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Annealing 4140 Steel: Process, Microstructural Changes and Applications


4140 steel is a chromium-molybdenum alloy steel that offers an excellent combination of strength, toughness, and wear resistance. It is widely used for parts that require high strength and fatigue resistance such as axles, shafts, gears, fasteners, and aircraft components.

In the as-hot-rolled condition, 4140 steel has a coarse grained structure with internal stresses that make machining difficult. Annealing is a heat treatment process commonly used to refine the grains, relieve internal stresses, and enhance machinability.

This article provides an in-depth look at the annealing process for 4140 steel. It covers the heating and cooling parameters, the resulting microstructural changes, effects on machinability and mechanical properties, and example applications that benefit from annealing. Recommendations are provided for annealing based on section size to achieve optimal characteristics.

Overview of the Annealing Process

1045 steel plate

Annealing involves heating 4140 steel to a specified temperature, holding at that temperature for a period of time, and then slowly cooling. The process softens the steel to improve machining and ductility.

The three main types of annealing used for 4140 steel are:

  • Full Annealing – Heating above Ac3 and slow furnace cooling
  • Process Annealing – Heating to subcritical temperature and slow cooling
  • Stress Relieving – Heating to 600-650°F to relieve residual stresses

Full annealing provides maximum softening and ductility enhancement. Process annealing provides an intermediate level suitable for many applications. Stress relieving just eliminates residual stresses from prior processing.

Annealing also prepares and refines the microstructure for further heat treatment if required. The temperature and time parameters determine the effectiveness.

Full Annealing Process

Full annealing is used when maximum ductility and softening is required in 4140 steel, such as prior to extensive machining operations.

The recommended full annealing process parameters are:

  • Temperature: 50-100°F (30-60°C) above Ac3 (~1550-1600°F)
  • Soaking Time: 1 hour per inch of thickness, 2 hours minimum
  • Cooling: Slow cool at 25-50°F per hour down to 1000°F, then air cool

Heating is commonly done in protective atmosphere furnaces to prevent oxidation scaling. Slow cooling allows time for full diffusion of carbon and equalization of temperatures throughout the part.

These full annealing conditions provide the lowest hardness and fully refined, uniform microstructure needed for further processing of 4140 steel.

Process Annealing Procedure

When maximum softening is not required, process annealing can be used to improve machinability with less processing time and cost versus full annealing.

Process annealing follows these guidelines:

  • Temperature: 100-200°F (50-100°C) below Ac1
  • Soaking Time: 1-2 hours
  • Cooling: Slow furnace cool at 25°F per hour

This subcritical temperature relieves residual stresses and provides partial grain refinement without complete transformation to austenite. Cooling rate is not as critical as for full annealing.

Process annealing provides intermediate hardness and sufficient ductility for most machining of 4140 steel while minimizing distortion and shrinkage.

Stress Relieving Parameters

Stress relieving can be utilized when the sole objective is to remove stresses in 4140 steel from prior cold working or machining operations.

Typical stress relief parameters are:

  • Temperature: 600-650°F
  • Time: 1 hour per inch of thickness
  • Cooling: Air cool

Soaking at this relatively low temperature eliminates residual stresses without significantly changing hardness or grain structure. Stress relieving should be done prior to finish machining to minimize distortion.

Effect of Annealing on Machinability

Annealing makes 4140 steel easier to machine by several mechanisms:

  • Reduces strength and work hardening
  • Refines grain structure
  • Eliminates directionality
  • Removes internal stresses
  • Softens and homogenizes carbides

This allows higher speeds and feeds to be used. Finish machining forces are lowered, resulting in reduced potential for distortion. Chip formation and breakability is improved.

Tool wear rates are reduced by the elimination of hard spots and stringers. Machining speeds can often be increased 2X versus hot rolled 4140, with feed rates up to 50% higher.

Annealing thus provides substantial machining productivity and cost advantages for 4140 steel components while improving quality.

Mechanical Properties after Annealing

While annealing improves machinability, it does result in decreased mechanical properties compared to the hot rolled or normalized condition:

  • Hardness is reduced to around 217 Brinell (95 Rockwell B)
  • Tensile strength declines to approximately 860 MPa (125 ksi)
  • Yield strength falls to roughly 655 MPa (95 ksi)
  • Elongation increases to 18% or higher
  • Toughness and fatigue strength are also lowered

These properties may be adequate for less demanding applications. If higher properties are needed, hardening and tempering heat treatments can be applied after annealing and machining.

Effect of Section Size on Annealing

For light sections under 1⁄2” thickness, stress relieving temperatures around 600°F are usually sufficient to reduce machining stresses.

Medium sections between 1⁄2” and 2” benefit from full process annealing just below Ac1 to refine the grain structure.

Heavy sections over 2” thick require full annealing above the Ac3 point to fully dissolve the microstructure and equalize temperatures.

The time at temperature should be increased for larger sizes to achieve results through the cross section. furnace cooling is critical to prevent reformation of coarse pearlite in heavy sections.

Double Annealing

For 4140 steel sections greater than 4” thickness, a double full annealing treatment is often used:

  • Anneal per recommendations in protective atmosphere furnace.
  • Air cool to room temperature.
  • Repeat full annealing process.
  • Slow furnace cool.

The second annealing cycle helps ensure complete diffusion of carbon and equalization of the microstructure through the thickness. This prevents soft surface layers and hard centers in heavy sections.

Annealing 4140 Steel Before or After Machining

For critical applications, 4140 steel should be annealed prior to any machining operations to avoid inducing stresses into the part.

However, for simpler parts or non-critical applications, rough machining may be performed before annealing. The annealing process will still relieve stresses and soften the steel for subsequent finish machining.

If substantial cold working such as heading or cold forming is done, then a stress relieving anneal should always follow such operations.

Annealing Compared to Normalizing

While annealing and normalizing both refine the grain structure, normalizing uses faster air cooling to produce a finer grain size. This results in higher mechanical properties for normalized 4140 versus annealed.

However, annealing provides better dimensional stability by minimizing distortion effects. Annealing also requires lower temperatures and less processing time.

For many applications not requiring ultimate strength, annealing offers a good compromise between properties, machinability, and cost.

Example Applications

Here are some examples where annealing is commonly used to improve machinability and properties of 4140 steel parts:

  • Transmission Gears – Annealing relieved stresses and enabled higher speed gear shaping. Achieved 95 ksi yield strength after carburizing.
  • Pump Shafts – Double annealing allowed faster OD turning and grinding with less distortion. Facilitated subsequent induction hardening.
  • Hydraulic Cylinders – Process annealing reduced machining forces during ID boring and honing. Provided adequate fatigue strength.
  • Large Fasteners – Full annealing enabled thread rolling of bolts up to 2” diameter without cracking. Optimized combination of strength and toughness.
  • Off-Road Vehicle Axles – Annealing reduced tool wear when splining axle shafts. Achieved through hardness above 30 HRC after quenching and tempering.

Proper annealing tailored to the section size and application requirements can optimize the machinability and resulting properties of 4140 components.

When to Avoid Annealing 4140 Steel

Annealing should generally be avoided or minimized in applications where:

  • Maximum strength and hardness are required
  • Hardness above 35 HRC is needed
  • High fatigue and impact resistance are critical
  • Part distortion must be minimized

For these situations, normalizing or quenching and tempering in the hot rolled condition will provide better results.

If annealing must be done, the 4140 steel should be re-austenitized and quenched after machining to restore proper heat treat response and properties.

Summary of Annealing 4140 Steel

Annealing transforms the coarse grained, stressed structure of hot rolled 4140 steel into a more refined, ductile, and machinable condition.

Key annealing process guidelines include:

  • Full Annealing above Ac3 for maximum softening
  • Process Annealing below Ac1 for intermediate properties
  • Stress Relieving at 600-650°F to remove residual stresses

For heavy sections, double annealing may be required. Annealing should be done before machining to avoid inducing stresses.

While lowering mechanical properties, annealing improves machinability by 2X and enhances subsequent heat treatment response. It allows optimum machining before further processing to achieve the required final part properties.


What is annealing?

Annealing involves heating steel like 4140 to a specified temperature, holding for a sufficient time, and then slowly cooling to soften the steel and refine the grain structure. This improves machinability and provides a starting structure optimized for subsequent heat treatment.

Why anneal 4140 steel?

Annealing improves the machinability of 4140 steel by reducing strength and hardness, eliminating internal stresses, removing directionality, and refining the coarse grains. This enables higher cutting speeds and feeds, lower machining forces, and improved chip breakability.

What are the different types of annealing?

Full annealing involves heating above Ac3 and slow furnace cooling for maximum softening. Process annealing uses subcritical temperatures below Ac1 and slower cooling to retain some strength. Stress relieving just involves heating to 600-650°F to remove residual stresses from prior work.

What microstructural changes occur during annealing?

Heating dissolves the pearlite bands into austenite. Slow cooling allows the austenite to transform into refined ferrite and cementite grains. This results in a more uniform, stress-free microstructure optimized for machining and subsequent hardening.

How does annealing affect the machinability of 4140 steel?

Annealing improves machinability by reducing strength and directionality, softening carbides, removing internal stresses, and refining the grain structure. This allows significantly higher cutting speeds and feed rates with lower machining forces and tool wear.

Does annealing reduce the strength of 4140 alloy steel?

Yes, annealing reduces the tensile strength to around 125 ksi and yield strength to about 95 ksi compared to the hot rolled condition. However, subsequent heat treatment can be applied after machining to increase strength as required.

How should annealing time vary for different section sizes?

For light sections under 1⁄2”, 1 hour soak times are sufficient to reduce machining stresses. Heavy sections over 2” thickness require 1-2 hours per inch of thickness to fully anneal and equalize temperatures through the cross section.

Is 4140 steel annealed before or after machining?

For critical applications, annealing should be done prior to any machining to avoid inducing stresses into the part. For non-critical components, rough machining may be done first, followed by annealing to facilitate finish machining.

Does annealing refine the grain size of 4140 alloy steel?

Yes, annealing transforms the banded as-hot-rolled structure into a more uniform, refined ferrite + cementite microstructure. This improves subsequent hardening response and properties after heat treating.

What temperature should be used to anneal 4140 steel?

For full annealing, temperatures of 50-100°F (30-60°C) above Ac3 (~1550-1600°F)are recommended. For process annealing, temperatures 100-200°F below Ac1 provide partial softening. Stress relieving can be done at 600-650°F.

Is normalized or annealed 4140 steel easier to machine?

Normalizing produces a finer grain size versus annealing, resulting in slightly better machinability. However, annealing provides better dimensional stability and less risk of distortion. For most applications, annealing offers a good balance of properties and machinability.

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