GB/T 13305-2008 — Micrographic Method for Determining Area Content of the α-Phases in Stainless Steels

Standard Number: GB/T 13305-2008
ICS: 77.040.99
Issued: August 19, 2008
Effective: April 1, 2009
Issuing Authority: Standardization Administration of the People’s Republic of China
Replaces: GB/T 13305-1991 and GB/T 6401-1986

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Preface

This standard replaces GB/T 6401-1986 and GB/T 13305-1991. The main changes compared with GB/T 13305-1991 and GB/T 6401-1986 are as follows:

• The standard name has been revised to: Micrographic method for determining area content of the α-phases in stainless steels
• Section 3.1 adds: “Specimens shall be cut by mechanical methods in cold state. When gas cutting or hot cutting methods are used, the melted zone, plastically deformed zone and heat affected zone must be completely removed. The distance between the cutting surface and the examination surface depends on the cutting method, generally not less than 0.5 mm.”
• Section 3.2 adds etching agents including NaOH or KOH solutions
• Section 4.1.2 adds provisions for ferrite-austenitic duplex stainless steel
• Section 4.2 adds “Quantitative analysis method for metallographic image determination”
• Section 5 adds “Test report” requirements

This standard was proposed by the China Iron and Steel Association.
This standard is under the jurisdiction of the National Technical Committee on Steel Standardization.
Drafting organizations: Central Iron and Steel Research Institute, Northeast Special Steel Group Co., Ltd. (Dalian), Metallurgical Industry Information & Standards Institute.

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1. Scope

This standard specifies the micrographic methods for determining the area content of the α-phase in stainless steels. These methods are applicable to austenitic stainless steels, ferritic-austenitic duplex stainless steels, and other stainless steel products.

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2. Normative References

The following referenced documents are indispensable for the application of this standard. For dated references, only the edition cited applies. For undated references, the latest edition applies.

• GB/T 18876.1 — Standard test method for application of automatic image analysis to determine volume fraction, inclusion content and inclusion size of other phases in steel and other metals

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3. Specimen Selection and Preparation

3.1 Sampling Method

Specimens shall be cut from steel materials (or billets) in the delivered condition. The sampling location and quantity shall be in accordance with the corresponding product standards or technical specifications. The examination surface of the specimen shall be parallel to the longitudinal section of the steel (or billet), with one edge coinciding with the axis of the steel (or billet).

Specimens shall be cut by mechanical methods in cold state. When gas cutting or hot cutting methods are used, the melted zone, plastically deformed zone and heat affected zone must be completely removed. The distance between the cutting surface and the examination surface depends on the cutting method, generally not less than 0.5 mm.

3.1.1 Bars and Round Steel

• 3.1.1.1 For steel with nominal diameter or side length greater than 40 mm: the examination surface shall coincide with the axis of the steel (or billet).
• 3.1.1.2 For steel with nominal diameter or side length ≤ 40 mm:
  • a) Diameter/side ≤ 30 mm: the examination surface is the longitudinal section through the axis
  • b) Diameter/side > 30 mm to 40 mm: the examination surface is half of the longitudinal section through the axis

3.1.2 Steel Pipes

The sampling method for steel pipes is specified in the standard with reference diagrams.

3.1.3 Steel Plates, Steel Strips and Flat Steel

The sampling method for steel plates, steel strips and flat steel is specified in the standard with reference diagrams.

3.1.4 Other Methods

For castings and other specimens without obvious axial symmetry, the examination surface orientation shall be specified by the corresponding product standards or technical specifications. The specimen examination surface area shall be not less than 10 mm².

3.2 Specimen Preparation

Specimens shall be ground and polished with appropriate grinding papers, using correct grinding and polishing procedures with strict operations. The specimen etching shall reveal the α-phase structure clearly. For chemical etching, the following etching reagents are recommended:

• a) Hot 60°C–90°C etchant: K₃Fe(CN)₆ 10g–15g, potassium hydroxide (solid) 10g–30g, H₂O 100 mL
• b) Copper sulfate etchant: CuSO₄·5H₂O 4g, HCl 20 mL, H₂O 20 mL
• c) Ammonium persulfate etchant: (NH₄)₂S₂O₈ 5g, H₂O 100 mL
• d) Electrolytic etching:
  • Oxalic acid 10g, H₂O 100 mL. Voltage 3V–12V, time 15s–45s
  • NaOH or KOH 20g, H₂O 100 mL. Voltage 3V–12V, time 5s–20s

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4. Determination Methods for α-Phase Area Content

The acceptable range of the α-phase area percentage shall be in accordance with the corresponding product standards or technical specifications.

4.1 Standard Rating Chart Comparison Method

4.1.1 Austenitic Stainless Steel α-Phase Determination

Place the specimen under a metallographic microscope in bright field. First observe the entire examination surface with lower magnification to select the area with the most severe α-phase content. For determination, the microscope magnification can be 280×–320×, preferably 300×, with an actual field diameter of 0.267 mm.

The standard rating charts for austenitic stainless steel consist of 4 grades (6 charts, see Appendix A):

Lớp	α-Phase Area Content
0.5	≤ 2%
1.0	> 2% ~ ≤ 5%
1.5	> 5% ~ ≤ 8%
2.0	> 8% ~ ≤ 12%
3.0	> 12% ~ ≤ 20%
4.0	> 20% ~ ≤ 35%

4.1.2 Ferritic-Austenitic Duplex Stainless Steel α-Phase Determination

Observe the entire examination surface under appropriate magnification. The magnification shall be 500×. The standard series images are divided into two series: Strip Series and Network Series. Each series contains charts with α-phase area content from 35% to 75% in 5% increments (see Appendix B).

4.2 Quantitative Analysis Method for Metallographic Image

For more precise measurements, quantitative analysis methods may be used in accordance with GB/T 18876.1, supplementing the comparison method in Section 4.1.

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5. Test Report

The test report shall include the following items:

• a) This standard number
• b) Specimen identification
• c) Sampling method and preparation
• d) Determination results
• e) Report drafter, date and examiner

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Appendix A — Standard Rating Charts for Austenitic Stainless Steel

The standard rating charts consist of 6 charts covering grades 0.5, 1.0, 1.5, 2.0, 3.0, and 4.0. The α-phase content ranges from ≤2% to >20%~35%.

Appendix B — Metallographic Spectrum for Ferritic-Austenitic Duplex Stainless Steel

Two series of standard images (Strip Series and Network Series), each containing charts with α-phase area content from 35% to 75% in 5% increments (35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%).

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Original PDF Document

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