THE INFLUENCE OF COOLING RATE ON THE STRUCTURE AND PROPERTIES OF G21CrMoV 4 6 CAST STEEL AFTER REGENERATIVE HEAT TREATMENT

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1 THE INFLUENCE OF COOLING RATE ON THE STRUCTURE AND PROPERTIES OF G21CrMoV 4 6 CAST STEEL AFTER REGENERATIVE HEAT TREATMENT GOLAŃSKI G.*, STACHURA S.,KUPCZYK J.,KUCHARSKA-GAJDA B. Institute of Materials Engineering, Czestochowa University of Technology, Czestochowa, Armii Krajowej 19, Poland * grisza@mim.pcz.czest.pl SUMMARY The paper presents research results of the influences of different cooling rates on the structure and properties of the G21CrMoV 4 6 cast steel exposed to regenerative heat treatment. The optimum range of cooling rate, in order to obtain bainitic structure, was determined on the basis of the CCT diagram. It has been proved that the structure of tempered bainite ensures obtaining optimum mechanical properties and impact energy in the investigated cast steel. 1. INTRODUCTION Long-term operation of the cast steel at the temperatures of o C contributes to mechanical properties reduction - slight decrease of strength and significant decrease of impact energy, below the minimum value of 27J [1 3]. Reduction of the properties is caused by changes occurring in the cast steel structure during operation, in particular: privileged precipitation on grain boundaries and phosphorus segregation to grain boundaries and the interfacial surface carbide/matrix [2 6]. The extent of properties reduction caused by the above-mentioned changes in the structure during long-term operation in elevated temperatures depends largely on the initial structure of the cast steel. However, changes of structure and properties of long-term serviced castings do not limit the possibilities of their further safe operation. Elongation of the operation time for longterm serviced cast steels is possible due to revitalization of the castings. Revitalization of the cast steel includes removing of deformations and fractures through welding and regeneration of the cast steel structure through heat treatment process [7, 8]. The structure of tempered bainite, as individual research proves [9, 10], enables to obtain optimum mechanical properties and impact energy of the cast steel. Since turbine frame and valve chamber casts have diverse wall thickness, thus it is necessary to determine the austenite cooling rate in order to obtain bainite structure in the entire casting. This work presents results of the research concerning influence of different austenite cooling rates on the structure and properties of G21CrMoV 4 6 cast steel. 2. INVESTIGATED MATERIAL The material of research was G21CrMoV 4 6 low alloy cast steel with the chemical composition given in table 1. Samples for investigation were taken from the internal turbine frame, serviced for over hours at the temperature of 540 o C and the pressure of

2 Table1. Chemical composition of the investigated cast steel (wt.%) C Mn Si P S Cr Mo V In the post-operational condition the cast structure was ferritic-perlitic. Inside and on ferrite grain boundaries numerous M 23 C 6 carbides were noticeable, often forming a continuous grid. In pearlite the process of fragmentation and spheroidization of M 23 C 6 carbides was visible (Fig. 1a). Inside ferrite grains also VC i M 6 C carbides were observed. a Fig. 3. Structure of the G21CrMoV 4 6 cast steel after: a) operation; b) cooling rate v 8-5 ~ K/s and tempering; b) cooling rate v 8-5 ~ K/s and tempering; c) cooling rate v 8-5 ~0.017 K/s and tempering Ferrite grain size in the investigated cast steel was within the limits of 88-31µm, which corresponds to the grain size 4 7 according to the ASTM standards. The cast steel after operation was characterized by very low impact energy of 10J and hardness of 156HV30, it also fulfilled elongation and tensile strength requirements for new products, however, the yield point was lower by 15 than the minimum requirements determined by the norm (Table 2). 2

3 Table 2. Structure and mechanical properties of G21CrMoV 4 6 cast steel after operation TS YS El. % KV J HV30 Structure after operation according to min. min. min. the Polish ** 197 Standard* *- PN - 89/ H ; ** - hardness according to Brinell 3. METHODOLOGY OF RESEARCH ferritic perlitic Investigation of the cast steel structure was performed by means of optical and scanning microscopy on the conventionally prepared nital etched metallographic microsections. Observation and image record of the structures was made by means of optical microscope Axiovert 25 with an attachment for digital image recording and a scanning microscope: JEOL JSM CCT diagram was constructed on the basis of the presupposed programme of different cooling rates of the samples in an optical dilatometer of LS 4 type. Static tensile test was performed on a servo-hydraulic testing machine: MTS 810. Hardness measurement was made by means of Future Tech FV 700 hardness tester. Mechanical properties research was made in accordance with Polish Standard. 4. RESULTS AND DISCUSSION Critical temperatures Ac 1 and Ac 3 of the investigated cast steel were determined at the heatup rate of 0.05 K/s. They amounted to 775 o C and 903 o C respectively. Assumed optimum austenitization temperature amounted to 910 o C and the austenitization time 3 hours. Such time of austenitization is applied for a few tons heavy frames and valve chambers casts. Heat up rate of the samples amounted to v 5-8 8,76 K/s. Austenitization temperature not higher than 30 o C above Ac 3, ensures fine austenite grain with the mean diameter of about 15µm [10]. Fig. 2. CCT diagram for the cast steel Applying different cooling rates for austenite at the temperature range o C, presented in Table 3, allowed to draw up a CCT diagram (Fig. 2). Table 3 also contains values of the received hardness and quantitative analysis of the structures. 3

4 Table 3. The influence of the cooling rate on type of structure and hardness of the cast steel V No. 8-5 K s -1 HV 30 Type of microstructure / percentage martensite/100 V k martensite/ bainite/ bainite/ 94, ferrite/ bainite/90, ferrite/ bainite/81, ferrite/ bainite/78, ferrite/ ferrite/53, bainite/44, pearlite / ferrite/84, bainite/12, pearlite / ferrite/80, pearlite/16, bainite/ Ferrite/76, pearlite/21, bainite/ ferrite/74, pearlite/ ferrite/74, pearlite/26 On the basis of the dilatometric curve analysis, it has been concluded that austenite cooled at the rate of K/s v K/s is transformed into ferrite and pearlite. Austenite cooling rate < v K/s allows to obtain ferritic pearlitic bainitic structures. While cooling rate at the range of K/s v K/s allows to obtain bainitic ferritic structure with increasing fraction of bainite as the cooling rate rises. Bainitic structure was obtained at the cooling rate v K/s. The critical cooling rate amounted to 71.43K/s (Table 3), which was calculated on the basis of chemical composition and austenitization temperature. The influence of cooling rate on the structure and hardness of the investigated cast steel is presented graphically in Fig. 2. Fig. 3. Influence of the cooling rate on structure and hardness of G21CrMoV 4 6 cast steel 4

5 In order to determine the influence of structure on G21CrMoV 4 6 cast steel properties, the samples were subjected to cooling at the rates which ensure obtaining of the following structures: bainitic, bainitic with about 20% of ferrite, and ferritic with about 20% of pearlite. After that, samples were tempered for 4 hours at the temperature of 720 o C. Obtained structures and properties are presented respectively in Fig.3 and Table 4. Table 4. Structure and properties of the cast steel after regenerating heat treatment Heat treatment parameters TS YS El. % KV J HV30 Structure 910 o C/ 3h/ K s o bainitic C/4h 910 o C/ 3h/ K s -1 bainitic o C/4h ferritic 910 o C/ 3h/ K s -1 ferritic o C/4h pearlitic according to Polish min. 320 min. 20 min. 27 Standard* ** 197 *- PN - 89/ H ; ** - hardness according to Brinell 5. CONCLUSIONS 1. Obtaining of the bainitic structure in the cast steel requires cooling of the castings at the rates higher than K/s. Bainitic structure after tempering ensures high mechanical properties and impact energy. 2. Ferrite content of about 20% in the bainitic structure contributes to impact energy reduction by about 40%, with an insignificant decrease of properties not larger than 5%. 3. Research has revealed an adverse influence of ferritic perlitic structure on the properties of G21CrMoV 4 6 cast steel. Mechanical properties and impact energy remained on the level of minimum demands defined by the norm. 4. It is important to minimize ferrite amount in bainitic structure because of its unfavourable influence on impact energy of the cast steel. Scientific work funded by the Ministry of Education and Science in the years as a research project No. DWM/46/COST/ LITERATURE 1. Dobosiewicz J. Influence of operating conditions on the change s in mechanical properties of steam turbine cylinders, Energetyka, 1, 1992, 27 (in Polish) 2. Stachura S., - "The changes of structure and mechanical properties steels and cast steels after long term serviced at elevated temperatures", Energetyka, 2, 1999, 109 (in Polish) 3. Golański G., Kupczyk J., Stachura S. - " Chamnges of structure and mechanical properties of long term serviced cast steels", VII International Scientific Conference "New technologies and achcievements in metallurgy and material engieneerig", Częstochowa, 2006, 172 (in Polish) 4. Stachura S., Stradomski Z., Golański G., - "Phosphorus in ferroalloys", Hutnik - Wiadomości Hutnicze, 5, 2001, 184 (in Polish) 5

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