MICROSTRUCTURE AND MECHANICAL PROPERTIES DEGRADATION confirmed the tested steel micro structure...

download MICROSTRUCTURE AND MECHANICAL PROPERTIES DEGRADATION confirmed the tested steel micro structure degradation after long time ... Microstructure and mechanical properties degradation

of 6

  • date post

    19-Mar-2018
  • Category

    Documents

  • view

    221
  • download

    5

Embed Size (px)

Transcript of MICROSTRUCTURE AND MECHANICAL PROPERTIES DEGRADATION confirmed the tested steel micro structure...

  • J. Miche, M. Burk, M. Vojtko: Microstructure and mechanical properties degradation of CrMo creep resistant steel operating under creep conditions

    Materials Engineering - Materilov ininierstvo 18 (2011) 57-62

    57

    MICROSTRUCTURE AND MECHANICAL

    PROPERTIES DEGRADATION OF CrMo CREEP

    RESISTANT STEEL OPERATING UNDER CREEP

    CONDITIONS

    Jn Miche1, Marin Burk1,*, Marek Vojtko1

    1Department of Materials Science, Faculty of Metallurgy, Technical University of Koice, Park Komenskho 11, 043 58 Koice, Slovak Republic

    *corresponding author: Tel.: +421 55 602 2776, Fax.: +421 55 602 2243, e-mail: marian.bursak@tuke.sk

    Resume In this contribution microstructure degradation of a steam tube is analysed. The tube is made of CrMo creep resistant steel and was in service under creep conditions at temperature 530C and calculated stress level in the tube wall 46.5 MPa. During service life in the steel gradual micro structure changes were observed, first pearlite spheroidization, precipitation, coagulation and precipitate coarsening. Despite the fact that there were evident changes in the micro structure the strength and deformation properties of the steel (Re, Rm, A5, Z), the resistance to brittle fracture and the creep strength limit, were near to unchanged after 2.1x105 hours in service. The steam tube is now in service more than 2.6x105 h.

    Available online: http://fstroj.uniza.sk/PDF/2011/10-2011.pdf

    Article info

    Article history:

    Received 13 July 2011 Accepted 27 July 2011 Online 28 July 2011 Keywords:

    Mechanical properties Creep resistant steel Creep properties Degradation ISSN 1335-0803

    1. Introduction

    Big power plant production capacities were built using CrMo and CrMoV steel grades. Their safe service life means high investment and production utilization and savings. That is why, such a high attention is paid to the monitoring and surveillance of their service conditions.

    During service life in creep conditions there is a gradual micro structure degradation of steel and this way some decrease of properties.

    The microstructure of creep resistant CrMo, CrMoV steel grades in the initial state with the common heat treatment of the used products (normalizing annealing followed by tempering) is not really the equilibrium state of the steel. Any thermal or mechanical influences are changing the microstructure in the direction to a higher level of equilibrium.

    Coagulation and coarsening of precipitates, carbides transformation, additional precipitation, and the evacuation of alloying elements from the matrix, are supposed to be the most detrimental processes [1,2,3,8,9]. Embrittlement, weakening of the micro structure and the final creep failure can be the result. The degree and intensity of creep degradation depend on both, in service conditions (temperature, stress, environment) and exposition time [2,3,4,6,7]. It is very important to study and know the time dependence of the performance of steel in service conditions and this way survey the possibilities of service life increase.

    The aim of this contribution is to consider the extent of microstructure degradation, the influence on mechanical and brittle fracture properties and first of all on creep strength limit for the tested CrMo creep

  • J. Miche, M. Burk, M. Vojtko: Microstructure and mechanical properties degradation of CrMo creep resistant steel operating under creep conditions

    Materials Engineering - Materilov ininierstvo 18 (2011) 57-62

    58

    resistant steel. Service life up to 2.6x105 h or more was considered in the given conditions.

    2. Material and methods

    The experimental material was cut out from pieces of the steam tube 335,6x41 mm. The tube was in service at temperature 530C and calculated stress level in the tube wall 46.5 MPa for exposition times 1.02x105 h, 1.57x105 h, and 2.21x105 h. The steam tube was made of CrMo creep resistant steel (10CrMo9.10). Chemical composition of the tested steel is in Table 1.

    Table 1

    Chemical composition of the tested steel

    [in weight %]

    Material C Mn Si Cr Mo V

    CrMo

    Steel

    0.11

    0.13

    0.46

    0.48

    0.24

    0.27

    2.06

    2.19

    0.96

    1.02

    0.005

    0.02

    Ranges of chemistry are given in Table 1. The different cut outs were from different parts of the steam tube showing slight differences in element contents.

    From the tested steel tube test samples were machined in longitudinal direction for tensile tests, creep strength limit tests, hardness tests, and polished surfaces for microstructure evaluation.

    Creep strength limit was determined by creep tests at 530C. The stress values for the test were calculated to end tests with defined experimental time to fracture. Results were in the range from 5.102 to 5.104 hours.

    After service life 2.5x105 and 2.6x105 h samples were cut out from the critical place in the steam tube. The shape of the cut outs was a spherical cap 0.6 mm high and 8 mm in diameter. The samples were tested for changes in the microstructure and micro hardness HV0,05. Microstructure was analysed in polished surfaces from the original material as

    well as on the samples cut out after the listed service life times by light microscope OLYMPUS and electron microscope JEOL JSM 7000 F.

    3. Results and discussion

    During service life there were in the steel gradual micro structure changes (degradation of the initial microstructure). It was confirmed by the microstructure analyses of the tested steel. In Figure 1 to Figure 3 are documented the microstructures starting from the initial state up to service lives times 102000 h, 157000 h and 260000 h.

    Based on metallography a conclusion can be made the initial micro structure is ferrite-pearlite, in condition after normalization annealing and tempering (Figure 1).

    Fig. 1. Microstructure of tested steel, initial state,

    REM

    By this annealing and tempering the pearlite is spheroidized, making the microstructure more stable.

    After 102000 h in service, the micro structure has changed. It is still ferrite-pearlite, but by the long time exposition to high temperature the pearlite was spheroidized more (Figure 2).

    Pearlite spheroidization continued with the growth of service life time. For 157000 h exposition the pearlite was completely spheroidized.

  • J. Miche, M. Burk, M. Vojtko: Microstructure and mechanical properties degradation of CrMo creep resistant steel operating under creep conditions

    Materials Engineering - Materilov ininierstvo 18 (2011) 57-62

    59

    Fig. 2. Microstructure of tested steel after 102000 h

    service, magnification 400 x

    Microhardness in the pearlite grains was 168 HV0,05 and in ferrite 157 HV0,05. The process heading towards the equilibrium state continued and the microstructure had changed to a ferrite-carbide mixture. In the ferrite matrix there were only scattered carbides after 260000 h, as documented in Figure 3.

    Fig. 3. Microstructure of tested steel after 260000 h

    service time, REM

    In grain boundaries coarse carbide particles were segregated, forming network like patterns in some localities.

    A more detailed microstructure and phase analysis was completed by the means of electron microscopy (TEM and REM).

    In Figure 4 the initial state microstructure is documented in more detail. In the ferrite fine precipitates are shown and the pearlite grains are spheroidized partially.

    The precipitates in the ferrite are prevailingly carbide particles based on Mo (Mo2C) and Cr (Cr3C7) in elongated stick like

    forms. Also complex carbide particles were found based on (Fe,Cr,Mo)C and they were globular. The carbide particles in cementite were characterized as cementite with alloying elements (Fe,Cr,Mo)C. Similar particles were in the grain boundaries, too. Chemical composition of particles analysed by EDX in locations marked in Figure 4 is given in Table 2.

    Fig. 4. Sub microstructure of the tested steel - initial

    state, with EDX analysis spots locations marked

    [REM]

    Table 2

    Chemical composition of particles found in the

    matrix initial state [in weight %]

    Element Spectrum

    13

    Spectrum

    12 Spectrum 5

    C

    Cr

    Fe

    Mo

    4.54

    2.0

    91.83

    1.64

    4.9

    3.19

    91.08

    0.83

    6.27

    8.92

    79.9

    4.91

    Sub microstructure of the tested steel after service time 102000 h is documented in Figure 5. The number of precipitates in the ferrite matrix was increased if compared to the initial state. In majority they are Molybdenum Mo(Mo2C) and Chromium carbides Cr(Cr7C3), but carbides of a number of other elements were found, too. They can be characterized as alloyed cementite. In grain boundaries in larger numbers an different sizes were precipitated particles first based on Cr, though there were others, too. The characteristic features of the sub microstructure

  • J. Miche, M. Burk, M. Vojtko: Microstructure and mechanical properties degradation of CrMo creep resistant steel operating under creep conditions

    Materials Engineering - Materilov ininierstvo 18 (2011) 57-62

    60

    did not change too much after 221000 h in service. See Figure 6.

    Fig. 5. Sub microstructure of tested steel after

    102000 h in service, TEM

    Fig. 6. Sub microstructure of tested steel after

    221000 h service, TEM

    Selective electron microscope diffraction identified in the matrix particles (precipitates) of the kind M7C3 and M2C. Complex and larger carbide particle