Metallurgical Evaluation

Overview

The 100,000+ replicated microstructure database. The reference base sits behind the in-situ replica field service: replicated structures 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 grades A through E).

The metallurgists at TCR have deep expertise in Metallographic preparation and examination to evaluate the characteristics of metals. They are highly skilled to assess a particular material’s heat treatment condition, microstructure, and forming process. The team undertakes macro and micro examination including Weld Examination, Case Depth and Decarburization Measurement, Micro Hardness Testing and Coating/Plating evaluation.

The Metallography department employs the 3 different SEM/EDAX, Inverted Metallurgical microscope, Olympus GX51 and the Leco 500 microscope with an Image Analysis System. 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 decarburization, phase/volume percentage, grain growth, inclusion rating, particle size, volume percentage, particle count, porosity and coating thickness.

TCR undertakes metallurgical evaluation using SEM, EDAX, XRD and TEM technologies

The ambit of frequently tested services in TCR metallography lab include:

• Microstructure Examination (Routine) with two photographs
• NDT microstructure with two photographs
• Microstructure with Comment on Heat Treatment
• Microstructure examination for failure related study
• Grain size distribution chart on Image Analysis (With print out)
• Prior austenite grain size measurement
• Prior austenite grain size measurement by Mc Quid Ehn method (including carburising)
• Oxide-scale/Nitriding/Carburising/Decarburising/ Coating – Measurements. (Avg. of 3 readings)
• Grain size Measurement as per ASTM E112 with photograph
• Linear measurement, up to 3 measurements
• Inclusion Rating as per ASTM E45 Method A with photograph
• Inclusion rating as per ASTM E45 with photograph
• Colour Metallography (With two Photos)
• Delta ferrite from SS weld microstructure, Sigma phase, volume fraction by microstructure examination (Avg. 3 frames)
• % Nodularity, Nodule Count as per ASTM A247 and IS 1865
• Porosity Analysis as per ASTM A 276
• Decarburization Level as per IS 6396 And ASTM E 1077
• Phase Distribution as per ASTM E 562 / 1245
• Powder Particle Size Measurement (Avg. 5 Frames)
• Coating Thickness Measurement as per ASTM B 487
• Retained Austenite Measurement with Electro Polish and Copper Deposition Method, And Calculation On Image Analysis Software from Microstructure Examination. (Avg. 3 Frames)
• Micro-Hardness Testing
• Micro Hardness Profile For Case Depth Measurement (Max. 10 Readings)
• Macro Etch Test Up To 100 Mm (Including Photo & Comments)
• Macro Etch Test Between 100 To 200 Mm (Including Photo & Comments)
• Macro Etch Test Over 200 Mm (Including Photo & Comments)
• Fractography by Stereo Microscope
• Fractography by SEM
• Coating Thickness by SEM
• Microstructure Examination Test With Photographs, Grain Size Comment On Carbide Precipitation, Nitrides & Intermetallic Phases In Haz, Parent, Weld As Per A-923 METHOD A, ASTM E-45 for Inclusion Rating
• Hydrogen Embrittlement on Copper
• Ferrite As Per ASTM E562 per Phase per Sample
• Intermetallic Phase (Chi, Sigma, Laves Nitrate Carbide) per phase per Sample
• Intermetallic Phases In Weld, Parent Material (PM), Heat Affected Zone (HAZ) per phase per Sample
• Microstructure Test with Photograph (For Sigma Phase)
• Microstructure Test With Photograph (For Ferrite Content)
• Analysis Of a Given SEM Image for Particle Size and Particle Size Distribution
• (Max/Min, Size/Frequency Information) Of the Dispersed Phase in a Continuous Phase Matrix.
• SEM Analysis with Single Image
• Delta Ferrite Measurement by Ferritscope
• Pit Dimension Measurement
• EDAX / EDS Analysis
• XRD Analysis
• In-Situ Replica Interpretation only On a Client Supplied Replica. (Please Note: TCR will not be held responsible for accurate data interpretation in areas where a TCR technician has not taken the replicas
• Structural Examination
• Inclusion Rating as ASTM E45 – Method D (Set Of Six Specimen)
• Volume Fraction Measurement (30 Frames) as per ASTM E 562
• Microstructure as per A 923 Method A
• Microstructure Carbide Network as per SEP 52100 Chart
• In-Situ Metallography
• Step Macro without Photograph – Each Step
• Step Macro with Photograph – Each Step
• Macro Measurement (MLP/Penetration) -Each
• Depth Of Attack
• Banding Index
• Intermetallic Phases
• Coating/ Plating Thickness/Mesh Size
• Austenitic Grain Size with Photographs (Up To 3 Samples)

Macro-Examinations

In Macro-etching a specimen is etched and macro-structurally evaluated at low magnifications. It is a frequently-used technique for evaluating steel products such as billets, bars, blooms and forgings. There are several procedures for rating a steel specimen by a graded series of photographs, showing the incidence of certain conditions and is applicable to carbon and low alloy steels. A number of different etching reagents may be used depending upon the type of examination. Steels react differently to etching reagents because of variations in chemical composition, the method of manufacturing, heat treatment, and many other variables.

Macro-Examinations are also performed on polished and etched cross-sections of welded material. During the examination, a number of features can be determined including the weld run sequence, which is vital for weld procedure qualifications tests. Apart from this, any defects on the sample are assessed for relevant specifications and compliance. Slag, porosity, lack of weld penetration, lack of sidewall fusion and poor weld profile are among the features observed in this type of examination.

It is procedural to identify such defects, either by standard visual examination or at magnifications of up to 50X. It is also routine to photograph the section to provide a permanent record and this is known as a photomacrograph.

Micro Examination

This is performed on samples that are either cut to size or mounted on a resin mould. These samples are polished to a fine finish, typically a one-micron diamond paste and prior to an examination on the metallurgical microscope, it is usually etched in an appropriate chemical solution. Micro-examination is performed for a number of purposes, the most common of which is to assess the structure of the material. It is also customary to examine for metallurgical anomalies such as third phase precipitates, excessive grain growth, etc. Many routine tests such as phase counting or grain size determinations are performed in conjunction with micro-examinations.

Weld Examination

Metallographic weld evaluations take place in many forms. In its most simple format, weld deposits can be visually examined for large-scale defects such as porosity or lack of fusion defects. On a micro scale, the examination can take the form of phase balance assessments from weld cap, weld root or can even be checked for non-metallic or third phase precipitates. Examination of weld growth patterns is also used to determine the reasons for poor mechanical test results. For example, an extensive central columnar grain pattern can cause a plane of weakness, giving poor charpy results.

Case Depth

Case hardening may be defined as a process for hardening ferrous materials in such a manner that the surface layer (known as the case) is substantially harder than the remaining materials (known as the core). This process is controlled through carburising, nitriding, carbonitriding, cyaniding, induction, and flame hardening. The chemical composition and mechanical properties are affected by these practices. The methodology utilised for determining case depth can either be chemical, mechanical or visual and the appropriate one is selected based on specific requirements.

Decarburization Measurement

This method is designed to detect changes in the microstructure, hardness or carbon content at the surface of steel sections due to decarburisation. To determine the depth, a uniform microstructure, hardness or carbon content of the specimen interior is observed. This method detects surface losses in the carbon content due to heating at elevated temperatures

Coating / Plating Evaluation (ASTM B487, ASTM B748)

A coating or plating application is used primarily for the protection of the substrate. Thickness is an important factor in the performance of the coating or plating. A portion of the specimen is cut, mounted transversely and is prepared in accordance with acceptable or suitable techniques. The thickness of the cross section is measured with an optical microscope. When the coating or plating is thinner than .00020, the measurement is taken with the scanning electron microscope.

Cross-sectioned metallographic examinations of substrates with plating, surface evaluations, thickness measurements, weight per volume and even salt spray testing can aid in the evaluation of plating.

Surface Evaluation

Surface inspection includes the detection of surface flaws along with the measurement of surface roughness. One of the methods used to perform this test is the use of a laser light. Measurement and analysis is possible when scattered light is reflected off the surface of a sample, An alternative method is the use of a motorized stylus (profilometer), where the stylus is placed on the surface and the texture of the material is measured in micro-inches or millimetres.

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

In order to establish a scale for grain size, ASTM E112 shows charts with outline grain structures for various dimensions. These universally accepted standards range from ASTM NO. 00 (very coarse) to 10 (very fine). A material's grain size is important as it affects its mechanical properties. In most materials, a refined grain structure gives enhanced toughness, and alloying elements are deliberately added during the steel-making process to assist with grain refinement. Grain size is determined from a polished and etched sample, using optical microscopy at a magnification of 100X.