SVF-67 CREEP DEFORMATION, CREEP DAMAGE ACCUMULATION AND RESIDUAL LIFE ...

71 Pages·2005·0.84 MB·English

SVF-67
CREEP DEFORMATION, CREEP DAMAGE
ACCUMULATION AND RESIDUAL LIFE
PREDICTION FOR THREE LOW ALLOYED
CrMo-STEELS
Anatoly Kondyr, Rolf Sandström and
Andars Samualawn
StudrWk, Nyköping, Fabruari 1979
Järnkontorets Varmhållfasthets-
kommitté. Projekt 56/1977.
Stiftelsen för Värmeteknisk
Forskning. Projekt 69.
"Förutsagning av återstående
livslängd hos varmhållfasta
material.
VHK-rapport nr 116» aug 1978
CREEP DEFORMATION, CREEP DAMAGE ACC..*I)
LATION AND RESIDUAL LIFE PREDICTIU' ^O
THREE LOW ALLOYED CrMo-STEELS
Anatoly Kondyr, Rolf Sandström •'• d
Anders Samuelsson
IM-1336 August 1T 8
AK/RS/AS/KJ/HA
CREEP DEFORMATION. CREEP DAMAGE ACCUMULATION AND
RESIDUAL LIFE PREDICTION FOR THREE LOW ALLOYED
CrMo-STEELS
Anatoly Kondyrx/r, Rolf Sandström , and Anders
Samuelsson
Swedish Institute fcr Metals Research
Drottning Kristinas väg 48
S-114 28 Stockholm, Sweden
SUMMARY
A detailed analysis of creep strain results for
three low alloyed steels of type 0,51*10, 1Cr-0.5Mo
and 2.25Cr-1Mo has been undertaken. The results
show that, excluding the primary stage, the true
strain rate can be described by a simple analytical
expression
e • A exp (Be)
where A and B are constants at constant stress and
temperature. A is approximately equal to the minimum
strain rate and B inversly proportional to the
fracture strain. Furthermore, 1/AB equals the time
t to rupture.
r
The residual life fraction in creep can be expressed
as
exp (-Be) = 1-t/tr
and a creep Jamage function y is introduced as
V * 1-ABt
x/ Presently at Lvov Polytechnical Institute,
Lvov, USSR
The expressions for strain rate and damage are
shown to be a special case of the Rabotnov-
Kachanov equations.
The analysis has been generalized to account for
multiaxial stress states, and as an example creep
in a tube with internal pressure is considered.
CONTENTS
Page
1 INTRODUCTION 1
2 MATERIALS
3 ANALYSIS OF DATA 5
3.1 Method of analysis 5
3.2 Prediction of residual life 11
3.3 The damage function 12
MULTIAXIAL STRESS STATES 15
4.1 Generalization of the creep 15
rate and creep damage equations
4.2 Example of creep under multi- 18
axial stresses. A tube with
internal pressure.
5 CONCLUSIONS 21
6 ACKNOWLEDGEMENTS 24
REFERENCES
TABLES
FIGURES
1 INTRODUCTION
Knowledge of the maximum creep strain that can
be safely allowed for a particular material is
of great practical and commercial interest. To
obtain this knowledge it is essential to be able
to describe the strain dependence of the tertiary
stage. Such knowledge is also a basic requirement
in methods for determining the residual lifetime
of creep deformed components based on creep strain
observations. Recently the value of such methods
has been emphasized (1}.
In order to account for the deterioration of a
material during creep, and to explain the shape
of the tertiary stage, the concept of creep
damage was introduced by Kachanov (2, 3). Kachanov's
theories have been further developed by Rabotnov (4).
The Rabotnov-Kachanov equations account for the
influence of damage on creep rate, and also
describes the rate of damage accumulation. The
theory is phenomenological and contains a number
of constants which are to be determined by compa-
rison to experimental results.
The Rabotnov-Kachanov equations deal with the
case of uniaxial creep, but have been generalized
to multiaxial stress states by Leckie and Hayhurst
(5,6).
The object of this work has been to find simple
analytical expressions for strain and strain rate
in the secondary and tertiary creep regions, and
to derive expressions for residu3l life time and
creep damage accumulation. These equations should
contain a low number of constants which can
easily be determined from experimental results.
2 MATERIALS
The materials studied in the present investigation
are three Ho- and Cr-fio-steels commonly used for
high temperature applications namely 0.5Ho. 1Cr-
0.5Mo and 2. 25Cr-1i"!a . Previously unpublished
creep strain data on these steels from Sandvik AB
and Stal Laval Turbin AB have been analysed. Chemical
composition, heat treatment, and mechanical properties
at room temperature of the investigated heats are
given in Tables 1 and 2.
The creep specimens had been machined from either
bar or tube material of commercial heats (Table 1).
The number of heats for each steel was: 0.5Mo -
two; ICr-O.Sf^o - three; 2.25Cr-Iflo - three.
The chemical composition of the steels was according
to standards in most cases. Two slight deviations
can be noticed, however. The two heats of 0.5f,o
steel have a Mo content which is slightly higher
than normal while the Mo content of heats 1 and 2
of the ICr-O.SMo steel is below the standardized
value.
For most heats the heat treatment consisted of
normalizing at 900-950°C for 15-30 mins and temper-
ing at 71G-770°C for 1-2 h (Table 1). One heat of
0.5Mo steel was annealed at J20°C. In most cases
the heats were air cooled. However, one heat of
1Cr-0.5Mo was oil quenched and in addition
tempered at such a low temperature as 640OC
(resulting in higher strength and lower elongation
at room temperature than for the other heats).
Identical values for the room temperature properties
have been given for the two heat treatments of the
0.5 Mo steel. It is likely that the mechanical
properties have only been determined for one heat
and both sets of values have therefore been put
in brackets in Table 2. Any systematic influence
of composition on room temperature properties is
difficult to detect using only the presented results.
The creep rupture properties of the analyzed steels
are presented in Table 3 and figs. 1-3. Rupture
stress, elongation, and area reduction are plotted
versus rupture time. The influence of the different
heat treatments on the creep rupture properties is
not very large. However, it seems that the annealed
condition of 0.5Mo and the oil quenched condition
of 1Cr-0.5Mo give slightly lower and higher
creep strength respectively and higher respectively
lower ductility than the normalized and tempered
condition. No systematic influence of composition
can be detected.
ANALYSIS OF DATA
3.1 Method of analysis
The creep data used for the investigation were
obtained in the form of graphs or tables of
engineering strain versus time. These were kindly
supplied by Sandvik AB and Stal-Laval Turbin AB.
and were the result of constant load testing in
the temperature interval 500°C to 60C°C with initial
loads ranging from 49 to 324 N/mm^. Only specimens
with times to rupture larger than 10^h were used
for the analysis.
The first step in tr.e analysis was to redraw the
strain-time curves obtained to curves of true strain
versus time. This was dona by using the relation
c = In (1 *6)
where e is true strain and 6 engineering strain.
The true strain rate e was then graphically obtained
from these curvr^, ->.r>d plots of log (true strain
rate) as a function of true strain were made, figs.
4-11. The slopes of these curves are approximately
the same for specimens of the same heat.
From these curves two interesting observations can
be made. One is that the secondary creep region,
i.e. the stage with constant strain rate, is very
short or in most cases non-existent. The often
stated claim that secondary creep can account for
a large portion of the total strain of these
materials (see e.g. ref. 1) is difficult to ju«;Lify
even though the creep strain versus time curves
can look quite straight over appreciable time intervals