DEGRADATION OF HEAT RESISTANT STEELS...

15. - 17. 5. 2013, Brno, Czech Republic, EU

DEGRADATION OF HEAT RESISTANT STEELS COMPONENTS OF POWER PLANT

Martin KRAUS, Jaroslav BYSTRIANSKÝ, Vratislav MAREŠ

VŠB-TU Ostrava, FMMI, RMTVC, 17. listopadu 15, 708 33 Ostrava, [email protected]

Abstract

Incidence of long term operating conditions with complex influence of many factors results in a properties

degradation of structural unit. These processes effect on all metals volume or surface only. And

influencing for example mechanical properties or materials decrease in various range. This paper deals

with materials degradation of tubular heat resistant steel components of the power plants pressure circuit

components after operational exposition range from 80 000 till 190 000 hours. A lot of types of destructive

and others testing methods and number of test for each of them were used to clarify of the extent and

mechanism of the damage these components. Changes of material properties were by those methods

evaluated in state after operational exposition and after heat reprocessing, always in the entire profile of

the wall thickness of structural part. Measured values of properties were compared with the assumed

properties of the non-degraded material in origin state. The specific problem is inhomogeneity (values

oscillation) by wall thickness and perimeter of tubular parts. Generally is possible to find, material failing

to satisfy several parameters of standardised required mechanical properties for new products from

mentioned steel.

Keywords: long-term exposition, degradation of properties, destructive testing, pressure circuit

1. INTRODUCTION

Materials and structural units are designed with a presumption of specific lifetime. For all this time

material should be satisfy the demands fully service requirements without danger of failure or destruction.

That is why informations, which were about actually state of the operating material and structural

components after their service life acquired, are very important [0, 0].

A damage of structural materials in conditions of the pressure circuit of power is a possible divided

according to a range of working solutions incidence. There are processes without prominent influence of

solution, as such rising as a voluminous incidences effect of stressors on metallic material (creep, fatigue,

etc.) – there is a possible quantitatively good [3, 4]. The second are processes with great solutions

influence. These is a possible describe only limited, because we do not know enters data (chemical

composition of working solution, etc.) – there are for example oxidizing or corrosion processes, hydrogen

influence and their interactions with a mechanical strain [0, 5]. For energetic equipment is to be

considered two different solution types - combustion products and steamwater [5, 6].

2. EXTRACTION OF TESTING MATERIAL FROM ZE STEAM POWER PLANTS EQUIPMENT

For the evaluation of materials properties power plants components, the material from shut-down thermal

power units was used. Components have been exposed to long-term real service loading. With service

time between 80 and 190 thousands hours and work temperatures 480°C to 550°C. This material was

liable to wide program of destructive testing. For testing were disposable cuts of heat-exchanging

surfaces as well as non-heated thick-walled components. They were working in undercreep conditions

[7], and creep conditions also. Units with which this article deals were exposure in creep conditions. And

were made from the steel 15128.5 (ČSN 41 5128) [8] Equivalents for this steel are 13MoCrV6 or EN

grade 1.7715. Materials (components) were assessed in various states. Most of them after long- term

exposure in a pressure circuit. Part of them was heat reprocessed aimed at refresh (at least partially)

degraded properties.

mailto:[email protected]


Available were cuts (tubes) of those thick-walled components (units) from pressure circuit:

- inlet / distribution super heated steam headers

- outlet / collecting super heated steam headers

- inlet / distribution reheated steam headers

- outlet / collecting reheated steam headers

- heat exchanging tube of steam super heaters

- super heated steam pipeline

- feed water headers

- out coming economizer headers

Cuts out from tubes, considered the thick-walled, were from the steel 15 128.5. Their dimensions in

nominal sizes varied: full diameter (D) from 250 mm to 521 mm; wall thickness from 25 mm to 56 mm. A

length of cutting part l = 300 – 400 mm. Consider carefully of position of test specimens with respect to

wide origin materials (after service loading) testing - state P was necessary in each of component cuts

(tube) or its lateral half. In that practically same testing volume was apply on the material after heat

reprocessing – state T. After detailed documentation of dimensions (Fig.1) the thickness of heat affected

zone (arisen by autogenous cutting by at the tube dividing) was examinated by the macroetch. HAZ is

surprisingly little (only in millimetres) with regard to tubes thickness, see Fig.2. Those layers (segments

A,B) were separated before testing. For each of tube cut exist cutting schemes (Fig.3) and testing

programmes [5].

Fig. 1 Parameters of Tube 40

Fig. 2 Macroetch – heat affected zone Fig. 3 The example of a cutting scheme


3. TESTING EXTENT

3.1 Chemical analysis

A chemical analysis for state P every time includes several measurements of a matrix on different

perimeter places. And after profile analysis in five levels of wall thickness was made (at two lateral cuts of

material minimally). Analysed were elements C, Mn, Si, P, S, Ni, Cr, Mo, V, Al, Sn, As, Sb, N.

3.2 Mechanical testing

Mechanical properties were monitored by following tests at shown parameters. Testing range was same

for state P and T.

Tensile test at room temperature according to ČSN EN ISO 6892-1 standard with extensometer at three

levels of wall thickness (three specimens for every level). Determined are ultimate strength, yield point,

elongation and reduction of area. Data record is used for determination of other deformation parameters.

Test specimens were round with threaded ends; diameter 10 mm and length of the reduced section min.

60 mm. Tensile tests at elevated temperature 600°C by ČSN EN ISO 10002 – 5 with same parameters

and specimens.

Notched bar impact tests at room and lowered temperatures according with ČSN EN ISO 10045 – 1 with

KCU 3 evaluation. At room temperature basic properties were taken, mainly variance of values in

connection with wall thickness level. The extraction of test specimens every time takes longitudinally and

transversally also. Again by three specimens at three levels.

For determination of transition temperature and transition curves results taken by testing at lower

temperatures are instrumental. Seven sets by three bars longitudinal and transversal were evaluated.

Results of those tests are next compared with values acquired on other specimen types (small punch

test, unnormalized test specimens) hereafter phase of research (out of this paper range).

Hardness HV 10 measurement on a lateral cut – in several (min. two) locations on a perimeter of each

tube. By three impacts at six wall thickness levels (segment P on Fig. 4).

A simplified example of test specimens location the Fig. 4 showed.

Fig. 4 The scheme example of test specimens location, segment P – HV10 measurement

3.3 Metalography

Metalographical analysis in longitudinal and transversal direction on a several segments. Always at all

wall thickness - for state P and T. Measurement of thickness of oxidic layer at all inside perimeter of a


tube - for state P. The schedule of heat reprocessing (state T) was determine as follows: 940 – 960 °C /

air cooling (thickness in mm = min on a temperature); following 680 °C , 15-20 min / air cooling. Selected

standardized properties for tubes from steel 15 128.5 are shown in the Table 1.

Table 1 Selected standardized properties for tubes from steel 15 128.5 by ČSN 41 5128

mech.properties at 20 °C mech.properties at 600 °C

Wall thick. (mm)

till 38 up 12;

D=70 -377 mm Temperature °C 600

min Re

(MPa) 365 430

The lowest yield point Rp0,2

(MPa) for tubes and forgings at standardized

lowest Re (MPa) at 20°C

315 < Re < 355 177

Rm

(MPa) 490 - 690 570 - 740 353 < Re < 430 181

A5 (%)

18 17

Re >430 216 KCU 3 longit.

(J.cm-2) 50 60

HB (inf.) 140 - 197 163 - 223

4. RESULTS AND DISCUSSION

In case of thick-walled components were evident in general these degradation trends:

- markant drift of transiting temperature,

- heterogeneity of mechanical properties by perimeter and wall thickness,

- structure decay; degree of desintegration and bainite volume depending on a position of wall

thickness; from almost compact bainitic blocks near outer surface to total crumbling of bainite near

inner surface),

- creep damage was evaluated at intervals grade 1 - starting creep without the occurrence of incipient

creep cavities to grade 2b – advanced creep with the occurrence numerous cavities (according with

VGB-TW 507) [9].

For documentation of some acquired results three components (tubes) with same full diameter and

different service hours pass were chosen (see Table 2 and Fig. 5)

Table 2 Parametres of chosen components

Tube no.

part

Work. pressure (ata)

Temp. (°C)

Service hrs.

Creep damage

(VGB-TW 507) Component (unit)

Dimension D x t (mm)

39 P39 T39

205 505 190742 1 1

outlet / collecting reheated steam hanging header

324 x 36

40 P40 T40

203 480 131127 1 1

inlet / distribution

super heated steam

header

324 x 32

43 P43 T43

194 550 81456 2a 1

outlet / collecting super heated steam header

324 x 56


Fig. 5 Tubes 39, 40 and 43 - delivered state

From the chemical analysis results showed that the material does not deviate from the prescribed

chemical composition and shows no a significant tendency to change composition over the wall

thickness. Comparison prescribed and average values of composition of each component shows Table 3.

Table 3 Chemical composition in wt.% (melt analysis)

Element C Mn Si Cr Mo V Al P S

standardized by ČSN 415128

0,10 till 0,18

0,45 till 0,70

0,15 till 0,40

0,50 till 0,75

0,40 till 0,60

0,22 till 0,35

max. 0,025 (over. 500°C)

max. 0,040

max. 0,040

Average 39 0,14 0,63 0,34 0,52 0,46 0,29 0,031 0,011 0,019

Average 40 0,12 0,62 0,29 0,56 0,39 0,31 0,007 0,014 0,026

Average 43 0,13 0,56 0,28 0,53 0,47 0,30 0,011 0,009 0,035

Mechanical properties (for example, tube 39 and 40 in Table 4) generally have a random distribution. For

these components, the yield strength and ultimate strength are not significantly different over the wall

thickness. Yield strength is lower than prescribed level properties for given material (see Table 1). An

interesting is a trend by the ratio Rm / Re, where the level of values after the heat reprocessing moves

away with increasing number of operating hours from initial state values (Fig. 6). Impact toughness values

vary considerably and for now failed to find significant relationship to operating time of components, or

other parameters (Fig. 7).

39

43

40


Table 4 Overview of the mechanical properties of selected tubes

part of tube P39 T39 P43 T43

test. temp. 20°C 600°C 20°C 600°C 20°C 600°C 20°C 600°C

Rp0,2

MPa) 336-351 225-230 424-442 255-281 325-344

168-195 397-424 279-291

Rm

(MPa) 551-559 276-289 558-565 344-364 503-512

213-257

596-619 357-391

A5 (%)

26-28 22-25 28-30 17-25 27-29 27-28 26-28 22-24

KCU 3 longitudinal (J.cm-2)

176-240 167-201 130-234 50-140

KCU 3 trnsversal (J.cm-2)

110-137 117-131 50-100 50-110

HV10 170-190 150-170 190-230 160-250

Fig. 6 Rm / Re ratio for tubes 39 and 43 in dependence on wall thickness levels

Fig.7 Tubes 40 and 43; KU3 dependence on wall thickness; last index: L – longitudinal, T - transversal

notched bars


Fig. 8 Microstructure: middle wall thick. of part P39 Fig. 9 Microstructure: middle wall thick. of part P39

Microstructure of part P39 (state P) tube was through a crosssection similar consisting of grains of ferrite,

pearlite nodules, excluded blocks of bainite and carbides along the grain boundaries and within grains

(Fig. 8, 9). Pearlite nodules and bainite blocks showing signs of tempering (decay). Ferrite grain size is in

the investigated regions quite similar and according to EN ISO 643 was rated G = 8.5. For state T

(designation T39) was detected in the investigated regions over the thickness, similar microstructure

consisting of ferrite grains , pearlite nodules and carbides excluded inside and along grain boundary

(Fig. 10). Other parameters are similar as in the original state. On the inner surface of all components

was observed an uneven oxide layer with a thickness of 30 to 320 um (Fig. 11).

Fig. 10 Microstructure of part T39 Fig. 11 Oxidic layer - inner surface of tube P40

On the longitudinal cut of part P43 was microstructure at the outer surface and in the central area similar,

composed mostly of ferrite grains, significantly decayed bainite and carbides along the grain boundaries

and within the grains. Toward the inner surface the proportion of bainite growing up. In places was the

microstructure similar through cross-section, consisting of ferrite, crumbling bainite blocks and excluded


carbides along the grain boundaries and within grains (Fig. 12). Microstructure was according to VGB-TW

507 standard evaluated by grade 2a - advanced creep with the occurrence isolated cavities. Ferrite grain

size in the investigated regions is quite similar and according to EN ISO 643 standard was rated G = 9.

After thermal reprocessing of T43 specimen was observed microstructure through the cross-section

similar and partially decayed. It consisted of ferrite grains, pearlite nodules and largely excluded carbides

within grains (Fig. 13). In the area of the outer surface and in the middle of the wall thickness, were

pronounced blocks of bainite. Ferritic grain size was G = 8. Microstructure was evaluated according to

VGB-TW 507 standard by grade 1 - without the occurrence of incipient creep cavities. This is an

interesting fact confirmed in all cases, when after the operating load detected was damage 2a,

respectively 2b. After a heat reprocessing succeeded in partially recover the structure of material.

It shows that the material degradation after long time of operation has not reached such a level as to

cause fully irreversible damage.

Fig. 12 Microstructure of part P43 Fig. 13 Microstructure of part T43

5. CONCLUSION

The evaluation of degradation processes is unfortunately hampered by lack of knowledge of the original

properties of the cutouts test pipes. Evaluation is therefore based on assumptions and standard (at the

time of production) properties of the tubes from steel 15 128.5 (13MoCrV6; 1.7715).

Overall, the material in some parameters does not meet standard mechanical properties for new products

from the given steel. This is an especially the low yield strength at ambient temperature and in some

cases the ultimate strength. This state is modified by the regime of thermal reprocessing.

Chemical composition does not vary with the requirements for given steel.

From the viewpoint of creep damage, creep symptoms appear here in a wide range - from beginning to

advanced.

A specific problem is the inhomogeneity (values fluctuating) of properties through the thickness and

circumference of the pipes. Here we must carefully consider whether the differences are determined by

the influence of exploitation, or original material inhomogeneity. Production of seamless pipes has its

specifics and similar structural (and subsequently others). Differences in these products often appear,

even though the material otherwise comply with the required (attested) properties. This effect was

subsequently solved by controlled sampling and comparative tests using small samples (out of scope this

article).

Research in this phase of the fundamental purposes of testing volume (systematic data collection) in the

order of several hundred specimens of each type, in particular mechanical tests of several dozen points

of supply.


The newly acquired knowledge, respectively. their analysis, only confirmed the considerable diversity of

method combined thermo-mechanical stress corrosion of components evolving from a number of factors.

Individual components are in fact exposed during operation, except the prescribed manner of loading

(temperature, pressure, etc.), also various step changes, whether planned outages, both in unexpected

outages and emergency situations. Also, during normal operation, it may not always be prescribed

conditions met. Undoubtedly play a role variables, which closely is related to the flow media (water,

steam), part geometry, etc.

In the current status research fills requirements and clearly defends the volume of performed testing.

Successfully is developed methodologies of assessment long-term degradation of energetic components

from alloyed steel. This methodology which should result in the ability to safely determine the remaining

service life of those products.

ACKNOWLEDGEMENT

This paper was created in the project No. CZ.1.05/2.1.00/01.0040 "Regional Materials Science and Technology Centre" within the frame of the operation programme "Research and Development for Innovations" financed

by the Structural Funds and from the state budget of the Czech Republic.

Reference

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[2] BYSTRIANSKÝ,J: Vliv chemických účinků prostředí a metalurgického stavu konstrukčních materiálů na

životnost klíčových komponent jaderných elektráren. VŠB-TU Ostrava, 2004

[3] VISWANATHAN,R.: “Damage Mechanisms and Life Assesment of High-Temperature Componentns”, ASM

International, Metals Park Ohio, Second Printing 1992

[4] SELIGER, P.; GAMPE, U.: Life Assessment of Creep Exposed Components, New Challenges for Condition

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Accessible from http://www.ommi.co.uk/PDF/Articles/50.pdf

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rámci projektu MPO ČR FD-K /088

[6] BYSTRIANSKÝ,J.; KRAUS, M.; BONČEK,R.; VEJVODA,S.: „Program destruktivních zkoušek konstrukčních

materiálů dlouhodobě exponovaných v podmínkách tlakového okruhu klasické elektrárny“in:Kotle a

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[7] VEJVODA,S.; JAROŠ, M.; BYSTRIANSKÝ,J.; ČMAKAL,J.; KUBOŇ,Z.; DOSTÁL,V.: Metodika stanovení

zbytkové životnosti tlakových zařízení pracujících na klasických elektrárnách v podcreepové oblasti.

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[8] ČSN 41 5128 - Ocel 15 128 Cr-Mo-V, ČNI Praha, 1986

[9] VGB-TW 507: Richtreihen zur Bewertung der Gefuegeausbildung und –schaedigung zeitstandbeanspruchter Werkstoffe von Hochdruckrohrleitungenund Kesselbauteilen. VGB Kraftwerkstechnik GmbH, Essen, 1992

http://www.ommi.co.uk/PDF/Articles/50.pdf

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