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. Brazing methods used range from hand-held torches for brazing one part at a time, to highly sophisticated, automated equipment that can braze hundreds or thousands of parts on a continuous basis or all at one time in large batch furnaces.
Brazing technology and research is steadily moving forward, and many end-use applications of heat- and corrosion-resistant alloys (referred to as HCR alloys in this article) are directly benefiting from these advances. HCR alloys today are not only being brazed to a variety of different metals but also to ceramics for use in conditions of extreme temperatures and/or corrosion. One proprietary recent example of this requires operations exceeding 2700°F (1500°C) in steady service. Brazing indeed has a bright future.
What are HCR alloys?
When referring to HCR alloys, what kind of metals are we talking about? The field is large and includes standard stainless steels, Inconels, Hastelloys, Rene and Haynes alloys, and titanium alloys (among others). The key consideration here is that each of these alloys is used in end-use applications where they must be tolerant of and strongly resistant to heat and/or chemical corrosion over extended periods of time.
To achieve this, the chemistries of these HCR alloys usually contain significant amounts of nickel and/or chromium. They often contain additions of molybdenum, tungsten, niobium, titanium and/or aluminum, and others. A few typical HCR-alloy chemistries are shown in Table 1.
It must be noted that any brazing filler metal (BFM) chosen to join an HCR alloy must not only be chemically compatible with that HCR alloy, but it must also be able to bond to and diffuse into that alloy and then be capable of handling the same end-use conditions to which the HCR alloy is exposed. It also needs to be understood that properly made brazed joints will be as strong as, or stronger than, the HCR alloys themselves. That means that if a component were to fail in service for any reason, the failure should occur in the HCR-metal itself and not in the brazed joint. Is this a tall order? Perhaps not.
For the successful brazing of HCR alloys, the principles of brazing must be thoroughly understood and conscientiously applied. If not, disaster awaits. I have seen too many examples, in too many industries, of half-hearted attempts at following the required steps for proper brazing that resulted in poor-quality brazements and premature failure of brazed assemblies out in the field.
Achieving High-Quality Brazing
Successful brazing is not difficult to achieve. There are a number of steps needed to achieve high-quality brazing of HCR alloys. The two most important steps are proper joint design and cleanliness of joint surfaces.
We will look at this in detail in part 2.