INTEGRATED EXAMINATION OF TECHNICAL CONDITION OF MOLYBDENUM BOATS

Transcription

1 INTEGRATED EXAMINATION OF TECHNICAL CONDITION OF MOLYBDENUM BOATS Yu. K. Karlov 1, V. I. Makarov 1, V. V. Rozhkov 1, V. G. Suluyanov 1, I. G. Chapaev 1 ; 1 JSC Novosibirsk Chemical Concentrates Plant, Novosibirsk, Russia Pelletize fuel is use in nuclear power engineering. After press-power being prepare an presse the obtaine green pellets are converte to high-strength ceramic fuel by sintering in the reucing meium. Sintering is performe continuously in tunnel-type furnaces at С. Before being sintere green pellets are subjecte to slow heating up to С, an after being sintere to slow cooling. Total process uration for ceramic pelletize fuel prouction makes hours. During all this perio pellets are locate in a special container i.e. molybenum boats. The important parameter of molybenum boats is their working capacity that shall comprise working cycles taking into account the reconitioning. In practice boats withstan consierably less working cycles. It is explaine by the fact that their working capacity is influence by the chemical composition of structure, construction an characteristics of prouction practice. The chemical composition of two suppliers is given as an example in Table 1. Table 1. Supplie Chemical composition, % r Мо Fе Аl Ni Si W Са Мg Na К О 2 С Alloy 1 Basic Alloy 2 Basic Alloys are similar accoring to the chemical composition, but Alloy 1 has no carbon content (С), an Alloy 2 has no oxygen (О 2 ). Carbon an oxygen are the most harmful impurities for molybenum because they have a consierable influence on molybenum liability to brittle failure. The formation of volatile an fusible oxies (Мо - МоО 3 ) at grain bounaries results in sharp ecrease of strength an plasticity. Molybenum carbies (Мо С, Мо 2 С), locating at grain bounaries, form brittle interlayers where cracks are initiate. In case of prolonge heating the strength of molybenum is ecrease through the process of seconary re-crystallization, grain coarsening up to wall thickness imensions of boats, concentrations of stresses at bounaries of coarse grains. Pressure an internal stresses are increase through hyrogen issolution, its accumulation in gaseous state in pores, lattice locations an at grain bounaries. It is assume that the rop of working temperature from 1750 to С ecreases hyrogen action an increases the number of working cycles in 1,5 2 times.

2 In Figure 1 the example of molybenum metallographic section structure in the beginning of operation an after 50 working cycles is given. а Figure 1 Structure of molybenum metallographic section before (а) an after (б) 50 sintering cycles. Fracture examination of molybenum boat after 70 working cycles witnesses the brittle fracture. The appearance of fragments is given in Figure 2. In the upper part the fracture has ark color (а), an in the lower part metallic luster (б) inherent to fresh fracture. Thus it can be suppose the fracture is occurre as a result of crack extension. б а Figure 2. Fragments of molybenum boat after 70 working cycles. Metallographic examination of fragment structure shows no substantial change of grain form an imensions. However segments with abnormally coarse grain are etecte in the zone of crack. Grain imension in these segments excees several times the average grain imension. In Figure 3 the microstructure of these segments is given. б

3 а Figure 3. Molybenum microstructure in abnormal sections а magnification х100; б magnification х25. The presence of segments with similar abnormally coarse grains in the material inevitably results in the appearance of seconary stresses (especially uring heating an cooling). Besies it shoul be mentione the growth of grain imension results in the consierable shrinkage of bounary length an thus in the growth of brittle phase volume fraction at bounaries. For these reasons the strength of such segments is much lower the strength of metal in total ue to the most likely formation of cracks. Furthermore brittle phase precipitates Мо Мо 2 С an Мо МоО 3 were etecte at grain bounaries. (Figure 4). б Figure 4 Molybenum microstructure with brittle phase interlayer (Мо Мо 2 С an Мо МоО 3 ) at grain bounary. х100 Micro-harness measurements of this phase show its harness makes kgf/mm 2, an it is much higher in comparison with molybenum kgf/mm 2. The presence of zone with heterogeneous structure an increase stress resulte in plasticity loss an crack formation.

4 Statistical ata on failure of molybenum boats was collecte uring a long perio of time. In Figure 5 the classical curve of number of working elements epening on time is given. M I II III t Figure 5. М number of working elements; t running time. On the given graphs «I» segment is etermine by failure of some working elements ue to some initial reject level an other reasons connecte with the beginning of operation an run-in of working elements. On «II» segment the number of failures is approximately stationary, therefore on this segment the number of working elements is linearly ecrease in the course of time. On «III» segment the number of failures begins to increase ue to the fact that elements start to failure because of physical wear. On «I» segment the number of working elements in the instant of time t can be calculate accoring to the following formula: M ( t) = M 0 F( t) where M 0 - initial number of working elements, M - current number of working elements, an F(t) - failure rate function for working elements. If it is assume that on «I» segment the failure of working elements occurs by accient, in this case the epenence F(t) is represente as follows: F( t) = exp( λt) where λ - failure rate function. Also the value λ (t) characterizes failure ensity function provie that the system has been working failure free uring time t. The value λ (t) t characterizes the probability of the fact that the system will fail within time interval t, t + t provie that it i not fail until the instant of time t. In this connection it shoul be mentione that the obtaine statistical ata relate to molybenum boats was approximate within the limits of the given moel. First of all on «I» segment we assume that failures of molybenum boats occur by accient, therefore the approximation was exponential that allowe us to estimate the value λ = On «II» segment we assume failure rate of molybenum boats to be constant resulting in the linear ecrease of number of working boats in the course of time, therefore the approximation was linear on «II» segment. It allowe us to etermine, firstly, the linear equation of regression, seconly, the amount of cycle to which the number of working boats will ecrease to zero. This way we etermine the maximum operation perio of molybenum boats. It will happen on 520 cycle. It can be also mentione that it is unlikely molybenum boats to be working for so long perio of time.

5 On «III» segment the failure rate highly increases ue to the physical wear of molybenum boats. The physical wear of molybenum boats first of all is etermine by the seconary recrystallization of molybenum that will result in the mechanical fracture of boat construction. Therefore the obtaine estimate allows us to etermine the approximate number of cycles when boats can operate in normal conition. This number of cycles can be approximately estimate as that is given as the operating life in the report of English researchers [7]. Segments «II» an «III» are the most interesting for the use of non-estructive testing methos in view of remaining life estimation. Methos of non-estructive testing such as visual measuring metho, ultrasonic testing an ey current metho were use. Wall thickness of molybenum boats was measure using ultrasonic thickness gauge ЭМАТ-100 an 36DL, an attenuation of ultrasonic waves in molybenum at frequencies 5МHz an 10 МHz was measure using ultrasonic flaw etector Еросh-III-В. Cracks were etecte as well as the epth of etecte efect was estimate using ey current flaw etector ВД-89НП. In Figure 6 results of wall thickness measurements are given.,мм 5,6 5,5 5,4 5,3 5,2 5,1 5 4, Number of cycles Figure 6. Depenence of wall thickness on the number of cycles Figure 6. Depenence of wall thickness on the number of cycles In Figure 7 Detection of efects in elements of molybenum boats

6 50 Number of cycles Number of boats Sum Bolts Rivets Cracks Literature: Figure 7. Frequency of efective boats 1. Johnson W.A., Mehl R.F., Trans. Amer. Inst. Min (metall.) Engrs, 1939, v.135, p I.L.Mirkin, Problems of theoretical material science, М, Oborongiz, 1938, p G.L.Davis, I. J. Buron, col. Molybenum, М, Foreign literature. 1962, p N.V.Chereshin, Basics of purification, maintenance an pumping in vacuum technology, М. Soviet raio, D. S. Moroz, B.B. Chechulin, Hyrogen embrittlement of metals, М, Metallurgy, 1967, p R. Barlow, F. Proshan, Mathematical theory of reliability, translation from English, М, BNFL report on use of molybenum boats for pellet sintering, «High temperatures high pressures», 1994, v. 26, p , Springfiel.