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Assessment of the Creep Rupture Data of the Welded Joints of P91

Steel using Several Evaluation Methods

Mohamed E. AbdEl-Azim and Omyma H. Ibrahim

Metallurgy Department, Atomic Energy Authority, Cairo, Egypt

Received: 24/7/2013 Accepted: 20/11/2013

ABSTRACT

The values of creep rupture strength of the welded joints of P91 steel, at 600

and 650°C were evaluated by several evaluation methods. Those were: (1) direct

comparison of the welded joints to the base metal, (2) stress reduction factor, (3)

Larson-Miller parameter and (4) lifetime assessment methods. It was found that the

creep rupture strength of the welded joints of P91 steel approaches that of the base

metal in short-term creep tests (<2000h). However, it approaches that of the

specimens simulating the fine-grained heat affected zone of the welded joints. At the

same time it is much lower than that of the base metal in the long-term creep tests

(>2000h), either at 600 or 650 C which could be related to the drastic

microstructural changes that occur during long-term creep tests.

Key Words: welded joints, creep rupture strength, P91 steel, evaluation procedures

INTRODUCTION

The majority of creep resisting steels used in steam and gas turbines and in the petrochemical

industry is based on low carbon, low alloy steels containing carbide forming elements such as Cr, Mo,

V, Nb, and W (1). Heat resistant martensitic steels of 9-12% Cr and with small additions of alloying

elements such as Mo, Nb, V, W, etc. are widely used for high temperature components for power

stations since they offer high operating temperatures (600-650°C) and pressures that lead to improved

thermal efficiency (2).

Remaining life prediction of heat resisting steels has recently become an issue in power

generation industry. The interest in the area of remaining life prediction arises from the necessity to

avoid costly forced outages, safety considerations and the need to extend the operating life of

structural components beyond the original design life (3, 4).

For the estimation of the lifetime of components for high temperature applications, it is

important to know the creep behaviour not only for the base metals, but also mainly for weldments.

Often, the weldment is the weakest part in a component, so it could determine its lifetime as a whole.

Recent research projects deal with long term testing and qualification of weldments of boilers

and piping materials. This includes 9% Cr steels (P91 and E911) (5, 6). The research programs comprise

long-term creep testing (up to 100000 h), mechanical testing after ageing and evaluation of creep

strain and creep rupture data (7). The aim of this work is to evaluate the existing creep rupture data for

the weldments of P91 steel at 600 and 650°C.

Material Data Sources:

Recent developments (8) in the analysis of creep rupture data for the prediction of long term

strength values have been directed towards the statistical assessment of both large population of data

from database (referred to as mean values in this work) and small number of experimental data

(referred to as specific values in this work).

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The mean values (MV) of the creep rupture strength of P91 steel at 600 and 650°C (8) are

shown in Fig. 1 as a function of the corresponding rupture times. The formulae shown in Fig.1 satisfy

the correlation between the mean values of the creep rupture strength and the rupture time at 600 and

650°C. The specific values (SV) of the creep rupture strength of P91 steel for heat Z of the welded

joints AZ and FZ are shown in Fig. 2 as a function of the corresponding rupture times at 600 and 650

°C. The formulae shown in Fig. 2 satisfy the correlation between the specific values of the creep

Fig. (1): Creep rupture strength (MV) of P91 steel as a function of rupture time

Fig. (2): Creep rupture strength (SV) of heat Z of P91 steel as a function of rupture time

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Rupture strength and the rupture time at 600 and 650 °C. These formulae are used for the

extraction of another data required in this work. Heat Z and its welded joints were creep rupture tested

at 600 and 650°C at Materialprüfungsanstalt (MPA), Stuttgart University. Table 1 gives the chemical

composition of heat Z. Table 2 gives an overview of the welded joints AZ and FZ of heat Z of P91

steel. In addition, an overview of the welded joints M1, M2, M3 and M4 is shown in Table 2. For

comparison with the welded joints the heat Z was stress relieved before creep tests at the same

condition of the post weld heat treatment condition of its welded joints. Some specimens of heat Z

were heat treated to have microstructure similar to the fine-grained heat affected zone (FGHAZ) of its

welded joint. The heat input during welding was simulated by inductive heating (heating time ~ 20 sec

from 22-850°C) of the specimens to a peak temperature of 850°C; then the specimens were held there

for 3 sec and finally quenched in oil.

Table 1: Chemical analysis of heat Z solution treated at 1050 C for 10 min and aged at 750C for 70

min.

Element wt.%

C 0.104

Fe Bal.

Si 0.27

Mn 0.46

Cr 8.17

Mo 0.90

V 0.194

Nb 0.064

Ni 0.16

P 0.014

S 0.003

N 0.055

Table (2): Overview of the welded joints of P91 steel

Welded

joint Heat

Welding

procedure

Weld

metal

Post weld heat

treatment

AZ Z

MAW P91 760°C / 2 h

FZ SAW P91 760°C / 2 h

M1* _ MAW P91 750°C / 2 h

M2* _ MAW P91 750°C / 2 h

M3* _ SAW P91 750°C / 4 h

M4* _ MAW- P91 750°C / 4 h

* The source of data is Materialprüfungsanstalt (MPA), Stuttgart University, Germany

Evaluation Methods:

(1) Direct comparison to the base metal method:

Figure 3 shows the logarithmic plot of rupture strength against rupture time plots for the

welded joints AZ, FZ, M1, M2, M3 and M4 of P91 steel. This is in comparison with those of the base

metal (mean values) at 600 and 650°C and that of the specimens of P91 steel simulated to FGHAZ of

the welded joints of P91 steel at 600 °C. At 600°C, the values of the creep rupture strength of the

welded joints lie between the mean values of the creep rupture strength of P91 steel and the values of

the creep rupture strength of the specimens simulated to FGHAZ of the welded joints. In short-term

creep rupture tests (<2000 h) the creep rupture strength of the welded joints approaches the mean

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values of the creep rupture strength of the base metal. In long -term creep rupture tests (>2000 h) the

creep rupture strength of the welded joints is lower than the mean value of the creep rupture strength

of the base metal and approaches the creep rupture strength of the specimens simulated to FGHAZ of

the welded joints. At 650 C in general the values of creep rupture strength of the welded joints are

much lower than the mean values of the creep rupture strength of the base metal.

Fig. (3): Creep rupture strength- rupture time plots of the base metal (BM) and the welded joints of

P91 steel at 600 and 650 C

(2) Stress reduction factor method :

Stress reduction factor (SRF) is the ratio of the creep rupture strength (Rf) of the welded joint to

the mean or the specific value of the creep rupture strength of the base metal (BM) at the condition in

which both the creep rupture strength of the welded joint and that of the base metal give the same

rupture time. The stress reduction factor of the welded joints AZ and FZ of P91 steel is given as a

function of the rupture time as shown in Figs. 4 and 5 depending on the specific values of the creep

rupture strength of their heat Z.

Figures 6 and 7 show the stress reduction factors of the welded joints AZ, FZ, M1, M2, M3 and

M4 of P91 steel against the rupture time at 600 and 650 C depending on the mean values of the creep

rupture strength of P91 steel. From Figs 4-7 the following notes were found. The values of SRF

depending on the specific values of the creep rupture strength of the base metal either at 600 C or 650

C are slightly higher than those depending on the mean values of the creep rupture strength of P91

steel. For example at 600 C the SRF depending on specific values of the creep rupture strength varies

from 0.69 to 1.08 while those depending on the mean values of the creep rupture strength varies from

0.65 to 0.98. In general the values of SRF depending on the mean values or specific values of the

creep rupture strength goes to lower values at 650 C than at 600C at the same rupture time. The SRF

values either at 600 C or 650 C approach 1 in short creep tests (<1000h). This means that the creep

rupture strength of the welded joints approaches the creep rupture strength of the base metal in short

term creep tests. In long-term creep tests, the creep rupture strength of the welded joint is much lower

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than the creep rupture strength of the base metal. Thus, the mean values of the creep rupture strength

of the base metal could be used for the prediction of the creep rupture strength of the welded joints

giving results approaching those obtained by using the specific values of the creep rupture strength of

the base metal.

Fig. (4): Stress reduction factor (SV)- rupture time plots of the welded joints of P91 steel at 600C

Fig. (5): Stress reduction factor (SV)- rupture time plots of the welded joints of P91 steel at 650C

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Fig. (6): Stress reduction factor (MV)- rupture time plots of the welded joints of P91 steel at 600C

Fig. (7): Stress reduction factor (MV)- rupture time plots of the welded joints of P91 steel at 650C

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(3) Larson-Miller parameter method:

The Larson-Miller parametric method is commonly used to predict the 105 h creep-rupture

strength from short-term creep data. Larson and Miller (9-11) introduced the following parameter, PLM as

follows:

PLM =T (C +log tr) (1)

which serves as one of the co-ordinates in a plot presenting the creep rupture data in terms of

their dependence on the applied stress, , T represents the test temperature , tr is the rupture time and

C is a constant related to the activation energy for creep. A value of C=20 was considered in the

original formulation and optimized from 10 to 40 depending on the material. It was found that the

best-fit constant value of C for P91 steel was around 31-36 (12, 13). However, the literature (14, 15)

indicates that the constant must be reduced from 31 or 36 for the data obtained under lower stresses

and high temperatures due to the microstructural changes which cause degradation in the creep

resistance for the martensitic steels.

Figures 8 and 9 show Larson-Miller plots for the creep rupture data (Specific Value) of the heat

Z of P91 steel, in addition to the mean values of the creep rupture data of P91 steel, at 600 and 650°C,

for two different values of C, 20 and 36. Figures 10 and 11 show Larson-Miller plots for the creep

rupture data (Specific Value) of the welded joints AZ, FZ, M1, M2, M3, M4 at 600 and 650°C, for

two different values of C, 20 and 36. The results for either the base or the welded joint give best fit

when the values of C equal to 36.

(4) Lifetime assessment method:

This method is proposed to evaluate the creep rupture strength of the welded joints from the

creep data of the base metal and the welded joint. The first method proposed in 1987 (ASME code

case N-47), suggested a safety factor, S defined by:

WJ=S BM (with S=2/3) (2)

where WJ is the engineering stress predicted to be imposed to the welded joint to reach the same creep

rupture time as that of the base metal for an applied engineering stress equal to BM (specific value).

This proposal does not take into consideration material properties or test temperature. It is possible to

evaluate the safety factor from the experiments as proposed by Tu et al. (16):

S= (tWJ/tBM) BMn/1 (3)

where tWJ and tBM, are the creep rupture times of the welded joint and of the base metal respectively,

when performing creep tests at the same engineering stress and at the same test temperature nBM is the

stress exponent of power low creep for the base metal.

Figure 12 shows the creep rupture time, tWJ of the welded joint AZ and FZ against the creep

rupture time, tBM of the heat Z of P91 steel at the same applied engineering stress and at the test

temperature of 600 C. We found a linear relationship between the tWJ and tBM for both the welded

joint AZ and FZ. The slope of the line is about 0.109. We propose that the slope of this line is equal to

tWJ/tBM in equation 3. The value of the stress exponent nBM of the base metal at 600 C equals 7.8 as

found by Gaffard et al. (5) which is not significantly different from that of the welded joint as found by

Wang et at. (17). By applying Equation 3 the value of the safety factor, S is 0.75

Figure 13 shows predicted values of the creep rupture strength for the welded joint of heat Z at

600 C. That was done by taking the value of the safety factor as 0.75 by applying Equation 3. Also

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by taking the safety factor equaling (2/3) as proposed by ASME code N-47 against the rupture time in

comparison with the values of the creep rupture strength of the welded joints AZ and FZ of the heat Z

of P91 steel against the creep rupture time. Considering the value of the safety factor as 0.75 gives

predicted values for the creep rupture strength of the welded joints of P91 steel more approaching the

experimental values of the creep rupture strength than using value of the safety factor as 2/3.

Fig. (8): Larson-Miller plots of the creep rupture data of P91 steel, taking C=20

Fig. (9): Larson-Miller plots of the creep rupture data of P91 steel, taking C=36

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Fig. (10): Larson-Miller plots of the creep rupture data of the welded joints of P91 steel, taking C=20

Fig. (11): Larson-Miller plots of the creep rupture data of the welded joints of P91 steel, taking C=36

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Fig. (12): Rupture time of the welded joints of P91 steel – rupture time of P91 steel at 600 C

1000 10000 100000

100

Predicted (S= 0.75)

AZ, Experimental

FZ, Experimental

Predicted (S= 0.667)

Cre

ep r

uptu

re s

tren

gth,

MP

a

Rupture time, h

Fig. (13): Logarithmic plot of experimental and predicted values of creep rupture strength of the

welded joints of P91 steel and rupture time at 600C

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General Discussion:

From the results shown in Fig. 3 (creep rupture strength- rupture time) and Figs 4-7 (stress

reduction factor) it could be found that the creep rupture strength of the welded joints of P91 steel at

600 C approaches the creep rupture strength of the base metal in short term creep tests (≤2000h).

However, it is much lower than the creep rupture strength of the base metal in long-term creep tests

(2000h) where the creep rupture strength approaches that of the specimens simulated to the FGHAZ

of the welded joints. At 650 C, the creep rupture strength of the welded joints is much lower than that

of the base metal. The significance in the creep rupture strength of the welded joints of P91 steel with

the rupture time in comparison with the base metal is due to the higher rate of change of the

microstructure of the welded joints during creep either at 600 C and 650 C than the base metal as

found in our previous work (18, 19). It is important to mention that the microstructure, of both the welded

joints after the post weld heat treatment at 760 for 2h and the base metal after tempering at 760 C for 2

h consists of tempered lath martensite (18). The microstructure of the welded joints shows four well-

defined zones. weld metal with tempered lath martensite within coarse prior austenite dendrites at the

centre of the welded joints. This is followed by coarse-grained heat affected zone (CGHAZ) with

tempered lath martensite within coarse prior austenite grains ( 50 µm) and fine grained heat affected

zone (FGHAZ) with tempered lath martensite within fine prior austenite grains ( 10 µm). Finally, the

base metal with tempered martensite with austenite grains of average size of 30 µm similar to the base

metal specimens (18). In our previous work (18, 19), we found that the creep resistance of the welded

joints of P91 steel is limited by that of the FGHAZ which is the weakest zone of the welded joints.

The lower creep resistance of the welded joints in comparisons with the base metals as shown in Figs.

3-7 is due to the lower creep resistance of its FGHAZ. The lower creep resistance of FGHAZ in

comparison with the other zones of the welded joint and the base metal is due to the higher rate of

recovery of tempered lath martensite of this zone in comparison with the other zones and the base

metal (18, 19). The fine structure of the tempered lath martensite consists of subgrains within the prior

austenite grains. Therefore, the lower creep resistance of FGHAZ is due to the higher rate of recovery

of subgrain structure of P91 steel, which is the main process, during creep resulting in higher rate of

reduction of dislocation density within the subgrains, and higher rate of growth of the subgrains as

found by Abd ElAzim et al. (18, 19)

The recovery of the subgrain structure is affected by the prior austenite grain size, the finer the

prior austenite grains as in FGHAZ in comparison with the other zones of the welded joint and the

base metal, the higher is the rate of recovery of the subgrain structure as found in our previous work (18,19). The microstructure of the martensitic P91steel is complex, either in the base metal or in the

welded joints with fine subgrain structure within the prior austenite grains after tempering the base

metal or after the PWHT of the welded joints. This implies higher values of the constant C in Eq. 1

(C 30) to give a good fit between the creep data at 600 and 650 C. In addition, the calculation of the

safety factor should take into consideration the test temperature and the material properties stress

exponent of power law creep as shown in Eq. 3 to give predicted values for the creep rupture strength

of the welded joints of P91 steel approaching the experimental ones.

CONCLUSIONS

It was concluded that:

1. Results of the direct comparison and the stress reduction factor methods reveal that the creep

rupture strength of the welded joints deviate much from that of the base metal in long term creep

test (>2000 h) at 600 C

2. A value of C equals 36 gives good fitting between the creep data of the base metal and the welded

joints at 600 and 650°C.

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3. The value of the safety factor of 0.75 taking into account the material properties and the test

temperature gives predicted values of the creep rupture strength for the welded joints of P91 at 600

C approaching the experimental values.

ACKNOWLEDGMENT

The authors would like to appreciate the support of Prof. E. Roos, Dr. H. Theofel and Dr. A.

Klenk, Materialprüfungsanstalt (MPA), Stuttgart University- Germany for rendering the present work

possible during the authors stay at MPA.

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