Vacaero

Manufacturers of heat treating and brazing vacuum furnaces and controls, complete hot zone and vacuum furnace retrofits, thermal spray coatings, plasma, HVOF and paint coating services.

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Temperature Uniformity Surveys

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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

Calculating Evacuation Time

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When a vacuum system is designed it is often necessary to select a mechanical vacuum pump or pump set that will evacuate the chamber and associated piping in a certain amount of time. In laboratory or research situations that may not be as necessary as in a production environment where the time to complete a process has quite a lot to do with the cost of the manufactured part. In addition, the cost of the vacuum system has to be taken into consideration as well. Larger pumps may reduce the evacuation time, but also are more expensive. There has to be a balance between all the parameters.

There are two simple methods for calculating evacuation time; one for a rotary vacuum pump, vane or piston, on its own, and a second for larger volumes when a vacuum booster pump may be used. Both methods give good results for simple vacuum systems where the mechanical vacuum pumps are located close to the chamber and the chamber is relatively empty.

Temperature Measurement in Vacuum Furnaces

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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

Oil for Vacuum Pumps

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In the world of mechanical oil sealed rotary vacuum pumps there is a need for a variety of oils and fluids to suit the specific type of pump, its duty and the process it is used on. This discussion covers high vacuum pumps only, such as are used in the heat treating and vacuum furnace industry. These same vacuum pumps are used in many other industrial and scientific applications and have to work under many different types of conditions including one that many people expose their pumps too – neglect!.

Rotary vane vacuum pumps are available as direct drive (usually 1800 rpm) and vee belt drive (between 400 and 500 rpm) versions. Rotary piston vacuum pumps are generally vee belt driven and run at about 500 rpm. The work duty of a vacuum pump can vary between intermittent use and running continuously. They can also be used for cyclic duty, to evacuate a loadlock for example, where the pump evacuates a chamber from atmosphere to vacuum every few minutes. The vacuum process can also vary, from clean air pumping to hazardous gas, wet vapor pumping and dirty/dusty atmospheres..

Vacuum Heat Treatment of Fasteners

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Fastener applications are demanding. Whether fasteners are being used in the petrochemical industry, in medical or mining applications, for assembly of marine or nuclear components or in the aerospace, automotive or construction world, vacuum processing allows us to repeatedly achieve the highest quality and metallurgy.

Most fastener materials, including stainless steels and superalloy grades, benefit from or actually require vacuum processing for heat treatment instead of being run under protective atmospheres. In general, there are three main sets of applications where vacuum heat treatment is used; processes that can be done in no other way than in a vacuum, processes that can be done better in a vacuum from a metallurgical standpoint and processes that can be done better in a vacuum from an economic standpoint. The absence of surface reactions or the ability to precisely control them is the main difference between vacuum heat treatment and all other forms of heat treatment. Vacuum processing can also remove contaminants from parts, and in some instances, degas or convert oxides found on the material’s surface.

Heat Treatment of Landing Gear

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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

Looking after Mechanical Vacuum Pumps

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This article is written for vacuum pumps such as the oil sealed rotary piston pumps used on many heat treating and vacuum furnace applications. The same information would also apply to the oil diffusion holding pump if it is used. This pump may be either a vee belt driven pump or a direct drive pump. The holding pump is used to keep the oil diffusion pump evacuated below the critical backing pressure when the main pump is in roughing mode.

All mechanical vacuum pumps need maintenance and the pump manufacturer usually lists the basic checks needed in the pump operation manual. This will vary with the application that the pump is used on but, at a minimum, will include the following: check oil level daily or weekly, depending on the application and use, change oil and check the shaft seal area for leaks every 6 months and inspect the exhaust valves and gas ballast valve seals every 12 months.