AISI 4130 is a versatile and widely used chromium-molybdenum alloy steel known for its excellent strength-to-weight ratio and exceptional weldability. To fully utilize its potential, proper heat treatment is crucial.
In this blog post, we will explore the recommended heat treatment process for AISI 4130 steel, including its chemical composition, mechanical properties, types of heat treatment, heating and cooling rates, and the necessary holding time for each process.
Chemical Composition and Mechanical Properties
AISI 4130 steel primarily consists of iron (Fe), chromium (Cr), and molybdenum (Mo). Other elements present in smaller quantities include carbon (C), manganese (Mn), silicon (Si), phosphorus (P), sulfur (S), and trace amounts of other elements. The composition of AISI 4130 steel provides it with excellent strength, toughness, and wear resistance.
| Element | Composition Range (% by weight) |
|---|---|
| Carbon (C) | 0.28 – 0.33 |
| Chromium (Cr) | 0.80 – 1.10 |
| Manganese (Mn) | 0.40 – 0.60 |
| Silicon (Si) | 0.15 – 0.35 |
| Phosphorus (P) | 0.035 (max) |
| Sulfur (S) | 0.040 (max) |
| Molybdenum (Mo) | 0.15 – 0.25 |
| Iron (Fe) | Remaining balance |
| Mechanical Property | Value |
|---|---|
| Tensile Strength | 85,000 – 120,000 psi |
| Yield Strength | 70,000 – 85,000 psi |
| Elongation at Break | 20 – 30% |
| Hardness (Rockwell C Scale) | 20 – 30 HRC (as quenched) |
| Impact Toughness (Charpy V-Notch) | 27 – 54 ft-lb |
AISI 4130 steel Heat Treatment
Heat treatment is a critical step in optimizing the properties of AISI 4130 steel. The specific heat treatment temperatures, heating and cooling rates, and soaking times can vary depending on factors such as the size and shape of the component.
Additionally, considerations like the type of furnace, quenching medium, and workpiece transfer facilities should not be overlooked during the heat treatment process.
Forging
When forging AISI 4130 steel, it is important to preheat the material carefully to avoid any potential issues. Raise the temperature to a range of 950-1200°C for the forging process. It is crucial not to forge the steel below 950°C to ensure the desired properties are achieved.
AISI 4130 steel Annealing Heat Treatment
For annealing, start by heating the SAE 4130 steel slowly to approximately 850°C, allowing sufficient time for thorough heating.
After reaching the desired temperature, cool the steel slowly in the furnace to around 480°C, followed by air cooling. This annealing process helps relieve internal stresses and promotes improved machinability.
AISI 4130 steel Hardening Heat Treatment
SAE 4130 steel is often supplied already heat treated with a hardness of 18-22 HRC. However, if further heat treatment is required, begin by heating the annealed SAE 4130 steel slowly to a temperature range of 870-890°C.
After adequate soaking at this temperature, quickly quench the steel in oil. It is crucial to temper the steel as soon as it reaches room temperature to enhance its strength and toughness.
AISI 4130 steel Tempering Heat Treatment
To temper AISI 4130 steel, carefully heat it to a suitable temperature based on a tempering chart or table. Generally, the temperature range falls between 400-570°C. Allow the steel to soak at this temperature for 2 hours per 25mm of the ruling section. After the soaking period, allow the steel to cool in ambient air. It is worth noting that tempering between 250-375°C is not advisable as it significantly reduces the impact value.
Types of Heat Treatment
The recommended heat treatment for AISI 4130 steel involves a two-step process: austenitizing and tempering. Austenitizing is the initial step, where the steel is heated to the austenitizing temperature, typically around 1600°F (870°C). This temperature allows the steel to transform its microstructure into austenite, which improves its machinability and facilitates subsequent heat treatment processes.
Heating and Cooling Rates
To achieve the desired microstructure transformation and hardness, the steel should be heated gradually and evenly to the austenitizing temperature. Once the desired temperature is reached, the steel needs to be rapidly cooled. Water quenching is recommended for AISI 4130 steel as it provides a higher as-quenched hardness compared to oil quenching.
Holding Time and Tempering
After the rapid cooling process, the steel is in a hardened state. To achieve the desired hardness range for AISI 4130 steel, tempering is necessary. For a water-quenched, ½-inch-diameter round part, tempering should be performed at approximately 1025-1200°F (550-650°C).
It is advisable to start tempering at the lower temperature and then check the as-tempered hardness. If needed, retempering at a higher temperature can be done to achieve the desired hardness range of 25-32 HRC.
Conclusion
Proper heat treatment is essential to unleash the full potential of AISI 4130 steel. By following the recommended process of austenitizing, water quenching, and tempering within the specified temperature range, you can achieve the desired hardness and mechanical properties.
Remember to adhere to the recommended heating and cooling rates, as well as the appropriate holding time for each step. With the right heat treatment, AISI 4130 steel can deliver exceptional strength, durability, and performance in various applications.