Acoustic emission monitoring of crack growth in dissimilar joints for corrosion resistant applications

Chiraz Ennaceur and Viviane Beaugrand

Paper presented at EWGAE 2008. 28th European Conference on Acoustic Emission Testing. Krakow, Poland, 17-19 Sept. 2008.

Abstract

In this paper investigation was undertaken to characterize the fusion boundary microstructure and to study the nature and character of boundaries that associated with cracking in dissimilar joints.

Testing has been performed to establish the threshold stress intensity factor at which cracking can occur, employing through-thickness single edged notched bend (SENB) specimens, with the notch placed at the dissimilar interfacebetween the forging and the butter weld. In order to establish the threshold stress intensity factor accurately a means is needed of detecting the initiating cracking events. Previous efforts have shown that direct current potentialdrop measurements were no more accurate at detecting the onset of cracking than monitoring crack mouth opening displacement. Thus, in the present work, the feasibility of acoustic emission (AE) monitoring to successfully detect theearliest stages of crack initiation was assessed. Multi-parametric analysis of the AE events was carried out for the classification and differentiation of the detected acoustic signature, correlated to fracture in differentmicrostructural zones. This analysis demonstrated that the most significant differentiating parameters were the amplitude and the energy of the wave emitted.

This work revealed the sensitivity of AE technique in monitoring and identifying the different mechanism of ruptures and crack propagation during different load steps.

1. Introduction

Subsea dissimilar joints typically comprise forged and machined hubs or tees made from low alloy steel, buttered with a nickel alloy, and then joined to pipeline micro-alloyed steels. The main advantage of buttering is that itenables the forgings to be post weld heat treated (PWHT) prior to making the completion weld in the field without PWHT, as the Heat Affected Zone (HAZ) of the completion weld lies wholly in the buttered layers. As for any fusion weldeddissimilar joint, a narrow partially mixed zone (PMZ) is observed at the dissimilar interface, where the composition is graded between that of the parent forging and the mid-bead chemistry of the butter runs. In order to preventcorrosion of piping systems, subsea, cathodic protection is applied. Whilst this does inhibit corrosion it can result in a certain amount of hydrogen charging. Whilst the vast majority of subsea joints have given successful service, asmall number have failed due to hydrogen embitterment. This subject has been the subject of extensive research at TWI ^{[Beaugrand and Smith]} , ^[Gittos] , ^{[Willingham and Gooch]} and the present publication reports on part of this work.

2. Experimental procedure

2.1 Material and microstructure

The environmental performance of an industrially fabricated dissimilar joint was investigated. The butter weld consisted of a build up of alloy 625 on a low alloy steel forging AISI 8630M, deposited using hot wire TIG welding withan applied preheat. The joint had been subjected to PWHT and a completion weld made, again using alloy 625, to a steel pipe. A metallographic section was taken through the dissimilar interface and the interfaced microstructures wereobserved and correlated with the fracture locations.

2.2 Environmental testing

Square section 12x12mm single edge notched bend (SENB) specimens were machined from the dissimilar joint and notched through-thickness by electro-discharge machining (EDM) at the dissimilar interface. Specimens were hydrogenpre-charged at -1100mV _SCE in 3.5%NaCl for a minimum of 48 hours prior to testing. Incremental, dwell loading tests were carried out by loading samples in three point bending to an initial stress intensity of 40MPam ^0.5, continued by further cumulatively applied load increments of 2.5MPam ^0.5 after an extended dwell of several hours at each load step. These tests were performed at 3°C in 3.5%NaCl aqueous solution under cathodic protection at -1100mV _SCE, applied by potentiostat, simulating subsea service. The specimens were instrumented with a pair of clip gauges, so that crack mouth opening displacement (CMOD) could be monitored. The test was terminated once crack initiationwas identified, by monitoring the response of either the clip gauges or the acoustic emission signal. The test set-up is shown in Figure 1.

Initial testing was performed to achieve significant crack extension (i.e. crack extension of a few millimetres) to calibrate the acoustic emission monitoring equipment (including noise emissions from the testing equipment) and todetermine the sensitivity of the method in detecting cracking at an early stage. This was compared with CMOD measurements. Subsequent tests were performed and stopped shortly after a significant signal was collected by AE. Samples werethen heat tinted and broken open in liquid nitrogen, prior to fractographic examination. The AE events (including energy) were correlated with features observed on the fracture faces, where possible.

Post-test examination included fractographic examination of the broken halves, to determine the depth of the crack, evidence of the location of the initiation event(s) and measurement of the extent of any cracking. Fracture featureswere correlated with initiation modes at the dissimilar joint interface/PMZ.

2.3 Acoustic emission equipment

The acoustic emission system used in the experiment was the PCI-2 board 2 channels. The acoustic emission signals were detected using two wide band piezoelectric transducers with a frequency range of 125-750 kHz (Nano 8mm diameter x8mm high). The acquired AE signals were amplified by a 40dB fixed gain preamplifier. The threshold selected was 40dB, which was well above mechanical noise level.

The sensors were placed on the top surface of the specimen within a distance of 90mm. The sensor calibration was carried out by using the standard pencil break technique ^[Hsu-Nilsen] , which generates an intense acoustic signal, (similar to a natural AE source), that the sensors detect as a strong burst. The purpose of this test is twofold. Firstly, it ensures that the transducers are in good acousticcontact with the part being monitored. Generally, the lead breaks should register amplitudes of at least 80dB for a reference voltage of 1 mV and a total system gain of 80dB. Secondly, it checks the accuracy of the source locationset-up. This last purpose involves indirectly determining the actual value of the acoustic wave speed for the object being monitored.

3. Results

3.1 Microstructure

The microstructure of the dissimilar interface between the butter weld deposit and the forging at the mid-bead position is showing in Figure 2a. At inter-pass positions, the microstructure was more complex with 'swirls' of diluted steel penetrating into the butter weld, as shown in Figure 2b. A full assessment of the microstructure is given elsewhere ^{[Beaugrand and Smith, 2008]} , but the main features of interest to the present work are the PMZ immediately adjacent to the interface at the mid-bead positions (Zone Φ in Figure 2a), and the zone between the swirls of diluted steel and the forging (Zone Δ in Figure 2b).

3.2 Acoustic emission

The results of five acoustic emission monitoring trials conducted during different environmental tests are shown in Table 1.

Table 1 AE events detected for each environmental test, together with details of when the test was stopped and the specimen broken open.

Acoustic emission monitoring was found to be significantly more sensitive to the detection of the initiation of cracking than monitoring the clip gauge. For example, during the initial test used for calibrating the AE, cracking wasdetected 4 hours before any appreciable clip gauge response was detected. During the experimental trials, three different types of signal were identified.

• A signal emitting waves with amplitude between 50 and 60dB and an absolute energy lower than 2,000aJ, during crack initiation and propagation
• A signal emitting waves with amplitude between 50 and 70dB and an absolute energy above 20,000 aJ, during crack initiation and propagation
• A signal emitting waves with amplitude between 80 and 100dB and an absolute energy above 20,000,000 aJ, in the latter stages of crack extension.

Examples of AE data acquired during testing are shown in Figure 3. Note that these data are correlated with fracture location, as described in sections 3.3 and 3.4.

3.3 Fracture location

Upon detecting an appropriate AE event and stopping the test, patches of crack initiation with a depth as small as 200microns were found.

Examples of the different fracture morphologies that had developed during testing are shown in Figure 4. These were correlated with the region of the interface microstructure where cracking occurred. The fracture morphology shown in Figure 4a developed in Zone Φ shown in Figure 2a and that in Figure 4b developed in Zone Δ shown in Figure 2b (i.e. at mid-bead and inter-run positions, respectively). Tearing was also observed in the forging during the later stages of crack extension.

3.4 Discussion

AE monitoring proved capable of capturing the earliest fracture initiation events that occur during environmental testing of dissimilar joints. This technique proved more capable than the other crack monitoring methods commonlyemployed for this type of environmental test.

Multi-parametric analysis of the AE events combined with examination of the fracture faces enabled the identification of specific crack features, corresponding to these acoustic signatures. Analysis of the AE events proved capableof differentiating between failure in different microstructural zones. Specifically, analysis of the amplitude and energy of the signal provided data that indicated where in a narrow 50µm or so wide microstructural zone failureinitiation occurred ( Table 2).

This capability has proven essential for characterising the failure modes that operate in subsea dissimilar joints and for qualifying a test method for determining threshold stress intensity factors for such fabrications ^{[Beaugrandand Smith, 2008]} .

Table 2 Correlation of the acoustic emission signals with cracking location.

Acoustic emission (AE) monitoring has been performed during bending tests of specimens containing dissimilar joints, these results showed that:

• AE monitoring was found to be more sensitive to the detection of the initiation of cracking than other methods investigated.
  
  
• Analysis of the AE data proved capable of correlating the microstructural region in which failure occurred with the cracking events detected.
• 
  

4. References

1. Beaugrand and Smith, 2008, TWI Research Board Report 17272.01/2008/1334.2
2. Gittos MF, Resistance of dissimilar joints between steel and nickel alloys to hydrogen-assisted cracking, corrosion 2008 New Orleans, arch 2008-06-18
3. Willingham DC and Gooch TG, the interface of stainless steel cladding deposited with ship electrode, the welding institute, Res.Bull, October 1971, 273-277
4. N N Hsu, F.R. Breckenridge. Characterization and Calibration of Acoustic Emission Sensors // Mat. Evaluation, 39, 60, (1981).

5. Acknowledgments

The work was funded by Industrial Members of TWI as part of the Core Research Programme. The authors are grateful for the technical support of Mike Gittos and Lee Smith, and acknowledge the contributions of David Seaman, JerryGolden, David Saul and Brian Kersey.