PWHT and Tempering Potential Issues

API934Seattle 2011

2Larson Miller ParameterThe use of an equivalence parameter, which involves both time and temperature, is of great interest. The most used tempering parameter is called the Larson- Miller Parameter LMP (or the Hollomon-Jaffe parameter).

The LMP formula is given for a single heat treatment:

with: temperature (K) and t time (hours)

It is also possible to extend this relationship to a multiple treatment by calculating an equivalent time teqi at a given temperature .

tLMP log20

n

ieqieq tLMP

1log20 )log20(10

iii teqit

3Larson Miller Parameter

4Larson Miller Parameter

Equivalent heat treatment parameter curves

17000

17500

18000

18500

19000

19500

20000

500505510515520525530535540545550555560565570575580585590595600605610615620625630635640645650655660665670675680685690695700705710715720725730735740745750

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20duration (hours)

t

e

m

p

e

r

a

t

u

r

e

(

C

)

LMP=T(K).(20+log(t(hours)))

5Influence of tempering parameter on UTSfor A542DCl4a ( Standard fabrications)

500

550

600

650

700

750

800

20000 20200 20400 20600 20800 21000 21200 21400 21600

Tempering parameter

U

T

S

(

M

P

a

)

Max UTS : 760MPa

Min UTS : 585MPa

Range of acceptable LMP conditions

LMP shall be :> 20100 in the as delivered condition, which dictates min Tempering< 21300 after max PWHT (can now be increased with fine-tuning)

Example of Acceptable LMP: CrMoV

6Examples from Industry

2 actual CrMoV examples from 2010-2011:

1. T 730C/4h* + ISR 675C/40h + PWHT 710C/32h

LMP 21333(T 1346F/4h* + ISR 1247F/40h + PWHT 1310F/32h)

2. T 750C/3h* + ISR 670C/50h + PWHT 710C/32h

LMP 21418(T 1382F/3h* + ISR 1238F/50h + PWHT 1310F/32h)

*Tempering duration is a function of thickness

71. T 730C/4h + ISR 675C/40h + PWHT 710C/32h

LMP 21333(T 1346F/4h* + ISR 1247F/40h + PWHT 1310F/32h)

Influence of tempering parameter on UTS

500

550

600

650

700

750

800

20000 20200 20400 20600 20800 21000 21200 21400 21600

Tempering parameter

U

T

S

(

M

P

a

)

Max UTS : 760MPa

Min UTS : 585MPa

Tempering 730C Tempering 730C +ISR +PWHT Max

Examples from Industry

82. T 750C/3h + ISR 670C/50h + PWHT 710C/32h

LMP 21418(T 1382F/3h* + ISR 1238F/50h + PWHT 1310F/32h)

Influence of tempering parameter on UTS

500

550

600

650

700

750

800

20000 20200 20400 20600 20800 21000 21200 21400 21600

Tempering parameter

U

T

S

(

M

P

a

)

Max UTS : 760MPa

Min UTS : 585MPa

Tempering 750C Tempering 750C +ISR +PWHT Max

Examples from Industry

9The counter proposal: 1. T 710C/4h + ISR 675C/40h + PWHT 710C/32h

LMP 21270 SAFE(T 1310F/4h* + ISR 1247F/40h + PWHT 1310F/32h)

2. T 710C/3h + ISR 670C/50h + PWHT 710C/32h

LMP 21257 SAFE( / * S / / )

Examples from Industry

Tempering parameter

Influence of tempering parameter on UTS

500

550

600

650

700

750

800

20000 20200 20400 20600 20800 21000 21200 21400 21600

U

T

S

(

M

P

a

)

Max UTS : 760MPa

Min UTS : 585MPa

Tempering 710C Tempering 710C +ISR +PWHT Max

10

Conclusions:

Requirements on Tempering very often lead to reduce the acceptable PWHT:

- This is risky for base metal properties after PWHTmax- This can lead to decrease the number of PWHT cycles- This can even lead to decrease the PWHT Temperature (welds properties?)- This is useless for as-delivered conditions as UTS max must be respected and thus a suitable minimum tempering must be applied.

More risky for base metal AND welds so what is the interest?

Conclusions

PWHT and Tempering Potential Issues Larson Miller ParameterLarson Miller ParameterLarson Miller ParameterExample of Acceptable LMP: CrMoVExamples from IndustryExamples from IndustryExamples from IndustryExamples from IndustryConclusions