Dimensional Changes After Heat Treatment
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
Selecting the Right Vacuum Furnace for the Job
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 and Fixing Vacuum Leaks
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
Backstreaming
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
Tips For Improving Vacuum Performance & Operation, Part Seven
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
Tips For Improving Vacuum Performance & Operation, Part Six
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
Tips For Improving Vacuum Performance & Operation, Part Five
We continue our discussion of ways to improve vacuum performance by understanding how to maximize the operation of our vacuum systems. Tip #5: Diffusion Bonding
Diffusion bonding is a solid-state joining process involving both vacancy and interstitial diffusion of atoms between surfaces in intimate contact with one another. The process is capable of intentionally bonding together a wide range of metal and/or ceramic part combinations, forming strong metallurgical bonds. Applications include hot compacting of oxides, nitrides, borides, carbides, sulfides and their mixtures to near theoretical densities as well as sintering ceramics and powder metals. By Dan Herring
