive crack-front markings.

    • Ratchet marks: Indicate multiple initiation sites coalescing.

    • Distinct area of origin (smooth) and fast fracture zone (rough).

Macroscopic Weld Examination

4.1 Zones to Inspect

  • Base Metal: Should show unaffected microstructure.

  • Heat-Affected Zone (HAZ): Check for grain coarsening or softening, visible color, and relief changes post-etch.

  • Fusion Zone (Weld Metal): Inspect filler-metal microstructure for potential porosity, cracks, and inclusions.

4.2 Key Indicators

  • Observations for defects:

    • Undercut, overlap, lack of fusion at root or cap.

    • Crater cracks or solidification cracks at weld toe.

    • Comparison of penetration depth versus throat thickness (must meet code requirements).

    • Weld bead profile (and how it can affect stress concentration: convex vs. concave).

Sulphur Print (Baumann) Expanded

5.1 Why Sulphur?

  • Rationale: Sulfide inclusions (e.g., MnS) concentrate sulfur, which upon acid activation, darkens photo paper.

5.2 Detailed Steps for Conducting Sulphur Print

  1. Soaking: Photographic paper in 5–10% H₃SO₃ for 3–5 minutes at room temperature.

  2. Draining: Hang paper vertically to remove excess acid, avoiding pooling.

  3. Application: Press damp paper (sensitive side) onto a clean, grease-free sample.

  4. Pressure: Use a soft rubber roller to eliminate air gaps for uniform contact.

  5. Development time: Varies with steel grade—taking longer for low-sulfur steels (up to 2 minutes).

  6. Washing: Gently roll with cotton wool in running water for 10 minutes.

  7. Fixing: Use standard photographic fixer (sodium thiosulfate) for 10 minutes, followed by water rinse (30 minutes) and drying.

5.3 Interpretation of Results

  • Visual Indicators:

    • Dark patches/lines suggest sulfide clusters or cracks opening to the surface.

    • Uniform light gray indicates low sulfur and good cleanliness.

Non-Destructive Flaw Detection Techniques

6.1 Ultrasonic Testing (UT)

  • Frequency range: 0.5–15 MHz pulses.

  • Modes:

    • Pulse-Echo: Same transducer sends and receives echoes.

    • Through-Transmission: Separate transmitter and receiver; defects attenuate signals.

  • Couplant: Requires gel or oil to couple transducer to test surface.

  • Advantages: Deep penetration (>500 mm in steel), quantitative depth measurement, real-time assessment.

  • Limitations: Requires good surface finish, expertise to interpret echoes, presence of geometric dead zones near surface.

6.2 Radiographic Testing (RT)

  • Energy Sources: X-ray (50 keV–10 MeV) or Gamma-ray sources (e.g., Ir-192, Co-60).

  • Film vs. Digital Detector: Digital systems provide faster processing and better dynamic range.

  • Sensitivity: Capable of detecting volumetric defects ≥0.5 mm in steel welds.

  • Advantages: Provides permanent records and superior resolution of porosity and inclusions.

  • Limitations: Safety hazards from ionizing radiation, complications with thick or dense sections, accessibility is restricted to one side.

6.3 Dye Penetrant Inspection (DPI/PT)

  • Types of Penetrants: Visible-dye, fluorescent (requiring UV lamp).

  • Types of Developers: Solvent-removable vs. water-washable, selected based on convenience and contamination considerations.

  • Advantages: Very sensitive to fine surface cracks (width <0.05 mm) and cost-effective.

  • Limitations: Only identifies surface-breaking flaws; porous coatings or rough surfaces can produce false results; additional time required for dwell and cleaning.

6.4 Magnetic Particle Inspection (MPI)

  • Magnetization Methods:

    • Longitudinal (current applied through the part).

    • Circular (using yoke or coil around the same part).

  • Particle Types: Dry or wet suspension in oil or water, available as colored or fluorescent under UV light.

  • Field strength: Typically ranges from 2 kA/m to 200 kA/m.

  • Advantages: Rapid and direct visualization, sensitivity to surface/near-surface cracks (depth under 1 mm).

  • Limitations: Only effective on ferromagnetic materials; geometry can restrict access; demagnetization is necessary afterwards.

Combining Methods & Best Practices

  • Multi-method Approach: Using various methods can provide higher confidence in assessments, for example, combining macroscopic etch, UT, and RT for critical welds.

  • Record Keeping: It is essential to maintain records of etch times, cut orientations, and NDT settings for reproducibility.

  • Safety Considerations: Proper ventilation for etching fumes, radiation protection during radiographic testing, and electrical safety for ultrasonic and magnetic particle inspections.

  • Training & Standards: Follow established criteria such as ASTM, ISO, or national codes related to sample preparation, inspection acceptance criteria, and qualifications of NDT personnel (e.g., standard ISO 9712).