No abstract

Tensile/impact behaviour of lower bainite obtained in high-Si steel 55Si3Mn2CrMoVNb was studied using SEM, TEM, and XRD. Specimens were austenitized at 900 • C and isothermally treated at 250, 270, and 300 • C with holding up to 600 min. The heat treatment results in the formation of cementite-free lower bainite/retained austenite structure, where retained austenite was found as blocky "islands" and interlaths "films". The width of bainitic ferrite laths decreases from 170-240 µm to 45-80 µm with holding temperature decreasing. This results in increasing UTS (to 1700 MPa) and hardness (to 52 HRC). The optimal combination of mechanical properties (UTS 1397-1522 MPa, hardness 45-47 HRC, total elongation 18-21 %, U-notched impact toughness 105-139 J cm −2 ) refers to holding at 300 • C to be associated with higher amount of retained austenite (30-33 %). With prolonging the bainitizing duration the hardness and ductility decreases while impact toughness increases. Prolonged holding at 300 • C leads to a continuation of bainite transformation and precipitation of transitional carbides within ferrite laths. K e y w o r d s : carbide-free lower bainite, retained austenite, phase transformations, microstructure, mechanical properties

Alloys ZhS6U-VI and ZhS3DK-VI modified jointly and individually with zirconium {0.05"i,) and hafnium (0.5%) arc described. It is established that complex modifying increases the strength and ductility properties of the alloys. The microhardness of alloy ZhS6U-VI is increased after tests for long-term strength. In alloy ZhS6U-VI tested for long-term strength cy-phase segregates in the form of flake and acicular particles. Modification of the alloy with hafnium increases the content of the flake o-phase, modification with zirconium increases the content of the acicular G-phase.

The high thermodynamic activity of zirconium with respect to some metals and nonmetals has made it a widely used additive for modifying various alloys. The present work is devoted to the effect of zirconium additives on the structure and properties of heat-resistant nickel alloys ZhS6U-VI and ZhS3DK-VI.The alloys studied were melted in a UPPF-3M vacuum induction furnace with a crucible with a capacity of 7.5 kg. Before casting into a ceramic mold the melts were modified in the furnace with a Zr -Ni alloying set (15% Ni, 85% Zr). A weighed portion of an ultradisperse alloying powder wrapped in aluminum foil was charged into the melt through the furnace charging device. 0, 0.05, 0.15, 0.25, 0.35, and 0.45 wt.% zirconium was added to both alloys. Mold sets for pin specimens were filled with the melt at 1560 -1580~We determined the chemical composition of the molten metal, studied the macro-and microstructure of the specimens before and after heat treatment (homogenizing at 1210~ for 4 h plus cooling in air). The morphology of the strengthening 7'-phase was studied 2 under a JSMT-300 scanning electron microscope at a 1000-fold magnification. The ultimate rupture strength, specific elongation, and impact toughness were determined at room temperature. The longterm strength (the time before failure "if) of alloy ZhS6U-VI was determined at t = 975~ and a stress cr = 230 N/mm 2, t = 760~ and er = 590 N/mm2; that of alloy ZhS3DK-VI was determined at t = 850~ and cr = 350 N/mm-'. The chemical composition of the initial heats of alloys ZhS6U-VI and ZhS3DK-VI (see Table 1) met the requirements of the corresponding standards.An analysis of macrofractures of heat-treated impact specimens after the tests showed that the amount of the brittle component decreases after modifying ZhS6U-VI and ZhS3DK-VI with zirconium (Fig. 1). This is especially noticeable in ZhS6U-VI specimens, which seems to be connected with the substantial change in the carbide morphology under the action of zirconium.In studying the dendrite structure of the alloys we established that with increase in the zirconium concentration the distance between the axes of the second-order dendrites decreased (Fig. 2). In this connection, a less developed dendritic segregation can be expected.A metallographic study showed that modification of alloy ZhS6U-VI with zirconium causes a change in the morphology of the carbides. At 0.05% Zr branches of eutectic carbides of"Chinese hieroglyphs" type were crushed and underwent a certain globularization. An increase in the zirconium content to 0.45% caused virtually complete disappearance of this type of carbide (Fig. 3).Etching in Marble reagent (40 g CuSO4, 200 cm 3 HCI, 200 cm 3 H20) revealed segregations ofa eutectic (y + 7' )-phase in spaces between axes of dendrites and on grain boundaries; the TABI.E

The present work is devoted to the effect of modification of high-temperature alloys ZhS6U-VI and ZhS3DK-VI with hafnium on their properties and structure. Alloys ZhS6U-VI and ZhS3DK-VI were molten in a vacuum induction furnace and modified with hafnium in an amount of 0 -1.5% (as calculated) using a nickel-hafnium alloying composition. It is shown that hafnium alloying diminishes the sizes and amount of the brittle component in a fracture and the distance between the second-order dendrite axes and changes the morphology of the carbides. With growth of the hafnium content the mechanical properties of both alloys and the high-temperature strength of alloy ZhS3DK-VI at room temperature increase.The alloys were melted by the method of remelting a measured preform in a UPPF-3M vacuum induction furnace in a crucible with a basic lining and a capacity of 7.5 kg. A weighed portion of a hafnium-nickel alloying composition of fraction i-3 mm was introduced into the melt heated to 1600~ 1.5 min before discharging. The composition provided the introduction of 0, 0.5, and 1.5% Hf 2 (as calculated) into alloy ZhS6U-VI and 0, 0.8, 1.0, and 1.2% Hf into alloy ZhS3DK-VI. Blocks of finger specimens were fabricated by the method of precise casting by melted models (the casting temperature was 1560 -1580~We determined the chemical composition of the alloys with modification by the mentioned variants and studied the macro-and microstructure before and after heat treatment (homogenizing at 1210~ for 4 h, cooling in air) and the i Motor-Sich Joint-Stock Company, Ukraine; Zaporozhe State Engineering University, Zaporozhe, Ukraine. 2 Here add below in the paper the content of elements is given in mass fro ctions.morphology of the main hardening intermetallic "/'-phase under a 10,000-fold magnification on a JSMT-300 scanning electron microscope. The ultimate rupture strength err, the specific elongation 8, and the impact toughness KCU were determined at room temperature. Tests for long-term strength were conducted on a DST-5000 installation. We determined the time before fracture of specimens of alloy ZhS6U-VI during a hold at t = 760~ o = 590 MPa and t = 975~ o = 230 MPa and that of alloy ZhS3DK-VI at t = 850~ and o = 350 MPa. The chemical composition of alloys ZhS6U-VI and ZhS3DK-VI (Table 1) met the requirements of the OST 1.90.126-85 standard. Hafnium was assimilated in the given alloys in an amount of 55%.Results of a fractographic analysis of heat-treated and fractured impact specimens of alloy ZhS3DK-V1 are presented in Fig. la-d, and ones for alloy ZhS6U-VI are presented in Fig. le-g. It can be seen that in fractures of alloy ZhS6U-VI without hafnium additives the brittle component TABLE 1