I was recently asked to help determine why a relatively new heat exchange that was part of an industrial boiler system had started springing leaks left and right. Since my client knows that I am more of a generalist-type failure analyst rather than a boiler-tube expert, I was asked if I thought I could do the work. I knew that it would take longer than my usual work, such as a fatigue crack in a conveyor system or a threaded fastener fracture during installation into an automotive assembly. But I told them that I thought my knowledge of their company would make it easier for them to take away useful advice from the technical information. They decided to give me a try.

Well, it was quite an experience. I have worked with high-temperature metallurgy a bit and corrosion problems more than a bit. One of the defined parts of my task was to review the other three reports that had been prepared since the problem first appeared. I found it extremely interesting that three reports performed by different individuals apparently reached three totally different conclusions as to the cause. By Debbie Aliya

In tensile loading, crack-opening stresses create cracks that are perpendicular to the loading direction. In Figure 1, we see a crack that runs perpendicular to the length of the cylinder, or to the axis of the cylinder. This was a fatigue crack, and fatigue cracks are inherently brittle on the macro (visible) scale. Most people who know even a little bit about fracture analysis would recognize this as a brittle crack.

Figure 2 shows the threaded portion of a bolt that has obviously behaved in a very ductile manner on the macro scale. Note the readily visible reduction in cross-sectional area. The initiation area of the crack is perpendicular to the axis of the part, however, so we would consider that portion of the crack itself to be brittle on the macro scale. Once the crack has modified the stress state of the bolt sufficiently, we see a change of crack plane to one that is 45 degrees with respect to the loading and part axis. This portion of the crack is macro-scale ductile. A large fraction of fracture surfaces show ductile cracking in some portion of the crack and brittle in another. By Debbie Aliya

What began as a simple labor of love years ago, namely to share what we’ve learned with the heat-treat industry, has reached an impressive milestone. This is the 100th column for “The Heat Treat Doctor”! So, how do we celebrate? By asking ourselves the most fundamental of questions: What is heat treating, and why do we do it? Let’s learn more.

Heat treating is a core manufacturing competency and can best be defined as “the controlled application of time, temperature and atmosphere to produce a predictable change in the internal structure (i.e. microstructure) of a material.” Thus, metallurgists are responsible to predict the microstructural changes that will occur in a component, while heat treaters are responsible for controlling the process and equipment variables so that the desired outcome will be achieved. By Dan Herring

“Everyone knows” that heat treating steels with a quench-and-temper process can make them stronger. Case hardening steels, when properly done, can also provide stronger components, even though the hard layer is confined to the surface or near-surface layers.

In fact, most steel, even when given a through-hardening treatment, have higher hardness and strength near the surface. (We will neglect decarburization and other surface “deviations from ideal practice” for now.) So why are there so many widespread benefits from making the surface harder or stronger? If there is a wear problem, maybe it is pretty obvious that (in many cases) making the surface harder will reduce loss of material due to wear. But even if there is no wear, hardening just the surface will often result in increased durability. There are many reasons for this, but today we will stick to those most relevant to structural applications, including beams for large civil structures and machinery components, among others. By Debbie Aliya

O-rings are an integral part of any successful vacuum system. Wherever detachable components (e.g., valves, pumps, etc.) are used, O-rings are necessary. But not all O-rings are created equal. Let’s learn more.

The choice of an O-ring is dependent on two factors: the end-use application and the vacuum/pressure range over which it is intended to operate. A typical O-ring is an elastomer – a polymer material with the property of elasticity – having a Durometer “hardness” in the range of 65–80 Shore (i.e. about the hardness of an automotive tire tread or soft skateboard wheel). The secret to their success is their ability to adapt to the unevenness of mating surfaces. The O-ring must be smooth, crack or scratch-free and properly lubricated. A vacuum system in which the construction materials are chosen carefully, welded or brazed joints are sound, static and dynamic seals are designed properly, and all materials and components are designed to withstand bake-out temperatures can operate with a leakage of less than 10-10 cubic centimeters/second. By Dan Herring

Maintenance of your furnace lining/insulation system can result in significant energy savings. In most cases, the extra cost of more efficient lining materials can often be recouped within a year or two.

Despite efforts by all major industries to “go green,” energy consumption in North America remains at record-high levels. Manufacturing costs, especially in energy-intensive processes requiring furnaces and kilns, have been negatively impacted by these rising fuel costs. Improving energy efficiency is key to staying competitive in today’s global economy, and now is the time for evaluating the performance of your furnaces and kilns. Key to the energy efficiency of your furnaces and kilns is the job the refractory insulation lining is doing. It is an unfortunate fact that although refractory insulation provides many heat-saving benefits, it needs to be maintained and repaired during service. Rather than haphazardly replacing materials, it is better to first conduct a thorough evaluation of the furnace-lining condition. By Thomas Rebernak and Steve Chernak

Aerospace thermal processing and Nadcap go hand in hand. Nadcap auditors are your key interface. Who are these people, and how can they help you? This article answers these questions and more.

Many Industrial Heating readers are familiar with the Nadcap program. In fact, nearly a quarter of respondents to a recent Industrial Heating online poll indicated that they are Nadcap accredited. In the same poll, over 40% indicated that they comply with AMS 2750D (Pyrometry), compliance with which is required to achieve Nadcap heat-treating (AC7102) accreditation. For those who do not know, Nadcap is the leading worldwide cooperative program of major companies designed to manage a cost-effective consensus approach to special processes and products and provide continual improvement within the aerospace and automotive industries. Nadcap is an industry-managed program administered by the not-for-profit Performance Review Institute and is part of their Customer Solutions & Support (CS&S). By Arshad Hafeez