fracture toughness characterisation of the martensitic...

21. - 22. 11. 2012, Plzeň, Czech Republic, EU

FRACTURE TOUGHNESS CHARACTERISATION OF THE MARTENSITIC CHROMIUM STEEL

P91

Pavel KONOPÍKa, Hans-Werner VIEHRIGb

a COMTES FHT a.s., Dobrany, Czech Republic, EU, [email protected]

b Helmholtz-Zentrum Dresden-Rossendorf, Dresden, Germany, EU, [email protected]

Abstract

In this paper J-R curves were measured with Charpy size SE(B) and C(T) specimens of P91 steel using the

unloading compliance technique. The fracture toughness parameters were determined according to the test

standards ASTM E1820 and ISO 12135 and whose evaluation differences and validity criteria were

assessed. Despite the available sophisticated instrumentation used for obtaining the J-R curves and strict

implementation of the recommended test procedure, a scatter in the initial portion of the curve is persistent.

This creates problems in the evaluation of the measured J-∆a values and in obtaining valid and

unambiguous JIC and J0.2BL values according to both standards. The influence of these scatter on the fracture

toughness values is discussed. Accompanying fractographic studies were performed to assess the J-R

curves and to determine the SZW.

Keywords: fracture toughness, J-R curve, unloading compliance, ASTM E1820, ISO 12135

1. INTRODUCTION

The thermal efficiency of power plants has been significantly enhanced by increasing the operation

temperature and the pressure up to 650°C and 300 bars, respectively. These operation conditions make high

demands on the durability and fracture toughness in this temperature range. These requirements meet heat-

resistant chromium steels due to its alloying elements and their heat treatment. One representative is the

X10CrMoVNb9-1steel, which is specified as P91 according to the US ASME Boiler and Pressure Vessel

Code. The range of loading requires the determination of fracture toughness values in the ductile range.

In case cracks in a structure initiate and propagate by ductile mechanisms, it is conventional to measure the

material’s resistance to crack growth, generally in terms of J, and to plot this against the crack extension, Δa,

to give a J-resistance curve [1]. The J-R curves can be constructed using either a multiple specimen or a

single specimen approach [2]. But since, in the most cases the material available for testing purposes is

limited; the application of the single specimen method becomes a necessity. The techniques available for

this purpose are unloading compliance, potential drop, acoustic emission, ultrasonic measurements among

others.

In ductile materials, first the blunting of pre-existing cracks occurs during loading followed by formation of

voids ahead of the crack tip at critical strain. These voids finally coalesce with the crack tip leading to crack

propagation [3]. Hence, the ductile crack initiation cannot be defined as a point in the J-Δa curve but rather

as a process which occurs over a range. The parameters JIc or J0.2BL are engineering estimates of fracture

toughness defined by the intersection of the 0.2 mm offset construction line and the J-R curve. They are

measured at a point near the initiation of ductile crack extension and provides a provisional JQ or J0.2BL(B)

value, which when properly qualified against the criteria proposed in the standard procedures like ASTM

E1820 [4] and ISO 12135 [5] becomes the fracture toughness parameter JIc or J0.2BL respectively. These

values are taken as the initiation fracture toughness of the material. Another criterion for prediction of

fracture toughness is measuring the stretch zone width (SZW) [6]. This procedure is considered to give an

accurate value of initiation fracture toughness, close to the onset of crack initiation with the degree of crack

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mailto:[email protected]


tip blunting increasing proportionally to the toughness of the material. But the determination of SZW is

subjective and prone to error due to different perceptions of the users.

2. J-R CURVE MEASUREMENT

In this work the single specimen unloading compliance method is used for J-R curve determination. The

paper attempts to discuss the applicability of single specimen unloading compliance method and the

problems associated with it in determining the J initiation fracture toughness values for P91 steels according

to the tests standards ASTM E1820 [4] and ISO 12135 [5].

Both standards are intended to determine fracture toughness values in the ductile region. However, there are

remarkable differences.

1. The blunting line according to ASTM E1820 is calculated as J = 2•σY•Δa while ISO 12135 calculates

those as J = 3.75•Rm•Δa with much steeper slope resulting in significantly lower J initiation values.

2. The calculation of the J-Integral values is slightly different in both test standards, for details see [4] and

[5].

3. The ISO fit line J=α+β(Δa)γ with an absolute value differs from the ASTM E1820 fit line which is

J=C1((Δa/k)C2

, where k=1mm.

4. ASTM E1820 compensates the uncertainties of the initial J-Δa values with the aoq fit. The ISO 12135

standard uses the coefficient λ to adjust the calculated a0 to the initial crack length measured on the

fractured surface.

5. There are large differences in the specimen size depending measuring capacities (Jmax and Jlimit).

3. MATERIAL AND SPECIMENS

The material P91 is a heat-resistant chromium steel. The main alloying contents are about 0.1% carbon, 9%

Chrome and 1% molybdenum. It contains further manganese and silicon, as well as micro-alloying elements

vanadium and niobium, which form thermally stable carbides and carbonitrides {V (C, N)} or {Nb (C, N)} of

the type MX. The interaction of these precipitates with dislocations in the matrix contributes to the high

strength at high temperatures. P91 is mainly used in the hardened condition. Chromium carbides of the type

M23C6 stabilize the structure by the location of carbides in the grains and sub-grains [7], [8], [9]. Good

oxidation resistance of P91 is also caused by the higher content of chromium. Manganese and silicon cause

an increase of mechanical strength values through solid solution hardening. Tab. 1 contains the chemical

composition of the specification and of the investigated heat.

Tab. 1 Chemical composition of the steel P91

Element C Cr N Si P Ni

Measured wt. %] 0,116 9,5 n.A. 0,464 0,0085 0,235

Specification [wt. %] 0,08 - 0.12 8 - 9,5 0,03-0,07 ≤0,5 ≤0,025 ≤0,4

Element V Mn S Al Mo Nb

Measured [wt. %] >0,230 0,507 <0,0006 0,0195 0,91 0,0903

Specification [wt. %] 0.18 - 0.25 0,3 - 0.6 ≤0,015 ≤0,03 0,85-1.05 0.06-0.10

The structure of investigated material was consisted of tempered martensite and carbides, see Fig. 1.


Fig. 1 Structure of P91

Tensile tests of P91 were performed. Round samples of diameter 5 mm and gauge length 10 mm were used.

The J-R curve testing was carried out according to the test standards ASTM E1820 and ISO 12135.

Compact tension, 1T-C(T), 0.5T-C(T) and Charpy size single edge bend, Charpy size 0.4T-SE(B),

specimens with T-L orientation according to ASTM E1823-07 [11].

4. TESTING AND EVALUATION

Tensile tests were executed with a cross head speed of 1 mm/min in the temperature range from -100°C to

+175°C.

The J-R curve tests were conducted and evaluated according to the test standards ASTM E1820-11 and ISO

12135-2002. The unloading compliance method was used to determine the actual crack length. Following

test conditions were applied:

test temperature range: -50°C to 200°C

loading velocity: 0.5 mm/min (1T-C(T); 0.2 mm/min (0.5T-C(T) and 0.4T-SE(B))

partial unloading: 25% of the actual load

relaxation time: 30 s

distance between the unloadings: 0.075 mm (1T-C(T); 0.04 mm (0.5T-C(T) and 0.4T-SE(B))

unload and reload ramp rate: 1000 N/s (1T-C(T), 100 N/s (0.5T-C(T) and 0.4T-SE(B))

end of test criteria: 4 mm crack extension or 6 mm COD (1T-C(T); 2 mm crack extension or 4 mm COD

(0.5T-C(T) and 0.4T-SE(B).

In addition, electronic scanning microscope investigations were used to assess the fractured surfaces and to

determine stretch zones widths.

5. RESULTS AND DISCUSSION

5.1 Young's elastic modulus and tensile testing

For fracture toughness determination using J-R curve the tensile properties and Young's elastic modulus

were measured. Young´s modulus was measured on a cube by the velocity of ultrasonic waves. Results are

as follows:

E = 217389 MPa and G = 84733 MPa. With considering of ultrasonic method results, the Young modulus (in

GPa) can be calculated by equation (1):

E = -0.058*T +218.547 (1)


The yield strength Rp0.2 (in MPa) and ultimate tensile strength Rm (in MPa) vs. temperature (in °C) behavior

was determined as follows:

Rp0.2=477+1476.25*EXP(-0.0121*(T+273)) (2)

Rm=582.6+1000.76*EXP(-0.00733*(T+273)) (3)

5.2 Fracture toughness testing

Tab. 2 summarizes the code, test temperatures and fracture toughness values of the tested material P91.

The validity criteria mentioned in the standards ASTM E1820 and ISO 12135 are presented in Tab. 3. As

seen in Tab. 2 the 1T-C(T) specimens have large differences between crack length determined by UC and

measured on the fractured surface. Nevertheless the validity criteria No. 12 of and No.13 of the ASTM

E1820 and ISO 12135, respectively are complied (see Tab. 3). Only 3 samples meet the ASTM validity

criteria – two 0.5-C(T) tested at 20°C and one 0.5-C(T) tested at 200°C. However, no sample meets the ISO

validity criteria. The biggest problem is ISO criterion number 3. This criterion is not met in all cases.

Tab. 2 Results of fracture toughness

unvalid unvalid

Da0 DaE J0.2/BL(25.4) KJ0.2/(25.4) Da0 DaE JQ KJQ JIC KJIC criteria criteria

°C GPa mm mm N/mm MPam mm mm N/mm MPam N/mm MPam ASTM ISO

P91-1-C(T)-2 20 217,4 0,21 0,33 113,9 165,0 0,27 0,44 334,1 282,5 11 3

P91-1-C(T)-3 200 207,0 0,43 0,58 121,0 165,9 0,63 0,79 260,5 243,4 5 3

P91-1-C(T)-4 200 207,0 0,43 1,07 150,0 184,7 0,65 1,29 307,9 264,6 5 3

P91-1-C(T)-5 20 217,4 0,22 0,86 55,5 115,2 0,24 0,97 522,6 353,3 11 3

P91-1-C(T)-6 20 217,4 0,23 0,51 75,6 134,4 0,26 0,63 288,9 262,7 11 3

P91-SE(B)-1 22 217,3 0,00 -0,05 168,5 200,6 -0,01 -0,01 324,1 278,2 7, 10 3, 5, 7

P91-SE(B)-2 22 217,3 0,00 -0,05 170,9 202,0 0,02 0,00 327,8 279,8 7, 10, 11 1, 3, 5, 7

P91-SE(B)-3 200 207,0 0,00 0,08 149,5 184,4 0,06 0,16 247,0 237,0 7, 11 1, 3, 5, 7

P91-SE(B)-4 200 207,0 0,00 0,04 135,4 175,5 0,11 0,16 235,4 231,4 7 3

P91-SE(B)-12 100 212,8 0,00 -0,02 149,5 187,0 0,00 0,02 295,2 262,7 7,10,11 1.3.5.7

P91-SE(B)-13 100 212,8 0,00 0,00 167,2 197,7 0,00 0,04 295,8 263,0 7,10,11 1,3,5,7

P91-SE(B)-14 -50 221,4 0,00 -0,06 162,1 198,6 -0,01 -0,05 272,4 257,5 4,7,11 1,5,7

P91-SE(B)-15 -50 221,4 0,00 -0,04 174,8 206,3 -0,02 -0,03 303,6 271,8 7 1,3,5,7

P91-0.5T-C(T)-9 22 217,3 0,06 -0,08 193,8 215,1 0,06 -0,05 401,0 309,4 3

P91-0.5T-C(T)-10 22 217,3 0,06 -0,06 205,6 221,6 0,06 -0,03 425,3 318,7 3

P91-0.5T-C(T)-11 200 207,0 0,22 0,14 182,1 203,5 0,31 0,25 359,8 286,0 3

P91-0.5T-C(T)-12 200 207,0 0,17 0,07 180,2 202,4 0,27 0,15 344,8 280,0 280,0 3 3

Specimen

ISO 12135-02 ASTM E1820-08T E-Modul

Tab. 3 Qualification of measured J-R curves according to validity criteria of ASTM E1820 and ISO 12135

No. ISO validity criteria ASTM validity criteria

1 Is J0.2BL < Jmax ? Number of point to determine aoq > 8

2 A minimum of six data points are used to define R-Curve?

Of all the points used for aoq at least 3 lie between 0.4 JQ and JQ

3 Is α > 0? Correlation coefficient of aoq fit >= 0.96

4 Is β > 0? Power coefficient C2 < 1.0

5 40 J0.2BL/(Rp0.2+Rm) ≤ a0 aoq lies between a0 ± 0.01W or a0 ± 0.5 mm

6 40 J0.2BL/(Rp0.2+Rm) ≤ B At least one point is between 0.15 mm and 0.5 mm lines

7 40 J0.2BL/(Rp0.2+Rm) ≤ (W-a0) At least one point is between 0.5 mm and 1.5 mm lines

8 3.75 Rm > 2(dJ/da)0.2BL At least five point is between Δamin and Δalimit lines

9 At least one datum point between 0.1 mm and 0.3 mm lines

Thickness B > 10 JQ / σY

10 At least two data points between 0.1 mm and 0.5 mm lines

Initial ligament, b0 > 10 JQ / σY

11 At least 1 point in all of the four equal crack extension regions

Regression line slope, dJ/da at ΔaQ < σY

12 Estimated a0/W is within 2% of measured initial a0/W ?

Δap validity check

13 Δap validity check


The ∆a values of the 0.4T-SE(B) specimen were crack length corrected as described elsewhere [12]. The

application of the correction procedure led to a good agreement between the calculated and measured crack

length. As demonstrated in Tab. 4 and graphically depicted in Fig. 2 the correction of the crack length has a

remarkable influence on the final crack length and the evaluated fracture toughness values for ductile crack

initiation.

Tab. 4 The influence of the correction procedure of the crack length

J0.2/BL(25.4) J0.2/BL(25.4) JQ JQ

N/mm N/mm N/mm N/mm

°C corrected non-corrected corrected non-corrected

P91-SE(B)-1 22 168,5 317,4 324,1 x

P91-SE(B)-2 22 170,9 355,9 327,8 x

P91-SE(B)-3 200 149,5 296,3 247,0 x

P91-SE(B)-4 200 135,4 268,5 235,4 363,9

P91-SE(B)-12 100 149,5 308,7 295,2 x

P91-SE(B)-13 100 167,2 329,9 295,8 x

P91-SE(B)-14 -50 162,1 334,7 272,4 x

P91-SE(B)-15 -50 174,8 338,0 303,6 471,7

SpecimenT

ISO 12135-02 ASTM E1820-08

Fig. 2 ISO evaluation – comparison J-R curves for SE(B) with and without corrected procedure for crack

length calculation for Charpy-size specimen

The fracture toughness values vs. temperature behavior can be seen in the Fig. 3. The J-R curves of all

tested geometries are collected in Fig. 4 and Fig. 5. The difference in the trend of the J-R curves evaluated

according to ASTM and ISO are depicted in Fig. 6.


Fig. 3 Fracture toughness values, ASTM and ISO evaluation

Fig. 4 Overview of J-R curves according to ASTM evaluation

Fig. 5 Overview of J-R curves according to ISO evaluation


Fig. 6 J-R curves tested at 20°C, comparison of ASTM and ISO evaluation

Specimens were investigated by SEM to determinate to assess the fracture appearance. For SEM

investigation see Fig. 7 a) and Fig. 7 b) as an example. In addition 3D pictures were made for specimens

1T-C(T)_06 tested at 20°C and 0.5-C(T)_09 tested at 20°C, see Fig. 8.

Generally, the fracture appearance shows:

1. Fatigue crack

2. First stretching

3. Ductile edges (small ductile seams)

4. Second stretching

5. Main ductile crack extension

a) b)

Fig. 7 Stretch zone widths; a) specimen 1T-C(T)_06 tested at 20°C

b) specimen 1T-C(T)_03 tested at 200°C


Fig. 8 3D picture of the fractured surface of the specimen 1T-C(T)_06 tested at 20°C (please use

polarization glasses)

The 1T-C(T) and the Charpy size SE(B) specimens tested at 20°C show a shallow slope at the beginning of

J-R curves. This shallow slope could be caused either by accuracy of measurement or the fracture behavior.

The left picture of Fig. 9 shows the first part of the J-R curve measured on specimen 1T C(T)_06 tested at

20°C. The shallow slope of the J-∆a data is reflected in the fracture appearance of the SEM pictures (a)

b)

Fig. 7a) and Fig. 8) as multiple stretching at the beginning. Multiple stretching means stretching after the

fatigue crack, small parts of ductile crack extension and continuation of the stretching till the ductile crack

extension. The two parts of the stretch zone are marked in the J-R curve of specimen 1T-C(T)-6 (left picture

in Fig. 9). The total stretch zone amounts 614 µm. However, at higher test temperatures this shallow slope is

not visible in the J-R curves and the fractured surfaces show remarkable smaller stretched zones (see

Fig. 7 b).

Fig. 9 Picture comparison with J-R curve, 1T-C(T)_06 tested at 20°C

This shallow slope has strong influence on J0,2BL determined according to ISO 12135, whereas it is smaller

for JIC determined according to ASTM E1820 because of the compensation by a0q correction procedure.

Fracture toughness evaluated according to ASTM decrease with higher temperature in all cases. However,

371µm

242µm

614µm


fracture toughness evaluated according to ISO decrease only in 0.5-C(T) and SE(B) specimen geometry but

the trend of the fracture toughness for 1T-C(T) geometry is opposite, see Fig. 3.

4. CONCLUSION

The heat resistant steel P91 was characterized by fracture mechanics. The focus was on the measurement

of J-∆a curves and the determination of fracture toughness values at ductile crack initiation according to the

test standards ASTM E1820 and ISO 12135. It was shown that the accuracy of the measurement of the

actual crack length by using the unloading compliance method strongly influences the engineering fracture

toughness values. In addition the structure of the investigated P91 steel influences the blunting of the loaded

crack. Following findings were made so far:

The fracture toughness values strongly scatter.

There is a special characteristic of the blunting appearance causing a shallow slope of the J-∆a

values, which strongly influences the crack initiation fracture toughness values. That let to low

fracture toughness values at test temperatures below 100°C.

Compared to 0.5T-C(T) and 0.4T-SE(B) specimens a different trend of the J-∆a values for 1T-C(T)

specimens was measured, in general opposite to [10].

Both test standards are designed to provide engineering fracture toughness for ductile crack initiation.

Nevertheless, the evaluated fracture toughness values differ strongly.

ACKNOWLEDGEMENTS

This work was done within the work on the projects FR-TI2/279 and CZ.1.07/2.3.00/20.0038

LITERATURE

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Fracture Resistance Curves and Engineering Applications, ASTM STP 803, C. F. Shih, and J. P. Gudas, Eds.,

ASTM International, West Conshohocken, PA, 1983, p. 278.

[2] Jones R. L., Gordon J. R., and Challenger N. V., "A Study of Specimen Size on J-R Curve Behaviour," Fatigue

Fract Eng M, Vol. 14, No. 7, 1991, pp. 777-788.

[3] Doig P, Smith R. F., Flewitt P. E. J., “The use of stretch zone width measurements in the determination of fracture

toughness of low strength steels,” Eng Fract Mech, Vol. 19, 1984, pp. 653–664.

[4] ASTM Standard E1820-119: Standard Test Method for Measurement of Fracture Toughness, Annual Book of

ASTM Standards, ASTM International, West Conshohocken, PA, 2008.

[5] ISO Standard 12135, Unified Method of Test for the Detremination of Quasistatic Fracture Toughness, Geneva,

Switzerland, 2002

[6] Ross E., “Anforderungen an werkstoffkennwerte für die bauteilauslegung bei statischer Beanspruchung und

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ISBN: 0-9552994-6-2.

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West Conshohocken, PA: ASTM International; 2007.


[12] Džugan, J, Corrected Procedure for Crack Length Calculation by the Unloading Compliance Technique for

Charpy-sized Specimens, Journal of Testing and Evaluation, Nov. 2005

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