(Table courtesy of the American Welding Society from their AWS Brazing Handbook, fourth edition, 1991)
Today’s brazing technology is based on a strong foundation of the brazing experiences of many people around the world over a period of many decades (even centuries). I’ve now been very active in the brazing world for almost 50-years, and, like my predecessors in the world of brazing, I’ve learned a lot about this fascinating joining process (and I’m still learning). In this article, I’d like to share with you one of my brazing experiences from many years back, one that involved high-temperature differential-expansion between an 18” (45 cm) diameter tool steel die and a thin carbide plate (round disc) that needed to be brazed to the die’s front surface for wear-protection.
When trying to braze together materials that have widely different Coefficients Of Thermal Expansions (COE’s), the material with the higher expansion rate (COE) will grow faster than the other when heated and shrink (contract) faster when cooled down from the brazing temperature. Once the two different materials have been brazed together and cooling begins, the shrinkage-rate differences between those two materials can produce significant shear stresses at the brazed interface between them. These stresses can, in some cases, be so strong that the thin brazed joint may be torn apart at either interface, or it might cause fracturing of either base metal, or perhaps open up a large crack through the BFM layer itself. It was this extreme — and very real — differential expansion problem that we had to address in today’s “history lesson”.
Shown in Fig. 1 is a photo of one of today’s modern plastic extrusion pelletizing dies, in which plastic is continuously extruded through the many small holes in the face of that small diameter die and cut into small pellets by a constantly rotating sharp cutter.
In the early 1980’s I was working on a brazing project for a client, a pelletizing company in New York State who made specialized high-temp extrusion dies with a unique rotating cut-off knife system. The overall process and unique application were confidential, so I’ll speak of it in general terms, but the brazing principles involved in making this “pelletizing” die assembly can be shared.
The extrusion die itself was a large diameter (18-inches/45 cm) tool-steel die, with many holes in its front face through which the end-use material was continuously extruded and cut off at a required length, all this being done at a high rate of speed. The rapidly rotating cutter, cutting the extruded product into about 15-in (40 cm) lengths, had a very sharp carbide cutting edge, which could quickly wear away the face of the tool steel die if that face were not protected by a very hard, wear-resistant surface. In our discussions with the client, we agreed to braze a large carbide disc onto the front of the tool steel die, which would significantly reduce the wear on the die surface.
In this particular case, the operating temp for this extrusion die was going to be high enough in temperature that the use of standard copper sheets between the carbide and die for the purpose of absorbing differential-expansion stresses between the die and the carbide could not be done, nor could low-temp silver-based brazing filler metals (BFMs) be used, even though the use of ductile pure copper sheets as a stress-reducing layer between the carbide and the die, and the use of silver-based BFMs to keep the brazing temps low, were both commonly being used for making cutting-tools at that time (still widely used today). Since neither of these low-temp materials could effectively be used in this high-temp die application, we had to look at higher temp shock-absorber material (such as pure nickel) and high-temp BFMs (such as nickel-based BFMs).
The final product was made via high-temp nickel brazing in a vacuum furnace, using a pure nickel sheet (Nickel 200) as the “shock-absorber” to absorb the differential-expansion stresses we knew would be present in this application. This is illustrated in Fig. 2.
Which Base-Metal Should I Use for Braze Fixturing so That It Will Last the Longest?
This question is not an uncommon one. Although I have never personally seen any kind of chart showing an “expected life” for fixture materials, it is important that people understand that there are a number of factors that will control the “life expectancy” of any fixturing material used in brazing, and all of these factors relate to the service conditions that the fixtures will encounter during the brazing process.
Whereas austenitic stainless steel fixtures may be fine for applications involving low to moderate temperatures, light loading, and slow heating and cooling situations, the use of high-temp super-alloys, such as Inconels, Hastelloy materials, and even ceramics, may be needed for the more aggressive brazing conditions. Shown below is a table listing some commonly used fixturing materials, and how they compare to each other relative to elevated temperature strength and thermal shock resistance, using 304 stainless as the starting “standard”. As can be seen, the materials that can handle these situations better also have a higher price-tag associated with that fixture material (notice the exception with graphite).
But, estimating an expected “life” (in months or years) of a fixture made from any of these materials would be very difficult, since the life of identical fixtures can vary extensively from brazing-shop to brazing-shop, depending on how they are used or abused. For instance, here’s five typical reasons (there are more) as to why it’s hard to put an “expected-life” on a brazing fixture: (1) shop abuse, (2) heating/cooling rates; (3) temperatures used; (4) loading conditions used, and then (5) atmosphere quality, among others:
1. Shop abuse is the phrase I use indicating how fixtures are stored and handled by various brazing shops. Some shops are quite careful, and stack the fixtures neatly on shelves, keep them indoors, and handle them carefully, whereas other shops may literally throw fixtures on the ground, store them outside their factory walls where they can be affected by the weather, and hammer them with large hammers to bend them back into shape, etc. Please note that the way the fixtures are handled and stored can dramatically affect fixture life.
2. Heating and cooling rates can significantly impact all fixture materials, since large temp-differentials can lead to significant distortion in the materials via thermal stresses. Thermally fatiguing the materials is very real in many shops, and will have a big effect on life of fixtures.
3. Brazing temperatures used will also affect life of fixtures, since the higher the temp of a furnace cycle, the greater will be the metal expansion factors, potential grain growth issues, diffusion of brazing filler metals (BFMs) into fixtures, temperature-differentials, distortion potential, etc.
4. Loading conditions relate to the pattern of placing components onto the fixtures for brazing. Evenly spaced, lighter-weight components will have a different effect than very heavy, massive parts crowded onto one side of a fixture, etc. What will the thermal effects be on the fixture in each case? Delta-T (temp-differentials) can be very significant when fixtures are unevenly loaded, thus leading to distortion (warpage), and subsequent early failure, etc.
5. Atmosphere quality. How much oxygen is present? Are any corrosive gases present? Is there moisture in the furnace atmosphere? Is significant outgassing occuring during heating? Each of these items can hurt the life of the fixture.
Normally, austenitic stainless is used for light-weight needs, and for when the furnace loading and cycles are at fairly low temp, evenly loaded, and atmosphere quality is very good (very little oxygen or corrosive gases present). For more potentially “abusive” situations (thermally and/or chemically), you would tend to move up to the “super-alloy” fixturing materials (such as Inconel or Hastelloy materials) that are much more thermally stable and corrosion resistant, or perhaps even to certain ceramic materials. But even that kind of a sequencing assumes careful handling, even loading, etc. Obviously, rough handling would make ceramic fixtures much more shortly-lived than even basic austenitic-stainless fixtures!
Thus, it is hard to predict the “expected life” of fixtures, per se. So much depends on the training of the workers doing the brazing in the shop, as well as the brazing cycles required.
In August’s article, I’ll address the commonly used method of adding a lot of “dead weight” onto parts in an attempt to keep them flat during brazing!