the effect of thermal history during fabrication on the ... 1420 rothwell.pdf · •welding...

Copyright © TWI Ltd 2011

The Effect of Thermal History During

Fabrication on the Mechanical

Properties of Weldments in Grade 91

Creep Resistant Steel

John Rothwell

TWI ltd


Drive for efficiency and reduced emissions

Temperature

(°C)

Pressure

(bar)

Sub-critical 538 167

Supercritical 540 - 566 250

Ultra-

supercritical 580 - 620 270 - 285

World Coal Institute 2007

1

21

T

TE

2% efficiency gain = 5% CO2 reduction


Evolution of 9-12% CrMoV steels

Years Alloy Modification 600°C /105h Creep

Rupture Strength,

MPa

Example

Alloys

Maximum

Metal Use

Temp, °C

1960-70 Addition of Mo or

Nb, V to simple

12Cr and 9Cr steels

60 EM12, HCM9M,

HT9, Tempaloy

F9, HT 91

565

1970-85 Optimization of C,

Nb, V

100 HCM12, T91,

HCM2S

593

1985-95 Partial substitution

of W for Mo

140 P92, P122,

P911 (NF 616,

HCM12A)

620

Emerging Increase W and

addition of Co, B

and N

180 NF12, SAVE

12, MARBN

NPM1

650

Adapted from Viswantathan, 2005


Fabrication - welding

• In principle, all the common

arc welding processes are

applicable

• TIG root + MMA is most

common


Typical heating cycle


Benefits of 9-12Cr steel weldments

• Relatively resistant to weld cracking

mechanisms

– Hydrogen cracking

– Temper embrittlement

– Hot cracking

• Clean steel practices and low carbon

• Low transformation temperature (Mf ~ 150)


2CrMo 9CrMo 316 SS

Temperature °C

Str

ess,

MP

a

Jones and Alberry, 1977

Residual stress during cooling


Fabrication issues

• Thermal control during

Steel making

• Welding procedure non

compliance

• Control of PWHT

• Repair welding


Type IV cracking of cross-weld specimens

WSF(t, T) = Ru(w)/t/T / Ru/t/T

Time

Cre

ep

Ru

ptu

re S

tres

s

Type IV Region (Outer HAZ)

Parent or Weld Metal

Failure

Threshold

Stress

Type IV

shortfall

Parent/Weld Metal

specimen rupture

130MPa, 630°C, 120 hours

80MPa, 630°C, 2436 hours


Objectives of investigation

• To evaluate the effect of prior and

post-weld heat treatment (PWHT) on the

cross-weld creep performance and HAZ

toughness of weldments in P91 steel.


Five conditions investigated

1. As-welded condition

• Cold fabrication (intentional or not)

2. Temper prior to welding no PWHT

• (cold repair)

3. PWHT

• conventional

4. Temper prior to welding + PWHT

• conventional repair

5. 2 x PWHT

• (Excessive PWHT)

PWHT =760°C 3hrs

Temper = 760°C 6hrsC


P1 P2 P4

W2

P1

W4

P2W1

P3

W3

P4

PWHT x2

TestToughness &Creep

PWHT x1

Temper

PWHT x1

W4

P2a

W4

P2b

P3

Conventional w

eld

or

repair (

PW

HT

)

‘Cold

’ we

ld (

AW

)

Excessiv

e

PW

HT

(PW

HT

x2)

Cold

repair in

tem

pere

d p

are

nt

(TA

W)

Conventional re

pair

in tem

pere

d p

are

nt

(T+

PW

HT

)

RPEB welded

PWHT =760°C 3hrs

Temper = 760°C 6hrsC


Reduced Pressure Electron Beam

(RPEB) welding• Produces ‘simple’ uniform HAZ

without reheated regions.

– Facilitates microstructural

inspection

– Likely to produce a worst case

scenario (continuous regions with

poor toughness)

• Capable of welding 150mm thick

P91 in one pass at 100mm/min!

– Low distortion

– High quality

– High productivity


RPEB

5mm Hi-Low

mismatchMulti pass arc weld and single pass

RPEB weld in 100mm thick steel


EB Welding for Improved Creep

Performance

Effect of EB welding

on the creep

properties of grade

122 at 650°C

Adapted from Abe and Tabuchi Science and Technology of Welding, 2004

Improvement

Associated With:

•Short thermal cycle

•Narrow HAZ

•Low weld angle20

30

40

50

60

70

80

90

100

110

120

130

140

150

100 1000 10000 100000

Time to Rupture, h

Cre

ep

Ru

ptu

re S

tre

ss

, M

Pa

Parent

GTA WeldedEB Welded

ASME 2179-6 Allowable Stress for P122 (Seamless)

Creep rupture data for Grade

122 at 650°C

(2.5mm HAZ)(0.5mm HAZ)


As welded microstructure

1.AW 2.TAW 3.PWHT 4.PWHTx2 5.T+PWHT

2mm


As welded combined

100

150

200

250

300

350

400

450

500

550

600

0 1 2 3 4 5 6 7

Distance from weld centre line, mm

Vic

kers

Hard

ness

HT1, H-J=0

HT2, H-J=21.51

HT3, H-J=21.20

HT4, H-J=21.61

HT5, H-J=21.51

Parent

HV10 kg

HAZ

As fabricated hardness across welds

PWHT

(HT 3,4,5)

No PWHT

(HT1,2)


P1W2 PWHT WM Charpy data

0

20

40

60

80

100

120

140

160

180

200

220

240

-40 -30 -20 -10 0 10 20 30 40

Temperature, °C

Ch

arp

y Im

pact

En

erg

y, J

HAZ, HT1 HAZ, HT2HAZ, HT3 HAZ, HT4HAZ, HT5 WM, HT2WM, HT3 WM, HT4Typical WM values, Metrode 2002 PM data V&M 2002Simulated CGHAZ 1300°C+1hr760°C Simulated CGHAZ 1300°C +5h@760°CSimulated CGHAZ 1300°C PM, HT1

Simulated

HAZWM

HAZ

HAZ

+PWHT


Fracture toughness (J) for all SENB specimens tested at 7°C

0

100

200

300

400

500

600

700

HT1 HT2 HT3 HT4 HT5

Heat treatment

Fra

ctu

re e

nerg

y (

J),

kJ/m

2

0

50

100

150

200

250

Ch

arp

y im

pact

en

erg

y, J

J (from CMOD)

HAZ Charpy impact energy

WM Charpy impact energy

7oC

As-welded


0.0

2.0

4.0

6.0

8.0

10.0

12.0

14.0

16.0

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

Clip gauge displacement, mm

Fo

rce, kN

7°C

15°C25°C60°C

HT2 (TAW)


Summary of toughness data

• Trends in Charpy and fracture toughness

results are similar

• Charpy toughness in the HAZ may appear

overly positive in view of code

requirements and the fracture toughness

in the same location.

• Simulated Charpy specimens and fracture

toughness tests appear to more

accurately identify the brittle zones

• Above 15oC there appears to be little

chance of brittle behaviour


630°C RPEB cross-weld creep rupture results for P91 material subject to various heat treatments

before and after welding

40

50

60

70

80

90

100

110

120

130

140

10 100 1000 10000

Time, hrs

Ru

ptu

re S

tress,

MP

a

HT1 X-WELD

HT2 X-WELD

HT3 X-WELD

HT4 X-WELD

HT5 X-WELD

X-WELD Jones 1990"

Parent ECCC 2005

630oC

Type IV failures


HAZ microstructure

• Thermal cycle forms refined HAZ

• MX coarsend in FG&ICHAZ

• Finer grains promote diffusion during service causing more rapid growth of precipitates and recovery of ferrite matrix

• Void formation in FG/ICHAZ

• Voids coalesce or ‘unzip’ to form creep cracks

60MPa, 630°C, 8491 hours


HAZ up close

As-welded P91 after creep test 60MPa, 630°C 8491hrs


Summary of Creep Results

• The effect of prior or post weld heat

treatment at 760o C had little effect on

creep strength for the durations

investigated (<10khrs)

• As precipitate growth is logarithmic with

time the effect on the precipitates in the

FG/IC HAZ is negligible as precipitates are

already present


Industrial implications

• Implications on toughness have to be

carefully considered. Can you afford to

have a brittle HAZ/weld metal?

– Careful procedural control

• Providing concerns over toughness and

resistance to fabrication cracking can be

overcome creep strength is unlikely to

suffer from a ‘cold’ weld procedure or

excessive PWHT.


Thank you for your attention


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