5140 ALLOY STEEL | 1.7035 | 41CR4 | SCR440

3. Key Heat Treatment Processes for 5140 Alloy Steel

Several established heat treatment processes can be applied to 5140 alloy steel to modify its microstructure and, consequently, its physical and mechanical properties. These treatments include annealing, normalizing, hardening and tempering, austempering, and induction hardening.

3.1 Annealing 5140 Alloy Steel

Annealing is primarily employed to soften 5140 alloy steel, enhancing its machinability and formability.

• Process: This involves heating the steel to a temperature range of 815°C to 870°C (1500°F to 1600°F), holding it at this temperature for an adequate duration, followed by a slow cooling process, typically furnace cooling.
• Outcome: This treatment aims to produce a microstructure of predominantly pearlitic or spheroidized carbides within a ferritic matrix. The maximum hardness achievable after annealing 5140 steel within this range is approximately 187 HB. This softened state, characterized by its specific microstructure, is ideal for subsequent manufacturing operations due to enhanced machinability and improved formability.

3.2 Normalizing 5140 Alloy Steel

Normalizing is a heat treatment process used to refine the grain structure of 5140 alloy steel, creating a more uniform and homogeneous microstructure.

• Process: It involves heating the steel to a temperature above its upper critical transformation point (Ac3) and then cooling it in still air.
• Outcome: This treatment results in a finer, more uniform grain size, which can improve the steel’s strength and toughness. Normalizing is often performed as a preparatory step before further heat treatments like hardening and tempering, ensuring a consistent and optimal response to subsequent thermal processing.

3.3 Hardening and Tempering of 5140 Alloy Steel

Hardening and tempering (often referred to as quenching and tempering) is a critical heat treatment for 5140 alloy steel to develop its optimal balance of strength, hardness, and toughness.

• Hardening Process: This involves austenitizing the steel, typically by heating to around 840°C (or as per specific application needs), followed by rapid cooling (quenching) in a suitable medium (e.g., oil or water). The goal is to transform the austenite into a hard martensitic or bainitic structure, which provides the initial high hardness.

• Hardenability: 5140 alloy steel exhibits good hardenability, superior to plain carbon steels like 1040, especially in achieving through-hardness in sections up to approximately 40mm in diameter when water quenched. For instance, a 15mm diameter 5140 sample shows consistent hardness across its cross-section due to martensite formation, a characteristic maintained even in 40mm diameter samples under similar quenching conditions. However, its hardenability is generally lower than that of higher-alloy grades such as 4140 or 4340; this should be considered when treating very large sections where achieving uniform through-hardness is critical, as those higher-alloy alternatives may offer deeper hardening.

• Tempering Process: After hardening, the steel is extremely hard but often too brittle for most applications. Tempering involves reheating the quenched steel to a specific temperature below its lower critical temperature (Ac1), typically between 175°C and 705°C (350°F and 1300°F). The chosen tempering temperature dictates the final balance of properties.
• Outcome: Tempering reduces hardness and strength to some extent but significantly increases ductility and toughness, relieving internal stresses induced during quenching. This process allows for precise control over the final mechanical properties of the 5140 alloy steel components, tailoring them to specific service demands.

3.4 Austempering 5140 Alloy Steel

5140 low-alloy steel is considered well-suited for austempering, an isothermal transformation process that produces a bainitic microstructure.

• Process: Austempering involves quenching the steel from its austenitizing temperature directly to a temperature above the martensite start temperature (Ms) but below the pearlite formation range (typically within the bainite transformation zone). The steel is held at this temperature until the austenite transforms completely into bainite. For 5140 steel, this transformation to bainite is generally completed within 1 to 10 minutes when held at temperatures between 315°C to 400°C (600°F to 750°F).
• Outcome: This process results in a bainitic microstructure, known for its excellent combination of strength, toughness, and ductility, often with reduced distortion compared to conventional quenching and tempering. 5140 alloy steel’s composition allows for austempering in larger section sizes compared to plain carbon steels like 1080, due to more time available to bypass pearlite formation during cooling.

3.5 Induction Hardening of 5140 Alloy Steel

For applications demanding high surface hardness and wear resistance combined with a tough core, 5140 alloy steel can be effectively induction hardened. This process is typically performed after an initial hardening and tempering treatment to establish the desired core properties.

• Process: Induction hardening is a surface hardening technique that uses electromagnetic induction to rapidly heat the surface layer of the steel above its transformation temperature, followed by immediate quenching.
• Outcome: This creates a hard surface case while maintaining the pre-established beneficial properties (like toughness or high strength) of the core. For 5140 alloy steel (often grouped with 4140 for this characteristic), induction hardening can achieve case depths of 1.0 mm to 2.0 mm, with core strengths typically ranging between 100-140 kgf/mm², depending on the component’s diameter and specific process parameters. This dual-property characteristic is highly advantageous for components subjected to surface wear and core stresses.

3.6 Summary of Heat Treatment Options for 5140 Alloy Steel

To provide a clear overview of you, the following table summarizes the key heat treatment processes for 5140 alloy steel: