VAC AERO Launches 2012 Series on “Vacuum Technology with Dan Herring”.

This is the first in a new series of articles entitled Vacuum Technology. The purpose of this series is to inform, educate and answer questions for a worldwide audience of individuals using vacuum methods and vacuum equipment. The series is intended for captive and commercial heat treaters, engineers, operators, metallurgists, quality control and quality assurance personnel, supervisory staff and anyone else who is interested in the subject of vacuum processing.

Brazing is a highly versatile joining technique that can be used to join many different types of metals, and can even be used to permanently bond engineered-ceramic materials, such as alumina, to a variety of metals. It is being done everyday in industry.

Alumina, which consists of aluminum-oxide powder granules imbedded in a glassy matrix binder system of calcium-oxide and silicon-dioxide (among others), can be joined to ceramic or metal structures primarily by two different methods, as shown in Fig. 1. By Dan Kay

OAKVILLE, Ontario, (October 24, 2011) – First established in 2003 in Kalisz, Poland to offer Metallurgical Services to the burgeoning aerospace industry in Poland, VAC AERO has now opened a second facility located in the town of Rzeszow in southeast Poland. VAC AERO’s Polish divisions offer special processing services, including vacuum heat  treating and brazing, vacuum carburizing, air plasma spray (APS) and HVOF protective coatings, as well as corrosion and oxidation resistant paint coatings. This new addition to the VAC AERO Group of Companies will be the sixth vacuum heat treating, brazing and coating facility worldwide.

The main components of a landing-gear structure are wheels and brakes, axles, bogie beams (a.k.a. truck beams), shock absorbers (a.k.a. shock struts), and drag and side braces. Primary design considerations on landing gear include maximum sink speed, spin up, spring back, lateral drift (on landing), towing, jacking, turning, braked roll, taxi, rebound, pivoting (main landing gear only), crashworthiness and fatigue.

Secondary loads include retraction/extension, aerodynamic loads, lock/unlock loads and emergency extension. In addition, nose landing-gear specific forces include dynamic breaking, nose-gear yaw and steering. Alloys used in these applications must have high strengths, normally between 260-300 ksi (1,792-2,068 MPa). By Carmine Filice, Daniel H. Herring, Paul Vanderpol

Wide Gap Brazing when Parts don’t Fit Together well for Brazing – A common occurrence (unfortunately) in the brazing world is the need to join two parts together by brazing in which the brazing gap is too large, i.e., in the range of 0.010-inches (0.25 mm) or larger. Ideal brazing clearances should be in the area of 0.000-inches to 0.005-inches (0.00mm to 0.125mm) maximum for most brazing filler metals (BFMs).
Brazing depends on capillary action to draw the liquid BFM into the brazing joint, and tight clearances are needed for best brazing to occur. If the BFM is pre-placed in the brazing joint prior to assembly of the parts then capillary action is not a major factor since the BFM will melt in-situ and join the two members without the need for flowing any distance through tight capillary spaces. By Dan Kay

Why does Brazing require Temperatures above 450C (840F)? Brazing, when performed correctly, is a joining process that produces a permanent bond between two or more materials by heating them to a temperature above 450C (840F), but lower than the melting-temperature of any of the materials being joined, and a permanent, metallurgical bond between these materials is produced when capillary action draws a molten brazing filler metal (BFM) through the clean, closely fitted faying surfaces of the joint.

The filler metal is not supposed to become fully liquid (i.e., have a “liquidus”) until the brazing temperature reaches at least 450C (840F). If the liquidus of the filler metal is below 450C (840F) then that filler metal would commonly be called a “soldering alloy”. People often wonder about the temperatures used to differentiate brazing from soldering. Why 450C (840F)? Is there some significance to these “exact” numbers? By Dan Kay

Nitriding is one of the most interesting and useful surface-hardening techniques. It is unique in that during the nitriding process, the specimen is not heated into the austenite phase, and it does not rely upon the formation of martensite to achieve high hardness and useful properties. It is heat treated prior to nitriding, forming tempered martensite to obtain the desired core properties unlike all other surface heat-treatment processes.

The processing associated with nitriding does have some advantages in avoiding problems such as quench cracking and distortion. It also has some side benefits in improved corrosion resistance and generation of beneficial residual compressive stresses, which improves fatigue resistance. Nitrided surfaces do exhibit high surface hardness, leading to improved wear resistance. By George Vander Voort

Although the fundamental relationships for stereology, the foundation of quantitative metallography, have been known for some time, implementation of these concepts has been restricted when performed manually due to the tremendous effort required. Further, while humans are quite good with pattern recognition, as in the identification of complex structures, they are less satisfactory for repetitive counting.

Many years ago, George Moore (1) and members of ASTM Committee E-4 on Metallography conducted a simple counting experiment. About 400 people were asked to count the number of times the letter “e” appeared in a paragraph without striking out the letters as they counted. The correct answer was obtained by only 3.8% of the people. Results were not Gaussian, however, as only 4.3% had higher values while 92% had lower values, some much lower. The standard deviation was 12.28. This experiment revealed a basic problem with manual ratings. In this case the subject was one very familiar to the test subjects, yet only 3.8% obtained the correct count. What degree of counting accuracy can be expected when the subject is less familiar, such as microstructural features? Image analyzers, on the other hand, are quite good at counting but not as competent at recognizing features of interest. Fortunately, there has been tremendous progress in the development of powerful, user-friendly image analyzers over the past two decades. By George Vander Voort

For most of its history, metallographic observations have been largely qualitative in nature. The structure might be described as being relatively coarse or fine, or layered, or uniform. Particles might be labeled as globular or spheroidal, lamellar, acicular, or blocky. Microstructures were single-phase or duplex, and so forth.

Forty some years ago when I entered industry, chart ratings and visual examinations were the main approach toward quantitation. I can well remember the mill metallographers looking at spheroidized carbide tool steel structures and stating that it was, for example, 90% spheroidized (many raters would never say 100%, just as some teachers would never grade an essay at 100%!) or that it was 60% spheroidized and 40% lamellar tending to spheroidize. Or, without looking at the chart (a seasoned rater never did!), they would pronounce that the grain size was, for example, 100% 6 to 8 or perhaps 70% 8 and 30% 3 to 5 if it was duplex in appearance. By George Vander Voort