GB/T 13298-2015Inspection Methods of Microstructure for Metals

Foreword

This Standard was drafted as per the rules specified in GB/T 1.1-2009.

This Standard replaces GB/T 13298-1991 Inspection Methods of Microstructure for Metals; compared with GB/T 13298-1991, this Standard mainly has the following technical changes:

--- Modify specimen preparation into “specimen preparedness” (see Chapter 3);

--- Increase two situations for specimen selection (see 3.1);

--- Add the schematic diagram for inspection surface (see 3.2.1);

--- Add “band structure rating” for the inspection of longitudinal section parallel to the rolling direction (see 3.2.3);

--- Adopt clear “mechanical setting fixture diagram” (see 3.7.2);

--- Add the vibration polishing (see 5.5);

--- Modify “specimen erosion” into “microstructure display”; add the general (see 6.1), optical method (see 6.2), thermal etching display method (see 6.3.5), anodic film method (see 6.4.2); delete the preparation and precautions of chemical etchants and electrolytic etchants (see 4.1.3 of 1991 Edition);

--- Delete “metallographic microscope is divided into desktop-type, vertical-type, horizontal-type” (see 5.2 of 1991 Edition);

--- Delete “precautions for protecting lenses when using a microscope” (see 5.5, use focusing magnifier to observe on the ground-glass plate” (see 6.6 of 1991 Edition);

--- Microstructure inspection part (see Chapter 7), delete photographing with black- and-white negative film and colour negative film, “black-and-white negative film and photographic paper processing” (see 6.10 of 1991 Edition), and “color negative film and color photographic paper processing” (see 6.11 of 1991 Edition); add microscope illumination mode (see 7.2.1); add image acquisition (see 7.3) and image analysis part (see 7.4).

--- Add “on-site metallographic inspection” (see Chapter 8);

--- Change “test records” into “test report” (see Chapter 9 of this Edition, and

--- Add etchant commonly used for metals (see Appendix A).

This Standard was proposed by China Iron and Steel Association.

This Standard shall be under the jurisdiction of National Technical Committee for Standardization of Steel (SAC/TC 183).

Drafting organizations of this Standard: Central Iron and Steel Research Institute, China Metallurgical Information and Standardization Institute, Fangda Special Steel Technology Co., Ltd., Daye Special Steel Co., Ltd., Shougang Corporation, and Xingtai Iron and Steel Co., Ltd..

Chief drafting staffs of this Standard: Li Jikang, Zhao Xiaoli, Luan Yan, and Ju Xinhua.

The historical edition replaced by this Standard is as follows:

1 Scope

This Standard specifies the specimen preparedness, specimen grinding, specimen polishing, microstructure display, microstructure test, on-site metallographic and test records for the metal microstructure inspection.

This Standard is applicable to the operation method of using metallographic microscope to inspect the metal structure.

2 Normative References

The following documents are essential to the application of this document. For the dated documents, only the versions with the dates indicated are applicable to this document; for the undated documents, only the latest version (including all the amendments) are applicable to this document.

3 Specimen Preparedness

3.1 Specimen selection

3.1.1 General

To ensure the effectiveness of inspection, the selected metallographic specimen shall represent the researched materials objectively and comprehensively as much as possible. The specimen cutting direction, location, quantity shall be determined by the metal manufacturing method, inspection purpose, relevant standards, and provisions of both parties’ agreement.

3.1.2 Routine inspection

In addition to the special provisions in the product standard, it is recommended to select from the position that can represent the characteristics of the materials; the specimen shall contain the complete processing and impact area. For instance, the specimen of steel strip or wire is suitable to cut from the end of the coil; cast specimen shall contain the maximum and minimum segregation zones; heat treatment specimen shall contain complete heat treatment layer; surface treatment specimen shall contain all surface treatment layers; welding specimen shall contain weld seam, heat affected zone, and base metal.

3.2 Inspection direction and selection of inspection surface

3.2.1 Refer to Figure 1 for the inspection surface schematic diagram.

D – Longitudinal section parallel to the rolling surface;

E – Longitudinal section perpendicular to the rolling surface;

F – Horizontal section perpendicular to the forging and rolling directions;

3.2.2 Generally, horizontal section perpendicular to the forging and rolling directions (F) can be used for inspection:

a) State and changes from the surface to the central microstructure;

h) Surface chemical heat treatment and coating structure and thickness.

3.2.3 Generally, longitudinal section parallel to the forging and rolling directions (D, E, G, H) can be used for inspection:

a) Content of non-metallic inclusions in steel;

d) Overall situation of heat treatment.

3.2.4 According to the requirements of analysis and research, multiple sections can be observed. For instance, when researching the deformation situation of the hot-rolled and cold-rolled metal structure, both horizontal and longitudinal sections can be observed comprehensively; for wire and rod materials, meanwhile the horizontal section is observed, the longitudinal section passing through the specimen axis can be observed.

3.3 Determination of specimen dimension

Specimen dimension shall depend on the rules with inspection area less than 400mm2; specimen height 15mm~20mm (smaller than the horizontal size).

3.4 Specimen cutting

Specimen can be cut by grinding wheel, electrospark wire, machine tooling (turn, mill, plane, grind), handsaw, and shearing method; if necessary, the specimen can be cut by oxyacetylene flame cutting method; while the hard and brittle metals can take sample by hammering method. When cutting the specimen, try to avoid the influence of cutting method on the structure (such as deformation, overheating, and etc.). […]

3.7 Specimen embedding

3.7.3 Resin embedding method

4 Specimen Grinding

4.1 Specimen planishing

The cut specimen shall be firstly planished to make preparation for the next procedure of grinding of sand paper. When grinding, use water to cool off the specimen, prevent the changes of the specimen structure due to heating.

4.2 Specimen burnishing

4.2.1 Manual burnishing

After planishing, cleaning, and drying, the specimen shall be ground successively on the different grain size of sand paper from coarse to fine; the sand paper shall be paved on the flat glass, metal or plate. Each time the sand paper is changed, the specimen shall rotate for 90° perpendicular to the old grinding mark; in such direction, burnish till the old grinding mark disappear totally, and the new grinding mark is uniform. Each time use water or ultrasound to clean and dry the specimen, then enter into the next procedure for the specimen preparation.

4.2.2 Burnishing of mechanical milling machine

Place the sand paper and grinding plate of different grain size from coarse to fine on the mechanical milling machine, and grind successively.

5 Specimen Polishing

5.1 General

Polish the grinding mark on the specimen to reach the mirror finish, and there are no grinding defects. The polishing method can take mechanical polishing, electrolytic polishing, chemical polishing, vibration polishing, micro-grinding and so on.

5.2 Mechanical polishing

5.2.1 Rough polishing

After the burnishing of sand paper, the specimen can be transferred to the polishing machine equipped with nylon, woolen cloth or fine canvas to conduct rough polishing; […]

5.2.2 Precise polishing

5.2.2.1 After rough polishing, the specimen can be transferred to the polishing disk equipped with nylon silk, velvet, or other fiber-uniform velour to conduct the precise polishing. According to the specimen hardness, different grain size of fine polishing paste, spray polishing agent, oxide suspension, and etc.. Pay attention to the polishing time and force, so as to avoid the specimen corner rounding or embossing. Generally, the polishing shall be continued till the grinding mark on the specimen is totally removed, and the surface becomes the mirror. After polishing, use water to wash; use absolute alcohol to clean and dry; so that the specimen surface has no water stains or dirt residue any more. 5.2.2.2 Precise polishing can select the manual or automatic methods. […]

5.3 Electrolytic polishing

Electrolytic polishing indicates that take the metal as the anode inserted in the electrolytic cell; its surface generates selective corrosion due to the electrolytic reaction; […]

5.5 Vibration polishing

Vibration polishing indicates driven by the industrial power (after half-wave rectification), the spiral vibration system makes the specimen do the circular motion on the grinding disk; meanwhile, it rotates itself; so that the polishing purpose is achieved. It is commonly used to remove the stress or residual deformation layer on the specimen surface, finally obtain high-quality surface.

5.6 Microscopic grinding

Microscopic grinding machine is made of replacing the microtome blade with grinding head. […]

6 Microstructure Displaying

6.1 General

After polishing, the specimen microstructure shall be directly displayed without treatment; or use physical or chemical method to conduct special treatment against the specimen, so that various structures can appear good contrast, and display clearly. The common methods include optical method, etching method, and interference layer method.

6.2 Optical method

Since different structures have different reflection intensity and colour against the light, which can be used to distinguish and display the metallographic microstructure. Specimen shall be observed without any other treatment; or use the polarized light differential interference and other accessories on the microscope to observe the specimen.

6.3 Etching method

6.3.2 Electrolytic etching

Take specimen as the anode of the circuit, and immerse it into appropriate electrolytic etching solution, so that display the metal microstructure. The etching conditions depend on voltage, current, temperature, and time.

NOTE: refer to Appendix A for the common etchants.

6.3.3 Constant potential etching

Constant potential etching is further development of electrolytic etching; use potentiostat to ensure the constant potential of the anode specimen during the etching process; […]

6.3.4 Ion etching (cathode vacuum etching)

The specimen surface is etched by the energetic ion bombardment; selectively remove partial atoms on the specimen surface, so that reveal the metal structure. It is particularly suitable for the composite specimen with significant difference of chemical properties, such as iron-nickel, stainless-gablock welding.

6.3.5 Hot-etching displaying method

It is one of the major high-temperature metallographic displaying methods; in case of vacuum (also can be under the inert gas protective atmosphere of nitrogen or argon) heating, grain boundaries and some phase boundaries can be displayed without the role of any media.

When heating the specimen in vacuum, due to the impact of temperature, if the thermal expansion coefficient of each phase or grain is greatly different, it may cause the embossment; under the illumination or ordinary light and polarized light, due to the different high-low projection and different potential grains, its optical characteristics can clearly reflect the structure characteristics.

6.4 Interference layer method

A thin film is formed on the polished surface of the metal specimen; […]

6.4.1 Film forming method by chemical etching

A method of forming a thin film on the metal specimen surface by using the chemical reagent. Different phases in the metal may form different-thickness thin film due to the different potential, so that make the various phases, orientations, different grains, subgrains, dendrites generate different interference color and display the structure difference due to the multiple reflections and interference phenomena. It is commonly used for phase identification, grain phase observation, and segregation structure.

7 Microstructure Inspection

7.1 Microscope

7.1.3 To ensure the accuracy of the inspection, operate and use the microscope properly as per the instruction manual.

7.1.4 Select the object lens and eye lens according to the desired magnification times. If there is scale in the eye lens, it shall be calibrated by the micrometer, which shall be tested according to the measurement requirement.

7.2 Microscopic observation

7.2.1 Generally, the total appearance of specimen shall be firstly observed at low magnification; then inspect at different magnification times as per the test purpose. According to the research requirements, observe in the following methods: a) Bright field illumination is used for the routine observation and inspection of microstructure; it is the most common observing mode; b) Dark filed illumination has high contrast difference, which is used for the identification and research of grain boundaries, defects, inclusions and etc.; c) Polarized light illumination is often used for the identification of phases in the heterogeneous alloys; it also has good effect to exhibit structure, inclusions, grain boundary twin boundaries of the anisotropic materials; place the sensitive color film into the optical path, so that it is possible to obtain the color image. d) Differential interference contrast (DIC) adjustment of prism not only can generate different colors, improve image contrast; but also exhibit the height difference between different positions on the specimen surface, so that make the image have uneven three-dimensional sense. The lens selection depends on the desired magnification times (choose properly according to the instruction manual of the microscope). Generally, in order to fully use the resolution of microscope object lens, the effective magnification times shall not be greater than 1000× numerical aperture (N, A) of object lens; after that select proper eye lens to match the object lens, so that prevent the false magnification. 7.2.3 The light source shall be adjusted properly, the emitted light shall be stable and have sufficient intensity. Adjust the location of light source and concentration, so that the light beam can shoot into the center of vertical luminaire entrance; and obtain the uniform brightness of the obtained image. 7.2.4 The color filter depends on the type of object lens. If it is achromatic lens, use the yellow, green color filter; if it is wholly achromatic lens, use either yellow, green or blue color filter. The color of the color filter shall be consistent with the color of the phase need to be identify. 7.2. […]

8 On-Site Metallographic Inspection

When conducting the structure non-destructive test on the large parts or components like large gears, shafts, and pipes, the inspection point can be selected directly on the workpiece, then perform the burnishing, polishing, etching and other processes. The portable metallographic microscope or film-coating method can be taken to copy the position need to be observed, and then bring to the laboratory to observe the structure.

9 Test Report

The test report shall contain the following contents:

process, sampling quantity, position and direction;

d) Structure displaying method; if the etching method is adopted, indicate the type of etchant;

g) Signature of inspector and auditor.