GB/T 6401-1986

Micrographic Method for Determining Area Fraction of α-Phase in Ferritic-Austenitic Duplex Stainless Steels Using Charts

Scope

This standard specifies the micrographic method for determining the area fraction of the α-phase in ferritic-austenitic duplex stainless steels.

1. Preparation of Test Specimens

1.1 Specimens shall be taken in the delivery condition. The sampling location and quantity shall comply with the relevant product standard or technical specification.

1.2 The examination surface of the specimen shall be a longitudinal section passing through the steel axis. For round bars and square bars, follow Figure 1; for steel plates, steel strips and flat bars, follow Figure 2; for steel tubes, follow Figure 3. The orientation of the examination surface for forgings and other steel products without an obvious axis shall be specified in the relevant product standard or technical specification. The area of the examination surface shall be not less than 10 mm². Large specimens may be divided into several sections, but shall still be regarded as one specimen.

1.3 Specimens shall be cut by mechanical means in a cold state. When flame cutting or hot cutting is used, the oxidized zone, plastically deformed zone and heat-affected zone must be completely removed. The distance between the cut surface and the examination surface depends on the cutting method and shall generally be not less than 0.5 mm.

1.4 Specimens shall be prepared in accordance with YB 28-59 Metallographic Examination Method for Ferrous Metals. Etching shall be performed to clearly reveal the α-phase. One of the following recommended etchants may be used:

a) Hot (60–90 °C) or boiling alkaline potassium ferricyanide solution:

• Potassium ferricyanide: 10–15 g (or 7–20 g)
• Potassium hydroxide (or sodium hydroxide): 10–30 g (or 7–20 g)
• Water: 100 mL

Use freshly prepared solution and immerse for several minutes. Austenite remains white, α-phase stains red to brown.

b) Ferric chloride hydrochloric acid ethanol aqueous solution:

• Ferric chloride: 5 g
• Hydrochloric acid: 100 mL
• Ethanol: 100 mL
• Water: 100 mL

After etching at room temperature, heat to 500–600 °C. The etched surface turns yellow.

2. Determination of α-Phase Content

2.1 Under bright-field illumination of a metallographic microscope, determine the area fraction of α-phase on the examination surface by comparison with standard reference charts. First, observe the entire examination surface at low magnification, select an area with moderate α-phase content, and perform the determination. The magnification shall be 500×, and the field-of-view diameter of the magnified image shall be 80 mm. When the scale of α-phase differs significantly from that on the standard charts, the magnification may be appropriately adjusted so that the α-phase scale in the field of view matches that on the standard charts.

Note: For arbitration, take the arithmetic mean of determinations from 10 randomly selected fields of view with moderate α-phase content.

2.2 The standard reference charts consist of two series: the Lamellar series and the Network series. If no specific series is stipulated in the relevant product standard or technical specification, either series may be selected for comparison based on the morphology and distribution of the α-phase.

2.3 Each series contains 9 reference charts. The α-phase area fraction for each chart is specified sequentially from 35% to 75%, in 5% increments (see Figures 4–12).

2.4 For determination, compare with the standard charts and determine the α-phase area fraction using the chart most closely matching the observed field of view. The permissible error is ±2.5% (absolute value). The acceptable range of α-phase area fraction shall be specified in the relevant product standard or technical specification.

Standard Reference Charts Summary

Supplementary Notes

• This standard was proposed by the Ministry of Metallurgical Industry of the People’s Republic of China.
• This standard was drafted by Central Iron and Steel Research Institute (CISRI) and Chongqing Special Steel Plant.
• Principal drafters: Wu Keqiu, Wang Daxing.

Issue Date: May 17, 1986 (State Bureau of Standards)
Implementation Date: May 1, 1987