Radiodiagnóstico

Base Metals and Base-Metal

Family Groups

THIS CHAPTER BEGINS by describing the general metallurgical considerations related to the selection of different base metals. The re- mainder of the chapter is devoted to describing specific considerations related to the groups of alloys that are most commonly joined by brazing.

Metallurgical Reactions

Some metals and alloys exhibit metallurgical phenomena that influence the behavior of brazed joints and base-metal properties and, in some cases, necessitate special procedures. These phenomena may be classified as: • Base-metal effects, including carbide precip-

itation

• Hydrogen embrittlement

• Heat-affected zone and oxide stability • Sulfur embrittlement

• Filler-metal effects, such as vapor pressure • Base-metal/filler-metal interactions, includ-

ing alloying

• Phosphorus embrittlement • Stress cracking

Other factors that cause interactions between base metals and filler metals include postbraz- ing thermal treatments, corrosion resistance, and dissimilar-metal combinations.

The extent of interaction varies greatly, de- pending on compositions (base metal and filler metal) and the duration and extent of the thermal cycles in the processing. There is always some interaction, except when mutual insolubility per- mits practically no metallurgical interaction.

In addition to the base-metal effects men- tioned previously and the normal mechanical

requirements of the base metal in the brazement, the effect of the brazing cycle on the base metal and the final joint strength must be considered.

Cold-worked-strengthened base metals are annealed and the joint strength reduced when the brazing process temperature and time are in the annealing range of the base metal being processed. Hot-cold-worked heat-resistant base metals can also be brazed; however, only the annealed physical properties are available in the final brazement.

The brazing cycle, by its very nature, usually anneals the cold-worked base metal, unless the brazing temperature is very low and the time at heat is very short. It is not practical to cold work the base metal after the brazing operation.

When a brazement must have strength after brazing that is above the annealed properties of the base metal, a heat treatable base metal should be selected. The base metal can be an oil- quench type, an air-quench type that can be brazed and hardened in the same or a separate operation, or a precipitation-hardening type that can be brazed and solution treated in a com- bined cycle. Parts already hardened may be brazed with a low-temperature filler metal, using short times at temperature to maintain the mechanical properties.

The strength of the base metal has a profound effect on the strength of the brazed joint; thus, this property must be clearly kept in mind when designing the joint for specific properties. Some base metals also are easier to braze than others, particularly by specific brazing processes.

Carbide Precipitation. If stainless steels

are heated to temperatures from 425 to 815 °C (800 to 1500 °F), the carbon in the base metal combines preferentially with chromium to form

chromium carbide, usually at the grain bound- aries. This chromium depletion reduces the cor- rosion resistance of the stainless steel. This con- dition has been defined as sensitization by some investigators. In certain corrosive environments, the mechanical properties may be impaired, with little or no apparent surface attack. A short braz- ing cycle keeps the chromium carbide precipita- tion to a negligible level with normal types of stainless steels. When this is not possible, one of the special grades of stainless steel may have to be used if its corrosion resistance is to be pre- served after brazing.

Precipitated carbides in stainless steels may be redissolved by heat treating at 1010 to 1120 °C (1850 to 2050 °F), followed by rapid cooling. Another stabilizing treatment that disperses the unprecipitated chromium uniformly throughout the structure consists of heating to 870 °C (1600 °F) for 2 h, followed by furnace cooling to 540 °C (1000 °F) and subsequent air cooling.

If the cooling from the brazing temperature is rapid, no appreciable amount of carbides is pre- cipitated. Where this cannot be done due to mass and it is necessary to braze stainless steels for corrosive service, one of the stabilized composi- tions, such as type 347 or 321, or an extra-low- carbon grade, such as 304L, should be used.

There are several ways to prevent or mini- mize the deleterious effects of carbide precipita- tion. First, because the reaction is time depen- dent, keeping the brazing thermal cycle as short as possible can minimize carbide precipitation. With short cycle times, such as would result from torch or induction brazing of small parts, even the unstabilized grades can be brazed with- out severe losses in corrosion resistance.

The susceptibility to carbide precipitation also depends on carbon content. Thus, type 304 is less susceptible than type 302, and the extra- low-carbon grades, such as type 304L, are rela- tively insensitive to carbide precipitation.

For critical applications, type 347, the nio- bium-stabilized grade, is recommended. It has good high-temperature strength and can be brazed without danger of impaired corrosion resistance. Type 321 is also a stabilized grade, but it has slightly lower general corrosion resistance than type 347 and is more difficult to braze, because titanium is used as the carbide- stabilizing element.

When high-melting-point filler metals are used, precipitated carbides can be redissolved by heat treatment after brazing.

Alternatively, corrosion resistance can be restored by diffusing chromium back into the depleted area around the carbide precipitates. Two hours at 870 °C is the recommended ho- mogenizing heat treatment.

Hydrogen Embrittlement. Hydrogen can

also be a source of trouble. Because of its small atomic size, it is able to diffuse quite rapidly through many metals, and the rate of diffusion increases with temperature. When hydrogen dif- fuses into a metal that has not been completely deoxidized, it may reduce the oxide of the metal, if the temperature is high enough. Metallic sponge and water vapor are the end products of this reaction.

Once hydrogen has diffused into the metal, several things can happen. If oxygen is present, the hydrogen may combine with it to produce water vapor. The water-vapor molecule, unlike the hydrogen molecule, is too large to diffuse out of the metal, and the high vapor pressures that develop can literally tear the metal apart by starting many fissures and blisters, mainly at the grain boundaries. The ultimate result is hydro- gen embrittlement. It commonly occurs in cop- per and copper-base alloys that have not been deoxidized. Pressures developed for tough pitch copper have been calculated to be as high as 620 MPa (90 ksi).

Electrolytic tough pitch copper, silver, and palladium, when they contain oxygen, are sub- ject to hydrogen embrittlement if heated in the presence of hydrogen. If tough pitch copper is to be brazed without embrittlement, hydrogen must not be present in the heating atmosphere. A better practice is to use deoxidized copper or oxygen-free copper where brazing is to be per- formed. Oxygen-free copper, if improperly heated, may also be oxidized and become sub- ject to hydrogen embrittlement. It is impractical to salvage hydrogen-embrittled copper.

A recently completed study (Ref 1) examined several commercial filler metals containing zinc, cadmium, or phosphorus that were found to cause embrittlement by migration of (copper) oxide to the grain boundaries, causing void for- mation and rupture of grain boundaries. (Oxides do not migrate as such but rather dissolve in the grains. The oxygen diffuses to the grain bound- aries, where it recombines, forming less- stressed particles.) The brazing was performed both by the conventional fluxed and fluxless methods without the presence of any source of hydrogen; however, this still resulted in the

same embrittlement. Therefore, it was con- cluded that the influence of flux is insignificant, because embrittlement also persisted in joints brazed without flux under argon.

To narrow the possibilities of embrittlement, a 72Ag-28Cu eutectic filler metal was used to fill several joints, and there was no such embrittle- ment, even in the most drastic brazing conditions employed. In comparing this filler metal with the others, the only difference was in the composi- tion. While BAg-8 contains silver as the only addition to copper, the other filler metals are a ternary or quaternary formulation, containing additions of zinc, cadmium, and phosphorus. The difference between silver and these other alloying additions is in their ability to reduce copper oxide; silver cannot act as a reducing agent. This indicates that embrittlement occurs as a result of the interaction of the other alloying elements with the copper base metal and not directly because of other factors, such as flux, atmosphere, and the time-temperature cycle. These parameters were identical for all the filler metals, and their influences were as expected; that is, the embrittlement was enhanced when more drastic conditions were employed.

Furthermore, embrittlement occurred only in tough pitch copper (containing oxygen as Cu₂O precipitates) but not in phosphorus-deoxidized copper, which is completely free of oxides. As a consequence, the coexistence of the additional alloying elements of zinc, cadmium, and phos- phorus, together with oxygen in the substrate, is the necessary prerequisite for embrittlement.

The results indicate that the responsible ele- ments are those that are capable of reducing copper oxide by a mechanism analogous to the hydrogen embrittlement of tough pitch copper. It seems quite certain that the thermodynamic activity of zinc, cadmium, and phosphorus is sufficiently high to cause the following reaction to take place:

CuO + (X) 3 Cu + XO

where X stands for one of these elements in the filler metal.

The mechanism suggested by this study is embrittlement induced by the coexistence of copper oxides in the base material together with certain alloying additions in the filler metal, such as cadmium, zinc, or phosphorus, capable of reducing the copper oxides.

Steel is especially prone to another mecha-

nism for hydrogen embrittlement. In this type, hydrogen diffuses into the steel as atomic hydro- gen in the same manner as it diffuses into copper, but it tends to accumulate in small voids, such as those around nonmetallic inclusions and at grain boundaries. Water vapor is not formed, as in copper, but the hydrogen atoms combine to form hydrogen molecules, which are less mobile and remain trapped at the discontinuities and, as a result, increase the concentration of molecular hydrogen, increase the vapor pressure, and lower the ductility of steel when stressed.

However, steel and other ferrous alloys may be salvaged by allowing the hydrogen to diffuse out by baking at slightly elevated temperatures (95 to 205 °C, or 200 to 400 °F) or by permitting the steel to stand for long periods of time until the ductility is regained.

A third type of embrittlement can occur when hydrogen combines with the metal to form hydride. The hydride lowers the notch tough- ness and affects the strain rate of the metal. Tita- nium, zirconium, niobium, tantalum, and their alloys are subject to this form of hydrogen embrittlement. Ductility can be restored if proper thermal treatments are followed after brazing; however, an inert or vacuum atmos- phere should be used for brazing to avoid any embrittlement. Most other metals and alloys whose oxides may be reduced by hydrogen con- tain an excess of deoxidizing elements and are not subject to hydrogen embrittlement.

Heat-Affected Zone. The heating of base

metals may cause changes in their properties, particularly if the metals are heated above their annealing temperatures. Base metals whose mechanical properties were obtained by cold working (hard tempers) may soften or undergo an increase in grain size if the brazing tempera- ture is above the recrystallization temperature. Where mechanical properties are obtained by thermal treatment, they may be altered by the brazing operation. Materials in the annealed condition generally experience no appreciable change due to brazing.

The width of the zone through which these changes may occur varies with the process used. If the heating is localized, as in torch or induc- tion brazing, the effects are confined to a narrow zone. If the whole assembly is heated, as in fur- nace brazing, the entire assembly is affected. In general, the heat-affected zone produced during brazing is wider and less sharply defined than those resulting from other welding processes.

Oxide Stability and Formation. When

clean metals are heated to brazing temperature, their surfaces may form metal oxides if the atmosphere around the part contains oxygen. Oxidized metal surfaces are usually difficult to wet with most filler metals. Fluxes and special atmospheres are designed to prevent oxide for- mation or to reduce at elevated temperature any oxidation that occurs during initial heating (Ref 2). Chromium, aluminum, titanium, silicon, magnesium, manganese, and beryllium all have oxides that are difficult to remove, and, there- fore, these metals usually require special prepa- ration (Ref 2). Fluoride-bearing fluxes can re- duce some oxides; hydrogen gas of sufficient purity can reduce them above certain tempera- tures, and techniques such as vacuum brazing may have to be used. Ideally, oxide formation should be prevented by brazing in low-dew- point or vacuum atmospheres.

Sulfur Embrittlement. Nickel and certain

alloys containing appreciable amounts of nickel, if heated in the presence of sulfur or compounds containing sulfur, may become embrittled. This occurs when a low-melting nickel sulfide is formed preferentially at the grain boundaries; this sulfide, being brittle and weak, cracks if subsequently stressed. Material so embrittled is usually scrapped, because the damage that has occurred cannot be salvaged.

Nickel and nickel-copper alloys are most sus- ceptible to this attack, whereas alloys containing chromium are less susceptible. It is important that alloys in which nickel is the major compo- nent be clean and free of sulfur-containing mate- rials (such as oil, grease, paint, and drawing lubricants) prior to heating and that heating be done in relatively sulfur-free atmospheres.

Vapor Pressure. Every metal is in equilib-

rium with its vapor pressure; some amount of the metal is present in the gaseous state. For most metals, at normal temperatures, this vapor pressure is so small as to be considered nonex- istent. For vacuum-tube applications, some met- als, such as zinc and cadmium, have relatively high vapor pressures, give off undesirable gases at normal brazing temperatures, and therefore cannot be permitted as constituents of the filler metal. Accordingly, special vacuum-tube-grade filler metals have become commercially avail- able (see Chapter 5, “Brazing Filler Metals”), and special fluxes are used in some situations.

Base-Metal/Filler-Metal Interactions.

There are always some interactions between the filler metal and the base metal. Although some

of this interaction aids in wetting the base metal, other detrimental effects may occur. Such effects include:

• Formation of brittle intermetallic compounds that lower joint strength

• Diffusion of the filler metal into the base metal to produce color changes

• Creation of a new alloy—with a higher melt- ing point than that of the original filler metal—that chokes off the flow of the filler metal

Researchers (Ref 3–6) have reported on their work whereby modified brazing processes for nickel-base materials were used to reduce the formation of brittle phases in the braze joint and also to speed the joining operation.

The three modified brazing processes devel- oped by the researchers included:

• Brazing under defined load • High-speed brazing

• Application of mechanical-excitation brazing Additives (silicon, boride) used to reduce the melting point of nickel-base materials brazed with nickel filler metals cause brittle phases, which exert a negative influence on the mechan- ical properties of the brazed joints. Diffusion annealing and subsequent aging merge the brit- tle phases in the braze joint, whereas the aging causes hardening of the base material. After this, the mechanical properties of the joint are comparable to those of the base material. How- ever, applying this heat treatment may cause the formation of coarse grains.

Brazing under defined load pressures differs from conventional brazing in that a previously defined load is set up quickly after the brazing temperature is reached. Brazing temperatures of 1150 and 1180 °C (2100 and 2150 °F) with tem- perature retention times of 1 and 10 min, respec- tively, have proved to be the best parameter combination.

This type of pressure-brazed joint with a more homogeneous microstructure produces good strength properties. Brazing time is of lit- tle significance; in contrast, the brazing temper- ature is of great importance to the strength prop- erties. However, strength values that can be attained without diffusion annealing are compa- rable with those of conventionally brazed spec- imens using cost-intensive and time-consuming heat treatments.

The formation of brittle phases can be influ- enced or even avoided by combining a consid- erable reduction of the brazing time with a simultaneous increase in the brazing pressure. The only useful method to achieve the required high gradient of temperature is a conductive heat treatment technique.

Another method to prevent the formation of brittle phases is by the mechanical excitation of the components during the brazing process. While brazing, a transducer directly connected to one specimen transfers high-frequency ener- gy into the brazing couple.

Using ultrasonic vibrations of approximately

30 kHz (amplitude of approximately 2 µm in

the longitudinal direction of the specimens), the accumulation of brittle phases is prevented. The wetting of the base metal with filler metal is improved, because the ultrasonic vibrations destroy any existing surface oxides. The super- position of mechanical excitation produces seams of a quality comparable with those joints heat treated (1100 °C, or 2010 °F, for 20 h) after conventional brazing.

Alloying is one of the significant base-

metal/filler-metal interactions that can deter- mine the behavior of brazed joints. The extent of interaction varies greatly, depending on the compositions of the base metal and the filler metal and on thermal cycles. There is always some interaction, except where mutual insolu- bility permits practically none.

The term alloying is a general term covering practically every aspect of interaction. Some of these aspects are as follows.

First, the molten filler metal can dissolve the base metal. Second, constituents of the filler metal can diffuse into the base metal, either through the bulk of the grains or along the grain boundaries, or can penetrate the grain bound- aries as a liquid. The results of such base-metal dissolution or filler-metal diffusion may be to raise or lower the liquidus or solidus tempera- ture of the filler-metal layer, depending on com- position and thermal cycle.

Examples include nickel, cupronickel, or Monel joined with pure copper filler metal; enough dissolution and diffusion occur so that the solidus of the copper filler metal is increased and flow is terminated. This also means that the remelt temperature of the filler-metal layer is higher than its original solidus temperature.

In brazing of ferrous-base high-temperature