AISI 4140 Chrome-Molybdenum High Tensile Steel

Topics Covered

Introduction
Chemical Composition
Physical Properties
Composition
Density
Mechanical Properties
Machining
Weldability
Heat Treatment
Forging
Stress Relieving
Annealing
Normalizing
Nitriding
Tempering
Flame and Induction Hardening
Hardening
Applications

Introduction

AISI 4140 is a chromium-molybdenum alloy steel. The chromium content provides good hardness penetration, and the molybdenum content ensures uniform hardness and high strength. AISI 4140 chrome-molybdenum steel can be oil hardened to a relatively high level of hardness. The desirable properties of the AISI 4140 include superior toughness, good ductility and good wear resistance in the quenched and tempered condition.

The AISI 4140 cold finished annealed chromium-molybdenun alloy steel can be heated using various methods to yield a wide range of properties, hence it is often used as stock for forging as it has self-scaling properties. AISI 4140 is capable of resisting creep in temperatures up to 538°C (1000°F) and maintaining its properties even after long exposure at comparatively high working temperatures.

The AISI 4140 cold rolled rounds are available in the 41L40 variant that contains 0.15-0.35 lead. The lead content improves machinability, but has significant effect on other desirable properties.

Chemical Composition

AISI 4140 is versatile because of its simple chemistry and has the following composition:

  • 0.40 % carbon and 0.85 % manganese which offers toughness and can be heat treated and hardened 0.1 % chromium adds to overall toughness but is not enough to be made into stainless steel
  • 0.25 % molybdenum and small amounts of sulfur, silicon and phosphorous
Element Content
Carbon, C 0.380 - 0.430 %
Chromium, Cr 0.80 - 1.10 %
Iron, Fe 96.785 - 97.77 % (As remainder)
Manganese, Mn 0.75 - 1.0 %
Molybdenum, Mo 0.15 - 0.25 %
Phosphorous, P ≤ 0.035 %
Silicon, Si 0.15 - 0.30 %
Sulfur, S ≤ 0.040 %

Physical Properties

Density

The density of AISI 4140 is 7.7 to 8.03 kg/m3

Mechanical Properties

Mechanical properties for AISI 4140 in the annealed state:

Mechanical Properties Metric Imperial
Hardness, Brinell 197 197
Hardness, Knoop (Converted from Brinell hardness) 219 219
Hardness, Rockwell B (Converted from Brinell hardness) 92 92
Hardness, Rockwell C (Converted from Brinell hardness. Value below normal HRC range, for comparison purposes only) 13.0 13.0
Hardness, Vickers (Converted from Brinell hardness) 207 207
Tensile Strength, Ultimate 655 MPa 95000 psi
Tensile Strength, Yield 415 MPa 60200 psi
Elongation at Break (in 50 mm) 25.7 % 25.7 %
Reduction of Area 56.9 % 56.9 %
Modulus of Elasticity (Typical for steel) 205 GPa 29700 ksi
Bulk Modulus 140 GPa 20300 ksi
Poissons Ratio (Calculated) 0.290 0.290
Machinability (Based on AISI 1212 as 100% machinability) 65 % 65 %
Shear Modulus 80.0 GPa 11600 ksi

Machining

AISI 4140 has good machinability. Operations such as turning, hobbing, drilling, sawing, broaching, tapping and milling can be easily performed using the recommendations provided by the machine's manufacturer for ideal tool type-feeds and speeds. AISI 4140 has the following properties:

  • Machinability rating at 66% of B1112
  • Average cutting speed of 110 ft/min.

Weldability

AISI 4140 is difficult to weld, but welding can be performed using any of the common welding practices. Welding can be done by preheating the section and ensuring that stress relieving takes place after welding. It is basically recommended that welding of AISI 4140 steel in the hardened and tempered condition should be avoided as it changes the mechanical properties within the weld heat affected zone. Likewise welding in the nitrided, flame or induction hardened condition is not recommended. However welding of AISI 4140 chrome-molybdenum steel in the annealed condition is widely preferred. In case welding in the hardened and tempered condition is required, the workpiece must be cooled instantly and the stress is relieved at 15°C below the original tempering temperature.

When welding AISI 4140 steel (in any condition) only low hydrogen electrodes should be used.

Follwoing welding, the maximum cooling rate that should be employed is 95°C /hr down to 95°C. This should be followed by cooling in still air. The cooling rate can be slowed by wrapping the work piece in a heat resistant blanket or burying it in sand.

Heat Treatment

There are various heat treatments of the AISI 4140 Chrome-Molybdenum High Tensile Steel which are:

  • Forging
  • Stress relieving
  • Annealing
  • Normalizing
  • Nitriding
  • Tempering
  • Flame and induction hardening
  • Hardening

Forging

  • Can be heated to 1150°C and maintained until uniform
  • Minimum forging temperature 850°C
  • Can be cooled slowly in sand or ash

Stress Relieving

  • Hardened: Heat to 500°C – 550°C
  • Annealed: Heat to 600°C – 650°C
  • Cool in still air

Annealing

  • Heat to 830°C – 850°C
  • Furnace cool

Normalizing

  • Heat to 870°C – 900°C
  • Cool in still air

Nitriding

  • Heat to 500°C – 530°C and hold for sufficient time to develop the depth of case required
  • Parts should be pre-hardened and tempered as required and also pre-machined leaving a small grinding allowance only.

Tempering

  • Heat to 450°C – 700°C cool in still air
  • Tempering in the range of 200°C – 420°C causes temper brittleness and should be avoided

Flame and Induction Hardening

All materials consisting of de-carburized surfaces should be removed to ensure best results.

  • Heat quickly to the required case depth at 860°C – 890°C and soak instantly in water or oil
  • Tempering at 150°C – 200°C helps reduce stresses in the case with marginal effect on its hardness

Hardening

  • Heat to 830°C – 880°C
  • Soak in oil or polymer

Applications

AISI 4140 is used extensively in most industry sectors for a wide range of applications such as:

Strippers bolts axle shafts
tool bodies bolsters holder blocks
stubs crankshafts tool holders
machinery parts and components mold bases couplings
connecting rods tie rods conveyor parts
ejectors reamer bodies chuck bodies
boring bars crow bars back up and support tooling
axles collets guides
logging parts fixtures piston rods
conveyor pins & rolls tracks shafts
jigs rams ejector pins
ways sprockets molds
hydraulic machinery shafts forks slides
studs cams gears
guide rods wear strips or parts pump shafts
drill collars sprockets torsion bars
forming dies ring gears gear racks
chain links lathe spindles brake dies
adapters valves spindles
logging parts trim dies arbors
milling spindles motor shafts pinch bars
pins pinions nuts
pump shafts tool holders sockets
worms    

Comments

  1. Gerald John Michrina II Gerald John Michrina II United States says:

    I have a question, rifle barrels are often 4140 Chrome Molly. What metal do I choose when I look for SFM for lathe or milling work. I see alloys buy that seems like a large category for one specific metal. Many aoos just say low, med high carbon, tool steel and allow. Can you help a newbie out?

  2. William Polozzolo William Polozzolo United States says:

    Would Nitriding, done at 1075 degrees F, alter the Rc hardness of 4140, that was tempered at 800 degrees F??

The opinions expressed here are the views of the writer and do not necessarily reflect the views and opinions of AZoM.com.

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