4140 Pre-Hardened Steel: Properties, Industry, Advantages, and Disadvantages

4140 steel is often supplied in a pre-hardened condition, meaning it undergoes heat treatment to achieve a specific hardness level before being used in downstream manufacturing processes. This steel alloy is valued for its hardness, toughness, high tensile strength, and more.

This article explores the properties and industrial uses of 4140 steel, as well as its advantages and disadvantages.

What Is 4140 Pre-Hardened Steel?

4140 steel is a medium carbon (nominally 0.4% C) chromium-molybdenum alloy steel (sometimes shortened to "chromoly") with small amounts of other elements added, such as manganese. It is known for its hardness and tensile strength. The chromium in the alloy adds good hardness penetration and the molybdenum imparts increased hardenability and strength. Pre-hardening means that the steel is delivered to users in a hardened condition 

How Is 4140 Steel Alloy Composed?

The composition of 4140 steel typically adheres to the following general specifications:

1. Iron (Fe): Approximately 96.785% - 97.77%
2. Chromium (Cr): 0.8% - 1.1%
3. Manganese (Mn): 0.75% - 1%
4. Carbon (C): 0.38% - 0.43%
5. Silicon (Si): 0.15% - 0.3%
6. Molybdenum (Mo): 0.15% - 0.25%
7. Sulfur (S): 0.04% max
8. Phosphorus (P): 0.035% max

The combination of these elements, particularly chromium and molybdenum, contributes to the alloy's high tensile strength, hardness, resistance to wear, and hardenability. The carbon content also plays the biggest role in the hardenability of the steel. 

To learn more, see our full guide on Alloy Steel.

What Are the Key Properties of 4140 Pre-Hardened Steel?

The key properties of 4140 pre-hardened steel are as follows:

1. Superior toughness
2. Good wear resistance
3. Good ductility
4. High tensile strength 
5. Good machinability 
6. Good heat resistance
7. High fatigue resistance 

How Is 4140 Pre-Hardened Steel Heat Treated?

A heat-treatable steel like 4140 can be supplied in several different pre-hardened conditions. The pre-hardening process can be tailored to provide steel with the exact balance of strength, ductility, and hardness that the buyer wants as raw material for his manufacturing processes. The most common pre-hardening heat treatment consists of:

1. Austenitizing: This step involves preheating and then soaking the material at a temperature at which the entire microstructure transforms into the face-centered-cubic austenitic phase. For 4140 steel, the austenitizing temperature is about 855 to 870 °C. The rule of thumb for the required hold time for uniform transformation and dissolution of carbides into the austenite matrix is about 30 minutes per inch of part thickness.

2. Quenching: 4140 steel is normally quenched in oil, which is a little less severe than quenching in water, while still producing the desired martensitic microstructure. Martensite is the hard phase that is produced when austenite is cooled quickly enough to below a critical temperature which depends on the steel’s composition. It is dependent only on temperature. The only time dependence for the martensite transformation is the time it takes to ensure the part’s temperature is reduced to the appropriate level for the desired depth of hardening into the part. In the as-quenched condition, 4140 steel has a hardness of approximately 54-59 HRC.

3. Tempering: After transforming to martensite, 4140 steel is at its hardest and strongest, but also its most brittle. Most applications of this steel require a compromise between hardness and ductility. This compromise is attained by reheating the quenched steel to a temperature of 200−650 ºC for 30 minutes to 2 hours, depending on the final mechanical properties desired. This reduces the hardness by transforming the microstructure to a mixture of tempered martensite, ferrite, and finely dispersed carbides.

Note that this discussion deals only with quench-and-temper heat treatments. Other types of heat treatments, such as normalization, are also options.

What Is the ASTM Standard for 4140 Steel?

The designation “4140” steel stems from earlier AISI (American Iron and Steel Institute) and SAE International (formerly Society for Automotive Engineers) specifications. Under the ASTM International (formerly American Society for Testing and Materials) system, 4140 steel is  described in the steel grade standard for ASTM A29/A29M, entitled “Standard Specification for General Requirements for Steel Bars, Carbon, and Alloy, Hot-Wrought.” ASTM A29/A29M covers a wide range of carbon and alloy steel grades and sets forth the general requirements for hot-wrought steel bars, including: dimensions, tolerances, chemical composition, and mechanical properties.

To learn more, see our ASTM Standard guide.

What Makes 4140 Pre-Hardened Steel Suitable for Industrial Use?

4140 pre-hardened steel is favored in industrial applications because of its desirable combination of properties. Its high level of hardness and strength, achieved through the pre-hardening, or heat treatment, process, is particularly advantageous in manufacturing industrial components and machinery. The hardening treatment boosts 4140 steel's wear resistance, making it ideal for abrasive or high-wear industrial applications. The heat-treated microstructure also contributes to good toughness and impact resistance. The machinability of this steel is good compared to that of other steels of the same hardness, and its heat resistance ensures the preservation of some of its mechanical properties at elevated temperatures. However, 4140 steel’s yield strength drops at elevated temperatures. At room temperature, the steel has a yield strength of about 655 MPa, which reduces to roughly 480 MPa at 200 °C and 350 MPa at 400 °C. Finally, 4140 pre-hardened steel comes in a variety of forms including: bar, plate, and sheet. 

How Does Machining Work With 4140 Pre-Hardened Steel?

4140 steel has good machinability, with a machinability rating of 66% of B1112. Operations such as hobbing, turning, sawing, broaching, drilling, tapping, and milling can be performed when adhering to the recommendations provided by the machine's manufacturer regarding the ideal tool types, feeds, and speeds. Cutting tools used for 4140 are usually either HHS (high-speed steel) or carbide cutting tools with an average cutting speed capabilities of around 110 ft/min. 

What Are Common Applications 4140 Pre-Hardened Steel?

 Here are some common applications of 4140 pre-hardened steel:

1. Mold and Die Making: 4140 pre-hardened steel is often used in the manufacturing of molds, especially in the car industry.
2. Gears and Shafts: This steel's combination of high tensile strength and good fatigue resistance makes it suitable for gears and shafts in mechanical systems.
3. Mining Equipment: 4140 steel is used in the mining industry for equipment such as crusher components, conveyor rolls, and drill bits. 
4. Automotive Components: 4140 pre-hardened steel is employed in the production of various automotive components, such as: axles, crankshafts, gears, pinions, and connecting rods. Its strength and toughness contribute to the overall performance and reliability of these parts.
5. Aerospace Components: Certain aerospace components are made from 4140 pre-hardened steel. These include: landing gear, structural elements, and engine components. 
6. Oil and Gas Industry: Components used in the oil and gas industry, such as drilling tools, valve components, and wellhead equipment, often utilize 4140 pre-hardened steel.
7. Firearms and Defense: Some firearm components, like barrels, receivers, and bolts, are made from 4140 pre-hardened steel. 
8. Machine Components: Various machine components, including shafts, spindles, feed and lead screws, and gears are made from 4140 pre-hardened steel due to their excellent mechanical properties.

What Are the Welding Considerations for 4140 Steel?

The major factors to consider in welding 4140 steel are:

1. Heat Treatment Condition: While both pre-hardened and annealed 4140 steel can be welded, both are challenging materials, and pre-hardened 4140 is more difficult to weld without weld cracking than annealed steel. The quenching and tempering process used in pre-hardening reduces the steel's ductility, making it less able to accommodate welding-induced stresses. This is compounded by the risk of hydrogen embrittlement, more pronounced in harder steels, where hydrogen atoms introduced during welding increase brittleness. Additionally, welding alters the properties of the heat-affected zone (HAZ) in pre-hardened 4140 more significantly than in its annealed counterpart, creating a property mismatch that can lead to cracking. The constrained thermal expansion, a result of the material's reduced ability to expand and contract without stress, further exacerbates this issue. It is generally recommended to anneal pre-hardened 4140 before welding, if possible, alongside employing appropriate preheating and post-weld heat treatments to minimize cracking risks.
2. Choice of Filler Metal: When welding 4140 steel, the choice of filler metal depends on the steel's pre-welding condition and the intended post-welding treatment. A low-alloy filler metal is often selected, which matches the tensile strength of the 4140 base metal but doesn't necessarily share its exact chemical composition. This ensures suitable properties for both the as-welded and post-weld heat-treated states. For components that will undergo multiple repairs, it's important to choose a filler metal that retains its mechanical properties after repeated stress-relief cycles. Under-matching the tensile strength with a weaker filler metal can improve ductility and fatigue life, but might not be suitable for high-stress applications. Overmatching, or using a filler metal stronger than the base metal, is generally avoided due to the risk of increased weld cracking. If the weld requires post-weld annealing or normalizing, a 4140 filler metal is usually good enough, as it can regain ductility through these heat treatments. For welds that will undergo quenching and tempering, a 4140 filler metal is necessary, as most low-alloy fillers lack the carbon content needed for these specific treatments. 
3. Preheating: Preheating is required for successful welding of 4140 steel. The preheating temperature is chosen to be about 15 °C less than the temperature of any previous tempering. The objectives of preheating are to decrease the driving force for cracking or distortion due to constrained thermal expansion and to slow the cooling rate of the weld and its heat-affected zone (HAZ) enough to avoid the formation of brittle martensite in and around the weld.
4. Interpass Heating: Once preheating is complete, the preheat temperature must be maintained if multiple weld passes are needed to complete the joint.
5. Post-Weld Heat Treatment: The post-weld heat treatment of 4140 steel involves a series of controlled heating and cooling steps designed to reduce the risk of hydrogen-induced cracking, relieve residual stresses, and restore or maintain the mechanical properties of the welded structure. This includes immediate post-heating (at 575-650 °C) after welding to diffuse hydrogen and prevent hydrogen-induced cracking, especially important for 4140's high hardenability. Proper preheating and maintaining interpass temperatures are vital to control the cooling rate, reducing residual stress and preserving metal toughness. Following welding, a slow cooling process or “hydrogen bake-out” is recommended, involving holding the weldment at preheat temperature, and then insulating it for gradual cooling. For thicker materials, stress-relieving treatments at specific temperatures and durations are also applied to release weld-induced stresses and maintain tight machining tolerances.
6. Heat Treatment: A part welded from annealed 4140 steel can be austenitized, quenched, and tempered after welding to produce the ideally desired microstructure, strength, and ductility for the application. It is not easy to heat treat a weld in situ, which is one of the reasons that welding 4140 that was previously heat treated, for example for a repair, can never match the strength and ductility of the original heat-treated material.

Additional factors should also be considered, such as: material thickness, dissimilar metal welding, joint configuration, and welding process. 

How Do Annealing and Normalizing Affect 4140 Steel?

Annealing and normalizing are heat treatment processes that have distinctly different effects on 4140 steel. Annealing involves heating the steel to a temperature above its Ac3 temperature (the temperature at which the microstructure can fully transform to the face-centered cubic phase, austenite) and holding it there before a slow cooling process. Several variations of this process are possible, depending on the primary objective of the operation. The goals of annealing 4140 include relieving internal stresses, softening and toughening the material, reducing warping and crack tendencies, and promoting a more uniform grain structure. As a result, annealed 4140 steel becomes softer and more machinable, but at the expense of some strength and wear resistance. The microstructure of annealed 4140 typically consists of coarse pearlite and ferrite.

The heat treatment known as “normalizing” also begins with heating the steel to a temperature 30-50 °C above its Ac3 temperature, followed by air cooling. The purpose of normalizing is to achieve a more uniform grain structure, enhancing the mechanical properties of the steel, including strength and toughness. Normalizing and annealing differ in their cooling rates, with normalizing having a faster rate than annealing. The accelerated cooling in normalizing contributes to a more refined microstructure, enhancing the overall strength and characteristics of the material in comparison to annealing. Normalized 4140 steel strikes a balance between hardness and machinability. The microstructure of normalized steel usually consists of fine pearlite and does include some bainite (a microstructure that forms in steel and other iron-based alloys during certain types of heat treatment, particularly during cooling from a high temperature), contributing to the enhanced mechanical properties. One trade-off of normalizing is a decrease in impact strength due to the thinning of the grain structure. 

What Are the Advantages of Using 4140 Pre-Hardened Steel?

4140 pre-hardened steel comes with a lot of benefits and advantages:

1. It has good corrosion resistance, particularly when compared to plain carbon steel.
2. It offers above-average machinability for its hardness.
3. It has good ductility, 
4. It has good wear resistance.
5. It is very hard (hardness up to 58 HRC). 
6. Pre-hardened 4140 steel has good impact resistance.
7. Despite its hardness, 4140 maintains good toughness, meaning it can absorb energy and resist fracturing or cracking under stress.
8. It has good fatigue strength. 
9. Because 4140 pre-hardened steel has already undergone thermal treatment, subsequent heat treatments for parts made from this material typically result in less distortion compared to those that haven't been pre-hardened.

What Are the Disadvantages of Using 4140 Pre-Hardened Steel?

Using pre-hardened 4140 steel has some disadvantages in addition to its benefits. Some of these are listed below:

1. It is difficult to weld. It is not recommended to weld pre-hardened steel due to its susceptibility to cracking even when preheated. 
2. Can cause some tool wear during machining and requires slower cutting speeds due to its hardness. 
3. Is more expensive compared to purchasing it in the annealed or normalized condition. 

Is 4140 a High Tensile Steel?

Yes, 4140 steel is considered to be a high-tensile steel. Its high tensile strength is attributed to its carbon level and its chromium-molybdenum alloy composition. Specific heat treatment processes f (quenching and tempering,  normalizing, and annealing), are needed to fully develop the desired mechanical properties. This steel is usually supplied in the quenched and tempered condition, with a range of tensile strength between 850 and 1000 MPa.

Summary

This article presented 4140 pre-hardened steel, explained it, and discussed its properties and various applications. To learn more about 4140 pre-hardened steel, contact a Xometry representative.

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