Quench hardening and tempering of gears

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Summary

1. Technical specification for quench hardening and tempering of gears...........................51.1 Field of application............................................................................................................51.2 General conditions.............................................................................................................51.3 Structures of the quench hardened and tempered gears....................................................61.4 Mechanical properties and tests........................................................................................7

2. Technical specification for case hardening and carbonitriding of gears followed by quench hardening and tempering............................................................................................8

2.1 Field of application............................................................................................................82.2 General conditions.............................................................................................................82.3 Measurement of the case hardening or carbonitriding hardness depth.............................92.4 Structure of the case hardened or carbonitrided layer.....................................................112.5 Mechanical properties of the case hardened or carbonitrided layer................................14

3. Technical specification for surface hardening treatment of gears.................................173.1 Field of application..........................................................................................................173.2 General conditions...........................................................................................................183.3 Treatment depth...............................................................................................................183.4 Surface hardening treatment profile................................................................................183.5 Structures of the heat treated layer..................................................................................193.6 Mechanical properties.....................................................................................................19

4. Technical specification for nitrided gears.........................................................................224.1 1.Field of application.......................................................................................................224.2 Definitions.......................................................................................................................234.3 General conditions...........................................................................................................244.4 Nitriding hardness depth.................................................................................................264.5 Structure of the nitrided case...........................................................................................274.6 Mechanical properties of the nitrided case......................................................................29

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1. Technical specification for quench hardening and tempering of gears

1.1 Field of application

This specification is applicable to quench hardened and tempered gears. The heat treatments may be carried out before or after cutting.

This specification defines the minimum conditions required prior to any mechanical finishing operation for any quench hardened and tempered gears with the exception of gears covered by a particular order or specification. This document assumes that the material was selected as being appropriate to the requirements of the quench hardening heat treatment in relation to the dimensions and shape of the gear to be treated.

The core mechanical characteristics of large dimension gears can be achieved only if the steel has a sufficient hardenability.

The wrought steels usually used by gear manufacturers are (EN 10083-3 standard):

30CrNiMo8 ;

36NiCrMo16 ;

42CrMo4 ;

35NiCr6.

Cast steels are also used (EN 10293 standard):

G26CrMo4 ;

G34CrMo4 ;

G35CrNiMo6-6.

1.2 General conditions

1.2.1 Austenitizing

The heating operation shall be carried out using one of the usually permitted processes:

furnace with or without controlled atmosphere ;

vacuum furnace ;

salt bath furnace ;

fluidized bed furnace.

The austenitizing temperature shall be the temperature recommended by EN 10083-3 standard for hot formed steels, or the temperature recommended by EN 10293 standard for cast steels. After agreement, a different austenitizing temperature may be used.

The austenitizing time shall make it possible to obtain a uniform temperature within the part as well as a homogeneous austenite. Once the core of the part has reached the required temperature, the temperature holding time shall depend on the mass of the part and it is usually set according to the following rule:

1 minute/mm of thickness with a minimum of 30 minutes

1.2.2 Quench hardening

Cooling shall be performed in a medium whose nature shall be function of the size of the part to be treated, the steel grade and the mechanical characteristics to be obtained.

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The media used can be as follows: water, water + additives, oil, pulsed air, calm air, gas quench hardening (vacuum furnace), salt bath, fluidized bed (martempering).

The type of quench hardening treatments used may be martensitic quench hardening (continuous cooling), martempering, or austempering. The steel grade used shall allow the selected type of quench hardening treatment to be carried out.

In the case of step quench hardening, the heat treatment parameters (temperature step, holding time) are determined form the ICT diagrams of the steel to be treated. These diagrams represent the kinetics of the transformations during cooling during an isothermal treatment. Cooling after the temperature step shall be carried out with air, unless particular agreements.

1.2.3 Tempering

Tempering is mandatory after martensitic quench hardening.

Tempering shall be carried out using one of the usually permitted processes: furnace with controlled atmosphere or not, vacuum furnace, salt bath furnace, fluidized bed furnace, etc.

The tempering temperature and time shall make it possible to obtain the required mechanical characteristics. However, the temperature range 260°C - 450°C shall be avoided (temper brittleness range), especially with chromium alloyed steels. A tempering temperature located within this range shall be covered by a particular agreement.

When the core of the parts has reached the required temperatures, the temperature holding time shall depend on the mass of the part and it is usually set according to the following rule:

2 minutes/mm of thickness with a minimum of 1 hour

Unless otherwise agreed, post-tempering cooling shall be carried out with calm air (furnace with controlled atmosphere or not, salt bath furnace, fluidized bed furnace).

1.3 Structures of the quench hardened and tempered gears

The austenitizing temperature and time shall not lead to excessive grain coarsening. The steel must have a fine-grained structure with a proportion of more than 90% of the surface with a grain size index above or equal to 5, measured in accordance with ISO 643 standard, according to the indications of ISO 6336-5 standard.

After quench hardening, the structure shall be martensitic or bainitic, depending on the type of heat treatment achieved, in the zone limited by a distance of 1.3 times the tooth height (h) from tooth tip (that is to say 2.9 times the normal module for A profile basic rack according ISO 53 standard) as defined in Figure 1.

Surface decarburization is not permitted, unless particular agreement. The decarburized depth shall be measured after quench hardening, using one of the methods described in ISO 3887 standard.

Figure 1: zone which shall be martensitic or bainitic

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A

B

M

~ 0.5 h

~ 0.9 h

h ~ 1.2 h

1.4 Mechanical properties and tests

1.4.1 Mechanical properties

After quench hardening and tempering, the hardness values specified shall correspond to that which can be obtained with the steel used, depending on the dimensions of the treated part (refer to EN 10083-3 and EN 10293 standards), the cooling medium and the tempering temperature.

The mechanical characteristics shall be measured in HB, HRC or HV30. When equivalences between the different hardnesses, or between post-treatment tensile strength values Rm and hardnesses are necessary, Table B.2 of ISO 18265 standard shall be used (steel hardness conversion values).

If decarburization or carburization is to be checked, it is possible to complete the measurements with a low-load hardness test whose definition shall be covered by a particular agreement.

1.4.2 Tests

The core and surface characteristics shall correspond to those which were specified for the steel used.

Unless otherwise agreed, surface hardness shall be measured on both lateral faces of the teeth at the point M defined below (see Figure 2) and, if possible, on a flank at mid-height.

After agreement, the hardness test may be carried out on a part cross section at tooth mid-width, or on a test coupon. The test coupons defined by the requesting party shall be covered by a particular specification.

material core, along the tooth axis, at point M located at a distance of approximately 1,2 times the tooth height (h) from tooth tip (that is to say 2.7 times the normal module for A profile basic rack according ISO 53 standard) ;

at points A and B when the distance of the hardness test on the surface is defined by the requesting party.

Figure 2: location of the measuring points on the cross section of a tooth

Hardness shall be measured in accordance with ISO 6506 (HB), ISO 6508 (HRC) or ISO 6507 (HV) standards.

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2. Technical specification for case hardening and carbonitriding of gears followed by quench hardening and tempering

2.1 Field of application

This specification defines the minimum conditions required prior to any mechanical finishing operation for any case hardening or carbonitriding of gears subsequently hardened by quenching and tempering, with the exception of gears covered by a particular order or specification.

This document is applicable to case hardened and carbonitrided layers with thickness above 0.3 mm and hardness above 58 HRC or 660 HV, in accordance with ISO 6336-5 standard.

This specification assumes that the material was selected as being appropriate to the requirements of case hardening or carbonitriding followed by quench hardening in relation to the dimensions and shape of the gear to be treated.

2.2 General conditions

2.2.1 Characteristics of the case hardened or carbonitrided layer

Case hardening or carbonitriding shall be obtained through one of the usually admitted processes (gaseous, low pressure, etc.).

The case hardened or carbonitrided layer shall have:

a thickness complying with the thickness required on the order (1), which shall take into account the machining allowance operation after heat treatment .

a regular profile reproduced on all teeth .

a healthy structure (refer to paragraph 2.2.4).

The mechanical characteristics of the case hardened or carbonitrided layer shall correspond to those specified on the order or, in default of, to the characteristics described in paragraph 5.

Carbonitriding is used for small- and medium-sized module gears whose treatment depth rarely exceeds 0.5 mm.

2.2.2 Core characteristicsThe core characteristics shall correspond to those which were specified for the steel used. If there are no particular specifications, the core characteristics shall comply with those which can normally be expected for the steel used, considering the mass of the part (refer to EN 10084).

Core hardness shall be checked on a part cross section (or a sample (2)) along the axis of the tooth, at a point M located at a distance approximately equal to 1.2 times the height (h) of the tooth when starting from the tip (that is to say 2.7 times the normal module for A profile basic rack according ISO 53 standard) (see figure 3).

1 The depth specified on the order, which includes the machining allowance, shall take precedence over the depth indicated on the drawing, this depth being understood as the depth on the finished workpiece.2 The specimens defined by the requesting party shall be covered by a particular specification.

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A A

BB

M

~ 0,5 h

~ 0,9 h

h ~ 1,2 h

Figure 3: indication of the locations of the measuring points on the cross section of a tooth

The presence of ferrite clusters is not acceptable within the limit which corresponds to the high stress zone, that is to say approximately 1.3 times the height (h) of the tooth when starting from the tip (that is 2.9 times the normal module for A profile basic rack according ISO 53 standard) (see Figure 1). In this zone, the accepted structures are: martensite, acicular ferrite and bainite.

Figure 4: zone in which the presence of ferrite clusters is not permitted

2.3 Measurement of the case hardening or carbonitriding hardness depth

By convention, the case hardening or carbonitriding hardness depth (CHD) is defined by the zone with hardness above 550 HV (if the core hardness is less than 450 HV) or 650 HV (if the core hardness is more than or equal to 450 HV) measured on a cross section to the quenched and tempered case hardened or carbonitrided surface.

This measurement shall be carried out by microhardness profile on a reference sample or on a part cross section in accordance with the requirements of ISO 2639 standard and using a load between 4.9 N (HV0.5) and 49 N (HV5) and usually set to 9.81 N (HV1).

The depth shall be measured at tooth mid-height, along direction AA, at approximately 0.5 times the height of the tooth from tooth tip (that is 1.1 times the normal module for A profile basic rack according ISO 53 standard) and at tooth root along direction BB at approximately 0.9 times the height of the tooth from tooth tip (that is 2.1 times the normal module for A profile basic rack according ISO 53 standard) (see figure 3).

On a same part, the case hardening or carbonitriding hardness depth shall be between a lower limit which is the nominal value of the case hardening or carbonitriding hardness depth and an upper limit which is the nominal value of the case hardening or carbonitriding hardness depth

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to which an upper limit deviation is added. The values of the upper limit deviations are given in Table 1 (3).

Case hardening or carbonitriding hardness depth

(CHD) (mm)

Upper limit deviation (mm)

0.3 0.2

0.5 0.3

0.8 0.4

1.2 0.5

1.6 0.6

2.0 0.8

2.5 1.0

3.0 1.2

Table 1: upper limit deviations in relation to the case hardening or carbonitriding hardness depth (CHD) (in accordance with ISO 15787 standard)

2.4 Structure of the case hardened or carbonitrided layer

The case hardened or carbonitrided layer shall be martensitic after quench hardening. The martensite shall be fine and mainly acicular. The bainite content shall be below 10% in accordance with the indications of ISO 6336-5 standard.

Tempering shall be carried out at a maximum temperature of 200°C. Once the core of the parts has reached the required temperature, the temperature holding time shall depend on the mass of the part and it is usually set according to the following rule: 2 minutes/mm of thickness, with a minimum of 1 hour.

3 After specific agreement between the requesting party and the heat treatment provider, a higher upper limit deviation may be granted.

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The steel shall have a fine-grained structure with a proportion of more than 90% of the surface with a grain size index above or equal to 5, measured in accordance with ISO 643 standard, according to the indications of ISO 6336-5 standard.

A maximum of 30% of retained austenite (4) shall be tolerated. This value can be measured by X-ray diffraction. The micrographic examination is not able to quantify the retained austenite content with a good precision. It is necessary to take into account the surface hardness drops visible on the hardness curve; these hardness drops shall not exceed 40 HV (difference of hardness between the maximum hardness point and the point located at a depth of 0,1 mm). If applicable, and subject to approval from the requesting party, the detected austenite quantity may be reduced by cryogenic treatment.

Note: the execution of X-ray diffraction tests shall be validated during the contract review after agreement between the requesting party and the heat treatment provider.

The presence of free carbides (cementite) semi-continuous or continuous networks (4) is strictly prohibited (see Photos 1 and 2). By concession, a few carbides dispersed in the form of cementite nodules may be tolerated at tooth tip (see Photo 3). The examination shall be carried out in accordance with NF A 05-150 standard “Micrographic examination Techniques”, using a magnification of 400 to 500.

4 After specific agreement, a concession may be granted by the requesting party, depending on the mechanical finishing method applied to the gears.

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Photo 1: semi-continuous carbide network. Prohibited structure (in accordance with standard ISO 6336-5).

Photo 2: discontinuous carbide networks. Structure prohibited without approval from the requesting party (in accordance with ISO 6336-5 standard).

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Photo 3: dispersed carbides in the form of cementite nodule. Acceptable structure after agreement between the requesting party and the heat treatment provider (in accordance with ISO 6336-5

standard).

Internal oxydetation, depending on the case hardening or carbonitriding hardness depth (see Table 2), in accordance with the indications of ISO 6336-5 standard. The inspection shall be carried out in accordance with NF A 05-150 standard after very slight 2% Nital etching.

Case hardening or carbonitriding depth

(mm)

Intergranular oxidation depth (µm)

< 0.75 12

≥ 0.75 and < 2.25 20

≥ 2.25 and < 3 25

≥ 3 and < 5 30

≥ 5 35

Table 2: thickness affected by intergranular oxidation after gas case hardening or carbonitriding heat treatment, in relation to the heat treatment depth. In accordance with ISO 6336-5 standard.

The presence of porosities is not acceptable. A micrographic inspection shall be carried out in accordance with NF A 05-150 standard, without etching and with 200X magnification. See the example in photo 4.

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Photo 4: example of porosities on a case carbonitrided layer (without etching)

2.5 Mechanical properties of the case hardened or carbonitrided layer

Unless particular specification on the order, the surface hardness of the case hardened layer shall be between 58 and 64 HRC, or between 660 HV and 800 HV.

The selected figure shall be the average of at least three measurements.

As regards hardness measurement, Table 3 indicates, for each test, the standard reference as well as the hardness values which must be obtained, while table 4 specifies the hardness measurement methods to be applied (5) depending on the required case hardening or carbonitriding hardness depth. Hardness conversions shall be carried out in accordance with Table A1 of ISO 18265 standard.

Hardness testHardness

values to be obtained

Standart reference for

the testHRC 58 to 64 ISO 6508HV 660 to 800 ISO 6507

HR 15 N 90 to 92 ISO 6508HR 30 N 77 to 81 ISO 6508HR 45 N 65 to 71 ISO 6508

HRA 80 to 83 ISO 6508

Table 3: hardness values to be obtained depending on the test carried out

The thickness of the case hardened or carbonitrided layer is determined according to the level of the transmitted forces. The case hardening or carbonitriding hardness depth shall be more than twice the depth of the maximum shear stress caused by contact pressure.

When the depth of the maximum shear stress is unknown, it is possible to assess the required heat treatment depth by referring to Figure 5 (extracted from ISO 6336-5 standard) which indicates the recommended case hardening hardness depth (CHD) as a function of the gear module (optimum value regarding surface load, and maximum value regarding embrittlment at tooth tip).

5 It is also possible to use hand-held microhardness testers to carry out these tests. In such a case, the test implementation conditions and the values to be obtained shall be covered by specific agreements.

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Figure 5: recommended values for case hardening hardness depth on finished part, in accordance with ISO 6336-5

The case hardening or carbonitriding hardness depths are those depths which remain after finishing. Table 5 gives a few examples of materials and depths usually used.

The depth values indicated are minimum values. As a general rule, the maximum case hardening or carbonitriding hardness depth values are set according to the following rule: maximum depth = 0.4 x module.

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Table 4: selection of test method for specifying hardness data according to minimum depth of hardening and minimum surface hardness in HV (in accordance with ISO 15787 standard)

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Type of gearGear module

(mm)Minimum depth

(mm)Examples of materials

used

Small module 0.6 to 2 0.3

17CrNi6-618CrNiMo7-6

20MnCr518NiCr5-415NiCr13

20NiCrMo2-2

Medium module

2 to 2.5 0.5

18NiCr5-417CrNi6-6

18CrNiMo7-620NiCrMo2-2

16MnCr520MnCr515NiCr13

2.5 to 3 0.6

3 to 3.5 0.7

3.5 to 4 0.85

4 to 4.5 1

4.5 to 5 1.2

5 to 5.5 1.4

5.5 to 6 1.6

Large module

7 to 8 1.7

18CrNiMo7-614NiCrMo13-4

8 to 9 1.8

9 to 10 1.9

10 to 12 2

Very large modules

12 to 20 2.2

18CrNiMo7-620 to 25 2.8

25 to 36 3.5

> 36 4

Table 5: examples of materials and case hardening or carbonitriding hardness depths usually used

3. Technical specification for surface hardening treatment of gears

3.1 Field of application

This specification defines the minimum conditions required before any mechanical finishing operation for any surface hardened and tempered gears with the exception of gears covered by a particular order or specification.

This document is applicable to hardened cases whose hardness is between 485 HV and 615 HV or between 48 HRC and 56 HRC, in accordance with ISO 6336-5 standard.

This specification assumes that the material was selected as being appropriate to the requirements of the gear teeth surface hardening heat treatment in relation with the dimensions and shape of the gears to be treated (6).

6 In particular, note that obtaining significant depths is possible only if the steel has a sufficient hardenability (see Table 3).

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3.2 General conditions

3.2.1 Surface hardened layer characteristics

Surface heating shall be carried out by one of the usually permitted processes (induction, flame, etc.).

The surface hardened case shall have:

a depth complying with that required on the order (7) (refer to paragraph Error: Referencesource not found), which shall include the machining allowance for any finishing operation after heat treatment ;

an regular profile reproduced on all teeth (refer to paragraph 3.4) ;

a healthy structure (refer to paragraph 3.5).

The case mechanical characteristics shall correspond to those specified on the order or, in default of, the characteristics described in paragraph 3.6.

3.2.2 Core characteristics

The core characteristics are dependent on the heat treatments carried out prior to surface hardening. Therefore, it is necessary to refer to the technical specifications which correspond to these heat treatments (annealing, through hardening, etc.).

Following particular agreements, these characteristics may be checked after the surface hardening operation. In that case, the core hardness test shall be carried out on a part cross section (or on a sample (8)) in conditions to be agreed by the parties.

The steel shall have a fine-grained structure with a proportion of more than 90% of the surface with a grain size index above or equal to 5, measured in accordance with ISO 643 standard, according to the indications of ISO 6336-5 standard.

3.3 Treatment depth

By convention, the surface hardening hardness depth (SHD) is defined between the surface and the zone with hardness equal to 0.8 times the surface minimum hardness required. This hardness is dependent on the steel grade (see Table 6).

This measurement shall be carried out on a part cross section of the treated surface (refer to paragraph 4) through microhardness profile, in accordance with NF A 04-203 standard, that is to say using a load of 9.8 N (HV1) and the hardness limit values indicated in Table 6 according to the steel grade.

For a given required depth, the hardened layer depth shall be between a lower limit which is the nominal value of the case depth and an upper limit which is the nominal value of the surface hardening hardness depth to which an upper limit deviation is added. The values of the upper limit deviations are given in Table 7.

3.4 Surface hardening treatment profileThe profile of the heat treatment which must be carried out on the gears shall be selected in Table 3 by the requesting party.

The profile may be checked on a part cross section (or on a sample ( 8)), as indicated in Figure 5, at a distance from the lateral face at least equal to three times the required surface hardening hardness 7 The case depth specified on the order, which includes the machining allowance, shall take precedence over the depth specified on the drawing, this depth being understood as the depth on the finished part.8 The samples defined by the requesting party shall form the subject of a particular specification.

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3 SHD

depth (SHD). It is recommended to carry out this check on the 2 lateral faces in order to verify the heat treatment homogeneity.

Figure 5: location of the cross section of a gear tooth for the check of the heat treatment depth

The locations of the measuring points for the various profiles are mentioned in Table 8. The measurements shall be carried out over the height of the tooth (h) in different sections normal to the surface (AA, BB and CC).

For tooth profiles which are not “through hardened” and in particular for the “tooth tip, flank and bottom” hardened profile, it is necessary to check that the thickness measured along the axis of the tooth does not exceed a distance of approximately 0.6 times the height (h) from tooth tip (that is to say 1.5 times the normal module for A profile basic rack according ISO 53 standard).

The regularity of the profiles may be qualitatively assessed on the cross section through macroscopic examination after chemical etching as defined in NF A 05-152 standard. However, in case of dispute, only the microhardness measuring method defined in paragraph 3.3 shall be taken into account.

3.5 Structures of the heat treated layer

The heat treated layer is martensitic; it shall not contain free ferrite. The case of large parts may form the subject of specific agreements.

A tempering treatment is recommended to limit brittleness. Tempering may be performed by induction or using a furnace. When it is carried out with a furnace, the stress relief tempering treatment shall be carried out within a temperature range from 160°C to 200°C maximum. When the core of the parts has reached the required temperature, the temperature holding time shall depend on the mass of the part and it is usually set based on the following rule: 2 minutes/mm of thickness with a minimum of 1 hour.

Adjusting tempering treatments may be performed upon request, at higher temperature, in order to obtain lower hardness values.

3.6 Mechanical properties

The post treatment surface hardness values are mentioned in Table 6, for information. They are expressed in HRC.

However, the hardness test cannot always be carried out with this testing method. When hardness conversions have to be performed, Table A1 of ISO 18265 standard shall be used.

Table 9 specifies the hardness testing methods to be applied according to the required surface hardening hardness depth SHD.

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Material grades (9) Surface minimum hardness HRC (10)

Surface minimum Hardness

(in HV) corresponding to HRC (minimum)

Usual surface hardening hardness

depth (SHD) (mm) (11)

C40E C40RC45E C45RC50E C50R

525457

435465510

222

38Cr246Cr237Cr441Cr4

53565356

450490450490

4.5566

42CrMo4 57 510 4

38MnB5 53 450 6

Table 6: characteristics of a few steels defined by NF A 35-563 and EN 10083 standards -“Special steels suitable for surface hardening heat treatments

Surface hardening hardness depth (SHD)

(mm)

Upper limit deviation (mm)

Induction hardening Flame hardeningLaser hardening and

electron beam hardening

0.1 0.1 - 0.1

0.2 0.2 - 0.1

0.4 0.4 - 0.2

0.6 0.6 - 0.3

0.8 0.8 - 0.4

1.0 1.0 - 0.5

1.3 1.1 - 0.6

1.6 1.3 2.0 0.8

2.0 1.6 2.0 1.0

2.5 1.8 2.0 1.0

3.0 2.0 2.0 1.0

4.0 2.5 2.5 -

5.0 3.0 3.0 -Table 7: upper limit values according to the surface hardening hardness depth (SHD) and the process

used (in accordance with ISO 15787standard) (12)

9 As the descriptions of the steels have changed since the publication of NF A 35-563 standard, the material grades indicated are equivalent designations still present in EN 10083 standard.10 According to NF A 35-563, these values correspond to the minimum case hardness values which can be achieved through surface hardening and after removal (if applicable) of the decarburized layer.11 These values correspond to the maximum heat treatment depths which can normally be achieved (on a round bar with diameter 100 mm) after surface hardening of steel grades whose hardenability is at the bottom of the range permitted by the standards. These values are given for information, to provide the mechanics operator with a guidance when selecting the appropriate steel. The operating conditions are likely to change these values in a very significant way.12 After particular agreement between the requesting party and the heat treatment provider, a higher upper limit deviation may be granted.

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AA

C C~ 0.3 h

~ 0.9 h

h

B B

~ 0.6 h

~ 0.9 h

h

~ 0.6 h

A A

Profile No.

Type and description of the required profile

Profile application Locations of the measuring points

1

“Tooth flank and root” hardening

“Ideal” profile, usual in case of tooth-by-

tooth treatment.(module > 6 mm)

2

“Tooth tip, flank and root” hardening

Usual profile in case of heat treatment

with a circular inductor.

3

“Through tooth” hardening

Usual profile in case of treatment of small modules (between

0.5 mm and 1.5 mm).

This profile does not make it possible to

obtain a high impact strength. These profiles shall be covered by a particular

agreement as regards their use and the locations of the measuring points.

4

“Tooth flank” hardening

This profile is not recommended as

fractures may initiate in the fillet

radius at tooth root.However, this profile may be suitable for large module gears used with low loads.

Table 8: profiles achievable by surface hardening, and location of measuring points

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Table 9: selection of test method for specifying hardness data according to minimum depth of hardening and minimum surface hardness in HV (in accordance with ISO 15787 standard)

4. Technical specification for nitrided gears

This document was prepared from data of ISO 15787 standard (update in progress) and NF A 02-052 draft standard.

4.1 1. Field of application

Nitriding is carried out on gears with small-sized modules (less than 12 mm) with 2 objectives:

improve friction, especially in cold condition ;

improve mechanical strength, especially in case of low loads.

This specification defines the minimum conditions required prior to any mechanical finishing operation for any nitrided gear teeth except gears covered by a particular order or specification.

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This specification assumes that the steel grade was selected by taking into account the type of nitriding treatment considered and according to the mechanical characteristics to be obtained on the part:

required core strength level after quench hardening and high temperature tempering (temperature at least 50°C higher than the nitriding temperature (13)) regarding the dimensions and the shape of the gear to be treated ;

hardness level to be obtained in the diffusion layer, and depth of the diffusion layer ;

thickness of the compound layer (usually called “white layer”).

This specification also assumes that the manufacturing process was defined so that there will be no carburization or decarburization left on the effective areas which have to be nitrided.Nitrided gears are usually made of steel from EN 10085 standard:

24CrMo13-6,

31CrMo12,

32CrAlMo7-10,

31CrMoV9,

33CrMoV12-9,

34CrAlNi7-10,

41CrAlMo7-10,

40CrMoV13-9,

34CrAlMo5-10,

to which a few material grades from EN 10083-3 standard can be added:

34CrMo4,

36NiCrMo16,

30CrNiMo8.

4.2 Definitions

In the scope of this specification, the term “nitriding” is applied to nitriding heat treatments and their derived processes, in particular nitrocarburizing heat treatments. As a matter of fact, nitrocarburizing is a variant of the nitriding process in which the type ε white layer is in larger quantity.

The main characteristic of these treatments is that they are carried out at temperatures for which iron is in the ferritic state (iron ), that is to say temperatures usually between 480°C and 580°C.

The treated layers (Figure 6) may include:

a layer in which nitrogen is in compound state, in the form of or ’ nitride. This layer is called “compound layer” (or also “white layer” at it appears white when viewed with an optical microscope after Nital etching) ;

a diffusion layer in which nitrogen is in solid solution or in compound state.

13 A difference of temperature less than +50°C shall be covered by a specific agreement.

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, ’ compound layer or mixed /’ compound layer

Diffusion layer reaching a depth of a few tenths of millimetre

4 possible configurationsX 500

Depending on whether the compound layer is present or absent and depending on the nature of the nitrides formed, four structural configurations are possible with these treatments (see Figure 6).

’ +’

Diffusion layer Diffusion layer Diffusion layer Diffusion layer

Mainly

+

diffusion layer

Mainly ’

+

diffusion layer

two-phase +’ layer

+

diffusion layer

Diffusion layer only

Figure 6: different structural configurations of the nitrided or nitrocarburized layers

4.3 General conditions

4.3.1 Characteristics of the nitrided case

Nitriding shall be carried out by means of one of the usually admitted processes (gas nitriding, salt bath nitriding, plasma nitriding, etc.).

The nitrided case shall have:

a depth complying to that required on the order (14) (compound layer thickness and/or diffusion layer depth), which shall include the required machining allowance for any finishing operation after treatment ;

a regular profile reproduced on all gear teeth ;

a sound structure (refer to paragraph 4 4.5).

4.3.2 Core characteristics

The core characteristics are dependent on the heat treatments performed before nitriding. Therefore, it is necessary to refer to the technical specifications which correspond to these heat treatments (quench hardening and high temperature tempering).

14 The depth specified on the order, which includes the machining allowance, shall take precedence over the depth specified on the drawing, this depth being understood as the depth on the finished part.

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A A

B B

M~ 0.5 h

~ 0.9 h

h ~ 1.2 h

Core hardness must be checked on a part cross section (or on sample (15)) along the axis of the tooth, at a point M located at a distance approximately equal to 1.2 times the height (h) of the tooth when starting from the tip (that is to say 2.7 times the normal module for A profile basic rack according ISO 53 standard) (see Figure 7), or at a point located at a distance from the surface at least equal to 3 times the diffusion layer depth.

For gear modules smaller than 2, if the core hardness is to be measured at less than 5  mm from the edge, then it shall be measured 5 mm below the surface.

The steel shall have a fine-grained structure with a proportion of more than 90% of the surface with a grain size index above or equal to 5, measured in accordance with ISO 643 standard, according to the indications of ISO 6336-5 standard. For the remaining 10%, the grain size shall be above or equal to 3.

Figure 7: indication of the locations of the measuring points in the cross section of a tooth

4.4 Nitriding hardness depth

The maximum depth usually required is 0.6 mm after grinding finish.

In accordance with ISO 6336 5 standard, the effective nitrided case depth is defined as the distance from the surface to a point where hardness is equal to 400 HV or 40.8 HRC. By convention, if the hardness exceeds 380 HV, the nitriding hardness depth (NHD) is defined by the zone where hardness is higher or equal to the core hardness + 50 HV.

This measurement shall be carried out by microhardness profile on a sample part or on a part cross section.

The depth shall be measured at tooth mid-height, along direction AA, at approximately 0.5 times the height of the tooth from tooth tip (that is to say 1.1 times the normal module for A profile basic rack according ISO 53 standard), and at tooth root, along direction BB, at approximately 0.9 times the height of the tooth from tooth tip (that is to say 2.1 times the normal module for A profile basic rack according ISO 53 standard) (see Figure 7).

On any part of the same batch and for the same location of the inspection cutting, the nitriding hardness depth shall be between a lower limit which is the nominal value of the nitriding hardness depth and an upper limit which is the nominal value of the nitriding hardness depth to which an upper limit deviation is added. The values of the upper limit deviations are given in Table 10 (15).

15 The specimens defined by the requesting party shall be covered by a particular specification.

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Nitriding hardness depth (NHD)

(mm)

Upper limit deviation(mm)

0.05 0.05

0.10 0.10

0.15 0.10

0.20 0.10

0.25 0.12

0.30 0.15

0.35 0.17

0.40 0.20

0.50 0.25

0.60 0.30

0.75 0.37

Table 60: upper limit deviations depending on the nitriding hardness depth (NHD) (in accordance with NF A 02-052 draft standard)

4.5 Structure of the nitrided case

4.5.1 Diffusion layer

The diffusion layer shall have a regular profile reproduced on all teeth.

The hardness of the diffusion layer shall correspond to the value usually expected with the steel used for usual nitriding conditions (refer to paragraph 4.6).

The presence of fine precipitates of cementite, carbonitrides or nitrides in the diffusion layer is permissible.

The continuous networks of aligned precipitates parallel to the surface (see Photo 5) are prohibited for low and medium depths (less than or equal to 0.3 mm), but permissible for high depths (more than 0.3 mm).

Continuous networks surrounding the grains (see Photo 6) are prohibited in the diffusion layer.

4.5.2 Compound layer

If the gear is not ground after heat treatment and if a specific compound layer is wanted, then its nature and thickness are specified on the order by the requesting party (as regards the nature of the compound layer, it is possible, for instance, to just require the presence of the compound layer or, if necessary, it is possible to specify the expected phases: , ’, presence of oxide, etc. (16)).

16 For more information on the types of compound layers to be used, it is possible to consult the articles published by Techniques de l’Ingénieur: M1224 “Theories of thermochemical treatments – Nitriding – Nitrocarburizing” and M1227 “Nitriding, nitrocarburizing and derived processes”.

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In these conditions:

the compound layer shall have a regular profile reproduced on all teeth ;

the nature and thickness of the compound layer shall comply with the specifications required on the order; in case of dispute, the nature of the compound layer may be determined through X-ray diffraction examination.

Note: the execution of X-ray diffraction tests shall be validated during the contract review after agreement between the requesting party and the heat treatment provider.

ISO 6336-5 standard specifies that the compound layer thickness (CLT) shall not exceed 25 µm for wrought and nitriding steels and shall be between 5 µm and 30 µm for nitrocarburized wrought steels (17).

The presence of microporosities is permitted only on half the compound layer thickness (Photo7).

Photo 5: continuous networks or aligned precipitates parallel to the surface, prohibited for small and medium diffusion layer depths

Photo 6: continuous networks surrounding the grains, prohibited in the diffusion layer

17 A higher upper limit deviation may be specified after agreement between the requesting party and the heat treatment provider.

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Photo 7: permissible presence of microporosities in the compound layer (BODYCOTE document)

4.6 Mechanical properties of the nitrided case

The surface hardness of the nitrided case shall correspond to that which can be expected with the steel used.

The hardness of the diffusion layer is dependent on the chemical composition of the steel and, in particular, the chromium, molybdenum, aluminium, and vanadium contents. The hardness also changes depending on the heat treatments previously achieved to the part. After nitriding, the diffusion layer hardness values of the parts made of through hardened and tempered steel are higher than those of the parts whose structure is annealed, as in the case of nitrocarburized wrought steels.

For information, Table 11 gives the hardness levels which can be reached for a few steel grades usually used to make gears.

After particular agreements, it is also possible to define surface hardness measuring tests to assess the overall quality of the heat treatment. In such a case, the hardness values to be specified may be selected by taking into account the indications of Table 11, and the selection of the recommended test for the measurement shall comply with the indications of Table 12. The test conditions shall also be specified (location of the measuring point, removal or not of the compound layer, etc.).

Material grade Vickers hardness (HV) of the diffusion layer

34CrMo4 500 – 800

42CrMo4 500 – 800

31CrMo12 800 – 1200

31CrMoV9 700 – 1000

30CrNiMo8 500 – 800

32CrMoV12-9 800 – 1200

Table 11: usual surface hardness of the diffusion layer after nitriding, depending on the steel in quench hardening and tempering condition

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The thickness of the nitrided case is determined according to the transmitted loads. As a general rule, the nitriding hardness depth shall be more than twice the depth of the maximum shear stress caused by contact pressure.When the depth of the maximum shear stress is unknown, it is possible to assess the required heat treatment depth by referring to Figure 8 (extracted from ISO 6336-5 standard) which indicates the recommended minimum nitriding hardness depth (NHD) according to the gear module.

Figure 8: recommended nitrided case depth values on finished part, in accordance with ISO 6336-5

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Minimum nitrided case

depth(mm)

Minimum surface hardness

≥ 300 HV≤ 400 HV

> 400 HV≤ 500 HV

> 500 HV≤ 600 HV

> 600 HV≤ 700 HV

> 700 HV≤ 800 HV > 800 HV

0.05 HV 0.1 HV 0.1 HV 0.1 HV 0.1 HV 0.1 HV 0.1

0.07 HV 0.1 HV 0.1 HV 0.3 HV 0.3 HV 0.3 HV 0.5

0.08 HV 0.1 HV 0.3 HV 0.3 HV 0.5 HV 0.5 HV 0.5

0.09 HV 0.3 HV 0.3 HV 0.5 HV 0.5 HV 0.5 HV 0.5

0.10 HV 0.3 HV 0.5 HV 0.5 HV 0.5 HV 1 HV 1

0.15 HV 0.5 HV 1 HV 1 HV 1 HV 1 HV 1

0.20 HV 1 HV 1 HV 1 HV 3 HV 3 HV 3

0.25 HV 1 HV 3 HV 3 HV 5 HV 5 HV 5

0.30 HV 3 HV 3 HV 5 HV 5 HV 5 HV 5

0.40 HV 5 HV 5 HV 10 HV 10 HV 10 HV 10

0.45 HV 5 HV 10 HV 10 HV 10 HV 10 HV 10

0.50 HV 10 HV 10 HV 10 HV 10 HV 10 HV 10

0.55 HV 10 HV 10 HV 10 HV 10 HV 10 HV 30

0.60 HV 10 HV 10 HV 10 HV 10 HV 30 HV 30

0.65 HV 10 HV 10 HV 10 HV 30 HV 30 HV 30

0.70 HV 10 HV 10 HV 30 HV 30 HV 30 HV 50

0.75 HV 10 HV 30 HV 30 HV 30 HV 50 HV 50

Table 12: selection of the test method for specifying hardness data according to minimum depth of hardening and minimum surface hardness in HV- (in accordance with NF A 02-052 draft standard)

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