Organizational characteristics of 42 CrMo ring billet Figure 1 is the computer numerical simulation of the solidified tissue by the vertical centrifugal casting at the casting speed of 480 rad/s, the casting temperature of 1500℃, the outer diameter of the casting of 1000mm, the internal diameter of 360mm and the process parameters of the cast water cooling. Figure 4. As can be seen from the figure, the outer wall of the annular casting is a uniform fine isopaxial crystal region. In the interior of the casting, the grain changes from the fine isopaxial crystal to the thick columnar crystal region, and the columnar crystal is getting bigger and bigger. This is due to the alloy in the process of centrifugal casting solidification, when the metal liquid into the casting, the cooling of the wall, start knot, crystal solidification, crystalline latent heat is perpendicular to the cast wall continuous outward heat dissipation, so the metal to the setting in the solidification of centrifugal casting process, because the casting wall is cold, and the cast wall is cold, but water spraying, when high temperature metal meets low temperature casting wall, near the metal wall by strong cooling effect, produce great overcooling degree and a large number of heterogeneous core. These core in cold melt in dendite growth, due to the heat dissipation of crystallization without directional, and a lot of, core block each other continue to grow up, so near the circular casting wall area form tiny, axial crystal organization, with the increase of solidification thickness and stable solidification shell formation, grain directly, by the surface of fine axial crystal some grain for the base growth, developed from outward and internal growth of columnar crystal area. Due to the coagulation layer barrier cooling, the alloy liquid cooling speed is reduced, resulting in lower alloy solidification speed, centrifugal casting process alloy liquid is mainly controlled by the one-dimensional cooling temperature, at this time, one-way heat dissipation conditions at the solid-liquid interface, under the action of one-way heat flow, the alloy along the radial ring casting growth, thus formed in Figure 1 (b) area column. Due to the selective growth, in the development of the columnar crystal, the farther the distance from the wall, the more the unfavorable crystals are eliminated, the more concentrated the direction of the columnar crystal, and the larger the size of the grain, as shown in Figure 1 (c).
3 Reasons of macroscopic segregation 3.1 During the solidification of the centrifugal casting, the rotating metal liquid is subjected to the radial centrifugal force and vertical, and the straight downward gravity. The centrifugal force is proportional to the radius of rotation, and proportional to the square of the angular rotation speed:
F is away from m2r
Where: F is the centrifugal force of the metal liquid; m is the mass of the metal liquid; it is the rotating angular velocity of the cast. As with the gravity acceleration in the gravitational field, the metal liquid will have a centrifugal acceleration in the rotating centrifugal force field.3.2 Composition segregation For centrifugal casting of 42 CrMo ring casting, the redistribution of the solute atoms will occur with the solidification temperature and the original composition of the solid phase and the liquid phase. Thus, it leads to drastic changes in the alloy composition. According to the macroscopic segregation equation p, the average velocity of the alloy flow and its solidification cooling velocity are the macroscopic bias
u 1
Analysis plays the main role. When its result is 1, no segregation. Due to the effect of centrifugal force, its flow speed is accelerated, when the alloy liquid flow speed v. Contrary to the solidification speed direction u, the liquid flows from the hot end of the two-phase region to the cold end, that is, from the internal flow with less solute to the root of the solute rich area, producing negative segregation, and otherwise, producing positive segregation back. In this study, under the influence of alloy elements, chromium has a strong affinity with carbon, which reduces the diffusion rate of carbon, increases the incubation period of supercooled austenite decomposition, and slows down the solidification speed, thus generating segregation.3.3 Layer segregation Figure 4 is a schematic diagram of the metal liquid flow on the cast axial section. When the alloy liquid into the cast, the metal liquid in the form of the flow of axial flow, motion, namely the first layer of metal liquid ① do axial flow, due to the temperature reduction, the flow speed, small, the speed of the first stream ② flow and beyond the first flow, flow, and so on, due to the conditions of the solidification of temperature, such as temperature, speed and tissue difference, formed in the form of concentric rings.
3.4 Measures to reduce segregation The measures to reduce macro segregation are: ① Choose the appropriate casting speed, so that the alloy liquid has
A suitable flow speed.② Add the incubation agent in the solidification process of the alloy liquid to accelerate the solidification.③ Take the appropriate cooling method to shorten the solidification time of the solid-liquid two-phase area, and increase the solidification cooling time as far as possible.
Tissue characteristics and macro-seanalysis of Cu-17Ni-3Al-X alloy casting of Cu-17Ni-3Al-X copper alloy with outer circle diameter of 265mm, thickness of about 50mm and height of 125mm were prepared by centrifugal casting method. The tissue characteristics and macro-segregation of copper alloy casting were studied by metallography and electronic probe. The results show that there are two forms of isoaxial crystal and cylindrical crystal in the casting, which are distributed in the outer wall and inner wall of the casting, and distributed in the center of the casting and the area near the inner wall of the casting. In the main combination, the gold elements Cu, Ni, and Al showed slight macroscopic segregation along the radial and axial directions of the casting, but were still within the design range of the alloy.2 Test Results 2.1 Tissue characteristics of the Casting The typical macroscopic tissue characteristics of the casting are shown in Figure 2. It can be seen from the figure that the macroscopic tissue morphology of the casting is symmetrical along the casting; the macroscopic organization of the casting can be divided into four obviously different areas: the fine isoaxial zone of the outer wall of the casting (①, parallel to the casting (ID, coarse isoaxial zone (II) near the inner wall of the casting, IV near the end to the inner wall of the casting). The organizational characteristics of these 4 regions are shown in Table 1.
The reason is that the flow of metal melt and the redistribution of solute atoms do not cause drastic changes in the alloy composition. According to the discriminant formula of macroscopic segregation, the conditions for eliminating macroscopic segregation are:
v / u / ( 1)
When the alloy is determined, the average velocity of the liquid flow in the dendrite and the solidification velocity of the alloy are determined u. For the solidified and contracted alloy, when the average velocity of liquid flow in the dendrite and the solidification velocity of the alloy are in the opposite direction, the smaller the average velocity of the liquid flow and the greater, the more favorable it is to reduce macroscopic segregation. During centrifugal casting, the flow velocity of centrifugal force and Coridian force is generally accelerated, which is unfavorable to inhibit macroscopic segregation. However, in this study, because the main components are copper and nickel, their density difference in the melt is very small, in addition, centrifugal casting water cooling, can effectively refine the dendrite, improve alloy solidification speed, especially because the wall thickness of the casting blank (about 55mm), can effectively inhibit the melt flow, so as to eliminate or reduce the purpose of macroscopic segregation of casting.
According to the requirements of RCC-M, the material of the main pipeline of the nuclear island of CPR1000 reactor pressurized water reactor nuclear power plant is austenite stainless steel (French brand Z3CN20-09M). This test determined the iron content of the main pipeline according to the MC1000 shevler diagram (Schaeffler) and MC1340 sections, and analyzed the reasons for the differences in the detection results of different methods. Based on the manufacturing experience of the main pipeline and the content of RCC-MM3406, the reasons of the increasing distribution of the iron content of the pipeline were analyzed. The scope of the application of the method of assessing ferrite content and MC1340, the determination of ferrite content, and the method of eliminating the uneven distribution of ferrite in the main canal are summarized.
The disadvantage of centrifugal casting is the rotating metal liquid under the action of centrifugal force, high density of elements, organization has a larger trend to the wall (the outer surface of the centrifugal tube) deposition, and the smaller density, element and organization has a larger trend to migrate to the inner surface, this phenomenon is called centrifugal casting floating, external phenomenon, will make the casting composition segregation, such as main centrifugal tube austenite stainless casting, carbon, nitrogen, phosphorus and sulfur elements to float on the inner surface of the centrifugal tube, and high density elements to the outer surface of the centrifugal tube. Combined with the material of the centrifugal tube in RCC-M Z3CN20-09M austenite one ferrite stainless steel, including carbon, nitrogen, phosphorus, sulfur and other elements density is far lower than that of iron, pouring metal steel in casting hollow cooling process (natural cooling), by the centrifugal force to the inner wall migration, so that carbon, nitrogen, phosphorus, sulfur and other elements from the appearance of the inner surface increase.Two groups of test samples were selected in Table 2, and the nitrogen content was determined by spectroscopic method. The results are shown in Table 3, and the above inference is further confirmed.
It can be seen from metallography and Schevler diagram and the DeLong diagram derived from it that carbon and nitrogen are the most influential elements that promote the formation or stabilization of austenite. In the DeLong diagram, carbon and nitrogen in the element of nickel equivalent calculation formula are 30%. It is concluded that the increase of ferrite content in the centrifugal tube from the inner wall to the outer wall is the increase of carbon and nitrogen from the outer surface to the inner surface and the internal floating and sedimentation of the centrifugal tube casting.2.3 Control of centrifugal casting components In industrial production, centrifugal casting components is often used to manufacture castings with gradient properties, such as metal composite castings with high wear resistance on the inner surface. In addition, after centrifugal casting, water spray rapid cooling (water cooled centrifuge tube) centrifugal casting pipe is smaller than the air cooling centrifugal casting tube, and the distribution of iron is relatively uniform. MP30 ferrite tester was used to determine the iron content at 1 / 4 of the inner surface, 1 / 2 and 1 / 4 of the outer surface of Yangjiang 1 unit. The detection point is shown in Figure 6, and the results are shown in Table 4. According to Table 4, water spraying rapid cooling was adopted after centrifugal casting, without obvious increasing iron distribution from the internal water wall to the external wall.(3) As can be seen from metallography and DeLong diagram, nitrogen is similar to carbon, which promotes the formation of austenite or stable austenite elements. In the composition analysis and control of the main centrifugal tube furnace, in addition to meeting the ferrite content of RCC-MM3406 assessed by shevler-diagram, nitrogen and other residual elements must also be controlled.(4) When the austenite ferrite stainless steel centrifuge tube is cast, the high temperature metal liquid is solidified by centrifugal force. Because the density of nitrogen and carbon is far lower than iron, it tends to migrate to the inner wall of the centrifuge tube, which leads to the segregation of ferite content due to the segregation of components. The method to control the composition and tissue segregation of the austenite-ferrite stainless steel centrifuge tube is water-cooled centrifugation. At the same time, centrifugal casting produces the characteristic of composition segregation, which can produce special products and castings with performance requirements.
