GB/T 41706-2022Metallic powders - Test method for determination of non-metallic inclusions in metal powders for powder forging (English PDF)

5.4

The use of the nearest neighbor concept will provide a more conservative assessment of inclusions: usually, their contents are overestimated rather than underestimated.

6.1 Sample preparation equipment

it is used for the preparation of samples for metallographic testing.

6.2 Metallographic microscope

It can be used for observation and measurement with a magnification of 100×; the wavelength of light is 544 nm (green filter), the magnification of the objective lens is 8×~12.5×, and the numerical aperture is 0.16~0.20.

NOTE: The designated optical element is very important as it determines the object features that can be resolved by the microscope, which is used for assessing inclusion sizes.

7.1

In order to evaluate the non-metallic inclusion content of powders for powder forging, the powders shall be mixed with appropriate quantities of graphite and lubricant and pressed to a specified green density. The test method requires as little lateral flow as possible (< 1%) during forging, so the diameter of the green compact relative to the diameter of the forging die shall meet this requirement accordingly.

7.2

The metallographic sample shall be removed from the powder-forged sample (part) for austenitizing and quenching treatment. The heat treatment of the sample can avoid the smearing effect and scratches caused by the soft inclusions, which is beneficial to the sample preparation.

7.3

The area of the polished surface of the sample (part) to be inspected shall not be less than 350 mm2. For small parts, multiple sections are allowed to be inspected to obtain the required area for measurement.

7.4

The polished surface shall be parallel to the direction of force, that is, the forging direction, and shall be able to represent the central area of the sample (part).

8.1 Sample preparation

When polishing samples, it is very important to obtain a clean polished surface and that the inclusions are not pitted, tailed, or obscured. Therefore, the procedures specified in

ASTM E3 and ASTM E768 shall be followed. An automatic grinding and polishing program is recommended. Samples are examined in polished condition and must not be affected by any prior corrosion (if any).

8.2 Determination of non-metallic inclusion content

8.2.1

The measurement area of the surface of the polished sample shall not be less than

350 mm2, measured at a magnification of 100×; the wavelength of light used is 544 nm (green filter), the magnification of the objective lens is 8×~12.5×, and the numerical aperture is 0.16~0.20.

8.2.2

Measure the size of inclusions according to the principle of nearest-neighbor separation.

Two inclusions within a distance of 30 µm (0.03 mm) (within 3 mm at 100× magnification) are regarded as the same inclusion.

8.2.3

According to the principle of nearest neighbor separation, three inclusions whose individual size is less than 30 µm and whose distance is less than 30 µm each other are regarded as an inclusion cluster.

8.2.4

According to the principle of nearest neighbor separation, if the distance between two inclusions is within 30 µm, then the individual inclusion with a size smaller than

30 µm is regarded as a part of the inclusions larger than 30 µm. An example is shown in Figure 2.

8.2.5

Detect and record the number of inclusion particles according to the principle of nearest neighbor separation, and use the maximum Feret diameter to determine the size, namely:

-- the length that is greater than or equal to 30 µm,

-- the length that is greater than or equal to 100 µm,

-- the length that is greater than or equal to 150 µm.

In addition to using the concept of nearest-neighbor separation to determine the size of inclusions, the size of individual inclusion particles can also be measured.

8.2.6

If the size of the inclusion is determined separately, the maximum Feret diameter […]