As we age, we sometimes outlive our joints. The aging of the largest segment of our population (the boomers) will surely increase the number of joint failures. When joints fail, the solution is often to replace the problem area with an artificial joint. Before that can happen, however, a number of thermal processes must take place because most of these replacement joints – prostheses – are made from a cast metallic alloy. A prosthesis is simply defined as "a device designed to replace a missing part of the body or to make a part of the body work better."

The use of alloys in surgical implants dates back about a century. Glass was used in very early hip-replacement trials for its biocompatibility, but it could not endure the wear and tear. Medical scientists experimented with plastic and stainless steel in the 1930s and 1940s. The development of 316 stainless and cobalt-chromium alloys in the 1920s and 1930s resulted in materials with superior characteristics to steel for many prosthetic applications. Titanium – a relative newcomer in the mid-20th century – was found to have excellent biocompatibility, and alloys of titanium were developed to optimize the mechanical properties.

When most of us think about health and safety in the heat-treat shop, we tend to focus on things that are hot, heavy or dangling overhead. The threats we can see, smell, hear or perhaps even taste are most often dealt with quickly. The old adage, “what you can’t see, won’t hurt you,” doesn’t apply in our industry. Let’s learn more.

Lesson Learned - I recall as a young engineer that when called upon to inspect the inside of a cold, well-ventilated batch integral-quench or pusher-type furnace that had been running endothermic or exothermic gas atmosphere, I would often experience mild headaches after an hour or so inside. I remember carrying aspirin with me for just such an occasion. Little did I realize at the time that furnace atmosphere was slowly being released from the brickwork and that, in this confined space, I was experiencing the early symptoms of gas poisoning. By Dan Herring

Proper selection of thermocouples and optimization of their performance in specific applications requires a good understanding of the construction, capabilities and limitations of the various types of thermocouples available.

Contact temperature-sensing devices, such a thermocouples, RTDs (resistance temperature detectors), thermistors, bimetallic thermometers and liquid-filled sensors, measure the temperature of their sensing tips; that is, they do not measure the temperature of gases, liquids or solids surrounding the sensing tip unless certain requirements are met. Differences in the thermal properties of the sensor and the media surrounding the sensor (e.g., thermal conductivity, thermal diffusivity and emissivity) can produce large differences in the observed temperature readings. Additionally, isotherms (areas of constant temperature) within the gas, liquid or solid material can produce large differences between observed and actual temperatures. The test engineer (user) must consider these factors before choosing the particular device design. This article describes various situations where temperature measurements are made, and studies temperature-sensor characteristics under dynamic and static conditions. Where appropriate, typical test data are presented to illustrate the performance of temperature sensors under actual tests. By Dan Nanigian, NANMAC Corp.

Nitriding is a case-hardening process in which nitrogen is introduced into the surface of a ferrous alloy such as steel by holding the metal at a temperature below that at which the crystal structure begins to transform to austenite on heating (Ac₁) as defined by the Iron-Carbon Phase Diagram (Fig. 1).

The material typically is placed in contact with ammonia, which allows the transfer of nitrogen to the surface during its thermal decomposition to nitrogen and hydrogen. Other special nitriding processes are also used for certain types of stainless steels involving the decomposition of nitrogen gas at high temperatures, but these will not be the focus of this discussion. By Dan Herring

This is the first and second part of a three part article. The nuclear industry is expanding, and the heat-treating community needs to keep pace. This article will discuss the current state of the industry including a brief overview of nuclear power and the various styles of reactors and types of components fit for nuclear service.

Specific heat-treating applications will be presented in detail including: annealing of zirconium pressure tubing; stress relief of steam valve bodies and pipe welds; hardening of pressure relief springs and internal valve components; heat treatment of bolts, seal rings and other types of fasteners/retainers; and the sintering of ceramic fuel pellets. Vacuum, atmosphere and induction hardening techniques will be discussed along with design requirements. New developments and industry trends will also be explored. By Dan Herring

Proper selection and maintenance of a sensing device for capturing and continuously measuring a process variable is essential for successful process control. Thermocouples are simple in form and function, but you must take great care in selection and use to get the optimum performance and accuracy.

The most common sensor in a thermal system is a thermocouple (TC), which is sensitive to a change in process conditions, reasonably accurate and offers the required precision at a modest cost. Thermocouples are rugged and can be used over a wide range of process temperatures-from subzero to over 4000F (2205C). However, a combination of many factors influence the successful application of a thermocouple, including thermocouple material purity and manufacturing quality, temperature exposure, thermal cycling, chemical exposure, the protection applied and physical abuse. This article provides a perspective on the selection and application of thermocouples. By Dan Herring

Modern quenching oils offer a wide range of capabilities and performance. However, heating and aging of the oil, as well as oil contamination by common contaminants, such as salt, water, hydraulic fluids and soot, can affect the quenching performance of a premium medium speed oil, resulting in fire, spotty work, cracking, distortion, and potential property damage and harm to personnel. This article discusses the effects of these contaminants on the cooling curve of a medium speed oil.

A high-performance quench oil must have oxidation resistance and low sludge formation, be nonstaining and have an elevated flash point and acceptable heat-transfer characteristics[1]. Except for some synthetics and vegetable oils, most modern quench oils are based on refined petroleum-base stocks. The use of higher fractions of naphthenic compounds result in lower flash points and greater sludge formation[2]. The presence of sludge reduces the heat transfer efficiency, which could produce inadequately hardened parts. Sludge is the result of oxidation and polymerization of quenching oils in use[3]. In general, the higher the sludge content, the older the quench oil. By D. S. MacKenzie, L. Gunsalus, and I. Lazerev, Houghton International Inc., Valley Forge, Pa.