Thermic treatments

Material treatments

We distinguish two different types of thermic treatments:
– the complete thermic treatment
– the surface thermic treatment

1. Complete thermic treatment

This thermic treatments have no surface hardening as a result. On the other hand, the hardness in the gear is not uniform. The hardening ability depends on two different concepts.

The hardenings intensity or maximum hardening possibility
The hardenings intensity depends on the carbon value at the warming in the solid phase. The higher the percent carbon the harder.

The penetration of the hardening
The penetration depends on the quantity of carbon and alloy elements that are formed at the austenisation phase defined by the size of austenisation grains and by the degree of cooling.

This hardening method can happen for or after the tooth hobbing: it should be ensured that the hardness of the workpiece before milling is limited with reference to the processing of the steel by the milling.

If the treatment is carried out after milling, because tooth hobbing is easier with soft steel, certain distortions may occur so that finishing such as tooth grinding becomes necessary. The alloy elements have a large influence on the penetration depth of the hardening: the main elements are Mn, Ni, Mo.

After hardening it is advisable to temper the pieces at a temperature between 200° C and 500° C.

Alloy steels are suitable to do this.

2. Thermal treatments of the surface

The thermal treatments of the steel surface is the result of the insertion of foreign elements into the surface of the steel by chemical reactions through various processes. These elements cannot be metallic like carbon C, nitrogen N, oxygen O, or semi-metallic such as silicon Si, bore B, or non-metallic elements like aluminum Al, etc. In case of gears the input of carbon and nitrogen are the most important elements.

Case-hardening

This process consists of spreading carbon in the surface of a low-carbon steel, after the teeth were soft cut. To obtain the percentage for the hardness similar to carbon one has to to make a hardening: see the left chart. The carbon can be applied in solid, liquid or gaseous form. The gaseous insertion is the most applied process for gears. For maximum hardness, the percentage carbon amounts approximately 0.9%. The following figure defines the effective case hardening depth limited to 550HV. The optimal case hardening depth is Ehtopt = 0.15. m. We draw the attention to the fact that the case hardening depth is bounded on Ehtlim = 0.4 m to prevent too high carbon concentrations.

Despite the fact that case-hardening can be applied on relatively soft steels, it is desirable to use steel alloys where a penetration depth can be obtained and whose core hardness after hardening can offer resistance to pressing of the cemented layer. The table gives an overview of the main case hardening steels.

Nitro carborizing

During the case-hardening process described above an adjusted amount of ammonia is added which admits an absorption of the latter in the outer layer. The ammonia is added in a quantity of 3% to 10% in gas form NH3. This process increases the hardenability to that extent that the hardness is obtained under less stringent conditions. One has to accept the residual material hardness in this process. The core hardness is also much higher: 1,300 to 1,500 N/mm2. This process is an intermediate solution between case-hardening and nitriding.

Nitriding

The traditional nitriding process is based on the use of an ammonia gas to insert nitrogen into the surface. The process temperatures will be between 480° C and the 590° C. The NH3 gas is cracked at this nitriding temperature into nitrogen and hydrogen.
Most steels try to form a nitride layer, the white layer. Elements such as aluminum (Al), chromium (Cr), vanadium (V) and molybdenum (Mo) aim to slow down the nitriding process. The hard top coat that is formed with the nitrading depends on the hardness of the base material for the nitriding process. Most steel alloys can be nitrided, but there is a range of nitriding steels that lend themselves especially for a good result as well as for the nitriding time. The table gives you a view of the main nitriding steel with their characteristics.

Surface induction hardening

Even though we are able to do flame hardening of pieces we will only talk about induction hardening. This process consists of the warming of a tooth surface by using an alternating current. This tooth surfaces are then cooled with an increase in the hardness of the surface as a result. Only the steel surface is warmed up in this process. The warming is done by a high or medium frequency current flowing through coils with adjusted tooth shape. The effective penetration depth of the induced current is inversely proportional to the square root of the frequency. Thus, the medium frequency is applied for large depths and the higher frequencies for smaller hardening layers. As already noted in the figure, the percentage carbon should amounts to at least 0.3% to obtain the hardness of 50 HRC.

oppervlakkig-inductieharden

Most steels that we used to have listed can be induction hardened. The different hardening principles have been introduced.
Figure 1: Rotary induction hardening without the tooth foot.
Figure a2: Rotary induction hardening with the tooth foot.
Figure b: Induction hardening of the tooth flanks with flocculent coi.l
Figure c: Induction hardening of tooth flanks with tooth feet. This solution is the most preferable. It improves the fracture resistance by 30% to 50%.
The warming-up phase should be followed by a hardening phase with an adjusted refrigerant. For a large gear with large module the hardening is done tooth after tooth. The coil follows the tooth parallelly and is immediately followed by an adjusted cooling.

Gas nitriding steels

Steel type Denomination Material N° Tensile strength Hardness HV3 Nitriding depth in mm
St52-3N
SKF280
1.0841 500-600
600-700
600-700
600-700
0,2-0,8
0,2-0,8
Steel alloys Ck45
25CrMo4
34CrMo4
42CrMo4
50CrMo4
50CrV4
34CrNiMo6
30CrNiMo8
32CrMo12
30CrMoV9
14CrMoV6.9
1.1191
1.7218
1.7220
1.7225
1.7228
1.8159
1.6582
1.6580
1.7361
1.7707
1.7735
650-750
750-900
800-950
850-1000
850-1000
850-1000
900-1200
900-1200
900-1300
900-1200
900-1050
300-400
600-750
600-750
600-750
550-700
600-750
650-800
650-800
800-900
750-850
800-900
0,2-0,8
0,1-0,7
0,1-0,6
0,1-0,6
0,1-0,5
0,1-0,6
0,1-0,6
0,1-0,6
0,1-0,8
0,1-0,8
0,1-1,0
Nitrided steel 31CrMo12
31CrMoV9
34CrAl6
34CrAlMo5
34CrAlNi7
1.8515
1.8519
1.8504
1.8507
1.8550
900-1300
900-1200
800-950
850-1000
900-1050
800-900
750-850
900-1100
900-1100
900-1100
0,1-0,8
0,1-0,8
0,1-1,0
0,1-1,0
0,1-1,0
Case hardening steel 16MnCr5V
20MnCr5V
1.7131
1.7147
600-800
600-800
650-750
650-750
0,1-1,0
0,1-1,0
100Cr6
X165CrMoV121
1.2067
1.2601
1000-1400
1400-1800
500-600
900-1050
0,1-0,4
0,1-0,15

Surface treatment by nitrogen diffusion

Diffusing of hard, wear-resistant and anti-corrosion surface layers of steel and cast iron, and hard and wear-resistant layers on stainless steel (inox).

Introduction QPQ

For improving the characteristics of parts and tools for machine and equipment constructions, there is an endless row of surface treatments available, both by thermic, chemical, thermo-chemical treatment, best known under the name “TENIFER”, since long an established name. By extending this treatment with polishing and oxidize – called “QPQ”, there has been added a very high corrosion resistance to the existing characteristics.

Characteristics

  • Thickness of the surface layers till 25 μm
  • Surface hardness till 1150 Vickers (dependent of the steel type)
  • High resistance against cold welding
  • Increment of the fatigue resistance
  • High corrosion resistance 100 hours salt spray test ASTM B117
  • Surface roughness: Ra = 0,5
  • Surface aspect: black
  • No transformations
  • Small size changes, order of magnitude: few microns
  • Temperature resistant till 450°C

QPQ replaces

  • Hard chromium plating
  • Zinc plating
  • Nickeling
  • Cadmium plating
  • Blackening
  • Phosphating
Metals Materialnr. HV1 HV3 HV30
Ck15
C45W3
Ck60
20MnV8
53MnSi4
90MnV4
42CrMo4X
19NiCrMo4
55NiCrMoV6
56NiCrMoV7
50NiCr13
X20Cr13
X35CrMo17
X210Cr12
X210CrW12
X156CrMoV12
45CrMoW58
X32CrMoV33
X38CrMoV51
X37CrMoW51
X30WCrV53
X30WCrV93
1.1141
1.1730
1.1221
1.7147
1.5141
1.2842
1.7225
1.2764
1.2713
1.2714
1.2721
1.2082
1.4122
1.2080
1.2436
1.2601
1.2603
1.2365
1.2343
1.2606
1.2567
1.2581
350
450
450
600
450
550
650
600
650
650
600
>900
>900
>800
>800
>800
800
>900
>900
>900
>900
>900
300
350
350
450
400
450
500
500
550
550
500
600
700
600
600
650
700
850
850
800
850
850
200
250
250
400
350
400
450
450
500
500
450
450
550
450
500
500
600
700
700
700
750
800

3. Tables

Hardening depth for nitriding and case-hardening compared to the module.
The following table lists the hardening depths for nitriding, case-hardening and surface hardening to the module according to DIN 50190.

Hardness comparison table

The following table returns the indicative figures for steel for comparison of material hardness values and tensile strength.