development of microstructure and mechanical properties of

206 INŻYNIERIA MATERIAŁOWA MATERIALS ENGINEERING ROK XXXVIII

Development of microstructure and mechanical properties of forgings in the thermomechanical treatment

Marek OpielaSilesian University of Technology, Institute of Engineering Materials and Biomaterials, Gliwice, Poland; [email protected]

The work presents research results of the influence of thermomechanical treatment via forging on microstructure and mechanical properties of newly elaborated microalloyed steel containing of 0.28% C, 1.41% Mn, 0.028% Ti, 0.027% Nb, 0.019% V and 0.003% B. The investigated steel is assigned to the production of forged elements for the automotive industry. Conditions of forging using the thermomechanical processing method were developed based on plastometric tests. Observations of the microstructures of thin foils were conducted using a TITAN80-300 FEI transmission electron microscope. The ap-plied thermomechanical treatment allows to obtain a fine-grained microstructure of the austenite during hot-working and production of forged parts. These acquire advantageous mechanical properties and guaranteed crack resistance after controlled cooling from the end plastic deformation temperature and suc-cessive tempering. Forgings produced using the thermomechanical treatment method, consecutively subjected to tempering in a temperature range from 550 to 650°C, reveal values of YS0.2 which equal from 994 to 939 MPa, UTS from 1084 to 993 MPa, KV from 69 to 109 J, KV–40 from 55 to 83 J, and a hardness ranging from 360 to 310 HBW. The results obtained in this paper make it possible to develop an industrial technology of forgings with high mechanical properties and guaranteed crack resistance, and also at a decreased temperature, by using the thermomechanical treatment method.

Key words: HSLA steels, thermomechanical treatment, structure, mechanical properties.

Inżynieria Materiałowa 5 (219) (2017) 206÷211DOI 10.15199/28.2017.5.1© Copyright SIGMA-NOT MATERIALS ENGINEERING

1. INTRODUCTIONThe pursuit to reduce production cost is the reason for implemen-tation of cost-effective technologies of constructional alloy steels forged products with the methods of thermomechanical treatment. Combination of hot-working carried out under properly selected conditions allows to produce fine-grained microstructure products.The point is the use of advanced degree of thermally activated dy-namic and static processes as well as dedicated cooling paths. The mentioned conditions ensure the combination of high mechanical properties, satisfactory ductility and toughness as well as guaranteed brittle fracture resistance without the necessity to implement expen-sive heat treatment or its reduction to tempering only. The HSLA type microalloyed steels are particularly suitable for production of forged elements with fine-grained microstructure with the use of thermomechanical processing; these steels contain elements with high chemical affinity to carbon and nitrogen, i.e.: Nb, Ti and V, and sometimes also increased concentration of N and B — improving hardenability. Metallic microadditions introduced into these steels form MX type (M — Nb, Ti, V; X — N, C) stable interstitial phases when interacting with C and N. Dispersive particles of interstitial phases, i.e. carbides, nitrides and carbonitrides of microadditions introduced into steel, inhibiting movements of grain boundaries of recrystallized austenite, create the opportunity to obtain metallurgi-cal products with high mechanical properties [1÷3].

Precipitations of interstitial phases, i.e. carbides, nitrides and the products of their mutual solubility — carbonitrides, play an impor-tant role in shaping mechanical properties of microalloyed steels. Stability of those phases depends mainly on their chemical constitu-tion, while the temperature of their dissolution can vary over a wide range. In the technological process of steel products manufacturing, it allows to obtain a variety of structural effects, such as: controlling austenite grain size at high temperature through undissolved parti-cles of precipitations, changing the kinetics of recrystallization and phase transitions induced by dissolved elements and precipitations, achieving the strengthening effect through fine-dispersive precipi-tations of carbonitrides formed at low temperature [4].

The studies of strengthening mechanisms indicate that most sig-nificant are: grain boundary strengthening, solution hardening, strain hardening and strengthening via second phase particles [3]. In case

of microalloyed steels, precipitation and grain boundary strengthen-ing have the most significant influence on shaping mechanical proper-ties, yet it is difficult to unequivocally separate the effects deriving from individual strengthening mechanisms [5÷7], thus controlling the strengthening mechanisms requires multi-scale analysis and computer aided modelling. The effectiveness of individual strengthening mecha-nisms is determined by chemical constitution of steel and the param-eters of plastic working. High strength and plastic properties of steels with ferritic-pearlitic microstructure are achieved by grain refinement and precipitation hardening. Whereas, mechanical properties of tough-ened steels depend on the strengthening coming from grain refinement of austenite induced with precipitation of interstitial phases, precipita-tion strengthening and martensitic type phase transitions, as well as from disintegration of metastable supercooled αʹ solution. Therefore, the knowledge of chemical constitution of austenite and the content of precipitations, controlling grain growth of this phase and influenc-ing the strengthening effect, has a very essential practical meaning. It makes possible to optimize the chemical constitution of steel allowing, at the lowest possible production costs, to attain appropriate micro-structure, which determines acquiring desired mechanical properties.

Meeting the increasing demands of forgings customers, espe-cially the requirements of the automotive industry, connected with an increase in strength and maintaining required ductility and brittle cracking resistance, is believed to be possible through full use of microadditions. In case of HSLA-type microalloyed steels, increase of strength is related to grain refinement and precipitation harden-ing via dispersive particles of carbides, nitrides and carbonitrides of Nb, Ti and V. While these microadditions are introduced into steels assigned for production of forgings with the use of conven-tional method [8÷10], their fully effective use can be achieved by applying forging with the method of thermomechanical processing [11, 12]. The analysis of the references related to manufacturing of forgings with the use of thermomechanical treatment method shows that the research conducted so far has been focused on introduc-tion of V [13÷15] and Ti and V [16÷18] microadditions into steel. Only few works apply to combined introduction of Nb, Ti and V microadditions [19, 20]. Limited references on the combined effect of these microadditions in the steel used for production of forg-ings creates the need for more comprehensive analysis and detailed

NR 5/2017 INŻYNIERIA MATERIAŁOWA MATERIALS ENGINEERING 207

understanding of the complex impact of MX-type interstitial phases on the microstructure and mechanical properties of steel.

2. EXPERIMENTAL PROCEDURE

Chemical composition of steel (Tab. 1) was designed taking into consideration the production of forged machine elements with method of thermomechanical processing. Investigated steel melts, weighing 100 kg, were done in VSG-100 type laboratory vacuum induction furnace.

Conditions of hot processing and cooling after its finish, allow-ing to obtain desired mechanical properties of forgings, were se-lected taking into consideration: – analyses of the kinetics of MX-type interstitial phases precipita-

tion in a solid state [7], – research of the influence of austenitizing temperature on γ phase

grain size [7], – investigation of the process of hot-working of steel with the

method of continuous compression of specimens at the rate of 1, 10 and 50 s–1 in a temperature range from 1100 to 900°C [21],

– examination of the kinetics of recrystallization processing (soft-ening of plastically deformed austenite in mentioned conditions),

– research of the kinetics of phase transformations of supercooled austenite [22].Flat bars with a 160×32 mm cross-section were forged into

14 mm thick flat bars in a temperature range of 1100 to 900°C ap-plying 50% of draft. The charge for forging was heated to 1150°C and held at that temperature for 45 min. In the first variant (I), seg-ments of flat bars were hardened in water directly from the tem-perature of forging finish. In the second variant, flat bars were iso-thermally held at a temperature of 900°C for 10, 60 and 100 s after forging finish and prior to hardening in water. The parameters of the thermomechanical processing (variant II) were selected in order to assess a various recrystallized austenite fraction before quenching on mechanical properties of forgings. Directly after quenching the obtained flat bar sections were subjected to tempering at a tempera-ture of 550 and 650°C for 1 h. Moreover, in order to compare the microstructure and mechanical properties of flat bars produced us-ing the thermomechanical treatment method, selected segments of flat bars were air-cooled after forging finish and successively sub-jected to conventional heat treatment, i.e. quenching in water from austenitizing temperature proper for steel, i.e. 900°C, and temper-ing in the same conditions as the segments of flat bars that were ob-tained in both variants of thermomechanical treatment (variant III).

Microstructure observations of the thin foils were carried out in a TITAN80-300 FEI ultra high resolution scanning transmission elec-tron microscope. The investigations was conducted in TEM mode. Identification of the chemical composition of dispersive precipita-tions was determined using an energy dispersion spectrometer EDS.

A static tensile test was carried out in order to investigate the influence of the implemented thermomechanical treatment and, in particular, the conditions of isothermal holding of forged flat bars on the mechanical properties. The study was conducted on an IN-STRON 1115 universal testing machine. The tests were done on samples 8 mm in diameter and with a gauge length of 40 mm. Im-pact testing at room temperature and at –40°C was carried out on a Charpy pendulum machine with an initial energy of 300 J, using V-notch specimens with a cross-section of 8×10 mm.

3. RESULTS AND DISSCUSION

Analyses of the microstructure of thin foils under a transmission electron microscope revealed that the microstructure of steel, hard-ened directly from the temperature of forging finish after the plas-tic deformation finish, consists of partially twinned lath martensite (Fig. 1), often with curved laths with faults as a result of uneven distribution of dislocation density in strongly plastically deformed austenite. The presence of dispersive carbides (Ti, Nb)C and TiC

which locate themselves mainly on the boundaries and inside the martensite laths (Fig. 2) was revealed in martensite with a diversi-fied spatial orientation of individual laths. The presence of retained austenite (Fig. 3) occurring in the form of thin films between the laths of martensite was also revealed in the microstructure of the studied steel.Table 1. Chemical composition of the investigated steel, wt %Tabela 1. Skład chemiczny badanych stali, % mas.

C Mn Si P S Cr

0.28 1.41 0.29 0.008 0.004 0.26

Ni Mo Nb Ti V B

0.11 0.22 0.027 0.028 0.019 0.003

Fig. 1. Microtwins bands in the martensite (a), dark field image (b); I variant of thermomechanical treatment: 900°C/water; TEM Rys. 1. Pasma mikrobliźniaków w martenzycie (a), pole ciemne (b); I wa-riant obróbki cieplno-plastycznej: 900°C/woda; TEM

TiC TiC

Fig. 2. TiC Carbide in the martensite (a), microdiffraction pattern of the precipitation (b); I variant of thermomechanical treatment: 900°C/water; TEMRys. 2. Węglik TiC w martenzycie (a), mikrodyfrakcja z wydzielenia (b); I wariant obróbki cieplno-plastycznej: 900°C/woda; TEM

Retained austenite

Fig. 3. Retained austenite as films located between martensite laths (a), dark field image from the (220) Fe-γ (b); I variant of thermomechanical treatment: 900°C/water; TEM Rys. 3. Austenit szczątkowy w postaci cienkich warstw pomiędzy listwami martenzytu (a), pole ciemne z refleksu (220) Fe-γ (b); I wariant obróbki cieplno-plastycznej: 900°C/woda; TEM


A similar morphology of lath martensite was found for steel coming from a flat bar section obtained during variant II of thermo-mechanical forging. Laths with a different width and quite diversi-fied orientation around a particular crystal zone are present in packs of martensite. Some of the martensite laths disclose fragmentation caused by the impact of subgrain boundaries with a considerable crystallographic misorientation angle on their growth, and others reveal curvilinear grain boundaries and variable width. As in the previous case, the presence of (Ti, Nb)C (Fig. 4) and (Ti, Nb, V)C dispersive carbides (Fig. 5) with variable sizes ranging from 40 to 100 nm was revealed in the martensite. The microstructure of steel hardened the conventional way after austenitizing of the samples at a temperature of 900°C also consisted of lath martensite with dispersive (Ti, Nb)C carbides.

Forgings produced in both variants of thermomechanical pro-cessing and forgings quenched conventionally from a tempera-ture of 900°C demonstrate quite a diversified microstructure after tempering in a temperature range from 550 to 650°C. The micro-structure of specimens taken from the flat bar sections, quenched directly after forging finish and after tempering at a temperature of 550°C, consisted of tempered martensite with precipitations of granular and lamellar Fe3C particles distributed inside the grains and on the lath boundaries. An increase in tempering temperature leads to coagulation of cementite. The M3C lamellar precipitations retain a privileged spatial orientation with ferrite, while this phase’s coagulated granular particles reveal a random crystallographic ori-entation with respect to the matrix.

Samples taken from the forging produced in variant II of thermo-mechanical treatment have a similar microstructure in the tempered state. In this case, after tempering at a temperature of 550°C there are thin lamellar precipitations of Fe3C on the lath boundaries, while

inside the laths of recovered ferrite there are dispersive lamellar parti-cles of the phase with privileged the relationships with matrix (Fig. 6).

Lamellar precipitations transform into granular Fe3C particles distributed on the lath boundaries and on the subgrain boundaries of the recovered ferrite along with an increase in the tempering temper-ature. The presence of (Ti, Nb)C carbides, (Ti, Nb)N nitrides (Fig. 7), and (Ti, Nb)(C, N) complex carbonitrides was also revealed in the microstructure of forging subjected to variant II of the thermo-mechanical treatment followed by tempering at 550°C. The micro-structure of steel subjected to conventional toughening also con-sisted of tempered martensite with dispersive particles of cementite.

Discussed microstructure of steel, both in hardened and tem-pered state, significantly affects mechanical properties of flat bar sections obtained in both variants of thermomechanical treatment. Results of investigation of mechanical properties and impact energy of Charpy V samples, taken from forgings produced in accordance to applied variants, are put together in Table 2.

The data presented in this table show that the flat bar section quenched in water directly from the temperature of forging finish (I variant of thermomechanical treatment), demonstrates the fol-lowing properties after tempering at the temperature of 550°C: YS0.2 of about 973 MPa, UTS of about 1057 MPa, TEl of about 13.5% and RA of about 51.5%. Flat bar sections obtained according to the II variant of thermomechanical processing reveal higher mechani-cal properties and distinctly higher crack resistance in tempered state, however, the best set of mechanical properties and crack re-sistance was noted for forging isothermally held at the temperature of 900°C for 60 s prior to hardening in water. The section of a flat bar obtained in such conditions reveals the following properties af-ter tempering in the temperature range from 550 to 650°C: YS0.2 from 994 to 911 MPa, UTS from 1084 to 974 MPa, KV from 95 to 109 J and KV–40 from 77 to 83 J.

Flat bar sections air cooled after forging, successively subject-ed to toughening (III variant) demonstrate the lowest mechanical

Fig. 4. Carbide (Ti, Nb)C in the martensite (a), EDS spectrum (b); II variant of thermomechanical treatment: 900°C/10 s/water; TEM Rys. 4. Węglik (Ti, Nb)C w martenzycie (a), widmo EDS (b); II wariant obróbki cieplno-plastycznej: 900°C/10 s/woda; TEM

Fig. 5. Carbide (Ti, Nb, V)C in the martensite (a), EDS spectrum (b); II variant of thermomechanical treatment: 900°C/60 s/water; TEMRys. 5. Węglik (Ti, Nb, V)C w martenzycie (a), widmo EDS (b); II wa-riant obróbki cieplno-plastycznej: 900°C/60 s/woda; TEM

Fig. 6. Disperse plate precipitations of Fe3C inside recovered ferrite laths (a), dark field image from the (100) Fe3C (b), electron diffraction pattern (c), solution of the diffraction from Figure 6c (d); II variant of thermomechanical treatment: 900°C/60 s/water, tempering tempera-ture: 550°C; TEMRys. 6. Dyspersyjne cząstki płytkowe Fe3C wewnątrz listew zdrowionego ferrytu (a), pole ciemne z refleksu (100) Fe3C (b), dyfrakcja (c); rozwią-zanie dyfraktogramu z rysunku 6c (d); II wariant obróbki cieplno-pla-stycznej: 900°C/60 s/woda, temperatura odpuszczania: 550°C; TEM


properties and crack resistance. Properties of the flat bar section formed in such conditions, after tempering in the investigated tem-perature range, are as follows: YS0.2 from 854 to 793 MPa, UTS from 895 to 830 MPa, TEl from 14.5 to 17.8%, RA from 58.4 to 62.5%, KV from 56 to 69 J and KV–40 from 38 to 49 J.

Conducted examinations of the influence of applied variant of processing as well as tempering temperature on hardness re-vealed that the highest hardness — of approximately 360 HBW — is demonstrated by a forging formed according to the II vari-ant of thermomechanical treatment with application of isothermal holding at the temperature of forging finish for 60 s prior to wa-ter-quenching, then subjected to tempering at the temperature of 550°C. Increase of tempering temperature for a forging formed under such conditions up to 650°C results in a mild decrease of hardness to about 330 HBW. The lowest hardness was revealed in case of segments of flat bars cooled in open air from the tempera-ture of forging finish. The hardness of flat bars obtained in such conditions, after tempering in a temperature range from 550 to 650°C, changes from 330 to 290 HBW (Fig. 8).

Fracture of Charpy V specimen taken from a forging obtained according to the I variant of thermomechanical treatment, sub-jected to tempering at the temperature of 650°C, after examination of impact resistance at the temperature of +20°C, is presented in Figure 9. Performed observations revealed that samples taken from forgings produced in such conditions have ductile fracture with nu-merous craters and voids and small amount of non-metallic inclu-sions on the fracture surface. Fractures of specimens after impact

(Ti,Nb)N

Fig. 7. Nitride (Ti, Nb)N in ferrite (a), microdiffraction pattern of the precipitation (b); II variant of thermomechanical treatment: 900°C/60 s/water, tempering temperature: 550°C; TEMRys. 7. Azotek (Ti, Nb)N w ferrycie (a), mikrodyfrakcja z wydzielenia (b); II wariant obróbki cieplno-plastycznej: 900°C/60 s/woda, temperatura odpuszczania: 550°C; TEM

Table 2. Mechanical properties and impact fracture energy using Charpy V samples after the thermomechanical forging and successive temper-ing (variant I and II) and conventional forging and successive toughening (variant III)Tabela 2. Właściwości mechaniczne oraz energia łamania próbek Charpy V po kuciu metodą obróbki cieplno-plastycznej i następnym odpuszczaniu (wariant I i II) oraz po kuciu konwencjonalnym i następnym odpuszczaniu (wariant III)

Variant

Forging conditionsTempering

temperature°C

Mechanical properties Impact energy

Charge heatingtemperature

°C

Finish of forgingtemperature

°C

Isothermal holdingtime

s

Coolingmedium

YS0.2MPa

UTSMPa

TEl %

RA%

KVJ

KV-–40

J

I 1150 ~ 900— water 550 973 1057 13.5 51.5 69.3 55.0— water 650 909 976 14.0 52.0 81.7 68.6

II 1150 ~ 900

10 water 550 967 1063 13.6 49.6 91.6 71.310 water 650 892 958 14.5 52.7 99.0 79.760 water 550 994 1084 14.3 50.9 95.0 77.360 water 650 911 974 15.1 50.2 108.7 82.6100 water 550 988 1077 13.0 49.6 95.7 75.7100 water 650 939 993 14.8 51.3 101.3 80.0

III 1150 ~ 900— air 550 854 895 14.5 58.4 56.3 37.6— air 650 793 830 17.8 62.5 68.6 49.0

Fig.8. Influence of the treatment variant and tempering temperature on hardness samples Rys. 8. Wpływ zastosowanego wariantu obróbki oraz temperatury odpuszczania na twardość próbek

0

100

200

300

400310 332

288

344 359328

Hard

ness

, HBW

550 oC

650 oCI variantII variant

III variant

Hard

ness

, HBW

550 oC


III variant

Hard

ness

, HBW

550 oC


III variant

Hard

ness

, HBW

550 oC


III variant

Hard

ness

, HBW

550 oC


III variant

Hard

ness

, HBW

550 oC


III variant

Fig. 8. Influence of the treatment variant and tempering temperature on samples hardnessRys. 8. Wpływ zastosowanego wariantu obróbki oraz temperatury od-puszczania na twardość próbek

resistance testing, taken from forgings produced according to the II variant of thermomechanical treatment, and obtained according to the III variant of processing, i.e. subjected to conventional forg-ing with free air cooling of forgings and successive toughening, have similar nature.

Dispersive particles of interstitial phases, mainly (Ti, Nb)C car-bides (Fig. 10) as well as Ti(C, N) carbonitrides, were revealed on the fracture surface of Charpy V specimens of studied steel.

4. CONCLUSIONS

Realized study showed that thermomechanical treatment (variant I and II), performed in conditions which ensure production of fine-grained microstructure of austenite prior to quenching and subse-quent high-temperature tempering, gives studied steel a better set of mechanical properties and, in particular, considerably greater resist-ance to cracking as compared with the state after open air cooling from the temperature of forging finish (variant III). Implemented thermomechanical processing allows to produce forged fabrications obtaining the following properties after controlled cooling from the temperature of plastic deformation finish and subsequent temper-ing in a temperature range from 550 to 650°C: YS0.2 from 994 to 939 MPa, UTS from 1084 to 993 MPa, TEl from 14.3 to 15.1%, RA from 51.5 to 52.7%, KV from 96 to 109 J and KV–40 from 77 to 83 J. High strength in such state at high crack resistance, also at decreased temperature, is noteworthy. The best set of mechanical properties and crack resistance was noted for a forging isothermally held at the temperature of 900°C for 60 s prior to quenching in water, subse-quently subjected to tempering at the temperature of 650°C.


Relatively low hardness of steel in state after high-temperature tempering should not cause difficulties during machining of forgings.

Microstructure observations of thin foils using transmission elec-tron microscope revealed that microstructure of steel in quenched state consists of lath martensite with high density of dislocations with considerable amount of dispersive carbides. Dispersive TiC, (Ti, Nb)C and (Ti, Nb, V)C carbides as well as Ti(C, N) carboni-trides revealed in examined steel, forming on dislocations during the plastic deformation, slow down the course of dynamic recovery and possibly also dynamic recrystallization and when hot-working is finished — decrease the rate of recovery and static or metadynamic recrystallization and limit grain growth of recrystallized austenite.

Obtained research results make it possible to develop industrial technology of forgings with high mechanical properties and guar-anteed crack resistance, also at decreased temperature, with the method of thermomechanical treatment.

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Fig.8. Influence of the treatment variant and tempering temperature on hardness samples Rys. 8. Wpływ zastosowanego wariantu obróbki oraz temperatury odpuszczania na twardość próbek

0

100

200

300

400310 332

288

344 359328

Hard

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550 oC


III variant

Hard

ness

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550 oC


III variant

Hard

ness

, HBW

550 oC


III variant

Hard

ness

, HBW

550 oC


III variant

Hard

ness

, HBW

550 oC


III variant

Hard

ness

, HBW

550 oC


III variant

Fig. 9. Micrograph of ductile fracture formed after impact test at the temperature of 20°C; I variant of thermomechanical treatment: 900°C/water; tempering temperature 650°CRys. 9. Mikrofotografia SEM przełomu ciągliwego powstałego po bada-niach udarności w temperaturze 20°C; I wariant obróbki cieplno-pla-stycznej: 900°C/woda; temperatura odpuszczania 650°C

Fig. 10. Carbide (Ti, Nb)C on the fracture surface of the specimen af-ter impact test performed at the temperature of 20°C: a) a view of the particle, b) spectrum of carbide; II variant of thermomechanical treat-ment: 900°C/60 s/water; tempering temperature: 650°C; SEMFig. 10. Węglik (Ti, Nb)C na powierzchni przełomu próbki po badaniach udarności w temperaturze 20°C: a) widok cząstki, b) widmo spektrome-tryczne węglika; II wariant obróbki cieplno-plastycznej: 900°C/60 s/woda, temperatura odpuszczania: 650°C; SEM

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Kształtowanie struktury i właściwości mechanicznych odkuwek w procesie obróbki cieplno-plastycznej

Marek OpielaInstytut Materiałów Inżynierskich i Biomedycznych, Politechnika Śląska, Gliwice; [email protected]

Inżynieria Materiałowa 5 (219) (2017) 206÷211DOI 10.15199/28.2017.5.1© Copyright SIGMA-NOT MATERIALS ENGINEERING

Słowa kluczowe: stale typu HSLA, obróbka cieplno-plastyczna, struktura, właściwości mechaniczne.

1. CEL PRACY

W pracy przedstawiono wyniki badań struktury i właściwości me-chanicznych odkuwek ze stali z mikrododatkami Ti, Nb, V i B typu HSLA wytworzonej w kontrolowanym procesie obróbki cieplno--plastycznej, poddanej następnie wysokiemu odpuszczaniu.

2. MATERIAŁ I METODYKA BADAŃ

Badania przeprowadzono na stali mikrostopowej typu HSLA o skła-dzie chemicznym zestawionym w tabeli 1. Wytopy badanej stali o masie 100 kg wykonano w laboratoryjnym próżniowym piecu in-dukcyjnym typu VSG-100S firmy PVA TePla AG. Uzyskane wyniki badań wstępnych posłużyły do opracowania dwóch wariantów ku-cia metodą obróbki cieplno-plastycznej płaskowników o przekroju 160×32 mm na płaskowniki o grubości 14 mm, w zakresie tempera-tury 1100 do 900°C z szybkością odkształcenia 3 s–1, stosując 50% stopień gniotu. Wsad do kucia nagrzewano do temperatury 1150°C i wytrzymywano w tej temperaturze przez 45 min. W pierwszym wariancie odcinki płaskowników hartowano w wodzie bezpośrednio z temperatury końca kucia. Natomiast w wariancie drugim po za-kończeniu kucia płaskowniki wytrzymywano izotermicznie w tem-peraturze 900°C przez 10, 60 i 100 s przed hartowaniem w wodzie. Bezpośrednio po hartowaniu wytworzone odcinki płaskowników poddano odpuszczaniu w temperaturze 550 i 650°C przez 1 h. Po-nadto w celu porównania struktury i właściwości mechanicznych płaskowników wytworzonych metodą obróbki cieplno-plastycznej wybrane odcinki płaskowników po zakończeniu kucia chłodzono na wolnym powietrzu, a następnie poddano konwencjonalnej obróbce cieplnej, tj. hartowaniu w wodzie z właściwej dla tej stali tempe-ratury austenityzowania wynoszącej 900°C i odpuszczaniu w iden-tycznych warunkach jak odcinki płaskowników wytworzone w obu wariantach obróbki cieplno-plastycznej (wariant III).

3. WYNIKI I ICH DYSKUSJA

Badania mikrostruktury cienkich folii przygotowanych ze stali zahartowanej bezpośrednio z temperatury końca kucia po zakoń-czeniu obróbki plastycznej ujawniły w mikrostrukturze osnowy martenzyt listwowy o dużej gęstości dyslokacji, częściowo zbliź-niaczony (rys. 1), wydzielenia, głównie węglików (Ti, Nb)C oraz TiC (rys. 2), lokujące się przede wszystkim na granicach i wewnątrz listew martenzytu. Ujawniono także obecność austenitu szczątko-wego (rys. 3) występującego w postaci cienkich warstw pomiędzy listwami martenzytu. Podobną morfologię martenzytu listwowego ma stal pochodząca z odcinka płaskownika wytworzonego w II wariancie obróbki cieplno-plastycznej. W pakietach martenzytu występują listwy o różnej szerokości i dość zróżnicowanej orienta-cji wokół określonego pasa krystalograficznego. Niektóre z listew martenzytu wykazują fragmentację spowodowaną oddziaływaniem

na ich wzrost podgranic o znacznym kącie dezorientacji krystalo-graficznej, a inne krzywoliniowe granice ziaren i zmienną szero-kość. Ponadto w martenzycie ujawniono obecność dyspersyjnych węglików (Ti, Nb)C (rys. 4) oraz (Ti, Nb, V)C (rys. 5) o zmiennej wielkości w zakresie od 40 do 100 nm.

Omówiona struktura stali zarówno w stanie zahartowanym, jak i odpuszczonym wywiera znaczący wpływ na właściwości mecha-niczne odcinków płaskowników wytworzonych w obu wariantach obróbki cieplno-plastycznej. Wyniki badań właściwości mecha-nicznych oraz energii łamania próbek Charpy V pobranych z odku-wek wytworzonych według zastosowanych wariantów zestawiono w tabeli 2. Z danych przedstawionych w tej tabeli wynika, że od-cinek płaskownika bezpośrednio hartowany w wodzie z tempera-tury końca kucia (I wariant obróbki cieplno-plastycznej) wykazuje po odpuszczaniu w temperaturze 550°C następujące właściwości: Rp0.2 około 973 MPa, Rm około 1057 MPa, A około 13,5% i Z około 51,5%. Większe właściwości mechaniczne oraz wyraźnie większą odporność na pękanie w stanie odpuszczonym wykazują odcinki płaskowników wytworzone według II wariantu obróbki cieplno-pla-stycznej, przy czym najlepszy zespół właściwości mechanicznych oraz odporność na pękanie ma odkuwka wytrzymana izotermicznie w temperaturze 900°C przez 60 s przed hartowaniem w wodzie. Wytworzony w tych warunkach odcinek płaskownika wykazuje po odpuszczaniu w zakresie temperatury od 550 do 650°C następujące właściwości: Rp0.2 od 994 do 911 MPa, Rm od 1084 do 974 MPa, KV od 95 do 109 J i KV–40 od 77 do 83 J.

4. PODSUMOWANIE

Zrealizowane badania wykazały, że przeprowadzona obróbka ciepl-no-plastyczna (wariant I i II) w warunkach zapewniających wy-tworzenie drobnoziarnistej struktury austenitu przed hartowaniem i następne wysokie odpuszczanie nadaje stali lepszy zespół wła-ściwości mechanicznych, a zwłaszcza wydatnie większą odporność na pękanie, w porównaniu ze stanem po chłodzeniu z temperatury kucia na wolnym powietrzu (wariant III).

Najlepsze właściwości mechaniczne oraz odporność na pęka-nie ma odkuwka wytrzymana izotermicznie w temperaturze 900°C przez 60 s przed hartowaniem w wodzie poddana następnie odpusz-czaniu w temperaturze 650°C.

Badania mikrostruktury cienkich folii w transmisyjnym mikro-skopie elektronowym wykazały, że stal w stanie zahartowanym ma strukturę martenzytu listwowego o dużej gęstości dyslokacji. W mar-tenzycie ujawniono węgliki TiC, (Ti, Nb)C i (Ti, Nb, V)C oraz węgli-koazotki Ti(C, N). Dyspersyjne cząstki tych faz, hamujące ruch granic ziaren austenitu zrekrystalizowanego dają możliwość wytwarzania wyrobów hutniczych o dobrych właściwościach mechanicznych.

Uzyskane wyniki umożliwiają opracowanie technologii prze-mysłowej obróbki cieplno-plastycznej odkuwek o dobrych właści-wościach mechanicznych i gwarantowanej odporności na pękanie, także w temperaturze obniżonej.

development of microstructure and mechanical properties of

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