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Discolored Work in a Vacuum Furnace – The Heat Treat Community Answers the Clarion Call

June 15, 2016 by Dan Herring

Discolored Work in a Vacuum Furnace – The Heat Treat Community Answers the Clarion Call

Question: Staining of titanium parts after vacuum heat treatment following a gas quench, your thoughts on possible causes and remedies? 

In response to this question, phenomenal suggestions by everyone! A wealth of great information here. So, what else could be happening? Let The Doctor add a few thoughts to the discussion. First, the fact that the discoloration (staining) is brown in coloration suggests that the oxide is forming on the part surface during cooling when the temperature is in the range of (approximate) 245ºC – 270ºC (475ºF – 520ºF). This is supported by the fact that the oxidation does not occur “during natural cooling” (which we assume to mean cooling under vacuum). Second, the fact that the discoloration is more evident at the bottom of the load suggests the phenomenon is (gas exposure) time dependent, that is, the longer the parts take to cool through the critical range, the greater the chance for discoloration. Third, a “steel-copper-stainless steel” test will be helpful in isolating if it is a water or air leak. Leaks in heat exchangers have been known to “open up” during the cooling cycle when exposed to hot gases and close up again at room temperature. The writer has personally experienced this – the solution being the replacement of the old heat exchanger at which time the problem went away. Fifth, look in all the places suggested by those who responded, but remember to make only one change at a time and evaluate its impact in order to find then correct the problem. Finally, as an unabashed promotion of my books on Vacuum Heat Treatment (Volume II of which comes out this fall), there are a number of sections that discuss this very issue in considerable detail covering subjects such as “Vacuum Furnace Contamination and Cleanup Cycles”, “Leaks External to the Vacuum Furnace Proper” and “Factors Affecting Performance: Discolored Work” to name a few.

Vacuum Diffusion Bonding – by Design

November 9, 2015 by Dan Herring

Vacuum Diffusion Bonding – by Design
Many of us who use vacuum furnaces are all too familiar with and have learned how to counteract the unintentional diffusion bonding that has been known to occur between component parts exposed to high temperatures and low vacuum levels.

By contrast, vacuum technology that has found an important niche is that of diffusion bonding by design2-6. Vacuum diffusion bonding relies on temperature, pressure, time, and (ultra low) vacuum levels to facilitate atomic exchange across the interface between the materials. The process will work on similar or dissimilar materials so long as they are in intimate contact with one another. Vacuum diffusion bonding can be performed with or without pressure being applied and with or without the assistance of a short-lived low melting point “filler metal” (i.e. “activation layers or interlayer”) to facilitate the joining process.

Evaporation

September 7, 2015 by Dan Herring

Evaporation

When performing any type of vacuum heat treatment it is always important to be aware of the possibility of evaporation and/or sublimation of elements, which can be present in the material being processed, introduced into the vacuum system with the workload, inherent in the equipment design or introduced during maintenance, repair or rebuilds. In cases where evaporation may be a concern, the vaporization rate is of prime importance and is directly related to the furnace pressure (the higher the pressure, the more frequent the collision of gas molecules and correspondingly, the few metal atoms leave the metal’s surface).

What is Evaporation? Vaporization is the process that occurs when a chemical or element is converted from a liquid (or a solid) to a gas. When a liquid is converted to a gas, the process is called evaporation or boiling; when a solid is converted to a gas, the process is called sublimation. The pressure exerted by the vapor of a liquid in a confined space is called its vapor pressure. As the temperature increases so too does its vapor pressure. Conversely, the vapor pressure decreases as the temperature decreases.

Vacuum Deposition Processes

August 10, 2015 by Dan Herring

Vacuum Deposition Processes

Vacuum deposition is a generic term used to describe a type of surface engineering treatment used to deposit layers of material onto a substrate. The types of coatings include metals (e.g., cadmium, chromium, copper, nickel, titanium) and nonmetals (e.g., ceramic matrix composites of carbon/carbon, carbon/silicon carbide, etc.), deposited in thin layers (i.e. atom by atom or molecule by molecule) on the surface.

Vapor deposition technologies include processes that put materials into a vapor state via condensation, chemical reaction, or conversion. When the vapor phase is produced by condensation from a liquid or solid source, the process is called physical vapor deposition (PVD). When produced from a chemical reaction, the process is known as chemical vapor deposition (CVD). These processes are typically conducted in a vacuum environment with or without the use of plasma (i.e., ionized gas from which particles can be extracted), which adds kinetic energy to the surface (rather than thermal energy) and allows for reduced processing temperature.

A Layman’s Guide to Understanding The Theory of Gases

July 9, 2014 by Dan Herring

A Layman’s Guide to Understanding The Theory of Gases

The movement of gases is an important and interesting subject but one often dismissed as a topic best left to scientists. However, the Heat Treater needs to know something about the basic nature (theory) of gases and in particular how they behave in vacuum. The main difficulty is that too much theory tends to become a distraction. Our focus here will be to better understand what goes on inside a vacuum furnace.

One definition of a gas is that it is simply a collection of molecules in constant motion (Fig. 2). The higher the temperature, the faster these molecules move, and as one might expect, the motion of gas molecules stops or dramatically slows down at or near absolute zero (0°K). As molecules speed up with an increase in temperature, there is an increase in their kinetic energy (or energy of motion). Molecular collisions occur between molecules and if contained, these molecular collisions against the walls of their container result in a pressure rise (which always occurs in a closed container when a gas is heated). In other words, pressure is simply the force per unit area that a gas exerts on the walls of its container.

Case Study: The Benefits of High Pressure Gas Quenching in Dimensional Control

June 12, 2014 by Dan Herring

Case Study: The Benefits of High Pressure Gas Quenching in Dimensional Control

Highly distortion prone gearing (Fig. 1) was the subject of an investigation into the dimensional changes which result from utilizing either oil or high pressure gas quenching following a low pressure vacuum carburizing process. For comparative purposes, the gears in question were also atmosphere gas carburized and plug quenched, which is standard practice for these geometries. Full production loads (Fig. 2) were run using two (2) different carburizing methods (atmosphere, vacuum) in combination with free quenching in either oil at 75°C (165°F) or high pressure gas (nitrogen) at 11 bar.

Gears were taken from multiple locations throughout each load for analysis. Parts for metallurgical evaluation were selected from the center of each load. Multiple areas on each part were then analyzed for microstructure, case depth, and hardness (surface, profile, core). Dimensional checks (out of round, gear tooth profiles) were conducted on the gears before and after heat treatment. For brevity, only a portion of the complete test program is presented here (see Reference 4 for more detail).

A Vacuum Heat Treater’s Library

May 14, 2014 by Dan Herring

A Vacuum Heat Treater’s Library

Over the years, many people have asked if we could recommend good books on the subject of Vacuum Heat Treatment. The following list includes books that we have found particularly useful  with respect to the scientific and practical aspects of vacuum, heat treatment, metallurgy and material science. Enjoy..

As you can tell from the list, some books are classics which have stood the test of time, some are relative newcomers, but all share the common trait that they are used each and every day by those of us who work in the fields of vacuum, heat treatment and metallurgy. The readers are encouraged to offer suggestions as to their favorite and most useful texts.

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