US3554791A - Covered welding electrode - Google Patents

Abstract

A COVERED WELDING ELECTRODE HAVING AN AUSTENTIC ALLOY CORE WIRE CONTAINING LESS THAN 0.01% CARBON AND ESSENTIAL FOR ACCEPTABLE CREEP-RUPTURE DUCTIBILITY OF WELD METAL IN THE AS-DEPOSITED CONDITION.

Description

United States Patent 3,554,791 COVERED WELDING ELECTRODE Edwin W. Johnson, Murrysville, and Frederick C. Hull, Pittsburgh, Pa., assignors to Westinghouse Electric Corporation, Pittsburgh, Pa., a corporation of Pennsylvania No Drawing. Filed Oct. 4, 1968, Ser. No. 765,037 Int. Cl. B23k 35/22 US. Cl. 117-205 11 Claims ABSTRACT OF THE DISCLOSURE A covered welding electrode having an austenitic alloy core wire containing less than 0.01% carbon and essential for acceptable creep-rupture ductility of weld metal in the as-deposited condition.

CROSS REFERENCE TO RELATED APPLICATIONS This application is related to copending application Ser. No. 765,159, filed on Oct. 4, 1968.

BACKGROUND OF THE INVENTION Field of the invention This invention is directed to covered welding electrodes prepared from a core wire of a family of austenitic stainless steel alloys and welds made therefrom. The weld deposits of this alloy are characterized by an exceptional combination of high creep-rupture strength and good rupture ductility. The welds are further distinguished by the fact that although they have a fully austenitic microstructure, they are highly resistant to hot cracking or microfissuring during the welding of restrained joints. Another highly desirable feature of these alloys is that the freedom from hot cracking of the welds is obtained without any preheating, while the high creep-rupture strength and stress-rupture ductility of the welds are obtained without the use of any post-weld heat treatment.

DESCRIPTION OF THE PRIOR ART The properties of creep-rupture strength and ductility are important when alloy parts are subjected to elevated temperatures of operation. To increase efficiency in steampower plants, for example, there is a long-term trend toward the use of higher steam temperatures. At the lower steam temperatures used in the past, the piping through which the steam is conducted was fabricated from lowalloy ferritic steels which were satisfactory. At the higher steam temperatures used in some of the more modern plants and contemplated for more extensive use in the future, the mechanical properties of ferritic steels are less favorable, due primarily to the fact that the strength of such steels declines rapidly within increasing temperature. With increasing steam temperature, therefore, the ferritic steel piping must-have thicker walls for a given steam pressure. This results in increased weight per unit length of pipe and requires the use of larger expansion loops, heavier supporting structures and more welding, which in turn results in greater costs.

It is well known that austenitic steels are much stronger than ferritic steels at all temperatures above about 1000 F. However, the use of austenitic steel steam piping in the past has been characterized by major difliculties peculiar to austenitic steel itself. One problem has involved weld hot cracking, wherein a weld bead tends to crack in a brittle manner at an early stage of cooling from its solidification temperature in the presence of ordinary stresses. This hot cracking has generally been a more troublesome problem in austenitic steels than in ferritic steels. Moreover, failures of welded joints in austenitic steel steam piping after a period of routine service have Patented Jan. 12, 1971 been found to occur under conditions symptomatic of severe or abnormal weakness and/or brittleness of the weld joint in a stress-rupture mode of failure.

A nominal range of compositions of fully austenitic steels which are disclosed in US. Pat. No. 3,201,233 is set forth in Table I as follows:

TABLE I Weight percent Chromium 1420 Nickel 15-30 Manganese 7.5-15 Molybdenum 0.5-3.75 Mn and Mo total 9-16 Carbon 0.010.08 Nitrogen 0.01-0.35 Silicon Up to 1 Vanadium Up to 0.3 Boron Up to 0.03 Zirconium Up to 0.06 Iron Balance Steam piping made from the alloy composition set forth in Table I includes a carbon content of from 0.01 to 0.08%, and a silicon content of up to 1%.

In the prepartion of heats for production, however, it has been found that the air melted heats from which pipe is fabricated usually have an average carbon content of 0.03% and a silicon content of 0.15%. The fully austenitric iron-base alloys having the compositions of Table I have been used as steam-turbine piping materials due to their high strength and ductility at elevated temperatures and low susceptibility to weld cracking. The usual method of welding such alloys has been the tungsten inert-gas (TIG) process. Sound welds with favorable mechanical properties have been produced in such alloys by the tungsten-inert-gas (TIG) rocess.

Although the TIG process can produce reliable welds either manually or automatically, there are many instances when covered electrode welding is preferred. For example, in field erection of equipment is confined spaces, the greater accessibility of the manual covered electrode is a distinct advantage. Moreover, the covered electrode provides its own protective shield so that the often cumbersome inert gas lines and water cooling lines of the TIG process are not needed. Another advantage of the covered electrode process is that there are available more welders who are qualified to use this process. Finally, the equipment for the TIG process, with all its automatic controls, is much more costly than the power supply for the covered electrode process.

US. Pat. 3,201,233 presents the stress-rupture properties of covered electrode Welds made with core rods of compositions within the range of Table I, having coating fluxes of commercially available compositions. The rupture strengths were considerably lower than those of TIG welds and the rupture ductilities of the covered electrode welds dropped with increasing rupture time to unacceptably low values around 1% elongation.

Early attempts to weld the fully austenitic alloys by the manual covered-electrode method resulted in unacceptably low values of the creep-rupture strength and particularly low stress-rupture ductility of the as-deposited weld metal such as indicated in Pat. No. 3,201,233. They are now known to be primarily attributable to high concentrations of carbon and silicon in the welds. The carbon and silicon contents of the covered-electrode welds were significantly higher than the respective concentrations of these same elements in TIG welds prepared from filler wires derived from the same alloy heats as the core Wires of the covered electrodes. The creep-rupture properties of the respective covered-electrode welds were far lower than those of the corresponding TIG welds.

Prior known commercially available coatings are the primary sources of both the carbon and the silicon contamination of the weld metal. The carbon is derived from carbonates, high-carbon master alloys and other such carboniferous constituents of the coating, while the silicon is derived from high-silicon master alloys such as ferrosilicon, from 'various solid silicates such as clays, and from silicate binders.

A source of silicon contamination of the weld metal derived from the invention is the silicate binder of the 4 DESCRIPTION OF THE PREFERRED EMBODIMENT To provide a covered-electrode weld between abutting ends of steam piping which weld has a high creep-rupture strength and satisfactory rupture ductility, it is desirable that the core wire of the covered electrode have a carbon content of less than 0.01%. The core wire of this invention has composition ranges such as shown in Table II as follows:

TABLE II.CORE WIRE COMPOSITION WEIGHT PERCENT Broad Option Typical Option Typical range A A B B Chrimium 13 20 15-18 16 16 Nickel 19 M 20 20 Manganese 7-15 10 10 Molybdenum. 0. 54 1. 753 2. 2 1. 75-3 2. 2 Carbon Less than Less than 0 008 Less than 0. 006

0. 01 0. O1 0. 01 Nitrogen Up to 0.30-.. 0. 03-0. 14.... 0.08 0.15-0.25 0.10 Silicon Up to 0.4 Up to 0.25 0.10 Up to 0.25 0.10 Vanadium Up to 1- O. 10 to 0.35 0. 2 Boron Up to0.03 Up to0.02... 0 010 0. 005-0. 02... 0.015 Zirconium- Up to 0.04. Up to 0.02 Iron 1 Balance.

electrode coating. This binder, which is constituted of sodiurn or potassium silicate or a mixture of these silicates, is responsible for a weld-metal silicon content of from about 0.1% to about 0.5%, depending on the other ingredients of the coating. The maximum stress-rupture ductility of the Weld metal is ultimately limited by this silicon contamination originating from the electrode coating binder.

In the absence of any significant sources of carbon in the electrode coating, the carbon content of the weld metal is ultimately determined by the carbon concentration in core wire of the electrode. In this respect it has been found to be extremely desirable to use core wires having carbon concentrations below 0.01%.

It has been found in accordance with this invention that an unexpectedly significant improvement in the creeprupture strength and ductility properties of welds occurs from the use of covered welding electrodes wherein the carbon content of the core Wire of the electrode is substantially less than 0.01%. This advantage is particularly important where the base metal, the weld, and the core wire consist of fully austenitic steel. Moreover, the advantage of improved creep-rupture strength and ductility is obtained in the as-deposited condition of the weld metal, and therefore the need for post-weld heat treatment is eliminated.

Accordingly it is an object of this invention to provide a covered Welding electrode having an austenitic alloy core wire containing less than 0.01% carbon.

It is another object of this invention to provide a covered welding electrode which provides acceptable creeprupture strength and ductility properties of a weldment in the as-deposited condition.

Other objects of the invention will in part be obvious and will in part appear hereinafter.

SUMMARY OF THE INVENTION Briefly, the invention comprises a covered welding electrode including a core wire containing less than 0.01% of carbon and a coating encasing the core wire, the coating being composed of an agglomeration of nonmetallic constituents including fluorides, oxides, and some metallic constituents, and the core wire being composed of an alloy comprising, by weight, from about 13 to 20% chromium, from about 13 to nickel, from about 5 to 18% manganese, from about 0.5 to 4% molybdenum, less than 0.01% carbon, up to about 0.30% nitrogen, up to 0.4% silicon, up to 1.0% vanadium, up to 0.03% boron, up to 0.04% zirconium, and the balance being iron and incidental impurities. Alloy welding rod with a carbon content of 0.008% and less gives exce l n results.

Generally, the fully austenitic weldable alloys for core wires of this invention comprise the compositions under the Broad Range lflOrl'n which satisfactory welds are obtained. For welds of strength at elevated temperatures comparable to that of AISI Type 316 stainless steel, along with excellent rupture ductility, the compositions listed under Option A and Typical A are useful. The alloys of Option B and Typical B provide considerably higher weld strength with good rupture ductility at elevated temperatures.

The coating for the electrode which is applied directly onto the core wire in a suitable manner such as extrusion, is an agglomeration of several ingredients held together by a suitable binder. The ingredients of nominal compositions of coatings are listed in Table III in concentration units of parts-by weight (p.-b.w.), where one (p.b.w.) is defined as one weight percent of the total amount of only the nonmetallic ingredients of the coating, exclusive of water.

The chemical and physical functions of the various coating ingredients are known only in varying degrees. CMC serves primarily as a slip agent for aiding extrusion while the soluble silicate serves as a binder to keep the coating intact until such time as the electrode is actually used in welding. The primary role of the manganese metal powder is presumed to be that of a chemical reducing agent or deoxidizer, alloying being an additional but secondary function. The fluorides, including both fluorspar and cryolite, are primarily fluxing agents that aid the impurity-scavenging action of the molten slag. The fluorides as well as the TiO and titanates also influence certain physical properties of the slag including the slags fluidity while molten and ease of removal after solidification. The small proportion of tferroboron (or other boroncontaining material) has the primary function of introducing a particular amount of boron into the weld deposit.

=Cr O and CaMoO are known to be sources of alloyed Cr and M0. The results of using other techniques of enriching the weld metal with the same elements has led to the conclusion that the alloying eflects alone are insufficient to explain the various metallurgical benefits gained from the presence of Cr O and CaMoO in the coating. These compounds are sources of oxygen, the liberation of which have the effect of limiting the contamination of the weld metal by oxidizable impurities including carbon, silicon, and sulfur. The beneficial effect of the CaMoO may be based on the known volatility of M00 at the welding temperature. The volatilization of the M00 has two desirable effects. First, the liberated vapor would serve as a gas atmosphere shield to protect the molten weld metal from chemical attack by the air, acting in an analogous way to the CO gas that is liberated from the CaCO used in many conventional electrode coatings. Another benefit is that the residual condensed material is CaO, which is a strongly basic ingredient of the slag that is extremely desirable for aiding the prevention of contamination of the weld metal by acid-oxide-fonming elements including sulfur, phosphorous, and silicon. Most of the molybdenum initially introduced in the form of CaMoO, was recovered in the deposit suggesting that complete volatilization of the M does not occur. A combination of all of these possible mechanisms is the best explanation for the beneficial effects of the CaMoO on the mechanical properties of the as-deposited weld metal.

The core wire was coated by applying a flux coating to the wire by extrusion. The batch size of each flux coating was about 1 kg. which yielded 60 to 70 electrodes per experiment. Three coatings Nos. 1, 4, 5, and 6, having compositions listed in Table III, were applied to the ten lots of core wire as shown in Table IV. The electrodes were baked at temperatures of up to 700 F. in two steps, the first of which occurred immediately after extrusion and terminated at 475 F. The latter at a higher temperature was performed just prior to welding.

Base materials were high strength stainless steels in the form of V-grooved /8 inch thick plate. Each grooved plate was formed by longitudinally sawing a 12" X 12" by /8 plate into two equal pieces 60 from the main surface, inverting one piece end-over-end, and welding it beside the other piece to a 1 inch thick backing plate.

TABLE IIL-COATING COMPOSITIONS PARTS BY WEIGHT Rutile (Ti02)....

1 Alkali silicate binder exclusive of H content. 2 A water soluble gum such as sodium earboxymethyl cellulose. 3 Balance.

The following example illustrates the practice of the invention.

EXAMPLE Ten lots of core wire for experimental electrodes were derived from various heats in heat sizes varying from to 3000 pounds. Most Otf the heats were prepared by partial vacuum induction melting, wherein the major elements iron, nickel, chromium, molybdenum, and vanadium were vacuum melted together after which the furnace was filled with argon or nitrogen and then manganese and master alloys supplying the nitrogen, boron, and zirconium were charged. The heats were poured under argon or nitrogen at atmospheric pressure. The ingots were converted into straight cut lengths of wire in commercial wire production facilities. The wire segments were usually finished to the final diameter by centerless abrasion. The core wire diameters in all experiments were The chemical wire analyses are listed in Table IV.

The grooved plates were filled by a down-hand manual welding procedure with a reverse polarity DC are of 145 amp. with the inch electrodes. To fill each groove requires about 16 passes of the 9, inch electrodes.

The as-deposited weld metal was converted into mechanical test specimens, most of which were longitudinally oriented tensile specimens of gage diameter 0.357 inch and gage length 1.5 inch. The specimens Were used in all of the tensile tests and most of the creep-rupture tests, particularly those expected to terminate in less than 1000 hours.

The creep-rupture properties of the as-deposited welds derived from the several electrodes are shown in Table IV.

The salient feature of Table IV is that the welds derived from core wires having carbon analyses of less than 0.1% displayed much higher values of rupture elongation than did those from the core wires with a carbon concentration exceeding 0.01% when the same coating flux TABLE IV.CREEP RUPTURE PROPERTIES OF WELDS 1,200 F. TEST TEMPERATURE Weld Deposit Number Coating analysis 1 N0 1 No.1 No.4 No.4 No. 4 No. 5 No. 5 No.5 No.5 No.6 No. 6

' 4 ht er t wuqlysis. can 15.1 15.1 15. 57 15. 3 15. 23 15. 39 15. 95 14. 9 15. 47 14. 7 15. 3 Ni: 21. 9 20.1 22.8 21.4 21. 15 13.13 21.12 21.0 20.51 21.5 21.2 Mn- 9. 95 9. s5 10. 15 11. 5 10. 02 10. 13 10. 9s 13. 44 10. 52 10. s 10. 0 Mo.- 2. 25 1. 95 2. 23 2. 1s 2. 25 2. 34 2. 25 2. 14 2. 19 2. 05 2. 25 N. 0. 31 0. 13 0. 122 0. 15 0. 25 0. 15 0. 13 0. 234 0. 21 0. 05 0. 25 v 0.19 0. 2 0. 2 0.155 0.15 0.18 0.07 0.19 0.15 B 0. 003 0. 002 0. 010 0. 010 0. 009 0. 010 0. 011 0. 000 0. 013 0.007 0. 009 Z17. 0. 003 0. 012 0. 003 0. 002 0.007 0. 103 0. 002 0. 015 0. 015 0. 005 0.007 c 0. 053 .008 0. 041 0. 003 0. 005 0. 03 0. 004 0. 025 0.005 0. 01s 0. 005 sf... 0. 00 .02 0. 03 0. 09 0. 05 0. 05 0. 02 0. 05 0. 02 0. 03 0. 05 s 0. 007 005 0. 007 0. 004 0. 003 0. 007 0. 010 0. 005 0. 00s 0. 007 0. 003 P1111: .002 0. 01 0 001 0. 004 0.005 0. 004 0.002 0. 004 0. 001 0.004 0 0.012 Al i i n 3 7 5 50'0' 50'0' 20 "47'5""40'0""50'0 "37*"17'5 40.0 40.0 .5

iii'iii nire" 12 433 252 179 391 411 133 440 133 43s Elongation (percent).. 3. 3 9. 3 2. 7 18.7 19. 6 4. 7 28. 7 5. 7 28.0 9. 3 18.0 Reduction of area (percent). 6. 0 24. 2 10.3 24. 0 19. 6 14. 4 43.0 9. 4 38.6 16.0 33. 4

1 Table III.

2 Iron base with incidental impurities.

was used with both wires. More specifically, deposit numbers 142B, 286-A, 297-A, 325B, 328-A, and 296A were derived from core wires having a carbon analyses of from 0.003 to 0.008%.

In view of the fact that the covered-electrode weld creep-rupture properties are sensitive functions of the electrode coating constitution as well as of the core-wire composition, the arrangement of the columns in Table IV is such that the properties of different welds derived elongation value for No. 102 is 3.3% as compared with that of No. 142B which is 9.3%.

Similarly the coating No. 4 was employed with a set of core wires having 0.041% carbon and with two sets of core wires having 0.003% and 0.006% carbon, respectively. The rupture ductility properties of weld No. 244-A, 286A, and 297-A, derived from core wire having carbon contents of 0.041%, 0.003%, and 0.005%, respectively, are characterized by elongation values of 2.7%, 18.7%, and 19.6%. This confirms the sensitive inverse response of the weld rupture ductility to the carbon content of the core wire. It particularly emphasizes the benefits obtained from the employment of core wire containing less than 0.01% carbon.

For coating No. 5 involving weld deposit Nos. 325C, 325B, 301-A, and 328A were derived from core wire containing 0.03%, 0.004%, 0.025%, and 0.006% of carbon, respectively. The resulting elongation values are 4.7%, 28.7%, 8.7% and 28.0%. These results are in accord with the sensitive inverse correlation indicated above with respect to coating Nos. 1 and 4.

In a similar manner, a coating No. 6 involved deposit Nos. 266B and 296A, derived from core wires having carbon contents of 0.018% and 0.006%, respectively. The resulting elongation values are 9.3% and 18.0%, respectively, indicating that there is a definite inverse correlation between carbon content and rupture ductility properties.

Accordingly, in accordance with this invention, superior stress-rupture ductility of the as-deposited weld metal is derived from covered welding electrodes in which the core wire carbon content is less than 0.01% and in which the coating on the core wire has a composition within the ranges indicated hereinabove.

Various modifications may be made within the spirit of the invention.

What is claimed is:

1. A covered welding electrode for weld deposits characterized by high stress-rupture strength and good rupture ductility up to at least 1200 F. and low cracking during welding, comprising a core wire and a coating therefor; the core wire comprising, by weight, from about 13 to about 20% chromium from about 13% to about 30% nickel, from about 5% to about 18% manganese, from about 0.5% to about 4% molybdenum, less than 0.01% C, up to about 0.30% nitrogen, up to about 0.5% silicon, up to about 1% vanadium, up to about 0.03% boron, up to about 0.04% zirconium, and the balance being iron with incidental impurities, the coating comprising from about 5 p.b.w. to 40 p.b.w. of a fluoride, up to about 30 p.b.w. of a titanate, up to about 15 p.b.w. of CaMoO up to about 25 p.b.w. of Cr O from about p.b.w. to p.b.w. of a silicate binder, up to about 1 p.b.w. of CaSiO up to about 1 p.b.w. of a water soluble gum, up to about 0.5 p.b.w. of boron, up to about 10 p.b.w. of manganese, and the balance being rutile.

2. The electrode of claim 1 wherein the core wire contains from about 0.0005% to 0.008% carbon.

3. The electrode of claim 1 wherein the core wire contains about 0.002% carbon.

8 from dilferent core wires, and tested with substantially identical coatings, can be conveniently compared. Coating No. 1 was employed on two sets of core Wire having clifferent carbon contents. Weld deposit, No. 102, involving one set which included 0.053% carbon and weld deposit No. 142-13 was derived from the other set of core wire containing 0.008% carbon. As indicated in the table the rupture ductility properties of the weld No. 102 are dis tinctly inferior to those of weld No. 142-3. Thus, the

4. The electrode of claim 1 wherein the core wire contains from about 15% to about 18% chromium, from about 19% to about 24% nickel, from about 7% to about 15 manganese, from about 1.75% to about 3% molybdenum, up to 0.008% carbon, from about 0.03% to about 0.14% nitrogen, up to about 0.02% boron, and the balance being iron.

5. The electrode of claim 1 wherein the core wire contains from about 15% to about 18% chromium, from about 19% to about 24% nickel, from about 7% to about 15 manganese, from about 1.75 to about 3% molybdenum, up to 0.008% carbon, from about 0.15% to about 0.25% nitrogen, from about 0.10% to about 0.35% vanadiurn, from about 0.005% to about 0.02% boron, and up to about 0.02% zirconium.

6. The electrode of claim 1 wherein the core wire contains about 16% chromium, about 20% nickel, about 10% manganese, about 2.2% molybdenum, about 0.008% carbon, about 0.08% nitrogen, about 0.10% silicon, about 0.01% boron.

7. The electrode of claim 1 wherein the core wire contains about 16% chromium, about 20% nickel, about 10% manganese, about 2.2% molybdenum, about 0.006% carbon, about 0.19% nitrogen, about 0.10% silicon, about 0.2% vanadium, about 0.015% boron, and the balance being iron.

8. The electrode of claim. 1 wherein the coating comprises from about 10 p.b.w. to about 35 p.b.w. of CaF up to about 5 p.b.w. CaMoO from about 10 p.b.w. to about 15 p.b.w. of silicate binder, up to about 0.5 p.b.w. CaSiO and from about 0.1 p.b.w. to about 0.2% boron.

9. The electrode of claim 1 wherein the coating comprises about 30 p.b.w. CaF about 10 p.b.w. titanate, about 5 p.b.w. CaMoO about 10 p.b.w. Cr O about 11.5 p.b.w. of K2Si30q, about 0.5 p.b.w. of a water-soluble gum, from about 0.05 p.b.w. to about 0.2 p.b.w. of boron, and about 3 p.b.w. manganese.

10. The electrode of claim 8 wherein the core wire contains from about 15% to about 18% chromium, from about 19% to about 24% nickel, from about 7% to about 15 manganese, from about 1.75 to about 3% molybdenum, from about 0.03% to about 0.14% nitrogen, up to 0.008% carbon, up to about 0.02% boron, and the bal ance being iron.

11. The electrode of claim 8 wherein the core wire contains about 16% chromium, about 20% nickel, about 10% manganese, about 2.2% molybdenum, about 0.006% carbon, about 0.19% nitrogen, about 0.2% vanadium, about 0.015% boron, and the balance being iron.

References Cited FOREIGN PATENTS 165,685 4/1950 Austria 117202 WILLIAM L. JARVIS, Primary Examiner U.S. Cl. X.R.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3: 55 179 Dated January 97 Inventor(s) Edwin w. Johnson and Frederick C. Hull It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 2, line 40, cancel "is" and substitute in Column 6, line 32, cancel "0.1%" and substitute 0.01% Column 7, line 9, after, "derived" insert the first nine lines of column 8. Column 7, line 51, after "chromium" insert a comma. Column 8, lines 1-9, cancel.

Signed and sealed this 13th day of July 1971 (SEAL) Attest:

WILLIAM E. SCHUYLER,

EDWARD M.FI.ETCIER,JR.

Commissioner of Paton Attesting Cfficer