D0I:10.13374/i.issn1001-053x.1989.06.020 北京科技大学学报 第11卷第6期 Vo1.11No.8 1989年11月 Journal of University of Science and Technology Betjing Nov.1989 The Behavior of Inclusions during the Special Melting Processes of Superalloys' Fu Jie(停杰)**，Zhao Junhua(赵俊华)，Xu Gaoyang（徐高阳) Wang Hu:(王忠)*◆* ABSTRACT:The behavior of nitride inclusions during the VIM,VAR and ESR processes of superalloys has been investigated.The experimental results have proved that the inclusions in the original metals,even if the pure TiN power added rapidly decompose at the superalloy liquid processing temperature and the nitrides in the remelted metals are mainly precipitated from solution during the solidification process. KEY WORDS:superalloy,inclusion,special melting process The type,quantity,morphology,size and distribution of inclusions have an important effect on the properties of superalloys.The oxide and other types of inclusions potentially decrease the low cycle fatigue life of nickle-base superalloys, As a result of the recent investigationt1l,it has been found that the increase of nitrogen content,consequently,the increase of the amount and size of multi- angular MC-type carbide inclusions in GH36 (31481)obviously decrease the reduction of area,impact toughness,stress rupture life and stress rupture elonga- tion of the alloy. To control the clcanliness of superalloys,it is necessary to know the beha- vior of inclusions,first of all,to determine the inclusion state of existing during the special melting processes of superalloys.The state of existing of inclusions determines their removal mechanism,consequently,the technique for removing inclusions.The solid inclusions in the liquid metal can be removed,relying on the appoximate interfacial property of ceramic materials or molten slag adhering Manuscript Received May 1,1989 ·,Dept of Metallurgy ··+Dcpt,of Material Scicace and Engineering 501
气 第 卷第 期 年 月 北 京 科 技 大 学 学 报 以 手 。 。 ‘ 傅 杰 专 ， 赵 俊华 不厂 王 惠 徐 高 阳 ， · ， 、 · · 口 ， ， 、 、 ， ， ， 几 一 ， ， ， 一 ， ， 。 ， ， ， 丫 ， ， · ， ， 一 … DOI ：10．13374／j ．issn1001－053x．1989．06．020
to inclusions to cause adsorption of them on the crucible wall (for VIM process), filter surface (for filtration process)or in the molten slag (for ESR process). If the inclusions in the original metal decompose in the liquid metal during the special melting processes,the inclusion removal will result from the interaction between the liquid metal and vacuum atmosphere (for VIM and VAR processes) or melten slag (for ESR process).Therefore,the techaique for removing inclu- sions or improving cleanliness will be different under the two conditions mentioned above. In this paper,the behavior of inclusions during the VIM,VAR and ESR processes of superalloys has been investigated.For most superalloys,the prime target for inclusion reduction is that of nitrides,since their content is usually an order of magnitude higher than that of the oxides.The study emphasized the behavior of nitride inclusions. 1 The Decomposition of Pure TiN Powder in the Liquid Metal during the VIM Process of Superalloys In work 2,it was supposed that TiN cannot be decomposed by vacuum induction melting at pressures realistic for superalloy processing (ie.,>1um)and some technique (e.g,filtration)for removing solid TiN from the liquid super- alloy must be proposed. For confirming this viewpoint,a special experiment was arranged. The experiment was carried out in a 10kg VIM furnace.The alloys tested were the commonly used forging superalloy GH169 (In-718)and GH132 (A- 286).During the experimental process,a piece of forging bar from 30kg VIM ingot remelted in a corundum crucible at pressure of higher than 1330Pa.When the temperature of liquid metal reached 1550C,the pure TiN powder weighing 0.2 wt%of charge was added into the melt.The melt remained in the crucible at 1550C for 2 min then poured into a cast iron mold.The ingot weight was about 1.7kg. The nitrogen contents of original forging bar and VIM ingot were analysed by TN-114.The size,morphology and composition of TiN powder and the character of inclusions in the ingot were investigated by JMS-35C SEM. The nitrogen content analysis results are shown in Table 1.It can be seen that 240ppm and 236ppm nitrogen have been added into GH169 and GH132, respectively. The size of TiN powder is more than 104m,predominately,within 20-30 um.The size and morphology of TiN powder are shown in Fig.1,a. The SEM wave spectrum and the energy spectrum observation results indicated 502
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that the TiN powder had a high purity which contained no carbon (see Fig.2, a)and other metallic elements except titanium. If the TiN powder did not decompose in the liquid metal during the VIM process,it would remain its original size and morphology in the VIM ingot, being the large inclusion.However,by observing the samples of the VIM ingot carefully,we have not found the original TiN powder in them,although the nitrogen content of the ingot is about 240 ppm higher than that of the original forging bar. Table 1 The Nitrogen Contents of Original Forging Bar and VIM Ingot (ppm) Alloy Forging Bar TiN Added Nitrogen Added VIM Ingot GH169 (1n-718) 96 2000 453 336 GH132 (A-286) 44 2000 453 280 10 10818初BPC Fig.1 Morphology of pure TiN powder (a)and Inclusions in the VIM Ingot of GH169 (In-718)(b)(um) The investigation result shows that the average size of inclusions in the VIM ingot for both GH169 and GH132 is about 2um smaller than that in the original forging bar,but the quantity larger and a number of inclusion clusters,consis- Table 2 The Composition of Inclusions in the VIM Ingot of GH169 (In-718),wt% Inclusion Ti Nb Fe Ni Cr 1008-1 6.85 72.13 4.01 11.44 5.56 2 32.17 16.10 19.38 29.21 12.14 3 53.75 12.07 7.12 17.00 9.26 -4 48.49 30.12 4.36 10.86 6.10 503
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ting of many individual particles,have been observed somewhere in the VIM ingots.The morphology of a typical inclusion cluster in the VIM ingot of GH169 is shown in Fig.1,b.These small particles contained carbon and niobium in a certain degree beside nitrogen and titanium (see Fig.2,b,and Table 2). Fig.2 Wave spectrun photos of pure TiN powder (a)and jnclusions in the VIM Ingot cf GH169 (In-718)(b) The results obtained demonstrated that the pure TiN powder added either adhered to the Al2O3 crucible wall,or decomposed in the liquid metal.From the experimental conditions and results,(ie.1060 ppm TiN powder,having a size of about 20um,completely decomposed in two minites),it was supposed that the decomposition rate of TiN powder be very high. Work 2 indicated that the TiN solubility corresponds to contents between 25 and 40 ppm N for the commonly used forging alloys.From Table 1,it can be seen that the nitrogen content in the original forging bar is higher than 40 ppm,but about 1000 ppm TiN (corresponds to about 240 ppm N)has decomposed in the liquid metals. To explain this result,the possibility of TiN powder decomposition in the liquid metal is discussed. During the VIM process,the reactions CN3-N2() (1) TiN(e)=〔Ti)+〔N) (2) simultaneously occur in the various regions.The denitrification reaction (1) occurs at the vaccum atmosphere/liquid metal interface.The nitrogen content of liquid metal in equilibrium with the partial pressure of nitrogen in vacuum atmosphere is very low,as the low nitrogen alloys (e.g.the In-718 containing 15 ppm N)can be melted in VIM furnace.The decomposition reaction (2)occurs within the melt.The nitrogen content of liquid metal in equilibrium with the 504
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K titanium contot,caleulated by Nmay i rory high,aithough cannot calculate the value accurately as the shortage of suifable thermodynamic data.When the overall system does nct reach the thermedynamic equilibrium, the pure TiN powder will decompese in the liquid metal. The equilibrium nitrogen content for reaction (2)decreases with decreasing the temperature of the alloy system,some new type nitride inclusions,concerning with the composition of the alloy melted,will precipitate from solution during the solidification process.As the liquid metal,in which the pure TiN powder was added,only remained in the crucible for 2 minites,the nitrogen distribution might be heterogeneous in the melt.At the place where the TiN powder just decomposed,the nitrogen content might be higher,consequently,the inclusicn cluster mentioned above intended to form. 2 The Behavior of inclusion during the VAR and ESR Processes of Superalloys The type of inclusions in the high Al,Ti alloy GH220 (311220),high Ti low Al alloy GH132,without Al,Ti alloy GH36 and the hot alloy GH169 has been studied by using optical microscope and scanning electron microscope.It was found that the main inclusions in the alloys tested are MC type inclusions inclu- ding primary carbides and nitride type inclusions.Usually,the former contains some degree of nitrogen and the latter is titanium nitride with some carbon solution (for GH132,GH220)or titanium,niobium nitride with some carbon solution (for GH169).It is to be noted that the alloy GH36 contains 700-1000 ppm nitrogen,but the main inclasions are MC type carbides containing V,Nb,C and N,no nitrides have been found in it (see Table 3). As the pure TiN powder can rapidly decompose in the liquid GH169 and GH132 at 1550C,the inclusions (carbides and nitrides)in the GH169,GH220, GH132 and GH36 should be able to decompose during the VAR and ESR pro- cesses. Table 3.The Composition of Inclusions in the ESR Ingot of GH36(3M481),wt% Inclusion Nb Cr Mo Fe Ni Mn Si 1 60.77.15.51.420.5 2.23.10.1 2 26.645.56.33.15.40.51.0 0.0 If the inclusions in the superalloys decompose during ihe VAR and ESR pro- 505
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