Discussion on Heat Treatment of Oil Drill Tool Crossover Sub

1. Introduction

The crossover sub is a crucial component of oil drilling equipment, requiring high safety and reliability. Made from alloy structural steel, it demands strict heat treatment procedures. Different heating techniques vary in operational requirements, steps, and technical expertise. This article introduces the heat treatment process of crossover subs, covering quenching heating, temperature determination, heating duration, holding time, and quenching cooling operations, aiming to enhance understanding of the heat treatment process for oil drilling crossover subs.

2. Heat Treatment Process of Crossover Subs

The standard heat treatment steps for crossover subs include:

1. Material preparation
2. Rough machining
3. Drilling
4. Normalizing (to refine grain structure and ensure uniform composition)
5. Quenching and tempering (to improve strength and toughness)
6. Final machining

Given the large size and high reliability requirements of crossover subs, normalizing before quenching and tempering is essential. This step helps refine the grain structure, enhance toughness, improve hardenability, and eliminate material defects like banded structures.

3. Quenching Heating Analysis

Based on the temperature at which the workpiece enters the furnace, quenching heating can be categorized into three methods:

1. Direct Heating: Cold workpieces are placed directly into a furnace preheated to quenching temperature. This method features a large cross-sectional temperature difference but fast heating speed, making it suitable for small workpieces and increasingly used for large workpieces as metallurgical quality standards improve.
2. Step Heating: The workpiece is heated along with the furnace, held at a constant temperature before reaching the transformation point, and then heated to the target temperature. This method is commonly used for large forgings but is less frequently applied to small workpieces.
3. Rapid Heating: The workpiece is placed in a furnace already heated beyond normalizing temperature by 100–120°C or even above quenching temperature. This method is a form of rapid heating applicable to small and medium-sized workpieces but rarely used for large forgings.

Given the production efficiency, size, and stress variations of crossover subs, the direct heating method is generally the most practical choice.

4. Quenching and Cooling Considerations

For crossover subs made of 40CrMnMo or other alloy steels, the critical oil quenching diameter is approximately 35–100 mm. Since crossover subs are large, oil quenching may not ensure a uniform bainite or martensite structure in the core, while water quenching risks cracking. Therefore, selecting an appropriate quenching medium is crucial to prevent cracking and achieve uniform section properties.

Recommended Quenching Medium:

• AQ251 water-soluble medium, which allows adjustable cooling curves based on steel composition and requirements, ensuring optimal cooling speed while minimizing thermal and structural stress to reduce cracking and deformation risks.

Cooling Methods:

• A single-quenching medium method is preferable, offering simplicity, cost-effectiveness, and suitability for large-scale production.
• Cooling time is a critical parameter that should be carefully controlled through experimental testing, as different quenching media require varying cooling durations.

5. Conclusion

Due to the large size and mass of crossover subs, as well as their high safety and reliability requirements, heat treatment is a crucial process. When selecting the heating treatment method, factors such as component size, shape, and performance requirements should be considered. A combination of heat treatment techniques for large and small forgings should be employed to improve overall heat treatment quality and efficiency.