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

Physical Properties

Density

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

Mechanical Properties

Mechanical properties for AISI 4140 in the annealed state:

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: