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FRACTURE STUDIES ON CARBON STEEL PIPING COMPONENTS AT

ELEVATED TEMPERATURE

M.Saravanan1, P.Gandhi1, S.Vishnuvardhan1, DM.Pukazhendhi1, G.Raghava1, M.K.Sahu2, J.Chattopadhyay2, B.K.Dutta2, K.K.Vaze2

1

CSIR - Structural Engineering Research Centre, Taramani, Chennai, INDIA-600113

2

Reactor Safety Division, Bhabha Atomic Research Centre, Mumbai, INDIA-400085 E-mail of corresponding author: sardiraj_m@sercm.org

ABSTRACT

In order to study the fracture behavior of piping components used in Primary Heat Transport (PHT) piping system at elevated temperature, experimental investigations were carried out on two carbon steel pipes and one elbow at an elevated temperature of 300°C. The pipes and elbow were made of SA 333 Gr.6 steel. Out of the two pipe specimens, one pipe specimen had 219 mm outer diameter (OD) and the other 406 mm OD. The nominal thicknesses of these pipe specimens were 15.1 mm and 26.9 mm, respectively. The pipe specimens had circumferential part-through notch at the centre. The initial notch angles of the pipe specimens were 34.1° and 37.8°, respectively. The elbow specimen had 219 mm OD and 14.8 mm thickness. The part-through axial notch in the elbow was located at intrados and the initial notch length was 64 mm. Prior to the fracture tests, all the pipes and elbow specimens were fatigue pre-cracked. The pipes and elbow were tested under four point bending and in-plane opening moment respectively. No significant crack growth was observed during the fracture tests on both the pipes and elbow.

Keywords: Leak before break, Fatigue pre-cracking, Plastic collapse load, Fracture, Elevated temperature

INTRODUCTION

Fatigue and fracture behaviour of various components used in power plants is of great importance to ensure adequate safety against operational and extreme loading conditions including improbable accidental loads. Piping components are subjected to cyclic loads due to thermal transients and larger cyclic loads during seismic events. The principle of Leak Before Break (LBB) behaviour is based on the fact that a crack will develop in such a manner that a detectable leak is produced, which has sufficient safety margin against critical through-wall crack size. Also, the initial surface crack should not become a through-wall crack after application of load cycles corresponding to one plant life.

SA 333 Gr. 6 carbon steel material is used for primary heat transport (PHT) piping system of Indian pressurized heavy water reactors (PHWR). Typically, PHT piping systems conduct D2O coolant, entering at 249ºC

and exiting at 293ºC under 10.5 MPa pressure. As it is a commercially produced C-Mn steel, SA 333 steel may be prone to dynamic strain ageing (DSA). In fact, it is reported (Tarafder & Ranganath 1997; Singh et al 1998) that the material shows small decrease in ductility and a concomitant increase in tensile strength over the temperature range of 200 to 300ºC [1].

To the best of author’s knowledge, no data was available in the literature at component level, on the fracture behaviour of components at elevated temperature. Hence, in order to study the fracture behavior of piping components used in PHT piping system at elevated temperature, experimental investigations were carried out on two carbon steel pipes and one elbow at an elevated temperature of 300°C.

MATERIAL PROPERTIES AND SPECIMEN DETAILS

Material Properties

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Table 1: Chemical composition of SA 333 Gr.6 carbon steel

Element C Mn P S Si

% weight 0.03 0.29 0.048 0.058 0.10

ASTM A 333 /A 333M - 05 (max.) 0.30 0.29 - 1.06 0.025 0.025 <0.10

Pipe Specimens

The pipes were made of SA 333 Gr.6 steel. Out of the two pipe specimens, one pipe specimen had 219 mm OD and the other 406 mm OD. The nominal thicknesses of these pipe specimens were 15.1 mm and 26.9 mm, respectively. The pipe specimens had circumferential part-through notch at the centre. The initial notch angles of the pipe specimens were 34.1° and 37.8°, respectively. Table 2 gives the details of length, outer diameter, average thickness and notch details of pipe specimens. Figure 1 shows a typical straight pipe having circumferential part-through notch.

Table 2: Dimensions of pipe specimens

Specimen No.

Specimen ID

Outer diameter

(mm)

Thickness of the

pipe (mm)

Length (mm)

Notch details

Notch length (mm)

Notch width (mm)

Notch depth (mm)

Initial notch angle (°)

1 TSPPTC 8-1 219 18.8 4300 65.0 3.0 6.0 34.1

2 TSPPTC 16-1 406 26.9 6087 134.5 3.3 13.4 37.8

C

C A

A

W

t

R

a

B

Section A-A

Detail-B

Section C-C

90° t

Surface crack

D

2c

Fig. 1: Details of a typical pipe specimen having circumferential part-through notch

Elbow Specimen

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induce stiffening effect against ovalization in the actual piping system. The average initial thickness of the elbow was 14.8 mm. The bend characteristic (h = tR/rm

2

) and radius ratio (R/rm) for the elbow were approximately 0.40

and 3.0. The elbow specimen was welded with end flanges at both the ends. The part-through axial notch in the elbow was located at intrados and the initial notch length was 64 mm. Figure 2 gives the details of the elbow specimen. 45° 580 TYP WELDED JOINT WELDED JOINT t FLANGE B C A OD 219 45

w = 3 B

C C

B

Detail A for axial notch at intrados

2C t Section BB

t

90º

a

Section CC 63.5 1.5 Ø270 162 Ø188.8 Ø470 Ø219 37.5°

12 HOLES Ø34.9 ON PCO 393.7 EQUISPACED + 2.5° 37.5 t 0.8 0.8

MAXIMUM MISMATCH AT ANY POINT ARROUND THE JOINT WITH CONCENTRIC CENTER LINES

D

+ 2.5

Detail B & C

Fig. 2: Details of a typical elbow specimen having axial part-through notch

EXPERIMENTAL INVESTIGATIONS

Fatigue Pre-cracking

Prior to fracture tests, both the pipe specimens and elbow were fatigue pre-cracked to create a sharp crack front. The fatigue pre-crack growth in the depth direction at centre of the notch was approximately 2.5 mm. The fatigue pre-cracking was carried out using servo-controlled electro-hydraulic actuators of ±1000 kN capacity. The theoretical plastic collapse load, Po, for the pipes with part-through notch (simply supported test condition) was

calculated using the following equation [3]. The tests were carried out under load control. The fatigue pre-cracking of pipes was carried out under four point bending. For the elbow, the cyclic load was applied in such a way that the notch was subjected to opening. The elbows were supported in hinged-hinged condition with one end connected to the actuator and the other end connected to the test floor through specially fabricated fixtures. The cyclic load was applied with a load ratio of 0.1. The maximum cyclic load was 10 to 20% of the plastic collapse load. The minimum cyclic load was 10% of the maximum cyclic load. Figures 3 and 4 show the test set-up of fatigue pre-cracking of pipe and elbow specimen respectively. The details of fatigue pre-pre-cracking for pipes and elbow are given in Table 3.

            −       −

= sinθ

t 2 a θ t 2 a cos t 16

L

L

m

R

σ

P

i o 2 f

0 (1)

Where

σf = flow stress of the material in MPa (average of yield and ultimate tensile strengths) Rm = mean radius of the pipe in mm

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Lo = outer span of loading in mm Li = inner span of loading in mm

θ = half the notch angle in radians

a = crack depth in mm

Table 3: Details of fatigue pre-cracking

Specimen ID Span (mm) Frequency

(Hz)

Cyclic load (kN)

No. of cycles

Outer Inner Max. Min.

Pipes

TSPPTC 8-1 4000 880 1.0 - 1.5

50 5.0 Initial - 70000

68 6.8 70000 - 272000

86 8.6 272000 - 330000

TSPPTC 16-1 5500 1620 1.0 150 15.0 Initial - 115000 225 22.5 115000 - 195000

Elbow TELPTAIN 8-2 - - 0.5 31.6 3.2 398213

Fig. 3: Test set-up for fatigue pre-cracking

of straight pipes Fig. 4: Test set-up for fatigue pre-cracking of an elbow

Fracture Tests

Subsequent to fatigue pre-cracking, fracture tests were conducted on the pipes and elbow at elevated temperature of 300° C. As mentioned earlier, the test set-up is similar to fatigue pre-cracking tests. Fracture tests were conducted under four point bending on 219 mm OD pipe and in-plane bending (opening moment) of elbow using servo-controlled electro-hydraulic actuator of ±1000 kN capacity. The pipe having 406 mm OD was tested under four point bending using ±5000 kN capacity hydraulic jack.

Test Set-up

The pipe specimens were subjected to four point bending load. The pipe specimens were simply supported over steel pedestals. The steel pedestals were firmly anchored to the strong floor of the laboratory with holding down bolts and nuts. A rigid steel distribution beam was connected to the piston end of the actuator and the same was used to apply two points loading on the pipe specimen. The distribution beam was supported on steel curved blocks at the two loading points, with rollers in between; this arrangement ensured smooth transfer of load to the pipe specimen without any local denting. In the case of elbow specimens, cyclic bending load under in-plane opening moment was applied under pin-pin support conditions. The elbow specimens were fixed to hinges at the ends, one end connected to the actuator and the other end to the test floor through specially fabricated fixtures. Figures 5 and 6 show the fracture test set-up for pipe and elbows.

Heating Arrangement

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used for setting the required temperature and maintaining the same throughout the test. The temperature was measured at regular intervals with a non-contact type laser based temperature measuring instrument. In addition, thermocouples were also used to measure the temperature.

Instrumentation

The total applied load was measured directly using a strain gauge based load cell connected to the actuator/hydraulic jack. The load-line displacement was measured by a Linear Variable Differential Transducer (LVDT) fixed to the actuator/hydraulic jack. Crack mouth opening displacement (CMOD) was measured across the crack centre line using a clip gauge, which was specially fabricated for these experiments. The clip gauge was calibrated prior to the experiments. Ovalization of the elbow was measured using ovality rings during fracture experiments. Crack initiation and crack growth were monitored using Alternating Current Potential Drop (ACPD) technique. Steel pins of 1 mm diameter were spot welded to both the pipes and elbow specimens along the notch at different locations. These are connected through instrumentation cables to the crack micro gauge. The crack micro gauge works on ACPD technique and gives the crack depth directly. All the data were acquired through a Data Logger which was interfaced to a computer system for on-line data acquisition. Fig.7 shows a close-up view of ACPD instrumentation and clip gauge used in measuring CMOD for the elbow. Fig. 8 shows the temperature indicator during fracture tests on pipes and elbow.

Details of Loading

Static monotonic load was applied for all the pipes and elbow specimens under displacement control. The fracture tests were conducted using computer controlled servo- hydraulic actuator of ±1000 kN capacity/±5000 kN capacity hydraulic jack. The rate of displacement was 0.05 mm/sec for actuator and 0.036 mm/sec for hydraulic jack. The various data continuously monitored and acquired during the fracture tests include the maximum load, load-line displacement, crack mouth opening displacement and ovality of the elbow.

Fig. 5: A view of fracture test on a 406 mm OD carbon steel pipe at elevated temperature

Fig. 6: Fracture test set-up on an elbow under opening moment

Fig. 7: Close-up view of ACPD instrumentation

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RESULTS AND DISCUSSION

The fracture test results are given in Table 4. The maximum measured load for 219 mm OD pipe was 279.0 kN; the corresponding load-line displacement and CMOD were 232.4 mm and 2.82 mm. The maximum measured load for 406 mm OD pipe was 985.8 kN; the corresponding load-line displacement and CMOD were 187.8 mm and 6.97 mm. The maximum measured load for 219 mm OD elbow as 221.9 kN; the corresponding load-line displacement and CMOD were 234.9 mm and 6.07 mm. Figures 9 and 10 show load vs. load-load-line displacement and load vs. crack mouth opening displacement curves for the pipe and elbow specimens. The deflected shape of the pipe specimen after fracture test is shown in Fig. 11. The pipe and elbow specimens failed by yieldingand the part-through notches have not developed into through thickness cracks. The experimental maximum measured load was compared with the closed form solution (Equation 1) and found to be conservative to the extent of 64.7% for the two pipes tested. Large scale ovalization has been observed in the elbow specimen at the end of fracture test. Figure 12 shows the close-up view of notch portion of pipe and elbow specimens after fracture test.

Table 4: Fracture test results

Specimen ID

Outer span (mm)

Inner span (mm)

MML (kN)

TPCL (kN)

Corresponding load-line displacement (mm)

CMOD (mm)

Pipes TSPPTC8-1 4000 880 279.0 169.4 232.4 2.82

TSPPTC16-1 5500 1620 985.8 706.9 187.8 6.97

Elbow* TELPTAIN 8-2 - - 221.9 - 234.9 6.07

MML: Maximum Measured Load; TPCL: Theoretical Plastic Collapse Load; CMOD: Crack Mouth Opening Displacement; * Opening moment (Tensile loading)

Fig. 9: Load vs. load-line displacement curves for pipe and elbow specimens

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406 mm OD Pipe

219 mm OD Elbow

Fig. 11: Deflected shape of the pipe after fracture test Fig. 12: Close-up view of notch portion of pipe and elbow after fracture test

CONCLUSION

Fracture tests were carried out on two carbon steel pipes and one elbow at an elevated temperature of 300°C. The fracture tests were carried out under four point bending in the case of pipe specimens and tensile opening moment in the case of elbow specimen. The maximum measured load, load-line displacement, crack mouth opening displacement were continuously acquired for the pipe specimens during the fracture tests. In addition to the above measurements, ovalization was also measured in the case of elbow specimen. Based on the experimental studies carried out the following conclusions are made.

The load vs. load-line displacement curves for the pipe and elbow specimens tested at elevated temperature have shown no signs of drooping from the peak load indicating large amount of energy absorption at peak load which is necessary for the justification of leak-before-break design. The experimental maximum measured load is conservative to the extent of 64.7% for the pipe specimens tested. Maximum ovalization of 16.9% has been observed during the fracture studies on elbow under in-plane opening moment. No significant crack growth was observed during the fracture tests on both the pipes and elbow.

ACKNOWLEDGEMENTS

The authors from CSIR-SERC thank Dr. Nagesh R. Iyer, Director and Dr S. Arunachalam, Advisor (Management), CSIR-SERC for their valuable guidance, encouragement and support in the R&D activities. The cooperation and support extended by the technical staff of Fatigue & Fracture Laboratory of CSIR-SERC in carrying out the experimental investigations is gratefully acknowledged. This paper is published with the kind permission of the Director, CSIR-SERC, Chennai.

REFERENCES

[1] Tarafder, S., Ranganath, V.R., Sivaprasad, S. and Johri, P., “Ductile fracture behaviour of primary heat transport piping material of nuclear reactors”, Sadhana, Vol. 28, Parts 1 & 2, February/April 2003, pp. 167-186

[2] ASTM A 333/A 333M - 05, “Standard specification for seamless and welded steel pipe for low-temperature Service”, American Society for Testing and Materials, U.S.A, 2009.

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[4] Vishnuvardhan, S., Pukazhendhi, DM., Saravanan, M., Muthuramalingam, G., Gandhi, P. and Raghava, G., ‘Fracture Tests of 219 mm OD and 406 mm OD carbon steel pipes having part-through notch at elevated temperature’, Report No. 4 on Sponsored Project SSP 6341, October 2009, CSIR - Structural Engineering Research Centre, Chennai, India.

Figure

Table 2: Dimensions of pipe specimens
Table 3: Details of fatigue pre-cracking
Fig. 8: A view of temperature indicator
Fig. 9: Load vs. load-line displacement curves for pipe and elbow specimens
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References

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