METHOD FOR JOINING METAL PARTS

Abstract

A method for joining a first metal part with a second metal part, the metal parts having a solidus temperature above 1100° C., includes applying a melting depressant composition on a surface of the first metal part, the melting depressant composition including a melting depressant component that includes at least 25 wt % boron and silicon for decreasing a melting temperature of the first metal part; bringing the second metal part into contact with the melting depressant composition at a contact point on said surface; heating the first and second metal parts to a temperature above 1100° C.; and allowing a melted metal layer of the first metal component to solidify, such that a joint is obtained at the contact point. The boron at least partly originates from a boron compound selected from any of the following compounds: boric acid, borax, titanium diboride and boron nitride. The melting depressant composition and related products are also described.

Claims

1. A method for joining a first metal part with a second metal part, the metal parts having a solidus temperature above 1100° C., the method comprising applying a melting depressant composition at least on a surface of the first metal part, the melting depressant composition comprising a melting depressant component comprising at least 25 wt % boron and silicon in total for decreasing a melting temperature of the first metal part, and optionally, a binder component for facilitating the applying of the melting depressant composition on the surface, bringing the second metal part into contact with the melting depressant composition at a contact point on said surface, heating the first and second metal parts to a temperature above 1100° C., said surface of the first metal part thereby melting such that a surface layer of the first metal part melts and, together with the melting depressant component, forms a molten metal layer that is in contact with the second metal part at the contact point, and allowing the molten metal layer to solidify, such that a joint is obtained at the contact point, wherein the boron at least partly originates from a boron compound selected from any of the following compounds: boric acid, borax, titanium diboride, boron nitride and/or combinations thereof.

2. The method according to claim 1, wherein the boron originates from any of the boric acid, borax, titanium diboride, boron nitride and/or combinations thereof in an amount 15-100 wt %, or 50 to 100 wt %, based on the total weight of the boron.

3. The method according to claim 1, wherein the silicon originates from any of elemental silicon and silicon of a silicon compound selected from at least any of the following compounds: silicon carbide, silicon boride and ferrosilicon.

4. The method according to claim 1, wherein the melting depressant component comprises at least 40 wt % boron and silicon.

5. The method according to claim 1, wherein the melting depressant component comprises at least 85 wt % boron and silicon.

6. The method according to any one of the preceding claims claim 1, wherein boron constitutes at least 10 wt % of the boron and silicon content of the melting depressant compound.

7. The method according to claim 1, wherein silicon constitutes at least 55 wt % of the boron and silicon content of the melting depressant compound.

8. The method according to claim 1, wherein the melting depressant component comprises less than 50 wt % metallic elements.

9. The method according to claim 1, wherein the melting depressant component comprises less than 10 wt % metallic elements.

10. The method according to claim 1, wherein the first metal part comprises a thickness of 0.1-1.0, or 0.6 to 1.0, or 0.3 to 0.6 mm and the applying of the melting depressant composition comprises applying an average of 0.02-0.12 mg boron and silicon per mm.sup.2 on the surface of the first metal part.

11. The method according to claim 1, wherein the surface has an area that is larger than an area defined by the contact point on said surface, such that metal in the melted metal layer flows to the contact point when allowing the joint to form.

12. The method according to claim 1, wherein the first metal part comprises >50 wt % Fe, <13 wt % Cr, <1 wt % Mo, <1 wt % Ni and <3 wt % Mn.

13. The method according to claim 1, wherein the first metal part comprises >10 wt % Cr and >60 wt % Ni.

14. A product comprising a first metal part that is joined with a second metal part according to the method of claim 1.

15. A melting depressant composition for joining a first metal part with a second metal part according to the method of claim 1, the melting depressant composition comprising i) a melting depressant component that comprises at least 25 wt % boron and silicon for decreasing a melting temperature, and ii), optionally, a binder component for facilitating applying of the melting depressant composition on the first metal part, wherein the boron originates from a boron compound selected from any of the following compounds: boric acid, borax, titanium diboride and boron nitride.

16. The method according to claim 2, wherein the silicon originates from any of elemental silicon and silicon of a silicon compound selected from at least any of the following compounds: silicon carbide, silicon boride and ferrosilicon.

17. The method according to claim 2, wherein the melting depressant component comprises at least 40 wt % boron and silicon.

18. The method according to claim 3, wherein the melting depressant component comprises at least 40 wt % boron and silicon.

19. The method according to claim 2, wherein the melting depressant component comprises at least 85 wt % boron and silicon.

20. The method according to claim 3, wherein the melting depressant component comprises at least 85 wt % boron and silicon.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0054] Embodiments of the present disclosure will now be described, by way of example, with reference to the accompanying schematic drawings, in which

[0055] FIG. 1 is a cross-sectional view of a first and a second metal part where a melting depressant composition is applied intermediate the parts,

[0056] FIG. 2 shows the metal parts of FIG. 1 during heating,

[0057] FIG. 3 shows the metal parts of FIG. 1 when a joint is formed,

[0058] FIG. 4 is a cross-sectional view of a first and a second metal part where a melting depressant composition is applied intermediate the components and when the second metal part abuts the first metal part,

[0059] FIG. 5 shows the metal parts of FIG. 4 during heating,

[0060] FIG. 6 shows the metal parts of FIG. 4 when a joint is formed,

[0061] FIG. 7 shows metal parts when a joint is formed and where the parts have been pressed towards each other during the forming of the joint,

[0062] FIG. 8 is a view corresponding to FIG. 7, where material from both metal parts have melted and formed the joint,

[0063] FIG. 9, corresponds to FIG. 1 and shows distribution of a contact point between the metal parts,

[0064] FIG. 10 shows an area of the contact point between the metal parts,

[0065] FIG. 11, corresponds to FIG. 3 and shows distribution of a joint between the metal parts,

[0066] FIG. 12 shows a cross-sectional area of the joint,

[0067] FIG. 13 shows metal parts used in the examples, and

[0068] FIG. 14 is a flow chart of a method for joining a first and second metal part.

DETAILED DESCRIPTION

[0069] FIG. 1 shows a first metal part 11 and a second metal part 12 where a melting depressant composition 14 as described above is arranged on a surface 15 of the first metal part 11. The second metal part 12 is, at a contact point 16, in contact with the melting depressant composition 14 on the surface 15. For the illustrated second metal part 12, which has a U-shape, a protrusion 28 is in contact with the melting depressant composition 14 at contact point 16. The second metal part 12 may comprise further protrusions and thus there may be correspondingly further contact points. The first metal part 11 may be made of a metallic element made of an alloy comprising elements such as iron (Fe), chromium (Cr), nickel (Ni), molybdenum (Mo), manganese (Mn), and may be for example an iron-based alloy. Examples of suitable metallic elements the first metal part 11 may be made of are for example alloys Nickel 200/201, Nicrofer 5923hMo, Hastelloy® C-2000 A1loy, hastelloy B3, A1loy C22, Iconel 625, A1loy C 276, Nicrofer 3033, Nicrofer 3127HMo, AL6XN, 254SMO, Monel 400, Mild Steel, Stainless steel type 316, Stainless Steel Type 304, but the alloys are not limited to these alloys. The second metal part 12 is also made of a metallic element, which may be the same metallic element as the first metal part 11 is made of. In FIG. 1 the first metal part 11 and the second metal part 12 are not yet joined.

[0070] Four planes P1-P4 are used for describing how the first metal part 11 and the second metal part 12 are joined. The first plane P1 defines the surface of the melting depressant composition 14. The second plane P2 defines the surface 15 of the first metal part 11, which is an “upper” surface 15 of the first metal part 11. This means that the melting depressant composition 14 has a thickness that corresponds to the distance between the first plane P1 and the second plane P2 (the surface 15). It should be noted that the thickness of the melting depressant composition 14 is greatly exaggerated in the illustrated figures. The real thickness, i.e. the amount of the melting depressant composition 14 on the surface 15 as well as the composition of the melting depressant composition 14, is discussed in detail below.

[0071] The third plane P3 defines a surface layer 21 of the first metal part 11, where the surface layer 21 extends from the surface 15 and to the third plane P3 which is located in the first metal part 11. Thus, the thickness of the surface layer 21 corresponds to the distance between the second plane P2 (the surface 15) and the third plane P3. The fourth plane P4 defines a lower surface of the first metal part 11.

[0072] The thickness of the first metal part 11 corresponds to the distance between the second plane P2 and fourth plane P4. The first metal part 11 has also a lower layer 22, which is the part of the first metal part 11 that does not include the surface layer 21 and which extends from the third plane P3 to the fourth plane P4. The illustrated shapes of the first metal part 11 and the second metal part 12 are just exemplifying shapes and other shapes are equally conceivable. For example, the metal parts 11, 12 may have curved shapes, such that the planes P1-P4 do not have the form of flat, two-dimensional surfaces, but instead the form of curved surfaces.

[0073] FIG. 2 shows the metal components 11, 12 when they are heated to a temperature above which the melting depressant composition 14 causes the surface layer 21 to melt and form a molten metal layer 210, but at a temperature that is below a melting temperature of the material in the first metal part 11 and in the second metal part 12. In brief, when heating the metal parts 11, 12, boron and silicon in the melting depressant composition 14 diffuses into the first metal part 11 and causes it to melt at a temperature that is lower than the melting temperature of the material in the first metal part 11 (and of the second metal part 12). The melting depressant composition 14 is applied on the surface 15 at an amount that causes the surface layer 21 to melt and form the molten metal layer 210. Thus, the amount of melting depressant composition 14 is chosen so that boron and silicon diffuses only into the surface layer 21 (too much boron and silicon may melt the entire first metal part 11). Suitable amounts of the melting depressant composition 14 are described in the examples below. Metal in the molten metal layer 210 then flows, typically by capillary action, towards the contact point 16 (and to other, similar contact points such as contact point 116).

[0074] FIG. 3 shows the metal components 11, 12 when all melting depressant composition 14 have diffused into the first metal part 11 and when metal in the molten metal layer 210 has flown towards the contact point 16 where a joint 25 now is formed. The joint now comprises metal that previously was part of the first metal part 11. As may be seen, the melting depressant composition 14 is no longer present on the surface 15 of the first metal part 11 since it has diffused into the first metal part 11 and, typically, to some extent into the second metal part 12. Since the joint 25 is formed from metal from the first metal part 11 the first metal part 11 is now thinner than before the heating. As may be seen, the first metal part 11 now has an upper surface 15′ that is not located at the second plane P2. Instead, the upper surface is now closer to the fourth plane P4. Generally, not all metal in the melted metal layer 210 flows towards the contact point 16 to form the joint 25, but some remains as an upper surface of the first metal part 11 and solidifies there simultaneously with the solidification of the joint 25. The solidification takes place when the temperature is decreased but also prior a decrease of the temperature, e.g. because the boron and silicon in the melting depressant composition gradually diffuse into and mix with the material of the first metal part 11. The physical process behind the melting of the metal in the first metal part 11 as well as the subsequent solidification is similar with the melting and solidification process that occur during brazing. However, compared to conventional brazing there is a great difference in that the melting depressant composition 14 comprises no or very small amounts of filler metal; instead of using a filler metal for creating the joint 25, metal from the first metal part 11 and, optionally as will be described, from the second metal part 12, is used for creating the joint 25. Additionally, the present boron sources which can be selected from boric acid, borax, titanium diboride and boron nitride are less harmful from a work-safety point of view than for example elemental boron. A1so, the boron sources of the present disclosure do not negatively affect corrosion properties of the metal parts as much as boron sources containing carbon (C), whereby strong joints with good corrosion properties can be provided.

[0075] FIGS. 4-6 corresponds to FIGS. 1-3 with the difference that the second metal part 12 is pressed into the melting depressant composition 14 to such an extent that it is basically in contact with or abuts to the first metal part 11 (some small amounts of the melting depressant composition 14 is still typically present between the metal parts 11, 12).

[0076] FIG. 7 corresponds to FIGS. 3 and 6 with the difference that the first metal part 11 and the second metal part 12 has been pressed towards each other during the forming of the joint 25. As a result the second metal part 12 has at the location of the joint 25 “sunk” into the melted metal layer 210 of the first metal part 11.

[0077] FIG. 8 corresponds to FIG. 7, where material from both the first metal part 11 and the second metal part 12 have melted and formed the joint 25. In practice, this is typically what happens during the forming the joint 25, especially if the first metal part 11 and the second metal part 12 are made of the same material, since the second metal part 12 also is in contact with the melting depressant composition.

[0078] Before heating, the second metal part 12 has an outer contour defined by the dotted line L2. During heating, a surface layer of the second metal part 12 forms a molten surface layer 26, where the metal of this layer flows to the contact point 16 and forms part of a joint 25 there. The molten surface layer of the second metal part 12 is represented by the layer between line L2 and line L1, where line L1 defines a boundary where the metal of the second metal part 12 has not been melted.

[0079] It should be noted that there is no real sharp boundaries between metal of the first metal part 11 and the second metal part 12 that is melted respectively is not melted. Instead, there is a gradual transition from “melted” to “not melted”.

[0080] FIG. 9 corresponds to joint forming situation of FIG. 1 and shows a distribution of the contact point 16 between the first metal part 11 and the second metal part 12. FIG. 10 shows the same metal parts 11, 12 but from above and in the first plane P1. FIG. 9 is a cross-sectional view as seen along line A-A in FIG. 10.

[0081] As may be seen, the contact point 16 has a distribution over the melting depressant composition 14 on the first metal part 11. The distribution of the contact point 16 has an area A2 that is significantly smaller than an area A1 of the melting depressant composition 14 on the surface 15. The area A1 comprises the area A2. The area A1 extends between two lines L3, L4 that are located at a respective side of the contact point 16. The area A1 of the surface 15 on which the melting depressant composition 14 is applied can be at least 10 times larger than the area A2 defined by the contact point 16. The area A1 may be defined as an area of the surface 15 on which melting depressant composition 14 is applied and from which area A1 metal is drawn to the form the joint 25. The area A2 may be defined as the area of the contact point 16, i.e. the area of contact between the melting depressant composition 14 and the second metal part 12, optionally including an area of contact (if any) between the first metal part 11 and the second metal part 12 at the contact point 16. The area A1 can be at least 10 times larger than the area A2.

[0082] FIG. 11 corresponds to FIG. 3 and shows a cross-sectional area A3 of the joint 25. The area A1 of the surface 15 on which the melting depressant composition 14 is applied may be according to another variant be at least 3 times larger than the cross-sectional area A3 of the joint 25. FIG. 12 shows the same metal parts 11, 12 but from above and in the second plane P2. FIG. 11 is a cross-sectional view as seen along line A-A in FIG. 12.

[0083] As may be seen, the joint 25 has a cross-sectional area A3 that is significantly smaller than the area A1 of the melting depressant composition 14 on the surface 15. As before, the area A1 may be defined as an area of the surface 15 on which melting depressant composition 14 is applied and from which area A1 metal is drawn to form the joint 25. The cross-sectional area A3 of the joint 25 may be defined as the smallest area the joint 25 has between the first metal part 11 and the second metal part 12. The cross-sectional area A3 may have the shape of a curved surface. Obviously, the areas A1 and A2 may have the shape of curved surfaces, depending on the respective shape of the first metal part 11 and the second metal part 12.

[0084] FIG. 13 shows a first metal part in the form of a plate 110 and a second metal part 120 which is a plate with the same outer dimensions, 20*40 mm, but was pressed to a shape of a U. The metal parts are used for exemplifying how two metal parts may be joined. The plate 110 is a rectangular plate, with a thickness of 0.4 mm and the dimensions of 20*40 mm and is made of stainless steel type 316L (SAE steel grade). With reference to FIG. 14 a flow chart of a method for joining a first and second metal part is illustrated. The metal parts may be made of any of the materials as described above.

[0085] In a first step 201 the melting depressant composition is applied on the surface of at least one of the metal parts (here the first metal part). The application per se may be done by conventional techniques, e.g. by spraying or painting in case the melting depressant composition comprises a binder component, and by PVD or CVD in case not binder component is used.

[0086] A next step 202 the second metal part is brought into contact with the melting depressant composition at a contact point on the surface. This can be done manually or automatically by employing conventional, automated manufacturing systems.

[0087] In a next step 203 the metal parts are heated to a temperature which is above 1100° C. The exact temperature can be found the examples above. During the heating a surface of at least the first metal part melt and, together with the melting depressant component, forms a melted metal layer that is in contact with the second metal part at the contact point between the first metal part and the second metal part. When this happen, metal of the melted metal layer flows towards the contact point.

[0088] A final step 204 the melted metal layer is allowed to solidify, such that a joint is obtained at the contact point, i.e. the metal that has flown to the contact point solidifies. The solidification typically includes decreasing temperature to normal room temperature. However, solidification also occurs during the physical process of redistribution of components (boron and silicon) in the joint area, before a temperature is decreased.

[0089] A number of experiments and examples are now presented for describing suitable materials for the boron source of the melting depressant composition.

EXAMPLES

[0090] In the following examples more details are presented for illustrating the invention.

[0091] The tests in these examples were made to investigate if silicon, Si, was able to create a “braze alloy” when the silicon was applied on the surface of a test sample of parent metal (i.e. on a metal part). A1so, different amounts of boron, B, were added for decreasing the melting point for the braze alloy. Boron is also used for changing the wetting behavior of the braze alloy. Properties of the tested blends were also investigated. In the examples wt % is percent by weight. Here, “braze alloy” is referred to as the alloy formed when the silicon and boron causes a part of, or layer of, the parent metal (metal part), to melt. The “braze alloy” thus comprises the blend and metallic elements from the parent metal.

[0092] If nothing else is stated the test samples of parent metal for all tests were cleaned by dish washing and with acetone before samples of the blends of silicon and boron were added to the test samples.

Test of Boron Sources.

[0093] Four commercially available new boron sources were tested, TiB.sub.2, Na.sub.2B.sub.4O.sub.7, H.sub.3BO.sub.3 and BN (the hexagonal form (h-BN)). SiB.sub.6 was used as reference.

[0094] The boron sources were blended with Si to a ratio of app. 2:10, B: Si.

[0095] By using the molecule weight of all components in each boron source the amount of source was calculated so that the ratio was obtained. The factor presented in the table is the quota between the molecule weight of the boron source and the weight of the boron in the source and it was used when calculating the needed amount boron source for the blends. The calculations are presented in table 1.

TABLE-US-00001 TABLE 1 Calculations: Molecule weight Weight of B % B in the of boron source in the source boron source. g/mol B % B factor Na.sub.2B.sub.4O.sub.7 201.22 43.24 21.5 4.653 TiB.sub.2 69.49 21.62 31.1 3.214 H.sub.3BO.sub.3 61.83 10.81 17.5 5.719 BN 24.82 10.81 43.6 2.296 SiB.sub.6 92.95 64.87 69.8 1.433

[0096] Blending of Powders

[0097] All boron sources used were powders. The used Si source was also a powder. After weighing the powders for each blend, the powders were firmly blended. The binder was then added with the weight and the blend was firmly blended again. The weights for the components are presented in table 2.

TABLE-US-00002 TABLE 2 measured weights of ingoing component: B-source Si tot Total Added (g) (g) weight (g) binder (g) Na.sub.2B.sub.4O.sub.7 9.3 10 19.3 15.14 TiB.sub.2 2.64 4.11 6.75 5.3 H.sub.3BO.sub.3 11.4 10.02 21.42 16.96 BN 4.60 10.01 14.61 12.92 SiB.sub.6 2.82 9.99 12.81 13.01

[0098] Applying Method and Sample Preparation

[0099] To obtain an even applying, a small hand screen printing equipment were used. The blends were screen printed on a plate sample of made of type 316L, with a thickness of 0.4 and the dimensions of 20*40 mm. The screen-printed area was 19*10 mm. The weight of all samples was measured before and after screen printing. The applied weight for braze cycle 1 (A), is presented in table 3 and for braze cycle 2 (B), in table 4. For the joining a second part were used. The second part was a plate with the same outer dimensions, 20*40 mm, but was pressed to a shape of a U. The samples were placed with the screen-printed area facing the pressed stainless-steel plate, so that a 2-dimensional joint was created between the screen-printed area and the pressed plate. The samples were placed in a fixture to ensure contact between the parts when brazed.

TABLE-US-00003 TABLE 3 Samples for the first braze, braze cycle (A) W (mg)/ Sample B Plate Applied Applied Area # source (g) Plate, (g) amount (g) (10*19 mm2) 1A BN 2.4953 2.5036 0.0083 0.044 2A BN 2.4941 2.5051 0.011 0.058 3A H.sub.3BO.sub.3 2.4918 2.4961 0.0043 0.023 4A H.sub.3BO.sub.3 2.4886 2.4928 0.0042 0.022 5A Na.sub.2B.sub.4O.sub.7 2.4883 2.4962 0.0079 0.042 6A Na2B4O7 2.4875 2.4977 0.0102 0.054 7A TiB.sub.2 2.4895 2.498 0.0085 0.045 zz8A  TiB.sub.2 2.4884 2.5026 0.0142 0.075 9A SiB.sub.6 2.4897 2.501 0.0113 0.059 10A  SiB.sub.6 2.4872 2.4993 0.0121 0.064

TABLE-US-00004 TABLE 4 Samples for the second braze, braze cycle (B) W (mg)/ Sample plate Applied Applied Area # B-source (g) Plate, (g) amount (g) (10*19 mm2) 1B SiB.sub.6 2.4822 2.4899 0.0077 0.041 2B SiB.sub.6 2.4848 2.4927 0.0079 0.042 3B SiB.sub.6 2.4817 2.4902 0.0085 0.045 4B SiB.sub.6 2.4856 2.4957 0.0101 0.053 5B SiB.sub.6 2.4959 2.5068 0.0109 0.057 6B TiB.sub.2 2.4962 2.5113 0.0151 0.079 7B TiB.sub.2 2.4937 2.504 0.0103 0.054 8B TiB.sub.2 2.4926 2.5048 0.0122 0.064 9B TiB.sub.2 2.4927 2.5042 0.0115 0.061 10B  TiB.sub.2 2.4887 2.4998 0.0111 0.058 11B  TiB.sub.2 2.4854 2.4973 0.0119 0.063 12B  H.sub.3BO.sub.3 2.4913 2.4957 0.0044 0.023 13B  H.sub.3BO.sub.3 2.494 2.499 0.005 0.026 14B  H.sub.3BO.sub.3 2.4974 2.5018 0.0044 0.023 15B  H.sub.3BO.sub.3 2.5004 2.5053 0.0049 0.026 16B  H.sub.3BO.sub.3 2.5002 2.5041 0.0039 0.021

[0100] Brazing

[0101] The brazing was performed in a vacuum furnace. The brazing temperature was 1225±5° C., for approximately 1 hour at the brazing temperature. Two brazing cycles were made, cycle 1 (A) and cycle 2 (B).

[0102] Results

[0103] The samples were analyzed by visual inspection and the results are presented in table 5 and 6.

TABLE-US-00005 TABLE 5 the results from braze cycle 1, (A). Sample B # source Results 1A BN Bright surface, braze joint 2A BN Bright surface, braze joint 3A H.sub.3BO.sub.3 Bright surface, melted started, no or smal joint 4A H.sub.3BO.sub.3 Bright surface, melted started, no or smal joint 5A Na.sub.2B.sub.4O.sub.7 Bright surface, melted started, no or smal joint 6A Na.sub.2B.sub.4O.sub.7 Bright surface, melted started, no or smal joint 7A TiB.sub.2 Dark surface, braze joint 8A TiB.sub.2 Dark surface, braze joint 9A SiB.sub.6 Bright surface, braze joint 10A  SiB.sub.6 Bright surface, braze joint

TABLE-US-00006 TABLE 6 the results from braze cycle 2, (B). Sample B # source Results 1B SiB.sub.6 Bright surface, braze joint 2B SiB.sub.6 Bright surface, braze joint 3B SiB.sub.6 Bright surface, braze joint 4B SiB.sub.6 Bright surface, braze joint 5B SiB.sub.6 Bright surface, braze joint 6B TiB.sub.2 Dark surface, braze joint 7B TiB.sub.2 Dark surface, braze joint 8B TiB.sub.2 Dark surface, braze joint 9B TiB.sub.2 Dark surface, braze joint 10B  TiB.sub.2 Dark surface, braze joint 11B  TiB.sub.2 Dark surface, braze joint 12B  H.sub.3BO.sub.3 Bright surface, melted started, no joint 13B  H.sub.3BO.sub.3 Bright surface, melted started, no joint 14B  H.sub.3BO.sub.3 Bright surface, melted started, small braze joint 15B  H.sub.3BO.sub.3 Bright surface, melted started, small braze joint 16B  H.sub.3BO.sub.3 Bright surface, melted started, small braze joint

CONCLUSIONS

[0104] The tests showed that braze joints were obtained when using BN, TiB.sub.2 and SiB.sub.6 as boron sources and can therefore be used as B sources. The test also showed that H.sub.3BO.sub.3 and Na.sub.2B.sub.4O.sub.7 had the potential to be used as alternative boron sources, but the effect was not as high as for the other B sources tested. This can probably be solved by inversing the amount of applied blend.

[0105] From the description above follows that, although various embodiments of the invention have been described and shown, the invention is not restricted thereto, but may also be embodied in other ways within the scope of the subject-matter defined in the following claims. Various melting depressant compositions can also be combined with various metals for the metal parts.