Nitriding

Nitriding

The purpose of this section is to provide information, means of specifying, and inspection of nitrided gearing. This section covers the selection and processing of materials, hardnesses obtainable, and definitions and inspection of depth of hardening.
Conventional gas nitride hardening of gearing, which has had a quench and temper pretreatment and is usually finish machined, involves heating and holding at a temperature between 950-1060_F (510-571_C) in a controlled cracked ammonia atmosphere (10 to 30 percent dissociation). Nitride hardening can also be achieved with the ion nitriding
process. During nitriding, nitrogen atoms are absorbed into the surface to form hard iron and alloy nitrides. The practical limit on case depth is about 0.040 inch (1.0mm)maximum, which requires a thorough stress analysis (for other than wear applications) of the effectiveness of the case for coarse pitch gearing.

Applications.
Nitrided gears are used when gear geometry and tolerances do not lend themselves to other case hardening methods because of distortion, and when through hardened gears do not provide sufficient wear and pitting resistance. Nitrided gears are used on applications where thin, high hardness cases can withstand applied loads. Nitrided gears should not be specified if shock loading is present, due to inherent brittleness of the case.
Materials.
Steels containing chromium, vanadium, aluminum, and molybdenum, either singularly or in combination, are required in order to form stable nitrides at the nitriding temperature.
Typical steels suitable for nitriding are 4140, 4150, 4340, the Nitralloy grades, and steels with chromium contents of 1.00 to 3.00 percent. Aluminum containing grades such as Nitralloy 135 and Nitralloy N will develop higher case hardness.
Pre-reatments.
Parts to be nitrided must be quenched and tempered to produce the essentially tempered martensitic microstructure required for case diffusion. Microstructure must be free of primary ferrite, such as is produced by annealing and
normalizing, which produces a brittle case prone to spalling. The nitriding process will cause a slight uniform increase in size. However, residual stresses from quench and tempering may be relieved at the nitriding temperature, causing distortion. This should be avoided by tempering at approximately 50_F (28_C) minimum above the intended nitrided temperature after quenching. In order to minimize distortion of certain gearing designs, intermediate stress relieving after rough machining at 25-50_F (14-28_C) below the tempering temperature may also be required prior to finish machining to relieve machining stresses before nitriding. In alloys such as series 4140 and 4340 steels, nitrided hardness is lessened appreciably by decreased core hardness prior to nitriding. This must be considered when selecting tempering or stress relieving temperatures.
If distortion control is very critical, the newer ion nitriding process should be considered. Nitriding can be accomplished at lower temperatures with ion nitriding than those used for conventional gas nitriding.
Nitriding over decarburized steel causes a brittle case which may spall under load. Therefore, nitrided surfaces subject to stress should be free of decarburization. Sharp corners or edges become brittle when nitrided and should be removed to prevent possible chipping during handling and service.
Where it is desired to selectively nitride a part, the surfaces to be protected from nitriding can be plated with dense copper 0.0007 inch (0.018 mm) minimum thickness, tin plate 0.0003 to 0.005 inch (0.008 to 0.13 mm) thick, or by coating with proprietary paints specifically designed for this purpose.
Nitrided parts will distort in a consistent manner when all manufacturing phases and the nitriding process are held constant. The amount and direction of growth or movement should be determined for each part by dimensional analyses both prior to and after nitriding.
Nitriding Process Procedures.
Variables in the nitriding process are the combined effects of surface condition, degree of ammonia dissociation,
temperature, and time of nitriding. Nitrogen adsorption in the steel surface is affected by oxide and surface contamination. In order to guarantee nitrogen adsorption it may be necessary to remove surface oxidation by chemical or mechanical means.
The nitriding process affects the rate of nitrogen adsorption and the thickness of the resultant brittle white layer on the surface.
A two stage nitriding process (two temperatures with increased percent of ammonia dissociation at the second higher temperature) generally reduces the thickness of the white layer to 0.0005-0.001 inch (0.013-0.026 mm) maximum. The white layer thickness is also dependent upon the analysis of steel.
The ion nitride process uses ionized nitrogen gas to effect nitrogen penetration of the surface by ion bombardment. The process can provide flexibility in determining the type of compound produced. The process can also be tailored to better control nitriding of geometric problems, such as blind holes and small orifices.
Specific Characteristics of Nitrided Gearing.
Nitriding does not lend itself to every gear application. The nitride process is restricted by and specified by case depth, surface hardness, core hardness and material selection constraints.
1. Material Selection.
  Selection of the grade of steel is limited to those alloys that contain metal elements that form hard nitrides.
2. Core Hardness.
  Core hardness obtained in the quench and temper pretreatment must provide sufficient strength to support the case under load and tooth bending and rim stresses. Core hardness requirements limit material selection to those steels that can be tempered to the core hardness range with a tempering temperature that is at least 50_F (28_C) above the nitriding temperature.
3. Surface Hardness.
  Surface hardness is limited by the concentration of hard nitride forming elements in the alloy and the core hardness of the gear. Lower core hardness does not support the hard,thin case as well as higher core hardness. Lower core hardness will result from less alloy, larger section size, reduced quench severity and a greater degree of martensite tempering. Lower core hardness results in a microstructure which causes a lower surface hardness nitrided case, since it limits the ability to form high concentration of hard metallic nitrides. Surface hardness will also increase with increasing nitride case depth.
4. Case Depth.
  The specified case depth for nitrided gearing is determined by the surface and sub-surface stress gradient of the design application. Surface hardness and core hardness will influence the design’s minimum required case depth. Since the diffusion of nitrogen is extremely slow, most specifications only specify a minimum case depth requirement.
  Case depth should be determined using a microhardness tester. At least three hardness tests should be made beyond the depth at which core hardness is obtained to assure that the case depth has been reached.
  A test bar, for example 1/2 to 1 inch (13 to 25 mm) diameter with a length 3¢ the diameter, disc or plate section, can be used for determining case depth of nitrided parts. The test section must be of the same specified chemical analysis range and must be processed in the same manner as the parts it represents.
  Sectioning of an actual part to determine case depth need only be performed when the results of the test bar are cause for rejection, or the surface hardness of the part(s) is not within 3 HRC points of the surface hardness of the test bar.
Specifications.
Parts which are to be nitrided should have the following specified:
1. Material grade
2. Preheat treatment
3. Minimum surface hardness
4. Minimum total case depth
5. Maximum thickness of white layer, if required
6. Areas to be protected from nitriding by masking, if required
7. Nitriding temperature
8. Metallurgical test coupons