1. Introduction

Power plants, which were originally intended to provide the base load, are frequently shut down and powered up. Variations in the steam temperature accompanying the power changes induce thermo-mechanical stresses in components, which lead to material degradation and consequently can cause failure, [1]. Results of some published investigations with low alloyed Cr-Mo-V cast steels reports about possibility of regenerative heat treatment after prolonged exploitation and influence of microstructure on properties, [2, 3]. Decrease of mechanical properties of exploited material is caused by changes in the steel microstructure due to long-lasting service operation. Reduction of toughness properties caused by long-term exploitation of the steel at elevated temperature depends to a large extent on the initial steel microstructure. Some published investigations reported that decrease of toughness properties caused by long-lasting operation is the least in the case of tempered bainite structure, [2]. It is important to emphasize that metallographic examinations of various steel grades after long-term service at elevated temperature revealed that transformations of carbides and morphological changes of phases have the most significant effect on service properties degradation, [4]. Considering assumption that post service regenerative heat treatment could change microstructure and improve decreased mechanical properties after prolonged exploitation, heat resistant steamline steel 14MoV6-3 is investigated in this paper. The low-alloyed steel 14MoV6-3 was chosen for this investigation, because it has been used for a long service period (194.207 service hours) with similar exploitation history and microstructure evolution as a previously mentioned Cr-Mo-V cast steels. Investigated material is taken from the Unit 5 main steamline ([??] 245x28mm) that operated at temperature 540[degrees]C and pressure 13,5MPa in thermal power plant Kakanj, Bosnia and Herzegovina.

2. Regenerative heat treatment

Chemical composition testing of investigated steel 14MoV6-3 was accomplished in order to confirm that all delivered specimens of steamline are made from the same material, so the results of predicted investigation on virgin, exploited and regenerative heat treated material could be comparable. Method for determination of chemical composition was spectral analysis. Chemical composition of steel 14MoV6-3 was in accordance to normative DIN 17175/79 for all material conditions. The goal of regenerative heat treatment of exploited steel 14MoV6-3 was obtaining improved mechanical properties that were degraded, particularly toughness properties. Regenerative heat treatment (hardening and tempering) was done according to normative DIN 17175/79 for high-temperature seamless tube steel 14MoV6-3 on specimen of exploited steel. Regenerative heat treatment cycle of exploited material was consisted of:

* Hardening, heating to temperature 950[degrees]C (1 hour) and accelerated cooling in oil because greater tube wall thickness, and

* Tempering, heating to temperature 700[degrees]C (3 hours) with slow cooling together with furnace for heat treatment.

3. Hardness and microstructure

Brinell hardness test was done on outer surface, 1.5 mm under the outer surface, and at longitudinal and transversal cross section of steamline pipe. According to European normative EN 10216-2:2002, steel 14MoV6-3 are delivering as seamless steel tubes for elevated temperatures with acceptable value of hardness in range of 145-190 HB30 at room temperature. Results of measured hardness values (HB30) for virgin, exploited and regenerative heat treated steamline steel 14MoV6-3 are presented in Table 1.

Considering the highly increased hardness of regenerative heat treated steel 14MoV6-3, that is obviously consequence of large amount of insufficiently tempered constituents, it implicitly indicate increase of strength properties, but also significant decrease of toughness properties after regenerative heat treatment, [5].

Metallographic testing was accomplished for virgin, exploited and regenerative heat treated steel 14MoV6-3 by optical microscope with magnifications 500x. This was done in laboratory at IWS Institute TU Graz (Institute for materials and welding at Technical University Graz), Austria. Fig. 1 shows microstructure of investigated material 14MoV6-3 at transversal cross section of steamline pipe with same magnifications.

Results of microstructure investigation that are presented in this paper mainly can confirm facts that the initial microstructure of the 14MoV6-3 low-alloyed steel features the mixture of bainite with ferrite, sometimes with a small amount of pearlite and significant amount of the M3C carbides and numerous, very fine MC type ones. The final structure image after long-term exploitation under service conditions is ferrite with rather homogeneously distributed precipitations inside grains and chains of the significant amount of precipitations on their boundaries, [6]. But in addition to mentioned microstructure evolution, there is also a significant growth of ferrite grain size after long- term operation at elevated temperature. As a consequence, grain growth has a significant influence on decrease of steel 14MoV6-3 impact toughness properties. Microstructure of regenerative heat treated steel 14MoV6-3 is ferrite structure with pearlite/bainite constituents on ferrite grain boundaries but also with insufficiently tempered constituents created by accelerated cooling. Comparing with virgin steel there is a less carbide precipitates in ferrite grains, less amount of ferrite at all and very small amount of pearlite and bainite.

4. Crack initiation and propagation energy

In order to investigate toughness properties of regenerative heat treated steamline steel 14MoV6-3, following temperatures were selected for impact toughness testing: 20[degrees]C, 150[degrees]C, 400[degrees]C and 540[degrees]C (service temperature). This was done by testing and comparison of crack initiation and propagation energy values of virgin steel, exploited steel and regenerative heat treated steel 14MoV6-3. For every testing temperature 3 Charpy V-notch specimens were used. In general, notch toughness is measured in terms of the absorbed impact energy needed to cause fracturing of the specimen. The change in potential energy of the impacting head (from before impact to after fracture) is determined with a calibrated dial that measures the total energy absorbed in breaking the specimen. Other quantitative parameters, such as fracture appearance and degree of ductility/deformation, are also often measured in addition to the fracture energy. Impact tests may also be instrumented to obtain load data as a function of time during the fracture event. The Charpy V-notch test continues to be the most utilized and accepted impact test in use in the industry, [6]. Results of average (3 specimens) impact crack initiation energy values per testing temperature for virgin, exploited and regenerative heat treated steel 14MoV6-3 are presented in Fig. 2.

Results of average (3 specimens) impact crack propagation energy values per testing temperature for virgin, exploited and regenerative heat treated steel 14MoV6-3 are presented in Fig. 3.

From the results of impact toughness testing it is notable that the impact crack initiation and propagation energy increases slightly up to 400 [degrees]C for virgin steel 14MoV6-3 and up to 150 [degrees]C for exploited steel 14MoV6- 3. It is reduces significantly, but not drastically above 400[degrees]C for virgin steel and above 150[degrees]C for exploited steel, so that its values are still more than sufficient at steamline service temperature 540[degrees]C. The most important result of exploited steamline steel impact toughness testing is extremely low value of impact crack initiation energy at room temperature 20[degrees]C, which is significantly beneath the allowed value, [6]. Considering regenerative heat treated steel, it is notable further decrease of toughness properties in overall testing temperature range. It should be mentioned that a large number of failures in engineering components occur due to preexisting defects, nonmetallic inclusions or other imperfections (casting, welding defects, etc.), [7]. However, it is of engineering interest to know how and why particular component has failed. The effort to extend designed lifetime of industrial plants operating for a long time at elevated temperatures is unthinkable without the knowledge of mechanical properties of materials prior to operation and mechanical properties after actual time of operation (actual mechanical properties), because the material properties can be reduced throughout the service life, [8].

5. Conclusion

post service regenerative heat treatment of exploited material was accomplished with relative success. Toughness properties of steamline steel 14MoV6-3 depends mostly on development of the precipitation processes and also on development of the microstructure changes and structure discontinuities, as well as grain growth, originated during the long period of exploitation at elevated temperature. For this reason some changes are necessary in the microstructure degraded by long-term exploitation. These changes are:

* Grain size reduction allowing to increase the crack resistance, decrease the transition temperature and raise yield strength,

* Dissolving of carbides in austenite, particularly the carbides precipitated on grain boundaries, and

* Obtaining of tempered ferrite/bainite structure.

Further investigation related with possibility of exploited steel 14MoV6-3 post service regeneration by heat treatment should be directed to search for more adequate heat treatment cycle.

DOI: 10.2507/28th.daaam.proceedings.049

6. Acknowledgement

This investigation was partly supported by IWS Institute at Technical University Graz and OEAD Austrian Agency for International Cooperation in Education and Research.

7. References

[1] Linn S. & Scholz A. (2013). Creep-fatigue Lifetime Assessment with Phenomological and Constitutive Material Laws. Procedia Engineering (6th International Conference on Creep, Fatigue and Creep-Fatigue Interaction CF-6), Vol. 55, 2013, pp. 607-611, ISSN1877-7058, DOI 10.106/j.proeng.2013.03.302

[2] Golanski G., Stachura S., Gajda-Kucharska B. & Kupczyk J. (2007). Optimisation of Regenerative Heat Treatment Parameters of G21CrMoV4-6 Cast Steel. Archives of Materials Science and Engineering, Vol. 28, No. 6, (June 2007) page numbers (341-344), ISSN 1897-2764

[3] Golanski G. & Wieczorek P. (2008). Electron Microscopy Investigation of the Cr-Mo-V Cast Steel. Archives of Materials Science and Engineering, Vol. 30, No. 2, (April 2008) page numbers (73-76), ISSN 1897-2764

[4] Dobrzanski J., Zielinski A. & Krtzon H. (2007). Mechanical Properties and Structure of the Cr-Mo-V Low-alloyed Steel after Long-term Service in Creep Condition. Journal of Achievements in Materials and Manufacturing Engineering, Vol. 23, No. 1, (July 2007) page numbers (39-42), ISSN 1734-8412

[5] Hodzic D., Hajro I. & Tasic P. (2014). Regenerative Heat Treatment of Heat Resistant Steel 14MoV6-3. Proceedings of 18th International Research/Expert Conference "Trends in the Development of Machinery and Associated Technology", 10-12 September 2014, Budapest, Hungary, ISSN 1840-4944, Ekinovic S., Yalcin S., Calvet J.V. (Ed.), pp. 117-120, Faculty of Mechanical Engineering in Zenica, Zenica

[6] Hodzic D. & Hajro I. (2016). Impact Crack Initiation and Propagation Energy of Prolonged Exploited Heat Resistant Steel. Proceedings of the 27th DAAAM International Symposium, pp.0308-0311, B. Katalinic (Ed.), Published by DAAAM International, ISBN 978-3-902734-08-2, ISSSN 1726-6979, Vienna, Austria, DOI: 10.2507/27th.daaam.proceedings.045

[7] Brnic J. & Brcic M. (2015). Comparison of Mechanical Properties and Resistance to Creep of 20MnCr5 Steel and X10CrAlSi25. Procedia Engineering (25th DAAAM International Symposium on Intelligent Manufacturing and Automation, 2014), Vol. 100, 2015, pp. 84-89, ISSN1877-7058, DOI 10.106/j.proeng.2015.01.345

[8] Matocha K. (2014). The Use of Small Punch Test for Determination of Fracture Behaviour of Ferritic Steels. Procedia Engineering (1st International Conference on Structural Integrity ICONS-2014), Vol. 86, 2014, pp. 885-891, ISSN1877-7058, DOI 10.106/j.proeng.2014.11.110

Caption: Fig. 1. Microstructure of virgin, exploited and regenerative heat treated steel 14MoV6-3 [5]

Caption: Fig. 2. Impact crack initiation energy

Caption: Fig. 3. Impact crack propagation energy

Table 1. Hardness values for investigated material 14MoV6-3

                              Virgin,   Exploited,   Heat treated,
Hardness of steel 14MoV6-3     HB30        HB30          HB30

Outer surface                 135-143    134-147        270-288
1.5 mm under outer surface    155-161    146-159        252-265
Transversal cross section     159-162    150-153        288-293
Longitudinal cross section    161-164    149-151        290-293