ASTM_E1268_01

Designation: E 1268 – 01 Standard Practice for Assessing the Degree of Banding or Orientation of Microstructures1 This standard is issued under the fixed designation E 1268; the number immediately following the designation indicates the year of original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A superscript epsilon (e) indicates an editorial change since the last revision or reapproval. INTRODUCTION Segregation occurs during the dendritic solidification of metals and alloys and is aligned by subsequent deformation. Solid-state transformations may be influenced by the resulting microsegre- gation pattern leading to development of a layered or banded microstructure. The most common example of banding is the layered ferrite-pearlite structure of wrought low-carbon and low-carbon alloy steels. Other examples of banding include carbide banding in hypereutectoid tool steels and martensite banding in heat-treated alloy steels. This practice covers procedures to describe the appearance of banded structures, procedures for characterizing the extent of banding, and a microindentation hardness procedure for determining the difference in hardness between bands in heat treated specimens. The stereological methods may also be used to characterize non-banded microstructures with second phase constituents oriented (elongated) in varying degrees in the deformation direction. 1. Scope 1.1 This practice describes a procedure to qualitatively describe the nature of banded or oriented microstructures based on the morphological appearance of the microstructure. 1.2 This practice describes stereological procedures for quantitative measurement of the degree of microstructural banding or orientation. NOTE 1—Although stereological measurement methods are used to assess the degree of banding or alignment, the measurements are only made on planes parallel to the deformation direction (that is, a longitudinal plane) and the three-dimensional characteristics of the banding or align- ment are not evaluated. 1.3 This practice describes a microindentation hardness test procedure for assessing the magnitude of the hardness differ- ences present in banded heat-treated steels. For fully marten- sitic carbon and alloy steels (0.10–0.65 %C), in the as- quenched condition, the carbon content of the matrix and segregate may be estimated from the microindentation hard- ness values. 1.4 This standard does not cover chemical analytical meth- ods for evaluating banded structures. 1.5 This practice deals only with the recommended test methods and nothing in it should be construed as defining or establishing limits of acceptability. 1.6 The measured values are stated in SI units, which are regarded as standard. Equivalent inch-pound values, when listed, are in parentheses and may be approximate. 1.7 This standard does not purport to address all of the safety problems, if any, associated with its use. It is the responsibility of the user of this standard to establish appro- priate safety and health practices and determine the applica- bility of regulatory limitations prior to use. 2. Referenced Documents 2.1 ASTM Standards: A 370 Test Methods and Definitions for Mechanical Testing of Steel Products2 A 572/A 572M Specification for High-Strength Low-Alloy Columbium-Vanadium Structural Steel3 A 588/A 588M Specification for High-Strength Low-Alloy Structural Steel with 50 ksi [345 MPa] Minimum Yield Point to 4 in. [100 mm] Thick3 E 3 Methods of Preparation of Metallographic Specimens4 E 7 Terminology Relating to Metallography4 E 140 Hardness Conversion Tables for Metals4 E 384 Test Method for Microhardness of Materials4 E 407 Test Methods for Microetching Metals and Alloys4 E 562 Practice for Determining Volume Fraction by Sys- tematic Manual Point Count4 1 This practice is under the jurisdiction of ASTM Committee E04 on Metallog- raphy and is the direct responsibility of Subcommittee E04.14 on Quantitative Metallography. Current edition approved Dec. 10, 2001. Published February 2002. Originally published as E 1268 – 88. Last previous edition E 1268 – 99. 2 Annual Book of ASTM Standards, Vol 01.03. 3 Annual Book of ASTM Standards, Vol 01.04. 4 Annual Book of ASTM Standards, Vol 03.01. 1 Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States. E 883 Guide for Reflected-Light Photomicrography4 3. Terminology 3.1 Definitions—For definitions of terms used in this prac- tice, see Terminology E 7. 3.2 Definitions of Terms Specific to This Standard: 3.2.1 banded microstructure—separation, of one or more phases or constituents in a two-phase or multiphase microstruc- ture, or of segregated regions in a single phase or constituent microstructure, into distinct layers parallel to the deformation axis due to elongation of microsegregation; other factors may also influence band formation, for example, the hot working finishing temperature, the degree of hot- or cold-work reduc- tion, or split transformations due to limited hardenability or insufficient quench rate. 3.2.2 feature interceptions—the number of particles (or clusters of particles) of a phase or constituent of interest that are crossed by the lines of a test grid. (see Fig. 1). 3.2.3 feature intersections—the number of boundaries be- tween the matrix phase and the phase or constituent of interest that are crossed by the lines of a test grid (see Fig. 1). For isolated particles in a matrix, the number of feature intersec- tions will equal twice the number of feature interceptions. 3.2.4 oriented constituents—one or more second-phases (constituents) elongated in a non-banded (that is, random distribution) manner parallel to the deformation axis; the degree of elongation varies with the size and deformability of the phase or constituent and the degree of hot- or cold-work reduction. 3.2.5 stereological methods—procedures used to character- ize three-dimensional microstructural features based on mea- surements made on two-dimensional sectioning planes. NOTE 2—Microstructural examples are presented in Annex A1 to illustrate the use of terminology for providing a qualitative description of NOTE 1—The test grid lines have been shown oriented perpendicular (A) to the deformation axis and parallel (B) to the deformation axis. The counts for N’, N||, P’, and P|| are shown for counts made from top to bottom (A) or from left to right (B). NOTE 2—T indicates a tangent hit and E indicates that the grid line ended within the particle; both situations are handled as shown. FIG. 1 Illustration of the Counting of Particle Interceptions (N) and Boundary Intersections (P) for an Oriented Microstructure E 1268 2 the nature and extent of the banding or orientation. Fig. 2 describes the classification approach. 3.3 Symbols: N’ = number of feature interceptions with test lines perpendicular to the deformation direction. N|| = number of feature interceptions with test lines parallel to the deformation direction. M = magnification. Lt = true test line length in mm, that is, the test line length divided by M. NL’ = N ’ L t NL|| = N|| Lt P’ = number of feature boundary intersections with test lines perpendicular to the deformation di- rection. P|| = number of feature boundary intersections with test lines parallel to the deformation direction. PL’ = P ’ L t > 2NL’ PL|| = P || L t > 2NL || n = number of measurement fields or number of microindentation impressions. N¯ L ’ = (N L’ n N¯ L|| = (NL || n P¯ L’ = (PL’ n > 2N¯ L’ P¯ L || = (P L || n > 2N¯ L || X¯ = mean values ( N¯ L’, N¯ L||, P¯L ’, P¯ L||) s = estimate of standard deviation (s). t = a multiplier related to the number of fields examined and used in conjunction with the standard deviation of the measurements to de- termine the 95 % CI. 95 % CI = 95 % confidence interval. 95 % CI = 6 ts =n % RA = % relative accuracy. % RA = 95 % CI X¯ 3 100 SB’ = mean center-to-center spacing of the bands. SB’ = 1 N¯ L’ . V V = volume fraction of the banded phase (constitu- ent). l’ = mean edge-to-edge spacing of the bands, mean free path (distance). l’ = 1 2 VV N¯ L’ AI = anisotropy index. AI = N¯ L’ N¯ L|| 5 P¯ L’ P¯ L|| V12 = degree of orientation of partially oriented linear structure elements on the two-dimensional plane-of-polish. V12 = N¯ L ’ 2 N¯ L || N¯ L’ 1 0.571 N¯ L || FIG. 2 Qualitative Classification Scheme for Oriented or Banded Microstructures E 1268 3 V12 = P¯ L’ 2 P¯ L || P¯ L’ 1 0.571 P¯ L|| 4. Summary of Practice 4.1 The degree of microstructural banding or orientation is described qualitatively using metallographic specimens aligned parallel to the deformation direction of the product. 4.2 Stereological methods are used to measure the number of bands per unit length, the inter-band or interparticle spacing and the degree of anisotropy or orientation. 4.3 Microindentation hardness testing is used to determine the hardness of each type band present in hardened specimens and the difference in hardness between the band types. 5. Significance and Use 5.1 This practice is used to assess the nature and extent of banding or orientation of microstructures of metals and other materials where deformation and processing produce a banded or oriented condition. 5.2 Banded or oriented microstructures can arise in single phase, two phase or multiphase metals and materials. The appearance of the orientation or banding is influenced by processing factors such as the solidification rate, the extent of segregation, the degree of hot or cold working, the nature of the deformation process used, the heat treatments, and so forth. 5.3 Microstructural banding or orientation influence the uniformity of mechanical properties determined in various test directions with respect to the deformation direction. 5.4 The stereological methods can be applied to measure the nature and extent of microstructural banding or orientation for any metal or material. The microindentation hardness test procedure should only be used to determine the difference in hardness in banded heat-treated metals, chiefly steels. 5.5 Isolated segregation may also be present in an otherwise reasonably homogeneous microstructure. Stereological meth- ods are not suitable for measuring individual features, instead use standard measurement procedures to define the feature size. The microindentation hardness method may be used for such structures. 5.6 Results from these test methods may be used to qualify material for shipment in accordance with guidelines agreed upon between purchaser and manufacturer, for comparison of different manufacturing processes or process variations, or to provide data for structure-property-behavior studies. 6. Apparatus 6.1 A metallurgical (reflected-light) microscope is used to examine the microstructure of test specimens. Banding or orientation is best observed using low magnifications, for example, 503 to 2003. 6.2 Stereological measurements are made by superimposing a test grid (consisting of a number of closely spaced parallel lines of known length) on the projected image of the micro- structure or on a photomicrograph. Measurements are made with the test lines parallel and perpendicular to the deformation direction. The total length of the grid lines should be at least 500 mm. 6.3 These stereological measurements may be made using a semiautomatic tracing type image analyzer. The test grid is placed over the image projected onto the digitizing tablet and a cursor is used for counting. 6.4 For certain microstructures where the contrast between the banded or oriented constituents is adequate, an automatic image analyzer may be used for counting, where the TV scan lines for a live image, or image convolutions5, electronically- generated test grids6, or other methods, for a digitized image, are used rather than the grid lines of the plastic overlay or reticle. 6.5 A microindentation hardness tester is used to determine the hardness of each type of band in heat-treated steels or other metals. The Knoop indenter is particularly well suited for this work. 7. Sampling and Test Specimens 7.1 In general, specimens should be taken from the final product form after all processing steps have been performed, particularly those that would influence the nature and extent of banding. Because the degree of banding or orientation may vary through the product cross section, the test plane should sample the entire cross section. If the section size is too large to permit full cross sectioning, samples should be taken at standard locations, for example, subsurface, mid-radius (or quarter-point), and center, or at specific locations based upon producer-purchaser agreements. 7.2 The degree of banding or orientation present is deter- mined using longitudinal test specimens, that is, specimens where the plane of polish is parallel to the deformation direction. For plate or sheet products, a planar oriented (that is, polished surface parallel to the surface of the plate or sheet) test specimen, at subsurface, mid-thickness, or center locations, may also be prepared and tested depending on the nature of the product application. 7.3 Banding or orientation may also be assessed on inter- mediate product forms, such as billets or bars, for material qualification or quality control purposes. These test results, however, may not correlate directly with test results on final product forms. Test specimens should be prepared as described in 7.1 and 7.2 but with the added requirement of choosing test locations with respect to ingot or continuously cast slab/strand locations. The number and location of such test specimens should be defined by producer-purchaser agreement. 7.4 Individual metallographic test specimens should have a polished surface area covering the entire cross section if possible. The length of full cross-section samples, in the deformation direction, should be at least 10 mm (0.4 in.). If the product form is too large to permit preparation of full cross sections, the samples prepared at the desired locations should have a minimum polished surface area of 100 mm2(0.16 in. 2) 5 Lépine, M., “Image Convolutions and their Application to Quantitative Metallography,” Microstructural Science, Vol. 17, Image Analysis and Metal- lography, ASM International, Metals Park, OH, 1989, pp. 103–114. 6 Fowler, D.B., “A Method for Evaluating Plasma Spray Coating Porosity Content Using Stereological Data Collected by Automatic Image Analysis,” Microstructural Science, Vol. 18, Computer-Aided Microscopy and Metallog- raphy, ASM International, Materials Park, OH, 1990, pp. 13–21. E 1268 4 with the sample length in the longitudinal direction at least 10 mm (0.4 in.). 8. Specimen Preparation 8.1 Metallographic specimen preparation should be per- formed in accordance with the guidelines and recommended practices given in Methods E 3. The preparation procedure must reveal the microstructure without excessive influence from preparation-induced deformation or smearing. 8.2 Mounting of specimens may be performed depending on the nature of the test sample or if needed to accommodate automatic polishing devices. 8.3 The microstructure should be revealed in strong contrast by any appropriate chemical or electrolytic etching method, by tinting or staining, etc. Test Methods E 407 list appropriate etchants for most metals and alloys. For certain materials, etching may not be necessary as the naturally occurring reflectivity differences between the constituents may produce adequate contrast. 9. Calibration 9.1 Use a stage micrometer to determine the magnification of the projected image or at the photographic plane. 9.2 Use a ruler to determine the length of the test lines on the grid overlay in mm. 10. Procedure 10.1 Place the polished and etched specimen on the micro- scope stage, select a suitable low magnification, for example, 503 or 1003, and examine the microstructure. Align the specimen so that the deformation direction is horizontal on the projection screen. Randomly select the initial field by arbi- trarily moving the stage and accepting the new field without further stage adjustment. 10.1.1 Bright field illumination will be used for most measurements. However, depending on the alloy or material being examined, other illumination modes, such as polarized light or differential interference contrast illumination, may be used. 10.1.2 Measurements may also be made by placing the test grid on photomicrographs (see Guide E 883), taken of ran- domly selected fields, at suitable magnifications. 10.2 Qualitatively define the nature and extent of the band- ing or orientation present in accordance with the following guidelines. Examination at higher magnification may be re- quired to identify and classify the constituents present. Fig. 2 describes the classification approach. 10.2.1 Determine if the banding or orientation present represents variations in the etch intensity of a single phase or constituent, such as might result from segregation in a tem- pered martensite alloy steel specimen, or is due to preferential alignment of one or more phases or constituents in a two-phase or multi-phase specimen. 10.2.2 For orientation or banding in a two-phase or multi- phase specimen, determine if only the minor phase or constitu- ent is preferentially aligned within the matrix phase. Alterna- tively, both phases may be aligned with neither appearing as a matrix phase. 10.2.3 For two-phase (constituent) or multiphase (constitu- ent) microstructures, determine if the aligned second phase (constituent) is banded in a layered manner or exists in an oriented, non-banded, randomly distributed manner. 10.2.4 For cases where a second phase or constituent is banded or oriented within a non-banded, nonoriented matrix, determine if the banded or oriented constituent exists as discrete particles (the particles may be globular or elongated) or as a continuously aligned constituent. 10.2.5 Describe the appearance of the distribution of the second phase (or, either lighter or darker etching regions within a single phase microstructure) in terms of the pattern present, for example: isotropic (nonoriented or non-banded), nearly isotropic, partially banded, partially oriented, diffusely banded, narrow bands, broad bands, mixed narrow and broad bands, fully oriented, etc. 10.2.6 The microstructural examples presented in Annex A1 illustrate the use of such terminology to provide a qualitative description of the nature and extent of the banding or orienta- tion. Fig. 2 describes the classification approach. 10.3 Place the grid lines over the projected image or photomicrograph of the randomly selected field (see section 10.17 ) so that the grid lines are perpendicular to the deforma- tion direction. The grid should be placed without operator bias. Decide which phase or constituent is banded. If both phases or constituents are banded, with no obvious matrix phase, choose one of the phases (constituents) for counting. Generally, it is best to count the banded phase present in least amount. Either NL or PL, or both (see 10.3.1-10.3.4 for definitions), may be measured, using grid orientations perpendicular (’) and par- allel (||) to the deformation direction, depending on the purpose of the measurements or as required by other specifications. 10.3.1 Measurement of NL’—with the test grid perpen- dicular to the deformation direction, count the number of discrete particles or features intercepted by the test lines. For a two-phase structure, count all of the interceptions of the phase of interest, that is, those that are clearly part of the bands and those that are not. When two or more contiguous particles, grains, or patches of the phase or constituent of interest are crossed by the grid line, that is, none of the other pha