Historical Articles

November, 1952 issue of Plating

 

Plating as an Aid in the Brazing of StainIess Steel

A. Korbelak, Editor, Plating Magazine and E.C. Okress, Advisory Engineer, Westinghouse Electric Corporation, Bloomfield, N.J.

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ABSTRACT
A method for the use of electrodeposits of nickel to facilitate the manufacture of large-sized intricate designs of stainless steel is described in some detail. The locking of slag lines and voids to obtain a good vacuum tight seal when such parts are used in evacuated electronic devices is indicated. A tabular summary illustrated with photomicrographs is included to outline the use of varying thicknesses of plated coatings for different brazing temperatures.

INTRODUCTION
In the manufacture of new equipment called for by the ever-increasing demands of today’s designers, frequent use is made of non-magnetic stainless steel alloys such as types 304, 347, etc. In the early design stages of a new electronic tube, the use of such alloys appeared necessary—with the understanding as is true of many new designs, that fabricating problems would occur. The size and complexity of the device was greater than anything previously attempted, and the fabricating problems were even tougher than those expected, because large sizes of various metals including a large glass window had to be assembled together and be vacuum tight up to the softening point of the glass. The large and intricate geometry of the subassemblies of the unit automatically ruled out a straightforward hydrogen furnace brazing operation common with much smaller tubes. So, the large subassemblies of the complicated device had to be assembled by commercial shielded inert gas arc welding techniques.

This feature required prior brazing of 304 and 347 stainless steel lips to the parts of various metals, in order that when corresponding subassemblies were fitted they could be welded together so that the assembly would be vacuum tight.

Fig. 1. Dissociation pressure of chromium oxide. Tracing, courtesy Westinghouse Research Laboratories	Fig. 2. Partial pressure of H2O vapor above the equilibrium mixture for the reactions 1/2 Cr2O3 (s) + 3/2 H2 -> Cr (s) + 3/2 H2O (g) Tracing courtesy Rand Report R-108

With conventional vacuum furnace techniques or with commercial hydrogen furnace techniques, the presence of chromium in the metals being brazed together or to copper and other metals posed a serious problem. The normal moisture content in either type of furnace is generally high enough to prevent reduction of very stable chromium oxide which is very undesirable. Good tight vacuum joints are not possible with such a condition, since chromium oxide is removed in vacuum or hydrogen only at temperatures which may be too high for the assembly, as indicated by Figs. 1 and 2, respectively. Through careful moisture control it is entirely possible to braze stainless steel directly in hydrogen (at dew points from —40° C ( -40° F) to—60° C (-76° F) but preferably at—80° C ( - 112° F) or lower if a sufficiently high melting point brazing alloy such as 10 per cent cupronickel is used.) This brazing alloy melts at 1100° C (2012° F) and flows at 1150° C (2102° F) and has the following percentage composition: copper—89.21, nickel—9.68, iron—0.80, balance zinc, manganese and lead. Obviously, the direct method cannot be used if a lower melting point alloy such as copper is being joined to the stainless steel with say silver-copper eutectic alloy, unless very low dew points of the order of—150° C ( -238° F) are maintained. Doing such a job on a commercial scale consistently is too costly and impractical. It was found very satisfactory to use a plated coating of nickel on the steel in this type of a brazing problem which was encountered in the electronic device described above. Lower brazing temperatures with considerably higher dew points, an economic consideration of importance, produced consistently good vacuum seals simply through the observation of certain values of non-porous deposit thicknesses. The sintering of the plated layer prior to actual brazing steps was found to be advantageous and imperative in some brazing operations. Through the use of this step, the added problem of slag lines and voids encountered even in the best of selected lots of stainless alloy stock was effectively overcome by the adoption of a slag locking technique as shown in Fig. 3. The shaded lines in the figure indicate slag line direction, with the dark areas representing brazing alloy fillets. Figs. 4 and 5 are photos of an actual simple subassembly prepared by the outlined techniques. Fig. 6 is a cross section of a typical brazed joint, between nickel plated 304 steel and O.F.H.C. copper, at 1000 magnifications with the dark layer of gold brazing alloy over the light area of sintered nickel plate. The alloyed portion of electroplated nickel is adjacent to the gray 304 stainless steel.

Fig. 3. Cross section of a typical subassembly illustrating slag and void locking technique by brazing

PROCEDURE FOR PLATING
The stainless steel alloy parts were first cleaned to remove surface and sub-surface contaminants. In the case of spinnings contamination was found to extend beneath the surface and was removed by an electrolytic etch in 1-1 sulfuric acid used at room temperature with the stainless parts being made anodic. Treatment time varied with the condition of the part and averaged about two minutes.

Fig. 6. Cross section of typical brazed joint. 1000X

After electropickling, the parts were rinsed thoroughly in clean water. A follow-up treatment in a Wood nickel chloride strike bath for between one and two minutes, a water rinse and final transfer to a modified Watts nickel solution completed the plating cycle.

The chloride strike was found to cause trouble unless purified free of copper and iron. Purification was simple with a low current density electrolysis, high surface area cathode method. Agitation during electro purification was helpful in speeding up the operation, which was performed periodically. Heavy copper contamination was found to result in thick spongy copper formation during purification and required frequent removal to prevent its sloughing off with resultant re-contamination. Anodes of graphite and/or nickel were bagged with a material of a special synthetic fiber.

The purified Watts nickel solution used was one containing approximately ten ounces of metal. Pitting control was exercised through the use of regular additions of a specially stabilized grade of hydrogen peroxide, on the recommendation that the use of peroxide stabilized with organics such as acetanilide be avoided. Current densities in the neighborhood of 25 amp/ft² (2.6 amp/dm²) were used. Thin spots such as would occur at contact points were overcome by shifting contacts when half the desired final thickness of plate was obtained. Care was observed in handling of the work during this phase of the plating operations, with operators wearing clean gloves and handling the work under water.

For ordinary brazed joints where short brazing times were encountered it was found adequate to plate less metal to obtain a satisfactory job. Thicker- coatings, approaching an average of 0.002 inch gave an extra margin of safety where long brazing times were encountered or when highly recessed work was processed. Table I summarizes the nickel coating thickness tested, in commercial hydrogen furnaces with dew points of the order of +25° C (+77° F).
Metallurgical studies showed that the nickel deposit began to alloy above 900° C and that it continued with time.

Fig. 7. Photomicrograph of sintered nickel electrodeposit on 347 stainless steel. 1000X

From the foregoing data .001 inch (and preferably .002 inch thick) nickel plating is recommended on 304 and 347 steel when sintered in commercial hydrogen furnaces with dew points of the order of +25° C for 30 minutes a 1000° C, provided the nickel plating is non-porous to begin with and the subsequent brazing temperature is below 1000° C for 30 minutes duration.

As an added protection against interface leakage in high vacuum work (.002 inch) coatings were heated in a—60° C dew point hydrogen furnace. Representative results are summarized in Table II.

Fig. 7 is a photomicrograph of a specimen with .002 inch nickel sintered to 347 steel at 1000° C (1832° F) for 30 minutes, in a—60° C dew point hydrogen furnace. This was a practice adopted to allow for more uniform alloying and is the recommended procedure for subsequent brazing at temperatures below 1000° C for 30 minutes’ duration. At 1116° C and up to 1171° C the alloying is quite rapid so that if a large plated mass is being brazed or heat treated to alloy the deposit to the steel, localized fusion may occur in those areas which come up to temperature immediately.

Through the use of the plating cycle outlined it was possible to put into production, the joining of large stainless alloy parts through simple commercial hydrogen brazing techniques with nominal or usual dew points of the order of +25°C, and employing brazing alloys such as silver-copper eutectic, silfos, gold-copper, etc.

The following references contain much specialized and valuable information on joint design, brazing and electroplating which is of a supplemental nature to the foregoing paper: S. Dushman, Scientific Foundations of Vacuum Technique, John Wiley & Sons, Inc., New York (1949). W. H. Kohl, Materials Technology for Electron Tubes, Reinhold Publishing Corporation, New York (1951).

ACKNOWLEDGMENT
Acknowledgment is hereby made to L. C. Werner, J. Corcoran, H. J. Ehringer, A. Baldi and R. Green for their contributions in the preparation of data for the foregoing paper.