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Advice on Charpy-fracture toughness correlations for EDF

The purpose of this literature review was to expand the number and range of the data used by EDF’s supplier in their Charpy-fracture toughness correlation for carbon manganese steels that are specified for European Pressurized Reactor (EPR) main steam lines and low alloy ferritic steels employed in primary circuit pressure vessel components.

The aim was to reduce the amount of fracture toughness testing required to qualify a particular forging/weld within the main steam line system and use Charpy data from the manufacturing acceptance testing programme instead.

The system line pipes are made from carbon manganese steels, which have minimum yield strength in the range 335-355MPa. Higher strength (400-420MPa yield), low alloys steels are also of interest to EDF. Since the system will be operated at 330°C, the Charpy-fracture toughness correlations of concern are those representative of the upper-shelf of the Charpy transition curve, and relevant to these steels.

Published data, as well as data available in TWI on Charpy-fracture toughness correlations and specimen testing orientation of carbon manganese and low alloy steels was collected. Charpy-fracture toughness correlations for the steels cited above were analysed.

Correlations between Charpy energy and fracture toughness have been reviewed. Various upper-shelf Charpy-fracture toughness correlations were investigated during the project. However as it can be seen, table 1, the impact test values to which the older equations for the upper shelf apply tend to be lower than is generally assumed with modern steels. This is because major improvements in toughness have been made in steel manufacture, especially with regard to cleanliness, over the last half-century.

The correlations of Barsom et al (5), Ault et al (4) and Priest (6) have restricted range of applicability with respect to impact energy and yield strength and were not applicable to EDF steels.

The correlations are plotted in figure 1 using the steel yield strength at 330°C, where applicable. This comparison shows that for Charpy energies above approximately 30J, the Roberts and Newton (7) correlation as quoted in API RP 579-1 / ASME FFS-1 (1) provides the most conservative correlation. Table 1 shows all the upper shelf correlations with their limitations.

Data for Charpy impact and fracture toughness tests for different steels were then collected from the literature and TWI reports. The data represent the upper shelf Charpy energy and upper shelf fracture toughness.

The data were compared with fracture toughness estimates using the BS 7910 (2) and Wallin’s (8) correlations (both included in the 2013 edition of BS 7910) and the results are shown in Figure 2. The “valid” points indicated in the figure refer to Kmat values meeting the small scale yield criterion given in ASTM E1921 so the results should be relatively size independent.

Both upper shelf equations J.5 and J.6 given in the BS 7910:2013 (3) are shown to be conservative with respect to the experimental data in predicting upper shelf fracture toughness. These correlations were considered to be applicable to EDF carbon manganese and low alloy steels.

In the Charpy upper shelf region, a conservative relationship such as those proposed by Roberts-Newton or BS 7910:2005 Equation J.5 is appropriate for old, high sulphur steels. However for modern, low carbon, low sulphur steels the new BS 7910:2013 Equation J.6 (3, 8) is recommended.

 

The upper shelf Charpy–fracture correlations derived by Wallin were derived from J R-curve data which included nuclear pressure steels but only for the temperature range from room temperature up to 100°C. Since EDF needed to estimate fracture toughness at the operating temperature of 330°C, the correlations could not be considered to be validated for the operating temperature. It is not certain whether other upper shelf correlations have been validated for temperatures as high as 330°C but it is highly unlikely.

There is little doubt that upper shelf fracture toughness will be achieved at 330°C, but because yield and tensile strengths will be lower at 330°C than at 100°C, initiation fracture toughness could be lower too.

If the new BS 7910:2013 equation J.6 is to be employed it is recommended that it is validated with fracture toughness data (J R‑curves) obtained at 330°C. This could be done by trying to locate appropriate data in the published literature or by conducting carefully controlled experiments on materials of specific interest.

EDF’s system line pipes are made from modern, low carbon, low sulphur steels and according to the BS 7910:2013 Equation J.6 (3, 8) could be used for fracture toughness estimation from Charpy testing data.

The actual ‘KIC value for the material can be significantly greater but is very unlikely to be less than the value calculated from the equation. 

References

  1. API 579-1 / ASME FFS-1, ‘Fitness-For-Service’, Second Edition, May 2007
  2. BS 7910:2005: ‘Guide to methods for assessing the acceptability of flaws in metallic structures’, British Standards (including Amendment 1).
  3. BS 7910:2013: ‘Guide to methods for assessing the acceptability of flaws in metallic structures’, British Standards (in preparation)
  4. Ault R T, Waid G M and Bertolo R B, 1971: ‘Development of an improved ultra-high strength steel for forged aircraft components’, Air force materials laboratory.
  5. Barsom J M and Rolfe S T, 1970: ‘Correlations between KIc and Charpy V notch test results in the transition temperature range’, ASTM-STP-466 (ASTM-STP) 466, pp281-302.
  6. Priest A H, Charnock W and Stewart A T, 1982: ‘The effect of accelerated irradiation on fracture behavior’, Edited by Perrin J S, American Society for Testing Materials Brager H R Effects of radiation on material: 11th Conference, ASTM-STP-782, pp475-491
  7. Roberts R and Newton C, 1981: ‘Interpretive Report on small-scale test correlations with KIc data’, Welding Research Council Bulletin, 265, pp362-367.
  8. Wallin K and Pisarski H, 2005: ‘The use of Charpy/fracture toughness correlations in the FITNET procedure’, Offshore Mechanics & Ocean Engineering, Halkidiki, Greece, ASME.

For further information, please email contactus@twi.co.uk.

 

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