All furnace equipment used for heat treating should be properly instrumented and periodically tested for uniformity. The temperature uniformity within the furnace must be regularly surveyed.  The frequency of surveying is largely dependent on the type of equipment in use and its previous history in accuracy and reliability.  Exact survey frequencies should be determined from applicable processing specifications.  However, quarterly temperature uniformity surveys are fairly standard.

The purpose of the uniformity survey is to determine the range of temperatures present at different locations in the furnace under normal operating conditions.  A furnace is normally qualified through an initial comprehensive survey.  This involves determining temperature variations by surveying at the maximum and minimum operating temperatures and at a series of intermediate temperatures not more than 167ºC (300ºF) apart.  After initial qualification, periodic surveys can be taken, usually on a quarterly basis.  Unless otherwise specified, periodic surveys can be performed at a single temperature that rotates between the minimum, mid-range and maximum operating temperatures of the furnace.  Uniformity surveys are also performed after any major repair to the furnace or when the operating integrity of the equipment may be in question. BY JEFF PRITCHARD

In any heat treating cycle, there are two important considerations concerning temperature: the temperature of the furnace hot zone which is generating the heat input, and the temperature of the actual workload. Heating by direct radiation, the main heating mechanism in vacuum, tends to be a slower process than other heating mechanisms such as convection or conduction.  As a result, there are times in the heat treating cycle, particularly during heat up, when the load will be at a lower temperature than the furnace hot zone.  This is known as temperature lag.  Hot zone temperature is controlled and measured through two (or more) thermocouples located close to the heating elements.  One thermocouple, the control thermocouple, is connected to the thermal process controller which transmits signals to control the amount of power directed to the furnace elements. BY JEFF PRITCHARD

The heat treatment of landing gear is a complex operation requiring precise control of time, temperature, and carbon control. Understanding the interaction of quenching, racking, and distortion contributes to reduced distortion and residual stress. Arguably, landing gear has perhaps the most stringent requirements for performance. They must perform under severe loading con­ditions and in many different envi­ronments. They have complex shapes and thick sections. Alloys used in these applications must have high strengths between 260 to 300 ksi (1,792 to 2,068 MPa) and excellent fracture toughness (up to100 ksi in.1/2, or 110 MPa×m0.5). To achieve these design and per­formance goals, heat treatments have been developed to extract the optimum performance for these alloys.

The alloys used for landing gear have remained relatively constant over the past several decades. Alloys like 300M and HP9-4-30, as well as the newer alloys AF-1410 and AerMet 100, are in use today on commercial and military aircraft. Newer alloys like Ferrium S53, a high-strength stainless steel alloy, have been proposed for landing gear applications. The alloy 300M (Timken Co., Canton, OH) is a low-alloy, vacuum-melted steel of very high strength. Essentially it is a modified AISI 4340 steel with silicon, vanadium, and slightly greater carbon and molybdenum content than 4340. The alloy is governed by standard AMS 6417. This alloy has a very good combination of strength (280 to 305 ksi, or 1,930 to 2,100 MPa), toughness, fatigue strength, and good ductility. It is a through hard­ening alloy to large thicknesses. . By D. Scott MacKenzie, Houghton International Inc. Valley Forge, PA

Once a good fixture design has been developed, careful consideration should next be given to the loading of the workpieces.

Heating in a vacuum depends mostly on the transfer of energy through radiation from the elements to the load.  For uniform heating and cooling, it is important that the workpieces are not shielded by one another.  Pieces within the load should be evenly spaced to ensure even exposure to radiation.  The size, shape and high-temperature strength of the workpiece should also be considered during loading.  Alloys with complex shapes and relatively low strength at heat treating temperatures may distort during processing.  In some cases, it may be necessary to support these components with specially designed fixtures. BY JEFF PRITCHARD

Knowledge of vapor pressure and rates of evaporation of various materials is valuable information for those operating vacuum furnaces, whether we are heat treating or brazing at high temperature and low vacuum levels or dealing with outgassing at very low temperatures and pressures.

When we think about a solid or liquid in a sealed vessel, we find that, even at room temperature and atmospheric pressure, there are molecules that leave the surface and go into the gaseous phase. The gas phase thus formed is called a vapor. The process of forming a vapor is known as evaporation and the rate of evaporation is determined by the temperature of the substance involved. In time, some of the evaporated molecules will, in all likelihood in the course of random movement, strike and stick to the surface of the vessel. This process is known as condensation and the rate of condensation is determined by the concentration of gas molecules (that is, the pressure of the evacuated gas). Eventually, the number of molecules leaving the surface of the substance is equal to the number returning to it (that is, the evaporation rate equals the condensation rate) and we have dynamic equilibrium. The (partial) pressure at which this occurs is known as the vapor pressure of the substance.² Below this pressure, surface evaporation occurs faster than condensation, while above it, surface evaporation is slower.