A vacuum Gauge is a pressure measuring instrument that measures pressure in a vacuum (i.e., in a vessel operating at sub-atmospheric pressure). Depending on the type of vacuum system (Fig. 1) and the required operating vacuum level, different vacuum gauges are required, often in combination with one another, to accurately determine and/or control the vacuum level of the chamber at any given moment in time.
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.
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.
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.
More and more application uses are being found for composite materials and their use is expanding beyond just aerospace applications. As such, you may be wondering how they are manufactured. Composites are manufactured using multiple layers of material, each with different properties, combined into a single structure. By joining them in this manner, the resultant “composite” material is unusually strong and light. It has greater strength, flexure, and more favorable mechanical properties than any of the individual materials used to make the composite. Metal matrix and ceramic matrix composites enjoy widespread use in the wind power, automotive and aerospace industries, among others (Fig. 1), as they offer unique advantages over steel and aluminum. Composite manufacturing is a growing industry and has strong continued prospects for expansion.
The most popular composites, valued for their very high strength to weight ratio, are made of a woven base material such as fiberglass, aramid (Kevlar®) or carbon fiber impregnated with a resin that is hardened into a plastic using heat and/or pressure. The woven material is known as a “prepreg” since it has not yet been impregnated with resin. The resin can be epoxy, polyester, polyurethane, or other plastic in liquid form. The base fiber, such as carbon, has high tensile strength but is not stiff enough on its own to be used as a structural element. The resin can be molded and formed into various shapes but is not strong enough on its own to serve as a structural member. When the base fiber and the plastic are combined, the resultant composite material has both the strength of carbon fiber and the retained shape of the cured epoxy or other hard plastic. This feature, the ability to incorporate the most favorable mechanical properties of each of the constituent materials, is what makes composites such an attractive choice for a wide variety of lightweight structures.
Most base metals typically brazed in vacuum furnaces have a natural oxide “coating” that can inhibit the flow of brazing filler metals.
Conversely, alloys containing appreciable amounts of reactive elements such as aluminum and titanium tend to form oxides at high temperatures which impede the flow of the brazing filler metal. Many of the nickel-base superalloys fall into this category and the severity of the problem varies depending on alloy composition. These materials should be brazed at high vacuum levels of 2 x 10-4 torr or better. There are several reliable techniques for improving the brazeability of difficult to braze materials. These include brush nickel plating of the joint surfaces, chemical etching techniques to remove aluminum and titanium from a shallow layer at the joint surface and using special aggressive braze filler metals with self-fluxing characteristics. The oxides of the less reactive metals like iron, nickel, and cobalt tend to dissociate (break down) under low pressure and high temperature. Therefore, alloys such as the 300 and 400 series stainless steels, carbon steels and many tool steels can be successfully brazed in vacuum at relatively high pressures (1 to 50 microns).
No matter what field you study, an accurate knowledge of its vocabulary is essential to understanding the subject. In the field of vacuum heat treatment, considerable emphasis is placed on the proper use, meaning, and interpretation of certain words or phrases. A brief summary of the terminology including common processes being run is presented below.