US2640792A - Inert monatomic-gas shielded refractory metal remelting surface-defect removal process - Google Patents

Description

June 2, 1953 w, o, BINDER 2,640,792

INERT'MONATOMIC-GAS SHIELDED REFRACTORY. METAL REMELTING SURFACE-DEF REMOVAL PROCESS Filed Jan 1951' ECT 29 Elecfrode Lead Gas Nozzle Tungsfen Elecirode Electric Power Source Inert Gas Surface defects such as seams, laps and entrapped oxldes.

Ground Le ad INVENTOR WILLIAM O. BINDER the metal. trides depletes the areas adjacent to the grain Patented June 2, 1953 um'rao starss "eraser OFFICE 1 INERT MEONATOMIC-GAS SHIELDED as resources! METAL REMELTING SUR. sacs-career REMOVAL raocuss William 0. Binder, Niagara Falls, N. Y., assignor. to. Union Carbide and Carbon Corporation, a corporation of New York Application January 29, 1951, Serial No. 208 ,395

1 Y This invention relates to metal working, and more particularly to the removal of surface defacts in refractory metal ingots by remelting the adjacent metal. I

High-chromium alloys require careful'surface conditioning prior to hot-rolling inorder to ensure satisfactory surface quality. Because of their inherent resistance to oxidation, these alloys do not scale readily, hence they retain surface defects which normally are eliminated in the reheating furnaces in the case of more freely scaling metals. Most oithe ingot'defects causing surface rejections are laps and seams due to metal splashing Since they are located near the surface, the standard commercial prac- Y tice is to remove these defects mechanically by hand grinding with abrasive wheels. More recen-tly, -powder scarfing techniques have been developed. However, both processes result in considerable loss .of metal, and the hand-grinding process is slow, relatively costly, and presents labor problems due to the disagreeable nature of the work.

The corrosion resistance of stainless steels and related high-chromium .alloys'is a function of chromium content; higher chromium contents imparting greater resistance. Any impoverishment of the surface by oxidation of chromium would reduce the general corrosion resistance of the metal. In borderline cases, oxidation could reduce the concentration to a dangerous minimum. Loss of easily oxidizable metals such as columbium, titanium, and tantalum must also be avoided as their loss would have a dangerous effect on the corrosion resistance of the :i'.

alloys.

High-chromium alloys always carry a certain percentage of carbon and nitrogen. These elements normally are kept in solid solution and do not seriously affect the corrosion resistance of the metal. On heating in certain temperature ranges, these elements combine with chromium precipitate along the grain boundaries of The formation of carbides and niboundaries of chromium. When exposed to certain media, these areas tend to dissolve leading to inter-granular corrosion of the metal. Commonly accepted means for preventing precipitation of chromium carbides and nitrides is to keep the percentages of carbon and nitrogen in the metal low. In some cases, columbium, tantalum, or titanium are added to combine with carbon and nitrogen as these elements form relatively 1 Claim. (01. ne -i0) cast.

2 fects from steel billets by melting the adjacent metal with an are protected from the air by, a

' stream of hydrogen; However, the latter is not suitable for high-alloy metals, due to contamination of the metal by such gas. Hydrogen, aswell as nitrogen, pickup during melting is detrimental because they dissolve readily in molten highchromium alloys and may produce porosity when the metal solidifies.

Excessive silicon pickup during melting must also be avoided, as it not only decreases general corrosion resistance in some media, but also it causes brittleness, decreases weldabi'lity and hotworkability. Silicon is apt to be picked up when slags are employed'to protect the metal during melting.

High-chromium alloys are susceptible to cracking during hot-working if improperly melted and Oxygen, carbon, and nitrogen have adirect influence, and the pickup of these elements should he kept to a minimum. Protective. slags tend to oxidize the metal unless they are kept on the'reducing side, and they do not prevent nitrogen pickup, particularly when the slag and metal are exposed to electric arc temperatures.

The main object of this invention to .provide an economical and eflicient method of removing all which yield the same composition in the melted as in the unmelted portions.

ity to melt the metal in the area of the surface defect to be removed. Such are and the molten 4 metal are continuously shielded from the air by a stream of inert monatomic gas during such are remelting operation. As a result, the defect. is removed, leaving the metal that has been melted of the same composition as that of the base metal.

Summarizing, surface melting according to the invention in an inert monatomic gas atmosphere has the following distinct advantages: there is no loss of easily oxidizable metals, hence the metal retains its corrosion resistance; there is no porosity due to pickup of hydrogen or nitrogen; there is no pickup of carbon or nitrogen to cause inter-granular corrosion suscepti bility; there is no loss of hot-workabilitydue to oxidation of the metal; and since no slag is used, there is no problem due to entrapmentrof slag'during solidification -or-to pickup of delerequired to remove the surface defects.

terious impurities present in the slag-making materials.

In the drawing, the single figure is diagrammatic view illustrating the invention.

As shown in the drawing, a metal body Ill, such as billet is moved horizontally under a tungsten electrode [2 provided with a gas nozzle or cup 14. The body l and electrode l2 are connected by leads l6 and I8 to an electric power source 20 of sufficient current-supplying capacity to energize an electric metal meltingarc 22 between the surface of the body and the tip of the electrode during such movement of the work l0. Simultaneously therewith the arc and heated surface portions of the electrode andwork are shielded from the air by an annular stream 24 of inert monatomic-gas which is delivered to the nozzle H from a suitable source via an inlet 26.

As the work l0 moves under the inert monatomi'c-gas shielded arc welding torch 28, the are 22 melts the adjacent metal containing defects, forming a. molten puddle 30 which pro nickel rich iron-molybdenum, and related alloys can be satisfactorily surface-conditioned prior to rolling by remelting the surface of the ingot with a shielded arc, according to the invention,

"by using either argon or helium gas as the protective atmosphere. The surface can be premelted manually, but mechanized premelting is more satisfactory as it is subject to better control.

The amperage value of the current during remelting depends on the depth of penetration An arc current of 450 amperes using argon, and a 1 in. diameter tungsten electrode will remove defects about in. deep. Somewhat lower amperage may be used with helium for substantially the same depth of penetration.

The speed of melting depends on the surface condition of the ingot and the amperage of the arc current. A work-speed of in./minute at 450 amperes has been found satisfactory for small ingots though somewhat faster speed can be -ing inch. Surfaces of satisfactory quality were obtained, according to the invention, with a gas flow of cu. ft./hour with argon, and

' 40 cu. ft./hour with helium. No significant difference in the quality of the surface was noted between the ingots premelted in argon and in v helium.

Precautions should be taken to minimize arc interruptions as the craters form areas of shallow surface cracking in the rolled surface.

However, the starting craters are relatively shallow and can be removed by grinding without too much difliculty prior to rolling, and in most instances, the craters formed at the end of each pass can be confined to the piped section which is eventually discarded. Another scheme is to tack-weld suitable metal pieces on the ends of the ingot for starting and stopping purposes.

In cases where the defects are relatively deep cracks, an electrode composed of metal of the same composition as that of the base metal can be used, without departing from the invention, to supply the additional metal required to fill such deep cracks. However, the electrode preferably is compossed of tungsten containing significant amounts of oxidic materials, such as thoria', which improve arc stability and/or other additions which improve the operation of the electrode.

I claim:

Process of removing surface and subsurface defects including laps and scams due to metal splashing, from ingots composed of relatively high-alloy refractory metals prior to hot rolling in order to insure satisfactory surface quality of the resulting product without the loss of any of the refractory alloy metal thereof and without any change in the composition of the base metal, which comprises positioning the ingot so that the surface to be conditioned is uppermost and substantially level, striking a relatively heavy duty are between such surface of the ingot and a refractory metal electrode composed of tungsten; continuously shielding such are and the adjacent portions of the electrodeand ingot with an annular stream of monatomic gas of the class consisting of argon and helium and mixtures of argon and helium discharged from a nozzle surrounding such electrode in spaced concentric relation therewith at a gas flow rate of twenty-five to forty cubic feet 'per hour, positioning the end of such nozzle as closely as possible to the surface of said ingot at a distance of not more than three-eighths of an inch, continuously moving said gas shielded arc with respect to the ingot surface at a speed of the order of about ten inches per minute while maintaining such critical spacing between said nozzle and the ingot surface, continuously supplying such are with a current of the order of 400-450 amperes, deeply melting the ingot metal under such are to a depth of the order of about one References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,512,787 Morton Oct. 21, 1924 2,125,172 Kinzel July 26, 1938 2,185,496 Brown Jan. 2, 1940 2,405,542 Wassell Aug. 6, 1946 2,474,023 Wyer June 21, 1949 2,475,357 Miller July 5, 1949 2,515,559 Lancaster et al. July 18, 1950 2,532,410 Kennedy Dec. 5, 1950

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