1. Shop abuse
2. Heating/cooling rates
3. Temperatures used
4. Loading conditions used
5. Atmosphere quality
These factors are detailed as follows:
1. Shop abuse is the phrase I use indicating how fixtures are stored and handled by the various shops I've visited. Some shops are quite careful and stack the fixtures neatly on shelves indoors and handle them carefully. Others literally throw them 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. Obviously, 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 temperature 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 the life of fixtures.
3. Temperatures used will also affect life of fixtures. The higher the temperature of a furnace cycle, the more the potential effect on expansions, potential grain-growth issues, diffusion of BFMs into fixtures, etc.
4. Loading conditions relate to the pattern of placing components onto the fixtures for brazing. Nicely spaced, light components will have a different effect than very heavy, massive parts crowed onto one side of a fixture and light parts loaded onto the other end of the same fixture. What will the thermal effects be on the fixture in that case? Delta-T (temperature differentials) can be very significant when fixtures are unevenly loaded, leading to distortion, warpage and subsequent early failure.
5. Atmosphere quality. How much oxygen is present? Are corrosive gases present? Is there moisture in the furnace atmosphere? Each of these items can hurt the life of the fixture.
Normally, austenitic stainless is used for lightweight needs. It is also used 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 abusive situations, you would tend to move up to the Inconel materials that are much more thermally stable and then to certain ceramics. But even that kind of sequencing assumes careful handling, even loading, etc. Obviously, rough handling would make ceramic fixtures much more short lived than even stainless fixtures.
Thus, it is hard to predict the life of fixtures. So much depends on the training of the people doing the brazing out in the shop.