Newsletter Mission Statement Keeping pace with the development of new technologies has always been one of VAC AERO’s main concerns. Given our role as a leader in vacuum heat treating and special processing, the aim of our Newsletter is (1) to facilitate discussion among our customers, technical and educational communities and suppliers on a variety […]
Since brazing plays an important part in your company’s products, plan to have your staff attend one of the high-powered, three-day seminars being held in 2019. Our Brazing Seminars cover all the essentials for successful brazing of commercial and aerospace components. The improvements to brazing operations that have resulted from these seminars have paid for the cost of the seminar many times over at many companies! Register your staff today! They WILL truly benefit from having attended this program!
Location and Dates for 2019:
April 9-11, 2019 Simsbury, Connecticut (CT)
May 14-16, 2019 Los Angeles, California (CA)
October 1-3, 2019 Spartanburg, South Carolina (SC)
November 12-14, 2019 Simsbury, Connecticut (CT)
To Register on the Kay & Associates’ Website CLICK HERE.
The type and reliability of instrumentation and process controls used on vacuum furnaces in the heat treatment industry is critical to both the performance of the vacuum furnace itself as well as the results that are achieved when processing critical components. It is not an understatement to say that given the life expectancy of vacuum equipment, instrumentation and controls should be updated every few years to take advantage of the most advanced technology possible (e.g., remote communication and diagnostics, process monitoring and control by Internet-based devices and the like).
Demands on Instrumentation and Control Packages
Temperature control and as a result temperature uniformity can be difficult because of the heat transfer characteristics of the furnace as it moves, for example, from convection to radiant heating and convective/conductive heat transfer during quenching. For example, the ability to vary the furnace heating rate (e.g., 3°C/min – 25°C/min) demands precise and accurate measurement and control, including setpoint program control with guaranteed soak features.
Vacuum furnaces are often used for a variety of products and processes by the heat treater making recipe management an important function. Temperature overshoot of set points is typically not allowed. Setpoint program control is often applied to the temperature, vacuum level and gas pressure with extensive interaction between these programs and also with the logic control.
This question comes up frequently and needs to be addressed again. As shown in Figure 1, the brazing filler metal (BFM) has filled the inside of the tubular joint but has a slight recess at the top edge of the joint. There is no large external fillet (or “meniscus”) of BFM on the outside of the brazed joint. Notice in the photo how the recessed material has a concave shape to it. The “meniscus” of any liquid is the curved shape of the surface of that liquid caused by surface tension. A meniscus can be either concave (desirable in brazed joints) or convex. In Fig. 1, there is a concave recessed meniscus to the BFM at the top edge of the joint. Is this okay?
Many people who see such a joint may incorrectly think that any fillet, whether it is for welding or for brazing, must extend beyond (outside) the joint in order to be acceptable and that any joint that has a recessed-meniscus, as shown in Fig. 1, has to be rejected as being “incomplete”. This is what I call “weld-think”, and has resulted in many such joints being re-brazed (unnecessarily) in order to add more BFM to the joint until the resulting joint shows a large external fillet. This is erroneous thinking that can actually hurt the brazed assembly.
The advantages of processing in vacuum including some of the materials and common processes have already been discussed (Process Applications Run in Vacuum Furnaces – Part One). Here we look at custom heat treatment processes conducted in vacuum furnaces including vacuum applications in the laboratory, Research & Development department and for light industrial requirements as well as a look at the future of vacuum processing.
Custom Heat Treatment Processes
There are many types of highly specialized processes that can be run in vacuum furnaces, and most are highly application specific. Some of these include:
Chemical Conversion – One of the applications not commonly considered in vacuum processing is that of chemical conversion. Sample material is loaded in non-reactive trays and placed inside the vacuum furnace or inside a retort (graphite or alloy). The material is then thermally processed under controlled temperatures and pressures to chemically convert a mixture of elemental materials into a compound. A typical chemical conversion process is run at 1370ºC (2500ºF) and requires up to several days for full transformation.
Creep and Compression Forming – Creep forming (aka hot sizing) is often used to flatten or form to a near-net shape and for correcting spring-back and/or inaccuracies in shape and dimensions of preformed parts. The part is suitably fixtured such that controlled pressure is applied to certain areas of the part during heating. This fixtured unit is then placed in a furnace and heated at temperatures and times sufficient to cause the metal to creep under its own weight until it conforms to the desired shape. Creep forming is done, for example, on titanium alloys, often in conjunction with compression forming.
Degassing – Vacuum degassing is a term often used to describe improved cleanliness in the steelmaking process. However, it is also used to reduce the hydrogen levels in many alloys such as titanium, tantalum, and niobium to avoid concerns over hydrogen assisted cracking (aka hydrogen embrittlement). Hydrogen is imparted into titanium during ingot, rolling and forging operations and can also be diffused into titanium during pickling or other chemical processes. Newer aerospace specifications demand that the hydrogen levels be no greater than 70 ppm. Vacuum degassing usually performed between 535°C – 790°C (1000°F – 1450°F) depending on the alloy, can achieve hydrogen levels of less than 20 ppm.
A number of years ago I wrote an article about the question of refrigerating brazing paste, something that has caused real problems for a number of people in the brazing industry, and which is still doing so today. A recent question about this topic was sent to me and indicated to me that it’s time to once again discuss this topic.
Many people in brazing shops today are still receiving brazing filler metal (BFM) pastes in containers (both large and small) indicating on the label that the BFM-paste must be refrigerated prior to use. Without proper explanation, a simple statement such as “Must be refrigerated prior to use” can lead to significant misunderstandings about what is meant by such a phrase, and has caused a lot of difficulties for brazing personnel who have erroneously believed that brazing paste, according to that warning, has to be “cold” when it is being used in the shop. Thus, before they use the brazing paste, they place it in a small refrigerator, such as that shown in Fig. 1, and then remove it the next day for use in their shop. That is completely wrong.
There is a constant need throughout the industry to produce the highest quality parts to the most stringent product specifications. Both long-established and new materials are being employed to meet the needs of lighter, stronger, smaller and more efficient designs, and the use of vacuum technology in manufacturing is of paramount importance in achieving these goals.
The strategy being adopted by manufacturing to meet these needs relies heavily on vacuum processes and equipment through:
- Process development – New materials, new products, and new applications demand absolute cycle repeatability, flexibility, and control, and as such, designers are specifying vacuum processing over other heat treatment methods.
- Process substitution – Older process technologies and the equipment associated with them are being replaced by vacuum equipment. The justification lies in reduced unit cost achieved by lowering the overall cost of manufacturing and/or through material and efficiency savings.
- Process replacement – Product-performance demands are forcing designers to look toward vacuum processing and its ability to offer a superior product for the same or very similar cost.