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I. Effects of diffused hydrogen: During welding, a certain amount of hydrogen is absorbed into the liquid metal. Some of this hydrogen escapes during the solidification of the molten pool, while the hydrogen that does not have time to escape remains in the final weld after the rapid cooling of the molten pool. In steel welds, hydrogen exists mostly in atomic or ionic form; these relatively small atoms or ions can diffuse within the lattice, and this is commonly referred to as diffused hydrogen ； The hydrogen that diffuses to lattice defects, microcracks, and voids combines to form a molecular state; due to its increased volume, it cannot diffuse any further and is known as residual hydrogen. Generally, diffused hydrogen accounts for 80%–90% of the hydrogen in welds; therefore, its impact on weld properties is greater than that of residual hydrogen. In welding operations, hydrogen can easily cause various welding defects, the most prominent of which are cold cracks. Since the diffusion and accumulation of hydrogen takes some time, cracks caused by diffused hydrogen sometimes also appear later, thereby causing greater damage to the weld. II. Detection method for diffused hydrogen: A single weld sample is prepared using the appropriate welding method; after welding, it is cooled rapidly and placed in a sealed container to collect the diffused hydrogen, which is then measured. The measurement results are converted to the volume of diffused hydrogen per 100 grams of deposited metal under standard conditions (0 °C, 101,325 Pa atmospheric pressure), expressed in mL/100g. Detection methods: Glycerin method, mercury method, chromatography method. III. Classification criteria for the diffusion hydrogen content in welding materials: ml/100g – HIIW (mercury method), HGB (glycerin method). Low hydrogen: 5-10, ≤5; Ultra-low hydrogen: ≤5 ---