Analysis of spiral steel pipe surface cracking

Surface cracking in spiral steel pipes can be a significant issue that arises unexpectedly during usage. This analysis explores the reasons behind such cracking and offers insights into prevention strategies.

Material Uniformity and Mechanical Performance

In the design phase, hydraulic (pneumatic) systems assume that materials exhibit uniform, continuous, and isotropic properties. However, in practice, spiral steel pipes can sometimes face unexpected cracking issues due to inherent material inconsistencies. The mechanical properties of these materials may not always align with the ideal assumptions, leading to potential failures under operational stress.

Brittle Cracking and Temperature

Research indicates that brittle cracking is closely related to the operating temperature of the material. When temperatures drop below a certain threshold, materials that are normally ductile can become brittle. This phenomenon, known as "cold brittleness," results in a significant decrease in the material's ability to absorb impact energy. High-strength metals, in particular, are prone to low-stress brittle fractures under these conditions due to their non-uniform and anisotropic microstructure, which may contain microcracks, inclusions, and porosity. Therefore, it is crucial to select materials that can withstand the lowest expected operating temperatures without becoming brittle.

Influence of Frequency and Circuit Parameters

The welding process for spiral steel pipes is influenced by the relationship between the frequency and the capacitance and inductance of the circuit. The frequency of the eddy current heating is inversely proportional to the square root of the circuit's capacitance and inductance, and directly proportional to the square root of the voltage and current. By adjusting these parameters, the current frequency can be controlled to achieve the desired welding temperature.

For low-carbon steel, the optimal welding temperature ranges from 1250°C to 1460°C, ensuring adequate penetration for pipe walls with thicknesses between 3mm and 5mm. Additionally, the welding speed can be adjusted to maintain the appropriate temperature, ensuring a strong and defect-free weld.

Cold Drawing and Hot Rolling Defects

Cracks in spiral steel pipes can also occur during cold drawing or hot rolling processes. High-precision drawing may lead to brittle fractures due to the material's limited plastic deformation capacity. Brittle cracking can result from various factors, including:

- Grain Boundary Separation: The separation of grain boundaries, whether due to strength differences or inherent weaknesses, can initiate cracks.

- Inclusion Segregation: Inclusions at the grain boundaries can serve as initiation points for cracks.

- Fatigue Cracking: Even at stress levels well below the material's yield limit, alternating loads can cause fatigue cracks to develop.

Welding Temperature and Heat Input

The primary factor influencing welding temperature in spiral steel pipes is the frequency of the eddy current heating power. This, in turn, is affected by the current frequency, voltage, capacitance, and inductance. Insufficient heat input can result in incomplete fusion or penetration of the weld, while excessive heat input can lead to overheating, burning, or the formation of voids in the weld.