INFLUENCY OF HEAT TREATMENT IN THE … Testing and Diagnostic -256- Table 1 - Chemical composition of P91 steel (% weight). C Si Mn P S Cu Cr Ni 0.108 0.33 0.53 0.013 0.002 0.190 8.560

Proceedings of the 6th International Conference on Mechanics and Materials in Design,

Editors: J.F. Silva Gomes & S.A. Meguid, P.Delgada/Azores, 26-30 July 2015

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PAPER REF: 5581

INFLUENCY OF HEAT TREATMENT IN THE MECHANICAL

PROPERTIES AT HIGH TEMPERATURES OF P91 STEEL-PIPE

WELDED JOINTS

Tatiane Chuvas1,2 (*)

, António Correia da Cruz3, Manuel Gomes

3, Maria Cindra Fonseca

1

1Federal Fluminense University - UFF/PGMEC, Niterói, RJ, Brasil

2Department of Mechanical Engineering - CEFET/RJ, Rio de Janeiro - RJ

3Instituto de Soldadura e Qualidade - ISQ, Oeiras, Portugal.

(*)Email: [email protected]

ABSTRACT

The aim of this work is the characterization of ASTM P91 steel-pipe joints Metal Cored Arc

Welding & Flux Cored Arc Welding (MCAW/FCAW) processes, which have been

increasingly applied due to high productivity and good surface quality of the welded joints

through the evaluation of mechanical properties at elevated temperatures under the conditions

with and without post-weld heat treatment. The results show that the welded joint hardness

and yield strength are the main properties changed by heat treatment.

Keywords: ASTM P91, FCAW, MCAW, mechanical properties, high temperature.

INTRODUCTION

The components exposed to high temperature fields, such as piping systems of petrochemical

industry, have strict requirements of the creep and fatigue at high temperatures resistance

besides the stable microstructure and mechanical properties that prevent premature and

catastrophic failures (Guodong Zhang et al, 2011; Hyde et al, 2012).

New steels, with high mechanical strength and stable microstructure at high temperatures,

have been developed in order to increase the thermal efficiency of power plants. However,

microstructural changes produced by manufacturing processes, such as welding, can be cause

changes in the steel properties and carry to premature and catastrophic failure (Divya et al,

2014; Isaac et al, 2011). Since they are applied at elevated temperatures, these steels require

good creep resistance (El-Azim, 2013). However, the heat input generated during the welding

process induces microstructural changes that lead to the formation of heat affected zone

(HAZ) composed of sub-regions denominated: coarse grains HAZ (CGHAZ), fine grains

HAZ (FGHAZ) and intercritical HAZ (ICHAZ).

Despite the high productivity, the Metal Cored Arc Welding (MCAW) and Flux Cored Arc

Welding (FCAW) processes are not widely exploited in the manufacture of pipe lines for high

temperature. (Arivazhagan et al, 2008). Thus, this study aims to evaluate the influence of

post-weld heat treatment (PWHT) on the mechanical properties at room temperature and

elevated temperature conditions in the MCAW/FCAW welding process in P91 steel.

MATERIALS AND EXPERIMENTAL TECHNIQUES

In the present work, it was used a P91 seamless pipe in the normalized and tempered

condition, with 152 mm outside diameter and a wall thickness of 18mm.The chemical

composition and mechanical properties of the base material are shown in Tables 1 and 2.

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Testing and Diagnostic

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Table 1 - Chemical composition of P91 steel (% weight).

C Si Mn P S Cu Cr Ni

0.108 0.33 0.53 0.013 0.002 0.190 8.560 0.300

Mo V N Al Nb As Sn Ti

0.870 0.221 0.053 0.012 0.067 0.006 0.150 0.003

Table 2 - Mechanical properties of P91 steel (room temperature).

σLE(MPa) σLR (MPa) Elongation (%)

ASTM 335 > 415 > 585 > 19

Laboratory 638 724 20

The samples were welded by the semiautomatic flux cored and metal cored arc welding

processes, which are being increasingly applied in tubular joints at site or pipe shop, due to its

high productivity.

For the root pass, in order to ensure the maximum integrity of the joint which is provided by

electrodes without slag formation, it has been used metallic electrode in the form of a metal

cored wire, within the SFA 5:28 specification and E90C-B9 classification with

1.2mmdiameter. The filling passes were performed using the E91T1-B9 flux cored electrode

that although it produces slag, provides greater productivity to the process when compared to

metal cored. The chemical composition of the electrode wires are shown in Tables 3 and 4.

Table 3 - Chemical composition of metal cored electrode E90C-B9(% weight).

C Si Mn P S Cu Cr

0.09 0.30 0.80 0.01 0.009 0.03 8.0

Ni Mo V N Al Nb Ni + Mn

0.30 0.87 0.18 0.05 0.008 0.03 1.10

Table 4 Chemical composition of flux cored electrode E91T1-B9.(% weight).

C Si Mn P S Cu Cr

0.11 0.34 0.89 0.020 0.05 9.4

Ni Mo V N Al Nb Ni + Mn

0.47 0.95 0.22 0.05 0.004 0.04 1.36

As the shielding gas was used a mixture formed of 98 % argon and 2 % CO2 with a flow rate

of 12 l/min. The purge gas used in the root protection against oxidation during welding was

commercial argon (99.998 % purity) with a flow rate of 16 l/min. The API 938-B standard

requires that the P91steel be welded at the minimum preheat temperature of 250 °C for

greater thickness and interpass temperature between 200 and 300 °C. In this work was used

preheat of 250 °C and the interpass temperature during welding was about 260 °C and



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heating was accomplished by electrical resistance, protected by insulating blanket. The sketch

with sample dimensions and sequence of weld passes is represented in Fig. 1.

(a) (b)

Fig. 1 - (a) Sample dimensions; (b) weld passes sequence.

The post welding heat treatment (PHWT) was carried out on the pipe in an electric furnace

where it was heated to a rate of 125°C/h until it was reached a temperature corresponding to

760°C ± 5°C, which was maintained for 2h. The cooling to 250°C was monitored at a

maximum rate of 125°C/h.

Tensile tests were performed in the base metal (BM) and the joint welded under the

conditions without PWHT (as welded) and with PWHT (after PWHT). The specimens

dimensions used are shown in the Fig. 2. Tensile tests were performed at four temperatures:

500, 550, 600 and 650 °C, beyond the ambient temperature.

Fig. 2 -Dimensions of tensile test specimens [mm].

RESULTS AND CONCLUSIONS

Firstly, data of the base metal behaviour were obtained, as shown in Table 5. Analyzing the

results it is evident the decrease in the yield and tensile strength. However, there is significant

strength stress decrease above 600 °C. This material can be applied to temperature conditions

at 600 °C and, in this case, the yield and tensile strength presented by even 600 °C are

satisfactory.

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Table 5 - Base metal properties.

Temperature (°C) �� (MPa) �� (MPa) Elongation (%) Area reduction

(%)

Room temperature (~25) 580 ± 14 734 ± 3 24.5 ± 2.7 47.0 ± 0.5

500 470 ± 7 516 ± 23 15.9 ± 2.1 48.8 ± 1.0

550 365 ± 14 409 ± 12 16.4 ± 1.3 65.4 ± 0.2

600 335 401 28.2 66.4

650 228 ± 11 314 ± 6 31.0 ± 2.9 72.8 ±0.6

With regard to elongation, it is expected that with increasing temperature the material has

higher ductility and consequently larger values of elongation, as noted in Table 5. At room

temperature the elongation values were higher and comply with the standard. The reduced

value observed at 500 and 600 °C is due to the fact that the tests were performed with the

same parameters, i. e., the displacement rate used at room temperature was reproduced at high

temperature, requiring more material. The area reduction increased significantly between 600

- 650 °C.

Fig. 3 compares the yield strength values of the joints with the base metal data where it is

possible to observe that up to 600ºC the joints yield strength with PWHT are slightly higher

than the base metal. Above this temperature, the standard is reversed and the welded joint has

lower yield stress values (about 10%) to the base metal. Regarding the welded joint without

PWHT, the yield strength is higher than the base metal was expected due to the martensite

formed in the weld bead as opposed to tempered martensite displayed in the base metal as

shown in the microstructural analysis. In 500ºC note a fall of yield strength in the condition

without PWHT.

Fig. 3 - Yield strength (MPa).

100 200 300 400 500 600 7000

100

200

300

400

500

600

700

800

Yie

ld S

tren

gth

(M

Pa)

Temperature (0C)

Base Metal

As welded

PHWT



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For tensile strength, again has a well homogeneous behavior, with the exception of 500 °C

temperature, where it is observed a decrease of tensile strength to the condition without

PWHT. Tensile strength should not fall below 585 MPa at 20 °C, below 290 MPa at 600 °C,

and below 215 MPa at 650 °C according to the ASTM Code. In all measurement, the tensile

strength values were above the limit values and all specimens with PWHT ruptured in the MB

region, describing the resistance of the weld and the influence of PWHT. Fig. 5 shows a

specimen macrograph tested at 600 °C with a joint PWHT where you can see that the fracture

occurred in BM.

Fig. 4 - Tensile strength (MPa).

Fig. 5 - Fracture region location: specimen at 600 ° C

100 200 300 400 500 600 7000

100

200

300

400

500

600

700

800

Temperature (0C)

Base Metal

As welded

PHWT

Ten

sile

Str

eng

th (

MP

a)

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Microstructural analysis showed that base metal (BM) consists of tempered martensite and

carbides (Fig. 6). In the as-welded sample, the microstructure of the weld metal is untempered

lath martensite (Figure 7a), which explains the high hardness values approaching

approximately 450 HV, as shown in Fig 7a and 7b. However, after performing the heat

treatment, it shown a microstructure of tempered martensite with high carbide precipitation,

as presented in Figure 8b, similar microstructural results are reported by Eggeler et al, 1992.

The PWHT operation results in a considerable decrease in hardness within the weld metal

region (~ 260 HV), as shown in Fig. 8a and 8b. However, this class of steel has a tendency to

form a soft zone in HAZ, specifically in the intercritical HAZ (ICHAZ). This region has only

partially austenitised during welding and subsequent PWHT operation produced a relatively

soft microstructure, which can reduce the creep resistance of the joint.

The Vickers hardness (HV10) was measured on welded samples at two levels, i.e. close to the

root pass and close to the cap pass, about 5 mm from the surface of the material.

Fig. 6 - Base metal microstructure.

(a) (b)

Fig. 7 - Weld metal microstructure (a) As welded; (b) after PWHT.



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-25 -20 -15 -10 -5 0 5 10 15 20 25

200

250

300

350

400

450

500Cap

Mic

roh

ard

nes

s (H

V)

Distance of center welding (mm)

Post Welding

After PWHT

HAZHAZ

-25 -20 -15 -10 -5 0 5 10 15 20 25

200

250

300

350

400

450

500

HAZ

Root

Mic

rohard

nes

s (H

V)

Distance of center welding (mm)

Post Welding

After PWHT

HAZ

(a) (b)

Fig. 8 Microhardness of weld joints (a) Cap; (a) Root.

CONCLUSIONS

The knowledge of the Cr-Mo steel mechanical behaviour, as the P91 steel, at high

temperatures is essential to establish safe conditions in service, especially when referring to

welded joints. Thus, this study allows the following conclusions:

1. The yield stress values are large influenced of the heat treatment. The welded joint with

heat treatment has mechanical properties very similar to the base metal. Regarding the

influence of temperature, up to 600 ° C the minimum requirements set in the standard are

obtained. However, after 600 ° C there is a significant fall of yield strength for all

conditions.

2. The tensile strength values are closer, both with respect to temperature as the heat

treatment condition.

3. The microstructural analysis of weld metal showed the presence of martensite laths,

which after PWHT were transformed into tempered martensite, very close to the base

material characteristics, that believed to contribute to reduce the hardness level of the

weld metal region.

4. After welding very high Vickers microhardness values were present in the weld metal

region (>400 HV) in the cap and root regions and the PWHT operation results in a

considerable decrease in hardness within the weld metal region (∼260 HV).

5. Regarding the soft zone observed in the HAZ, a detailed study of the presented

microstructure to reach better conclusions is required.

ACKNOWLEDGMENTS

The authors would like to thank the Brazilian research agencies (CNPq, CAPES and

FAPERJ) for their financial support.

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REFERENCES

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2817-2820.

[2]-Divya M, Das CR, Albert SK, Sunil Goyal, Ganesh P, Kaul R, Swaminathan J, Murty BS,

Kukreja LM, Bhaduri AK. Influence of welding process on Type IV cracking behavior of P91

steel. Materials Science & Engineering A, 2014, 631, p. 148-158.

[3]-Eggeler G, Tato W and Jemmely P. Creep rupture of circular notched P91 specimens -

influence of heat treatment and notch geometry. Scripta Metallurgica et Materiala. 1992, 27,

p. 1091-1096.

[4]-Guodong Z, Yanfen Z, Fei X, Jinna M, Zhaoxi W, Changyu Z, Lu Z. Creep–fatigue

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Engineering and Design, 2011, 241, p. 4856-4861.

[5]-Hyde CJ, Sun W, Hyde TH, Saad AA. Thermo-mechanical fatigue testing and simulation

using a viscoplasticity model for a P91 steel. Computational Materials Science, 2012, 56, p.

29-33.

[6]-Isaac Samuel E, Choudhary BK. Tensile work hardening behaviour of P91 steel. Materials

Science and Engineering A. 2011, 528, p. 7827-7830.

Documents

INFLUENCY OF HEAT TREATMENT IN THE … Testing and Diagnostic -256- Table 1 - Chemical composition of P91 steel (% weight). C Si Mn P S Cu Cr Ni 0.108 0.33 0.53 0.013 0.002 0.190 8.560