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 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. Notice in Table 1 that the other grades of austenitic stainless steels all are limited to no more than 0.030 sulfur maximum, which means that regular 303 stainless contains a minimum of six (6) times the usual amount of sulfur that is contained in all the other types of austenitic stainless steel. Remember, that’s a minimum amount; it can be higher! by Dan Kay

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. By George Vander Voort

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. By Dan Herring

We continue our discussion of vacuum gauges by focusing on the types of gauges in use throughout the heat treatment industry including their function and application.

Types of gauges. There are three common types of gauges used in heat treatment, namely mechanical, thermal conductivity, and ionization gauges. Each is unique and well suited for its intended purpose. Mechanical gauges. The use of mechanical displacement to indicate pressure is considered a type of “direct acting” gauge. Examples are Bourdon gauges, U-tube and capacitance diaphragm manometers. They measure the force per unit area utilizing Dalton’s law: “Total pressure of a mixture of gases equals the sum of the partial pressures of each gas”. Direct acting gauges are not subject to the issue of gas sensitivity and do not need to be calibrated for the gas being measured. Their sensitivity is such that theyare not used in the high vacuum range (below ~10-4 mbar). By Dan Herring

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. By Dan Herring 

Burlington, Ontario, October 19, 2015 – A VAV6648 HV-2 bottom loading vacuum furnace with a work chamber of 66″ diameter x 48” high has been shipped to an international Aerospace company. The furnace includes a high vacuum pumping system and a 2-bar argon quench system designed to incorporate unique performance features; the cooling nozzle configuration and location have been engineered to improve gas penetration into the workload, gas velocity and cooling speed. The energy-efficient graphite hot zone lightweight design and low thermal mass enables faster quenching and longer life. The heavy duty hearth is constructed with quickly removable rails of pure molybdenum capable of handling distributed loads of up to 4400 lbs. The furnace operating system is based on VAC AERO’s versatile Honeywell HC900 interactive hybrid control package with SCADA and complete network integration capabilities and remote monitoring and control.

When designing a vacuum system it is important to take into account the system conductance. What is Conductance and Why Does it Matter? Conductance is the characteristic of a vacuum component or system to readily allow the flow of gas and can be thought of as the inverse of resistance to flow. It must be closely considered when designing a vacuum system and selecting the pump and other components, otherwise your vacuum chamber will take too long to reach the pressure required.

Well-designed piping of vacuum equipment, as well as proper component selection, increases production efficiency by minimizing the vacuum pumping time. It also minimizes energy use, making your equipment less expensive to operate. Ignoring the principal of conductance and designing the system with only physical configuration and flow rates in mind, can cause delayed equipment startup, plant downtime and process inefficiency because if a problem is found after startup, it can take considerable time and money to correct. By Dan Herring

One of the most critical components on any vacuum furnace is the pressure relief value. While its function is clear, the fact that it needs to be inspected – and tested is either not well understood or simply ignored. Normally positioned atop a vacuum furnace it is in an area that is not always conducive to maintenance, and complicated in many instances by the fact that only the manufacturer can service them. 

What is a Pressure Relief Valve? A pressure relief valve is a safety device designed to protect a vacuum furnace from over-pressurization. An overpressure event refers to any condition that would cause the pressure to increase beyond the specified design pressure (the so-called maximum allowable working pressure or MAWP). The pressure relief valve is an integral part of the safety system provided on most vacuum furnaces. Vacuum vessels, including evacuated chambers and associated piping pose a potential hazard to personnel and the equipment itself from collapse, rupture, or implosion. By Dan Herring

A number of companies who are currently using vacuum-furnaces for many of their brazing processes are also using induction-brazing equipment to join some of their other production parts. Let’s take a brief look at the induction-brazing process to see what it is, and how it can be effectively used by brazing shops today to meet some of their production needs.

This article is written without a lot of complex language in order to make this process as simple and easy to understand as possible, and to therefore encourage people to use it more. For a deeper, more thorough engineering-study of the principles and theory of induction heating, the reader is referred to other technical books and articles on the subject. This current article will give you a good, basic understanding of induction brazing, and how to apply it in your brazing shops. by Dan Kay

For many years, ASTM E384 has stated that the load range for microindentation hardness testing with both Knoop and Vickers indenters is 1 to 1000 gf. But, it also states that tests that produce a Knoop long diagonal or a Vickers mean diagonal of < 20 gf should be avoided as the precision in measuring such small indents is poor. The standard recommends using loads no lower than 25 gf. This article shows that the Knoop test exhibits better measurement precision at loads of 200 gf and below because the long diagonal is 2.7 times greater in length than the Vickers mean diagonal length for the same specimen tested at identical loads. Knoop, however, does not produce constant hardness values as the load changes, which is a major problem. If one can use a 100X objective with a numerical aperture of 0.95 – while obtaining adequate image contrast – indents as small as 14.7 μm in length could be measured. But, the challenge is to obtain acceptable image contrast at 1000X magnification so that the indent tips can be clearly seen. Realistically, a minimum diagonal length of 20 μm is a better target. By George Vander Voort

Image Caption: Aluminum brass, Cu-22% Zn – 2% Al that was solution annealed at 850C (producing a nice coarse grain size – average of ASTM 3.3). The specimen was etched with Beraha’s PbS tint etched and viewed with Nomarski DIC to bring out the deformation around the Vickers indents. The hardness was 57 HV – very soft!  A 500 gf load was used.  And it was taken at 100X.