4140 steel is an alloy steel that provides an excellent combination of strength, toughness, and wear resistance. It is widely used in applications requiring high fatigue strength such as gears, fasteners, axles, and structural aircraft components.
The properties of 4140 steel are strongly influenced by its carbon content. Increasing carbon levels improves hardenability for greater through-thickness properties, but can reduce notch toughness if excessively high. Proper control of carbon is essential to balance strength with ductility and impact strength.
This article provides a detailed examination of how carbon content affects the hardenability, strength, toughness, and other mechanical properties of 4140 steel. The effects of carbon on heat treatment response, microstructure, and service performance are discussed. Optimal carbon ranges are suggested based on application requirements.
Overview of 4140 Steel
4140 steel belongs to the family of chromium-molybdenum low alloy steels. The composition is designed to provide high hardenability for increased strength after heat treating:
- Carbon – 0.38-0.43%
- Manganese – 0.75-1.0%
- Silicon – 0.15-0.30%
- Chromium – 0.8-1.1%
- Molybdenum – 0.15-0.25%
The medium carbon level combined with the chromium and molybdenum additions enable 4140 steel to attain high strength through larger cross sections while retaining good toughness.
The effects of raising or lowering the carbon content from the nominal range are covered below.
Effect of Carbon on Hardenability
One of the main effects of increasing carbon content is to improve the hardenability of 4140 steel by slowing the formation of pearlite and widening the temperature range allowing martensite formation.
As illustrated by the continuous cooling curves in Figure 1, higher carbon 4140 steel converts to martensite at progressively slower cooling rates. This enables hardening through thicker sections.
Figure 1. Continuous cooling curves showing increased hardenability from higher carbon content.
For example, a 0.45% C 4140 bar may remain fully martensitic at center during oil quenching up to 3” diameter. At 0.40% C the limit may be 2” diameter before a softer core results.
Hardenability as measured by ideal critical diameter or Jominy end quench testing consistently increases with higher carbon levels in 4140 steel.
Strength Effects of Carbon Content
The strength of 4140 steel increases progressively as the carbon content is raised from 0.35 up to 0.45% and beyond. Some typical tensile strength values based on carbon level are provided in Table 1 for quenched and tempered 4140 steel:
|Tensile Strength (ksi)
Table 1. Effect of carbon content on tensile strength of heat treated 4140 steel.
The higher carbon levels increase the amount of martensite formed during quenching to provide greater strengthening. This results in higher achievable hardness and strength capability.
However, for carbon levels above 0.45%, the rate of strength increase declines while toughness drops rapidly as covered in the next sections.
Effect on Hardness
Increasing the carbon content has a direct impact on the as-quenched hardness achieved in 4140 steel, enabling higher attainable hardness levels:
- 0.35% C – Hardness up to 47 HRC
- 0.40% C – Hardness up to 50 HRC
- 0.43% C – Hardness up to 52 HRC
- 0.45% C – Hardness up to 54 HRC
Again, the rate of hardness increase slows above 0.45% carbon as the steel becomes saturated with excess carbon in martensite.
After tempering, the hardness range simply shifts lower by a few points but the trend of higher hardness at increased carbon levels remains.
Reduction of Area and Ductility
While higher carbon content improves the strength and hardness of 4140 steel, it conversely results in lower ductility and notch toughness. This effect becomes pronounced above 0.45% carbon.
Reduction of area (ROA) on a tensile test is a measure of ductility in steels. Figure 2 shows how ROA decreases significantly as carbon rises past 0.45%.
Figure 2. Reduction of area versus carbon content in heat treated 4140 steel.
Likewise, elongation during tensile testing follows the same trend of decreasing sharply as the carbon level is increased beyond 0.45% due to the formation of excess brittle martensite.
Impact Toughness Effects
The notch toughness and impact strength of 4140 steel, as measured by Charpy V-notch testing, declines noticeably as the carbon content is raised past approximately 0.43%.
As illustrated in Figure 3, the impact energy drops from over 20 ft-lbs down to 10-12 ft-lbs range as the carbon approaches 0.50%.
Figure 3. Decline in Charpy impact energy with increasing carbon content.
This demonstrates the sensitivity of impact resistance to carbon levels exceeding 0.43% in 4140 steel. Even though strength increases further, the loss of ductility and toughness limits practical carbon range to 0.45% maximum.
Effects on Fatigue Strength
Higher carbon content generally results in lower fatigue strength in 4140 steel, especially in larger section sizes.
Reasons for reduced fatigue life include:
- Increased susceptibility to quench cracking
- Higher potential for retained austenite
- Lower fracture toughness
- Higher tensile residual stresses
To avoid fatigue issues, carbon is often kept on the lower end of the 4140 range for components with thickness over 1 inch or requiring maximum fatigue resistance.
Shot peening is particularly beneficial for increasing fatigue strength in higher carbon 4140 steels.
Recommended Carbon Ranges
To summarize the effects of carbon content:
- 0.35-0.40% C – Best balance of properties for most applications
- 0.40-0.43% C – Higher strength with good toughness
- 0.43-0.45% C – Maximum hardness capability with some loss of ductility
- Over 0.45% C – Avoid due to poor notch toughness
The low end of the standard 4140 range provides the optimum combination of hardness, strength, ductility, and toughness needed for critical components such as aircraft landing gear and fracture-critical structures.
Higher carbon levels up to 0.45% maximize hardness and strength in smaller sections. However, toughness and fatigue life may be compromised.
Effect on Machinability
As the carbon level increases in 4140 steel, machinability declines somewhat due to higher strength and hardness in the annealed condition prior to heat treating.
Finish machining forces will be higher for a 0.45% carbon 4140 bar versus a 0.40% C bar in the same pre-hardened condition. Tool wear rates will also increase.
Annealing can help offset the lower machinability of higher carbon 4140. Adjustments to feeds, speeds, and tooling may also be needed for carbon above 0.43% to compensate for shorter tool life and higher cutting forces.
Influence on Corrosion Resistance
The corrosion resistance of 4140 steel in ambient conditions tends to decrease slightly as carbon content increases due to reduced chromium availability with more carbide formation.
However, for most applications the differences are relatively small. Providing sufficient tempering and using shot blasting or other surface treatments to induce compressive stresses can minimize any corrosion sensitivity from higher carbon levels.
Reducing the carbon below the standard 4140 range also has effects on properties and performance:
- Carbon below 0.35% – Hardness and strength capability is lowered. Hardenability is reduced which limits bar sizes that can be fully transformed. Not recommended for most applications.
- 0.35-0.38% C – Marginal loss of through hardness for larger sections. Provides maximum toughness and fatigue strength. Allows extensive cold working prior to heat treating without cracking. An option for fracture-critical components.
While carbon levels down to 0.35% sacrifice some hardening potential, the improvement in machinability, fatigue strength, formability, weldability, and corrosion resistance provide benefits for some applications where hardness below 50 HRC is tolerable.
Effect on Prior Microstructure
Higher carbon 4140 steels generally have a finer as-rolled ferrite grain size compared to lower carbon variations. For example, 0.45% C may have an ASTM grain size of 7-8 versus 8-9 for 0.38% C.
This finer initial grain structure improves mechanical properties after hardening and tempering. Normalizing or annealing further refines the grains for optimal toughness and ductility.
Care must be taken to avoid excessive grain coarsening during austenitizing at higher carbon levels. Slow cooling after normalizing is also beneficial.
Effect on Cost
Increasing the carbon content makes 4140 steel more expensive due to higher alloying costs. Typically for every 0.01% increase in carbon, the material cost rises by about 3-5%.
However, the improved properties may justify the extra cost for certain applications requiring maximum hardness, strength, or wear resistance where a higher carbon 4140 grade is suitable.
Example Applications and Grades
To summarize some example uses:
- Gears, fasteners, shafts: 0.38-0.40% C range provides the best all-round properties.
- Structural aircraft components: 0.38-0.40% C recommended for optimal fracture toughness.
- Small pins, knives, shear blades: 0.45% C allows up to 54 HRC hardness when needed.
- Larger shafts, rods over 2” diameter: 0.35-0.40% C maintains fatigue strength.
Some industry examples of higher carbon 4140 grades include:
- AMS 6414 – 0.40-0.48% C aerospace bolting with 210 ksi tensile strength
- 9310 – 0.43% C bearing and gear steel
- AISI 4340 – 0.42% C heavily used for aircraft landing gear
Matching the carbon content to the specific size, properties, and performance requirements is key to optimizing the use of 4140 alloy steel.
The carbon content has a key influence on the properties and performance of 4140 alloy steel:
- Increasing carbon from 0.35 to 0.45% improves hardenability and strength, but reduces ductility and toughness
- Levels above 0.45% result in poor impact resistance and fatigue strength
- Through a range of 0.38-0.43%, an optimal balance of properties is achieved
- Smaller sections can utilize higher carbon for maximum hardness
- Larger sections benefit from lower carbon for fracture toughness and fatigue resistance
By controlling carbon in the ideal range and matching to the intended application, 4140 can be tailored to provide the precise combination of strength, toughness, and hardenability required.
How does carbon content affect the properties of 4140 steel?
Increasing carbon improves strength and hardenability but reduces ductility, toughness and fatigue strength. Optimal properties are achieved in a range of 0.38-0.43% carbon. Higher levels over 0.45% significantly lower fracture resistance.
What is the main benefit of higher carbon in 4140 steel?
The main advantage of increased carbon content is to improve hardenability for greater through-thickness strength and hardness capability in larger sections sizes. With higher carbon, larger parts can be fully transformed to martensite during quenching.
What carbon level provides the best strength-toughness balance?
For most applications requiring a good balance of high strength, hardness, ductility, and toughness, carbon levels of 0.38-0.40% are recommended. This is the most widely used range for critical components like aircraft landing gear.
Why does toughness decrease at higher carbon levels?
With increased carbon, more brittle martensite is formed during quenching resulting in reduced ductility, impact energy, and fatigue resistance. High carbon martensite also promotes cracking. To avoid this, carbon is typically kept below 0.43% for fracture critical uses.
What is the maximum carbon level normally used for 4140 steel?
To maintain a reasonable level of notch toughness and fatigue strength, the carbon content in 4140 steel is usually limited to a maximum of 0.45%. Greater than 0.45% carbon results in poor impact resistance and cracking tendency which limits practical applications.
When can higher 0.43-0.45% carbon grades be used?
Higher carbon 4140 steels are suitable for smaller sections, non-critical components, and applications requiring maximum hardness like cutting tools and wear-resistant parts where lower toughness is tolerable.
Why is carbon kept on the lower end for larger sections?
For 4140 components over 2” thick, carbon levels from 0.35-0.40% ensure good through-thickness hardenability and provide the best toughness and fatigue strength. The higher carbon variations lose too much ductility in large sizes.
Does higher carbon improve machinability of 4140 steel?
No, increasing the carbon content tends to lower the machinability of 4140 somewhat due to higher strength and hardness. More difficult machining characteristics, higher cutting forces, and shorter tool life are experienced at carbon levels over 0.40-0.43%.