Effect of the rate of electrocontact heating on the structural state of alloy 36NKhTYuM8

Abstract

The use of rapid electrocontact heating (by an electric current) in quenching dispersion-hardening alloys opens new prospects for substantial improvement of their properties by providing the requisite degree of dissolution of the excess phases with retention of the fine-grained structure of the supersaturated solid solution, which is difficult to attain under the conditions of conventional heating. The advantage of such quenching is connected with the possibility of short-term superheating of the alloy above the temperature of heating for quenching conventionally used. Therefore, such a Ixeala-nent is expedient foremost for alloys that contain poorly soluble excess phases. The present work concerns the role of the heating rate for quenching in formation of the phase composition and slxucture of quenched dispersion-hardening spring alloy 36NKhTYuM8.The composition of dispersion-hardening spring alloy 36NKhTYuM8 has been designed to employ the reinforcing effect of segregation of two intermetallic phases in aging, namely, a y'-phase of the Ni3(Ti, AI) type and a Laves phase ofthe Fe2Mo type [1].The main difficulty in quenching alloy 36NKhTYuM8 consists in dissolution of the poorly soluble Laves phase. The conventional furnace heating (in a protective atmosphere) to 1000-1050~ used in quenching virtually cannot dissolve the excess Laves phase contained in the alloy in any as-received state, and a higher temperature of heating for quenching is not used due to the susceptibility of the alloy to grainThe use of rapid electrocontact heating for quenching alloy 36NKhTYuM8 turned out to be very effective precisely because of the greater dissolution of the excess Laves phase as compared to conventional furnace heating [2]. It has been shown that heating at a rate v h = 300 K/sec to 1200~ with a 6-9-sec hold almost halves the content of the excess Laves phase relative to the initial content with retention of the finegrained structure (d< 15 mm). It has been shown in [2] that rapid quenching is characterized by an unusual kinetics of the dissolution of the excess Laves phase, because the predominant part of this phase dissolves precisely in the heating stage. The dissolution is especially intense if the alloy has been deformed before quenching. Thus, an increase in the heating rate in quenching the alloy can be an important resource of additional intensification of the dissolution of the Laves phase, because it can be accompanied by acceleration of the primary recrystallization and a change in the temperature range of the process [3].We studied the effect of the rate ofelectrocontact heating I in quenching on the phase composition and structural state of alloy 36NKhTYuM8 melted at a plant (0.04% C, 35.5% Ni, 13.5% Cr, 7.82% Mo, 2.93% Ti, 1.16% A1, the remainder Fe).We tested a 0.3-mm-thick ribbon after cold plastic deformation with a 50% reduction (as received). A detailed description of the method of research can be found in [2]. The volume fraction of the particles of the Laves phase in the structure of the alloy was deter...

Special features of dissolution of the laves phase under the conditions of rapid electron heating for quenching

“…This fact first mentioned in work [4] is quite unusual because the elevated stability of the grain structure manifests itself under the conditions of full dissolution of the excess phase, which occurs in the heating stage. It is known that only a homogeneous grain structure can be resistant to collective recrystallization in the absence of excess phases [7].…”

“…Despite the assumptions we have made, it is obvious that the tensile stresses are commensurable in order of magnitude with the ultimate rupture stress and the yield stress of the Laves phase, and, consequently, with allowance for the anisotropy of its properties due to the h.d.p, lattice, there is a possibility of twinning and even cracking of the particles under the conditions of rapid heating, which has been observed experimentally in [4]. It can be assumed that the considerable stresses can change the conditions of phase equilibrium in the system and become a cause of the dissolution of the Laves particles in the austenite matrix under rapid heating.…”

“…A detailed description of the methods o f study is presented in works [3,4]. The volume fraction of particles of the Laves phase in the structure of the alloy was determined by Glagolev's method of points [6] with detecting particles with a size of_> 2 ~tm under a magnification o f x 300.…”

“…In particular, quenching alloy 36NKhTYu from rapid heating has given an especially fine-grain structure, which improves the technological ductility of the quenched alloy and stimulates a more effective, i.e., interrupted, mechanism of segregation of the hardening ,{'-phase in subsequent aging in the entire volume of the alloy [1,2]. The use of rapid quenching for alloy 36NKhTYuM8 has made it possible to reduce considerably the volume fraction of the difficultly soluble Laves phase of the Fe2Mo ~pe and simultaneously preserve the fine-grain structure [3,4], which is impossible under the conditions of conventional furnace heating of such alloys [5].…”

“…Searching for the possible causes of the unusual kinetics of the dissolution of the Laves phase we established that the process of primary recrystallization in a deformed alloy occurs and ends fully just in the stage of rapid heating to 1200~ [4]. This coincidence allowed us to presume that there exists a direct dependence between two processes, namely, the dissolution of the Laves phase and the primary recrystallization in the austenite matrix.…”