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Report number: 1-FSFT
Evaluation of phase relations in weld overlays of 316, 309MoL and SKWAM
Fredrik Stenarson Fritjof Tibblin
Supervisors: Professor Malin Selleby, Tomislav Buzancic
and PhD Sten Wessman
2013
Dept. of Material Science and Engineering Royal Institute of Technology
Stockholm, Sweden
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Abstract AREVA NP Uddcomb AB wants to replace the material used for a specific valve seat used in boiling water reactors, BWR. Their solution is a weld overlay of different stainless steels composed of two buffer layers of the steel 309 MoL followed by two layers of the filler material SKWAM welded on type 316 stainless steel or carbon steel. The report focuses on the long term structural effects in the weld overlay due to the operating temperature in BWRs, in this case 270 °C. To investigate the thermodynamic stability in the weld overlay the computer software Thermo-‐Calc was used and a metallographic examination was carried out. The results from these procedures were compared and possible long term effects were discussed. Most likely spinodal decomposition is the most severe structural change that may appear in the material. At equilibrium conditions at the operating temperature ferrite is decomposed into Fe-‐rich and Cr-‐rich ferrite but since the kinetics is not included in the calculations it is not possible to determine the rate of decomposition.
Keywords: SKWAM, 316, 309MoL, 475°C embrittlement, spinodal decomposition, intermetallic phase, welding filler material, Thermo-‐Calc, stainless steel, thermodynamic stability.
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Table of Contents
Abstract ......................................................................................................................................................... 2
Introduction .................................................................................................................................................. 4
Background ................................................................................................................................................... 5
Materials ................................................................................................................................................... 5
Austenitic stainless steels ..................................................................................................................... 5
Ferritic stainless steels .......................................................................................................................... 5
Sigma phase .............................................................................................................................................. 5
475 °C embrittlement ............................................................................................................................... 6
Carbides .................................................................................................................................................... 8
Computations and experiments .................................................................................................................... 8
Thermo-‐Calc calculations .......................................................................................................................... 8
Metallographic examination ..................................................................................................................... 9
Light optical microscope ....................................................................................................................... 9
Scanning electron microscope, SEM ..................................................................................................... 9
Assumptions ............................................................................................................................................ 10
Results and discussion ................................................................................................................................ 11
Sigma phase at 300 °C ............................................................................................................................. 11
Carbides at 300 °C ................................................................................................................................... 13
Spinodal decomposition at 300 °C .......................................................................................................... 14
Comparing layers in the weld overlay ................................................................................................. 17
Metallographic examination ................................................................................................................... 17
Sources of error ...................................................................................................................................... 21
Conclusions ................................................................................................................................................. 22
Acknowledgements ..................................................................................................................................... 22
References .................................................................................................................................................. 23
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Introduction Cobalt based Stellite alloys, have traditionally been used as hard facing materials for nuclear plant valves (mainly gate valves) owing to their high corrosion resistance and superior wear resistance under sliding conditions. However, the need to avoid the use of Stellite alloys has emerged since they are the main source of cobalt, which is the largest contributor to the occupational radiation exposure. Isotope cobolt59, which may be released from cobalt containing surfaces in the form of wear and corrosion products, is transported to the reactor vessel where it is activated to the radioactive isotope cobalt60 by neutron capture in the fission process. In the light of these findings and as a most effective way to reduce cobalt contamination, many cobalt-‐free hard facing alloys, such as iron-‐based and nickel-‐based alloys, have been developed in order to replace Stellite. After annual routine testing of the BWRs security system, cracks were detected in manually welded valve seats. An investigation took its start to find a new material combination with better resistance against crack formation. Repeatedly welding maintenances were done but every year new cracks were detected. Under normal conditions the valve seats are exposed to 69 bar and 270 °C and in worst-‐case scenarios 80 bar and 300 °C.
The old weld overlay consisted of SKWAM welded directly on carbon steel. In this case the structure probably gets completely martensitic and therefore brittle. The new material combination that is under consideration is done with mechanized welding consist of a base material of type 316 covered with 309MoL and a few layers of the filler material SKWAM. All compositions including Stellite 6 can be found in table 1. Type 316 and 309MoL are primarily austenitic and SKWAM is predominantly ferritic. They are all stainless steels and are highly alloyed with chromium and none of them contains cobalt. When joining these grades there will be a mixing between the materials and new phases may occur that can cause problems.
Table 1: Composition of used materials in wt%
Element Carbon steel 316 309MoL SKWAM Stellite 6 Fe 97.65 65.495 58.83 80.48 0 C 0.25 0.08 0.02 0.02 1.2 Si 0.5 0.75 0.45 0.7 0 Mn 1.6 2 1.5 0.7 0 Cr 0 17 21.5 17 30 Ni 0 12 15 0 0 Mo 0 2.5 2.7 1.1 0 S 0 0.03 0 0 0 P 0 0.045 0 0 0 N 0 0.1 0 0 0 Co 0 0 0 0 63.8 W 0 0 0 0 5
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In this project the thermodynamic stability of the weld overlay was investigated with Thermo-‐Calc a software for equilibrium calculations [1], metallographic examination of weld overlay was performed and possible upcoming phases and problems that may occur in the future after long operation times were discussed. Evaluation of thermodynamic stability was focused on different compositions depending on cooling conditions, which phases were to be expected after long periods of time and how chromium is distributed in the different phases. Metallographic examination was made with focus on determining the phases in the overlay and comparing with thermodynamic calculations.
Background
Materials The materials of interest are 309MoL, 316 and SKWAM which are all stainless steels. SKWAM is a product name and the others are material groups. 309MoL and 316 are both austenitic steels while SKWAM is ferritic-‐martensitic.
Austenitic stainless steels Austenitic stainless steels are the most produced and largest category of stainless steels. Generally austenitic steels have good mechanical properties such as high toughness and ductility. The corrosion resistance is good in most environments but decreases when exposed to elevated temperatures, the maximum service temperature is approximately 760 °C. The austenite phase is promoted by addition of nickel, carbon, nitrogen and manganese where the most important addition is nickel. Austenitic stainless steels generally contain about 8-‐20 wt% nickel but some austenitic stainless steels are free of nickel. In this case nickel is replaced with manganese and nitrogen. Austenitic stainless steels are used in a series of applications, most of them at low temperatures, such as structural support, kitchen equipment and medical products. Austenitic stainless steel is considered to have good weldability [2].
Ferritic stainless steels Ferritic stainless steels consist mainly of ferrite phase. Ferritic stainless steels generally have better corrosion resistance compared to austenitic stainless steels but do not have as good mechanical properties. The corrosion resistance does not depend on the ferritic phase but rather the chromium and molybdenum content. Ferritic stainless steels are used for applications where corrosion resistance is more important than good mechanical properties, such as exhaust systems for cars and in chemical industries. The ferrite phase in stainless steels is favored by high chromium and molybdenum contents and low nickel content. Compared to austenitic stainless steels they are cheaper due to fewer alloy elements but they are relatively more expensive since they are hard to manufacture. Ferritic stainless steels are sensitive to embrittlement, such as 475 °C embrittlement and are therefore used at relatively low temperatures, up to 400 °C but as low as 280 °C pressure vessel. The weldability of ferritic stainless steel is not as good as for austenitic because grain growth reduces toughness and ductility [2].
Sigma phase When stainless steels are exposed to elevated temperatures for an extended period of time intermetallic phases may precipitate e.g. sigma phase. The sigma phase consists of mainly iron and chromium but its
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composition is varying depending on the alloying elements. It is precipitated when heat treated at about 570-‐1000 °C [2]. The same applies for the interpass temperature during welding and the heat input should be minimized [8].
Precipitation of sigma phase causes embrittlement of the material since the sigma particles are harder than the surrounding matrix and therefore reduces the ductility and toughness. Since the sigma phase contains 15-‐70 % chromium it is most likely that precipitation will occur in chromium rich environments within the material e.g. the ferrite phase [2]. Generally the chromium content needs to be above 20 wt% for the precipitation to take place and if the chromium content is raised to 25-‐30 wt% sigma phase forms rapidly [3].
The main transformation mechanism for the precipitation of sigma phase is the transformation of ferrite to sigma phase. Sigma phase will form directly in chromium rich regions of the ferrite grains. It is possible for sigma phase to form in austenite but it is not as usual since it is harder for chromium to diffuse in FCC than BCC. Except for chromium other ferrite stabilizing elements such as silica and molybdenum will accelerate the formation of sigma phase [3].
475 °C embrittlement It is mainly ferritic stainless steels that experience 475 °C embrittlement if they are exposed to temperatures in the interval of 425-‐550 °C [2]. 475 °C embrittlement only takes place in stainless steels in the ferritic phase during annealing around 475 °C [5]. Most common is that 475 °C embrittlement does not occur when welding because long time exposure to high temperatures is required. It will therefore be important to know the environment, such as the temperature range the material will be exposed to in its application [2]. A broader framing of the material suggests that 475 °C embrittlement can occur in steels that are ferritic, austenitic-‐ferritic and in filler materials that contain δ-‐ferrite. 475 °C embrittlement is due to spinodal decomposition [5].
Alloying elements affect time and temperature for the maximum embrittlement of the ferritic phase. Silicon, aluminum, chromium and molybdenum do all accelerate the maximum embrittlement. Carbon has the opposite effect and reduces the maximum effect of embrittlement when forming chromium-‐carbides. Alloys that contain titanium and niobium form stable carbides before chromium-‐carbides are formed so the embrittlement effect is enhanced as long as there are such stable carbides formed instead of chromium-‐carbides [5]. Nitrogen and manganese seems to have no impact on the 475 °C embrittlement, while nickel increases the effect [5], [6].
The dominant theory of why 475 °C embrittlement occurs is the coherent precipitate below 550 °C because of the miscibility gap in the iron-‐chromium phase diagram as can be seen in Fig 1. Iron-‐chromium alloys with compositions in the range of the miscibility gap and being annealed below 550 °C tend to precipitate two phases, α-‐ferrite and α’-‐ferrite. α-‐ferrite is an iron-‐rich phase with BCC-‐lattice. α’-‐ferrite is a chromium-‐rich phase with BCC-‐lattice, which contains about 61-‐83 % chromium and is nonmagnetic [2]. The two phases are said to have different morphologies where the newly formed phase of α’-‐ferrite is embedded in the chromium depleted α-‐ferrite. There are two ways for α’-‐ferrite to form, either through nucleation and growth or through spinodal decomposition [7].
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Figure 1. Parts of the iron-‐chromium system [1].
Velocity and rate of embrittlement can be seen as a function of the chromium content. High chromium content and higher temperatures results in more embrittlement whereas stainless steels with low chromium content can be almost exempt from 475 °C embrittlement [2]. In this mechanism, activation energy of aging is similar to the activation energy of Cr diffusion in the ferrite phase. The kinetics for 475 °C embrittlement precipitation can be tested by measuring the hardness and impact strength in ferrite with Charpy-‐V. The kinetics of the embrittlement can be of significant importance in certain construction parts in BWRs [6]. Studies of both ferritic-‐ and duplex stainless steels have shown that spinodal decomposition is faster in duplex steels. Radiation has been found to accelerate the spinodal decomposition and also effect volume fraction and morphology [7].
Cold working affects stainless steels so that precipitation of α’-‐ferrite increases which accelerates the embrittlement. 475 °C embrittlement also makes the steel less resistant to corrosion since the chromium depleted α-‐ferrite is particularly susceptible to corrosion. There are some alternatives to reduce the embrittlement and restore the mechanical and corrosion properties. By heat treatment of the embrittled material in the temperature interval of 550-‐600 °C for a short period of time the original properties of the stainless steel can be restored and α-‐ferrite and α’-‐ferrite can form ferrite again [2]. There will be more 475 °C embrittlement in materials with high chromium content when it has been exposed to elevated temperatures for long periods of time. Therefore stainless steels with high chromium content should not be heat-‐treated at too high temperature[8].
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Carbides In cases when the amount of carbides in austenitic stainless steels is critical, a solution annealing treatment can bring carbides back into solution. By quenching, a low amount of carbides can be obtained. The alloying elements that precipitated as carbides earlier are now in a non-‐equilibrium state. Depending on how high temperature the stainless steel will be exposed to in its application the diffusion coefficient changes and the kinetics for alloying elements determine if stable carbides can form ones again [5].
For stainless steels carbon has low solubility at low temperatures. Excess of carbon may result in precipitation of iron-‐chromium-‐carbides such as M23C6 and M6C. The chromium content in M23C6 is often in the range of 42-‐65 wt%. Since the chromium content in M23C6 is two to four times as much as the average matrix content the close surroundings of M23C6 will be depleted in chromium. Variation of chromium content can be evened out by heat treatment. During heat treatment the temperature should be higher than the temperature range where M23C6 is precipitated otherwise the diffusion for chromium and iron is to slow. Precipitation of M23C6 is mostly concentrated to grain boundaries which make adjacent areas chromium depleted. Chromium contents below 11,5 wt% increases the risk of corrosion. Since chromium depletion is concentrated to grain boundaries activation potential of intergranular corrosion increases and propagation will progress along chromium depleted grain boundaries [5].
Increased carbon content increases the risk of intergranular corrosion. Nickel contributes to increased precipitation of M23C6 because it reduces the solubility of carbon and increases the carbon activity. Silicon influence carbide precipitation the same way as nickel but with stronger effect. M23C6 precipitation is mildly affected of increased chromium content, the intergranular corrosion resistance increases since the closest surroundings have enhanced chromium content. Molybdenum reduces carbon solubility and carbides can be precipitated to a greater extent. Manganese increases carbon solubility and reduces carbon activity but seems to have no influence on corrosion resistance [5].
Computations and experiments
Thermo-‐Calc calculations The composition of each layer in the weld overlay was calculated from a principle of 70 %-‐30 % mixing between layers. Each layer composition was input data in Thermo-‐Calc 3.0 beta 2, thermodynamic calculations were made and output data such as plots and tables were extracted. Thermo-‐Calc was set to use TCFE7 [1] database in all calculations. Calculations were carried out with the main goal to extract plots of stable phases in each layer considering two different methods, 70 %-‐30 % principle and 70 %-‐30 % principle with subsequent Scheil calculations when 3 % melt remained. The composition used in the last method was the composition of the melt when 97 % had solidified. Reality is expected to be somewhere between equilibrium of the 70 %-‐30 % principle and 70 %-‐30% with subsequent Scheil calculations. Focus has also been on determining how much chromium that was expected to be distributed in each phase since it affects 475 °C embrittlement. Chromium distribution diagrams for each phase and all layers were calculated using Thermo-‐Calc. The driving force for precipitation of other phases than BCC and FCC were also calculated with Thermo-‐Calc. By using previous equilibrium
9
calculations stable phases at 300 °C were detected. All stable phases except BCC and FCC were excluded in equilibrium calculations but still the driving force was calculated, the rest was suspended from all calculations.
Metallographic examination The examined sample was a weld overlay consisting of a base layer of carbon steel covered with two buffer layers of 309 MoL and two layers of SKWAM.
Light optical microscope Sample preparation started with cutting a piece of the weld overlay with a saw. Then the piece was casted in a polymer matrix. The piece was later grinded with two different papers and later polished. Last step in the preparation was etching with a 10 % solution of electrolytic chromic acid until phases could be easily detected. The sample was examined with light optical microscope and pictures were taken to examine included phases, for further discussions and results.
Scanning electron microscope, SEM The polished and etched sample was put in a beaker containing ethanol. This was done to clean the sample. Then the sample was horizontally fixated with conducting clay. The sample was then examined with a Hitachi S-‐3700N scanning electron microscope. The composition in each layer was measured using the software Brunker Quantax 800. Also a picture of each layer was taken with the SEM. Carbon and nitrogen cannot be measured with this instrument since these elements are too light.
Figure 2. Schaeffler-‐diagram, phases to be expected in each layer of the weld overlay. [9]
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Assumptions The weld overlay consists of several layers that will interact during welding. Whenever a new layer is added the heat will partially melt the base layer and the two will mix. In this report the mixture is assumed to be 70 %-‐30 % between the layers. This means that if material A is welded onto material B the new layer will consist of 70 % material A and 30 % material B. This percentage was used after recommendations from AREVA NP Uddcomb AB. It is also assumed that the mixture is 70 %-‐30 % all over the layer.
The operating temperature for the valve seat in a BWR is about 270 °C and the calculated worst-‐case scenario gives a temperature of about 300 °C. All tables in this report are calculated at 300 °C. The most interesting temperature is the operating temperature since this report is focusing on long term effects but since the difference between operating and worst case temperature is small and temperatures are low it is assumed that 300 °C is representative. During operation the valve seat experiences a pressure of about 69 bar and in the worst-‐case scenario the pressure increases to about 80 bar. In this report it is assumed that the pressure does not affect the calculations and all calculations are done using atmospheric pressure. Calculations using Thermo-‐Calc with a pressure of 80 bar were carried out and there was negligible difference as when carried out with atmospheric pressure.
During the Scheil calculations it was assumed that carbon is fast diffusing. From the Scheil calculations the composition of the liquid phase were acquired, which was used to create plots of stable phases. The composition that was used in this report is for the liquid phase when 97 % of the system is solid. In this case it is assumed that the diffusion rate will be low and the remaining 3 % will solidify with another composition than the rest of the system. This composition is assumed to be a worst-‐case scenario.
It is taken into account that it is not possible to perform calculations on diffusion free phase transformations using Thermo-‐Calc. In this particular case irradiation effects on the weld overlay can be excluded since the valve seat is situated in an area of the plant with low radiation. This assumption was made after discussions with AREVA NP Uddcomb AB.
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Results and discussion
Sigma phase at 300 °C After evaluation of Fig 3 it can be stated that the sigma phase has no thermodynamic stability at 300 °C. Therefore sigma phase will not be precipitated even after long periods of time at 300 °C. If any sigma phase is present it has been an effect from the welding thermal cycle but sigma phase precipitate after long time and welding usually concerns rapid cooling.
Buffer layer First SKWAM layer
Second SKWAM layer Third SKWAM layer
Figure 3. Amount vs. temperature of all stable phases for all layers. [1]
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The equilibrium calculations using the composition from Scheil calculations of the third SKWAM layer when 3 % melt remains show that sigma phase is thermodynamically stable, as can be seen in Fig 4. Even if the sigma phase would precipitate in this layer the volume of sigma phase would be small considering the whole sample, also the kinetics of the reaction must be taken in to account since the temperature is low.
Buffer layer First SKWAM layer
Second SKWAM layer Third SKWAM layer
Figure 4: Amount vs. temperature of all stable phases for liquid composition in all layers during Scheil calculations when 3 % of the system is in liquid phase. [1]
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Carbides at 300 °C The carbon content is decreasing from base material to top layer. At equilibrium the amount of carbides at 300 °C follows the carbon content tendency. Carbides M23C6 and M6C are both thermodynamically stable but the total amount of them never surpass 1 mole%. M23C6 and M6C do not seem to coexist in the same layer at equilibrium. In the buffer layer and the first SKWAM layer M23C6 is thermodynamically stable, for the second and the third SKWAM layers M6C is thermodynamically stable. The equilibrium calculation using the composition from the Scheil calculations when 3 % of melt remains shows that all layers contain a higher amount of carbides, both M23C6 and M6C can coexist. The increased concentration of carbides is an effect of about six time’s higher carbon content. Even though the carbide content is high in the 3 % melt the carbide concentration in the whole sample is low. The amount of carbon and carbides decreases from the base layer to the top layer in the same way as in the calculations at equilibrium. The opposite is true considering the driving force for precipitation of carbides. The driving force for carbide precipitation increases from the base layer to the top layer as seen in table 2. Even though carbides are thermodynamically stable at 300 °C AREVA NP Uddcomb AB has not had any problem with carbides in the weld overlay. This states that no substantial amount is formed during welding and that the kinetics is slow at the operating temperature.
Table 2: Driving force for precipitation of carbides at 300 °C for each layer.
Buffer layer 1st SKWAM layer 2nd SKWAM layer 3rd SKWAM layer M23C6 0,045 0 0 3,7 M6C 0 2,09 3,14 3,14
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Spinodal decomposition at 300 °C Spinodal decomposition is thermodynamically stable at 300 °C. If the weld overlay reaches equilibrium Fe-‐rich BCC and Cr-‐rich BCC will be dominating phases in all layers which can be seen in Fig 3. If the system reaches equilibrium the absolute majority of the total Cr-‐content will be in the Cr-‐rich BCC phase as can be seen in Fig 5.
Figure 5. Weight percentage of total chromium content in α’-‐ferrite for all layers at 300 °C when equilibrium is reached.
67.7% 77.1% 78.2% 78.2%
0.0%
20.0%
40.0%
60.0%
80.0%
100.0%
Buffer layer First SKWAM layer Second SKWAM layer Third SKWAM layer
Weight-‐% of total Cr-‐content in Cr-‐rich BCC at equilibrium for each layer
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Fig 6 is displaying the Cr-‐content in all phases for the different layers. The Cr-‐content in the Cr-‐rich BCC is increasing while decreasing in the Fe-‐rich BCC. This shows that there is a driving force for spinodal decomposition as the temperature decreases.
Buffer layer First SKWAM layer
Second SKWAM layer Third SKWAM layer
Figure 6. Amount of Cr in all stable phases at equilibrium for all layers. [1]
Considering only the thermodynamics the separation between iron and chromium into two different BCC phases will be greater as temperature decreases. Fig 7 shows that there is a substantial amount of Cr-‐rich BCC in all layers. It also shows that the amount of Cr-‐rich BCC stays basically the same even though the total Cr-‐amount in each layer is decreasing towards the third SKWAM layer. The decreased
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chromium content in the SKWAM layers is probably compensated by an increased amount of ferrite, which can decompose. In the buffer layer a large part of the system is austenite.
Figure 7: Amount of Cr-‐rich and Fe-‐rich BCC in each layer at equilibrium.
Even though the thermodynamics states that the ferrite should be separated into one Cr-‐rich and one Fe-‐rich phase at 300 °C the calculations do not consider the kinetics for the reactions. For instance it is not likely to have spinodal decomposition right after welding since high temperatures under longer periods of time is required. In reality the reaction for spinodal decomposition is slow and requires chromium diffusion in solid state. The valve seats within the nuclear plant will be exposed to a somewhat elevated temperature, 270 °C under normal circumstances, which will enhance spinodal decomposition but it still is below the most critical temperatures. The most critical temperature according to literature is approximately 475 °C, the reaction rate for spinodal decomposition is highest at this temperature. The operating temperature for the valve seat is lower than 475 °C but since nuclear plants run day and night all year around it will be exposed to this elevated temperature for long periods of time. With all certainty the kinetics is lower at the operating temperature but since all calculations in this project is done assuming equilibrium it is not possible to determine the decomposition rate at 270 °C.
The results from the Scheil calculations are not relevant when talking about spinodal decomposition since the composition used in calculations only represent the 3 % of liquid phase remaining. The composition of the remaining 97 % that is solidified has almost the same composition as at the original composition and is assumed to behave the same way.
0,151 0,155 0,154 0,152
0,575
0,785 0,833 0,837
0
0,1
0,2
0,3
0,4
0,5
0,6
0,7
0,8
0,9
Buffer layer 1st SKWAM layer 2nd SKWAM layer 3rd SKWAM layer
Cr-‐rich BCC Fe-‐rich BCC
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Comparing layers in the weld overlay According to Fig 7 the spinodal decomposition is similar in all SKWAM layers, but the buffer layer differs and has lower amount of Fe-‐rich BCC. The main reason that the buffer layer does not contain much Fe-‐rich BCC is because there is large amount of austenite present, which does not decompose. Since the Cr-‐content is higher in the buffer layer it suggests that the amount of Cr-‐rich BCC should be higher compared to the SKWAM layers. But it follows the opposite trend, the buffer layer does contain more chromium but much of it is found in austenite and other Cr-‐rich phases. Fig 5 shows that in the buffer layer less chromium are absorbed in Cr-‐rich BCC. In the SKWAM layers lower amount of austenite is found and other Cr-‐rich phases are also found in smaller amounts, this result in more BCC.
The general trend for all layers is that Fe-‐rich BCC is reduced and more Cr-‐rich BCC is precipitated at lower temperatures as can be seen in Fig 3.
Metallographic examination In Fig 2 the composition for each layer is pointed out in a Schaeffler-‐diagram. In the metallographic examination no precise determination of the amount of each phase was performed so there can only be a brief discussion of expected and actual precipitated phases.
• All layers: An overview of all layers in the weld overlay can be seen in Fig 8.
Figure 8. All layers. Magnification x12.5.
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• Buffer layer 1: From Schaeffler-‐diagram, 100 % austenite was to be expected and Fig 9 show that there is probably a few percent of ferrite present in the sample.
Figure 9. First buffer layer, material 309MoL. Magnification x200.
• Buffer layer 2: From Schaeffler-‐diagram, 5 % ferrite and 95 % austenite were to be expected and Fig 10 shows that austenite and ferrite are present. The two buffer layers has approximately same ratio between austenite and ferrite.
Figure 10. Second buffer layer, material 309MoL. Magnification x200.
White areas:Dendrite of austenite
Dark areas:Ferrite
Dark areas:Primaryprecipitationof ferrite
White areas:Dendrites ofaustenite
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• SKWAM layer 1: From Schaeffler-‐diagram, a mixture of austenite, ferrite and martensite with approximately 80 % ferrite can be expected. The ratio is hard to determine from Fig 11 but it is clear that ferrite, austenite and martensite is present. Ferrite seems to be the dominating phase.
Figure 11. First SKWAM layer. Magnification x500.
• SKWAM layer 2: From Schaeffler-‐diagram, only ferrite should be present. Fig 12 shows that there
are three phases present, ferrite, austenite and martensite. Ferrite is the dominating phase.
Figure 12. Second SKWAM layer. Magnification x100.
White areas:Ferrite
Grey areas:Dendrites ofAustenite
Dark areas:Martensite
White area:Ferrite
Grey/Dark area:Martensite andaustenite
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15,05
19,57 17,77 17,23
9,29
17,71 17,16 16,96
Calculated wt% Cr in each layer SEM valvue of wt% Cr in each layer
Buffer layer SKWAM 1 SKWAM 2 SKWAM 3
Martensite is present in all SKWAM layers as seen in Fig 11 and 12. It is not possible to see martensite in the Thermo-‐Calc calculations since it is not thermodynamically stable but if precipitated the decomposition is slow. Martensite is an effect of welding and rapid cooling from the austenitic region. During operation in the nuclear plant more martensite will not form in the SKWAM layers since rapid cooling from high temperatures is required to form martensite. In this case the valve seat will be exposed to a somewhat elevated temperature for a long time but not high enough.
Fig 13 shows that the assumption of a 70 %-‐30 % mixture is quite accurate for the chromium content in each layer.
Figure 13. Calculated wt% chromium with 70 %-‐30 % mixture in each layer of the examined sample and measured wt% chromium from SEM.
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The calculated values and the values measured with SEM for all elements are summarized in table 3.
Table 3: Calculated values from 70 %-‐30 % mixture and composition of elements using SEM.
Fe Si Mn Cr Ni Mo Buffer layer 1 calculated
70,47 0,47 1,53 15,05 10,5 1,89
Buffer layer 1 from SEM
82,07 0,18 1,06 9,29 6,65 0,76
Buffer layer 2 calculated
62,32 0,45 1,51 19,57 13,65 2,46
Buffer layer 2 from SEM
66,22 0,3 1,47 17,71 11,55 2,67
SKWAM layer 1 calculated
75,03 0,63 0,94 17,77 4,1 1,51
SKWAM layer 1 from SEM
72,66 0,33 0,84 17,16 7,4 1,55
SKWAM layer 2 calculated
78,85 0,68 0,77 17,23 1,23 1,22
SKWAM layer 2 from SEM
78,91 0,45 0,6 16,96 2,04 0,95
Sources of error The mixture between layers in the weld overlay was assumed to be exactly 70 %-‐30 % mixture. It is unreasonable that the mixture is exactly 70 %-‐30 % in the whole layer. Reasonable is that the area closest to the layer beneath is more mixed than at the top of the new layer, as a gradient.
During Scheil calculations it was assumed that the melt segregates until 3 % of the melt is remaining. When the 3 % melt remains calculations were aborted because otherwise temperature of solidification would be unrealistically low. 3 % melt were discussed with our supervisors and it was decided that it was a reasonable amount. This was thought to be a worst case scenario for the weld overlays composition.
When the Scheil calculations were performed carbon was assumed to be a fast diffusing element because of its small size. Since nitrogen has approximately the same size as carbon it is possible that it also should have been considered to be fast diffusing.
All calculations in Thermo-‐Calc were done with the constitution of three SKWAM layers and one buffer layer. Unfortunately the samples from AREVA NP Uddcomb AB consisted of two SKWAM layers and two buffer layers. Also the base material was carbon steel instead of stainless steel type 316. This makes the comparison between the calculations and the samples less meaningful.
The samples from AREVA NP Uddcomb AB have not been in operation in a nuclear plant. Comparing samples with the calculations makes them less accurate since calculations are focusing on long-‐term effect due to an elevated temperature. The samples only show the structure right after welding.
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Conclusions The method was to perform equilibrium calculations using Thermo-‐Calc to gain information on which phases that are present in the different layers of this particular weld overlay. A metallographic examination was carried out to compare the calculations with the samples. One shortcoming in this project was that the sample that was examined has not been in operation and because of that no long-‐term effects could be observed. One way to improve the method would be to use a sample that had been in operation. During the metallographic examination martensite was observed in the SKWAM layers. This was assumed to be an effect from welding and is not possible to predict using Thermo-‐Calc. Since martensite will influence the mechanical properties of the valve seat an improvement would be to find a way to predict the amount of martensite formed.
Among the thermodynamic effects that occur after long time exposure to the operating temperature spinodal decomposition seems to be the most severe. At equilibrium the spinodal decomposition is extensive but in the calculations performed in Thermo-‐Calc the kinetics was not considered. This is a shortcoming with the method and to get more accurate results kinetic calculations should be performed. For example if the kinetics for the spinodal decomposition at the operating temperature is slow this might not be a problem but it can have large impact on the mechanical properties if the kinetics is fast. The chromium composition is crucial for the spinodal decomposition since it is depending on chromium diffusion. By using SEM the calculated wt% of chromium in each layer could be controlled. Fig 13 shows that the approximation is good when the composition between layers is similar but between the carbon steel and the highly alloyed buffer layer the difference is large.
The method also offers some advantages. Phases that do not exist in the weld overlay after welding can be disregarded if they are not thermodynamically stable at the operation temperature. For example the sigma phase will not be a problem in this case since it is not stable at the operating temperature according to Fig 3 and was not detected in the samples. Using this method it is possible to exclude several phases but not to get an exact result. The most important improvement in this case would be to learn more about the kinetics for spinodal decomposition at the operating temperature.
Acknowledgements Thanks for all help and support from supervisors’ professor Malin Selleby and PhD Sten Wessman at Dept. of Material Science and Engineering at KTH. Thanks to Wenli Long for your help with SEM. For helping us with the preparation of the samples thanks to Ian Patterson and Jonas Guldbrandsson for demonstrating welding procedure at AREVA NP Uddcomb AB. Most of all thanks to Tomislav Buzancic for assigning us this project, support and the field trip to AREVA NP Uddcomb office in Karlskrona.
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