Study of microstructure and mechanical properties of 14% ...

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126 INŻYNIERIA MATERIAŁOWA MATERIALS ENGINEERING ROK XXXVII Study of microstructure and mechanical properties of 14% Cr ODS steel produced by hot isostatic pressing followed by hot forging Zbigniew Oksiuta Faculty of Mechanical Engineering, Bialystok University of Technology, Bialystok, Poland, [email protected] Consolidation by hot isostatic pressing followed by hot forging of a mechanically alloyed ODS Fe–14% Cr–2% W–0.3% Ti–0.3% Y 2 O 3 (in wt %) ferritic steel powder was applied to find the effect of those methods on the microstructure and mechanical properties of the alloy. Microstructure investigations of the ODS steel revealed that after hot isostatic pressing bimodal-like microstructure with fine and coarse grains up to a few microns was observed. Hot forging process, performed at about 900°C with 50% of deformation, did not improved microstructure homogeneity and grains elongated towards radial direction were observed. High temperature tensile tests revealed that hot forging improves the tensile strength of the as-HIPped ODS steel. However, the ductility and upper shelf energy decreased. This can be related to the microstructure of the ODS steel observed for both consolidation methods and emphasises a necessity of further optimization of hot forging parameters. Key words: ODS ferritic steel, hot isostatic pressing, hot forging, tensile testing, Charpy impact properties. Inżynieria Materiałowa 3 (211) (2016) 126÷130 DOI 10.15199/28.2016.3.6 © Copyright SIGMA-NOT MATERIALS ENGINEERING 1. INTRODUCTION An oxide dispersion strengthened (ODS) reduced activation ferritic (RAF) steel is an advanced superalloy considered to be used as the most promising candidate material for structural components in the future fusion reactor. The ODS reduced activation ferritic steel can be obtained through selection of the appropriate alloying elements with a low activation ratio. Therefore, such widely alloying ele- ments used in steels production as Al, Ni, Mo, Cu, Nb and N have to be eliminated and replaced by low activation elements, e.g. Fe, Cr, W, V, Ti, Si and C. These elements also ensure fully ferritic (BCC) structure. In addition, in the ODS RAF steel harmful elements like Ag, Ho, Bi, Co, Sm, Lu, Dy, Gd, and Cd must be restricted to a very low level [1]. The ODS RAF steel can replace an older generation of such steels as PM2000, MA956 and MA957 [2, 3]. The ODS RAF steel has many advantages such as a high thermal conductivity, low expansion coefficient, excellent irradiation resist- ance and superior mechanical properties at high temperature [4÷6]. This kind of steel is generally produced by powder metallurgy (PM) route that consists of mechanical alloying (MA) of a pre-alloyed powder with yttrium oxide nanoparticles followed by hot isostatic pressing (HIP) or hot extrusion (HE). However, this PM processing route of the ODS steel is a source of the most of the problems ex- hibited by the material. These problems encompass brittleness, rela- tively high ductile to brittle transition temperature (DBTT), porosity, inhomogeneous microstructure and anisotropy of mechanical prop- erties. Therefore, thermomechanical treatments, such as hot rolling (HR) or hot forging (HF), can be applied to homogenize the micro- structure of the ODS steel. Most of the authors prefer multistep hot rolling process [7, 8], and according to this author’s knowledge few articles about hot forging process of the ODS steel were published. However, Ukai et al. [9] reported that HIPing followed by forging can significantly increase Charpy impact properties of the ODS fer- ritic steels by porosity reduction and grain size refinement. Therefore, the main goal of this work is to develop and testing a new generation of 14% Cr ODS RAF steel reinforced by yttrium oxide nanoparticles. The article describes the powder metallurgy manufacturing route that consists of mechanical alloying of a pre- alloyed, argon atomized, ferritic steel powder and further consolida- tion by HIP followed by hot forging. The microstructure as well as tensile and Charpy impact properties of the ODS RAF steel were investigated and will be discussed. 2. EXPERIMENTAL A pre-alloyed Fe–14Cr–2W–0.3Ti (in wt %) gas atomized powder was mechanically alloyed with 0.3% Y 2 O 3 nanoparticles in a plan- etary ball mill, for 30 h, in high purity argon atmosphere. After me- chanical alloying the 14Cr ODS steel powder was canned, degassed at 450°C in vacuum of 10 –2 Pa, hermetically sealed and HIPed at 1150°C under pressure of 200 MPa for 2 hours. After HIP the ingot was annealed at 900°C for 1 h and hot forged at 100 T??? press with 50% of deformation with cooling down slowly to room temperature. Microscopic observations of the 14Cr ODS steel were performed using a light microscopy (LM), a scanning electron microscopy (SEM) and a transmission electron microscopy (TEM). Microhard- ness measurements were carried out using a Vickers diamond pyra- mid (JENOPHOT 2000) by applying a load of 0.98 N for 15 s. At each condition, 10 measurements were performed. High tempera- ture tensile tests were carried out at temperature of 20°C, 600°C and 750° C for flat 0.5×1.5×25 mm 3 specimens with a strain rate of 10 –4 s –1 in argon. At least three tensile specimens at each tem- perature were tested. Charpy impact tests were performed between –100°C and 300°C using an impact machine with an energy capaci- ty of 30 J on KLST (3×4×27 mm 3 ) specimens. The ductile-to-brittle transition temperature (DBTT) was determined at the half-value of the upper shelf energy (USE). Chemical analyses were conducted using wavelength dispersive X-ray fluorescence spectroscopy (WD-XRF) as well as LECO TC-436 and LECO IR-412 device. 3. RESULTS AND DISCUSSION Chemical composition of the tested ODS ferritic steel after HIP is summarized in Table 1. Table 1. Chemical composition of the ODS steel after HIP Tabela 1. Skład chemiczny stali ODS po HIP Element Cr W Ti Mn C Al Y O Fe wt % 13.8 1.9 0.33 0.38 0.056 0.02 0.26 0.17 bal.

Transcript of Study of microstructure and mechanical properties of 14% ...

126 INŻYNIERIA MATERIAŁOWA MATERIALS ENGINEERING ROK XXXVII

Study of microstructure and mechanical properties of 14% Cr ODS steel produced by hot isostatic pressing

followed by hot forgingZbigniew Oksiuta

Faculty of Mechanical Engineering, Bialystok University of Technology, Bialystok, Poland, [email protected]

Consolidation by hot isostatic pressing followed by hot forging of a mechanically alloyed ODS Fe–14% Cr–2% W–0.3% Ti–0.3% Y2O3 (in wt %) ferritic steel powder was applied to find the effect of those methods on the microstructure and mechanical properties of the alloy. Microstructure investigations of the ODS steel revealed that after hot isostatic pressing bimodal-like microstructure with fine and coarse grains up to a few microns was observed. Hot forging process, performed at about 900°C with 50% of deformation, did not improved microstructure homogeneity and grains elongated towards radial direction were observed. High temperature tensile tests revealed that hot forging improves the tensile strength of the as-HIPped ODS steel. However, the ductility and upper shelf energy decreased. This can be related to the microstructure of the ODS steel observed for both consolidation methods and emphasises a necessity of further optimization of hot forging parameters.

Key words: ODS ferritic steel, hot isostatic pressing, hot forging, tensile testing, Charpy impact properties.

Inżynieria Materiałowa 3 (211) (2016) 126÷130DOI 10.15199/28.2016.3.6© Copyright SIGMA-NOT MATERIALS ENGINEERING

1. INTRODUCTION

An oxide dispersion strengthened (ODS) reduced activation ferritic (RAF) steel is an advanced superalloy considered to be used as the most promising candidate material for structural components in the future fusion reactor. The ODS reduced activation ferritic steel can be obtained through selection of the appropriate alloying elements with a low activation ratio. Therefore, such widely alloying ele-ments used in steels production as Al, Ni, Mo, Cu, Nb and N have to be eliminated and replaced by low activation elements, e.g. Fe, Cr, W, V, Ti, Si and C. These elements also ensure fully ferritic (BCC) structure. In addition, in the ODS RAF steel harmful elements like Ag, Ho, Bi, Co, Sm, Lu, Dy, Gd, and Cd must be restricted to a very low level [1]. The ODS RAF steel can replace an older generation of such steels as PM2000, MA956 and MA957 [2, 3].

The ODS RAF steel has many advantages such as a high thermal conductivity, low expansion coefficient, excellent irradiation resist-ance and superior mechanical properties at high temperature [4÷6]. This kind of steel is generally produced by powder metallurgy (PM) route that consists of mechanical alloying (MA) of a pre-alloyed powder with yttrium oxide nanoparticles followed by hot isostatic pressing (HIP) or hot extrusion (HE). However, this PM processing route of the ODS steel is a source of the most of the problems ex-hibited by the material. These problems encompass brittleness, rela-tively high ductile to brittle transition temperature (DBTT), porosity, inhomogeneous microstructure and anisotropy of mechanical prop-erties. Therefore, thermomechanical treatments, such as hot rolling (HR) or hot forging (HF), can be applied to homogenize the micro-structure of the ODS steel. Most of the authors prefer multistep hot rolling process [7, 8], and according to this author’s knowledge few articles about hot forging process of the ODS steel were published. However, Ukai et al. [9] reported that HIPing followed by forging can significantly increase Charpy impact properties of the ODS fer-ritic steels by porosity reduction and grain size refinement.

Therefore, the main goal of this work is to develop and testing a new generation of 14% Cr ODS RAF steel reinforced by yttrium oxide nanoparticles. The article describes the powder metallurgy manufacturing route that consists of mechanical alloying of a pre-alloyed, argon atomized, ferritic steel powder and further consolida-tion by HIP followed by hot forging. The microstructure as well as

tensile and Charpy impact properties of the ODS RAF steel were investigated and will be discussed.

2. EXPERIMENTAL

A pre-alloyed Fe–14Cr–2W–0.3Ti (in wt %) gas atomized powder was mechanically alloyed with 0.3% Y2O3 nanoparticles in a plan-etary ball mill, for 30 h, in high purity argon atmosphere. After me-chanical alloying the 14Cr ODS steel powder was canned, degassed at 450°C in vacuum of 10–2 Pa, hermetically sealed and HIPed at 1150°C under pressure of 200 MPa for 2 hours. After HIP the ingot was annealed at 900°C for 1 h and hot forged at 100 T??? press with 50% of deformation with cooling down slowly to room temperature.

Microscopic observations of the 14Cr ODS steel were performed using a light microscopy (LM), a scanning electron microscopy (SEM) and a transmission electron microscopy (TEM). Microhard-ness measurements were carried out using a Vickers diamond pyra-mid (JENOPHOT 2000) by applying a load of 0.98 N for 15 s. At each condition, 10 measurements were performed. High tempera-ture tensile tests were carried out at temperature of 20°C, 600°C and 750° C for flat 0.5×1.5×25 mm3 specimens with a strain rate of 10–4 s–1 in argon. At least three tensile specimens at each tem-perature were tested. Charpy impact tests were performed between –100°C and 300°C using an impact machine with an energy capaci-ty of 30 J on KLST (3×4×27 mm3) specimens. The ductile-to-brittle transition temperature (DBTT) was determined at the half-value of the upper shelf energy (USE). Chemical analyses were conducted using wavelength dispersive X-ray fluorescence spectroscopy (WD-XRF) as well as LECO TC-436 and LECO IR-412 device.

3. RESULTS AND DISCUSSION

Chemical composition of the tested ODS ferritic steel after HIP is summarized in Table 1.Table 1. Chemical composition of the ODS steel after HIPTabela 1. Skład chemiczny stali ODS po HIP

Element Cr W Ti Mn C Al Y O Fe

wt % 13.8 1.9 0.33 0.38 0.056 0.02 0.26 0.17 bal.

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LM and TEM images of both ODS as-HIPped and HF speci-mens are presented in Figures 1 and 2. There is a notable dissimi-larity in grain size and dislocation structure observed. As HIPped ODS material has bimodal distribution of coarse up to 30 µm grains with smaller subgrain structure, clearly observed by TEM in Figure 1b and 1c. Coarser and elongated towards radial direction (up to 200 µm) grains has material after HIP and HF (Fig. 2a). However, as expected, the dislocation density in this specimen is significantly higher than in as-HIPped material. TEM observations of this speci-men also revealed subgrains microstructure (Fig. 2b) with the grain size about 500 nm. In general, the results of microstructural inves-tigations suggest that hot forging at 900°C causes the microstruc-tural anisotropy and on the other hand increases dislocation density. This can be confirmed by Vickers microhardness tests of both ODS

specimens. HIP and HF materials are about 10% harder in compari-son with the as-HIPed alloy, 450±15 HV0.1 and 415±20 HV0.1, respectively.

Table 2 summarizes the results of the tensile properties as a func-tion of testing temperature. Figures 3 and 4 also show an example of stress–strain curves of both ODS steels tested at RT, 600°C and 750°C, respectively.

It is clear that ultimate tensile strength (UTS) and 0.2% yield stress (YS0.2) of the HIP + HF material are considerably higher, with-in a range of testing temperature but the total elongation is much lower than that for as-HIPed ODS steel. The effect of work hard-ening after HF is obvious. In comparison to the literature data the results of tensile tests are rather comparable [10]. Reported by Y. Wen et al. [10] tensile properties of a 14YWT ODS steel, consoli-

Fig. 1. Microstructure of the ODS ferritic steel after HIP: a) after etch-ing; LM, b) dislocations structure; TEM, c) higher magnification of TEM imageRys. 1. Mikrostruktura stali ODS po prasowaniu izostatycznym (HIP): a) zgład trawiony; mikroskop świetlny, b) struktura dyslokacji; TEM, c) zdjęcie TEM przy dużym powiększeniu

Fig. 2. Microstructure of the ODS ferritic steel after HIP and HF: a) after etching; LM, b) general view of microstructue; TEM, c) dislo-cations structure; TEMRys. 2. Mikrostruktura stali ODS po prasowaniu izostatycznym (HIP) i kuciu na gorąco (HF): a) zgład trawiony; mikroskop świetlny, b) widok ogólny struktury; TEM, c) struktura dyslokacji; TEM

a) a)

b) b)

c) c)

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dated by hot forging followed by rolling, are not statistically differ-ent (UTS = 1150 MPa and Tel = 10.8%, tested at RT). However, the influence of the rolling process on the ductility of the ODS ferritic steel, especially at elevated temperature cannot be neglected.

Results of Charpy impact energy vs testing temperature are sum-marized in Figure 5. Unexpectedly, the upper shelf energy of as-HIPped ODS steel is about 25% higher in comparison to the HIP and HF specimen (USE = 1.7 J). However, the lower shelf energy (LSE) and transition temperature are better for the HIP and HF spec- imen. This is in coincidence with the microhardness and tensile re-

sults obtained for both materials, and indicate that inhomogeneous microstructure and higher dislocation density decrease the absorbed impact energy at upper shelf temperature and on the other hand no-tably shifts the DBTT toward lower value, probably by closing re-sidual porosity of as-HIPped alloy.

The fracture surface morphologies of broken specimens at the LSE (–100°C) and USE (200°C) are presented in Figure 6. Cleav-age-type of brittle fracture is clearly observed for both materials tested at LSE temperature. The cleavage fracture proceeded through the specimens with no shear lips at the edge of the specimens and with no lateral expansion. The cleavage steps and river patterns in-dicate that in both materials the grain boundaries tend to be stronger than the grains.

The fracture mode changes from brittle to ductile at the tempera-ture range 35÷85°C, that is considerably above the safety require-ments for application of the ODS steel in the future fusion reactor. At 100°C and above, low magnification SEM images (not presented here) revealed a typical ductile cups and cone fracture mode. Higher magnification of SEM images presented in Figure 6, taken from the centre area of the fracture surfaces of all specimens, show dimpled fracture morphology.

The results presented here clearly indicate that mechanical prop-erties of the ODS ferritic steel, produced by powder metallurgy route, are influenced by the consolidation method applied. The mechani-cal properties of both materials are complex and difficult to be un-ambiguously explained. As expected, the tensile strength is higher for HIP and HF material within a range of testing temperature. On the contrary, the ductility presents reverse dependence. The HIP and HF material exhibits inferior ductility. Assuming that oxide nano-particles morphology in both ODS specimens are similar, the higher strength can be attributed to a large number of the dislocations and elongated (textured) grains observed in the material after hot forg-ing. On one hand, higher dislocation density strengthened the tensile properties and increases the hardness of as-HIP and HF steel; on the other hand it decreases ductility. Also, the impact energy of as-HIPed and HF steel has not been improved considerably. It is difficult to find an explanation for lower USE values measured for the hot forged specimen. Probably, the USE decrease is related to the elongated (textured) grains observed in the ODS steel after hot forging. There-fore, further tests have to be conducted towards optimization of the hot forging process parameters, especially the temperature. However, a significant improvement of the mechanical properties and fracture of the ODS RAF steel was also reported in Ref. [8, 10, 14], when after powder consolidation additional multi-steps hot rolling treat-ment was applied.

Table 2. Summary of the tensile properties of the ODS steels as a func-tion of testing temperatureTabela 2. Wyniki badań wytrzymałości na rozciąganie stali ODS w funk-cji temperatury

T °C

HIP HIP and HFUTS MPa

YS0.2 MPa

Tel %

UTS MPa

YS0.2 MPa

Tel %

RT 950±32 865±23 12.5±1.4 1120±26 970±25 6.2±1.9

600 505±15 470±10 13.2±1.2 570±25 540±20 10.4±2.4

750 230±10 210±15 6.2±1.6 310±20 275±12 4.5±1.2

Fig. 3. Tensile stress-strain curves of the ODS steel after HIPping Rys. 3. Krzywe naprężenie-odkształcenie uzyskane dla stali ODS po HIP w trzech różnych temperaturach

HIPed RT

Fig. 3. Tensile stress–strain curves of the ODS steel after HIPRys. 3. Krzywe naprężenie–odkształcenie uzyskane dla stali ODS po pra-sowaniu izostatycznym (HIP) w różnej temperaturze

Fig. 4. Tensile stress-strain curves of the ODS steel after HIPping and HF Rys. 4. Krzywe naprężenie-odkształcenie uzyskane dla stali ODS po HIP i kuciu na gorąco

RT

Fig. 4. Tensile stress–strain curves of the ODS steel after HIP and HFRys. 4. Krzywe naprężenie–odkształcenie uzyskane dla stali ODS po pra-sowaniu izostatycznym (HIP( i kuciu na gorąco (HF)

Fig. 5. Charpy impact properties of the HIP and HF specimens Rys. 5. Wyniki testów udarności stali ODS po HIP i kuciu na gorąco w funkcji temperatury

Fig. 5. Charpy impact properties of the HIP and HF specimensRys. 5. Wyniki prób udarności stali ODS po prasowaniu izostatycznym (HIP) i kuciu na gorąco (HF) w funkcji temperatury

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4. SUMMARYThe mechanically alloyed Fe–14Cr–2W–0.3Ti–0.3Y2O3 ODS ferri-tic steel powder was consolidated by HIP followed by HF. Compar-ative characterization of the microstructure and mechanical proper-ties of both ODS materials was carried out. Microstructural analysis revealed important differences in grain size and dislocation density of tested materials. Bimodal grain size distribution was observed in as-HIPed steel whereas material after HIP and HF exhibited elon-gated and coarser microstructure. As expected, the as-HIPed and HF material has substantially higher hardness, tensile strength and better (lower) DBTT. However, the total elongation and USE were not improved. Further optimization of the hot forging process pa-rameters is necessary to obtain an improvement of USE, keeping compromise between good strength and ductility of the ODS steel.

ACKNOWLEDGMENTS

This work was supported by Bialystok University of Technology through a grant No. S/WM/1/2015.

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Fig. 6. SEM images of the surface fracture of the ODS steel: a) and b) after HIPping at -100 C and 200 C, respectively, c) and d) HIP and HF at -100 C and 200 C, respectively Rys. 6. Zdjęcia SEM przełomów próbek stali ODS: a) i b) po HIP odpowiednio w -100C i 200C oraz c) i d) po HIP i kuciu, odpowiednio w -100C i 200C

Fig. 6. SEM images of the surface fracture of the ODS steel: a) and b) after HIPping at -100 C and 200 C, respectively, c) and d) HIP and HF at -100 C and 200 C, respectively Rys. 6. Zdjęcia SEM przełomów próbek stali ODS: a) i b) po HIP odpowiednio w -100C i 200C oraz c) i d) po HIP i kuciu, odpowiednio w -100C i 200C

Fig. 6. SEM images of the surface fracture of the ODS steel: a) and b) after HIPping at -100 C and 200 C, respectively, c) and d) HIP and HF at -100 C and 200 C, respectively Rys. 6. Zdjęcia SEM przełomów próbek stali ODS: a) i b) po HIP odpowiednio w -100C i 200C oraz c) i d) po HIP i kuciu, odpowiednio w -100C i 200C

Fig. 6. SEM images of the surface fracture of the ODS steel: a) and b) after HIPping at -100 C and 200 C, respectively, c) and d) HIP and HF at -100 C and 200 C, respectively Rys. 6. Zdjęcia SEM przełomów próbek stali ODS: a) i b) po HIP odpowiednio w -100C i 200C oraz c) i d) po HIP i kuciu, odpowiednio w -100C i 200C

Fig. 6. Fracture surface of the ODS steel: a), b) after HIP, at –100°C and 200°C, respectively, c), d) HIP and HF, at –100°C and 200°C, respec-tively; SEMRys. 6. Przełomy próbek stali ODS: a), b) po prasowaniu izostatycznym (HIP), odpowiednio w –100°C i 200°C, c), d) po prasowaniu izostatycznym (HIP) i kuciu na gorąco (HF), odpowiednio w –100°C i 200°C

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130 INŻYNIERIA MATERIAŁOWA MATERIALS ENGINEERING ROK XXXVII

Badania mikrostruktury i właściwości mechanicznych ferrytycznej stali ODS wytworzonej metodą HIP

i kucia na gorącoZbigniew Oksiuta

Wydział Mechaniczny, Politechnika Białostocka, Białystok, [email protected]

Inżynieria Materiałowa 3 (211) (2016) 126÷130DOI 10.15199/28.2016.3.6© Copyright SIGMA-NOT MATERIALS ENGINEERING

Słowa kluczowe: stale ODS, udarność, wytrzymałość na rozciąganie, prasowanie izostatyczne i kucie na gorąco.

1. CEL PRACY

Artykuł przedstawia problematykę związaną z otrzymywaniem nowej generacji stali ferrytycznych umacnianych nanocząstkami tlenku itru (Oxide Dispersion Strengthened, ODS). Produkowane metodą mechanicznego stopowania (MS) stale ODS ze względu na swoje właściwości: żarowytrzymałość, odporność na pełzanie, mały współczynnik rozszerzalności cieplnej oraz odporność na pęcznienie podczas promieniowania jonizującego są uważane za atrakcyjny materiał strukturalny do budowy reaktora termojądro-wego.

Zasadniczym problemem dotyczącym otrzymywania stali ODS jest to, że do ich produkcji można użyć tylko pierwiastków, któ-rych czas połowicznego rozpadu jest krótki (Reduced Activation Elements). Do grupy pierwiastków stanowiących podstawę do pro-dukcji stali należy zaliczyć: żelazo, chrom, wolfram, tytan, wanad, krzem i węgiel oraz stabilne temperaturowo tlenki Y i Ti. Użycie innych pierwiastków, np.: Mn, Ni, Al, Co, Cu i B, stosowanych po-wszechnie do produkcji innych gatunków stali nie jest wskazane.

Dodatek tlenku itru oraz zanieczyszczenia pochodzące z procesu MS powodują pogorszenie udarności i plastyczności stali. Ponadto stale ferrytyczne o strukturze regularnie przestrzennie centrowa-nej (RPC) mają wysoką temperaturę progu kruchości (Ductile to Brittle Transition Temperature, DBTT), która jest znacznie powyżej temperatury pokojowej. Na przykład w żaroodpornych stalach fer-rytycznych typu PM2000 lub MA957 stwierdzono, że temperatura przejścia ze stanu kruchego w stan plastyczny jest rzędu 100°C. Jednym ze sposobów obniżenia temperatury progu kruchości jest zastosowanie dodatkowej obróbki cieplno-plastycznej. Dlatego ce-lem pracy było otrzymanie przez MS i konsolidację (metodą HIP) stali ODS oraz zastosowanie dodatkowej obróbki cieplno-plastycz-nej, kucia na gorąco, w celu określenia wpływu tej metody na wy-brane właściwości stali.

2. MATERIAŁ I METODYKA BADAŃ

Materiałem użytym do badań był rozpylany w argonie proszek stopowy Fe–14% Cr–2% W–0,3% Ti mechanicznie stopowany w młynie kulowym w atmosferze argonu z dodatkiem 0,3% Y2O3. Po MS proszek został odgazowany w próżni i hermetycznie za-mknięty w metalowej kapsule i poddany procesowi HIP w tempera-turze 1150°C, pod ciśnieniem 200 MPa przez 2 h. Skład chemiczny stali po HIP przedstawiono w tabeli 1. Po procesie HIP kapsułę wy-grzewano w 900°C przez 1 h i poddano kuciu na gorąco. Po kon-solidacji przygotowano zgłady metalograficzne w celu porównania mikrostruktury stali zarówno po HIP, jak i po HIP i kuciu. Mikro-strukturę badano za pomocą skaningowej (SEM) i transmisyjnej mikroskopii elektronowej (TEM). Twardość próbek wyznaczono

sposobem Vickersa, stosując obciążenie 0,1 kG (0,98 N) przez 15 s. Próby wytrzymałości na rozciąganie prowadzono w temperaturze 20, 600 i 750°C ze stałą szybkością odkształcenia 10–4 s–1 w ar-gonie. Badania udarności metodą Charpy prowadzono z użyciem miniaturowych próbek o wymiarach 3×4×27 mm w zakresie tem-peratury od –100°C do 300°C.

3. WYNIKI I ICH DYSKUSJA

Mikrostrukturę stali ODS po HIP i po kuciu na gorąco przedstawio-no na rysunkach 1 i 2. Stal ODS po HIP ma bimodalny rozkład zia-ren (rys. 1a i 1b), natomiast wydłużone w kierunku promieniowym ziarna zaobserwowano w przypadku materiału po HIP i kuciu (rys. 2). Obserwacje mikrostruktury sugerują, że proces kucia spowodo-wał anizotropię mikrostruktury oraz zwiększenie gęstości dysloka-cji. Potwierdzają to badania twardości obu próbek, z których wyni-ka, że stal w wyniku kucia ma o około 10% większą twardość w po-równaniu ze stalą po HIP, odpowiednio: 450 HV0,1 i 415 HV0,1.

Typowe krzywe wytrzymałości na rozciąganie w funkcji tem-peratury oraz wyniki badań udarności przedstawiono odpowiednio na rysunkach 3, 4 i 5. W tabeli 2 przedstawiono średnie wartości wyników badań wytrzymałości na rozciąganie. Rysunek 6 prezen-tuje zdjęcia SEM przełomów próbek stali po procesie HIP i kucia na gorąco.

Z prób wytrzymałości na rozciąganie wynika, że zarówno Rm, jak i Re0,2 stali po HIP i kuciu są większe w zakresie badanej temperatury w porównaniu z materiałem po HIP, natomiast całkowite wydłuże-nie stali uległo pogorszeniu. Efekt ten można wytłumaczyć umoc-nieniem stali ODS spowodowanym kuciem.

Próby udarności ujawniły, że USE (Upper Shelf Energy) stali po HIP jest o 25% większa w porównaniu z materiałem po kuciu (USE = 1,7 J). Jednak wyniki LSE (Lower Shelf Energy) oraz DBTT stali po kuciu są lepsze (rys. 5). Efekt zmniejszenia USE można wy-jaśnić niejednorodnością struktury i większą gęstością dyslokacji zaobserwowaną po kuciu. Natomiast obniżenie temperatury progu kruchości jest prawdopodobnie spowodowane zmniejszeniem po-rowatości szczątkowej w stali po procesie HIP.

4. PODSUMOWANIE

Porównawcza charakterystyka mikrostruktury stali ODS po HIP i po kuciu na gorąco ujawniła znaczące różnice w wielkości ziaren i gęstości dyslokacji badanych materiałów, co spowodowało zwięk-szenie twardości, wytrzymałości na rozciąganie i zmniejszenie DBTT. Jednakże wydłużenie i USE uległy pogorszeniu. Aby uzy-skać poprawę właściwości mechanicznych stali ODS będą prowa-dzone dalsze badania optymalizacji parametrów kucia na gorąco.