Investigation of postweld heat treatment of quenched and tempered pressure vessel steels

Transcription

1 University of Wollongong Thesis Collections University of Wollongong Thesis Collection University of Wollongong Year 2003 Investigation of postweld heat treatment of quenched and tempered pressure vessel steels Zoran Sterjovski University of Wollongong Sterjovski, Zoran, Investigation of postweld heat treatment of quenched and tempered pressure vessel steels, PhD thesis, Materials Engineering, University of Wollongong, This paper is posted at Research Online.

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3 67 PART B EXPERIMENTAL INVESTIGATION

4 68 CHAPTER 3 EXPERIMENTAL PROCEDURE

5 MATERIALS SELECTION AND WELDING Materials selection BIS80PV, a quenched and tempered (QT) steel, was selected for the research project because of its widespread use in the transportable pressure vessel industry in Australia. Bisalloy Steel Pty Ltd provided BIS80PV plate in two thicknesses, 11 mm and 20 mm. BIS80PV is classified as pressure vessel grade plate in accordance with AS Bisalloy Steels Pty Ltd also provided 12 mm QT steel plate. The 12 mm plate is QT structural plate considered a possible candidate for pressure vessel plate because of its high impact toughness. The weld consumable selected was LACM2 manufactured by The Lincoln Electric Company (Australia). It is a fluxed core wire that is 2.4 mm in diameter. The chemical compositions of all the plates and weld metals are shown in Tables 3.1. The flux used in conjunction with LACM2 was 880M, which is a neutral flux that protects the arc from the atmosphere.

6 70 Table 3.1: Chemical compositions (wt%) of the materials used in this project (balance is Fe). PM 11 mm BIS80PV WM 11 mm LACM2 PM 12 mm BIS80 WM 12 mm LACM2 PM 20 mm BIS80PV WM 20 mm LACM2 Weld Wire LACM2 C P Mn Si S Ni Cr Mo Cu Al Sn <0.002 < < < Nb < < < Ti * V <0.003 <0.003 < <0.003 * B Ca N O * Ti+V+Zr=0.0140Wt% Welding process and weld procedures Submerged arc welding (SAW) was used to weld the test plates for this project because this process is used to manufacture the majority of transportable pressure vessels in Australian industry. SAW is an automated process that can produce quality welds that are relatively free of defects. In SAW the molten metal and the arc are both shielded from the atmosphere by the flux. This process hence has the advantage of preventing the rapid escape of heat and inturn it is classed as a relatively low hydrogen welding process. A national survey of manufacturers and repairers of transportable pressure vessels was conducted and it was evident that a low heat input, multiple weld run procedure and

7 71 single vee butt weld (welded from both sides) preparation is typically used. Figure 3.1 shows a schematic representation of the weld joint preparation and Table 3.2 shows the key process settings. The 11 mm and 12 mm plate required 7 and 10 runs, respectively and there was no pre-heat treatment. The weld for 20 mm plate consisted 15 runs and required a preheat treatment at a minimum temperature of 50 C. A lower heat input was used for the root run, 0.9 kj/mm, to provide a weld bead shape that would be favourable to further weld runs. Manual Metal Arc Welding (MMAW) was used to tack the plates into position. The detailed weld procedures are shown in Appendix A. Back Gouge 60 o Weld Face 0 mm mm Weld Root Back gouge to sound metal before welding from this side Tacking Figure 3.1: Schematic representation of the joint preparation Table 3.2: Key weld process variables Wire Travel Flux Feed Run Polarity Current Feed Voltage Speed Heat Input Stickout Height Angle (Amps) (mm/min) (V) (mm/min) (kj/mm) (mm) (mm) ( ) Root runs DC Fill runs DC Figure 3.2 shows a photograph of the weld process and the resulting weld. Weld integrity was checked via weld macros and 10% ultrasonic weld scan of the total weld length. There was no evidence of defects in the weld macros and no recordable discontinuities in the ultrasonic testing (complied to AS1210-Class ).

8 72 (a) ~12 mm (b) Figures 3.2: Photographs of (a) SAW process and (b) the resulting weld.

9 SIMULATED POSTWELD HEAT TREATMENT Box furnace heat treatment Postweld heat treatment (PWHT) for impact, bend, fatigue, and fracture toughness testing samples was carried out in a box furnace. Heat treatment conformed to AS Pressure Equipment Manufacture, and was carried out in an argon atmosphere. The temperature of treatment was 570±5 C (validated by two thermocouples attached to the plate) and holding time used was 30 minutes for both the 11 mm and 12 mm plates, and 50 minutes for the 20 mm plate. The ramp up rate was 200 C/hour and two cooling rates were used. Samples were cooled in still air (fast cooling) for all categories of mechanical testing and others were cooled at 250 C/hour down to 400 C and then cooled in still air (slow cooling) for impact toughness testing of the QT parent plate. An integral part of this research was to examine the effect of multiple or repeated PWHT cycles on various mechanical properties. A PWHT cycle involves ramping up to 570 C and holding for 30 or 50 minutes (depending on plate thickness) and then cooling to room temperature. A maximum of 4 PWHT cycles were carried out because this is the number of PWHT cycles a transportable pressure vessel would be expected to undergo during its service life. BIS80 plate that was plastically strained to determine the effects of strain on impact toughness was postweld heat treated at 545±5 C. The ramp rates and conditions of heat treatment were the same as described for PWHT at 570 C. This temperature was selected to minimise the effects of oxidation on the surface of the sample. BIS80PV PM was austenitised at 950 and annealed to reveal microstructural banding that was not clearly evident in a Nital etch of the tempered martensite microstructure.

10 Dilatometer heat treatment Simulated PWHT on BIS80PV PM was carried out with the dilatometer set up in quench mode as shown in Figure 3.3. In the quench dilatometer the specimen (see Figure 3.4) is held between two quartz tubes and heated by a water-cooled induction coil in a vacuum. A thermocouple spot-welded on the surface of the sample controls the temperature. Stress relieving temperatures in the dilatometer were varied from 540 to 620 C and holding times varied from 0 to 16 hours. Hardness values for each sample were plotted versus the Holloman parameter (HP). The HP (Equation 3.1) is a combined temperature time parameter commonly used to study creep, tempering and stress relieving (Lochhead and Speirs, 1972). HP = T (20 + log t (Equation 3.1) 3 10 ) 10 where T is temperature (K), and t is time (hours) Heat treatment in the dilatometer complied with AS , as a heating rate of 200 C per hour before the temperature of the sample reached 400 C was applied. Also, the cooling rate of 250 C/hour down to 400 C (slow cooling) was used.

11 75 thermocouple induction coil quenching gas outlet LVDT quenching gas quartz tube holder specimen Figure 3.3. Schematic diagram of the set-up for quench dilatometry. 10 mm 1.5 mm 5 mm Figure 3.4. Schematic diagram of the dilatometer sample

12 MECHANICAL TESTING Various types of mechanical tests were selected to determine the effect of PWHT on properties that would be relevant to the transportable pressure vessel industry of Australia. Impact toughness testing was selected because of its similarity to a high velocity road tanker accident. Fracture toughness tests were selected to determine toughness in the presence of a crack (whether it be an actual crack, weld defect or inherent design flaw) and to obtain a correlation with impact toughness. Hardness testing was selected to compare the different zones of the weldments, and tensile and bend testing were carried out to ascertain ductility of the weldment before and after simulated PWHT. Mechanical testing was conducted in the PM, WM and HAZ regions of the weldment. In the PM region, particular attention was given to rolling direction because properties measured in the direction of rolling are inferior to properties measured transverse to the direction of rolling. Fatigue and CTOD fracture toughness testing was only conducted in the PM on account of real-life failures occurring in this region and a limited quantity of WM and HAZ material. The presence of error bars in graphs/figures in the experimental results represents the standard deviation for the data point Hardness testing Samples were prepared for hardness testing in the same manner as for microscopy (see Section 3.4.2). Leco micro-hardness tests were performed at a load of 500 g or 1 kg. A load of 500 g was used for traverses across all the zones of the weldment. The hardness traverse was directed at obtaining hardness values across five weldment zones, namely: parent metal (PM), weld metal (WM), coarse grained HAZ (CGHAZ), defined as the region adjacent to the WM intercritical HAZ (ICHAZ), defined as the HAZ region adjacent to the PM, and fine grained HAZ (FGHAZ), defined as the region between the CGHAZ and ICHAZ.

13 77 A load of 1 kg was used for hardness testing before and after 1 PWHT cycle in all the zones of the weldment (except ICHAZ). A minimum of 5 hardness values was taken in each zone tested. Micro-hardness testing (1 kg) was also conducted on dilatometer samples exposed to various combinations of temperature and time (listed in Table 3.3). Table 3.3: Temperature and time combinations for which hardness tests were carried on BIS80PV PM plate heat treated in the dilatometer. Temperature ( C) Time (minutes)

14 Bend testing transverse guided bend test of weldment Bend testing was performed in accordance with AS on all welded plates before and after 1 PWHT cycle. Root bends of 180 were achieved by bending weldments slowly and uniformly with a former 6.7 times the thickness of the plate (see Figure 3.5). In a root bend test the root of the weld is in tension and the face of the weld is in compression. For the weld procedure used in this project the root side was the side that was back gouged and filled (see Figure 3.1). The purpose of the bend tests was to ensure the ductility of the weld with and without PWHT. Bend test samples were ground flush and linished to remove weld reinforcement. The samples were then machined to the required width of 30 mm as specified for samples less than or equal to 20 mm in thickness. Table 3.4 shows the test plan for the bend tests. t 6.7t 6.7t+2.2t Figure 3.5: A schematic representation of the set up for a bend test (t=plate thickness (mm)). Table 3.4: Bend test plan (cross-weld samples only). No PWHT 1 PWHT Cycle 11 mm BIS80PV 12 mm BIS80 20 mm BIS80PV =tested

15 Impact testing Charpy V-notch Impact testing of the base plate was carried out in accordance with AS A striking energy of 325J was used and the test temperature selected was 20 C as required by the Australian Pressure Vessel and QT Steel Standards (Australian Standard 3597, 1997). Impact energy was averaged from 5 test pieces in each group of tests. Samples were machined to AS (see Figure 3.6 (a)) from the middle of the plate to assess impact energy where banding and segregation effects would be most severe. Impact test results are only presented for the as-received plate for the orientations shown in Figure 3.6 (b). In the L-T and L-S orientations the length of the sample is longitudinal to the rolling direction and notch plane is perpendicular to the rolling direction, and in the T-L and T-S orientation the length of the sample is transverse to the rolling direction and the notch contains the rolling direction. Samples designated with an S indicate the notch root is perpendicular to the short transverse or through-thickness direction. Charpy V-notch impact tests were also carried out to determine the effect of multiple or repeated PWHT cycles on impact energy (fast and slow cooling) and the ductile-brittle transition temperature (DBTT) in T-L and L-T orientations (slow cooling only). The DBTT is defined as the temperature at which the impact energy is equal to 27.1J. To achieve the test temperature a Julabo Bath machine (see Figure 3.7) was used. This is a temperature bath capable of reaching 50 C, and any testing required below this temperature was achieved using a mixture of ethanol and powdered dry ice. The test temperature was accurate to ±1 C. The holding time in the temperature bath was 10 minutes and the impact test was then completed within 6 seconds (as required by the Standard for impact testing (Australian Standard , 1989)).

16 80 R0.25 mm 45 2 mm 10±0.06 mm 55±0.6 mm 10±0.06 mm (a) L-T L-S RD T- L T-S (b) Figure 3.6. (a) Schematic diagram of the Charpy V-notch specimen and (b) the specimens in relation to the rolling direction of the base plate.

17 81 Julabo Machine Figure 3.7: Julabo Machine and the Impact Tester set on 325J scale. Additionally, Charpy impact testing was carried out before and after PWHT on plastically strained (3.5% total strain/3.2% plastic strain) 12 mm BIS80 samples in the T-L orientation. The samples were strained transverse to the direction of rolling (uniaxially) and then machined into standard size Charpy V-notch specimens. The plastic strain was completed on a computer controlled Instron machine using a 10 mm gauge length extensometer near the future location of the notch. Charpy V-notch impact testing was also conducted on the WM and HAZ regions of the weldment. Figure 3.8(a) shows the way in which samples were cut from the WM and Figure 3.8(b) shows a schematic representation of a HAZ sample. HAZ samples are comparable to PM samples in the T-S orientation. The fracture surface of the majority of HAZ Charpy V-Notch samples contained both HAZ and WM due to the curvature in the weld and HAZ profile (see Figure 3.8(b)). The notch in the HAZ was positioned to initiate fracture in the CGHAZ. This zone is

18 82 the most brittle of all the sub-zones in the HAZ, and it generally has the highest hardness value. Weld (a) WM PM HAZ RD (b) Figure 3.8: Schematic diagram of impact specimens in relation to (a) weld and (b) HAZ. Lateral Expansion (mm) and percent crystallinity (percentage of the fracture surface that is shiny at low magnification) were measured on all PM and WM samples that were tested at 20 C. Lateral expansion was measured using a dial indicator (see Figure 3.9) and percent crystallinity was measured using image analysis. In the HAZ samples, lateral expansion and percent crystallinity were not measured because the fracture surface contained regions of both WM and HAZ. The impact testing experimental testing program conducted at -20 C is shown in Tables B1 to B5 in Appendix B. For all tests conducted at 20 C an average of 5 samples was taken from each group.

19 83 W First fractured half of Charpy Sample Notch Second fractured half of Charpy Sample A Lateral Expansion = A-W (a) (b) Figure 3.9: (a) Photograph of dial indicator set up to measure lateral expansion (mm) and (b) Schematic diagram defining the lateral expansion.

20 Tensile testing parent plate and cross-weld specimens Tensile properties were measured in accordance with Australian Standard AS Firstly, parent metal samples were tested at room temperature with samples machined transverse to the direction of rolling. This was done to ascertain the relationship between tensile properties and the number of PWHT cycles. For 11 mm PM BIS80PV samples tensile testing was carried out before and after 1PWHT cycle. For 12 mm PM BIS80 and 20 mm PM BIS80PV tensile testing tensile testing was carried out before PWHT and up to and including 4 PWHT cycles. The strain rate used for these tests was 3.5x10-3 s -1 and Figure 3.10 shows the schematic representation of all the samples used. Reduction in area (%) and elongation (%) were also measured in these samples. 40 mm 10 mm (min) 220 mm Figure 3.10: Schematic representation of PM tensile tests for 11, 12 and 20 mm plates (samples have rectangular cross section). Tensile properties were also measured on cross-weld specimens in accordance with AS , titled, Methods for destructive testing of welds in metal Method 2.1:Transverse butt tensile test. Similarly to the PM tensile test, a strain rate of 3.5x10-3 s -1 was used and below is a schematic diagram of the samples used. Testing was carried out before and after 1 PWHT cycle in the cross-weld specimens, and reduction in area (%) and elongation (%) was also measured.

21 85 30 mm 50 mm W Weld with reinforcement removed P For 11 mm samples W=20 mm & P=32 mm For 12 mm and 20 mm samples W=25 mm & P=37 mm Figure 3.11: Schematic representation of cross-weld tensile test specimens for 11, 12 and 20 mm plates (samples have rectangular cross section). Finally, yield stress (MPa) and ultimate tensile strength (MPa) at a test temperature of 20 C were measured on rod shaped test pieces of 11, 12 and 20 mm PM after exposure to 0, 2 and 4 PWHT cycles. These properties were required to determine the CTOD values for 11, 12 and 20 mm parent plate (CTOD procedure is discussed in Section 3.3.6). The strain rate used was 3.5x10-3 s -1 and the samples were circular in crosssection with a 5 mm diameter and 25 mm gauge length. Testing was carried out in accordance with AS Methods for tensile testing of metals. Figure 3.12 shows photographs of the set up for this series of tensile tests.

22 86 Extensometer Sample (a) (b) Figure 3.12: Test set up for tensile testing at 20 C in gaseous nitrogen. Photograph (a) is a close up of sample and photograph (b) shows the temperature chamber.

23 Fatigue testing crack growth rates Fatigue crack propagation data was collected on standard CTOD PM samples exposed to 0, 2 and 4 PWHT cycles. These samples required fatigue pre-cracking at room temperature for subsequent CTOD testing. Figure 3.13 shows the test set-up for fatigue cracking and the test configuration is shown and discussed in more detail in Section Fracture Toughness Testing CTOD. Fatigue pre-cracking was carried out at a frequency of 30 or 60 Hz and an R ratio (minimum load: maximum load) of 0.1. Table 3.5 quantifies the loads defined in Figure Figure 3.13: Photograph of fatigue testing set up.

24 88 Table 3.5: Fatigue cycle parameters for all PM samples. Maximum Minimum Sample Load, P max Load, P min (kn) (kn) Mean Load, Amplitude, P m P a (kn) (kn) 11 mm PM mm PM mm PM Load (kn) Pr Pa Pmax Pm Pmin 0 0 No. of Cycles 1 Figure 3.14: The fatigue stress cycle (sinusoidal) used for pre-cracking CTOD samples and collecting fatigue crack growth data (P r = load range). The ultimate aim of collecting fatigue crack growth data was to calculate fatigue crack growth rates and the resistance to fatigue crack growth. Crack propagation rate, da/dn is found to follow an equation of the form: da dn = C m a n (Equation 3.2) σ a where C, m and n are constants, σ a is the alternating stress, and a is the crack length.

25 Testing of 11 and 12 mm samples For the 11 and 12 mm PM samples data for da/dn and stress intensity factor range, K, were generated. The first step in obtaining this plot is to set up data acquisition and loading/unloading cycles (30 Hz). After exposure to 1000 cycles the fatigue crack length was estimated using a compliance method (ASTM E1820, 2001). In this method sequential loads and unloads are carried out after every 1000 cycles. After every 1000 cycles, the sample was unloaded to the minimum allowable fatigue load and then loaded to the maximum allowable fatigue load. This was repeated three times and the compliance, C i, was taken as an average of six values (first unload to load, first load to unload, second unload to load, second load to unload, final unload to load and final load to unload) to determine the final crack length with accuracy. The compliance, C i, is the slope of the curve generated by the load (N) versus COD (clip gauge displacement) plot. The final fatigue crack length, a i, is calculated when C i is combined with other parameters as is shown in the following Equations ( K K U + K U K U + K U K U )W 5 a i = (Equation 3.3) where U = BW E 2 C (1 ν ) i S , E= Young s Modulus (Pa), ν= Poisson s Ratio (0.33), B=thickness of sample, W=width of sample, S=spacing between two outer rollers, K 1 = , K 2 = , K 3 =2.9821, K 4 = , K 5 = , K 6 = , and a i =final fatigue crack length (mm). The next step was to determine fatigue crack growth rate, da/dn (mm/cycle). This was simply calculated by the following equation: a = dn a da 1000( x+ 1) 1000( x) (Equation 3.4) 1000

26 90 At this point the crack growth rate, da/dn, is calculated every 1000 cycles until a desired fatigue crack length is achieved for CTOD testing. The next step was to determine the corresponding stress intensity factor range, K, in order to plot the graph of da/dn versus K. K = K max K min (Equation 3.5) ( a ) Pmax OR mins Kmax OR min = f (Equation 3.6) 3 2 W BW where P max =maximum fatigue load (N) and P min =minimum fatigue load (N) f ( a ) W ( 1 a ) a 2.7( a ) W + W W 3 ( + 2a )( 1 a ) a 1.99 a W W = (Equation 3.7) 2 1 W W Once a plot of da/dn versus K is obtained then an equation is assigned to the linear portion of the curve using Microsoft Office 2000 (add trendline function). The power law equation is of the form shown in Equation 3.8 and n represents the slope of the curve and this is indicative of the resistance to fatigue crack growth (Paris Law) (Dieter, 1988). da dn n = or C K n y = Cx (Equation 3.8) where C=constant, n=slope of the linear portion of the curve Testing of 20 mm samples For 20 mm samples exposed to 0 and 4 PWHT cycles, fatigue crack growth rate was measured by dividing the crack length (mm) by the number of fatigue cycles. Although this technique of measuring fatigue crack growth rate absorbs Region 1 (initiation) and Region II ( macroscopic crack growth) in the da/dn versus K plot, valuable insight into fatigue crack growth behaviour is still gained. The samples were fatigued at a

27 91 frequency of 60 Hz and the crack length was measured at nine equally spaced points across the specimen thickness, centred about the specimen centre and extending to 0.005W from the specimen surfaces (see Figure 3.15). The two near surface measurements were then averaged and added to the remaining 7 measurements, which were then averaged to determine the final crack length (ASTM E ). Fatigue Crack Measurement indicators Figure 3.15: Location of measurements of fatigue crack length on 20 mm PM sample.

28 Fracture toughness testing CTOD CTOD testing was carried out to obtain a correlation with impact toughness and compare the fracture toughness against plate thickness and the number of PWHT cycles. CTOD fracture toughness was determined for PM plate exposed to 0, 2 and 4 PWHT cycles. The samples were taken transverse to the direction of rolling with the notch in the direction of rolling (T-L orientation, Figure 3.6). Testing was conducted in accordance with ASTM E The procedure for CTOD testing is outlined below: preparation of samples, fatigue pre-cracking and validation, testing, and CTOD value and validation Sample preparation Figure 3.16 shows a schematic representation of the samples used for CTOD testing. The specimen configuration is termed rectangular section SE(B) specimen (ASTM E1290, 1999). Figure 3.17 shows an actual 20 mm CTOD sample with the direction of rolling indicated.

29 93 Single Wire Cut a i ±0.1 mm w±0.1 mm 2.25W min 2.25W min b mm Notch Detail For b=11 mm ai =7 mm, w=22 mm ±0.04 mm 1±0.1 mm 2±0.1 mm For b=12 mm ai =8 mm, w=24 mm For b=20 mm a i =16 mm, w=40 mm Figure 3.16: Schematic representation of CTOD samples. RD Notch Figure 3.17: Photograph of 20 mm CTOD PM sample Fatigue pre-cracking The loads, frequencies, stress cycles and method of measuring crack length in fatigue pre-cracking were as described in Section 3.3.5, Fatigue Testing Fatigue Crack Growth Rates. Appendix C details the steps in the selection of the maximum fatigue load used.

30 94 The length of the fatigue crack, a o, required is between 0.4W to 0.7W (ASTM E1290, 1999). Before testing, the crack length was estimated by the compliance method described in Section After testing the fatigue crack length was measured by the same technique as that for the 20 mm PM samples in Figure 3.15, Section CTOD the test Crack tip opening displacement testing was carried out in accordance with ASTM E The test temperature selected was 20 C and this was achieved by testing in an Instron temperature chamber using liquid nitrogen gas as shown in Figure 3.18(a). The specimen was set up as shown in Figure 3.18(b) or schematically in Figure The notch centreline was mid-way between the rollers to within 0.5% of the span, and the sample was positioned square to the roller axis within 2 (ASTM E1290, 1999). Testing was carried out in position (or cross-overhead) control and the velocity of the crosshead for all samples tested was 1 mm/minute. A variety of data was collected from this test, namely: Time (seconds) Clip gauge opening displacement (COD) (microns) Load (kn), and Position (mm). Side grooving was used on the 11 mm and 20 mm samples to ensure qualification of the fatigue pre-crack in accordance with ASTM E Appendix D shows the side grooving procedure in detail.

31 95 (a) (b) Figure 3.18: Photograph of (a) machine and temperature chamber and (b) sample set up on rollers.

32 96 For 11 mm samples r =10 mm, d = 20 mm, S=88 mm, W=22 mm For 12 mm samples r =10 mm, d = 20 mm, S=96 mm, W=24 mm For 20 mm samples r =4 mm, d = 20 mm, S=160 mm, W=40 mm clip gauge d W ao 0.6d r Figure 3.19: Schematic representation of CTOD test set up. S Post test procedure CTOD value qualification Samples were either heat tinted or fatigue cracked to mark the amount of slow stable crack extension after test completion. For fatigue marked samples the force applied was less than 70% of that used for the maximum cyclic force in fatigue pre-cracking (Standard requirement) (ASTM E1290, 1999). Samples for heat tinting were placed on a hot plate until a blue coloured oxide layer formed, then plunged into liquid nitrogen and completely fractured to promote minimal additional deformation to the sample (Standard recommendation) (ASTM E1290, 1999). Figures D.4 to D.6 in Appendix D show typical low magnification images of various heat tinted and fatigue marked samples. The next step was to measure the fatigue pre-crack length and the amount of slow stable crack extension. The length of the fatigue crack (discussed in Section Fatigue Testing Fatigue crack growth rates) plays a pivotal role in the determination of the final CTOD value, δ, and the amount of slow stable crack extension determines the

33 97 subscript given to δ. There are three subscripts that may be assigned to CTOD, δ, namely: 1. δ c is given at the onset of unstable brittle crack extension or pop-in when the final crack length less the original crack length ( a p ) is less than 0.2 mm. 2. δ u is given at the onset of unstable brittle crack extension or pop-in when a p is greater than 0.2 mm. 3. δ m is given at the attainment of a maximum force plateau for fully plastic behaviour. The first step in assigning the subscript is to plot force (kn) versus clip gauge displacement (mm). This graph indicates whether δ m, or δ c or δ u is assigned. If the graph indicates that δ c or δ u have to be assigned then a p is measured. Appendix E shows the force (kn) versus clip gauge displacement (mm) plots that were typical of the 11 mm, 12 mm and 20 mm PM samples transverse to the direction of rolling. Three typical low magnification images for each group of samples are also given. After measuring the fatigue crack length, a o, and determining the subscript of δ, then the equation for determining δ is: 2 ( 1 ν ) rp ( W a0 ) p + σ E [ r (( W a ) + a z] K δ = (Equation 3.9) 2 2 ν YS p o o + where K is shown in Equation 3.10, ν=0.33, r p =0.44, ν p =plastic component of clip gauge opening displacement (see Appendix E), σ YS =yield or 0.2% proof stress and z=0. ( a ) PS f K = W (Equation 3.10) 3 2 ( BB ) 1 2W N where P=maximum force (N), B N =thickness of plate after side grooving, f(a/w) is shown in Equation 3.7. Averages of the CTOD values were taken and then plotted against the number of PWHT cycles (see Section 4.7.6).

34 MICROSCOPY Low magnification microscopy Low magnification microscopy was used to observe the fracture and/or test surface features of impact, tensile, bend, fatigue, and CTOD specimens. Surface features such as splitting or delamination, and crystallinity (%) or fibrosity (%) were examined using low magnification microscopy Optical microscopy Optical microscopy was used mainly for the microstructural evaluation of samples exposed to varying postweld heat treatments. Optical microscopy was used to evaluate the effect of PWHT on microstructure in the parent metal, weld metal and HAZ regions of the weldment. Optical microscopy was also used to characterise features that may have an effect on the mechanical properties of QT steels and their weldments, for example, inclusions, microstructural banding, and delamination or splitting in Charpy samples Scanning electron microscopy (SEM) Scanning electron microscopy (SEM) was used to study the fracture surface of the Charpy V-notch PM and WM samples and EDS was employed to investigate compositional segregation. Figure 3.20 shows a schematic representation of the various regions that are present in the fracture surface of a Charpy V-notch specimen. Note that the splits or delaminations are parallel to the rolling plane and only occur in PM samples. All of these regions were investigated in the SEM.

35 99 Notch Shear Region General Fracture Area Split Figure 3.20: Schematic diagram of typical regions investigated in Charpy V-notch fracture surfaces (splits only occur in PM or HAZ samples) of L-T or T-L orientations. SEM was also used to study the fracture surface of all PM CTOD samples exposed to 0, 2 and 4 PWHT cycles (fast cooled).

36 RESIDUAL STRESSES Hole drilling technique for measuring residual stresses Two 12 mm BIS80 welds were sent to HRL Materials/ETRS for residual stress measurements by the hole drilling method. There was a 12 mm weld test plate that had received no PWHT and 12 mm weld test plate that had received 1 PWHT cycle. Residual stress was measured on the weld centre-line longitudinal and transverse to the direction of welding using a rosette strain gauge. This technique has been described in some detail in Chapter 2 - Literature Review. The test was carried out in accordance ASTM Standard Test Method E Residual stress measurements were also carried out on the weld centre-line of 20 mm BIS80PV without PWHT and the PM region (100 mm away from the weld centre-line) of 12 mm BIS80 without PWHT. All residual stress measurements in the weld metal were conducted on the final weld run, indicated by the weld procedures in Appendix A Stress relaxation testing A Gleeble 3500 thermomechanical simulator was used to carry out stress relaxation testing on 12 mm and 20 mm BIS80PV cross-weld samples. Cross-weld samples (schematically shown in Figure 3.23) were selected for stress-relaxation testing because, in addition to their well-defined yield point, it is in this region of transportable pressure vessels where the relaxation of residual stresses is most critical. The Gleeble 3500 thermomechanical simulator uses direct resistance heating and generates a uniform hot zone in the middle of the test samples. This limited hot zone is an advantage in stress-relaxation testing because machine stiffness will be higher than for conventional stress relaxation testing in which the entire sample and equipment is at the testing temperature. The test set up is shown in Figure Thermocouples (Type K) were welded to the centre of the sample and copper grips hold the specimen in the jaws of the Gleeble to

37 101 ensure precise, controlled resistive heating. Two C shaped clamps were used to firmly hold the grips and the sample in position (Figure 3.24 (b)). Initially hot tensile tests (at 570 C) on 12 mm BIS80 and 20 mm BIS80PV cross-weld samples were carried to determine the position (or stroke) at which yielding occurs. In the hot tensile tests at 570 C a strain rate of 8x10-4 s -1 was used for both the 12 and 20 mm cross-weld samples and yielding occurred at a stroke (or position) of mm. Stress relaxation tests were then set using the same strain rate (8x10-4 s -1 ) to a stroke of mm. The subsequent relaxation of the stress was plotted as a function of time, providing an insight into the relaxation of a stress close to the yield point at the test temperature. Furthermore, a stress relaxation test was carried out on 20 mm cross-weld sample loaded to a stress well within the elastic region (~250 MPa) at 570 C. 115 mm 12 mm sample Hot Zone Weld Metal 10 mm φ 20 mm sample 10 mm mm Figure 3.23: Schematic representation of Gleeble cross-weld samples that show the extent of hot zone for a set temperature of the sample of 570 C.

38 102 (a) Thermocouple Copper Grips Sample C Clamp (b) Figure 3.24: Photograph of (a) Gleeble 3500 Thermomechanical Simulator and (b) close up of the sample set up.