influence of structure on mechanical properties of...

METAL 2008 13. –15. 5. 2008, Hradec nad Moravicí

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INFLUENCE OF STRUCTURE ON MECHANICAL PROPERTIES OF

REGENERATIVE G17CrMoV5 – 10 CAST STEEL

Grzegorz GOLAŃSKI

Czestochowa University of Technology, Institute of Materials Engineering,

Armii Krajowej 19, 42 – 200 Czestochowa, Poland

e-mail:[email protected]

Abstract

The paper presents results of research on the influence of regenerative heat treatment on the

structure and properties of G17CrMoV5 – 10 cast steel. The cast steel after service revealed

degraded bainitic-ferritic structure and was characterized by mechanical properties below

the minimum requirements. It has been proved that the structure of high-temperature

tempered bainite ensures optimum combination of mechanical properties and impact energy.

However, tempered bainitic-ferritic structure obtained through normalization was

characterized by impact energy twofold lower than the cast steel with bainitic structure, with

mechanical properties being on similar level. While ferritic-bainitic structure obtained

through slow cooling ensured only required impact energy of KV > 27J with mechanical

properties below demanded minimum.

1. INTRODUCTION During long-term operation of low-alloy Cr – Mo – V cast steels at elevated

temperatures there is slow decrease of their mechanical properties (hardness, yield point and

tensile strength) and decrease of impact energy below required minimum of 27J, frequently

down to the level of 4 ÷ 6J. Changes in the applied properties of long term serviced cast steels

are related to the processes of structure degradation, mostly [1÷4]: privileged precipitation of

carbides on grain boundaries, changes of morphology, chemical composition as well as

dispersion of carbides and segregation of phosphorus and other trace elements on grain

boundaries.

Processes of structure degradation and, thus, unfavourable changes in the applied

properties of long term serviced cast steels, do not limit the possibilities of their further safe

operation. Self study [5] carried out on the cast steels with various service periods, revealed

lack of creep changes in their structure. One of the conditions of extending safe operation

time of the steel casts entails applying a process of their revitalization. The process consists in

regenerative heat treatment in order to obtain “new” regenerated structure which would

enable improvement of plastic properties – increase of impact energy and decrease of DBTT

temperature, with the mechanical properties similar to the properties of new casts [6, 7]. In

order to obtain optimum structure from the point of applied properties, the regenerative heat

treatment should provide:

� maximum grain size reduction in order to increase crack resistance, decrease DBTT

temperature and raise the yield point;

� dissolving carbides in austenite, especially those precipitated on grain boundaries,

which enables obtaining required mechanical properties;

� elimination of grain boundaries’ brittleness caused by phosphorus segregation to grain

boundaries during operation [8, 9].

The purpose of the work was to determine the influence of structure on properties of

the cast steel subjected to heat treatment and to prove that the structure regenerated through


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heat treatment could enable obtaining of mechanical properties on the level comparable to the

properties of new casts.

2. INVESTIGATED MATERIAL

Material for investigation was taken from high-pressure turbine frame body serviced

for ca. 251 678 hours (total running time) at the temp. of 535 oC and pressure 9.0 MPa. Frame

was made of low-alloy Cr – Mo – V cast steel G17CrMoV5 – 10 (G17). Chemical

composition of the investigated cast steel is presented in Table 1, while Table 2 includes

mechanical properties of G17 cast steel after operation along with the standard requirements.

Table 1. Chemical composition of the cast steel, %wt

Investigated

material C Mn Si P S Cr Mo V

G17CrMoV5 -10 0.15 0.65 0.26 0.012 0.018 1.60 1.17 0.30

Table 2. Mechanical properties of G17CrMoV5 –10 cast steel after operation

and requirements according to Polish Standard

After service

TS

[MPa]

YS

[MPa]

El.

[%]

KV

[J] HV30

491 241 14.4 12 133

According to

Polish

Standard [10]

TS

MPa

YS

MPa

El.

%

KV

J HV30

590 ÷ 780 min. 440 min.

15 27 ---

In the post-operational condition the investigated cast had degraded bainitic-ferritic

structure, where bainite occurrence was indicated by characteristic arrangement of carbides.

Prevailing phase in the structure was quasipolygonal ferrite of diverse amount and carbides

dispersion precipitated inside grains. On grain boundaries and inside ferrite grains numerous

carbide precipitates of diverse morphology could be observed. In some areas the amount of

carbides precipitated on grain boundaries was so large, that they formed the so-called

“continuous grid” of precipitates (Fig. 1a). The size of ferrite and bainite grain was diverse,

within the range of 125 ÷ 62.5µm, which corresponds to the 5/3 size according to ASTM

standard scale. The cast steel after service was characterized by low impact energy of 10J and

hardness of 133HV30. Moreover, other properties, such as: yield point, tensile and elongation

strength, were lower than the minimum standard requirements (Table 2).


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Fig. 1. Structure of G17CrMoV5- 10 cast steel after service (a); cast steel fracture (b)

3. METHODOLOGY OF RESEARCH

Critical temperatures Ac1 and Ac3 of the investigated cast steel, amounting to 809 and 937 oC

respectively, were determined by means of optical dilatometer LS – 4, applying heat up rate

of 0.05K/s. Heat treatment of G17 cast steel consisted in cooling of the samples after four-

hour austenitization at the temperature of 960 oC at the cooling rates corresponding to the

following processes: bainitic hardening, normalizing and full annealing. Four-hour tempering

was carried out at the temperatures of 700, 720 and 740 oC. Observation and record of

microstructures and fractures was done by means of Axiovert 25 optical microscope and

JOEL JSM – 5400 scanning microscope. Research of mechanical properties (static tensile

test, hardness and impact energy) was performed according to currently obeyed standards.

4. RESULTS

4.1. Structure of G17 cast steel after heat treatment

Structures obtained in G17 cast steel, due to various cooling rates applied after

austenitization process and subsequent high-temperature tempering, are presented in Fig.1a

and 1b.

Observed structures after heat treatment differed in morphology and bainite amount. In

the cast steel after bainitic hardening only “needle-shaped” bainite form occurred and was

morphologically similar to martensite, which indicated lower bainite appearance in the

structure. After normalization, however, bainitic-ferritic struture could be observed in the cast

steel, with about 10% of ferrite amount. In this structure bainite had “feathery” form, which

indicated upper bainite appearance. Apart from “feathery” bainite there were also single areas

of “needle-shaped” bainite occurrence.

High-temperature tempering of both structures caused precipitation of carbides on former

austenite grain boundaries and lath boundaries as well as inside bainite laths.

In both cases obtained structures were characterized by fine grain with the mean diameter of

former austenite grain amounting to 44.2 ÷ 22.1 µm, which corresponds to 7/8 size according

to ASTM standard scale.


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Fig. 2. The G17 cast steel after heat treatment: a) tempered bainite structure; b) tempered

bainitic – ferritic structure; nithal etched,

The G17 cast steel after full annealing and tempering revealed ferritic-bainitic structure

Fig. 3a. Prevailing phase in the structure was quasi-polygonal ferrite in a diverse amount and

with carbide dispersions precipitated inside the grains. Bainite amount in the structure did not

exceed 10 ÷ 15%. On grain boundaries there were numerous large carbide precipitates. Ferrite

grain size after annealing amounted to 44.2 ÷ 31.2µm, which corresponded to 6/7 grade

according to ASTM scales. The size decreased by about two grades in comparison with ferrite

grain size after service. Applied heat treatment in this case contributed to similar changes

which take place in the structure after long-term operation.

Fig. 3. a) The G17 cast steel structure after full annealing and tempering; b) fracture of the

cast steel with ferritic-bainitic structure

4.2. Research of mechanical properties

Heat treatment of G17 cast steel after service, regardless of the cooling-rate applied,

contributed (after tempering) to an increase of crack resistance reflected in the impact energy

– Fig. 4, Table 3. Tempering at 740 oC (maximum temperature of tempering for this grade of

material) causes ca. double increase of impact energy in comparison with the temperature of

700 oC, i.e. for bainitic structure impact energy KV increases from 55J up to 101J, while for

bainitic-ferritic structure impact energy rises from 27J up to 55J, however, for ferritic-bainitic

structure – from 19 to 43J (Fig. 4a). Along with the growth of impact energy there is also

about 15% decrease of hardness for cast steels with prevailing bainite fraction in the structure,


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and about 7% - for cast steels with ferritic-bainitic structure (Fig. 4b). Similar tendency is

noticeable for yield point and tensile strength for bainitic structures. (Table 3).

Fig. 4. Influence of tempering temperature on impact energy KV (a) and hardness HV30 (b)

Obtained mechanical properties met the standard requirements, regardless of the

tempering temperature, however the tensile strength exceeded required minimum

significantly. High temperature of tempering ensures obtaining of high impact energy and

mechanical properties above the required minimum. Therefore, it is applied in order to

increase the structure’s stability in the cast steel assigned for long term service at elevated

temperatures.

Full annealing applied in the investigated cast steel after austenitization appeared to

be definitely unfavourable. Mechanical properties decreased below lower limit of

requirements, with the impact energy meeting the demanded minimum of KV > 27J (Table 3).

4.3. Fractography

After service the cast steel was subject to decohesion through transcrystalline fissile

mechanism with characteristic „longitudinal river profiles”. On the fracture numerous

secondary cracks were observed, the so-called fracture toward the depth, running on the grain

boundaries. Occurrence of these cracks proved feebleness and fragility of boundaries.

Moreover, micro fields of continuous form were observed (Fig. 1b). Fissile cracking requires

little energy, which resulted in low impact energy of the cast steel after service. Applied heat

treatment caused change of the cracking mechanism in the investigated cast steel (Fig 5a, 5b).

In the cast steel with high-temperature tempered bainite, on the entire surface under the

fraction, there was a transcrystalline ductile fracture initiated by fine-dispersion precipitates of

carbides and sulfide inclusions. A characteristic of plastic cracking is the ability to absorb

significant amounts of energy connected with plastic deformations preceding decohesion.

High impact energy of the cast steel with purely bainitic structure results from a large total

amount of grain boundaries (boundaries of bainite packets) and high ductility after tempering

of lower bainite structure (Fig. 5a).

The cast steel with bainitic – ferritic structure was subject to decohesion through

mixed mechanism. Directly under the notch, at a depth of about 1.0 ÷ 1.5 mm, cracking

proceeded in plastic manner through transcrystalline ductile mechanism. Beneath, the fissile

cracking went on through transcrystalline fissile mechanism with micro fields of ductile type

(Fig. 5b).

700 710 720 730 740

20

30

40

50

60

70

80

90

100a)

KV

, J

Temperature of tempering, oC

bainite

bainitic - ferritic

ferritic - bainitic

700 710 720 730 740

160

180

200

220

240

260

b)

HV

30

Temperature of tempering, oC

bainite

bainitic - ferritic

ferritic - bainitic


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Table 3. Parameters of heat treatment, properties and structure of G17CrMoV5 – 10 cast steel

Type of heat

treatment

TS

[MPa]

YP

[MPa]

EL.

[%]

KV

[J] HV30 Structure

After service 491 241 14.4 12 133 bainitic -

ferritic

Bai

nit

ic

har

den

ing

700 794 606 16.7 55 256 bainite

740 675 486 19.2 101 220 bainite

No

rmal

izat

ion

700 769 561 17.1 27 255 bainitic -

ferritic

740 658 453 20.13 55 217 bainitic -

ferritic

Fu

ll a

nn

eali

ng

720 548 274 22.7 35 168 ferritic -

bainitic

740 525 269 24.6 43 167 ferritic -

bainitic

According to PN

[10]

590

÷

780

min.

440 min. 15 min. 27 ---- ___

Lower impact energy of the cast steel with bainitic-ferritic structure in comparison to

the cast steel with bainitic structure results from the occurrence of ferrite which contributes to

fissile cracking and upper bainite which is characterized by higher brittleness than in the case

of lower bainite [11, 12]. In the cast steel with ferritic-bainitic structure the decohesion

occured through mixed mechanism. Straight under the notch cracking could be observed

through transcrystalline ductile mechanism which at the depth of ca. 0.7 mm under the notch

was developing into transcrystalline fissile fracture. Inside this fracture there were micro

fields revealing ductile character (Fig. 3b).

Fig. 5. Fractography of fracture: a) of tempered bainite structure; c) of tempered bainitic-

ferritic structure


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5. CONCLUSIONS

1. Long term service of cast steel elements at elevated temperatures contributes to strong

decrease of mechanical properties – higher in the case of yield point than tensile

strength, as well as impact energy decrease.

2. Tempering of bainitic structure at 740 oC, which is beneficial for large stability of the

structure during long term service, ensures optimum combination of high mechanical

properties and impact energy.

3. Tempered bainitic-ferritic structure of G17 cast steel, with mechanical properties

similar to properties in the case of bainitic structure, was characterized by impact

energy KV almost two times lower. Negative influence of ferrite on impact energy is

evident.

4. Tempered ferritic – bainitic structure of G17 cast steel obtained through slow cooling

does not ensure minimum mechanical properties at the required impact energy (for the

tempering temperature of above 720 oC).

5. Bainitic structure enables applying high tempering temperatures without concern for

decrease of mechanical properties below the required minimum.

6. Mechanical properties on the level of the new casts’ properties can be obtained in the

process of regenerative heat treatment by receiving of a “new”, regenerated structure.

Acknowledgements

Scientific work funded by the Ministry of Education and Science in the years 2006 ÷

2009 as a research project No. DWM/46/COST/2005.

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[2] STACHURA S., STRADOMSKI Z., GOLAŃSKI G., 2001, Hutnik - Wiadomości

Hutnicze, 5, p.184

[3] STACHURA S., 1999, Energetic, 2, p.109

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p. 531

[5] STACHURA S., GOLANSKI G., Report BZ – 202 – 1/01 researches unpublished

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[9] GOLANSKI G., STACHURA S., KUPCZYK J., ZYSKA A., GAJDA B., 2006,

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[10] PN – EN 10213 – 2 Technical delivery conditions for steel castings for pressure

purposes – Part 2: Steel grades for use at room temperature and elevated temperatures

[11] KOTILAINEN H., 1980, The Micromechanisms of Cleavange Fracture and their

Relationship to Fracture Toughness in a Binitie Low Alloy Steel, VTT, Espoo, Finland,

ISBN 951- 38 – 1014 – 3

[12] BHADESHIA H. K. D. H., 2001, Bainite in steels, 2nd

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Cambridge, ISBN 1 – 86125-112-2

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