Fatigue Crack Growth Studies on Narrow Gap Pipe Welds of Austenitic Stainless Steel Material

Available online at www.sciencedirect.com

ScienceDirect Procedia Engineering 86 (2014) 203 – 208

1st International Conference on Structural Integrity, ICONS-2014

Fatigue Crack Growth Studies on Narrow Gap Pipe Welds of Austenitic Stainless Steel Material Punit Aroraa,*, Saroj Kumarb, P.K. Singha, V. Bhasina, R.K.Singha and K.K. Vazea a

Reactor Design & Development Group, bIsotope Applications Division, Bhabha Atomic Research Centre, Mumbai, 400085, India * E-mail ID: [email protected]

Abstract The objective of the present study is to understand the Fatigue Crack Growth (FCG) behavior in austenitic stainless steel pipe welds. The tests have been carried out on Compact Tension (CT) specimens machined from the actual narrow gap pipe welds. The notch was located in Weld Centre Line (WCL), Heat Affected Zone (HAZ), HAZ-Fusion Line (FL) interface and the parent metal. The comparisons have been made for the FCG rate for different locations of the crack. The tests were carried out using standard procedure of ASTM E647. The results of FCG rate of narrow gap welds (HAZ, WCL, HAZ-FL interface and parent material) have been compared with that of conventional V-groove welds of Shielded Metal Arc Welding (SMAW) process. The comparison shows that the fatigue crack growth resistance of narrow gap welds is better than conventional groove welds. The FCG resistance of weld and parent material is better than HAZ location in the Paris regime. However, parent material shows superior threshold crack driving force range. © © 2014 2014 The The Authors. Authors.Published Publishedby byElsevier ElsevierLtd. Ltd.This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). Peer-review under responsibility of the Indira Gandhi Centre for Atomic Research. Peer-review under responsibility of the Indira Gandhi Centre for Atomic Research Keywords: Fatigue Crack Growth, Stress Intensity Factor range, Fusion Line, Heat Affected Zone

1. Introduction Shielded Metal Arc Welding (SMAW) is one of the widely used joining processes in piping system of the nuclear power plants. This welding process is manual in nature and flux is used to shield the molten weld pool during welding. The weld joints prepared using this process may have reduced fracture toughness and higher fatigue crack growth compared to the parent metal because of the entrapment of slag inclusion and porosity. Although utmost care is taken by following the various standards to produce a defect free weld joints but owing to the manual nature of the process, some defects may be undetected during the inspection at the time of welding due to either limitation of the inspection system or human error. Therefore, improving the fracture toughness and the fatigue crack growth of the weld joints are always desirable to enhance the life of the piping components/ system. Usually

1877-7058 © 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license

(http://creativecommons.org/licenses/by-nc-nd/3.0/). Peer-review under responsibility of the Indira Gandhi Centre for Atomic Research doi:10.1016/j.proeng.2014.11.029

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Fatigue Crack Growth Rate (FCGR) tests are carried out on the base material. Effects of various parameters on FCGR are available in literature based on the standard specimen tests [1]. Earlier few works have been carried out on the carbon steel pipe and stainless steel pipe welds of SMAW and GTAW [2,3]. In view of the various investigations on significantly higher FCG rate of SMAW in comparison to parent material, automated Hot Wire Gas Tungsten Arc Welding with Narrow Gap (HW-GTAW-NG) technique was adopted for welding of the pipes. Already heated filler wire when fed to groove does not subject HAZ region to sensitization temperature range. In addition, narrow groove of the weld leads to reduction in weld distortion and residual stresses. In view of this, fatigue crack growth behavior of the pipe welds using HW-GTAW-NG was investigated. A systematic study of fatigue crack growth behavior of hot wire narrow groove welds and its comparison with that of conventional weld grooves has been carried out. The comparison has also been made between the fatigue crack growth resistance of HW-GTAW-NG welds and SMAW welds. 2. Experimental Details Pipe material was austenitic stainless steel of SA312 type 304LN. The compact tension specimens have been machined from the seamless pipe and pipe welds of nominal outer diameter 324 mm (designated as 300NB) having thickness of 25 mm. The pipes were in solution annealed condition and conforming to the SA312 specifications of ASME Section II and Section III, 2010. Welding of pipes was carried out as per ASME Section IX of the Boiler and Pressure Vessel code. Gas Tungsten Arc welding (GTAW) with hot wire and narrow gap technique was followed for welding of 300NB pipe. ASME Section II does not specify the filler wire for (GTAW) for welding of SS304LN material. Therefore filler wire ER308L as specified for 304L was used for welding of pipes under this study. The details of the welding consumables, process and parameters are given in Table 1. The chemical composition of the pipe and pipe weld materials are given in Table 2. Table 1. Welding consumables, process and parameters Welding process Filler wire type Filler wire diameter (mm) Electrode diameter (mm) Welding Current (A)

Hot wire GTAW ER 308L 1.2 mm 3.2 mm Peak/base = 110/100 to 160/140 8.4 90-110 mm/min Argon

Welding voltage (V) Welding Speed (mm/min) Inert Gas

Table 2: Chemical composition of material in weight % Pipe size ASME

Material 304LN

300NB 300NB

304LN Weld

C 0.03 max 0.024 0.02

Mn 2.0 max 1.73 1.76

Si 1.0 max 0.55 0.37

P 0.045 max 0.022 0.016

S 0.03 max 0.001 0.005

Cr 18-20 18.8 19.52

Ni 8-12 9.25 9.91

N 0.1-0.16 0.15 0.1

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Table 3. Tensile properties of materialss

Material 304LN HTGTAWNG

269

Ultimate Tensile Stength (MPa) 528

64.2

320

653

-

Yield strength (MPa)

Figure 1. Typical schematic of CT specimen machined from 300NB pipe weld

El (%)

Tests have been carried out on o compact tension specimens having notch at the weld f fusion centre and at different locations from line covering HAZ-FL interfacee, HAZ and parent material. The tensile propeerties of the pipe and pipe weld for each size of pipes are T specimen given in Table 3. Sketch of CT with dimensions has been shown n in Figure 1(a). The specimens were machin ned from the actual pipe weld in L-C orieentation as indicated in schematic of pipe weld w (Figure 1(b)).

Figure 1(b). Schematic for the machining of from full scale pipe weld

specimen

2.1 Microstructure Studies The micrographs for the different regiions such as base, weld and HAZ-FL interface for hot wire w GTAW welds are shown in Figures 2(a) and 2(b).

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Figure 2(a). Typical grain structure of parent material

Figure 2(b). Weld and HAZ-FL interface by hot wire GTAW (Narrow Gap)

2.2 Fatigue Crack Growth Rate Test Studies Fatigue crack growth tests have been carried out on compact tension specimens machined from the actual pipe welds. Notch in the CT specimen has been located at different locations from weld center line which covers HAZ and parent material regions. The specimens were designed and tested using standard ASTM E647 procedure. The typical etched CT specimen has been shown in Figure 3.

The tests were carried out using ΔK-increasing (increasing crack driving force) and ΔK-decreasing (decreasing crack driving force) test methods. All the tests have been carried out at room temperature and in air environment. The load ratio (R) that is the ratio of maximum stress intensity factor to minimum stress intensity factor in a cycle was kept as 0.1. The crack growth was measured using COD gauge. The loading and test details are given in Table 4. The ΔK increasing tests have been carried out nearly from 15MPa√m to 50MPa-√m and ΔK varied from 35 MPa-√m to 10 MPa-√m. in ΔK-decreasing tests.

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Punit Arora et al. / Procedia Engineering 86 (2014) 203 – 208

3. Results and Discussions The fatigue crack growth behavior of different locations for HW-GTAW-NG welds has been shown in Figure 4. The variation of crack growth rate (da/dN) for a given crack driving force range (ΔK) is comparatively higher for HAZ locations than weld and base in the Paris regime.

Figure 4. The fatigue crack growth rate versus SIF range curves for base (SS-B-3-RT), weld(SS-WCL-2-RT), HAZ (SS-HAZ-3-RT) and HAZ-FL interface (SS-FL-1-RT, SS-FL-2-RT). This figure shows that, FCGR of the weld shows better fatigue crack growth resistance compared to base and heat affected zone in the Paris regime. These crack growth observations are consistent with the observation by various investigators who have compared crack growth in weld and HAZ on full scale pipe component of austenitic stainless steel [3]. Singh et. al. have studied the fatigue crack growth on 168 OD pipe having circumferential notch on the outer surface in weld/ HAZ location subjected to pure alternating bending moment. The crack growth in their study brought out that weld has better FCG resistance compared to HAZ. The dendritic structure of the weld metal leads to directional effect. In this case crack front may not be oriented along the weak link of the dendrite. Here also crack growth is transdendritic or inter-dendritic depending upon the orientation of the dendrite ahead of the crack tip. The crack growth direction with respect to dendrite orientations is to be investigated for understanding the lower crack growth in weld location. Weld metal also has few percent of ferrite which helps in retarding the crack growth. Jang C. et. al. [4] have discussed the fatigue crack growth paths within the dendrite microstructure of the welds. They observed dependence of fatigue crack path on the weld microstructure, especially the dendrite alignment. The effects of weld microstructure were also studied by Tsay and Tasy [5] and Rao et. al. [6]. It has also been reported that at lower ΔK, fatigue crack growth is significantly affected by microstructure and continuous striations are not formed on the fracture surface. This leads to higher ΔKth for welds as compared to HAZ and base as shown in Figure 4.

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Figure 5. Comparison of crack growths of welds by HW-GTAW-NG (SS-WCL-2-RT) with SMAW having conventional groove (SS-WCL-SMAW) [3] Figure 5 indicates that SMAW welds [3] leads to higher crack growth rate as compared to hot wire GTAW with narrow groove. Singh et. al. [7] have also shown on full scale pipe that the fatigue resistance of HW-GTAWNG welds is superior to SMAW with convetional groove. 4. Conclusions The fatigue crack growth rate tests carried out on hot wire GTAW narrow groove welds at room temperature show higher crack growth in HAZ region as compared to weld and parent material. However, further investigations are required to understand the crack path with respect to dendrite orientation in welds. The ΔKth of weld centre line is higher than HAZ and parent material. The FCG resistance of hot wire GTAW with narrow groove is superior to SMAW welds with convectional V-groove. References [1] Ellyn Fernand (1997), "Fatigue damage, crack growth and life prediction", Chapman & Hall, pp 381. [2] Ho-Jung Lee, Minu Kim, Changheui Jang, Sun-Young Cho, Jun-Seog Yang, (2011) “Fatigue Crack Growth Rate and Fracture Resistance of Heat Affected Zone of Type 316L Stainless Steel with Narrow Gap Welds” Proceedings of the ASME 2011 Pressure Vessels & Piping Division Conference, Maryland, USA [3] P.K.Singh, V.Bhasin, K.K.Vaze, A.K.Ghosh (2008) “Fatigue Studies on Austenitic Stainless Steel Pipe Welds” International Congress 2008, International Institute of welding, Chennai. [4] Changheui Jang, Pyung-Yeon Cho, Minu Kim, Seung-Jin Oh, Jun-Seog Yang, Effects of microstructure and residual stress on fatigue crack growth of stainless steel narrow gap welds, Materials and Design 31 (2010) 1862–187. [5] Tsay LW, Tasy CY. The effect of microstructures on the fatigue crack growth in Ti–6Al–4V laser welds. Int J Fatigue 1997;19:713. [6] Rao EJ, Guha B, Malakondaiah G, Radhakrishnan VM. Effects of welding process on fatigue crack growth behaviour of austenitic stainless steel welds in a low alloy (Q&T) steel. Theor Appl Fract Mech 1997;27:141–8 [7] P.K.Singh, Punit Arora, V.Bhasin, K.K.Vaze, D.M.Pukazhendi, P.Gandhi, G.Raghava “Fatigue crack growth and fracture resistance of narrow gap pipe welds of type 304ln stainless steel” Transactions, SMiRT-22, San Francisco, California, USA - August 18-23, 2013