HEAT TREATMENT OF REACTOR VESSEL STEEL AISI 321

Transcription

1 HEAT TREATMENT OF REACTOR VESSEL STEEL AISI 321 Pavel PODANÝ a, Petr MARTÍNEK a, Petr NACHÁZEL a, Martin BALCAR b a COMTES FHT a.s., Dobřany, Czech Republic, EU, comtes@comtesfht.cz b ŽDAS a.s., Žďár nad Sázavou, Czech Republic, EU, zdas@zdas.cz Abstract Article deals with the heat treatment of AISI 321 steel used in reactor vessels. AISI 321 is typical austenitic stainless steel with combination of main alloying elements Cr-Ni in ratio ca 18/10 of weight percents. Experiment was focused on influence of heat treatment a thermomechanical processing on the microstructure and especially on the morphology and distribution of titanium carbo/nitrides and its clusters. Three experimental heats with various amounts of carbon, titanium and boron were prepared and subjected to different heat treatment regimes. Also different solution annealing after forging was applied. Microstructure of the samples was analysed by means of optical and scanning electron microscopy. Numerical simulation in DEFORM HT software was used for simulation of cooling in different environments after forging. Keywords: austenitic steels, heat treatment, titanium carbonitrides 1. INTRODUCTION Stainless steels with austenitic microstructure and with the approximate composition 18 wt.% chromium, 10 wt.% nickel and additions of molybdenum, titanium or niobium are today widely used in components designed for high temperature applications like nuclear power stations, boilers, superheaters. [1] Main goal of this experiment was to reduce the amount of large carbo/nitrides clusters. Large clusters usually causes echoes during ultrasonic inspections and thus the almost finished products have to be rejected. In normal practice, sufficient titanium is added to the steel to combine with all the carbon. Titanium prevents the formation of Cr23C6 carbides, which locally deplete the matrix from chromium. [2] Titanium and niobium carbides are much less soluble in austenite than is chromium carbide, so they will form at much higher temperatures as relatively stable particles. These should remain relatively inert during commercial heat treatments involving solution temperatures no higher than 1050 C and thus minimize the possible nucleation of Cr23C6. However, TiC and NbC have some solubility in austenite at 1050 C and can subsequently precipitate at lower temperatures. During high-temperature processes, these carbides dissolve to a greater extent in austenite and can then reprecipitate at lower temperatures. Therefore, NbC and TiC will not always form inert dispersions, and are often likely to be redistributed by heat treatment. They do, however, have the great advantage of not depleting the matrix of chromium, particularly at sensitive areas such as grain boundaries. [3] The precipitation of carbides and nitrides also occurs during the rapid quenching from high solution temperatures. [3] 2. EXPERIMENT Three different heats of austenitic stainless steel were prepared. The chemical composition meets requirement of Russian standard 08Ch18N10T and American standards AISI 321 and AISI 321H (in case of heat enriched with boron). Chemical composition of experimental material is summarized in table 1. Main variation of chemical composition consists in different amounts of titanium, carbon and boron.

2 Tab. 1 Chemical composition of experimental heats Element Heat number [wt%] C 0,0400 0,0400 0,0600 Mn 1,5100 1,6100 1,7000 Si 0,5300 0,5100 0,5700 P 0,0250 0,0220 0,0210 S 0,0070 0,0020 0,0020 Cr 17,600 17,650 17,600 Ni 10,300 10,100 10,100 Ti 0,2200 0,2800 0,3900 B 0,0001 0,0037 0,0045 H 1,3000 1,5000 1,6000 N 0,0215 0,0265 0,0116 O 0,0051 0,0062 0,0063 Experiment was focused mainly on microstructure examination and image analysis. This was done on ingots in three localities (ingot axis - 1, one half of radius - 2 and edge 3), on ingot after different heat treatment, on samples forged from ingot and cooled in two different environments and on these samples after high temperature annealing. Microstructure examination and image analyses were focused on the amount and distribution of titanium carbonitrides, on the volume fraction of delta ferrite and the grain size after forging and heat treatment. Particular parameters of experiment described above are shown on table 2. Tab. 2 Parameters of experiment Experiment procedure Parameters Analyses Heat treatment of ingots (solution annealing) Cooling after forging Annealing temperature [ C] Temperature [ C] Time [h] Vermiculite or 900 and 1; 5; or water 1100 and 1; 5; quenching ; 5; 10 Microstructure examination and image analysis Ingots were cut on conventional cutting saw. Samples for microstructure examination were subjected to conventional metallographic procedure (grinding and subsequent polishing up to alumina suspension). Beraha 2 etchant was used for revealing delta ferrite. Austenite grains were revealed by means of V2A etchant (10 H 2 0, 10 HCl, 1 HNO 3, 1 wetting agent). Identification of carbonitrides was done by means of HKL Nordlyss EBSD camera and EDS detector INCAxsight on JEOL 7400F scanning electron microscope. Image analysis was made on Nikon MA 200 optical microscope with NIS Elements software for image analyses. All samples were evaluated at 500x magnification and 30 image fields on each sample were inspected (one field represents the area of µm 2 ). All experimental heat treatment was done by means of annealing furnace Heraus. Forging of experimental ingots was done by means of hydraulic press Zeulenroda (PYE 40).

3 3. RESULTS AND DISCUSSION 3.1 Microstructure of ingots Quantitative image analysis of carbonitrides count and carbonitrides size found, that there is no essential difference between particular heats. But there was found that there is a substantial difference between the localities within the ingot. Highest amount of carbonitrides is concentrated on the edge (near surface) of the ingot. On the other hand, the size (average area) of these particles near surface is the lowest (see figure 1). Fig. 1 a) amount of carbides per one field (in ingots b) average area of particles in ingots Clusters of carbonitrides are classified as one large carbonitride during image analyses. Histograms show the distribution of carbonitrides into 10 size factors (from 0 10 µm 2 with 1 µm 2 step). The class other means carbonitrides clusters. There is clearly visible that the amount of large carbonitrides clusters decreases from the ingot axis to its surface (see figure 2). Fig. 2. Histograms of carbonitrides classification in ingot a) Ingot axis, b) one half of radius, c) surface of ingot The delta ferrite formed complex network around the as cast grains in the middle of the ingot. Near surface of ingot, delta ferrite forms more separate islands and its volume fraction is lower. Both delta ferrite and carbonitrides distribution within the ingot is related to ingot solidification. Carbonitrides forms clusters thanks to the reduced speed of solidification and so as delta ferrite is formed thanks to the segregation of alpha phase forming elements. 3.2 Heat treatment of ingots Samples from ingots were annealed at 900 C, 1100 C and 1300 C for 1, 5 or 10 hours. Results of the image analyses after solution annealing are shown table 3. b)

4 Tab 3. Results of the image analyses after solution annealing ( object count represents the amount of particles per one analysed image field, object area represents average area of one particle) TiCN Obj. count Obj. area Obj. count Obj. area Obj. count Obj. area Temperature Time [-] [μm2 ] [-] [μm 2 ] [-] [μm 2 ] 0 h 13,00 3,49 14,1 3,67 14,70 3, C 1100 C 1300 C 1 h 12,10 3,48 12,2 3,21 14,17 3,05 5 h 12,87 3,37 13,67 3,04 14,47 3,33 10 h 10,57 5,40 6,8 5,90 14,53 5,33 1 h 16,57 4,15 8,93 5,77 12,80 4,88 5 h 16,33 4,60 10,53 4,68 18,33 5,67 10 h 12,37 4,06 7,67 6,64 14,77 5,32 1 h 36,13 1,72 30,13 2,10 25,43 2,05 5 h 37,03 1,98 28,23 2,31 26,27 1,84 10 h 20,53 2,50 12,77 3,44 15,20 2,60 Annealing at 900 C for short time (1 and 5 h) does not substantially affect the size and count of carbonitrides. Effect of annealing on this temperature is only visible after annealing for 10 hours when the amount of carbonitrides decreases and its average area slightly increases. Increasing of solution temperature to 1100 C led to the similar effect on particles size even after one hour. But the average amount of carbonitrides per one inspected field did not change substantially. Longer annealing at this temperature did not bring another effect. Best result was reached thanks to the annealing at 1300 C for 1 and 5 hour. The average area of one particle decreased and the average count of particles increased. But prolonging of this annealing to 10 hours returns almost the same values of particles count and average area is it was in initial state before annealing. Nevertheless annealing at this temperature for 10 hours led to more homogeneous distribution of carbonitrides. 3.3 Forging of samples from ingots and different cooling Samples were heated up to 1200 C and solution annealed for 2 hours and after that they were forged with a degree of reduction 2,5. Two different cooling processes were applied: quenching in water and simulation of cooling of whole forged bar with 250 mm diameter on the air. Inserting the forged sample in the insulating vermiculite simulated this process. The condition of cooling was verified by means of DEFORM HT software, which proved the similarity of cooling conditions of small sample in vermiculite. Influence of forging and way of cooling on the carbonitrides size and distribution were then studied. The forging itself has substantial influence on the size and distribution of carbonitrides. Increasing of the amount of carbonitrides was observed in all three heats. Heat showed the strongest increasing of carbonitrides thanks to the forging. Only low increasing of particles count was observed after cooling in vermiculite in comparison to cooling in water. Forging of all samples led to decreasing of the size of carbonitrides. 3.3 Annealing of forged samples Forged samples were afterward annealed at two different temperatures 1020 and 1100 C for further possible dissolution of carbonitrides. It was found, that in case of heats and annealing at 1020 C for forgings quenched in water led to further increasing of particles count and decreasing of average area of particle (see table 4). Increasing of annealing temperature for these heats was beneficial for further particles refinement but was negligible for final grain size. Grain size increased from 8 (after forging) to 5

5 (after annealing at 1100 C) according to ASTM E 112. This result was the same for all heats, there were only minor differences between them. Tab. 4 Average values of particles count, and size for initial state, after forging (1 quenching in water, 2 cooling in vermiculite) and annealing TiCN Obj. count Obj. area Obj. count Obj. area Obj. count Obj. area state [-] [μm 2 ] [-] [μm 2 ] [-] [μm 2 ] Initial state 13,0 3,49 14,1 3,67 14,70 3,31 forging 1 27,97 2,17 18,13 2,86 166,53 0,92 ann C 23,9 2,63 24,87 2,83 192,57 0,90 ann C 23,73 2,95 45,13 2,10 203,83 0,89 forging 2 25,7 3,05 46,20 2,70 171,73 0,95 ann C 23,03 2,81 32,73 1,98 180,37 0,97 ann C 19,43 3,05 57,23 1,56 215,57 0, Mechanical testing There was not enough of material for standard mechanical testing, thus minitensile test samples were made from forged specimens. The sample for minitensile testing is flat with very small dimensions (functional body length is in units of millimetres). Tests were done on minitensile test machine MTS with video-extensometer MESSPHYSIK. Results of testing of samples annealed at 1020 C are in table 5. The yield strength increases thanks to the increasing count of carbonitrides particles (precipitation strengthening). Thus the heat shows highest YS and TS of all heats. Tab. 5 Results of mechanical testing after forging 1 (subsequent quenching in water) and annealing at 1020 C 4. CONCLUSIONS Specimen Rp0,2 Rm A [MPa] [MPa] [%] ,5 609,3 70, ,4 595,4 56, ,2 628,7 63,0 Complex experiments on austenitic stainless steel were performed. Three experimental heats of AISI 321(H) with various amounts of carbon, titanium and boron content were subjected to particular annealing heat treatment, forging with various cooling and further annealing. Three different temperatures of solution annealing were applied on ingots 900 C, 1100 C and 1300 C with different holding time 1, 5 and 10 hours. Forged samples were cooled in two different environments rapid quenching in water and slow cooling in vermiculite. Last heat treatment solution annealing was applied on forged samples C and 1100 C for one hour. Results of this experimental program are following: As cast state ingot: Highest amount of carbonitrides is concentrated on the edge (near surface) of ingots where the rapid solidification takes place. Carbonitrides are coarsening and form clusters towards the ingot axis together with decreasing of solidification velocity. Delta ferrite forms network structure near the ingot axis; it divides into separate islands near the ingot surface. Ingots after heat treatment: Short time annealing at 900 C (1 and 5 h) does not substantially affect the size and count of carbonitrides. Ten hours at 900 C brings the dissolution of greatest carbonitrides. Increasing of solution temperature to 1100 C led to the similar effect on particle size even after one hour. Best result was reached thanks to annealing at 1300 C for 1 and 5 hour where the average area of particle

6 decreased and the average amount of particles per one inspected image filed increased. Increasing of both annealing temperature and holding time lead to substantial decreasing of delta ferrite volume fraction. Forged samples: Forging at 1200 C substantially increased the amount of carbonitrides in all heats. Minimal change was observed on heat 46471, the maximum increasing of carbonitrides was recorded in heat Substantial increasing of particles count was observed in heat after cooling in vermiculite in comparison to cooling in water. However the difference in other heats was not crucial. Forged samples after solution annealing: Annealing at 1020 C led to increasing of particles count and decreasing of average particle area on heats and (with higher carbon and boron content). Using of higher annealing temperature (1100 C) was not beneficial because of coarsening of grain size. Grain size increased from 8 (after forging) to 5 (after annealing at 1100 C) according to ASTM E 112. Heats with higher carbon and boron content proved good mechanical properties. Precipitation of very fine carbonitrides has positive effect on the yield strength. ACKNOWLEDGMENTS The results presented in this paper are supported by: project FR-TI1/222 of Ministry of Industry and Trade of the Czech Republic and project West-Bohemian Centre of Materials and Metallurgy CZ.1.05/2.1.00/ co-funded by European Regional Development Fund. LITERATURE [1] Kaishu G. et al. : Effect of aging at 700 C on precipitation and toughness of AISI 321 and AISI 347 austenitic stainless steel welds, Nuclear Engineering and Design 235, Nuclear Engineering and Design 235 (2005) [2] Kaneko, K. et a..: Formation of M 23C 6-type precipitates and chromium-depleted zones in austenite stainless steel. In: Scripta Materialia, Volume 65, Issue 6, September 2011, Pages , ISSN [3] Honeycombe. W.K., Bhadeshia, H. D. K. H, Steels: Microstructure and Properties, 3rd Edn, Elsevier Ltd., 2006, pp. 267