Getting to the “Hearth” of It: 5 Hearth Tips
Want a free tip? Read some of the top 101 Heat Treat Tips that heat treating professionals submitted over the last few years. These handy technical words of wisdom will keep your furnaces in optimum operation and keep you in compliance. If you want more, search for "101 heat treat tips" on the website! This selection features 5 tips all about the hearth of your furnace!
Also, check out Heat Treat Resources in the September 2021 magazine to check it out yourself!
Hacksaw Your Hearth!
When loading parts, carefully place the workload on the center of the hearth (front-to-back and side-to-side). Make sure it is stable and no part of the load is close to or touching the heating elements. This can create arcing and damage your parts.
Tip: Once the load is in place, mark the hearth posts with a hacksaw to quickly find the front and back measurements each time.
(Ipsen USA)
TZM Moly Grids
A very commonly observed failure mechanism with a moly post hearth assembly is bending of the moly posts. They will stay fairly straight at the center of the hearth area, but they can distort badly toward the outer sides of the work zone. The outer rows of vertical posts end up leaning away from each other. This is due to the very high linear thermal expansion coefficient of nickel-iron alloy grids (usually 330 SS or Inconel). With a high load on the nickel alloy grid, it is not able to slide on the perpendicular hearth beams as the temperature rises. The outer hearth post rows are forced in an outward direction. The quenching of the furnace load does not reverse all of this effect and over time results in the severe bending of the hearth posts.
By replacing the stainless steel or nickel alloy grids with a moly or TZM alloy moly grid, which exhibits very low thermal expansion, the hearth life can be increased. For comparison, the figure shows the coefficients of linear thermal expansion for commonly used grid materials. For example, a 36” wide 330 SS grid at 70°F grows to 36.6” wide at 2200°F.
Another significant benefit of TZM moly grids is use at higher furnace process temperatures without the problem of a softened, sagging grid that cannot support the load properly.
(Grammer Vacuum Technologies, Inc.)
How to make thru-process temperature monitoring robot friendly!
In modern rotary hearth furnaces, temperature profiling using trailing thermocouples is impossible as the cables would wind up in the furnace transfer mechanism.
Due to the central robot loading and unloading and elimination of charging racks/baskets the use of a conventional thru-process system would also be a challenge.
Faced with such loading restrictions it is necessary to fit the thermal barrier inside the cavity of the product (engine block shown) and allow automated loading of the complete combined monitoring system and product.
To allow miniaturization of the thermal barrier to fit, but also provide sufficient thermal protection, the use of phased evaporation technology is critical. Such a system allowed BSN Thermoprozesstechnik GmbH in Germany to commission such a furnace accurately and efficiently and thereby optimize settings to not only achieve product quality but ensure energy efficient, cost effective production.
(PhoenixTM)
Hearth Height Adjustment
The available width and height of the work zone in a vacuum furnace with a round hot zone is determined by the elevation placement of the top of the furnace hearth. This distance is determined by the length of the vertical hearth support posts. By having spare, interchangeable hearth post sets of varying lengths, one can extend the work zone width or height as needed. The figure shows a variety of work zone dimensions that are possible with a standard 36” wide x 36” tall typical work zone as an example. The important thing in choosing your work zone shape is to maintain an (approximately) 3” clearance between the elements and the work zone to avoid part to element contact.
Note: With the symmetric shapes of modern, round hot zones there is good reason to expect good temperature uniformity anywhere within the 3” clearance ring shown in Figure 1. If you can build a survey fixture capable of surveying all the space you want to use, you theoretically could use more than just the rectangular space shown in the examples. Getting an auditor to accept the survey is a separate task.
(Grammer Vacuum Technologies, Inc.)
TZM Moly Hearths
In the case of furnaces with all-molybdenum hearths or of graphite hearths with molybdenum (“moly”) support posts, a direct replacement of those moly posts with TZM alloy moly posts will both increase strength of the hearth assembly and eliminate problems with recrystallization-induced embrittlement of the posts. (For an all-moly hearth, replacement of the horizontal load beams with TZM would have a similar benefit.) The comparative strengths vs. temperature of TZM alloy and pure moly are shown in the graph. Whereas at room temperature the strengths are very similar (around 110KSI-120KSI), once you exceed the 2000F recrystallization temperature of pure moly, the difference becomes dramatic. At 2000F the pure moly is about 40% of the strength of TZM alloy. By the time it reaches 2300F the pure moly is only about 25% of the strength of TZM alloy.
Not only is the TZM alloy much stronger than pure moly at temperature, but it also does not suffer from the same embrittlement problems. Pure moly, once it has recrystallized, forms very brittle grain boundaries in its microstructure. Its behavior begins to resemble that of glass. This is the primary mode of failure of moly components in vacuum furnaces – breakage due to intermetallic grain boundary embrittlement. TZM’s recrystallization temperature is around 2500F, and even when it does recrystallize, it forms very fine new grains that still have decent ductility. Hence, we recommend TZM alloy as a replacement for pure moly in all structural applications for vacuum furnaces. It is the “right stuff.”
Note that all metals used in a vacuum furnace, moly and TZM alloy included, will suffer from distortion due to the numerous thermal cycles they experience. Moly hearth beams are a good example. Once distorted moly hearth beams can be very difficult if not impossible to straighten without breaking them. To have any chance at all they must be heated to forging temperatures. TZM hearth beams however, due to their good ductility can often be heated to forging temperatures and successfully straightened. Most heat treating shops scrap out the moly hearth beams rather than even trying to straighten and re-use them. With a TZM hearth the hearth components can typically be re-used with a newly re-lined hot zone saving a large additional expense.
(Grammer Vacuum Technologies, Inc.)
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