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 contentApplication Examples
CCrNiMoV
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 CodeSAE/ASTM% Alloy content
CCrNiMoV
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.

Effect of Cr and CrMo additions on yield strength of quenched and tempered steel

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)

Effect of Cr and CrMo additions on fracture toughness of quenched and tempered steel

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.

Effect of Cr and CrMo additions on mechanical properties

Fig 3: Effect of Cr and CrMo additions on mechanical properties