Atmosphere Furnaces
Atmosphere furnaces are characterized by their use of a protective atmosphere to surround the workload during heating and cooling. The most common furnace atmosphere, however, is air. Often times,nothing more is needed. When an air atmosphere is used, such as in a low temperature tempering operation, the final condition of the material’s surface (or skin) is not considered important. Furnace atmospheres play a vital role in the success of the heat treating process. It is important to understand why we use them and what the best atmosphere for a specific application is. There are many different types of atmospheres being used, and it is important to understand how a particular atmosphere is chosen as well as its advantages and disadvantages and to learn how to control them safely.The purpose of a furnace atmosphere varies with the desired end result of the heat treating process. The atmospheres used in the heat treating industry have one of two common purposes:
1. To protect the material being processed from surface reactions, i.e., to be chemically inert(or protective)
2. To allow the surface of the material being processed to change, i.e., to be chemically active(or reactive)
Some atmospheres such as argon and helium are often associated with vacuum furnaces and are used at partial pressure (pressure below atmospheric pressure). Others, such as sulfur dioxide are used for very special applications (Geng H., Manufacturing Engineering Handbook, p. 18.51)
Visual Testing
Visual testing (VT) is by far the most powerful inspection method available. Because of its relative simplicity and lack of sophisticated equipment, some people discount its power. However, it is the only inspection method that can and should be applied during each step of the welding process,rather than after the weld has been made, it is the only method that can actually increase the qualityof fabrication and reduce the generation of welding defects.Most standards require that all welds be visually inspected. Visual inspection begins long before an arc is struck. Materials that are to be welded must be examined for quality, type, size, cleanliness,and freedom from defects. The pieces to be joined should be checked for straightness, flatness, anddimensions. Alignment and fit up of parts should be examined. Joint preparation should be verified.Procedural data should be reviewed, and production compliance assured. All of these factors precede any welding that will be performed.During welding, visual testing includes verification that the procedures used are in compliance with the welding procedure specification. Upon completion of the weld bead, the individual weld passes are inspected for signs of porosity, slag inclusion, and any weld cracks. Bead size, shape, and sequences can be observed. Interpass temperatures can be verified before subsequent passes are applied. Visual inspection can ensure compliance with procedural requirements. Upon completing the weld, the size, appearance, bead profile, and surface quality can be inspected. Visual testing should be performed by the weld inspector, as well as by the welder. Good lighting and eye sight is imperative. In most fabrication shops, some type of auxiliary lighting is required for effective visual inspection. Magnifying glasses, gauges, and workmanship samples all aid in visual testing. (Geng H., Manufacturing Engineering Handbook, p. 21.39)
Hack Sawing
In hack sawing, a straight, relatively short, blade is tensioned in a bow, powered back and forth via an electric motor and a system of gears, and fed through a stationary, clamped workpiece either by gravity or with hydraulic assistance. The hacksaw therefore basically emulates the manual saw ingaction. Cutting is generally done on the “push stroke,” i.e., away from the pivot point of the bow. In more sophisticated models, the bow is raised slightly (relieved) and speeded up on the return, or noncutting, stroke, to enhance efficiency. By its very nature, however, hack sawing is inherently inefficient since cutting is intermittent. Also, mechanical restrictions make it impossible to run hacksaws at anything but pedestrian speed. The advantages of hack sawing are: low machine cost; easy setup and maintenance; very low running costs; high reliability and universal application—a quality hydraulic hacksaw can cut virtually anything, albeit very slowly. For this latter reason, in mainstream industrial applications hack sawing has all but disappeared in favor of band sawing or circular sawing . (Geng H., Manufacturing Engineering Handbook, p. 32.1)
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