Supply Conditions of AISI 4140 Round Bar
AISI 4140 is a chromium-molybdenum alloy steel with roughly 0.40 percent carbon, 1 percent chromium and 0.2 percent molybdenum. Its chemistry is controlled by ASTM A29/A29M, general requirements for alloy bar are given in ASTM A322, cold finished bar is covered by ASTM A331, hardenability bands follow SAE J1268 and the equivalent European grade is 42CrMo4 in EN 10083. The mechanical properties of the same chemistry can vary widely, because they depend on the supply condition in which the mill delivers the bar.
Three conditions account for most industrial demand: annealed, normalized, and quenched and tempered. A fourth practical state, quenched and tempered bar that is subsequently nitrided, is produced by the component manufacturer rather than by the mill.
Typical Mechanical Properties by Supply Condition
The table gives representative room temperature values for round bar tested according to ASTM A370. Property values depend on bar diameter and on the location of the test specimen, so a specification should always state the size range together with the property.
| Condition | Tensile strength (MPa) | 0.2% yield strength (MPa) | Elongation in 50 mm (%) | Reduction of area (%) | Hardness (HBW) | Charpy V-notch (J) |
|---|---|---|---|---|---|---|
| Annealed | 655-850 | 415-550 | 25-30 | 50-60 | 197-235 | 40-60 |
| Normalized | 700-900 | 450-650 | 20-25 | 45-55 | 210-255 | 35-55 |
| Quenched and tempered, medium strength | 850-1000 | 650-800 | 18-22 | 45-55 | 248-302 | 25-45 |
| Quenched and tempered, high strength | 1000-1200 | 800-950 | 12-17 | 35-45 | 302-375 | 15-30 |
Impact energy is strongly dependent on test temperature and notch geometry, and the values above are representative rather than guaranteed. Bar diameters above about 100 mm develop slightly lower core properties than small diameters in the quenched and tempered condition, because the cooling rate at the centre is slower.
Annealed Condition
Annealed bar is heated to approximately 840-870 degrees Celsius and cooled slowly in the furnace. The structure is a coarse pearlite and ferrite, hardness is low and residual stress is limited, which gives the best machinability and the highest ductility of the three supply conditions. Annealed bar is normally chosen when the part will be extensively machined, or when a soft condition is needed before a subsequent forming operation.
Normalized Condition
Normalizing heats the bar to about 850-900 degrees Celsius and cools it in still air. The faster cooling refines the grain and produces a more uniform structure than annealing, so strength and hardness are higher while ductility is only moderately reduced. Normalized bar is used as a pre-treatment before hardening, and as a final condition for parts that need better strength than annealed bar without the distortion associated with quenching.
Quenched and Tempered Condition
Quenching and tempering is the route that produces the high strength levels used for shafts, studs, gears, couplings and tooling. The bar is austenitized at about 830-850 degrees Celsius, quenched in oil to form martensite, then tempered to the required strength and toughness balance. Because tempering temperature controls the result, the strength condition is normally specified by hardness range or by tempering temperature rather than by a single tensile value.
| Tempering band | Indicative tensile strength | Indicative hardness | Character |
|---|---|---|---|
| Below 430 degrees C | Above about 1400 MPa | Above about 42 HRC | Highest strength, lowest toughness |
| 430-540 degrees C | About 1100-1400 MPa | About 34-42 HRC | High strength with moderate toughness |
| 540-650 degrees C | About 850-1100 MPa | About 26-34 HRC | Balanced strength and toughness |
| Above 650 degrees C | Below about 850 MPa | Below about 26 HRC | Maximum toughness and ductility |
Higher tempering temperatures raise toughness and ductility at the expense of strength and hardness. This trade-off is the single most important selection decision in specifying quenched and tempered AISI 4140, and it explains why two bars of identical chemistry can behave completely differently in service.
Elastic and Physical Properties
For structural and machine design calculations, the properties below are the values normally used for this grade. They are essentially unaffected by the supply condition.
Young's modulus: approximately 205 GPa (190-210 GPa is the usual design band)
Poisson's ratio: approximately 0.29
Density: approximately 7.85 g per cubic centimetre
Thermal expansion coefficient: approximately 12.3 micrometres per metre per degree Celsius over 20-100 degrees Celsius
Thermal conductivity: approximately 42 W per metre per kelvin at 100 degrees Celsius
Specific heat capacity: approximately 460 joules per kilogram per kelvin
These values support deflection, interference fit, thermal growth and heat treatment distortion calculations, and they should not be confused with the strength properties that change with heat treatment.
How to Specify Properties on an Order
State the supply condition explicitly: annealed, normalized, or quenched and tempered
For quenched and tempered bar, state a hardness range at a defined location rather than a single tensile value
State the bar diameter range, because core properties fall as diameter increases
Require the test method, for example tensile testing to ASTM A370 and hardness testing to ASTM E10
State non-destructive or microstructure verification requirements where the part is safety related
Frequently Asked Questions
Q: Which supply condition gives the highest strength in AISI 4140 round bar?
Quenched and tempered bar gives the highest strength, with tensile values from about 850 MPa up to more than 1200 MPa depending on tempering temperature. Annealed bar is the softest and normalized bar sits between the two.
Q: Why does bar diameter affect the mechanical properties?
Quenching removes heat from the surface first, so the core of a large bar cools more slowly and may not fully transform to martensite. The resulting core properties are lower than those measured near the surface, which is why properties are specified for a diameter range.
Q: How does hardness relate to tensile strength in this grade?
Tensile strength and hardness track each other closely for quenched and tempered alloy steel. As a working approximation, tensile strength in MPa is about 3.3 times the Brinell hardness number for this grade, which is why a hardness range is often an adequate acceptance criterion.
Q: Does annealing reduce the strength permanently?
No. Annealing sets a soft condition that can be changed at any time by re-austenitizing and quenching followed by tempering. The annealed state is chosen for machinability, not as a final service condition for highly stressed parts.
Q: What causes the loss of ductility in the high strength condition?
Low temperature tempering leaves a fine carbide distribution in a highly stressed martensitic matrix. This raises strength and hardness but restricts plastic deformation, so elongation and impact energy fall as tempering temperature is reduced.
Q: How should properties be verified after delivery?
By tensile and hardness testing of specimens taken from the bar in accordance with the specified standard, supported by the mill certificate. Where the application is critical, hardness is checked on the finished part as well, because further machining and heat treatment can alter the condition.