Post-welding heat treatment and hydrogen removal are crucial steps in ensuring the quality and performance of welded parts after welding, playing a vital role in preventing weld cracks and improving structural stability. The core purpose of post-welding heat treatment is to slow down the release rate of residual welding stress, preventing cracks caused by stress concentration. Hydrogen removal treatment focuses on dispersing hydrogen incorporated into the weld metal during welding, preventing hydrogen-induced cracking. These two processes complement each other, jointly improving the reliability of welded parts.
Post-welding heat treatment is usually performed immediately after welding. It involves heating the welded parts to a certain temperature and holding it for a period of time to homogenize the metal structure of the weld and heat-affected zone, while simultaneously reducing the peak value of residual stress. The selection of the heating temperature needs to be determined comprehensively based on the material, thickness, and welding process of the parts, generally not exceeding the tempering temperature of the base metal. For example, for low-carbon steel welded parts, the post-heat treatment temperature can be controlled between 200-300℃, which effectively relaxes stress without causing material degradation. Heating methods can include flame heating, resistance heating, or induction heating, ensuring temperature uniformity and avoiding localized overheating or underheating.
Holding time is a key parameter in post-heat treatment, directly affecting stress relaxation. Insufficient holding time prevents sufficient release of residual stress; excessive time may lead to performance degradation due to prolonged exposure to high temperatures. In practice, holding time needs to be adjusted based on part thickness, heating temperature, and material properties, typically based on the completion of austenitization and the onset of uniform cooling in the weld metal. For example, for thicker welding parts, the holding time needs to be appropriately extended to ensure sufficient heat penetration and uniform stress relaxation.
The core of hydrogen removal treatment lies in accelerating hydrogen diffusion and escape through high-temperature baking, reducing the hydrogen content in the weld metal. Hydrogen is a major factor initiating cold cracking in welding, especially in materials prone to hydrogen-induced cracking, such as high-strength steel and low-alloy steel. The temperature for hydrogen removal treatment is typically set between 250-350℃. This temperature range promotes hydrogen diffusion without triggering material phase transformation or performance degradation. The holding time needs to be determined based on the part's thickness and hydrogen content, generally no less than 2 hours to ensure sufficient hydrogen escape. For thick plates or welding parts with high hydrogen content, the holding time can be appropriately extended, or a staged heating method can be used to improve the hydrogen removal effect.
The order of post-heating and hydrogen removal treatment needs to be flexibly adjusted according to the part's material and process requirements. For materials prone to hydrogen-induced cracking (such as high-strength steel), hydrogen removal treatment is usually performed first, followed by post-heating to prevent hydrogen from redissolving at high temperatures. For parts where residual stress is the primary control target (such as large structural components), post-heating treatment can be performed first, followed by hydrogen removal treatment to further reduce the hydrogen content. In practice, post-heating and hydrogen removal treatment can also be combined, completing stress relaxation and hydrogen dissipation within the same temperature range, but temperature and time parameters must be strictly controlled to avoid sacrificing one for the other.
Environmental control during the treatment process is also crucial. During the heating and holding stages, welding parts must avoid contact with humid air or corrosive media to prevent surface oxidation or hydrogen re-penetration. For example, dry air or an inert gas (such as nitrogen) can be introduced into the heating furnace to create a protective atmosphere and reduce oxidation and hydrogen contamination. Furthermore, parts must be thoroughly cleaned of surface oil, rust, and other impurities before and after treatment to prevent the decomposition of impurities at high temperatures, which could generate hydrogen or other harmful gases.
The effectiveness of post-heating and hydrogen removal treatment needs to be verified through non-destructive testing and performance testing. Non-destructive testing (such as ultrasonic testing and magnetic particle testing) can detect the presence of cracks or hydrogen-induced defects within the weld; mechanical property tests (such as tensile tests and impact tests) can assess whether the strength and toughness of the treated material meet the standards. If the test results do not meet the requirements, the treatment parameters must be readjusted or remedial measures (such as partial repair or reheat treatment) must be taken to ensure that the welding parts meet the design requirements. Through scientific and reasonable post-heating and hydrogen removal treatment, the reliability of welding parts can be significantly improved, their service life extended, and a solid guarantee for engineering safety.
