Abstract
Titanium alloy seamless pipes are crucial in industries such as military and aviation, where exceptional strength, corrosion resistance, and low density are paramount. Traditional manufacturing techniques, including hot extrusion and cold rolling, are expensive and inefficient, limiting mass production potential. To overcome these challenges, research into the PQF continuous rolling process has gained traction. Ultrasonic testing plays an essential role in ensuring the quality of these pipes, but it faces unique difficulties due to the material’s microstructure and surface conditions. This article explores these challenges, examines existing standards for ultrasonic testing, and highlights advancements in detection methods tailored for hot-rolled titanium alloy seamless pipes.
Titanium alloys offer high strength, low density, corrosion resistance, and exceptional performance in environments with hydrogen sulfide and carbon dioxide, making them ideal for demanding industrial applications. Currently, seamless pipes for military, aviation, and other industries are produced worldwide using hot extrusion and cold rolling processes. However, these methods are inefficient and costly. To address these challenges and enable mass production, research institutes and enterprises are exploring the PQF continuous rolling process for manufacturing titanium alloy seamless pipes.
Ultrasonic testing is a critical technique for ensuring the quality of these pipes. Contact ultrasonic testing employs a bidirectional oblique probe, featuring both transmitting and receiving capabilities. Two probes transmit ultrasonic waves simultaneously in opposite directions, allowing the waves to travel in a zigzag pattern along the pipe wall. When the probe's incident angle is precisely calibrated to the pipe's specifications, the opposite probe receives the transmitted wave, producing an echo signal. This signal, following a zigzag acoustic path along the pipe wall, is referred to as a "through wave."
Challenges arise during ultrasonic testing due to the titanium alloy's microstructure, which generates significant noise, and the poor surface conditions of hot-rolled pipes. Longitudinally distributed rolling micro-defects on the inner pipe wall can create clutter signals that lead to false alarms or misjudgments. In automated online testing, signal instability often results from rapid sound energy attenuation and high clutter levels. Manual ultrasonic testing further reveals issues such as missing "through waves," excessive clutter signals, or detection limited to primary or secondary defect waves, which significantly deviates from the norms of seamless steel pipe testing. These challenges underscore the necessity of developing a specialized ultrasonic flaw detection method tailored to titanium alloy seamless pipes, particularly for hot-rolled applications.
Standards for Ultrasonic Testing
The widely used ultrasonic testing standard for steel pipes is GB/T 5777-2019, which specifies the requirements for full-circumference automatic ultrasonic testing to detect longitudinal and transverse defects in seamless and welded steel pipes (excluding submerged arc-welded pipes). This standard applies to steel pipes with an outer diameter of 6 mm or more and a diameter-to-wall thickness ratio of at least 5.
For titanium alloy pipes, the primary ultrasonic testing standard is GB/T 12969.1-2007, which outlines methods for detecting flaws in titanium and titanium alloy pipes by analyzing reflected signals from artificial reference samples. This standard covers seamless or welded pipes used in condensers and heat exchangers, with outer diameters ranging from 6 mm to 80 mm, wall thicknesses between 0.5 mm and 4.5 mm, and a wall-to-diameter ratio of no more than 0.2.
The flaw detection process employs a line-focusing probe using the shear wave immersion technique, similar to the approach used for carbon steel pipes. Artificial defect depths in sample pipes are specified as either 12.5% of the nominal wall thickness or 0.1 mm, whichever is greater. The recommended probe frequency ranges from 5 MHz to 15 MHz, with optimal rectangular chip sizes of 8 mm × 6 mm or 10 mm × 8 mm.
Ultrasonic Testing for Titanium Alloy Pipes
The GB/T 12969.1-2007 standard applies to small-diameter, thin-walled pipes. The manufacturing process for titanium alloy pipes is now well-established. Titanium alloy tube billets are produced through drilling extrusion and oblique rolling perforation, followed by rolling, drawing, spinning, and other processes to create pipes for various applications and specifications.
A Steel Pipe Company utilizes a PQF continuous rolling pipe unit. Through an industrialized assembly line that includes perforation, tube rolling, and sizing (or reduction), the company has successfully manufactured titanium alloy oil pipes with dimensions of 88.9 mm × 7.34 mm, surpassing standard detection limits, particularly in terms of flaw depth.
Currently, corrosion-resistant pipes follow the GB/T 5777-2019 U2 flaw detection level, specifying flaw depths of 5% of the nominal wall thickness—this is significantly stricter than the requirements outlined in GB/T 12969.1-2007. As there are no specific standards for testing titanium alloy seamless pipes in the petroleum industry, consultations with oilfield users have determined that ultrasonic testing methods and grades can refer to GB/T 12969.1-2007 while adopting the U2 flaw detection level.
Titanium alloy pipes generally require higher probe frequencies, ranging from 5 MHz to 15 MHz, due to their material properties and stringent application requirements. These conditions assume smooth surface finishes and relatively uniform microstructures. However, hot-rolled titanium alloy seamless pipes often have suboptimal surface conditions and significant microstructural variations, reducing the effectiveness of high-frequency probes. Comparative testing of probes with frequencies of 1.25 MHz, 2.5 MHz, and 5 MHz revealed that a 2.5 MHz probe is optimal for detecting flaws in hot-rolled titanium alloy seamless pipes. The recommended chip size for the 2.5 MHz probe is 12 mm × 10 mm.
