University of Wollongong Thesis Collections

University of Wollongong Thesis Collection

University of Wollongong Year

Investigation of postweld heat treatment

of quenched and tempered pressure vessel

steels

Zoran SterjovskiUniversity of Wollongong

Sterjovski, Zoran, Investigation of postweld heat treatment of quenched and temperedpressure vessel steels, PhD thesis, Materials Engineering, University of Wollongong, 2003.http://ro.uow.edu.au/theses/464

This paper is posted at Research Online.

http://ro.uow.edu.au/theses/464

NOTE

This online version of the thesis may have different page formatting and pagination from the paper copy held in the University of Wollongong Library.

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67

PART B

EXPERIMENTAL

INVESTIGATION

68

CHAPTER 3

EXPERIMENTAL PROCEDURE

69

3.1 MATERIALS SELECTION AND WELDING

3.1.1 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 AS3597-1993.

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.

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

LACM2C 0.1700 0.1150 0.1550 0.0900 0.1800 0.0950 0.0600 P 0.0110 0.0100 0.0100 0.0080 0.0150 0.0100 0.0110

Mn 1.4300 1.5500 1.1000 1.5200 1.4200 1.4900 1.2400 Si 0.2000 0.4000 0.1900 0.4700 0.2100 0.4000 0.6300 S 0.0035 0.0060 0.0030 0.0080 0.0020 0.0070 0.0050 Ni 0.0290 1.1000 0.0200 1.6000 0.0270 1.3000 1.9600 Cr 0.2000 0.1100 0.0160 0.0290 0.2000 0.0870 0.0900 Mo 0.2100 0.4200 0.2100 0.5500 0.2000 0.4700 0.6400 Cu 0.0110 0.0160 0.0090 0.0190 0.0330 0.0160 0.0300 Al 0.0310 0.0300 0.0040 0.0240 0.0310 0.0280 - Sn <0.002 <0.002 0.0050 <0.002 0.0080 <0.002 - Nb <0.001 0.0010 <0.001 0.0010 <0.001 0.0010 - Ti 0.0250 0.0090 0.0260 0.0030 0.0240 0.0050 * V 0.0030 <0.003 <0.003 <0.003 0.0050 <0.003 * B 0.0005 0.0004 0.0013 0.0003 0.0011 0.0003 -

Ca 0.0008 0.0011 0.0008 0.0005 0.0009 0.0005 - N 0.0031 0.0054 0.0033 0.0074 0.0036 0.0053 - O 0.0020 0.0290 0.0022 0.0310 0.0018 0.0320 -

* Ti+V+Zr=0.0140Wt%

3.1.2 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

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.

6.0-6.5 mm

60

0 mm

Back gouge to soundmetal before weldingfrom this side

Back Gouge

Tacking

Weld Root

Weld Faceo

Figure 3.1: Schematic representation of the joint preparation

Table 3.2: Key weld process variables

Run

Polarity

Current

(Amps)

Wire

Feed

(mm/min)

Voltage

(V)

Travel

Speed

(mm/min)

Heat Input

(kJ/mm)

Stickout

(mm)

Flux

Height

(mm)

Feed

Angle

(°)

Root runs DC+ 340 340 26 600 0.9 25 25 90 Fill runs DC+ 340 340 28 600 1.0 25 25 90

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 1 – 1997).

72

(a)

(b)

Figures 3.2: Photographs of (a) SAW process and (b) the resulting weld.

~12 mm

73

3.2 SIMULATED POSTWELD HEAT TREATMENT

3.2.1 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 AS4458-

1997-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.

74

3.2.2 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).

3

10 10)log20( −×+= tTHP (Equation 3.1)

where T is temperature (K), and t is time (hours)

Heat treatment in the dilatometer complied with AS4458-1997, 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.

75

quenching gas

thermocouple

induction coil

specimen

quenching gas outlet

quartz tube holder

LVDT

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

76

3.3 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.

3.3.1 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.

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)

15

30

540 45

60

360

15

30

560 45

60

360

15

30

580 45

60

360

960

15

30

620 45

60

360

78

3.3.2 Bend testing – transverse guided bend test of weldment

Bend testing was performed in accordance with AS2205.3.1-1997 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.

6.7t+2.2t

6.7tt

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

79

3.3.3 Impact testing – Charpy V-notch

Impact testing of the base plate was carried out in accordance with AS1544-1989. 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 AS1544.2-1989 (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 1544.2, 1989)).

80

55±0.6 mm10±0.06 mm

10±0.06 mm

2 mm

R0.25 mm

45°

(a)

L-T L-S

T-L T-SRD

(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.

81

Figure 3.7: Julabo Machine and the Impact Te

Additionally, Charpy impact testing was ca

plastically strained (3.5% total strain/3.2% pla

T-L orientation. The samples were strained

(uniaxially) and then machined into standard

plastic strain was completed on a computer co

gauge length extensometer near the future loca

Charpy V-notch impact testing was also condu

weldment. Figure 3.8(a) shows the way in wh

Figure 3.8(b) shows a schematic representatio

comparable to PM samples in the T-S orientati

The fracture surface of the majority of HAZ C

HAZ and WM due to the curvature in the we

The notch in the HAZ was positioned to initi

Julabo Machine ster set on 325J scale.

rried out before and after PWHT on

stic strain) 12 mm BIS80 samples in the

transverse to the direction of rolling

size Charpy V-notch specimens. The

ntrolled Instron machine using a 10 mm

tion of the notch.

cted on the WM and HAZ regions of the

ich samples were cut from the WM and

n of a HAZ sample. HAZ samples are

on.

harpy V-Notch samples contained both

ld and HAZ profile (see Figure 3.8(b)).

ate fracture in the CGHAZ. This zone is

82

the most brittle of all the sub-zones in the HAZ, and it generally has the highest

hardness value.

Weld

(a)

PM

WM

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.

83

(a)

W

A

First fractured half of Charpy Sample

Notch

Second fractured half of Charpy Sample

Lateral Expansion = A-W

(b) Figure 3.9: (a) Photograph of dial indicator set up to measure lateral expansion (mm)

and (b) Schematic diagram defining the lateral expansion.

84

3.3.4 Tensile testing – parent plate and cross-weld specimens

Tensile properties were measured in accordance with Australian Standard AS1391-

1991. 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

220 mm

10 mm (min)

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

AS2205.2.1-1997, 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.

85

30 mm

P

50 mm

W

For 11 mm samples W=20 mm & P=32 mm

For 12 mm and 20 mm samples W=25 mm & P=37 mm

Weld with reinforcement removed

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 cross-

section with a 5 mm diameter and 25 mm gauge length. Testing was carried out in

accordance with AS1391-1991-Methods for tensile testing of metals. Figure 3.12 shows

photographs of the set up for this series of tensile tests.

86

(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.

Extensometer Sample

87

3.3.5 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

2.3.6 – 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 3.14.

Figure 3.13: Photograph of fatigue testing set up.

88

Table 3.5: Fatigue cycle parameters for all PM samples.

Sample

Maximum

Load, Pmax

(kN)

Minimum

Load, Pmin

(kN)

Mean Load,

Pm

(kN)

Amplitude,

Pa

(kN)

11 mm PM 8.12 0.81 4.47 3.66

12 mm PM 9.50 0.95 5.23 4.28

20 mm PM 14.64 1.46 8.05 6.59

No. of Cycles

Loa

d (k

N)

Pmax

Pa

Pmin

Pr

Pm

Figu

and

The

grow

is fo

dNda

whe

0

0

re 3.14: The fatigue stress cycle (sinusoidal) used fo

collecting fatigue crack growth data (Pr = load range).

ultimate aim of collecting fatigue crack growth data

th rates and the resistance to fatigue crack growth. C

und to follow an equation of the form:

namC aσ=

re C, m and n are constants, aσ is the alternating stress

1

r pre-cracking CTOD samples

was to calculate fatigue crack

rack propagation rate, da/dN

(Equation 3.2)

, and a is the crack length.

89

3.3.5.1 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, Ci, 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, Ci, is the slope of the curve generated by the load (N) versus COD (clip

gauge displacement) plot. The final fatigue crack length, ai, is calculated when Ci is

combined with other parameters as is shown in the following Equations 3.3.

( )WUKUKUKUKUKKai5

64

53

42

321 −+−+−= (Equation 3.3)

where

1

14

)1( 2

−

+

−= S

iCEBW

Uν ,

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, K1=0.9997481, K2=3.59504,

K3=2.9821, K4=3.21408, K5=51.51564, K6=113.031, and ai=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:

−= +

1000)(1000)1(1000 xx aa

dNda (Equation 3.4)

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.

minmax KKK −=∆ (Equation 3.5)

( )WafBW

SPK OROR ×=

23minmax

minmax (Equation 3.6)

where Pmax=maximum fatigue load (N) and Pmin=minimum fatigue load (N)

( )( ) ( )( )( ) 2

3

2

1212

7.293.315.2199.13

Wa

Wa

Wa

Wa

Wa

Wa

Wa

Waf

−+

+−−−

= (Equation 3.7)

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).

nKCdN

dA ∆= or nCxy = (Equation 3.8)

where C=constant, n=slope of the linear portion of the curve.

3.3.5.2 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

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 E1290-99).

Fatigue Crack Measurement indicators

Figure 3.15: Location of measurements of fatigue crack length on 20 mm PM sample.

92

3.3.6 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 E1290-99. The procedure for CTOD testing is outlined below:

� preparation of samples,

� fatigue pre-cracking and validation,

� testing, and

� CTOD value and validation.

3.3.6.1 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.

93

2.25W min

Single Wire Cut

2.25W min

ai ±0.1 mm

w±0.1 mm

40°1±0.1 mm

2±0.1 mm

2.5±0.04 mm

b mm

Notch Detail For b=11 mm

ai =7 mm, w=22 mm

For b=12 mm

ai =8 mm, w=24 mm

For b=20 mm

ai =16 mm, w=40 mm

Figure 3.16: Schematic representation of CTOD samples.

RDNotch

Figure 3.17: Photograph of 20 mm CTOD PM sample

3.3.6.2 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.

94

The length of the fatigue crack, ao, 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 3.3.5. 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 3.3.5.

3.3.6.3 CTOD – the test

Crack tip opening displacement testing was carried out in accordance with ASTM

E1290-99. 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 3.19.

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 E1290-99. Appendix D shows the side

grooving procedure in detail.

95

(a)

(b)

Figure 3.18: Photograph of (a) machine and temperature chamber and (b) sample set up

on rollers.

96

ao

W

r

d

S

0.6d

clip gauge

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

Figure 3.19: Schematic representation of CTOD test set up.

3.3.6.4 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 3.3.5 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

97

subscript given to δ. There are three subscripts that may be assigned to CTOD, δ,

namely:

1. δc – is given at the onset of unstable brittle crack extension or pop-in when the

final crack length less the original crack length (∆ap) is less than 0.2 mm.

2. δu – is given at the onset of unstable brittle crack extension or pop-in when ∆ap

is greater than 0.2 mm.

3. δm – 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 δm, or δc or δu is assigned. If the graph

indicates that δc or δu have to be assigned then ∆ap 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, ao, and determining the subscript of δ, then the

equation for determining δ is:

( ) ( )( )[ ]zaaWr

aWrE

K

oop

pp

YS ++−−

+−=(2

1 022 ν

σνδ (Equation 3.9)

where K is shown in Equation 3.10, ν=0.33, rp=0.44, νp=plastic component of clip

gauge opening displacement (see Appendix E), σYS=yield or 0.2% proof stress and z=0.

( )( ) 2

32

1 WBBWafPS

KN

×= (Equation 3.10)

where P=maximum force (N), BN=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).

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3.4 MICROSCOPY

3.4.1 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.

3.4.2 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.

3.4.3 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.

99

Notch

Shear Region

General FractureArea

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).

100

3.5 RESIDUAL STRESSES

3.5.1 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 E837-85.

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.

3.5.2 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 3.24. Thermocouples (Type K) were welded to the

centre of the sample and copper grips hold the specimen in the jaws of the Gleeble to

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 3.105 mm.

Stress relaxation tests were then set using the same strain rate (8x10-4 s-1) to a stroke of

3.105 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

15-20 mm

10 mm φHot Zone Weld Metal

12 mm sample

20 mm sample

10 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.

102

(a)

(b)

Figure 3.24: Photograph of (a) Gleeble 3500 Thermomechanical Simulator and (b)

close up of the sample set up.

Sample C Clamp Copper Grips

Thermocouple