Scholarly article on topic 'The effect of long term exposition at 540°C on the empirical correlations for determination of mechanical properties of low alloy CrMoV steel from the results of Small Punch tests'

The effect of long term exposition at 540°C on the empirical correlations for determination of mechanical properties of low alloy CrMoV steel from the results of Small Punch tests Academic research paper on "Materials engineering"

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{"Small Punch (SP) test" / "Load–displacement curve" / "Exposure at high temperature" / "Empirical correlation" / "Yield strength" / "Tensile strength" / FATT}

Abstract of research paper on Materials engineering, author of scientific article — Ondřej Dorazil, Karel Matocha, Jinbin Zhu, Yuan Chen

Summary The present paper describes the effect of long term exposure at 540°C on the empirical correlations for determination of yield strength, tensile strength and FATT of 14MoV6-3 low alloy steel. Empirical correlations obtained for the pipe in as received state were compared with the results of standardized tensile, impact and Small Punch tests carried out on the testing materials cut of the three pipes of significantly different metallurgical quality and time of exposure at 540°C.

Academic research paper on topic "The effect of long term exposition at 540°C on the empirical correlations for determination of mechanical properties of low alloy CrMoV steel from the results of Small Punch tests"

Perspectives in Science (2016) 7, 156—160

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The effect of long term exposition at 540 °C on the empirical correlations for determination of mechanical properties of low alloy CrMoV steel from the results of Small Punch tests*

Ondrej Dorazil3*, Karel Matochaab, Jinbin Zhuc, Yuan Chenc

a Material &Metallurgical Research, Ltd., Pohranicni 693/31, 703 00, Ostrava-Vitkovice, Czech Republic b Faculty of Metallurgy and Materials Engineering, VSB — Technical University of Ostrava, Czech Republic c School of Mechanical Engineering, East China University of Science and Technology, 130 Meilong Rd., Shanghai 200237, China

Received 1 October 2015; accepted 19 November 2015 Available online 10 December 2015

KEYWORDS

Small Punch (SP) test;

Load—displacement

curve;

Exposure at high temperature; Empirical correlation; Yield strength; Tensile strength; FATT

Summary The present paper describes the effect of long term exposure at 540°C on the empirical correlations for determination of yield strength, tensile strength and FATT of 14MoV6-3 low alloy steel. Empirical correlations obtained for the pipe in as received state were compared with the results of standardized tensile, impact and Small Punch tests carried out on the testing materials cut of the three pipes of significantly different metallurgical quality and time of exposure at 540 °C.

© 2015 Published by Elsevier GmbH. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Introduction

The effort to extend the design lifetime of fossil power plants operating for a long time at elevated temperatures

* This article is part of a special issue entitled "Proceedings of the 1st Czech-China Scientific Conference 2015''.

* Corresponding author.

E-mail address: ondrej.dorazil@vsb.cz (O. Dorazil).

requires the knowledge of residual lifetime of the critical components. Residual lifetime assessment is unthinkable without the knowledge of (Matocha, 2015):

1) Mechanical properties of materials prior to operation, respecting all technological operations realized throughout the manufacturing of the component.

2) Mechanical properties after actual time of operation (actual mechanical properties), because the material

http://dx.doi.org/10.1016Zj.pisc.2015.11.025

2213-0209/© 2015 Published by Elsevier GmbH. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/ licenses/by-nc-nd/4.0/).

Displacement [mm] Displacement [mm]

Figure 1 Load-displacement curve recorded during a time independent Small Punch (SP) test.

properties can be reduced throughout the service life by ageing due to the service loading and temper ageing.

The need for evaluating the actual mechanical properties of structural components by direct testing method has led to development of innovative techniques based on miniaturized specimens. Among these, a technique called the Small Punch (SP) test has emerged as a promising candidate (Hurst and Matocha, 2010; Lucon, 2001; Lucas, 1990; Small, 1993). It is mechanical testing method used at the present time to obtain tensile, fracture and creep data from very small quantities of experimental material. In 2007 CWA 15627 ''Small Punch Test Method for Metallic Materials'' (CEN Workshop Agreement, 2007) was issued by CEN (European Committee for Standardization).

The objective of the SP test is to produce a load—displacement (punch displacement, crosshead displacement) record and/or load-specimen deflection record (see Fig. 1), which contains information about the elastic-plastic deformation and strength properties of the material.

The following parameters are determined from the load—displacement curve during the time independent SP tests (Small, 1993):

• Fm [N] — maximum load recorded during SP test,

• Fe [N] — load characterizing the transition from linearity to the stage associated with the spread of a yield zone through the specimen thickness. It is determined according to the Code by two tangents method (see Fig. 1),

• um [mm] — displacement corresponding to the maximum load Fm,

• Uf [mm] — displacement corresponding to 20% load drop,

• ESP [J] — SP fracture energy obtained from the area under the load—displacement curve up to the Uf.

The load—displacement curves obtained can be utilized to derive empirical correlations between SP and standardized test results (Mao and Takahashi, 1987; Hurst and Matocha, 2014; Rodriguey et al., 2012; Dymacek and Jecminka, 2014; Matocha et al., 2014a) or they can be analyzed in terms of elastic-plastic finite element methods (Nakataetal., 2010; Hulkaetal., 2012; Prakash andRamesh, 2012; Madia et al., 2013).

The present paper describes the effect of long term exposure at 540 °C on the empirical correlations for determination of yield strength (YS), tensile strength (UTS) and FATT of 14MoV6-3 low alloy steel. Empirical correlations obtained for the pipe in as received state were compared with the results of standardized tensile, impact and Small Punch tests carried out on the testing materials cut of the three pipes of significantly different metallurgical quality and time of exposure at 540°C.

Testing material

A pipe 0 457 mm x28mm in as received state heat treated to seven significantly different tensile properties (Matocha et al., 2014b), superheater outlet header (SH) 0 521mm x36mm exposed for 90,000h at 540°C, Output steam piping 0 324 mm x 48 mm exposed for 151,062 h at 540°C (CH) and steam piping elbow 0 245 mm x36mm exposed for 270,000 h at 540°C (B6) made of low alloy steel of type 14MoV6-3 were used as the testing materials. Chemical composition of the testing materials is shown in Table 1.

Metallurgical quality of the testing materials has been expressed by BRUSCATO factor X=(10.P + 5.Sb + 4.Sn + As)/ 100and J factor J=(Si+Mn) x (P + Sn) x104 (see Table 2).

Table 1 Control chemical analysis.

C Mn Si S P Cr Mo Ni V Al N

AS 0.12 0.57 0.19 0.005 0.009 0.57 0.52 0.08 0.32 0.020 0.013

SH 0.14 0.64 0.29 0.009 0.013 0.51 0.59 0.17 0.33 0.005 0.010

CH 0.13 0.58 0.26 0.016 0.014 0.68 0.43 0.17 0.31 0.028 0.009

B6 0.14 0.59 0.24 0.014 0.011 0.66 0.44 0.13 0.24 - -

Table 2 Metallurgical quality of the testing materials.

Pipe 0 457 x 28 mm in X=12.0ppm J= 114

as received state

Super heater outlet X=26.4ppm J = 223

header (SH)

Output steam piping X= 17.8 ppm J = 160

Steam piping elbow X=13.4ppm J=116

Table 3 Mechanical properties of the tested materials. Component YS [MPa] UTS [MPa] A [%] Z [%] FATT [°C]

AS SH CH B6

350 323 292

492 532 494 553

31.8 81

30.0 72

32.0 76

25.1 66

-5 +71 +36 +84

Figure 3 Empirical correlation for determination of UTS from the results of SP tests for low alloy steel of type 14MoV6-3 in as received state (blue rhombuses) together with results obtained on steam pipes after long term operation at 540°C (brown squares).

500 700 900 1100 1300

Pe/(h02) [MPa]

Figure 2 Empirical correlation for determination of YS from the results of SP tests for low alloy steel of type 14MoV6-3 in as received state (blue rhombuses) together with results obtained on pipes after long term operation at 540°C (brown squares).

Tensile properties at laboratory temperature in the longitudinal direction and FATT in the transverse direction are summarized in Table 3.

The results obtained show that FATT temperature has significantly affected both metallurgical quality of the material and long term exposition at 540 °C.

Results and discussion

Figs. 2 and 3 show the empirical correlations for the determination of YS and UTS from the results of SP tests for the pipe in as received state heat treated to seven significantly different tensile properties together with the results of SP tests and standardized tensile tests obtained on the materials after long term exposure. The empirical correlation for determination of yield strength from the results of SP tests was expressed as a dependence of yield strength on Fe/(h0)2 because the parameter Fe/(h0)2 eliminates differences in disc specimen thicknesses on load Fe. The empirical

correlation for estimation of material tensile strength from the results of SP tests was expressed as a dependence of tensile strength on Fm/(um h0) because the parameter Fm/(um h0) eliminates differences in disc specimen thicknesses on Fm and um (Hurst and Matocha, 2012).

Disc test specimens 8 mm in diameter and 0.5 mm in thickness were oriented in R—L direction (simulating removal of testing material by the scoop cutter sampling technology) (Parker et al., 1989). It is evident that the empirical correlations obtained for determination of YS and UTS from the results of SP tests for the steel in as received state are also valid for the determination of actual tensile properties after long term operation at elevated temperatures.

Empirical correlations for FATT estimation are based on the fact that steels exhibiting standard Charpy impact ductile to brittle transition behaviour also show the transition behaviour in the temperature dependence of fracture energy of the Small Punch tests, but shifted to a lower temperature (Hurst and Matocha, 2014).

Fracture energy of the Small Punch test is calculated from the area under the load—displacement curve until the specimen failure (punch displacement of uf). Small Punch transition temperature TSP is defined as the temperature corresponding to half of the sum of the maximum and minimum fracture energy calculated by the method of least squares from the temperature dependence of fracture energy derived from the experimentally measured data in transition areas (CEN Workshop Agreement, 2007). Transition temperature TSP, measured in a series of SP tests typically ranging from -193 °C to laboratory temperature, is correlated with the FATT determined from a series of Charpy V impact tests in the form (CEN Workshop Agreement, 2007; Hurst and Matocha, 2014)

FATT = a ■ TSPor FATT = a ■ TSP + p

where a, p are constants.

Fig. 4 shows the empirical correlation for estimation of FATT for the pipe in as received state, FATT and TSP for the

Figure 4 Empirical correlation for the estimation of FATT from the results of SP tests for low alloy steel of type 14MoV6-3 in as received state (blue rhombuses) together with results obtained on steam pipes after long term operation at 540°C (brown squares).

materials under investigation after long term operation at 540°C and 95% confidence interval of the empirical correlation for FATT estimation obtained in Hurst and Matocha (2014) for the steel 14MoV6-3 after long term operation at 540°C.

From the results it is evident that the correlation obtained for the as received state of this material cannot be used in all cases for evaluation of the actual FATT level after long term operation at 540°C. SP test technique is in this case more sensitive in detection of degradation processes occurring during long term operation at elevated temperatures in comparison with standardized Impact tests (Hurst and Matocha, 2014).

Conclusions

1) The empirical correlation for determination of material yield strength from the results of SP tests should be expressed as a dependence of yield strength on Fe /(ho)2 because the parameter Fe/(h0)2 eliminates differences in disc specimen thicknesses on load Fe.

2) The empirical correlation for estimation of material tensile strength from the results of SP tests should be expressed as a dependence of tensile strength on Fm/(um h0) because the parameter Fm/(um h0) eliminates differences in disc specimen thicknesses on Fm and um.

3) The empirical correlations obtained for determination of YS and UTS from the results of SP tests for the 14MoV6-3 low alloy steel in as received state are also valid for the determination of actual tensile properties after long term operation.

4) The empirical correlation for estimation of FATT from the results of SP tests obtained for the zero state (as received state) of this material cannot be used in all cases for evaluation of the actual FATT level after long term operation at 540 °C.

Conflict of interest

The authors declare that there is no conflict of interest.

Acknowledgements

This paper was created in the project Support of VSB — TUO activities with China with the financial support from Moravian-Silesian region and in the Project No. L01203 ''Regional Materials Science and Technology Centre — Feasibility Program'' funded by the Ministry of Education, Youth and Sports of the Czech Republic.

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