ap-6-10.dvi Acta Polytechnica Vol. 50 No. 6/2010 Microstructure of the Transitional Area of the Connection of a High-temperature Ni-based Brazing Alloy and Stainless Steel AISI 321 (X6CrNiTi 18–10) R. Augustin, R. Koleňák Abstract This paper presents a detailed examination of the structure of the transitional area between a brazing alloy and the parent material, the dimensions of the diffusion zones that are created, and the influence on them of a change in the brazing parameters. Connections between Ni-based brazing alloys (NI 102) with a small content of B and AISI 321 stainless steel (X6CrNiTi 18–10) were created in a vacuum (10−2 Pa) at various brazing temperatures and for various holding times at the brazing temperature. Various specimens were tested. First, the brazing alloys were wetted and the dependence of the wetting on the brazing parameters was assessed. Then a chemical microanalysis was made of the interface between the brazing alloy and the parent material. The individual diffusion zones were identified on pictures from a light microscope and REM, and their dimensions, together with their dependence on the brazing parameters, were determined. Keywords: NI 102 brazing alloy, AISI 321, wetting, chemical microanalysis, diffusion zones. 1 Introduction High-temperature brazing of stainless steels is a spe- cific method for connectingmaterials that can create high-quality, demountable connections without local thermal impact on the connected material. High- temperature brazing can be applied where it is not desirable to use welding because of the thermal im- pact or because of the complexity of the connections that will be created [1]. Nickel-based brazing alloys are the most suitable for high-temperaturebrazingof stainless steel. When partsmade of stainless steel are connected by a high- temperature nickel brazing alloy, the metallurgical connection has characteristics similar to those of a welded connection. However, in contrast to welding there is no melting of the parent material (PM) due to the significantly lowermelting point of the brazing alloys [2]. A high-vacuum atmosphere in high-temperature brazing prevents the brazing alloy and PM interact- ing with the ambient atmosphere, so it is not nec- essary to use fluxes with this method. At the same time, the vacuum atmosphere has no effect on the physical characteristics of PM. Brazing in a vacuum complies with the latest world trends in machine technology, and is therefore also the most favoured high-temperature brazing method [3]. High-temperature brazing of stainless steels with nickel-based brazing alloys in a vacuum has already been used for a long time in practical applications. It is therefore desirable to know the influence of the basic brazingparameters on the characteristics of the brazing alloy and the connection that is formed. It is then possible to optimize the initial parameters in such a way that the connections meet the require- ments made on them [4]. 2 Materials and methods Austenitic stainless steel AISI 321 (X6CrNiTi 18–10, DIN 1.4541) was selected for the purposes of the ex- periment. High alloyed austenitic steels cannot be refined, so they can also be brazedwith slow cooling. The exact chemical composition of the 17246 steel is shown in table 1 [5]. Two brazing alloys of similar chemical compo- sition containing B were selected from the series of high-temperature nickel-based brazing alloys, see Tab. 2. According to the STN EN 1044 standard, they both fall under code NI 102. Table 1: Chemical composition of AISI 321 Fe Cr Ni Mn Si C W Ti Mo 67.00 % 19.30 % 8.12 % 1.27 % 0.41 % 0.02 % 0.63 % 0.36 % 0.06 % 7 Acta Polytechnica Vol. 50 No. 6/2010 Table 2: Chemical composition of brazing alloy NI 102 Brazing alloy Ni Cr Si B C Fe NI 102-01 83.5 % 6.5 % 4.5 % 3.0 % 0.05 % 4.0 % NI 102-02 82.0 % 7.0 % 4.0 % 2.0 % 0.15 % 4.5 % The brazing parameters shown in Tab. 3 were se- lected with reference to the objective of the paper, in order to be able to assess the influence of specific variables. The specimens were heat treated in a PZ 810 vacuum furnace with heating rate 26.88◦C/min and cooling rate 2.15◦C/min at 10−1 to 10−2 Pa. Table 3: Experimental parameters Specimen No. 1 Brazing Brazing Brazing alloy temperature time [◦C] [min.] NI 102-01 1200 10 NI 102-02 1200 10 Specimen No. 2 Brazing Brazing Brazing alloy temperature time [◦C] [min.] NI 102-01 1050 30 NI 102-02 1050 30 Specimen No. 3 Brazing Brazing Brazing alloy temperature time [◦C] [min.] NI 102-01 1100 120 NI 102-02 1050 120 The finished specimens were divided using a cut- ting disc, and after marking they were sealed with bakelite on a Buehler SimpliMet 1000 device. Then they were ground with grinding paper with 240, 600 and1200grit andpolishedwith diamondpasteswith 9, 6 and 3 μmgrit on a semi-automaticBuehler Beta machine. Finally, the specimens were polished us- ing colloidal silica 2 μm (Mastermet). To make the structure visible, an etching agent was used (10 ml H2SO4, 10mlHNO3, 20mlHF, and50mlH2O).The etching time was about 20 seconds. Specimens pre- pared in this way could be scanned and analyzed by a Neophot 30 lightmicroscope. Whenmeasuring the diffusion zones, 11 vertical parallel lines weremarked out in eachpicture, and on eachof thempoints defin- ing the boundaries of the given zone were drawn in. The resulting values are the arithmetic mean of the 11 measurements. Line profiling and element distri- butionmappingweremeasuredusing anARLSEMQ device. 3 Results 3.1 An evaluation of the wetting of the brazing alloys First of all, the wetting was measured on the speci- mens. Angle α was determined on the pictures cre- atedby the lightmicroscope, using graphics software. An example of these measurements is shown in Fig. 1, 2 and 3, where the changes in this angle as a result of different parameters can also be recog- nized. The results of the measurements are summa- rized in Tab. 4. Tangent to the surface of PM – σSL Tangent to the surface of the brazing alloy – σLV Wetting angle – α Fig. 1: NI 102-02, α ∼ 3.62◦ (1200◦C/10 min.) Fig. 2: NI 102-02, α ∼ 5.63◦ (1050◦C/30 min.) Fig. 3: NI 102-02, α ∼ 1.82◦ (1050◦C/120 min.) 8 Acta Polytechnica Vol. 50 No. 6/2010 Table 4: Values of the wetting angles of the examined brazing alloys at the given parameters Brazing alloy Holding time at brazing temperature [min] Brazing temperature [◦C] Contact angle of wetting [◦] NI 102-01 10 1200 1.54±0.5 NI 102-02 10 1200 2.34±0.5 NI 102-01 30 1050 3.40±0.5 NI 102-02 30 1050 5.14±0.5 NI 102-01 120 1100 1.45±0.5 NI 102-02 120 1050 1.50±0.5 3.2 Chemical microanalysis A global chemical microanalysis was then performed on the specimens. The objective was to determine the proportion of the basic components of the exam- ined materials in the mutual diffusion and creation of the connection. The samediffusion reactions took place for all pa- rameters, but they are most visible at brazing tem- perature 1100◦C and 120 min. holding time at this temperature — Fig. 4 and Fig. 5. Fig. 4: Cr concentration in transitional area NI 102 – AISI 321 The zone of Cr diffusion from PM into the braz- ing alloy can be observed (white points left from the interface) in Fig. 4, where the interface is indicated. Placeswith the highestCr content arewhite. Cr also diffuses along the grain boundaries from the brazing alloy into PM (visible grain boundaries right from the interface). This is probably caused by B. Cr with B creates CrB2 borides, and B diffuses very quickly from the brazing alloy into PM along the grain boundaries. Fig. 5 shows the concentrationofFe inPMand in the transitional area. The zone of Fe diffusion from PM into the brazing alloy, similarly as Cr, can be observed. However, the dark areas along the grain boundaries of PM indicate that the concentration of Fe in these places is significantly reduced, and this is also confirmed by linear microanalysis, see Fig. 6. Fig. 5: Fe concentration in transitional area NI 102 – AISI 321 Fig. 6: Linear microanalysis of transitional area NI 102- 01 – AISI 321 (1100◦C/120 min.) 9 Acta Polytechnica Vol. 50 No. 6/2010 The map of elements Ni and Si shows no higher participation in the creation of the brazed connection at any parameters. 3.3 Measurements of the diffusion zones The pictures from the lightmicroscope show the sep- arate diffusion zones in the interface between brazing alloy and PM, see Fig. 7. Specifically the PM solu- bility zone in the brazing alloy, the diffusion zone of the brazing alloy in PM, the diffusion on the bound- aries of the grains, and the zonewith a visible change in the microstructure. By measuring these zones for various parameters, it was possible to assess their dependence on the parameters The measurement re- sults are shown in Tab. 5. Fig. 7: Diffusion zones in the interface NI 102 – AISI 321 The figures show examples of pictures from the light microscope on which the specific zones were measured. Fig. 8 shows that the solubility zone of PM and the diffusion zone of the brazing alloy into PM cannot be distinguished with the given param- eters. They were therefore measured as one whole. Fig. 9 shows that these zones are clearly distinguish- able after the parameters have been changed. Fig. 8: Interface of NI 102-01 and AISI 321 (1200◦C/10 min.) Fig. 9: Interface of NI 102-01 and AISI 321 (1050◦C/30 min.) With longer holding time atbrazing temperature, the PM solubility zone and the diffusion zone of the brazingalloy inPMcanbeclearlydistinguished, even when the brazing temperature is 100◦C lower, and a significant growth in the width of specific zones can also be seen – Fig. 10. Table 5: Recorded dimensions of diffusion zones with various parameters Brazing alloy Holding time at brazing temperature [min] Brazing temperature [◦C] Zone of PM solubility [μm] Zone of diffusion into PM [μm] Diffusion along grain boundaries [μm] Zone of changed structure [μm] NI 102-01 10 1200 23 93 38 NI 102-02 10 1200 23 109 49 NI 102-01 30 1050 9 16 101 50 NI 102-02 30 1050 5 18 194 72 NI 102-01 120 1100 23 50 203 78 NI 102-02 120 1050 13 24 332 132 10 Acta Polytechnica Vol. 50 No. 6/2010 Fig. 10: Interface of NI 102-01 and AISI 321 (1100◦C/120 min.) 4 Discussion Brazing alloysNI 102on stainless steelAISI 321were distinguished by very good wetting at all brazing parameters. It is known that the angle of wetting decreases with rising temperature and holding time at brazing temperature. However it was found that temperature has amuch greater influence onwetting than the holding times at brazing temperature. It was also found that a slightly increased content of B inbrazingalloyNI102-01 in comparison toNI 102-02 caused better wetting of the surface of PM. A chemical microanalysis showed that the solu- bility zone of PM in the brazing alloy and also the interface between the brazing alloy and PM are cre- ated to a significant extent only by Fe and Cr, and the results indicate that the proportion of Ni and Si is significantly lower. Interstitial atoms of B along the grain boundaries diffuse most quickly into PM, because they are not soluble in the solid solution of Ni. B creates CrB2 phase with Cr. These areas are characterized by a higher concentration of Cr. The concentration of Fe in these places is significantly re- duced. By analysing the diffusion zones, it was found that, when there is a short holding time, even at a higher brazing temperature it was not possible to distinguish the solubility zone ofPM into the brazing alloy from the diffusion zone of the brazing alloy into PM. For the same brazing parameters, brazing alloy NI 102-02 achieveddeeper diffusion of the brazing al- loy into PM, whereas better penetration of PM into the brazing alloy occurredwith brazing alloyNI 102- 01. The influence of brazing temperature ondiffusion depth appeared to be approximately the same as the influence of the holding time at brazing temperature. Even a slight change in the brazing temperature has a big influence on diffusion of the brazing alloy into PM. 5 Conclusion From the measured values of wetting shown in Tab. 4, it can be stated that: • The wetting of both brazing alloys, NI 102-01 and NI 102-02, is excellent, or even perfect, for all parameters. • Although the two brazing alloys fall under code NI 102 according to their chemical composition, they have different wetting values. • The better wetting of brazing alloy NI 102-01 is probably due to a higher content of B. • The angle of wetting becomes smaller with ris- ing brazing temperature and longerholding time at this temperature. The proportion is therefore inverse. • According to the measured values, the influence of the brazing temperature on the wetting is more distinct than the effect of the holding time at brazing temperature. As a result of the chemical microanalysis of the specimens: • The zone of solubility of PM in the brazing alloy and the interface between the brazing alloy and PMare created to a significant extent only byFe andCr. The results show that the proportion of Ni and Si is significantly lower. • Interstitial atoms of B along the grain bound- aries diffusemost quickly intoPM, because they are not soluble in the solid solution of Ni, where they formaphasewithCr. These areasare char- acterized by a higher concentration of Cr. The concentrationofFe in theseplaces is significantly reduced. On the basis of the measured diffusion zone values shown in Tab. 5, the following conclu- sions can be drawn: • With a holding time of 10 minutes at brazing temperature 1200◦C, it was not possible to dis- tinguish the solubility zone of PM into the braz- ing alloy from the diffusion zone of the brazing alloy into PM on the interface of either of the alloys. • Using the same brazing parameters, brazing al- loy NI 102-02 achieved deeper diffusion of the brazing alloy intoPMand, conversely, therewas better penetration of PM into the brazing alloy in the case of brazing alloy NI 10-01. • The influence of brazing temperature on diffu- sion depth is directly proportional to the influ- ence of the holding time at brazing temperature. • A change or drop in brazing temperature, even a relatively small change (50◦C), has a great in- fluence on the diffusion of the brazing alloy into PM. 11 Acta Polytechnica Vol. 50 No. 6/2010 Acknowledgement This paper was prepared with support from the VEGA 1/0381/08 project — Research of the in- fluence of physical metallurgical aspects of high- temperature brazing on the structure of connections of metal and ceramic materials and from the APVT 20-010804 project — Development of lead free soft active solder and research of material solderability of metal and ceramic materials with the use of ultra- sonic activation. References [1] Available on: http://www.pva-lwt-gmbh.de/ englisch/sites/te vakuum verfah.php [2] Ruža, V., Koleňák, R., Jasenák, J.: Spájkovanie vo vákuu. Trnava, SZS, 2005. [3] Available on: http://www.aws.org/ w/a/wj/2004/10/030/index.html [4] Available on: http://cdsweb.cern.ch/record/1151308?ln=sk [5] Ruža, V., Koleňák, R.: Spájkovanie materiálov. Bratislava, STU, 2007. Ing. Robert Augustin doc. Ing. Roman Koleňák, PhD. Phone: +421 949 358 111 Faculty of Materials Science and Technology in Trnava PavilonT Bottova 23, 917 24 Trnava, Slovak Republic 12