1310001-9898-ijet-ijens

International Journal of Engineering & Technology IJET-IJENS Vol:13 No:01 86

1310001-9898-IJET-IJENS © February 2013 IJENS I J E N S

Abstract— This work investigated the elevated temperature

low cycle fatigue (LCF) properties of GX12CrMoVNbN9–1

(GP91) cast steel. Fatigue tests were performed for five levels of

total strain amplitude ξac and temperature of 400, 550 and

600oC. In addition, the preliminary results from tensile test

were presented. Strong cyclic softening was observed in all

fatigue tests at elevated temperatures without stabilization

period of loop parameters. The plastic strain amplitudes during

cyclic strain loading were measured and correlated with the

fatigue lifetime using Coffin – Mason – Basquin plots at each

test temperature. The fatigue lifetime decreased as the

temperature test increased. The temperature effect on l ifetime

was more pronounced at low strain amplitudes.

Index Term— Fatigue, Mechanical properties, Lifetime

estimation, Cast.

I. INTRODUCTION

Thermal – mechanical fatigue occurring with the participation

of elastic – plastic strains is the basic mechanism of damage in

many elements serviced at elevated temperatures. These

elements include for example: electric power boilers, boiler

pipes, superheaters, engine elements. Temperatures of service

for the steels, of which these elements are made, reach 600 0C.

The basis for forecasting the fatigue lifetime of such elements

is the knowledge of low cycle properties of these materials,

determined at elevated temperatures. Material characteristics

are most often determined for the so-called period of

stabilization of cyclic properties. If this period does not occur,

they are determined from the period corresponding to half the

fatigue lifetime. Therefore, they do not take account of mutual

interactions of stress and temperature appearing during low

cycle fatigue and their influence on the course of low cycle

properties. This is the reason why the results of calculations

and tests of fatigue lifetime of construction elements subject to

changing load at elevated temperatures are characterized by a

considerable scatter [1], [2]. The required reliability, necessary

This paper was realized in the framework of the grant No.

1215/B/T02/2011/40 funded by Ministry of Science and Higher

Education in the years 2011-2013.Stanisław Mroziński is with the

University of Technology and Life Sciences in Bydgoszcz, Kaliskiego 7,

85-791 Bydgoszcz, Poland corresponding author (phone:+48 52 340-82-

64; fax:+48 52 340-82-71; e-mail: [email protected]).

Grzegorz Golański is with the Institute of Materials Engineering,

Czestochowa University of Technology, Armii Krajowej 19, 42–200

Czestochowa, Poland (e-mail:[email protected]).

for these elements, is mostly achieved through selection of

adequately high factor of safety.

The fundamental purpose of this study is to determine the

influence of elevated temperature on low cycle properties of a

cast steel. Additional aim is experimental verification at

elevated temperature of the analytical models used for the

description of low cycle properties of steels at ambient

temperature.

II. EXPERIMENTAL PROCEDURE

The research material was high-chromium GX12CrMoVNbN9-1

(GP91) cast steel of the following chemical composition

(%mass): 0.12C; 0.47Mn; 0.31Si; 0.014P; 0.004S; 8.22Cr;

0.90Mo; 0.12V; 0.07Nb; 0.04N. The investigated GP91 cast steel

was after heat treatment (as-received condition) with the

following parameters of temperature and time: 1040oC/12h/oil +

760oC/12h/air + 750

oC/8h/furnace. The influence of heat

treatment parameters on the properties and microstructure of

the examined cast steel is presented inter alia in the work [3].

Low cycle tests were performed using testing machine, the 8502

Instron type, with strain control (ac = const). The tests were

carried out at elevated temperature: 400, 550 and 600oC. Fatigue

tests were preceded by the static test of tension run at the

abovementioned temperatures. The test samples prepared for

research were round and threaded (Fig. 1). Fatigue tests as well

as the static test of tension at elevated temperature were

realized using the heating chamber. The test pieces were

resistance-heated, the temperature of test pieces was controlled

using thermoelements Pt – Rh/Pt. Loading applied during the

tests was oscillating sinusoidally with the strain ratio R = - 1.

The tests were carried out at five levels of total strain amplitude

ac: 0.25; 0.30; 0.35; 0.50 and 0.60%.

The frequency of load change f during the tests amounted to

0.2Hz. Assumed as the criterion for the end of a fatigue test and

at the same time the fatigue lifetime Nf at a given strain level,

was the number of cycles N at which the occurrence of

deformation on the hysteresis loop arm in the compression

half-cycle was observed. The analysis of fatigue properties of

GP91 cast steel under the conditions of changing loads was

performed using the parameters of hysteresis loop which

included: total strain amplitude - ac, plastic strain amplitude -

ap, elastic strain amplitude - ae, stress amplitude -a. Their

values were determined on the basis of the values of loading

force and strains recorded during the fatigue tests.

On the basis of the recorded values of stress amplitude a in

the following stress cycles, the graphs of changes in the

Elevated Temperature Low Cycle Fatigue

Properties of Martensitic Cast Steel Stanisław Mroziński, Grzegorz Golański



characteristic hysteresis loop parameters were plotted, as the

function of the number of stress cycles N. The characteristic

quantities for the loop, determined for half the number of cycles

to failure (N/Nf=0.5), were used while preparing the basic

fatigue characteristics of the cast steel. The obtained data were

also used to determine the slopes of regression lines applied

for the description of dependence between stress a and strain

ap (Manson’s criterion).

III. RESEARCH RESULT AND ANALYSIS

A. Initial microstructure

The investigated cast steel in the as -received condition (after

heat treatment) was characterized by the microstructure of

high-temperature tempered martensite with elongated

subgrains whose shape was inherited from the lath martensite

with numerous precipitations. On the boundaries of prior

austenite grain and on the boundaries of subgrains, M 23C6

carbides were precipitated. Inside the subgrains, many

precipitations of the MX type were observed. Such a

microstructure is a typical microstructure of quenched and

tempered 9÷12%Cr steels [4], [5]. Detailed information on the

microstructure of high-chromium steels/cast steels is provided

in the work [3], [6]. Example of the microstructure of the

examined cast steel in the as-received condition is presented in

Fig. 2.

B. Tensile properties at elevated temperatures

The low cycle fatigue tests of GP91 cast steel in the as -

received condition were preceded by the static test of tension

at elevated temperature. Table I includes the results obtained

from the test of mechanical properties.

An increase in the temperature of testing leads to a decrease

in yield strength (YS) from the level of 419 MPa to 303 MPa, as

the temperature of testing increases from 400 to 600oC. A similar

dependence was observed for tensile strength (TS), where a

significant decrease from 536 MPa to 338 MPa could be

noticed. A decrease in the values of strength properties was

accompanied by the growth of the values of plastic properties

– elongation and reduction of area.

C. LCF properties at elevated temperatures

The tests carried out have proved that the process of low cycle

fatigue of GP91 cast steel is characterized by strong cyclic

softening (an increase in the width of hysteresis loop ap

and a strong decrease in the stress amplitude a), whose

intensity grows along with the growth of temperature of the

fatigue test. Regardless of the fatigue test temperature, there

was no period of stabilization of the hysteresis loop parameters

observed in the following stages of cyclic strain. Cyclic

softening of the examined cast steel continued until the

occurrence of a crack in the test piece, which proves cyclic

exhausting of fatigue lifetime of the cast steel. Example of

changes in the hysteresis loop parameter – stress a, as the

function of the number of cycles N for three levels of strain ac,

is presented in Fig. 3.

Cyclic softening that occurred during low cycle fatigue was

also observed in high-temperature creep resisting martensitic

steels of 9÷12%Cr grade. In these steels however, contrary to

the investigated cast steel, there was a clear period of

stabilization of the hysteresis loop parameters observable –

stress amplitude a and strain amplitude ac [7].

Due to the lack of a clear period of stabilization of the

hysteresis loop, analytical description of fatigue properties of

the examined cast steel is considerably difficult. Considering

the changes observed in the parameters of hysteresis loop, in

the function of the number of stress cycles, the values of

hysteresis loop parameters necessary for analytical

descriptions of characteristics of the examined cast steel were

determined for the number of cycles N corresponding to

Fig. 1. Test sample for the low cycle fatigue tests

Fig. 2. Microstructure of tempered martensite of GX12CrMoVNbN9-1

cast steel in the as-received condition (after heat treatment ), SEM,

etched with ferric chloride

T ABLE I

Mechanical properties of GX12CrMoVNbN9 – 1 cast steel at

elevated temperature

Temperature oC

YS

MPa

TS

MPa

El.

%

RA

% E

MPa

400 419 536 29 49 182100

550 395 339 47 83 161460

600 303 338 64 87 146200



0.5Nf (points 1, 2 and 3 in Fig. 3). Examples of hysteresis loop

obtained at three levels of strain for this number of stress

cycles is shown in Fig. 4.

On the basis of comparative analysis of hysteresis loops

obtained at the tested temperatures for five values of strain

(Fig. 3 and 4), it can be concluded that the temperature has an

influence on two basic parameters of the loop, i.e. εap (width

of the loop) and σ (height of the loop). For the same values of

strain, along with the temperature growth, the loop width

increases and the loop height decreases. For the analytical description of dependence between stress

σa and strain εap, Morrow’s equation was applied (1):

apa lg'n'Klglg (1)

where: K’- cyclic strain hardening coefficient, MPa; n’- cyclic

strain hardening exponent.

The graphs obtained as a result of approximation of hysteresis

loop parameters (a and εap) from the periods corresponding to

half the fatigue lifetime (N/Nf =0.5) are shown in Fig. 5. While

Table II includes the values of parameters of Morrow’s

equation (n’ and K’).

Mathematical model of cyclic softening of GP91 cast steel,

described with Morrow’s equation (1), is given in Table II.

Fig. 3. Influence of temperature on the changes in stress a: a) ac=0.25%, b) ac=0.35%, c) ac=0.60%

a) b) c)

Fig. 4. Influence of the temperature of fatigue test on the loop shape: a) ac=0.25%, b) ac=0.35%, c) ac=0.60%

Fig. 5. Influence of temperature on the characteristics of cyclic strain of

GP91 cast steel



Growth of the temperature of fatigue test leads to an evident

decrease in the value of coefficient K’ and increase in the value

of hardening coefficient n’ for the temperature of 600ºC and its

decrease at the temperature of 550ºC.

The values of parameters n’ and K’ – the basic material

characteristics used during the calculations of low cycle loads

– in the case of materials not revealing the period of

stabilization, depend on the period of lifetime (N/Nf), for which

they were determined. In the work [8], [9] it has been shown

that the values of parameters n’ and K’ depend on the number

of cycles of changing loads for which they were determined.

The values of these parameters determined at half the lifetime

(N/Nf =0.5) are not the average values for the whole fatigue

test, which in the Authors’ view shows the scale of

simplification and can lead to considerable errors . This remark

gains particular meaning when describing the fatigue

properties of a cast steel at elevated temperatures in which the

range of changes in these properties is bigger, compared to

ambient temperatures.

Fatigue lifetime of the investigated cast steel is described

using the equation of Manson-Coffin-Basquin (MCB) (2).

f

N2'f

b

fN2

E2

ap

2

ae

2

ac'

f

(2)

where: b - fatigue strength exponent;

c - fatigue ductility exponent;

f’- fatigue strength coefficient, MPa;

f’ - fatigue ductility coefficient;

E - Young’s modulus, MPa.

For graphic illustration of the influence of temperature on

lifetime, Fig. 6 shows the obtained results in the form of fatigue

graphs, whilst Table III includes the parameters of MCB

equation (2).

Performed analysis of the obtained characteristics (Fig. 6)

shows that the abscissa 2Nt, the point of intersection of two

curves: ae=f(2Nf) and ap=f(2Nf), in the analyzed cases,

amounts to 4620 and around 5700 cycles, respectively, for the

temperature of 400oC and for temperatures of 550

oC and 600

oC.

This proves that with the values of total strain ac applied in

fatigue tests, the process of cyclic s train in the examined cast

steel for all temperatures ran with the dominant role of plastic

strain component ap.

Therefore, it can be assumed that for these strain levels ac the

cyclic strain resistance of the investigated cast steel mostly

depends on its plastic properties. Similar dependence was also

observed in the case of high-temperature creep resisting

martensitic steels of the P91 and P92 type [10], as well as HB20

–type cast stainless steel [11], however, the point of

intersection Nt for these steels at room temperature amounted

to about 1000 cycles, while for the HB20 steel at the

temperature of 600oC – 4012 cycles.

Analysis of the performed fatigue graphs (Fig. 6) obtained at

room temperature and elevated temperature allows to state that

the influence of temperature on lifetime depends on the level of

total strain amplitude. This influence is slight in the area of the

largest strains realized in the research (ac=0,60%) and

increases as the value of strain ac falls (Fig. 7).

Characteristics of low cycle fatigue of the examined cast steel

is provided in Table IV. Due to the lack of a clear stabilization

period of the fatigue characteristics (Fig. 3), the value of stress

a was determined from the period corresponding to half the

fatigue lifetime (N/Nf=0.5) [12], [13].

T ABLE II

Functions describing the course of cyclic strain of

GP91 cast steel, described with Morrow’s equation (1)

Temperature, oC

Regression function and correlation

coefficient

lga = lgK’ + n’ lgεap;

n’ – cyclic strain hardening exponent;

K’ – cyclic strain hardening coefficient.

400 lga = lg763 + 0.0956lgεap; R2= 0.97

550 lga = lg416 + 0.0743lgεap; R2= 0.96

600 lga = lg496 + 0.1384lgεap; R2= 0.96

Fig. 6. Low cycle fatigue life of GP91 cast steel at 400 and 600

oC

temperature

T ABLE III

Mathematical model of fatigue life of GP91 cast steel

Fatigue

strength

coefficient

’f

Fatigue

ductility

coefficient

’f

Fatigue

strength

exponent

b

Fatigue

ductility

exponent

c

Number of

strain

reversals

2Nt

ºC MPa - - - -

400 659 0.346 - 0.060 - 0.585 4620

550 526 0.814 - 0.055 - 0.728 5717

600 248 2.210 - 0.022 - 0.851 5700



Increase in the temperature of testing in the scope of low

cycle fatigue, apart from reducing the fatigue lifetime of GP91

cast steel, also contributes to the straining of the investigated

material while the values of stress are lower and lower σa (Table

IV). This has an influence on the reduction of low-cycle stress

transfer capacity, along with the temperature growth.

During the tests, regardless of the temperature, the changes in

fatigue characteristics of cast steel were observed (changes in

the hysteresis loops parameters). The changes in the loops

parameters as the function of a number of stress cycles and the

lack of a clear stabilization period, make it difficult to determine

the representative material data, used further for the

calculations of fatigue life. The results obtained while testing

the cast steel confirm the results included inter alia in the work

[2], where the difficulties in determining the period of

stabilization of cast steel at elevated temperature as well as the

influence of the extent of fatigue damage on material data were

signaled.

The changes in fatigue characteristics of steels/cast steels

occurring at elevated temperatures are the reason why

calculating the fatigue life of construction elements serviced at

elevated temperatures using material data, determined e.g. at

half the fatigue life, raise doubts. This data reflect only the

instantaneous properties of the material. The work [14],

proposes a method of calculating fatigue life that considers the

changes in cyclic properties occurring during loading. A new

calculation method requires special analysis of the results of

low cycle fatigue tests. The proposal of method for analyzing

the test results is provided inter alia in the work [8].

IV. CONCLUSION

1. Martensitic GX12CrMoVNbN9-1 cast steel during low

cycle fatigue at elevated temperatures of 400, 550 and

600C is subject to cyclic softening and does not

reveal a clear period of stabilization.

2. The extent of changes in cyclic properties is

influenced by the level of strain ac and the

temperature. At the temperature of 600C, the extent

of changes in cyclic properties is definitely higher

than at the temperature of 400C. At both

temperatures, the extent of changes in fatigue

properties decreases along with the growth of total

strain ac.

3. Fatigue lifetime of martensitic cast steel is influenced

by the level of strain ac,, as well as the temperature of

testing. Influence of the temperature on fatigue

lifetime depends on the level of strain. It is slight in

the area of very large strains and increases as the

level of strain falls.

4. The changes appearing in the parameters of

hysteresis loop and the lack of clear stabilization

period at elevated temperatures of the examined cast

steel makes it difficult to determine the basic material

data. Their values depend on the period of fatigue

lifetime assumed to determine them. Assuming them

from the period corresponding to half the fatigue

lifetime makes them reflect only the instantaneous

cyclic properties of the cast steel from this period of

fatigue lifetime.

5. During the service of power plant facilities,

interactions connected with the changes in stress and

temperature occur. The tests presented in this paper

were carried out under the conditions of constant

amplitude stress and constant temperatures. In order

to formulate detailed conclusions on the fatigue

properties of the cast steel, further studies should

take into consideration the changes in stress and

temperature occurring during the tests .

T ABLE IV

Fatigue characteristics of GP91 cast steel in the as-received condition

Strain

amplitude εac,

%

Temperature 400ºC Temperature 550ºC Temperature 600ºC

Number of

cycles to

failure Nf

Stress a

(N/Nf=0.5),

MPa

Number of

cycles to

failure Nf

Stress a

(N/Nf=0.5),

MPa

Number of

cycles to

failure Nf

Stress a

(N/Nf=0.5),

MPa

0.25 8990 319 4533 249 3548 193

0.30 4400 336 3340 257 2505 206

0.35 3001 347 1819 261 1945 208

0.50 955 371 889 273 947 237

0.60 855 378 773 278 683 236

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

Nu

mb

er

of

cy

cle

s t

o f

ailu

re N

f

0.35 0.30 0.35 0.50 0.60

Strain ac, %

400

550

600

Fig. 7. Low cycle fatigue life of GP91 cast steel in the as-received

condition: 400 – at the temperature of 400ºC; 550 – at the

temperature of 550ºC; 600 – at the temperature of 600ºC



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[4] A. Zielińska – Lipiec, T . Kozieł and A. Czyrska – Filemonowicz,

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[6] F. Abe, ―Bainitic and martensitic creep-resistant steels‖, Current

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P92 microstructural stability during fatigue tests at high

temperature‖, Procedia Engineering, vol. 2, 2010, pp. 2141–2150

[8] S. Mroziński and A. Lipski, ―Method of low-cycle fatigue test

results processing‖, Physicochemical Mechanics of Materials, vol.

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analytical description of cyclic properties of martensitic cast

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Polish)

Stanisław Mroziński was born on 7th May 1961 in Bydgoszcz

(Poland). In 1986 he graduated from University of Technology and Life

Sciences (the UTP) in Bydgoszcz, Faculty of Mechanical Engineering.

Since 1986 he has been employed in the UTP Faculty of Mechanical

Engineering, where he got the following academic degrees: PhD degree in

technical sciences (1995), post -doctoral degree in technical sciences

(2008).

Since 2010 he has been employed as an associate professor of the

UTP. In the years 2009-2012 he performed the function of a deputy

dean in the Faculty of Mechanical Engineering for organization and

development issues. He is an author or co-author of around 60 articles

and scientific papers presented in conferences and seminars, home and

abroad. At present he manages a research laboratory accredited in 2001

by the Polish Centre for Accreditation. His research work deals with the

issues of fatigue of materials and constructions, as well as methods of

experimental study on the structure and operation of machines.

Since 2002 Stanisław Mroziński, Ph.D. (Eng) has been an expert

auditor of the Polish Centre for Accreditation, assessing the research

laboratories in Poland. He actively participates in the work of

associations acting in the university as well as in the country: Polish

Society of Mechanical Engineers and Technicians (SIMP), Polish

Society of Theoretical and Applied Mechanics (PTMTiS), European

Structural Integrity Society (ESIS). His achievements for the University

were awarded several t imes, for instance with state medals (Silver Cross

of Merit in 1996) and First -Class Rector Awards.

Grzegorz Golański was born on 7th October 1973 in Wieluń

(Poland). In 1998 he graduated from Czestochowa University of

Technology in the Faculty of Metallurgy and Materials Engineering.

Since 2001 he has been employed in this Faculty, first as an assistant ,

and next as an assistant professor. In 2003 he got a degree of PhD in

technical sciences in the field of materials engineering.

He is an author or co-author of around 110 articles and scientific

papers presented in conferences and seminars, home and abroad. Either

as a supervisor or executor, he was involved in realization of over 50

scientific research works connected with production problems. He took

part in the realization of 7 research projects (in two of them as a

supervisor). His research work deals with the issues related to the

processes of degradation of high-temperature creep resisting materials,

as well as methods of shaping of microstructure and properties of steels

and cast steels through heat treatment.

For his remarkable achievements in t he field of science he was

awarded several t imes, for instance with Rector Awards.

1310001-9898-ijet-ijens

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