Vacuum furnaces come in all shapes and sizes but common to each is that they require some type of vacuum vessel. Most vacuum vessels in the heat treating industry are cylindrical in shape and either horizontal or vertical in orientation. In general, vacuum vessel designs are made as small as possible so long as they don’t hamper the process being run in the vessel. There are several obvious reasons for this including the cost of the vessel itself as well as the vacuum hardware involved. In addition pumping systems become larger and operating costs tend to be higher (e.g. more gas is required for quenching).

Designs for vacuum chambers involve two distinct aspects; one structural and one related to the process application. The basic structural design falls in the realm of mechanical engineering following established industry guidelines, an example of which is the ASME Pressure Vessel Code for vacuum furnaces utilizing high pressure gas quenching (above 14.7 psig). The design must take into account both the external and internal forces acting on the vessel to prevent buckling (i.e. structural instability) and deal with overpressure issues, both major concerns from a strength of materials standpoint. By Dan Herring

Low temperature vacuum heat treatment offers unique advantages to a variety of industries including Aerospace, Automotive, Electronics, Household Appliances, Machine Tools and Tool and Die as well as Commercial Heat Treaters who must serve all of these customer.

This article will discuss the key factors required to optimize processing parameters in low temperature heat treatments (e.g. tempering, stress relief) by focusing on a variety of topics. Low temperature processing can be batch or continuous, either as stand-alone units or “modules” incorporated into a continuous vacuum furnace system. The following is a basic description of the operation of a typical batch vacuum furnace. By Dan Herring

Vacuum furnaces can use a variety of different gases during the processing cycle in partial pressure operation, for backfilling to atmospheric pressure at the end of the processing cycle and for cooling/quenching. The most common of these gases (in order of frequency of use) are nitrogen, argon, hydrogen and helium. Other common gases include various hydrocarbons and ammonia (for vacuum carburizing/carbonitriding) and specialty gases such as neon (for certain electronics applications).

In vacuum heat treatment nitrogen is used primarily for cooling/quenching, as a partial pressure gas and for backfilling to atmospheric pressure at the end of the heat treating cycle. A common misconception, however, is that nitrogen gas is a true inert gas. It is not and under the wrong circumstances it can react with the surface of the material being heat treated with deleterious effects. By Dan Herring

One of the questions all Heat Treaters are asked is, “How much, if at all, will my part change (i.e. shrink or grow) during heat treatment?” While the heat treater would love to be able to give a precise answer to this question, in most situations volumetric size change during heat treatment cannot be accurately predicted, at least not accurately enough to allow for final machining and/or grinding to close tolerances prior to heat treatment.

Experimental work has been done on many materials to show the effects of heat treatment on size change. As one might expect, the effects are different for every material grade. For example, an 80 mm (3.15”) cube of D-2 tool steel (Fig. 2) reveals growth (0.08%) in one dimension and shrinkage in the other two dimensions as a result of vacuum hardening. This graph demonstrates how knowing the part orientation from the mill-supplied bar is important when trying to plan for size change during heat treatment. By Dan Herring, THE HERRING GROUP Inc., and Patrick McKenna, Nevada Heat Treating, Inc.

All of the common (and several uncommon) heat treatment processes can be run in vacuum, from annealing and brazing to sintering and tempering. Many companies that currently outsource vacuum heat-treating ask themselves if they would be better served by setting up this capability in-house. Others who already have an in-house heat treat department wonder if switching to vacuum processing will offer them a competitive edge. This article will help address these questions.

Vacuum furnaces are typically characterized by their method of loading, horizontal or vertical, as well as if there is internal load movement, being classified as either batch or continuous (i.e. multi-chamber) types. The various furnaces sizes, production capabilities and feature configurations are almost endless and detailed extensively elsewhere. Since most vacuum furnaces have a life expectancy of 40 – 50 years, decisions as to what to purchase, and from whom, become very important. By Dan Herring

Finding leaks in vacuum furnaces is a task that few of us cherish but all of us know is important and necessary. Leaks are a problem experienced by almost every vacuum user. Leaks can occur suddenly or develop slowly over time but in either case they are damaging both to product quality and to furnace internal components.

In extreme cases, the problem is obvious: the furnace will not pump down and/or the hot zone (or heating elements) shows obvious signs of attack. Tiny leaks, however, are more common often going undetected because of the pumping systems ability to overcome them. However, even small leaks can cause continuous and sometimes catastrophic damage. Thus, routine leak checking and leak repair should be a part of any good vacuum furnace maintenance program. By Dan Herring

In simplest terms, backstreaming is the movement of pumping fluid back into the vacuum furnace chamber, that is, oil vapor molecules attempt to reverse course and move up and back toward the vacuum vessel, opposite to the direction of the desired gas flow.

Backstreaming is not limited to the pumps themselves, but encompass the entire pumping system (e.g. plumbing, valves, baffles and traps). The oil type and characteristics play a role as well. In all cases, the result of backstreaming, namely the contamination of the work chamber or workload, is totally unacceptable and often catastrophic. Backstreaming is often due to: Incorrect start-up or shutdown procedures – the far most common operator mistake as far as the writer is concerned; Exceeding maximum throughput capacity for long periods of time; Exceeding the critical discharge pressure in the foreline. By Dan Herring

“It was only a tiny drop of water, now and then,” lamented the home owner. “How was I to know that all those little drops would add up to a huge water bill?” The same can be said of a heat treat furnace that is always down for this reason or that. Avoiding the hidden costs associated with equipment downtime is the key to saving money.

Maximizing furnace productivity requires a proactive approach, which must continue throughout a unit’s operational lifetime. This requires careful planning and anticipation of problems. The process should begin even before the purchase of a piece of equipment by matching equipment and supplier capabilities with production and process needs. Buying good, well-built, high-quality equipment and operating and maintaining it properly will avoid most hidden costs. By Daniel Herring

We continue our discussion of ways to improve vacuum performance by understanding how to maximize the operation of our vacuum systems.

Tip #7: Materials Selection for Grids, Baskets & Fixtures. General Design Considerations. To obtain the most cost effective design requires a thorough understanding of the service conditions under which the material will be exposed. Important considerations include: Normal operating (exposure) temperature as well as the maximum (and minimum) usage temperatures; Metallurgical stability over the expected duty/thermal cycle (period, frequency, and rate of heating/cooling); Thermal expansion characteristics; Fabrication (or casting) methods (with respect to development of thermal or chemical gradients in the material). By Dan Herring

We continue our discussion of ways to improve vacuum performance by understanding how to maximize the operation of our vacuum systems. Tip #6: Controlling Partial Pressure Additions.

Introducing a partial pressure gas into a vacuum furnace at a pressure in excess of the materials vapor pressure will help avoid significant evaporation or “boiling away” of elemental constituents in the materials being processed. Without this control, surface integrity and in some cases the chemical composition of metal (or filler metal) can be affected. By Dan Herring