Filler composition for high yield strength base metals

Abstract

A weld wire of the present invention comprises a steel sheath encapsulating a fluxed core having a combination of fluxing compounds and alloying elements. The fluxing compounds comprise up to 2% Wt of fluoride compounds and up to 49% Wt of oxide compounds. The alloying elements comprise Mn, Ni, Co, Ti and up to about 0.98% Wt of C. The amount of Co is sufficient to produce a ferrite-bainite weld metal morphology of a resulting weld. A yield strength of the resulting weld was measured from about 95 ksi to about 111 ksi.

Claims

1. A weld wire for use in a flux-cored arc welding (FCAW) process of welding high strength steel, comprising: a fluxed core including, by weight: alloying elements comprising (i) about: 11-13% of Ni and 11-16% of Fe, and (ii) up to about: 1% of C, 4.98% of Co, 0.1% of B, and 0.25% of Zr; and fluxing compounds comprising: (i) about 0.5-2% of metal fluorides and (ii) about 20-49% of oxides; and a steel sheath encapsulating the fluxed core, wherein an amount of Co in the fluxed core of the weld wire is used in the FCAW process of welding the high strength steel for controlling a fine-grained morphology of acicular ferrite-bainite in a produced weld metal.

2. The weld wire of claim 1, wherein the alloying elements further comprise about 12-20% of Mn and up to about: 1% of Ti, 2% of Mg, and 3.0% of Si.

3. The weld wire of claim 1, wherein the steel sheath comprises, by weight, up to about: 0.02% of C and 0.5% of Mo.

4. The weld wire of claim 1, wherein the alloying elements comprise 11.18% of Ni.

5. The weld wire of claim 1, wherein the alloying elements comprise 11.1-11.9% of Fe.

6. The weld wire of claim 1, wherein the alloying elements comprise 0.98% of C.

7. The weld wire of claim 1, wherein the alloying elements comprise 4.98% of Co.

8. The weld wire of claim 1, wherein the alloying elements comprise 0.06% of B.

9. The weld wire of claim 1, wherein the alloying elements comprise 0.00% of B.

10. The weld wire of claim 1, wherein the alloying elements comprise 0.23% of Zr.

11. The weld wire of claim 1, wherein the fluxing compounds comprise about 1.9-2.0% of metal fluorides.

12. The weld wire of claim 11, wherein the fluxing compounds comprise 1.96-1.98% of metal fluorides.

13. The weld wire of claim 1, wherein the fluxing compounds comprise about 45-49% of oxides.

14. The weld wire of claim 13, wherein the fluxing compounds comprise 47.4-47.9% of oxides.

Description

BRIEF DESCRIPTION OF THE DRAWING FIGURES

(1) FIG. 1 is a cross-sectional view of the fluxed-core wire of the present invention.

(2) FIG. 2 is a schematic illustration of the FCAW process with the fluxed-core welding wire of the present invention.

(3) FIG. 3a is a table (Table 1) providing percentages of components and alloying elements in the fluxed core.

(4) FIG. 3b is a table (Table 2) of weld metal compositions.

(5) FIG. 3c is a table (Table 3) which shows actual weights of the fluxing components in the fluxed core.

(6) FIGS. 4(a)-(d) are the resulting microstructures of various weld metal samples.

(7) FIG. 5 provides data of a physical test of an exemplary fluxed-core wire.

DETAILED DESCRIPTION OF THE INVENTION

(8) The fluxed core wire of the present invention comprises a metal sheath and a fluxed core composition with the preferred fill of about 16% by weight. Other fill percentages are possible if preferred by particular applications of different metal bases. The fluxed is characterized by a composition which results in a fine-grained morphology of the acicular ferrite+bainite in the produced weld metal. The detailed chemical and elemental composition of the fluxed core of the wire of the present invention is provided in Table 1 shown in FIG. 3a.

(9) The primary function of Si in the present fluxed core composition is to deoxidize the weld pool during welding. If oxygen is present in the welding process and remains in the resulting weld metal, it will cause porosity in the weld metal, which would be an undesirable result. Si also plays a role in substitution strengthening and increased hardeanability of the produced weld. The present invention limits the concentration of Si in the weld metal composition to about 0.3% to minimize the amount of inclusion growth and its adverse effect on the weld microstructure. The primary role of Mn is to influence substitution strengthening and increased hardeanability of the weld microstructure. It also removes S from the weld pool (S forms undesirable low melting point inclusions on the grain boundaries of the weld metal). The primary role of C, the main alloying agent of the present invention, is to perform interstitial strengthening. The primary role of Ni is to increase toughness of the resulting weld metal. As the amount of Ni in an alloy increases, the lower shelf impact energy is raised while the upper shelf impact energy is lowered.

(10) The main function of Ti and Zr in the present fluxed core composition is to control the size and distribution of oxide inclusions, since both elements are strong oxide and nitride formers. Formation of smaller oxide inclusions in the weld microstructure is important for the heterogeneous nucleation of acicular ferrite, which requires less energy. Lower energy produces acicular ferrite grains with smaller grain size in the weld metal. Oxide inclusions also minimize the grain growth at elevated temperatures by providing a drag force to the grain boundaries, slowing their growth and reducing the grain size in the solid state. The resulting small grain size of the acicular ferrite increases the surface area of the formed grain boundaries, which causes the impurities present in the weld metal to be less concentrated and, therefore, less detrimental to impact toughness of the weld. The smaller grain size and the larger number of grains in the weld metal also increases toughness, because the large number of smaller grains impedes the concentration and growth of the cracks as well as formation and travel of dislocations inside the weld.

(11) Use of alloying element Co in the fluxed core of the inventive wire is particularly important for controlling the morphology of the weld metal produced by the FCAW process. The present invention contemplates the amounts of Co in the core to be up to 4.98% Wt. In particular, the fluxed core combination of the wire comprises a combination of Ni and Co. The resulting compositions of the welds (% by weight) produced in the experimental welding runs are presented in Table 2 shown in FIG. 3b The best formulation turned out to be number 17-020 which resulted in weld joints of good yield and acceptable impact strength (in the [ksi] units). The actual weights of the fluxing compounds and elements are provided in Table 3 of FIG. 3c. The total weight of the fluxing mix is about 5 lbs for all the formulas (from 17-019 to 17-024).

(12) The process of manufacturing a fluxed core wire of the present invention involves a series of steps in which a strip (or a sheath material) is fed through the shaping dies which bend the strip and form it into a shape that can later be filled with the ingredients of the fluxed-core composition. Usually, the shape is a U-shape. The shaped sheath is then filled with the fluxed-core composition which has a combination of fluxing compounds and alloying elements. The fluxing compounds comprise up to 2% Wt of fluoride compounds and up to 49% Wt of oxide compounds. The alloying elements comprise Mn, Ni, Co, Ti and up to about 0.98% Wt of C. The wire then travels through the closing dies which close it into a tubular form in which the sheath 30 completely encapsulates the core 32, forming a fluxed-core wire as illustrated in FIG. 1. The ingredient of the fluxed-core composition are often powdered, which is compacted when the encapsulated wire is fed through the drawing dies to reduce the wire's diameter to the final desired size.

(13) Various fluxing components listed in Table 1 (shown in FIG. 3a) are added to produce the welds characterized by the minimum yield strength of 100 ksi for both high and low heat input welding, as well as the minimum Charpy impact of 30 ft-lbs at 40 C. The fluxed core wire of the present invention makes it possible to produce the welds at welding interpass temperatures as low as 200 F (93 C) while minimizing the risk of hot or cold cracking. (Interpass temperature is the temperature maintained during welding, until completion of the weld joint. Minimum and maximum interpass temperatures are typically the same as the minimum and maximum preheat temperatures). In particular, the fluxing compounds react with hydrogen before it enters into the molten weld pool, reducing the concentration of hydrogen pockets in the weld metal and, therefore, the possibility of cold cracking. The resulting weld metal has a low concentration of diffusable hydrogen, typically less than 4 ml/100 g.

(14) A welding apparatus for FCAW utilizing the wire of present invention is shown as an illustrative example in FIG. 2. The welding apparatus comprises a direct current power supply 50, a welding gun 10 with an electrode 14 and means for feeding the electrode into the welding gun. An example of the means for feeding the electrode shown in FIG. 2 is a wire drive 20 and a wire reel 22. It should be understood, of course, that any other way of feeding the wire electrode into the welding gun falls within the scope and spirit of the present invention. A shielding gas 16 is supplied to the welding process through gas nozzle 12 in the welding gun. Electrode 14 has a sheath and a core having a fluxed core composition comprising fluxing compounds and alloying elements in percentages as shown in Tables 1 and 2, shown, respectively, in FIGS. 3a and 3b. For the FCAW process in which the welding gun is coupled to a direct current power supply, the preferred shielding gas is a mixture of Ar and CO.sub.2 mixed in the 75% Ar/25% CO.sub.2 or 90% Ar/10% CO.sub.2 or 95% Ar/5% CO.sub.2 proportions. It is also possible to use 100% of CO.sub.2 as a shielding gas, as well as 95% Ar/up to 5% of O.sub.2 to stabilize the arc. The arc 18 is formed between the wire electrode Lt. of the present invention and the work pieces (sheets 11 and 13 in FIG. 2) to form a molten weld pool 15. The shielding gas 16 can be supplied to the welding process from an external source 17, as shown in FIG. 2.

(15) To form a weld on a work piece using the welding apparatus with a novel flux-cored wire electrode of the present invention, a welding process uses a welding apparatus with means for feeding the wire electrode and means for supplying a shielding gas into the apparatus. The means for feeding the wire into the welding apparatus can comprise a wire drive and a wire reel, or any other suitable arrangement supplying the wire into the apparatus with the speed sufficient to replace the portion of the wire consumed during the welding process. It is contemplated that the means for feeding the wire into the welding apparatus can be internal or be located outside of the apparatus. The welding apparatus is coupled to a direct current power supply and the arc is formed between the electrode and the work piece on which the weld is to be formed. Supplying the shielding gas into the welding process can be done from an external gas supply feeding the gas into a gas nozzle of the welding apparatus.

(16) Feeding the wire electrode of the present invention into the welding apparatus involves providing the wire with a sheath and a fluxed core having a core composition as provided in Table 1 (FIG. 3a) and Table 32 (FIG. 3b). The preferred mixture of shielding gas is a mixture of Ar and CO.sub.2 mixed in the 75% Ar/25% CO.sub.2 or 90% Ar/10% CO.sub.2 or 95% Ar/5% CO.sub.2 proportions. It is also possible to use 100% of CO.sub.2 as a shielding gas, as well as 95% Ar/up to 5% of O.sub.2 to stabilize the arc.

(17) The above-described welding process is preferably used in the direct current FCAW process. The work piece used in the FCAW process comprised two base steel plates. The steel plates used for the experimental runs were of the type HY-100 and HY-80, 2.5 cm thick. The composition of the steel plates in weight percentages is provided in Table 4.

(18) TABLE-US-00001 TABLE 4 Base metal compositions. Element (% Wt) HY-100 HY-80 C Up to 0.193 Up to 0.149 Mn Up to 0.322 Up to 0.312 P Up to 0.001 Up to 0.005 S Up to 0.002 Up to 0.006 Si Up to 0.207 Up to 0.202 Cu Up to 0.109 Up to 0.188 Cr Up to 1.236 Up to 1.103 V Up to 0.005 Up to 0.004 Ni Up to 2.249 Up to 2.149 Mo Up to 0.251 Up to 0.248 Al Up to 0.016 Up to 0.015 Ti Up to 0.001 Up to 0.001 Zr Up to 0.001 Up to 0.001 Nb Up to 0.001 Up to 0.001 Co Up to 0.008 Up to 0.007 B Up to 0.0001 Up to 0.0001 W Up to 0.001 Up to 0.001 Sn Up to 0.005 Up to 0.006 Pb Up to 0.0024 Up to 0.0022

(19) Turning now to FIG. 4, the representative morphology of a weld (a cross-section of a tensile specimen coded P22549 at different resolutions) is shown there in the photos of microstructures 4(a) 4(d) of a specimen that was weld using electrode formula 17-005 (Table 2, shown in FIG. 3b). Such morphology is typical for the fluxed core wires listed in Table 2 shown in FIG. 3b. A tensile plate was welded using a 1 inch (2.5 cm) thick HY-100 grade steel. 22 welding passes were used to join the plate. The welding parameters used to complete the weld are as follows from Table 4:

(20) TABLE-US-00002 TABLE 4 Welding Parameters used to weld P22549 Voltage 28 Amperage 250 Wire Feed Speed 480 Root opening of plate inch Included angle of plate 45 degrees Preheat before welding 200 F. Interpass temperature 225 F. Backup bar thickness inch
The flux composition of the fluxed-core wire of formula 17-005 is as follows from Table 5:

(21) TABLE-US-00003 TABLE 5 Flux composition by weight % of formula 17-005 C 0.60 Si 2.60 Mg 2.00 Ti 0.79 Mn 11.78 B 0.09 CO 9.99 Ni 5.99 Fe 12.91 Flourides 0.61 Oxides 52.65 Total 100.00

(22) In the microstructures shown in FIGS. 4(a)-4(d) the white areas correspond to the ferrite and upper bainite phases, the dark etching areas show lower bainite with inclusions of carbides dispersed through the matrix as black spots. No significant presence of martensite seemed to be present in the microstructure. The results of physical tests of the same specimen are shown in FIG. 5.