Burlington, Ontario, August 17, 2015 - A new column, written by vacuum heat treating expert Dan Herring “The Heat Treat Doctor”® in collaboration with Edwards Vacuum will be published monthly and offer helpful tips and techniques on vacuum pumping systems to a worldwide audience of individuals using all types of vacuum equipment. The first article will appear in the September 15th “What’s Hot!”. This Vac-Aero exclusive publication launched back in September 2007 and contains hundreds of archived articles can now be found in the “Resources” section of the VAC AERO website where anyone can sign up for the monthly newsletter. Keeping pace with the development of new vacuum processing technologies is the reason VAC AERO takes pride in its continued support of the metal treating community by helping to promote, collaborate and assist others in their search for solutions on important issues in the practice and application of vacuum processing.
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.
303 stainless steel is a machinable grade of 304-stainless steel. As mentioned in my earlier article (about 321-stainless), austenitic stainless steels are essentially iron-based alloys with at least 10.5% (or more) chromium added to it, as well as from 8-12% nickel, have inherent corrosion resistance, are usually very brazeable, are generally non-magnetic, and do not require (or effectively respond to) subsequent heat-treatment after brazing. They are primarily used in the “annealed” (soft) condition in end-use service. 303-Stainless - This machinable version of 304-stainless is generally available in either of two chemistries, standard 303, or 303Se. The use of 303Se has apparently decreased significantly over the years, but it is still available. The standard grade of 303 contains a minimum of 0.15-percent sulfur added to its chemistry, the sulfur being added for machinability purposes.
ASTM Test Method E 112 says it covers test methods to determine the average grain size of specimens with a uni-modal distribution of grain areas, diameters or intercept lengths. It says that these distributions are approximately log-normal. But, it does not describe how one can determine if their specimen’s grain size distribution is a uni-modal normal (Gaussian) distribution. ASTM E 1181, Standard Test Methods for Characterizing Duplex Grain Sizes, says it covers test methods to characterize grain size in products with any other distribution (other than a “single log-normal distribution of grain sizes”). But, the only example given in Appendix X2 shows the percentage of the number of intercept measurements in 38 length classes from 0 to 1 to 37 to 38 mm. Thirty eight classes is far too many to properly reveal the grain size distribution. This procedure reveals a log-normal distribution but it is not in terms of ASTM grain size numbers, which makes it less useful.
Selecting the correct vacuum gauge or gauges is critical to the success of a heat treatment process. It is important to know how they work and what options are available so that the correct choice can be made. There are several important considerations when using a vacuum gauge. They include the method of operation, the gas composition (inert or reactive, corrosive), the gas sensitivity (calibration factor), and the process being performed in your system. Given the wide range of pressures encountered when running processes in vacuum furnaces (a staggering 9 orders of magnitude), no one gauge is adequate over the entire range of possible vacuum levels. As with vacuum pumps, multiple gauges are necessary to properly cover the entire operating range with the needed precision and accuracy. Given that it is critical to monitor the vacuum pressure at various points in the process and perhaps multiple locations throughout the vacuum system, the correct selection of each gauge ensures that we achieve optimal results.
Constructed of the finest materials and craftsmanship, VAC AERO’s high performance vacuum furnaces are operator friendly and designed to minimize maintenance and downtime to deliver outstanding quality and value to commercial and in-house heat treaters alike. VAC AERO’s vacuum furnaces are designed for rapid heating rates to very uniform temperatures at high vacuum levels and can be customized to suit unique applications such as high pressure gas quenching, high temperature heat treating, ultra-clean processing and more. VAC AERO’s high efficiency hot zones are designed for easy maintenance and reduced energy consumption. Our external quench system allows for easy maintenance of the heat exchanger and quench motor. A high efficiency blower and motor combine fast cycle times and quenching speeds to provide uniform gas distribution and superior cooling performance from processing temperatures at pressures of up to 10 bar.