Metallurgical Testing Laboratory

What does a metallurgical testing laboratory examine?

The metallurgical testing laboratory, part of TCR's materials testing laboratory at Mahape, examines metals for microstructure, grain size, inclusion content, phase balance, case depth, decarburisation and coating thickness, on parent metal, welds and heat-affected zones. Each result is reported against the standard the specification names, with photomicrographs.

• Microstructure: routine examination, comment on heat treatment, and examination for failure investigation.
• Grain size: ASTM E112, ISO 643 and IS 4748, including prior austenite grain size.
• Inclusion rating: ASTM E45 Methods A and D, DIN EN 10247 and IS 4163.
• Phases: ferrite and volume fraction to ASTM E562, detrimental intermetallic phases to ASTM A923, graphite and nodularity in cast iron to ASTM A247 and IS 1865.
• Surface layers: case depth to IS 6416, decarburisation to ASTM E1077 and IS 6396, coating thickness to ASTM B487.
• Macro examination: macro etch to ASTM E340 and ASTM E381, and weld macro examination to ASME Section IX.

A database of 100,000+ replicated microstructures sits behind the in-situ replica field service: the structures are retained physically and image-archived since the early 1990s, with calibrated reading metallurgists carrying institutional memory across creep cavitation classes, decarburisation, sigma phase, theta phase, M23C6 carbide morphology, and reformer-tube specific damage signatures (carburisation, carbide coarsening, internal oxidation, reformer-tube creep damage graded 0 to 5 on the Neubauer scale).

TCR's metallurgists prepare and examine metallographic specimens to evaluate the characteristics of metals, assessing a material's heat treatment condition, microstructure, and forming process. The team undertakes macro and micro examination including Weld Examination, Case Depth and Decarburisation Measurement, Micro Hardness Testing and Coating/Plating evaluation.

The metallography department uses an inverted metallurgical microscope, an Olympus GX51 and a Leco 500 microscope with an image analysis system, and draws on the four SEM/EDAX instruments at TCR Advanced, Vadodara. The technical team has indigenously developed a microstructure characteriser software that assists with the analysis of images to determine microstructural degradation due to creep. The software can also calculate the graphitisation, depth or width of decarburisation, phase/volume percentage, grain growth, inclusion rating, particle size, volume percentage, particle count, porosity and coating thickness.

TCR undertakes metallurgical evaluation using SEM, EDAX and XRD.

Frequently requested metallographic tests include:

• Routine microstructure examination with photomicrographs, including comment on heat treatment and NDT microstructures
• Microstructure examination for failure related study
• Grain size to ASTM E112, with grain size distribution by image analysis
• Prior austenite grain size, including the McQuaid-Ehn method for carburising steels
• Inclusion rating to ASTM E45, Methods A and D
• Oxide scale, nitriding, carburising, decarburising and coating depth measurement
• Decarburisation level to IS 6396 and ASTM E1077
• Phase distribution and volume fraction to ASTM E562 and ASTM E1245
• Delta ferrite in stainless steel welds, by microstructure and by Feritscope
• Sigma phase and intermetallic phases (chi, sigma, Laves, nitride, carbide) in weld, parent metal and HAZ, including ASTM A923 Method A
• Nodularity and nodule count to ASTM A247 and IS 1865
• Carbide network to the SEP 52100 chart, and banding index
• Porosity analysis, powder particle size measurement and particle size distribution from SEM images
• Retained austenite by electro-polish and copper deposition, calculated on image-analysis software, performed under NADCAP AC7101
• Colour metallography
• Coating and plating thickness to ASTM B487, by optical microscope and by SEM
• Micro-hardness testing, and micro-hardness profiles for case depth
• Macro etch tests, step macros and macro measurement (MLP and penetration)
• Depth of attack and pit dimension measurement
• Fractography by stereo microscope and by SEM
• SEM imaging, EDAX/EDS analysis and XRD analysis
• Hydrogen embrittlement of copper
• In-Situ Metallography, and interpretation of client-supplied replicas (where a TCR technician has not taken the replica, TCR is not responsible for the accuracy of interpretation in that area)
• Structural examination

Macro-Examinations

Macro etching grades billets, bars, blooms and forgings of carbon and low-alloy steel against the graded photographs of ASTM E381 and ASTM E340. On a weld cross-section it records the weld run sequence that a weld procedure qualification asks for, and assesses slag, porosity, lack of penetration, lack of sidewall fusion and weld profile against the specification, with a photomacrograph for the record.

Micro Examination

Micro examination assesses the structure of the material and looks for metallurgical anomalies such as third-phase precipitates and excessive grain growth. Phase counting and grain size determination are reported from the same section.

Weld Examination

Metallographic weld evaluation ranges from large-scale defects such as porosity and lack of fusion to phase balance in the weld cap and root, and non-metallic or third-phase precipitates. Weld growth patterns also explain poor mechanical results: an extensive central columnar grain pattern can create a plane of weakness that gives poor Charpy values.

Case Depth

Case depth measurement establishes how far a carburised, nitrided, carbonitrided, cyanided, induction-hardened or flame-hardened surface layer extends towards the core, by the microscopic or the hardness method of IS 6416, chosen to suit the specification.

Decarburisation Measurement

Decarburisation measurement establishes the depth to which the surface of a steel section has lost carbon on heating at elevated temperature, read from the change in microstructure, hardness or carbon content against the interior, to ASTM E1077 and IS 6396.

Coating / Plating Evaluation (ASTM B487, ASTM B748)

Coating and plating thickness is measured on a cross-section by optical microscope to ASTM B487, and by scanning electron microscope where the layer is too thin to resolve optically. Cross-section examination, surface evaluation and salt spray testing together show how a plating will protect its substrate.

Surface Evaluation

Surface evaluation detects surface flaws and measures surface roughness, by laser light scattering or by stylus profilometer.

Grain Size Determination (ASTM E112, ISO 643, IS 4748)

Grain size is rated against the charts of ASTM E112, which run from ASTM No. 00 (very coarse) to 10 (very fine), and to ISO 643 and IS 4748. It matters because grain size governs mechanical properties: in most steels a refined grain structure gives better toughness.