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Flame and Induction Hardening

Flame or induction hardening of gearing involves heating of gear teeth to 1450-1600_F(788-871_C) followed by quench and tempering. An oxyfuel burner is used for flame hardening. An encircling coil or tooth by tooth inductor is used for induction hardening. These processes develop a hard wear resistant case on the gear teeth. When only the surface is heated to the required depth, only the surface is hardened during quenching. Material selection and heat treat condition prior to flame or induction hardening significantly affects the hardness and uniformity of properties which can be obtained.
  • Methods of Flame and Induction Hardening.
    Both of these methods of surface hardening can be done by spin hardening, or by tooth to tooth hardening. Spin hardening of gearing involves heating all of the teeth across the face simultaneously by spinning the gear element within the heat source (flame or induction coil) which envelopes the entire face width. Gearing is removed from the heat source and immediately hardened by the quenchant. Shafting and gearing can also be progressively spin hardened by spinning the shaft or tooth section within the heat source and following quench head. The heat source and quench head traverse axially along the length to be hardened.
    Gearing can also be tooth to tooth, progressively hardened by passing the flame or inductor and following quench head between the roots of teeth. Inductor or flame heads or burner may be designed either to pass in the root diameter between flanks of adjacent teeth, to heat the root diameter and opposite flanks of adjacent teeth, or may fit or encompass the top land to heat the top land and opposite flanks of each tooth.
    Heat sources designed to pass between adjacent teeth followed by quenching are desirable from both endurance or bending strength and wear considerations, because both the flanks of teeth and root diameter are hardened. Only the non-critical top lands of teeth are not hardened. An inductor or flame head which encompasses only top lands of teeth and adjacent flanks followed by quenching provide wear resistance to the flanks, but endurance or bending strength in the roots is not enhanced. Residual tensile stress in the roots of teeth may also prove detrimental. It is, therefore, recommended that both the designer and heat treater know what type of hardening pattern is desired.
    Gearing may also be tooth to tooth, progressively hardened by passing the inductor between the roots of adjacent teeth, while the gear element is submerged in a synthetic quench (termed “Delapena Process”). This process, like other tooth to tooth hardening techniques, is time consuming and is not economical for small, finer pitch gearing (finer than 10 DP). Spin hardening is more economical for smaller gears.
    Three basic gases are used for flame heating, which include MAPP, acetylene and propane. These gases are each mixed with air in particular ratios and are burned under pressure to generate the flame which the burner directs on the work piece.
    Simple torch type flame heads are also used to manually harden teeth. Since there is no automatic controlof this process, high operator skill is required.
    Induction hardening employs a wide variety of inductors ranging fromcoiled copper tubing to forms machined from solid copper combined with laminated materials to achieve the required induced electrical currents.
    Coarser pitch teeth generally require inductors powered by medium frequency motor generator sets or solid state units. Finer pitch gearing generally utilizes encircling coils with power provided by high frequency vacuum tube units.
    Wide faced gearing is heated by scanning type equipment while more limited areas can be heated by stationary inductors. Parts are rotated when encircling coils are used.
    Induction heating depth and pattern are controlled by frequency, power density, shape of the inductor, work piece geometry and work piece area being heated.
    Gearing can be obtained by dual frequency spin coil induction heating using both low (audio) frequency (AF) of 1-15 kHz and higher (radio) frequency (RF) of approximately 350-500 kHz. Initially low audio frequency is used to preheat the root area, followed by high radio frequency to develop the profile heated pattern, followed by quenching.
    Quenching after flame or induction heating can be integral with the heat source by use of a separate following spray, or separate by using an immersion quench tank. Oil, water or polymer solutions can be used, in addition to air, depending upon hardenability of the steel and hardening requirements.
  •  Application.
    Flame and induction hardening have been used successfully on most gear types; e.g., spur, helical, bevel, herringbone, etc. These processes are used when gear teeth require high surface hardness, but size or configuration does not lend itself to carburizing and quenching the entire part. These processes may also be used when the maximum contact and bending strength achieved by carburizing is not required. These processes are also used in place of more costly nitriding which cannot economically generate some of the deeper cases required.
    Contour induction is preferred over flame when root hardness and closer control of case depth is required. Contour flame hardening of the flanks and roots is not generally available. The general application of flame hardening is to the flanks only, except when spin flame hardening is applied. The spin flame process generally hardens below the roots, but hardens teeth through the entire cross section, reducing core ductility of teeth and increasing distortion.
    If high root hardness is not required, flame hardening is more available and more economical than induction hardening for herringbone and spiral bevel gearing.
    NOTE: AGMA quality level will be reduced approximately one level (from the green condition) after flame or induction hardening unless subsequent finishing is performed.
  • Material.
    A wide variety of materials can be flame or induction hardened, including (cast and wrought) carbon and alloy steels, martensitic stainless steels, ductile, malleable and gray cast irons. Generally, steels with carbon content of approximately 0.35-0.55 percent are suitable for flame or induction hardening. Alloy steels of 0.5 percent carbon or higher are susceptible to cracking. The higher the alloy content with high carbon, the greater the tendency for cracking. Cast irons also have a high tendency for cracking.
    Selection of the material condition of the gearing can affect the magnitude and repeatability of flame and induction hardening. Hot rolled material exhibits more dimensional change and variation than hot rolled, cold drawn material because of densification from cold working. A quench and tempered material condition or preheat treatment, however, provides
    the best hardening response and most repeatable distortion.
  • Prior Heat Treatment.
    For more consistent results, it is recommended that coarser pitched gears of leaner alloy steels receive a quench and temper pretreatment; for example, 4140 steel with teeth coarser than 3 DP. In both carbon and alloy steels, normalized or annealed structures can be hardened. These structures do, however, require longer heating cycles and a more severe quench which increase the chance of cracking. The annealed structure is the least receptive to flame or induction hardening.
    Successful induction hardening of either gray or ductile cast iron is dependent on the amount of carbon in the matrix. The combined carbon in pearlite will readily dissolve at the austenitizing temperature. Pearlite microstructures are desirable. Pearlite promoting alloy additions such as copper, tin, nickel or molybdenum may be necessary to form this microstructure.
  • Hardening Patterns.
    There are two basic methods of flame or induction hardening gears, spin hardening and tooth to tooth hardening.
    The hardening patterns shown are not possible for all sizes and diametral pitches. For coarser pitches, requirements should be worked out with the supplier. For induction hardening, the kW or power capacity of the equipment limits the pattern which can be attained. Root flame hardening by the tooth by tooth process is difficult and should be specified with care.
    The induction coil method is generally limited to gears of approximately 5 DP and finer. The maximum diameter and face width of gears capable of being single shot induction coil hardened is determined by the area of the outside diameter and the kW capacity of the equipment. Long slender parts can be induction hardened with lower kW capacity equipment by having the coils scan the length of the part while the part is rotating in the coil.
    Flank or root and flank induction scan hardening (contour) can be applied to almost any tooth size with appropriate supporting equipment and kW capacity. However, for pitches of approximately 16DP and finer, these methods are not recommended. Spin hardening in an induction coil is recommended. Spin hardening of finer pitches is also required when using flame burners.
    The allowable durability and root strength rating for the different hardening patterns should be obtained from appropriate AGMA rating practices. These bending strength ratings are lower at the roots of teeth when only the tooth flanks are hardened.
  • Process Considerations.
    Several areas must be considered when processing. Some of the more critical requirements are outlined below.
    1. Repeatability.
      Repeatable process control is essential for acceptable results. With induction, this is usually not a problem with properly maintained equipment since electrical power characteristics, inductor movement and integral quench intensity can be readily controlled.
      Repeatabiltiy becomes more difficult with flame hardening. Equipment varies from hand held torches to tailor made machine tools with well controlled movement of burner heads. Equipment must be such that heating rates across the burner face are consistent from cycle to cycle. Gas pressure and mixing of heating gases must be uniform. Burner head location must be precise from cycle to cycle.
    2. Heating with Flame or Induction.
      Accurate heating to the proper surface temperature is a critical step. Burner or inductor design, heat input and cycle time must be closely controlled. Under heating results in less than specified hardness and case depth. Overheating can result in cracking. Flame hardening may also cause burning or melting of tooth surfaces.
    3. Quenching.
      Heat must be removed quickly and uniformly to obtain desired hardness. The quenchant should produce acceptable as quenched hardness, yet minimize cracking.  Quenchants used are: water, soluble oil, polymer, oil and air.
      Parts heated in an induction coil are usually quenched in an integral quench ring or in an agitated quench media. When the part is scanned while rotating in a coil, a spray quench usually follows behind the coil.
      Flank hardened teeth usually have an integral quench following the inductor, or the gear is submerged in liquid during heating.
      Quench time and temperature are critical and in-spray quenching, pressure velocity and direction of the quench media must be considered. When localized or air quenching is used, a coolant is used on a portion of themetal away from the heating zone to maintain the base metal near ambient temperature so the part mass can absorb heat from the heated zone.
    4. Tempering.
      Tempering is mandatory only when specified. However, for particular processes, judgment should be exercised before omitting tempering. It is good practice to temper after quenching to increase toughness and reduce residual stress and crack susceptibility. Tempering should be for a sufficient time to insure that hardened teeth reach the specified tempering temperature. Flame hardened parts which are air quenched are self tempered, and separate tempering is unnecessary.
    5. Surface Hardness.
      Surface hardness isthe hardness measured on the immediate surface and is primarily a function of the carbon content. Hardness may be lower as a result of prior heat treatment, alloy content, depth of hardening, heating time, mass and quenching considerations.
    6. Effective Case Depth.
      Effective case depth for flame and induction hardened gears is normally defined as the distance below the surface at the 0.5 tooth height where hardness drops 10 HRC points below the surface hardness.
      When a tooth is through hardened, effective case depth does not apply. When root is also to be hardened, depth of case at the root may be specified.
  • Rating Considerations.
    Designers should be aware that AGMA decreases load ratings for gears which do not have hardened roots. AGMA gear rating standards should be consulted for appropriate stress numbers.
    1. Heat Affected Zone.
      In flame hardening, the heat affected zone (HAZ) is a region that is heated to 1300-1400_F, (704_C-760_C) but does not get hardened and thus has lower strength. This zone should be located either a minimum of 1/8 inch up the flank from the critical root fillet or well below the root diameter.
      Contour induction hardening results in case depth at the root to be approximately 60 percent of the depth at the pitch line due to mass quench and hardenabiltiy effect. Profile hardening of fine pitched gearing using a submerged quench decreases the difference between pitch line and root case depth.
    2. Case Depth Evaluation (Hardness Pattern).
      Although it is not always practical, particularly on larger gearing, the only positive way to check case depth is by sectioning an actual part. For tooth by tooth hardening, a segment of a gear can be hardened and sectioned. Case depth should be determined on a normal tooth section, using an appropriate superficial or micro-hardness tester. When a gear cannot be sectioned, hardness pattern and depth can be checked by polishing end faces of teeth and nitric acid etching. Grit blasting is also occasionally used. Hardness can also be checked on end faces at flank and root areas.
      NOTE: During tooth by tooth induction hardening, power is lowered and travel is sometimes increased as the inductor approaches the end faces. This is to prevent edge burning and cracking. In these instances, hardness may be lower at the ends, particularly at the root area. In this case, existence of a hardness pattern can be demonstrated by acid etching, but actual depth cannot be accurately measured.
  • Specifications.
    The drawing, order, or written specification should include the following information:
    1. Chemical analysis range of the material or designation.
    2. Prior heat treatment.
    3. Hardening pattern required.
    4. Minimum surface hardness required. (Maximums may be specified for induction hardened parts).
    5. Those areas where the surface hardness is to be measured and the frequency of inspection.
    6. Depth of hardening required and the location(s) at which the depth is to be obtained.
    7. Whether destructive tests are to be used for determining the depth of hardening and the frequency of such inspection.
    8. Tempering temperature, if required.
    9. Magnetic particle inspection, if required.
  • Documentation.
    The heat treater should submit the following information:
    1. Surface hardness range obtained and the number of pieces inspected.
    2. Depth of hardening obtained at each location specified when destructive tests are required, and the number of pieces inspected.
    3. Results of magnetic particle inspection, if required.

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