Brazing Heat- and Corrosion-Resistant Alloys for Robust Service (part 2)
This article was originally published on September 2, 2014.
In part 1 (June), corrosion-resistant alloys were defined and techniques for obtaining the best brazing joints were discussed. Part 2 addresses issues critical to these steels and helps readers to select the correct brazing filler metal (BFM).
Brazing is a highly versatile and cost-effective high-temperature method to permanently join a wide range of heat- and corrosion-resistant alloys for service in such diverse industries as automotive, aerospace, medical, electrical, hand-tools, cutlery and food-handling.
When brazing stainless steel assemblies, such as those made from 304 or 316 stainless, it is very important for the brazing shop to only specify the “L” (low-carbon) versions of these alloys for brazing (or welding). A common mistake made by many shops is to only specify, and braze, the standard versions of these alloys. That is an incorrect choice and has resulted in premature corrosive failures of stainless components in the field. As shown in Table 1, the “L” version of 304, for example, typically contains only about one-third (or less) of the amount of carbon as does the standard grade.
During brazing processes, the stainless steel can easily dwell long enough between 800-1500?F (425-815?C) – known as the “sensitization” range – for carbon in the stainless to quickly react with the chromium in the metal to form chromium carbides. This depletes those regions of the chromium-oxide protective layer, resulting in oxidation/corrosion of that depleted region. This often exhibits itself as a rust-band on each side of a weldment or as a generalized oxidation/corrosion on furnace-brazed components. Be sure your purchasing personnel are made aware of this, and understand the need to only buy the “L” version of these alloys.
Some designers specify 321 stainless for brazing, and I strongly urge caution in so doing. Refer once again to Fig. 4 in part 1 (metal/metal-oxide curves) for the titanium-oxide curve on the far right side. 321 stainless uses excess titanium additions (a minimum of 5x the carbon in the alloy) so that the titanium will react with the carbon to prevent the carbon from depleting the chromium oxides.
Once the carbon has been “tied-up” so to speak, what happens with the rest of the excess Ti present in the stainless matrix? A number of brazing shops report that the excess titanium then reacts directly with any oxygen present in the furnace atmosphere, forming a dark gray/brown surface discoloration that is then difficult to braze. For this reason, I do NOT recommend specifying 321 stainless for any brazing applications. Instead specify 304L, 316L or stabilized 347 stainless, which uses niobium instead of titanium in its matrix and is therefore preferable to 321.
Which BFM should I use?
This is a commonly asked question, and it has many possible answers. The most metallurgically sound response is to use the BFM that as much as possible matches the chemistry and corrosion resistance of the HCR alloy being joined.
Let’s look at copper BFM. Unfortunately, the heat- and corrosion-resistance characteristics of copper prevent its use in many brazing applications for HCR alloys, but a very important exception stands out – copper BFM in the automotive industry. For many years the automotive industry has preferred pure-copper BFM (BCu-1 and BCu-1a) for brazing carbon-steel and 304L stainless components, which are used to carry/transmit fuel and for oil-cooler assemblies. From fuel-sender components inside the fuel tanks to the fuel rails injecting fuel into the engine cylinders, copper-brazed stainless components are very common today in automotive fuel-transmission lines.
Figure 5 shows a portion of a new, high-pressure fuel-rail assembly. Higher pressures and higher temperatures are keys to better fuel economy. Even so, copper is the preferred BFM. According to experts in the automotive industry, the temperatures and corrosive conditions in field use are such that pure copper is still quite acceptable for use as a BFM for joining their HCR alloys. One of the keys for this success is the need to keep the brazed-joint clearances as tight as possible to minimize the size of the exposed edges of any copper-brazed joint.
Silver brazing has been used for many years to join a wide variety of stainless steel components in the medical, food-handling and electrical industries and even in some aerospace applications. Temperature limitations must be observed, however, so as not to remelt any of the silver-based BFMs used in those parts. Thus, the use of silver-based BFMs is usually limited to applications that do not see any significant temperature excursions.
Gold-based BFMs are still used to braze a number of HCR alloys for aerospace components due to their ease of use, corrosion/oxidation resistance, forgiving nature when joint gap fit-ups are not ideal, and higher-temperature capabilities (as compared to any silver-based BFM). These benefits are obviously offset by the extremely high cost of such BFMs.
Figure 6 shows an example of an Inconel 625 JT9D fuel nozzle that is gold-brazed using the BAu-4 BFM. Three threaded connectors on the right and the nozzle tip on left were brazed into an Inconel 625 forged nozzle used in the combustion chamber of a JT9D jet engine. Fuel flows through the center fitting of the three on the right side and compressed air flows through the two outer fittings.
Nickel-based BFMs exhibit some of the finest characteristics for joining HCR alloys used at elevated temperatures in service (sometimes exceeding 2000?F/1100?C). These BFMs have chemistries that often closely match those of the HCR alloys they are joining, and their remelt temperatures (when properly brazed) often exceed the initial brazing temperature used by several hundred degrees. It is not uncommon to have an aerospace component brazed at about 2000?F (1100?C) and see service temperatures in excess of 2100?F (1150?C) without remelting. Some of the boron-containing nickel-based BFMs, when brazed such that isothermal solidification occurs during brazing, are capable of handling temperature over-runs significantly higher than their original brazing temperature. This is rarely possible to achieve with any other type of BFM (in the author’s experience).
Cobalt-based BFMs (BCo-1) can be used on HCR alloys where nickel cannot be used because of health concerns, such as in dental braces and similar mouth appliances. Some governments do not allow nickel-based BFMs to be used in such applications, and BCo-1 is a suitable alternative. Cobalt has shown itself to be less erosive than nickel in a number of high-temperature brazing applications, particularly where HCR alloys are used as thin sheet metal.
HCR Alloys Used in Honeycomb Brazing
Brazed metallic honeycomb structures are used in a wide number of high-temperature aerospace applications, with a number of HCR alloys having a long history of success in such usage, including Hastelloy, 321 stainless, Inconels and several titanium alloys.
One unique application developed in the 21st century is a special honeycomb manufacturing process in which a very thin strip of BNi-2 nickel-based BFM foil is embedded into the honeycomb structure as the honeycomb is being formed. This is clearly seen in Fig. 6, where the close-up of the honeycomb shows the BNi-2 foil within the honeycomb structure. This specialized product was designed to provide the exact amount of BFM needed to effectively bond the honeycomb to its substrate but with no excess BFM material to cause any erosion of the HCR honeycomb structure.
Titanium has been an HCR alloy of choice for aerospace applications for many years. Its light weight and high strength relative to steels and Inconels is a major advantage. Consequently, it has found major use in aerospace honeycomb structures as well.
However, titanium’s strong reaction with oxygen at elevated temperatures requires that brazing be done only in vacuum furnaces. Further, only vacuum furnaces in which the brazing hot zone is extremely clean and leak-tight should be used. As mentioned earlier, electrolytic-nickel plating is often required on faying surfaces of brazed joints so that the molten BFM can adequately flow and create a strong metallurgical bond.
Figure 8 depicts an example of a titanium-brazed honeycomb structure for use in commercial jet engines on many of the Airbus A340 and A380 aircraft today. The entire exhaust system is constructed from titanium (grade 21). Perforated sheets on the right side of the honeycomb structure in the photograph allow for excellent acoustic-attenuation (noise-abatement) characteristics for this material. The entire assembly handles exhaust gases up to about 1200?F (650?C).
Strength of Brazed Joints in HCR Alloys
This topic is critically important, since I still hear too many people saying, “Sure, brazing is OK, but if you really want a joint to be strong, you should weld it!” I can understand this statement if designers do not follow the guidelines I’ve mentioned earlier in this article and if brazing shops fail to properly clean, fit up and braze the components.
When properly done, brazing will be every bit as strong as a weld, and, as mentioned earlier in this article, any failure of the part should be in the HCR alloy itself and not in the brazed joint. An early, unique and very important milestone in jet-engine brazing occurred in 1950-51. Engineers at Curtiss-Wright Corporation brazed together 431 stainless compressor rotor wheels consisting of a series of cast 431 stainless steel blades that were brazed into a 431 stainless hub (Fig. 9a). Brazing was done using a nickel-based BFM (later known as BNi-1). This was part of a program to develop jet engines utilizing a significant number of brazed components in the rotating members inside the engine.
Whereas the blades normally spun at about 12,000 rpm, a special “whirl-test” evaluation was conducted to see what would happen to the brazed joints when they were sped to destruction. In a special underground chamber the engine was tested at steadily increasing rotor speeds until, finally, the engine destructed at 41,800 rpm.
When all the broken engine pieces were finally put back together and evaluated (Fig. 9b), it was found that not one brazed joint failed! The 431 stainless itself broke apart due to tangential (centrifugal) stresses that exceeded the ultimate tensile strength of the 431 stainless.
Yes, brazed joints, when properly made, can indeed be “stronger” than the HCR base metals being joined.
New Developments in Joining HCR Alloys to Ceramics
A lot of work is currently under way (much of it still proprietary) in developing high-temperature applications requiring the use of ceramics joined to HCR alloys. One example recently developed is shown in Fig. 10. It involves Inconel 625 bonded to a 3-inch-diameter (75-mm) alumina-ceramic filter for heated gases operating at temperatures up to about 1100?F (600?C). CTE problems between the Inconel and the alumina were high. Ceramic rings were built to keep the ceramic window from warping. The ceramic ring with the window was brazed together and then brazed into the Inconel ring. This required special tooling to keep the thermal mismatch from cracking the ceramic, and it was brazed with an “active” silver-based BFM at 1600?F (870?C) in vacuum.
Another current example of such ceramic joining is shown in Fig. 11, in which a Haynes 230 HCR alloy is joined to alumina for high-temperature insulator applications in diesel or jet engines. Bonding was done at 1925?F (1050?C) in a vacuum brazing furnace using a special gold-based BFM. IH