WO2000075391A1

WO2000075391A1 - A welding electrode, a welded article, and a steel weldable with the welding electrode

Info

Publication number: WO2000075391A1
Application number: PCT/SE2000/001118
Authority: WO
Inventors: Björn Holmberg
Original assignee: Avesta Sheffield Aktiebolag (Publ)
Priority date: 1999-06-07
Filing date: 2000-05-30
Publication date: 2000-12-14
Also published as: SE517771C2; SE9902100D0; SE9902100L; AU5434800A

Abstract

The invention concerns a welding electrode consisting of a core and a coating. The core wire consists of an austenitic stainless steel, which contains in weight %: 0.001-0.03 C, 0.01-0.3 Si, 0.5-2.5 Mn, 19-21 Cr, 9-12 Ni, 0.01-0.2 N. The coating contains in weight %: 6-10 CaCO3, 2-5 F, 10-30 SiO2, 4-10 Al2O3, 25-35 TiO2, 7-12 Fe, max 5 CrN, max 5 Ni, max 10 Cr, max 5 Mn, balance essentially only sodium- and potassium compounds existing as components and natural minerals; binding agents; and unavoidable impurities. The invention also relates to a welded article of an austenitic stainless steel, a new austenitic stainless steel and a welding wire made of such steel.

Description

A WELDING ELECTRODE, A WELDED ARTICLE, AND A STEEL WELD ABLE WITH THE WELDING ELECTRODE

TECHNICAL FIELD

The invention relates to a welding electrode consisting of a core and a coating. The invention also concerns a welded article of an austenitic stainless steel and an austenitic stainless steel which is weldable with the use of the welding electrode.

BACKGROUND OF THE INVENTION

Coated electrodes for the welding of austenitic stainless steels normally has such a composition in the core and coating that a certain amount of ferrite is obtained in the deposited weld metal during the solidification of the weld, which ferrite dissolves existing impurities which therefore do not cause the formation of heat checks. The ferrite content in the weld metal usually is expressed through the so called ferrite number, FN, which can be expressed graphically in the Schae-ffler de Long diagram for deposited weld metal. Typical ferrite numbers for austenitic weld metals are 5-10 FN. Also 0 FN occurs, but then it is usually the matter of very high alloyed austenitic weld metals.

It is a drawback of deposited weld metals, which contain ferrite, that the weld metal can be embrittled and that cracks may arise, if the welded article is subjected to alternately high and low temperatures.

DISCLOSURE OF THE INVENTION

It is an object of the invention to address the above complex of problems through the provision of a welding wire which has a new composition of the coating, resulting in a desired composition in the deposited weld metal. The invention also concerns a new austenitic stainless steel, which has a composition corresponding to the desired composition which can be obtained in the deposited weld metal, when an austenitic stainless steel is welded by means of the welding electrode according to the invention. The claimed protection for the austenitic steel of the invention, however, is not restricted to the use of the welding electrode of the invention for the welding of the steel. Correspondingly, the use of the welding electrode is not restricted to the steel of the invention but can be used for the welding of a plurality of different austenitic stainless steels. As a matter of fact, the welding electrode has been developed for solving problems in connection with the use of known austenitic steels, which are subjected to alternately high and low temperatures, when the known steels have been welded with today common welding materials, particularly heat and creep resistant austenitic steels, such as ASTM S30815 and 309S, but also other austenitic stainless steels, such as ASTM 304, 321, and 347.

The core in the welding electrode according to the invention may have a composition which is known per se and consists of a wire of an austenitic stainless steel, which contains in weight-% 0.001-0.03 C, 0.01-0.3 Si, 0.5-2.5 Mn, 19-21 Cr, 9-12 Ni, 0.01-0.2 N, balance essentially only iron and unavoidable impurities. Possibly, the core wire also may contain cerium in an amount of 0.005-0J %. The existence of cerium in the core wire is, however, not an absolute condition according to the general aspect of the invention. A more preferred composition of the core wire is given in the patent claims and in the description.

The coating of the welding electrode according to an aspect of the invention shall contain in weight-%: 6-10 CaCO₃, 2-5 F, 10-30 SiO₂, 4-10 Al₂O₃, 25-35 TiO₂, 7-12 Fe, max 5 CrN (chromium alloyed with nitrogen), max 5 Ni, max 10 Cr, max 5 Mn, balance essentially only compounds of sodium- and potassium which are included as components in natural minerals; binding agents and unavoidable impurities. More preferred compositions of the coating are given in the dependent claims and in the description.

At the welding, the core of the welding wire is melted and combined with components in the coating such as to form the metal in the weld. In the weld root also components from the base material, that are melted, will be included, i.e. components of the material which are being welded, but in the subsequently deposited weld beads the addition by dissolution from the base material will be ever lower so that the top bead will be substantially free from components which have been included from the base material through dissolution. This material is defined as pure weld metal. In the compositions which will be presented below always pure weld metal is referred to, which is the result of alloying the welding electrode core with components from the coating but without addition of melted material from the base material.

The accompanying Fig. 1 shows a modified Schaeffler de Long diagram, in which the chromium and nickel equivalents, Cr_eq and Ni_eq, respectively, are defined in the following way: Cr_eq = % C + % Mo + 1.5 x % Si + 0.5 x % Nb Nieq = % Ni + 0.5 x % Mn + 30 x % C + 30 x % N

It has been proved that the deposited weld metal has a very small tendency to embrittlement and therefore a good impact strength in combination with other important features, including a good creep resistance if the alloy elements in the deposited weld metal (the pure weld metal) are balanced such that the values of the chromium and nickel equivalents will lie within the frame of the area A-B-C-D-A in the modified Schaeffler de Long diagram shown in Fig. 1 (the region having said area is shown at a larger scale in Fig. 1A), in which the coordinates of the corner points A, B, C, and D in the said area A-B-C-D-A- are

A 19.1/13.9

B 21.1/16.3 C 21.1/20.5

D 19.1/17.9

The line A-B in the area A-B-C-D-A lies on the line which in the Schaeffler de Long diagram corresponds to the ferrite number FN + 0.5, while the line C-D lies on the line corresponding to the ferrite number FN - 10.

The line A-D is determined by the requirements on resistance against oxidation, while the line B-C indicates the upper limit to the risk of formation of sigma-phase.

Preferably, the amounts of the alloy elements of the deposited weld metal should be balanced such that the chromium and nickel equivalents will be found within the frame of the area A'-B'-C'-D'-A' in the modified Schaeffler de Long diagram, Fig. 1 and Fig. 1 A, in which the coordinates for the corner points A, B', C, and D' are

Cr_eq/Ni_eq

A' 19.1/14.5

B' 20.5/16.0

C 20.5/18.4

D' 19.1/17.1

In the area A'-B'-C'-D'-A, the line A'-B' lies on the line of FN - 0.5, while the line C'-D' lies on the line FN - 6, which means that the risk of formation of ferrite and formation of sigma-phase is very small. As far as the conditions for the lines A'-D' and B'-C are concerned, the same considerations apply as has been mentioned in connection with the area A-B-C-D-A.

The good results which have been achieved with the deposited weld metal according to the invention also should be possible to be reproduced in an austenitic stainless steel, which is manufactured to have a composition corresponding to the composition of the deposited weld material (the pure weld metal) according to the invention. What has been mentioned above concerning the composition of the weld metal and which also is stated in the appending patent claims, and which also will be explained more in detail in the following description of performed experiments, also applies for such a new steel material. That material may have any conceivable shape, such as sheets, strips, bars, tubes, castings, wires, etc, including also welding wires (so called bare wire, also referred to as solid wire) for welding not only the steel of the invention but also for welding other, new or known, austenitic stainless steels.

Further characteristics and aspects of the invention will be apparent from the following description of performed experiments and from the appending patent claims.

BRIEF DESCRIPTION OF DRAWINGS In the drawings Fig. 1 shows a modified Schaeffler de Long diagram, in which the indicated areas represent the composition of the deposited weld metal, and of the steel or the welding wire, respectively, of the invention; Fig. 1A shows the said areas at a larger scale;

Fig. 2 is a diagram which shows the ferrite content in deposited weld metal versus the ageing of the material; Fig. 3 is a diagram which illustrates the creep resistance of the examined materials; and Fig. 4 shows the impact strength of the examinted materials and how the impact strength can be changed as the material is aged.

DESCRIPTION OF PERFORMED EXPERIMENTS

The base material used in the experiments consisted of the austenitic steel which is known under the trade name 253 MA^®, corresponding to ASTM S30815. That steel has the following nominal composition in weight-%: 0.09 C, 0J65 N, 1.7 Si, max 0.70 Mn, 21 Cr, 11 Ni, max 0.5 Mo, balance iron and unavoidable impurities and a small but O 00/75391

functional amount of cerium, about 0.05%. The ferrite number of the base material is 3.9 ± 3.0. This is a typical high temperature steel, which has an excellent resistance against oxidation also at high temperatures in most environments, an excellent creep resistance and rupture strength, a comparatively good toughness and in principle a good weldability. However, the strength of the weld joints, of course, depends on the material features of the deposited weld metal, which in its turn depend on the composition of the filler metal.

The investigations included testing of different filler metals. Welding tests were performed with coated electrodes consisting of a core, which in all electrodes was made of the same type of steel, and on the core there were coatings with different compositions in order to provide different filler metals and hence different weld metals (deposits). The core consisted of a wire which was manufactured of a material according to the specifications stated in Table 1.

Table 1

Com osition of the core wire, wei ht-%, in tested electrodes

In the welding electrode of the invention, the core wire more specifically was made from a heat having the composition 0.012 C, 0.02 Si, 1.58 Mn, 0.013 P, 0.008 S, 19.66 Cr, 10J Ni, 0J6 Mo, 0.03 N, balance iron and unavoidable impurities. In this connection, it shall be mentioned that the presence of 0J6 % Mo is an impurity emanating from used raw materials and is not an intentionally added element.

Samples of the weld metals (deposits) were made through butt welding (MMA) of sections (100 + 200 + 200 + 100) of a 0 455 x 15 mm pipe of the base material. This welded pipe was then cut up into test specimens, suitable for the subsequent ageing treatment.

The ageing treatment was performed in a chamber furnace. The temperature was about 840°C, which corresponds to the mean temperature for a typical use of the material. The holding times were 1, 2, 4, 8, 16, and 32 weeks.

The compositions of the examined weld metals (pure weld metals) are stated in Table 2. O 00/75391

Table 2

Chemical com osition wei ht-% of examined material

In Table 2, weld metal No. 1 is a weld metal formed through welding of the base material with a coated electrode, which is manufactured by Avesta Welding AB, and which known by its trade name 253 MA^®. Weld metal No. 2 was obtained with a welding electrode of the invention. Weld metal No. 3 was obtained through welding with another electrode which was manufactured at a laboratory scale.

Measurements of the ferrite content were carried out on sawn surfaces through a magnetic method (Ferritscope), whereafter a mean value over the whole of the weld was determined. The results are shown in Fig. 2. The ferrite content in weld metal No. 1 is considerably lower than the calculated FN number. This may depend on transformations during the cooling down to room temperature and also on mixing of the weld metal with the molten base metal. Not until after 16 weeks the measured ferrite content had been reduced to about 0 %.

The weld metal No. 2 of the invention has a negative, calculated FN number, but nevertheless exhibits a certain, though low, ferrite content. The reason for this can be explained by mixing with molten base metal. Already after 2 weeks of ageing the ferrite content, however, was down at the zero level.

The weld metal No. 3 had a considerably lower FN value and nor did it contain any measurable amounts of ferrite prior to ageing.

The results from the creep strength measurements are illustrated in Fig. 3. The diagram in that drawing shows that the weld metals No. 1 and No. 2 exhibited equally good creep resistance (within a normal scattering band). The results also correspond to the results from previous tests with welding with electrodes of the type Avesta Sheffield 253 MA^®. The base metal had a creep strength which is somewhat lower than that of previously tested materials of the same type but still within the scattering band. With weld metal No. 3, on the other hand, there were obtained times to fracture which were shorter, corresponding to more than a factor 10. In all the welding test specimens, the fracture occurred in the weld with low coefficients of elongation and ductility; in weld metal No. 3 as a matter of fact equalling zero.

The results from impact tests are shown in Fig. 4. The toughness of the base metal is shown as well as the toughness of those weld metals No. 1 and No. 2, which both exhibited a good creep resistance, see above. The superiorly best impact strength was achieved with weld metal No. 2 of the invention, which was not impaired because of ageing.

The following conclusions can be drawn from the stated results.

The ferrite content measurements as well as studies of the structures, which are not discussed above, showed that only weld metal No. 1 contained considerable amounts of ferrite, which comparatively rapidly was transformed to sigma-phase during the ageing. In the two other weld metals, No. 2 and No. 3, there did not occur any significant changes of the structure, while an increasing amount of precipitation could be detected in the base metal. All this indicates that the weld metal No. 1 is pronouncedly embrittled, as well as the base metal, but not as much. Weld metals No. 2 and No. 3, on the other hand, are not very susceptible of ageing as judged from measurements of ferrite contents and from structure studies.

The creep rupture test evidently shows that weld metal No. 3 is not a suitable filler metal. It is evident that the purely austenitic solidification of weld metal No. 3 has caused the weld metal to be afforded significantly poorer mechanical features than weld metal No. 2 of the invention.

Finally, the impact tests verified the experiences from the other measurements, and the diagram in Fig. 4 clearly illustrates the superiority of the filler metal of the invention, weld metal No. 2.

The conclusion from the stated results of the experiments therefore is that the invention provides a very good weld metal; a good creep resistance and an improved impact strength because of a lower susceptibility to embrittlement in comparison with the comparison material. A further conclusion can be drawn from the tests, namely that a steel with a composition corresponding to the weld metal of the invention is well suited also as a material for a base metal as well as for a welding wire, and that a base metal and a welding wire of that type most probably can be welded also with other filler metals, and can be used also for welding of other base metals, respectively.

Claims

PATENT CLAIMS

1. A welding electrode consisting of a core wire and a cover coating, c h a r a c t e r i s e d therein that the core wire consists of an austenitic stainless steel which contains in weight-%:

0.001-0.03 C

0.01-0.3 Si

0.5-2.5 Mn

19-21 Cr 9-12 Ni

0.01-0.2 N optionally cerium in an amount of max.0J %, balance essentially only iron and unavoidable impurities, and

that the coating contains in weight-%:

6-10 CaC0₃

2-5 F

10-30 Si0₂ 4-10Al₂O₃

25-35 Ti0₂

7-12 Fe max 5 Cr'N ma 5Ni max 10 Cr max 5 Mn balance essentially only sodium- and potassium compounds existing as components in natural minerals; binding agents; and unavoidable impurities.

2. A welding electrode according to claim ^characterised therein that the core wire contains in weight-%: max 0.02 C max 0.2 Si 1.5-2.0 Mn 19.5-20.5 Cr 10.0-11.0 Ni 0.02-0. ION.

3. A welding electrode according to any of claims 1 and 2, characterised therein that the coating contains in weight-%:

7-9 CaC0₃ 3-4 F

15-25 Si0₂ 5-8 A120₃ 28-32 Ti0₂ 8-11 Fe 1-3 CrN 0.5-2 Ni 5-9 Cr 1-3 Mn.

4. A welding electrode according to claim 3, characterised therein that the coating contains in weight-%:

about 8 CaC03 3.5 F 20 Si0₂ 7A120₃ 30 Ti0₂ 10 Fe 2 CrN 7Cr 2Mn.

5. A welding electrode according to any of claims 1-4, characterised therein that the core wire corresponds to 60-80 % and the coating to 20-40 % of the mass of the welding electrode.

6. A welded article of an austenitic stainless steel, c h a r a c t e r i s e d therein that the weld metal, as not mixed with the base metal, contains in weight-%:

0.02-0J C 0.2-1.5 Si

0.2-1.7, preferably max. 1.5 Mn

18-21, preferably max. 20.5 Cr

8-14 Ni

0.10-0.22 N optionally cerium in an amount of up to max 0.1 %, balance iron and unavoidable impurities,

and that the amounts of the alloying elements in the weld metal, as not mixed with the base metal, are balanced so that the values of the chromium and nickel equivalents fall within the frame of the area A-B-C-D-A in the modified

Schaeffler de Long diagram shown in Fig. 1, and partially at a larger scale in Fig. 2, in which the chromium and nickel equivalents, Cr_eq and Ni_eq, respectively, are defined in the following way:

Cr_eq = % Cr + % Mo + 1.5 x % Si + 0.5 x % Nb Ni_eq = % Ni + 0.5 x % Mn + 30 x % C + 30 x % N,

wherein the coordinates of the corner points A, B, C, and D in said area A-B-C- D-A are Cr_eq/Ni_eq

A 19J/13.9

B 21.1/16.3

C 21.1/20.5

D 19.1/17.9.

7. A welded article according to claim 6, c h a r a c t e r i s e d therein that the chromium and nickel equivalents fall within the frame of the area A'-B'-C'-D'- A\ in which the coordinates of the corner points A', B', C, and D' in said area are v- g / I lgq

A' 19.1/14.5

B' 20.5/16.0

C 20.5/18.4

D' 19.1/17.1

8. A welded article according to claim 6 or 7, characterised therein that the weld metal contains in weight-%:

0.035-0.065 C

0.45 - 1.0, preferably max.0.75 Si

0.5-1.5, preferably max.1.0 Mn

18.5-20.5, peferably max.20.0 Cr

9.5-12.5 Ni 0.12-0.20 N.

9. An austenitic stainless steel, characterised therein that it contains in weight-%:

0.02-0.1 C 0.2-1.5 Si

0.2-1.5 Mn 18-20.5 Cr 8-14 Ni 0.10-0.22 N optionally cerium in an amount of up to max 0.1% balance iron and unavoidable impurities, and that the amounts of the alloying elements in the weld metal, as not mixed with the base metal, are balanced so that the values of the chromium and nickel equivalents fall within the frame of the area A-B-C-D-A in the modified Schaeffler de Long diagram shown in Fig.1, and partially at a larger scale in Fig. 2, in which the chromium and nickel equivalents, Cr_eq and Ni_eq, respectively, are definited in the following way:

Cr_eq = % Cr + % Mo +1.5 x % Si + 0.5 x % Nb Ni^ = % Ni + 0.5 x % Mn + 30 x % C + 30 x % N, wherein the coordinates of the corner points A, B, C, and D in said area A-B-C- D-A are

Cr_eq/Ni_eq A 19J/13.9 B 21.1/16.3

C 21.1/20.5

D 19J/17.9.

10. An austenitic stainless steel according to claim 9, characterised therein that the chromium and nickel equivalents fall within the frame of the area A'-B'-C'-D'-A', in which the coordinates of the corner points A', B', C, and D' in said area are

A' 19.1/14.5 B' 20.5/16.0

C 20.5/18.4

D' 19.1/17.1

11. An austenitic stainless steel according to claim 10, characterised therein that it contains in weight-%:

0.035-0.065 C 0.45 - 0.75 Si 0.5-1.0 Mn 18.5-20.0 Cr

9.5-12.5 Ni 0.12-0.20 N.

12. A welding wire made of an austenitic stainless steel according to any of claims 9-11.

13. An austenitic stainless steel, characterised therein that it is weldable with a welding electrode according to any of claims 1-5.