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AISI 4140 Steel: Optimized Heat Treatment Process

Alloy steel 4140 is a high-quality material with high demand for procurement and application, so its quality is crucial to various manufacturing industries. Therefore, many partners have been purchasing 4140 steel from Songshun for a long time, precisely because we can achieve better quality and price than others. This is due to Songshun’s optimization of the heat treatment process of aisi 4140 steel. In this article, we will explore in depth the heat treatment process of 4140 alloy steel, which is also an important reason why Songshun’s aisi 4140 steel is chosen.


First, put the aisi 4140 steel into the furnace for uniform preheating. The temperature is usually controlled between 400°C-700°C, depending on the size and shape of the steel. We will formulate standardized temperature, speed and time parameters according to different needs. Avoid excessively high temperatures, which may cause grain coarsening, while too low temperatures may affect the subsequent quenching effect.

By optimizing the preheating time and temperature, Songshun can ensure that the 4140 steel in this process can be evenly heated, thereby eliminating the residual stress and grain boundary carbides and structural changes inside the material, and reducing the deformation and cracking that may be caused during quenching.


SAE 4140 steel is generally heated to 850℃-900℃ and then cooled in air. Normalizing cools faster than annealing. By normalizing aisi 4140 steel, a more uniform grain structure can be achieved, improving the mechanical properties of the steel while maintaining a certain toughness.

The key to optimizing the normalizing process is to keep the steel heated thoroughly for the appropriate time to complete the transformation from ferrite to austenite. Another way is to optimize its cooling rate, which helps to form a finer microstructure, improving the overall strength and properties of the material compared to annealing.


4140 tool steel is usually hardened by cold working or heat quenching. The quenching process is also an important step for aisi 4140 to obtain high hardness and strength. Generally, the quenching temperature is controlled at 815℃-860℃ or 860℃-880℃, and is maintained until the temperature of the entire part is uniform. It can be quenched in oil, water or polymer. Songshun is usually quenched in oil, which will Slightly milder than quenching in water. In the quenched condition, the hardness of 4140 steel is approximately 54-59 HRC.

This process mainly makes aisi 4140 steel form a martensite structure, thereby significantly improving its hardness and strength properties. The cooling and quenching medium has an important influence on martensite transformation, and a faster cooling rate can achieve better hardening effect. In addition to using water or oil quenching, Songshun also uses new quenching media, such as polymer quenching fluid, to obtain better quenching effects. This optimizes the quenching effect and meets some performance requirements that cannot be achieved by water or oil quenching.


The tempering temperature of 4140 material depends on the required hardness level of the steel, generally 550-700℃, for at least 1 hour, and then air, water or oil cooling. Avoid tempering within 200°C – 420°C, which will cause temper embrittlement (“blue brittleness”).

The ASTM 4140 steel provided by Songshun is optimized through tempering temperature and time. Through rich experience and data obtained from multiple operations, the best tempering temperature and time combination for different specifications and steel types is summarized, which can ensure the elimination of internal stress and brittleness generated during quenching, and further improve the toughness and fatigue resistance of the material.


4140 is usually annealed at 800℃-872℃ and maintained at this temperature for 1 hour per inch (25.4 mm) of maximum thickness and a minimum of 2 hours. It is then slowly cooled in a furnace.

Annealing 4140 can release internal stress, soften and toughen the material, and promote a more uniform grain structure, so that its structure is usually composed of coarse pearlite and ferrite. Therefore, in order to optimize the process, in addition to the precise control of annealing temperature and time, the choice of annealing atmosphere is also important. Vacuum annealing or protective atmosphere annealing can reduce oxidation and decarburization on the steel surface. And optimizing the uniformity of the temperature and cooling rate of the steel during the annealing process can improve the uniformity of the performance of steel 4140.

In addition to the above main heat treatment process, Songshun has optimized the heating method, cooling, insulation, heat treatment auxiliary technology, etc. during the heat treatment of aisi 4140 steel. Therefore, choosing to cooperate with Songshun Steel can not only provide high-quality aisi 4140, but also competitive prices, abundant inventory, first-class services, etc., and it is a trustworthy partner in choosing a steel supplier.


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