FILLER FOR THE WELDING OF MATERIALS FOR HIGH-TEMPERATURE APPLICATIONS

Abstract

A filler for welding including (in % by weight): C: ≦0.036, Ni: 15.0-20.0, Cr: 15.0-22.0, Mn: 0.75-2.0, Zr: 0.1-1.45, Si: 0-1.5, Al: 0-2, N: <0.06, and a balance of Fe and inevitable impurities.

Claims

1. A filler for welding, comprising (in % by weight): C: ≦0.036 Ni: 15.0-20.0 Cr: 15.0-22.0 Mn: 0.75-2.0 Zr: 0.1-1.45 Si: 0-1.5 Al: 0-2 N: <0.06 balance Fe and inevitable impurities.

2. The filler according to claim 1, wherein C is ≦0.030 by weight.

3. The filler according to claim 2, wherein C is ≦0.020% by weight.

4. The filler according to claim 1, wherein Ni is 15-17 by weight.

5. The filler according to claim 1, wherein Ni is 17-20% by weight.

6. The filler according to claim 1, wherein Cr is 15-22% by weight

7. The filler according to claim 6, wherein Cr is 17-22% by weight.

8. The filler according to claim 6, wherein Cr is 15-19% by weight.

9. The filler according to claim 1, wherein Mn is 0.75-1.75% by weight.

10. The filler according to claim 1, wherein Zr is 0.35-1.45% by weight.

11. The filler according to claim 10, wherein Zr is 1.15-1.45% by weight.

12. The filler according to claim 10, wherein Zr is 0.35-1.39% by weight.

13. The filler according to claim 10, wherein Zr is 0.35-0.65% by weight.

14. The filler according to claim 1, wherein Zr is 0.1-1.3% by weight.

15. The filler according to claim 14, wherein Zr is 0.5-0.7% by weight.

16. The filler according to claim 14, wherein Zr is 0.35-0.65% by weight.

17. The filler according to claim 1, wherein Si is 0.3-1% by weight.

18. The filler according to claim 1, wherein Al is 0-1.

19. The filler according to claim 18, wherein Al is 0.3-1% by weight.

20. The filler according to claim 1, wherein N is 0-0.03% by weight.

21. The filler according to claim 1, wherein N is 0% by weight.

22. The filler according to claim 1, wherein the filler is in the form of a welding band or a welding wire.

Description

DESCRIPTION OF DRAWINGS

[0048] FIGS. 1-6: SEM images of welded joints produced from the filler according to the invention.

[0049] FIG. 7: Drawing of test bar used in the experiments.

[0050] FIG. 8: Tabulation of chemical composition of the fillers according to the invention used in the experiment.

[0051] FIG. 9: Chemical composition of parent metal APMT, Incoloy800HT as well as 253Ma.

DEFINITIONS

[0052] In the present application, with “filler”, reference is made to the material that upon joining two or more workpieces forms the welding seam between the workpieces.

[0053] With “parent metal” or “workpiece”, in the present application, reference is made to the materials that are joined with “the filler”.

Examples

[0054] In the following, the welding material according to the invention will be described with reference to concrete experiments. Before the experiments, first the parent metals were determined. These became APMT, Incoloy 800HT, and 253MA. Chemical analysis of the parent metals used in the experiments is seen in FIG. 9.

[0055] In order to get a sufficient amount of material for making tensile test pieces and creep test pieces, it was determined that the parent metals should be in the form of tubes in lengths of 15 cm having an outer diameter YD of 88.9 mm and a wall thickness of 5.0 mm. The parent metals are commercially available.

[0056] Next, the fillers were produced. A tabulation of all melting experiments and their composition is seen in FIG. 8. The melts were produced in the following way:

[0057] First, the incorporated alloying materials were weighed. Each metal was weighed on a balance of the make Sartorius BP 41005. The accuracy of the weighing was +0.3 g. The total weight of each experimental melt was 1100 g.

[0058] Melting was effected inductively in a furnace of the make Balzers. First, the container, in which the crucible is situated, was pumped down to a pressure of 0.1 torr. Then, a preheating of the crucible and the alloying materials was made. Before the melting was initiated, the container was filled with the protective gas Ar to a pressure of 400 torr. In the end of the melting, a part of Zr was added to the melt via a lance in the lid of the container. This procedure is called spiking and is made because Zr has a very high reactivity with oxygen. Although it is a deliberately low partial pressure of 0 in the container, Zr reacts rapidly with the small amount of 0 present and disappears from the usable part of the melt.

[0059] For every melting experiment, chemical analysis was made to check the actual composition in finished ingot. Two melts of No. 1 and No. 4 were needed to get a sufficient amount of welding wire for welding APMT to 253MA also.

[0060] After casting, the ingot was turned into cylindrical blanks, which were hot-rolled into a diameter of 6 mm. Then, they were drawn into a diameter of 1.6 mm. The two last steps were made for only a seventh part of the wires.

[0061] Next, the wires were used to weld together tubes of APMT to Incoloy 800HT and APMT to 253MA by means of TIG welding. Before welding, the tubes were cleaned and pickled.

[0062] Root gas was used to protect the root bead from oxidizing and forming slag. To get to an effective root gas protection, end portions for the tubes were needed. All tubes were edge prepared in both ends for providing a second chance should the first welding attempt fail. Therefore, end portions were needed having a diameter corresponding to the new inner diameter of the tubes plus two times the thickness of the lip in the single U groove. The result was a diameter of 82 mm of the end portions. The material of the end portions was plain carbon steel and a thickness of 2 mm was enough. In the middle of the end portions, there should be a hole having a diameter of 7 mm to introduce/discharge the protective gas. On the inlet side, a tube was welded over the hole as an adapter to the protective gas hose.

[0063] The tubes were prepared before the welding by attaching the tube end portions by spot-welding and by attaching each material pair by spot-welding. Upon spotting, the tungsten electrode is used to melt together the parent metals. Then, the tubes were put in a furnace for preheating to 300° C. The welding was made with seven beads. For the root bead, a welding current of 80 A was used, and for the rest of the beads, a welding current of 100 A. For the root bead, the welding rod with Ø 1.6 mm was used, and for the rest of the beads, the welding rod with Ø 2.0 mm. In the welding, the voltage was approximately 11 V and the positioner had a constant advancing speed of 100 mm/min. This gave a heat input of about 0.5 kJ/mm for the root bead and about 0.65 kJ/mm for the rest of the beads. The protective gas was pure Ar both in the welding gun and the root protection. The gas flow was 10 l/min in the welding gun and 81/min for the root gas.

[0064] After welding, the tubes were heated in a furnace at 850° C. for 30 min and then they were allowed to cool down slowly to room temperature.

EDS Analysis of Material Composition in Welding Seam

[0065] After the welding, before heat treatment, EDS analysis of the welding seams was made with the purpose of determining their chemical composition. The EDS analysis was made of a sample sized 600 μm times 400 μm, which was taken from the middle of each welding seam. Table 1 shows the result from EDS analysis of the different combinations of materials.

TABLE-US-00001 TABLE 1 Result from EDS analysis (Weight %) Weld joint Ni Cr Al Si Mn Zr Fe APMT-Nr.1-800HT 9.6 20.7 0.9 0.5 1.0 0.5 rest APMT-Nr.2-800HT 8.1 20.5 1.0 — 1.2 1.2 rest APMT-Nr.3-800HT 17.2 20.6 0.8 0.5 1.4 0.4 rest APMT-Nr.4-800HT 16.0 20.4 0.5 0.3 1.7 1.0 rest APMT-Nr.1-253MA 4.2 20.5 0.8 0.6 1.4 0.3 rest APMT-Nr.4-253MA 11.3 20.9 0.8 0.7 1.3 1.1 rest

Tensile and Creep Testing

[0066] Before the tensile and creep testing, test bars were produced by cylindrical blanks being sawn out from the welded blanks. The cylinders were 100 mm long with the welding seam in the middle. Then, the cylinders were machined into test bars with dimensions according to FIG. 7.

[0067] The tensile testing was made with a machine of the make Zwick/Roell Z100. The APMT ends of each test bar were always mounted in the lower drawing jaw. All tensile testing was carried out at room temperature. The creep test pieces were applied in rigs, and beforehand, the diameter of each test bar had been measured with an accuracy of thousands of millimetres.

Tensile Testing

[0068] Tensile testing was made both with tensile test pieces, which had been manufactured by turning after welding, and tensile test pieces, which had become heat-treated before tensile testing. The heat treatment went on for 500 h at 750° C.

[0069] Table 2 shows ultimate tensile strength and elongation values for the different combinations of materials after heat treatment 500 h at 750° C. Three tensile tests were carried out for each material combination.

TABLE-US-00002 TABLE 2 Ultimate tensile strength and elogation values for the different combinations of materials after heat treatment 500 h at 750° C. Material combination Bar no. Rm [Mpa] Rupture elongation [%] APMT-Nr. 1-800HT 1 568 20.67 2 531 17.40 3 570 10.82 APMT-Nr. 2-800HT 1 587 12.48 2 629 6.7 3 596 17.11 APMT-Nr. 3-800HT 1 468 13.75 2 401 8.98 3 445 10.45 APMT-Nr. 4-800HT 1 559 17.37 2 540 8.44 3 380 4.44 APMT-Nr. 1-253MA 1 710 23.02 2 651 8.93 3 659 12.01 APMT-Nr. 4-253MA 1 508 3.16 2 618 14.77 3 585 14.67

[0070] From table 2, it is seen that the welding material according to the invention has sufficient strength to be used in welded joints. The strength of a welded construction of different materials is generally set by the strength of the weakest material. Incoloy 800HT has a specified tensile strength of 536 MPa at room temperature (Special Metals datasheet, P. No. SMC-047, Copyright © Special Metals Corporation, 2004 (September 04)). Thus, it is seen that Fillers 1, 2, and 4 have higher and essentially higher, respectively, strength than the parent metal Incoloy 800HT. The strength of Filler 3 is lower than the strength of Incoloy 800HT. However, Filler 3 is sufficiently strong to be used in welded joints.

[0071] The parent metal 253MA has a tensile strength of 650-850 MPa. In table 2, it is seen that the strength of Filler 1 corresponds to the strength of 253MA. Filler 4 has sufficiently high strength in comparison with 253Ma to be usable in welded joints.

[0072] Rupture elongation is a measure of the ductility of the weld metal. The rupture elongation in table 2 exceeding 8% are considered be sufficient for the weld or welding seam to be usable. From table 2, it is seen that the rupture elongation of the inventive materials 1-4 is sufficiently ductile.

[0073] Test bar No. 2 of APMT-No. 2-800HT had several pores, which is the explanation why this test bar got so low values.

Creep Testing

[0074] Creep testing was carried out at 800° C. with a tensile stress of 28 MPa. Table 3 shows the results from creep testing at 800° C. All samples were subjected to a tensile stress of 28 MPa.

TABLE-US-00003 TABLE 3 Creep testing at 800° C. Test Time to Creep Rupture Material combination position rupture [h] vel.[1/s] elongation [%] APMT-Nr. 1-800HT C306-1 150.0 2.22*10.sup.−8 2.8 APMT-Nr. 2-800HT C307-2 23.5 1.58*10.sup.−7 7.79 APMT-Nr. 3-800HT C308-3 174.0 1.65*10.sup.−8 2.43 APMT-Nr. 4-800HT C309-4 273.0 8.07*10.sup.−9 4.33 APMT-Nr. 1-253MA C310-5 7.5 1.51*10.sup.−6 18.89 APMT-Nr. 4-253MA D087 267.0 1.26*10.sup.−8 4.7

[0075] The creep strength of the inventive samples can be compared with the creep strength of APMT, which at 800° C. and 28.8 MPa is 100 h to failure.

[0076] From table 3, it is seen that Fillers 1, 3, and 4 exceed the value of APMT. In particular, Filler 4 shows excellent creep resistance, both in combination with Incoloy 800HT and 253MA.

[0077] The low creep values of Filler No. 2 in combination with Incoloy 800HT and Filler 1 in combination with 253MA are assumed to depend on the presence of much ferrite in the welding seam. The formation of ferrite may in turn depend on the relatively low amount of nickel in the filler.

Study of Oxide Growth after 500 h of Heat Treatment at 1050° C.

[0078] An examination was made of the oxide formation on samples having been heat treated for 500 h at 1050° C. The following material combinations were studied: APMT-No. 1-Incoloy 800HT, APMT-No. 2-Incoloy 800HT, APMT-No. 1-253MA, and APMT-No. 4-253MA. The oxide formation on the respective sample was estimated ocularly by an experienced laborant.

[0079] The result indicated a strong oxide growth on the combinations of materials APMT-No. 1-Incoloy 800HT, APMT-No. 2-Incoloy 800HT, and APMT-No. 1-253MA. The strong oxide growth on these samples may be assumed to be connected to the low content of Ni in these fillers, which only was 3.09 and 2.52% by weight, which should be compared with 15.26 and 15.37% by weight in Fillers 3 and 4. From table 1, which shows the content of nickel from EDS analysis, it is seen that the content of Ni is approximately 9% by weight in the welding seams with the combinations of materials APMT-No. 1-Incoloy 800HT and APMT-No. 2-Incoloy 800HT. There is apparently too a low content upon use at 1050° C. APMT-No. 1-253MA has even as low a content of Ni as 4% by weight.

[0080] The weld metal in the material combination APMT-No. 4-253MA has 11% by weight of Ni and has not been affected by corrosion. It is reasonable to assume that the lower limit for how much Ni that is needed for devastating corrosion in the joints not to arise is 10% by weight.

Microscopy

[0081] Finally, the microstructure of the welding seams was evaluated by optical microscope and SEM. Before microscopy, the welding seam was cut out into a 25 mm long piece, was encased in 30 mm Bakelite pellet, and was ground and polished. Microscopy was made on samples taken directly after welding as well as on samples, which were heat-treated for 500 h at 750° C.

[0082] FIG. 1 shows a SEM image in 440 times magnification of a sample from a welded joint between 253MA-Filler No. 1-APMT taken in the interface between the weld metal and parent metal 253MA. The sample has been taken directly after welding without heat treatment. The position of the sample is seen in FIG. 1. In the image, small AlN precipitations in the form of about 2 μm large black dots can be observed in the interface between parent metal and the weld metal, see the encircled area in FIG. 1. The weld metal also contains small round white precipitations. By means of SEM, it could be established that these precipitations have a high content of Zr and nitrogen and hence it may be assumed that the same consist of ZrN.

[0083] FIG. 2 shows a SEM image from a sample from a welded joint between Incoloy 800HT-Filler No. 2-APMT. The sample has been taken in the interface of weld metal of Filler 2 and the parent metal APMT directly after welding without heat treatment. In this sample, no AlN precipitations could be found. However, in the image, small white precipitations appear, which are evenly distributed across the weld metal. Analysis in SEM shows that these precipitations consist of a Ni.sub.xZr.sub.x phase. Since the content of nitrogen is low in the parent metal both in APMT and Incoloy 800HT, nickel and zirconium form precipitations of Ni.sub.xZr.sub.x instead of AlN. In the finished welding seam, Ni.sub.xZr.sub.x will constitute a reservoir of zirconium. This zirconium will take care of nitrogen that diffuses into the welding seam from the atmosphere in use of the welded joint at high temperatures, thereby preventing and minimizing, respectively, the formation of brittle AlN precipitations.

[0084] FIG. 3 is a SEM image of a sample taken from the interface between weld metal of Filler 1 and the parent metal 253MA, which has been heat treated for 500 h at 750° C. Also this sample shows small precipitations of AlN in the interface between weld metal and filler.

[0085] FIG. 4 is a magnification of the weld junction in FIG. 3. In FIG. 3, it is seen that, in addition to AlN, also small white precipitations have been formed, which are assumed to consist to of ZrN.

[0086] FIG. 5 is a SEM image of a sample taken from the interface between weld metal of Filler 4 and the parent metal 253MA, which has been heat treated for 500 h at 750° C. In this figure, no AlN precipitations can be observed in the interface between weld metal and parent metal. However, a relatively great amount of white precipitations in the weld metal are seen. These are assumed to be ZrN. The lack of AlN precipitations and the great amount of ZrN are assumed to depend on the high content of Zr in Filler 4.

[0087] FIG. 6 shows a SEM image from a sample from a welded joint between Incoloy 800HT-Filler No. 3-APMT, which has been heat treated for 500 h at 750° C. The sample has been taken in the interface of weld metal of Filler 3 and the parent metal APMT. In this sample, precipitations of Ni.sub.xAl.sub.x (nickel aluminide) have been formed in the weld junction between the filler and the parent metal (APMT). The formation of nickel aluminide is assumed to depend on the filler having high content of nickel as well as the parent metal having high content of Al. Furthermore, the content of zirconium is relatively low in Filler 3-0.63% by weight.

[0088] To sum up, the SEM images show that Fillers 2 and 4, which have a high content of zirconium, contribute to minimize the formation of aluminium nitride (AlN) in the weld metal. It should also be noted that in the cases austenitic steel with high content of nitrogen is used as parent metal, the Zr content in the filler should be high in order to avoid the formation of AlN, cf. FIGS. 3 and 4.