Low temperature carbon steel pipe

Low temperature carbon steel pipes are medium carbon structural steel, known for their good mechanical properties, affordability, and wide availability. Despite these advantages, their biggest limitation is low hardenability, making them unsuitable for high cross-sectional dimension workpieces. These pipes are suitable for low temperature operations and can withstand certain low-temperature impacts.

Quenching Process for Low Temperature Carbon Steel Pipes

- Heating and Quenching:

Quenching Temperature: Typically set to A3 + (30~50)°C, often on the higher end in practice. Higher quenching temperatures enhance heating speed, reduce surface oxidation, and improve efficiency.

Holding Time: Uniform austenite formation requires sufficient holding time. For larger furnace capacities, extend holding time appropriately to ensure even heating and avoid hardness inconsistencies. However, excessively long holding times can lead to coarse grains, oxidation, and decarbonization, compromising quenching quality. For larger equipment, extend holding time by 20% beyond specified process documents.

- Cooling:

Cooling Medium: Use a 10% salt solution for its high cooling rate. The workpiece should harden without chilling to prevent cracking, which can occur due to rapid transformation at around 180°C.

Cooling Method: After rapid cooling to a low temperature region, switch to slow cooling. Monitor water temperature carefully; experienced operators should know when to switch from water to air cooling to prevent excessive stress and cracking.

Movement During Cooling: Workpieces should not remain static in the cooling medium. Move them according to their geometry to ensure even hardness and stress distribution, preventing large deformations or cracking.

Post-Quenching Hardness and Tempering

- Hardness Post-Quenching:

Target hardness is HRC56~59. Larger sections may have slightly lower hardness but should not be below HRC48. Insufficient hardening can leave ferritic and sorbitic structures, which are undesirable.

- Tempering:

Post-quenching, temper at 560600°C, aiming for hardness of HRC2234. The goal is to achieve desired mechanical properties, with the exact hardness depending on specific application requirements.

Drawing Hardness: Adjust by varying tempering temperature. For example, high-strength shafts may require higher hardness, while gears or shafts with keyways, which undergo further machining, may need lower hardness.

- Tempering Time:

While it minimally impacts hardness, ensure overall tempering time is more than one hour to achieve uniform properties.

Low temperature carbon steel pipes offer excellent mechanical properties and are suitable for a variety of applications. Proper quenching and tempering processes are crucial to optimize their performance, ensuring they meet specific engineering requirements while preventing common issues such as cracking and uneven hardness.