Carburizing

Carburizing

Gas carburizing consists of heating and holding low carbon alloy steel (0.07-0.28 percent Carbon) at normally 1650-1800_F
(899-982_C) in a controlled atmosphere which causes additional carbon to diffuse into the steel (typically 0.70-1.10 percent carbon at the surface).
Gear blanks to be carburized and hardened are generally preheated after the initial anneal by a subcritical anneal at 1100_F-1250_F (590-675_C), normalize, normalize and temper or quench and temper to specified hardness before carburize hardening.
This is done for machinability, dimensional stability and possible grain refinement considerations. An intermediate stress relief before final machining before carburizing may be used to remove residual stress from rough machining.
After carburizing for the appropriate time, gearing will usually be cooled to 1475-1550_F (802-843_C), held at temperature to stabilize while maintaining the carbon potential, and direct quenched. Gearing may be atmosphere cooled after
carburizing to below approximately 600_F (315_C) and then reheated in controlled atmosphere to 1475-1550_F (802-843_C) and quenched. After quenching, gearing is usually tempered at 300-375_F (149-191_C). Gearing may be subsequently given a refrigeration treatment to transform retained austenite and retempered.

Applications.
Carburized and hardened gearing is used when optimum properties are required. High surface hardness, high case strength, favorable compressive residual stress in the hardened case, and suitable core properties based on selection of the appropriate carburizing grade of steel, result in the highest AGMA gear tooth ratings for contact stress, pitting resistance and root strength (bending).
Carburized gear ratings are higher than the ratings for through hardened and other types of surface hardened gearing because of higher fatigue strength. Improved load distribution can be obtained by subsequent hard gear finishing. Conventional hard gear finishing (skiving and grinding) results in some sacrifice of beneficial compressive stress at the surface and substantially increases costs.
Carburized gearing is used in enclosed gear units for general industrial use, high speed and aerospace precision gear units and also large open gearing for mill applications. Carburized gearing is also used for improved wear resistance. Specified finish operations after hardening depend upon accuracy and contact requirements for all applications.
Carburizing technology is well established and the available equipment and controls make it a reliable process. Surface hardness, case depth, and core hardness can be specified to reasonably close tolerances, and the quality can be audited.
Some gearing does not lend itself to carburize hardening because of distortion. Gearing which distorts and cannot be straightened without cracking, rack gears, thin sections, complex shapes, parts not designed for finishing or where finishing is cost prohibitive, present manufacturing problems. Press quenching after carburizing can be used to minimize distortion. Selected areas of gearing can be protected from carburizing (masked) to permit machining after hardening, or can be machined after carburizing and slow cooling before hardening.
Gearing beyond 80 inch (2032 mm) diameter is difficult to carburize due to the limited number of available furnaces for processing. Maximum size of carburize gearing is currently in the 120 inch (3048 mm) diameter range. Most of this large gearing requires tooth finishing (skiving and/or grinding) after carburizing and hardening.
Materials.
Material selection is an integral part of the design process. Selection should be made on the basis of material hardness and hardenability, chemistry, cleanliness, performance, and economical considerations. Performance criteria include, but are not limited to, the following: toughness, notch sensitivity, fatigue strength, bending strength, pitting wear resistance, and operational characteristics.
Control With Test Bars.
Test bars are used to show that the case properties and, when required, core properties meet specifications. Test bars should be of the same steel type as the gear(s), but not necessarily the same heat. Bars should accompany gearing through all heat treatments, including all post hardening treatments. Consideration should be given to evaluation of that portion of the case that is not removed during tooth finishing.
A section, with a ground and polished surface (normal, at mid length of a test bar), is considered satisfactory for determining effective case depth of carburized helical and spur gearing to 4 1/2DP. The test bar should have minimum dimensions of 5/8 inch (16 mm) diameter by 2 inch (50 mm) long. One inch (25 mm) diameter ¢2.0 inch (50 mm) long bar may be used for coarser pitch carburized gearing to 1.5 DP. The size of the bar for coarser than 1.5 DP gearing should be mutually agreed upon, and should approximate the inscribed diameter at mid height of the tooth cross section. The bar length should be 2-3 times the diameter.
Test discs or plates may also be used whose minimum thickness is 70 percent of the appropriate test bar diameter. The minimum in scribed diameter on a test disc (or plate dimensions) should be a minimum of three times its thickness.
The recommended test bar diameter for bevel gearing is to be approximately equal to the inscribed diameter of the normal tooth thickness at mid face width.
When disagreement exists as to the properties obtained on the test bar and the parts, an actual part may be sectioned for analysis.
1. Case Hardness.
  Case hardness should be measured with microhardness testers which produce small shallow impressions, in order that the hardness values obtained are representative of the surfaces or area being tested. Those testers which produce Diamond Pyramid or Knoop hardness numbers (500 gram load) are recommended. When measuring directly on the surface of a case hardened part or test bar, superficial or standard Rockwell A or C scale may be used. Other instruments such as Scleroscope or Equotip are also used when penetration hardness testers can not be used. Consideration must be given to the case depth relative to the depth of the impression made by the tester.
  Low readings can be obtained when the indentor penetrates entirely or partially through the case.
  Microhardness tests for surface hardness should be made on amounted and polished cross-section at a depth of 0.002 to 0.004 inch (.05 to .10 mm) below the surface. Care must be taken during grinding and polishing not to round the edge being inspected and not to temper or burn the ground surface.
  NOTE: Direct surface hardness readings (ASTM E18-79) or file checks at the tooth tip or flank will generally confirm the case hardness. However, if secondary transformation products are present below the first several thousandths of the case, direct surface checks will not necessarily indicate their presence.
  Microhardness inspection 0.002 to 0.004 inch (.05 to .10 mm) from the edge on a polished cross section of the tooth is more accurate. This type of inspection may be necessary for accurate micro-hardness readings near the surface.
2. Core Hardness.
  When required, core hardness may be determined by any hardness tester, giving consideration to the size of the specimen.
  NOTES: Occasionally banding, which results from the steel melting practice, can cause variations in core hardness during testing with amicrohardness tester. These variations should not fall below the minimum, when core hardness is specified.
3. Case Depth - Effective.
  The procedures used to prepare the cross sectioned specimen for case hardness (refer to 5.3.3) should be used to prepare the specimen for case depth evaluation. The microhardness traverse should be started 0.002 to 0.004 inch (.05 to .10 mm) below the surface and extend to at least 0.01 inch (.25mm) beyond the depth at which 50HRC is obtained. Usually an interval of 0.005 inch (0.13 mm) is used. Care should also be exercised in establishing the perpendicular to the mid tooth point when starting the traverse. Effective case depth at roots are typically 50-70 percent of mid tooth height case depths, and tips may be 150 percent of mid tooth height case depths.
  When steels of high hardenability such as 4320, 4327, 8627, 4820, 9310, and 3310 are used for fine pitches, the high through hardening characteristics of the steel may prevent obtaining a hardness less than 50 HRC across the tooth section. The case depth should then be determined in the following manner: Measure the base material hardness at mid tooth height at the mid face. For each one HRC point above 45 HRC, one HRC point should be added to the 50 HRC effective case depth criterion (example, core hardness equals 47 HRC, effective case depth should bemeasured at 52HRC).
  NOTE: Through carburized fine pitch teeth have several disadvantages. Favorable compressive surface stresses are lowered. Excessive tooth distortion and a loss of core ductility can also occur. Parts of this type should be carefully reviewed for case depth specifications and for use of lower hardenability steels such as 4620 and 8620.
4. Case Carbon Content.
  Surface carbon content may be determined from a round test bar by taking turnings to a depth of 0.005 inch (0.13 mm).
  Spectrographic techniques have also been developed for this purpose. Carbon gradient can also be determined on the bar by machining chips at 0.002 to 0.010 inch (0.05 to 0.25 mm) increments through the case, depending on accuracy desired and depth of case. Grinding in steps through the case would be used with spectrographic techniques.
  Test specimens should be carburized with the parts. Care should be exercised to maintain surface integrity during cooling or in tempering for subsequent machining. Bar should be straightened to within 0.0015 inch (0.038 mm) (TIR) before machining.
  Test specimens must be clean and machined dry. Care must be taken to ensure that the turnings are free of any extraneous carbonaceous materials prior to analysis.
5. Microstructure.
  The microstructure may be determined on a central normal section of the test bar or tooth, preferably mounted, after being properly polished and etched.
  Microstructure will vary with the core hardness as related to steel hardenability, section size and quench severity.
Specifications.
To aid in obtaining the above characteristics, the heat treater should be given the following as a minimum:
1. Material.
2. Case depth range.
3. Surface hardness range.
When additional characteristics are required, the following additional items may be specified in whole or part:
1. Core hardness. Approximate minimum tooth core hardness, which can be obtained from some typical carburizing grades of steel and good agitated oil quenching.
2. Core microstructure.
3. Case microstructure.
4. Surface carbon content.
5. Subzero treatment.
6. Areas to be free of carburizing by appropriate masking by copper plating or use of commercial stop-off compounds.
Carburizing Process Control.
Precision carburizing requires close control of many factors including:
1. Temperature Control.
  Furnace equipment with temperature uniformity, close temperature control, and accuracy of temperature recording and control instruments. Controls should be checked and calibrated at regular intervals.
2. AtmosphereControl.
  Furnaces should be capable of maintaining a carburizing atmosphere with controllable carbon potential. Instrumentation for continuous atmosphere control is preferred, but other approved methods may be used.
3. Subzero Treatment (Retained Austenite Conversion Treatment).
  When the surface hardness is low due to excessive retained austenite in the case microstructure, it may be necessary to refrigerate the parts to transform the retained austenite to martensite. The refrigeration treatment may vary from 20_F (-7_C) to -120_F (-84_C). To minimize microcracking, parts should be tempered before and after refrigeration.
  NOTE: Caution should be exercised in the use of refrigeration treatment on critical gearing. Microcracks can result which can reduce fatigue strength to a moderate degree. Use of refrigeration may require agreement between the customer and supplier.
4. Carbide Control.
  When high surface carbon results in a heavy continuous carbide network in the outer portion of the case, parts should be reheated to typically 1650_F(900_C)in a lower carbon potential atmosphere, typically 0.60 percent carbon, to diffuse and break up the excess carbide. Carbide networks should be avoided whenever possible as they tend to reduce fatigue strength of the material.
5. Decarburization.
  Surface decarburization as defined for carburized gearing is a reduction in the surface carbon in the outer 0.005 inch (.13 mm) below the specified minimum. This is characterized by an increase in carbon content with increasing depth; for example, when the peak carbon content is subsurface.
  Gross decarburization can be readily detected microscopically as a lighter shade of martensite and clearly defined ferrite grains. Hardness in this area will be substantially lower.
  Partial decarburization will result in a lighter shade of martensite, but may not show discernible ferrite. It will result in reduced hardness if the carbon content falls below approximately 0.60 percent.