FRACTURE TOUGHNESS PROPERTIES OF THE HEAT-AFFECTED-ZONE IN FERRITIC STEEL WELDMENTS

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1 Key Engineering Materials Online: ISSN: , Vols , pp doi: / Trans Tech Publications, Switzerland FRACTURE TOUGHNESS PROPERTIES OF THE HEAT-AFFECTED-ZONE IN FERRITIC STEEL WELDMENTS R Moskovic 1, N P O Dowd 2, M Priest 1, P E J Flewitt 1, 3 1) BNFL Magnox Generation, Berkeley, Gloucestershire, UK 2) Imperial College, South Kensington Campus, London, UK 3) University of Bristol, Bristol, UK Keywords: Heat-affected-zone, fracture toughness, CrMoV steel, weldment, finite element analysis, crack growth. Abstract, For safety critical applications of plant it is necessary to ensure that conservative values of mechanical properties are used for structural integrity assessments. Assessment procedures require fracture toughness often of the weldments as one of the input parameters. The main constituents of weldments are steel plate, weld metal and the heat-affected-zone (HAZ) in the plate. This paper considers the evaluation of upper shelf fracture toughness properties of HAZ. During fracture toughness testing using three point bend specimens the cracks tended to deviate from higher strength coarse grain HAZ into lower strength regions comprising weld metal or fine-grained HAZ. The tests in which cracks started in the HAZ and propagated mainly through this region yielded values of J 0.2 in the range 118MPam 1/2 to 177MPam 1/2. These experimental results are discussed with respect to the predictions of a finite element model that considers particularly the path followed by the crack within the weldment. 1 Introduction Welded joints are widely used in the manufacture of pressure vessels, pipework and other components forming the pressure boundary of nuclear plant. The heat-affected-zone (HAZ) of structures is a common location for potential defects. It is also a region where a lower level of fracture toughness can exist. The HAZ, which is typically ~3mm wide, exhibits a large range of microstructures across this small distance. The microstructure within this region of the weldment is complex and influenced the welding thermal cycle. For multipass weldments, a series of parameters including weld consumable, preheat temperature, heat input, cooling rate etc, will influence the finally produced microstructure. Associated with these microstructural variations are differences in the mechanical properties [1]. The microstructure of the HAZ is closely linked to both the peak temperature of the welding thermal cycle and the cooling rate through temperature range between 800 and 500 C. The region of the plates that is closest to the weld experiences peak temperatures of ~1100 C which leads to some dissolution of the carbide precipitates and to ferrite grain growth. In low alloyed steels, further changes of microstructure and, thereby, mechanical properties may occur during the stress relief heat treatment as a result of secondary hardening which involves formation of chromium, molybdenum and vanadium rich carbides. Due to the wide range of microstructures encountered in the HAZ, it is very difficult to obtain reproducible values of fracture toughness by testing real weld HAZ. Hence, weld thermal cycle simulation techniques are often used to compare the fracture toughness of various microstructures observed in the HAZ and provide ranking of fracture toughness for different HAZ microstructures [1]. The simulated HAZ regions are relatively wide and it is likely that the plastic deformation will be contained within the HAZ unlike in a real weld HAZ. The coarse grained HAZ (CGHAZ) has high strength and low ductility and is generally susceptible to one of several forms of cracking during either welding or the post weld heat treatment cycle. When a crack is located in the CGHAZ, the region of high stress and strain at the crack tip is larger than the spatial region over which both the microstructure and the mechanical properties vary. As a result, yielding in different microstructures occurs at different stress levels and the size and the shape of the plastic zone varies accordingly. A considerable All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (# , Pennsylvania State University, University Park, USA-16/09/16,10:52:40)

2 42 Advances in Fracture and Damage Mechanics III amount of theoretical work involving finite element modelling has been carried out [2,3] to describe the distribution of strain accumulated within this region of variable microstructure defined as the HAZ. Moreover, there have recently been further theoretical considerations, albeit limited, to address how a crack will extend within this critical region of a weldment [2]. Structural integrity assessments of cracked components employ fracture mechanics principles. Mechanical properties and fracture toughness in particular are important input parameters for these assessments [4]. Unfortunately, because the HAZ occupies only a small volume of the overall weldment, it is difficult to measure the mechanical properties and fracture toughness. To obtain fracture toughness properties of the HAZ, a number of workers [5, 6] carried out experimental work using specially manufactured weldments with the weld preparation designed to produce a HAZ perpendicular to the plate surface. These tests have demonstrated that there is a tendency for the crack to grow out of the higher strength HAZ into the lower strength weld metal. In this paper, in Section 2, we describe the experimental procedure to measure the fracture toughness for the HAZ of a CrMoV steel weldment. In Section 3, we present the results of fracture toughness tests undertaken on three point bend specimens with a precrack positioned in the HAZ of this weldment. In Section 4, the results of a finite element model to describe the crack path are presented and compared with the experimental results in the discussion, Section 5. 2 Experimental Procedure The material for testing was taken from double V butt manual arc weldments manufactured using a ferritic steel parent plate and Babcock, H1, welding consumable. For the plate the concentration of the main alloying elements, in weight %, obtained from chemical analysis is 0.12%C, 1.34%Mn, 0.27%Si, 0.59%Cr, 0.29%Mo, 0.10%V, 0.18%Ni and 0.013%S. Typical chemical composition of Babcock H1 electrode is 0.048%C, 0.96%Mn, 0.32%Si, 0.39%Cr, 0.31%Mo and 1.60%Ni. After extracting the samples they were heat treated for 2 h at 675 C using a heating rate of 100 C/h up to 650 C and 50 C/h for the final 25 C. Cooling to 650 C took 35 h followed by cooling at 15 C/h to ambient temperature. Figure 1 shows schematically the procedure adopted to extract fracture toughness test samples from these weldments. It was necessary to ensure that the HAZ was aligned so that a precrack could be introduced into this region of the microstructure for testing. These samples containing the HAZ, parent plate and weld metal were typically 11mm thick and were reconstituted into three point bend Charpy sized specimens. Test specimen blanks were obtained by electron beam welding ferritic steel extension pieces of comparable yield strength, to the width and length of the extracted samples. The electron beam weld joints were located at least 5mm from the fusion boundary to prevent microstructural changes in the HAZ during welding. In general, three point bend specimens with a cross section of 20mm x 10mm were used. A notch was introduced into the prescribed region of the specimen by spark erosion and then extended by approximately 0.75mm by fatigue to achieve an initial crack to width ratio of ~ 0.5. Precracked specimens were side grooved by 10% on each side using a Charpy V-notch profile cutter. Fracture toughness tests were carried out at 150 C using 100kN Mayes servo-hydraulic and screw driven machines. Single specimen fracture toughness tests were performed using the unloading compliance technique for continual crack monitoring and the procedure is described elsewhere [7, 8]. The fractured specimens were examined optically. For this one half of each fractured specimen was sectioned in three positions with cuts parallel to the side-faces to identify the microstructure at the original crack tip and in the region of the subsequent crack growth. The specimens were sectioned in the centre and close to the side-grooves.

3 Key Engineering Materials Vols Experimental Results The weldment was manufactured by a multipass manual metal arc process and a schematic macrosection is shown in Figure 2. The individual weld beads consist of coarse columnar (shaded) and grain refined microstructures. The coarse columnar grains in the weld metal were typically 100 m wide and 400 m long with a hardness of approximately 236HV 10. The microstructure of the parent plate is inhomogeneous consisting of bands of polygonal ferrite grains and tempered bainite with a hardness of 213HV 10. The microstructure for the HAZ, generally 3mm wide, varied with distance from the weld fusion boundary and, due to the intersection of weld cusps, periodically throughout the depth of the weld. The 1mm wide region immediately adjacent to the fusion boundary contains unaltered GCHAZ, with a typical hardness of 323HV 10, supercritically grain refined HAZ, SCGRHAZ, with a typical hardness of 248HV 10 and intercritically reheated grain coarsened HAZ, ICGHAZ, with hardness typically in the range from 244HV 10 to 287HV 10. The microstructure of GCHAZ is predominantly tempered martensite and bainite with a prior austenite grain size of ~ 100 m. SCGRHAZ comprises bainite with a prior austenite grain size of ~ 30 m. The microstructure of ICGHAZ is coarse grained martensite and bainite decorated with fine grained ferrite and carbide precipitates. The grain size is very variable but typically between 10 m and 100 m. The prior austenite grain size was found to decrease with the increasing distance from the fusion boundary to about 30 m. To distinguish it from unaltered GCHAZ microstructure with 100 m grain size, this microstructure is referred to as the medium grain coarsened HAZ. At the edge of the HAZ, furthest from the fusion boundary, is intercritical HAZ with substantial grain refinement giving a grain size of 10 m and with a hardness of 256HV 10. Finally subcritically reheated HAZ is adjacent to the unaltered parent plate. The fracture toughness tests were terminated just after a decrease in the force maximum. The test specimens were then fractured at a temperature of -196 C. Metallographic sections were then prepared as described earlier. Optical examination of the crack paths showed that frequently there was a tendency for the cracks to grow from high to low yield strength material. The view of the fracture surface in Fig 3(a) shows a uniform fatigue crack and ductile crack extension which is uniform across most of the specimen thickness except at one side where it is greater over a small region of the specimen thickness. Examination of the sections perpendicular to the fracture surface established that the fatigue pre-crack was entirely in the HAZ and that subsequent ductile crack growth occurred. This is illustrated in Fig 3 (b) which depicts a fatigue pre-crack initially located in the GCHAZ and then propagating in the HAZ and changing direction into the weld metal. The results of the fracture toughness tests performed at 150 C on a material in as received condition are presented in Table 1. Table 1. J(MPam 1/2 ) and a(mm) results obtain at 150 C for post weld heat-treated weldment with the precrack positioned in the HAZ or part in HAZ and part in weld metal. (HAZ) (2/3 HAZ) (1/3 HAZ) (Weld Metal) J0.2 J a J0.2 J a J0.2 J a J0.2 J a * > * > * * * * denotes specimens in which the initial fatigue crack and subsequent ductile crack growth occurred only in the HAZ

4 44 Advances in Fracture and Damage Mechanics III The data are subdivided into four groups depending on proportions of HAZ and weld microstructure at the fatigue crack tip. The table gives the final values of J and a. Only in one specimen was the fracture precrack entirely located in the weld metal. Most the values of J and a for 2/3 HAZ and 1/3 weld and for 100% weld metal are within the scatter observed for specimens in which the fatigue precrack was entirely in the HAZ. The lowest values of fracture toughness were observed when one third of precrack was located in the HAZ and two thirds in the weld despite the fact that the values of a at the end of the test are relatively high. 4 Modelling Crack Path A finite element model to describe the development of plasticity and ductile crack growth has been presented elsewhere [9]. This model considers a welded joint as a composite of these materials comprising the parent steel plate, the weld metal and the HAZ. This simplification allows each component of the overall weldment to be addressed where the stress strain behaviour is described by a power law relationship. In the model, it is assumed that all these materials have the same elastic modulus but different yield strengths and strain hardening components. The local approach implemented through the Gurson-Tvergaard model, developed for porous materials, was used to model crack initiation and growth [10, 11]. When the void volume fraction reaches locally an assumed critical value, the stiffness of the element at that point is gradually reduced to zero and the element removed from the analysis resulting in crack growth. It was assumed that all voids were present at the start of the analysis (no void nucleation) and the finite element mesh size was chosen to be representative of the crack tip opening displacement for the HAZ material. Prior to modelling crack initiation and growth for the welded joint, each of the constituent materials was modelled separately as a homogenous material using typical values of 200GPa for the Young modulus of elasticity and yield strengths, y, for the HAZ, plate and weld metal of 800MPa, 533MPa and 267MPa respectively. The strain hardening component for the plate and weld metal was assumed to be 10 and that for HAZ to be 5. Contours of plastic strain in the joint determined from the finite element analysis are illustrated in Fig 4 (a) and (b) for P/P L ratios of 0.9 and 1.2 respectively. The limit load for the joint was determined numerically to be P L = 0.26B(W-a o ) y3, where a o is the initial crack length and W the plate width. Note that the deformation in the HAZ is not symmetrical about the centre line despite the crack lying at this location. At a value of P/P L = 1.2 there is a large region of intense plastic deformation in the weld. Despite the fact that the yield stress of the HAZ is three times higher than that of the weld metal, due to the stress concentrations at the crack tip, the peak strains are still located in the HAZ, indicating that it is likely that ductile fracture will initiate within this region of the weldment. The predicted crack path for the welded joint at the end of the analysis which allowed for 0.8mm of ductile crack growth is shown in Fig 5. Here, the crack grows out of the original plane towards the weld metal side of the joint. The analysis indicates that with a crack angle of 30 and the HAZ width of 6mm, crack will reach the weld metal after 12mm of ductile crack growth. 5 Discussion Fracture toughness of a material is conventionally evaluated by considering a critical value of a crack tip field characterising parameter at initiation of crack growth. To describe upper shelf ductile crack growth a critical strain based microscopic criterion was first proposed by McClintock and Irwin [12]. To date there has been little experimental measurement of the fracture toughness and crack growth path within the HAZ of a weldment because of difficulties associated with specimen manufacture and testing. In this particular case, some of the difficulties were overcome by using blocks extracted from the CrMoV steel weldment that were reconstituted into three point bend specimens. These specimens could then be used to measure the upper shelf fracture toughness of HAZ material. Due to the overall narrow width of the real weldment HAZ, ~3mm, three point bend 10x20mm or Charpy size geometry specimens are, in general, the largest that can be produced with a precrack for fracture toughness

5 Key Engineering Materials Vols testing. These specimens have sampled small volumes of material, giving rise to microstructure dependent variation in fracture toughness. In particular, by placing the notch in the CGHAZ it was also necessary to develop the plastic zone in the weld metal with a coarse columnar microstructure. In heterogeneous materials such as weldments, the plasticity development and crack growth behaviour depends on the ratio of the height of the HAZ to the remaining ligament. When the height of the HAZ is small compared with the remaining ligament, then it is not possible to contain both the plasticity and the crack growth within the HAZ. For the present ferritic steel weldment, it was found that the plastic deformation occurred preferentially in the lower yield strength weld metal. This is in agreement with the results of numerical analysis. As a result, ductile void growth occurred initially in the HAZ but then progressed into the weld metal causing the crack to propagate out of the HAZ into weld metal. Inspection of the fracture toughness data in Table 1 shows that in general the highest values were obtained for specimens in which the fatigue precrack was entirely in the HAZ. In general the results show a trend for the fracture toughness to decrease as the proportion of the fatigue precrack in the weld metal increases. However, a single result obtained on a specimen in which the crack was initially located and then propagated in the weld metal is comparable to the results obtained on specimens for which the entire fatigue precrack was in the HAZ. It is in fact not possible to establish the crack growth resistance of the weldment from these small specimens. The main reason for this is that it is difficult to establish the trend when J controlled crack growth occurs over a small range of crack extensions and there is a large random scatter in the data introduced by microstructure variability both from specimen to specimen and during crack growth when sampling small volumes of material. However, an insight into the fracture behaviour can be obtained from modelling. It is possible to compare these experimental results for the initiation and growth of the cracks within these three point bend specimens containing the HAZ and the predictions of the finite element model described in Section 4. This model incorporates Gurson-Tvergaard void growth with element removal to describe the softening behaviour in the vicinity of the crack tip and this predicts the crack growth resistance behaviour of the weld joint. The trends in fracture behaviour and the crack path observed experimentally are consistent with the predicted path from the numerical analysis see Figures 3b and 5. Indeed, it has been found that the results are relatively insensitive to the properties of the plate provided that the plate has higher yield strength and fracture toughness than the weld metal [2]. Although the importance of the mechanical properties and, in particular, the fracture toughness of the material contained in the HAZ of weldments is recognised for assessing the integrity of structures and components the present work demonstrates why these properties are difficult to measure. Certainly within the literature fracture toughness data for extracted or even simulated HAZ material is sparse [5,6,13,14]. Moreover, it is important that the measurements are undertaken so that the HAZ is contained within the original composite macrostructure of the weldment. If the measurements are made on bulk material subject to a simulated heat treatment then unrealistic values of upper shelf fracture toughness may be obtained. It is clearly extremely difficult to ensure that the precrack is introduced into, and continues to grow within the HAZ material to make evaluation of the initiation toughness appropriate. In general, after a limited amount of crack growth the crack diverts into either the weld metal or parent plate depending upon the relative yield strength of these materials to that of the HAZ. In these particular tests, the cracks are diverted from the higher strength grain coarsened HAZ into the lower strength regions of either the weld metal or the fine grained HAZ. 6 Conclusions (i) Fracture toughness testing of the HAZ in a CrMoV steel weldment shows that the cracks tended to deviate from high strength GCHAZ, into lower strength regions of weld metal or fine grained HAZ.

6 46 Advances in Fracture and Damage Mechanics III (ii) (vi) The regions of weld metal into which cracks deviated were coarse columnar grained. There is good correlation between the crack path predicted by a finite element model and the experimental measurement for a crack positioned in the HAZ of a weldment. The present results indicate the importance of undertaking fracture toughness tests on specimens that contain HAZ material within the overall composite macrostructure of the weldment to provide realistic values of fracture toughness of use in fracture mechanics assessments of structures and components Acknowledgement: This paper is published with the permission of the Head of Reactor Services Organisation, BNFL Magnox Generation. 7 References [1] C L Davis, and J E King, Met. and Mat. Trans., Vol. 25A (1994), p563. [2] K H Schwalbe, Y J Kim, A Cornec and M Kocak, EFAM ETM-MM 96: The ETM Method for Assessing the Significance of Crack-Like Defects in Joints with Mechanical Heterogeneity (Strength and Mismatch), Eds K H Schwalbe, and M Kocak, GKSS Research Centre Pub. GKSS/97/E/9, Geesthact FRG, [3] C Thaulow and M Toyoda, EFAM ETH-MM 99, Mismatching of Interfaces in Welds, Eds K H Schwalbe and M Kocak, GKSS Research Centre Pub., Geesthact FRG, 1999, p75. [4] P E J Flewitt, Mechanical Behaviour of Materials, Ed. Bakker A, Delft Univ. Press (Delft) 1995, p143. [5] I Milne and D A Curry, Proceedings of International Conference on Pressure Vessel Technology, Inst. of Mech. Eng., London, [6] C Fossati and S Ragazzonit, Welding International, No.12 (1988), p1091. [7] ESIS Procedure for Determining the Fracture Toughness of Materials. European Structural Integrity Society, Document ESIS, 2, [8] B K Neale, D A Curry, G Green, J R Haigh, and K N Akhurst, Int.J. of Pressure Vessels and Piping. Vol.20 (1985), p155. [9] R Moskovic, N O Dowd and P E J Flewitt, Smirt 18, Prague, 2003 (To be published). [10] A.L.Gurson, J.Eng. Materials and Technology, Vol.99 (1997), p2. [11] A. Needleman, and V Tvergaard, J.Mech. Phys. Solids, Vol.32 (1984), p461. [12] F A McClintock and G R Irwin, Fracture Toughness Testing and its Applications, ASTM STP 381, ASTM, (Phila). 1965, p84. [13] R Moskovic, L F Exworthy, and B J C Ellis, Fracture Toughness of HAZ in Ducol w30 and BW87A Plate Steels, TE/SXA/REP/0087/97, [14] G Wardle, R P Birkett and B K Neale, Mismatching of Interface and Welds, Ed. K H Schwalbe and M Kocak, GKSS Research Centre Pub., Geesthacht, FRG, 1999, p 371.

7 Key Engineering Materials Vols Specimen configuration after re-construction Slice from which weld section is extracted Heat Affected Zone Weld Metal Heat Affected Zone Fusion boundary MMA weld metal Plate Plate Parent plate Weld section extracted for re-construction Figure 1 Schematic illustration of HAZ material extraction from the weldment. Figure 2 Schematic illustration of the multipass weld showing the HAZ. (a) (b) Figure 3(a) Fracture surface from a test specimen and (b) corresponding metallographic section

8 48 Advances in Fracture and Damage Mechanics III plastic strain (%) plate HAZ weld plastic strain (%) plate HAZ weld (a) (b) Figure 4 Plastic strain distribution predicted from the FE modelling in welded joint near tip at different fractions of the plastic collapse load. (a) P/P L = 0.9 (b) P/P L = 1.2 [where P is the applied load and P L is the limit load = 2.6 B (W a o ) y3] plastic strain (%) mm Figure 5 Crack tip profiles and contours of plastic strain after a ~ 0.8mm crack growth