Measuring Method for δ Ferrite Content: In Austenitic Stainless Steel Welds

Austenitic stainless steel is one of the main materials of nuclear reactor structure, in which there are a large number of welded parts.

A small amount of δ ferrite in stainless steel weld can improve the strength and intergranular corrosion resistance of the weld, and prevent welding hot cracks;

But too much δ ferrite will cause σ phase embrittlement and δ phase selective corrosion.

δ ferrite content is one of the important technical indexes for the development and application performance evaluation of austenitic stainless steel welding materials.

At present, the commonly used methods of δ-ferrite content in austenitic stainless steel welds are metallographic method, chemical method and magnetic method.

The metallographic method calculates the area ratio by directly observing the δ-ferrite in the metallographic sample, and then calculates the volume fraction.

It belongs to destructive inspection, which requires enough measuring points to obtain data with high reliability and high detection cost.

The chemical method can indirectly obtain δ ferrite content (mass fraction) by calculating the nickel equivalent and chromium equivalent in the material and comparing the empirical diagram.

Schaeffer diagram, Delong diagram and WRC-92 diagram are three kinds of diagrams commonly used in chemical methods at present.

Schaeffer diagram was first applied, but the influence of nitrogen and copper was not considered;

In Delong diagram, nitrogen is regarded as the forming element of austenite and included in nickel equivalent, and the curve accuracy is improved;

The WRC-92 figure also introduces nitrogen and copper.

Chemical method also has some problems, such as: the accuracy of alloy element content directly affects the accuracy of δ ferrite content calculation;

The effect of alloying elements on δferrite content is not linear.

These will lead to a certain deviation between the measured value and the actual value.

The magnetic method is to determine the content of δ ferrite by measuring a certain magnetic physical quantity related to the content of δ ferrite.

This method is greatly affected by the principle of the measuring instrument.

If the content or morphology of δ ferrite in the material is uneven, the reproducibility and accuracy of the results are poor.

The magnetic method is easy to operate and can realize on-site nondestructive testing, which is commonly used.

In actual detection, one or two methods are usually selected for measurement.

Researchers from the Key Laboratory of Reactor Fuel and Materials of China Nuclear Power Research and Design Institute simultaneously used the above three methods to measure the δ-ferrite content of austenitic stainless steel surfacing layer for nuclear power, and compared and analyzed the difference of measurement results of different detection methods.

1. Test contents

1.1 Test materials

The research object is 308 stainless steel overlay, and the sample size is 50mm × 25mm × 10mm, and its chemical composition meets the requirements of ASTM A276-2006 Stainless Steel Bars and Shapes.

1.2 Test standards

The metallographic test is carried out according to GB/T 1954-2008 Measuring Method for Ferrite Content of Chromium Nickel Austenitic Stainless Steel Welds and GB/T 15749-2008 Quantitative Metallographic Method.

On the basis of existing detection methods, chemical composition analysis is carried out, and Schaeffler diagram and WRC-1992 diagram are selected to calculate δ ferrite content.

The magnetic method is used for measurement according to GB/T 1954-2008 and JB/T 7853-1995 Measurement of Ferrite Number in Chromium Nickel Austenitic Stainless Steel Weld Metal.

1.3 Test equipment

The metallographic method adopts Olympus GX71 metallographic microscope and its supporting TIGER3000 metallographic image analysis system for detection and analysis;

For chemical method, carbon/sulfur analyzer is used to detect carbon and sulfur, spectrophotometer is used to detect silicon, phosphorus and boron, and inductively coupled atomic emission spectrometer is used to detect other metal elements;

The content of δ ferrite was directly read by ferrite measuring instrument in magnetic method.

2. Test process and results

2.1 Metallographic method

Metallographic method to measure the content of δ ferrite mainly includes standard sample atlas contrast method and measurement method.

These two methods are used to measure the content of δ ferrite in 308 stainless steel surfacing layer.

2.1.1 Map comparison method

With reference to GB/T 1954-2008 standard, the prepared metallographic sample is observed under a microscope, and the area with a relatively uniform distribution of δferrite is selected for photographing (see Fig. 1).

Measuring Method for δ Ferrite Content: In Austenitic Stainless Steel Welds 1
Measuring Method for δ Ferrite Content: In Austenitic Stainless Steel Welds 2

Fig. 1 Microstructure morphology of prepared and standard samples

As specified in the standard GB/T 1954-2008, the magnification should not be less than 500 times, so the microstructures of samples prepared at 500 times and 1000 times are obtained respectively, and compared with the microstructures of standard samples at 500 times and 1000 times, it can be determined that the content of δ-ferrite is 7.5%~10%.

2.1.2 Measurement method

The GB/T 15749-2008 standard is the national standard “applicable to the determination of phase volume fraction in various alloy microstructures”, which covers a variety of phase content calculation methods such as grid number point method, grid section method, line segment calibration method (including four line method, eight line method, etc.).

Compared with the metallographic secant method in GB/T 1954-2008, the former has more dividing lines, larger coverage and higher accuracy.

Measuring Method for δ Ferrite Content: In Austenitic Stainless Steel Welds 3

Fig. 2 Microstructure of δFerrite in Weld at the Same Position

According to the standard GB/T 15749-2008, 300 times, 500 times and 1000 times of the sample are measured by the grid section method.

Fig. 2 shows the microstructure at the same location.

At different multiples, the average δ ferrite content measured is 11.0%, 7.6% and 9.5% respectively (see Table 1).

Table 1 δ Ferrite Content at the Same Location under Different Magnification Times

Amplified

Mass fraction

1

2

3

4

5

6

7

8

9

Mean value

300 times

10

9.7

11.6

11.7

12.7

10.8

11.5

10.6

10.8

11

 500 times

8.2

10

8.8

6.1

10.1

6.4

9.3

9.8

9.3

7.6

 1000 times

10.6

11

8.2

7.7

10

7.8

10.5

10.8

9.6

9.6

It can be seen from Table 1 that the content of δ ferrite measured at different magnification varies greatly.

The content of δ ferrite measured at 500 times is the lowest, and the content of δ ferrite measured at 300 times and 1000 times is higher.

The test results show that the magnification in the metallographic method has a great influence on the results:

The δ ferrite grain in the weld is usually very small. When the magnification is low (300 times), the microstructure in the field of view is too dense, the edge of image segmentation calculation is not obvious, and the results are generally large;

When the magnification is too high (1000 times), the selected field of view area is small.

Because the tissue itself is uneven, more points may be measured to obtain more accurate results.

As GB/T 1954-2008 stipulates that the magnification shall not be less than 500 times, it is appropriate to select 500 times according to the actual measurement.

Measuring Method for δ Ferrite Content: In Austenitic Stainless Steel Welds 4

Fig. 3 Schematic Diagram of δFerrite Content Measurement Method

In the test, grid number point method, grid section method, four line method and eight line method were used respectively, and 9 locations were randomly selected under 500 times conditions to measure the δferrite content , and the measurement method is shown in Fig. 3.

The measurement results of these methods are 7.6%, 7.6%, 6.7% and 7.6% respectively (see Table 2).

Table 2 Grid point method, grid section method, four line method and eight line method

δ-Ferrite content measured at random locations under 500 ×

Position and mean

Grid number point

Grid section

Four line

Eight line

1

6.9

8.2

4.1

6.9

2

8.2

10

6.6

8.8

3

9.2

8.8

5.1

5.8

4

5.6

6.1

6.5

5.9

5

10.2

10.1

7.7

7.9

6

4.2

6.4

5.5

6.3

7

7.9

9.3

9

9.3

8

8.3

9.8

5.9

8.5

9

7.5

9.3

9.6

9.2

mean

7.6

7.6

6.7

7.6

Table 2 shows that except for the four wire method, the measurement results of the other three methods are identical.

2.2 Chemical method

The chemical composition of 308 stainless steel surfacing layer is shown in Table 3, which meets the requirements of technical indicators.

Table 3 Chemical Composition of 308 Stainless Steel Overlay

CCr NiMoMnCoCuVSBPSi
0.02819.2510.40.0651.380.020.030.040.00780.00060.0130.73

According to Schaeffler diagram and its calculation formula, the mass fractions of chromium and nickel in the test material are 20.41% and 10.93% respectively, corresponding to the content of δ-ferrite of about 8.5%;

According to WRC-1992 figure, the mass fractions of chromium and nickel in the test material are 19.315% and 10.24% respectively, corresponding to the content of δ-ferrite of about 13%.

It can be seen that under the influence of many factors, the calculation results of chemical method will be different if different experience charts are selected.

It can be seen from the analysis that since copper is introduced as nickel equivalent in WRC-1992 diagram, and 308 stainless steel contains 0.03% copper, the calculated δ-ferrite content is significantly higher than that calculated in Schaeffler diagram.

2.3 Magnetic method

According to the requirements of magnetic method measurement in GB/T 1954-2008 standard, randomly measure 6 points along the weld bead direction of 308 stainless steel surfacing layer, take the average value of 5 readings on each point as the measurement result, and the average δ-ferrite content is 3.4%.

3. Analysis and discussion

The a-ferrite content of 308 stainless steel surfacing layer was measured by metallographic method, chemical method and magnetic method respectively (see Table 4).

The δ-ferrite content of austenitic stainless steel weld structure is generally 4%~12%.

It can be seen that the magnetic method measurement results are low, and the chemical method WRC-1992 diagram measurement results are high.

Table 4 308 Stainless Steel Overlay Measured by Different Methods δ Ferrite content

Measuring method

δ -Ferrite mass fraction

Metallography (500 times)

Atlas contrast method

10.0

Measurement method

7.6

Chemical method

Schaeffler chart

8.5

WRC-1992 chart

13.0

Magnetic method

3.4

When measuring with different methods such as grid number point method, grid section method, four line method and eight line method, due to the uneven distribution and shape of δ-ferrite in each field of view, the four line method has the least measurement data on the same map, so the deviation is also large.

Although the field of view selected by the metallographic method is large and random, the reliability of the results is generally high.

In order to reduce the adverse effects of the method and improve the reliability of the measurement results, the appropriate measurement method should be selected by comprehensively considering the magnification, field of view position and the differences between the principles of different methods.

At 500 ×, the average values of the measured results of the grid number method, grid section method and eight line method are the same, that is, the δ-ferrite content of 7.6% is a more reliable result.

The chemical method is based on the detection results of element content. The δ-ferrite content is obtained by calculating and comparing the experience chart.

The accuracy of the chemical element measurement directly affects the accuracy of the results.

The chemical method is directly affected by such factors as the accuracy of element measuring instruments and the selection of experience maps.

The measurement result of magnetic method is relatively small, which may be caused by: different physical quantities measured by different instruments are different, and the instruments themselves have certain measurement errors;

For materials with low content of δ-ferrite, the morphology and distribution of δ-ferrite are uneven.

If the sampling location is inappropriate, the reproducibility and accuracy of the determination results are poor.

In the process of test and production, it is better to avoid using one method to measure the δ-ferrite content of welds, and it is better to use multiple methods for mutual verification.

4. Conclusion

(1) Each of the three methods has advantages and disadvantages, which should be selected according to the actual situation:

Metallographic method is a destructive testing method, but the measurement results are highly reliable.

When testing, it is necessary to comprehensively consider the magnification, field of view position and the difference between different methods and principles, and select the appropriate measurement method;

Based on the known chemical composition of the material, the detection value can be obtained easily and quickly by chemical method, but it is necessary to select appropriate experience chart and calculation formula;

The magnetic method belongs to non-destructive inspection, which is suitable for rapid inspection of large material components on site, but its measured value is low.

(2) The measurement results of the metallographic atlas comparison method and the chemical Schaeffler diagram method are similar, and the measurement results of the grid number method, grid section method and eight line method in the metallographic measurement method are the same, which can be used as alternative methods in actual detection.

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