Machinery's Handbook, 31st Edition
484 Heat Treatment of Steel the mixture stirred until the soda is thoroughly dissolved. A still more effective method of cleaning is to dip the work into a mixture of 1 part sulfuric acid and 2 parts water. Leave the pieces in this mixture about 3 minutes; then wash them immediately in a soda solution. Flame Hardening.— This method of hardening is especially applicable to selective hardening of large steel forgings or castings that must be finish-machined prior to heat- treatment or, because of size or shape, cannot be heat treated by using a furnace or bath. An oxyacetylene torch is used to quickly heat the surface to be hardened, which is then quenched to secure a hardened layer that may vary in depth from a mere skin to 1 ⁄ 4 inch (6.35 mm) and with hardness ranging from 400 to 700 BHN (Brinell Hardness Number). A multiflame torchhead may be equipped with quenching holes or a spray nozzle behind the flame. This is not a carburizing or a case-hardening process as the torch is only a heat - ing medium. Most authorities recommend tempering or drawing of the hardened surface at temperatures between 200 and 350°F (93 and 177°C). This treatment may be performed in a standard furnace, in an oil bath, or with a gas flame. It should follow the hardening process as closely as possible. Medium-carbon and many low-alloy steels are suitable for flame hard - ening. Plain carbon steels ranging from 0.35 to 0.60 percent carbon will give hardnesses of from 400 to 700 BHN. Steels in the 0.40 to 0.45 percent carbon range are preferred, as they have excellent core properties and produce hardnesses of from 400 to 500 BHN without checking or cracking. Higher-carbon steels will give greater hardnesses, but extreme care must be taken to prevent cracking. Careful control of the quenching operation is required. Spinning Method of Flame Hardening: This method is employed on circular objects that can be rotated or spun past a stationary flame. It may be subdivided according to the speed of rotation, where the part is rotated slowly in front of a stationary flame and the quench is applied immediately after the flame. This method is used on large circular pieces, such as track wheels and bearing surfaces. There will be a narrow band of material with lower hardness between adjacent torches if more than one path of the flame is required to harden the surface. There will also be an area of lower hardness where the flame is extinguished. A second method is applicable to small rollers or pinions. The work is spun at a speed of 50 to 150 rpm in front of the flame until the entire piece has reached the proper temperature; then it is quenched as a unit by a cooling spray or by ejecting it into a cooling bath. The Progressive Method: In this method the torch travels along the face of the work and the work remains stationary. It is used to harden lathe ways, gear teeth, and track rails. The Stationary or Spot-hardening Method: When this method is employed, the work and torch are both stationary. When the spot to be hardened reaches the quenching temperature, the flame is removed and the quench applied. The Combination Method: This approach is a combination of the spinning and progres sive methods, and is used for long bearing surfaces. The work rotates slowly past the torch as the torch travels longitudinally across the face of the work at the rate of the torch width per revolution of the work. Equipment for the stationary method of flame hardening consists merely of an acetylene torch, an oxyacetylene supply, and a suitable means of quenching; but when the other methods are employed, work-handling tools are essential and specially designed torches are desirable. A lathe is ideally suited for the spinning or combination hardening method, whereas a planer is easily adapted for progressive hardening. Production jobs, such as the hardening of gears, require specially designed machines. These machines reduce handling and hardening time, as well as assuring consistent results. Induction Hardening.— The hardening of steel by means of induction heating and subsequent quenching in either liquid or air is particularly applicable to parts that require localized hardening or controlled depth of hardening and to irregularly shaped parts, such as cams that require uniform surface hardening around their contour. Advantages offered by induction hardening are: 1) a short heating cycle that may range from a fraction of a second to several seconds; heat energy can be induced in a piece of steel at the rate of 100 to 250 Btu/in 2 /min (165 to 410 J/mm 2 /min) by induction heating, as
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