Molybdenum and its applications
Heat treatable engineering steel
The use of heat treatable steels covers the full range of mechanical engineering, especially where dynamic stresses are involved.
Application examples include:
- Automotive parts such as crankshafts, axle shafts, steering components
- Shafts in locomotive construction, shipbuilding and heavy engines
- Parts for machine tools and general mechanical engineering
- Turbine and generator shafts in power stations
- Components and accessories for the oil and gas industry
- Fastening elements such as high strength bolts
- Landing gear and control elements in aviation
- Tools in oil and gas exploration
Standard heat treatable engineering steels
European
Code | USA Code
AISI/SAE/
ASTM | % Alloy content | Application Examples |
|---|
| C | Cr | Ni | Mo | V |
|---|
| C22 |
|
0.22 |
|
|
|
|
Low Stress structural components |
| C35 |
1035/1038 |
0.35 |
|
|
|
|
Standard Stress structural components |
| C45R |
1049 |
0.45 |
|
|
|
|
Standard Stress structural components |
| C55E |
|
0.55 |
|
|
|
|
Gear shafts and wheels |
| 1% Cr and CrMo Steel |
|---|
| 28Cr4 |
|
0.25 |
1 |
|
|
|
Driving wheels and shafts |
| 25CrMo4 |
|
0.25 |
1 |
|
0.25 |
|
Axle arbors, turbine components |
| 34Cr4 |
5132 |
0.34 |
1 |
|
|
|
Axle, axle arms |
| 34CrMo4 |
4135/4137 |
0.34 |
1 |
|
0.25 |
|
High toughness components, incl. crank shafts, axle arbors |
| 41Cr4 |
5140 |
0.41 |
1 |
|
|
|
Axles, control components |
| 42CrMo4 |
4140/4142 |
0.41 |
1 |
|
0.25 |
|
High toughness components for automobiles and aircraft |
| 48CrMo4 |
|
0.5 |
1 |
|
0.25 |
|
Steel for induction hardening up to 250 mm Diameter |
| 50CrMo4 |
4150 |
0.5 |
1 |
|
0.25 |
|
High toughness components for automobiles and aircraft |
| CrNiMo Steel |
|---|
| 36CrNiMo4 |
4340/4980 |
0.36 |
1 |
1 |
0.25 |
|
Highly charged components for automobiles and aircraft |
| 34CrNiMo6 |
4337/4340 |
0.34 |
1.5 |
1.5 |
0.25 |
|
Crank shafts, eccentric shafts, gear components |
| 30CrNiMo8 |
|
0.3 |
2 |
2 |
0.4 |
|
Structural components for heavy demands |
| NiCrMo Steel |
|---|
| 28NiCrMo4 |
|
0.28 |
1 |
1 |
0.25 |
|
Structural components for very heavy demands |
| 33NiCrMoV14-5 |
|
0.33 |
1.3 |
3.5 |
0.5 |
0.2 |
Generator shafts, high strengt & toughness components |
| 36NiCrMo16 |
|
0.36 |
1.8 |
4 |
0.7 |
|
High strength mechanical engineering components |
| CrMoV Steel |
|---|
| 14CrMoV6-9 |
|
0.14 |
1.5 |
|
0.9 |
0.3 |
High strength welded components |
| 30CrMoV9 |
|
0.3 |
2.25 |
|
0.25 |
0.2 |
High toughness crank shafts, screws, bolts |
| All grades with Mn between 0.5 and 0.9% |
Table 1: Standard heat treatable engineering steels
Steel grades are selected to meet property requirements for a given application.
Demand of higher strength and toughness require increasing alloy content for improved hardenability:
- Carbon content is systematically increased in unalloyed grades from 0.22% to 0.55%
- Next are a series of 1% Chrome (Cr) and 1% Cr / 0.25% Mo grades with carbon content increasing again from 0.25% to 0.55%
- For higher stressed components CrNiMo steels are used with both nickel and chromium stepping up between 1% and 2%
- In NiCrMo steels nickel is present up to 4% and Mo up to 0.7% to ensure through-hardening of components like generator shafts
- In CrMoV steels carbon is partly replaced by alloys - up to 0.9% Mo - to retain good weldability or extra high toughness
Molybdenum’s most important role in these grades is to increase the hardenability and to promote a uniformly hardened microstructure across the full cross section.
This is illustrated with the following series:
Standard heat treatable engineering grades
| EN Code | SAE/ASTM | % Alloy content |
|---|
| C | Cr | Ni | Mo | V |
|---|
| C35 |
AISI/SAE/ASTM 1035/1038 |
0.35 |
|
|
|
|
|---|
| 34Cr4 |
AISI/SAE/ASTM 5132 |
0.34 |
1 |
|
|
|
|---|
| 34CrMo4 |
AISI/SAE/ASTM 4135/4137 |
0.34 |
1 |
|
0.25 |
|
|---|
Table 2: Unalloyed carbon - 1 Cr -1Cr0.25Mo steel
Adding 1% Cr increases the yield strength by approximately 50%; further addition of 0.25% Mo again raises the strength and extends through-hardening from 100 to 500 mm.
Fig 1: Effect of Cr and CrMo additions on yield strength of quenched and tempered steel
Adding 1% Cr improves the toughness values by 15%; further addition of 0.25% Mo extends excellent toughness to 500 mm cross section.
(The determination of the ruling heat treatment diameter for various geometries is standardized in DIN EN 17201)
Fig 2: Effect of Cr and CrMo additions on fracture toughness of quenched and tempered steel
Fig. 3 demonstrates the effect of Cr and CrMo additions on the mechanical properties at various carbon contents.
The desired combination of high strength and high notch impact energy is located in the upper right corner of the diagram.
Adding 1% Cr to plain carbon steel moves the property range into the desired direction. A further boost in the direction of high strength combined with adequate toughness is achieved through the addition of 0.25% Mo.
Fig 3: Effect of Cr and CrMo additions on mechanical properties
