In welding heat treatment, hydrogen elimination treatment is a key process step, and its core purpose can be summarized as: preventing hydrogen induced delayed cracks in welding joints, ensuring the safety and integrity of the structure.
Below, I will provide a detailed explanation from several aspects including principles, hazards, processes, and applicable situations.
1、 Sources and Forms of Hydrogen Existence
Firstly, to understand why hydrogen needs to be 'eliminated', we need to know where hydrogen comes from:
Welding materials: The coating of the welding rod, the flux, and the oil and moisture on the surface of the welding wire all contain hydrogen. At high arc temperatures, these substances decompose to produce hydrogen atoms.
Base metal surface: rust, oil stains, paint, and moisture on the workpiece groove and its surrounding areas.
Environmental atmosphere: When welding in a humid environment, water vapor in the air will also decompose into hydrogen under the action of an arc.
During the welding process, these hydrogen will dissolve into the high-temperature molten metal. When the weld seam cools, the solubility of hydrogen in the steel sharply decreases.
2、 The hazards of hydrogen: hydrogen induced delayed cracking
This is the core problem that hydrogen elimination treatment needs to solve. The formation process of hydrogen induced delayed cracks is as follows:
Hydrogen dissolution and supersaturation: During welding, a large number of hydrogen atoms dissolve in the molten pool. When the weld is rapidly cooled, hydrogen does not have time to escape, resulting in supersaturation and solid solution in the weld metal.
The diffusion and aggregation of hydrogen: supersaturated hydrogen atoms have extremely high activity and diffusion ability. They will diffuse and aggregate into micro defects (such as grain boundaries, dislocations, inclusions) or stress concentration areas (such as weld toes, undercuts) in the weld seam.
Formation of hydrogen molecules, generating enormous pressure: When aggregated hydrogen atoms combine to form hydrogen molecules, their volume rapidly expands, creating enormous pressures up to hundreds of atmospheres locally.
Induced cracking: There is residual tensile stress in the welded joint itself. When the pressure generated by hydrogen molecules is combined with residual tensile stress, exceeding the local fracture strength of the material, it will lead to the formation of microcracks.
Delay: The diffusion and aggregation of hydrogen require time, so cracks do not appear immediately after welding, but only a few hours, days, or even weeks after welding. This "delay" characteristic makes it more concealed and dangerous, and it may only break after the product passes inspection or even during service, causing serious accidents.
3、 The principle and function of hydrogen removal treatment
Hydrogen elimination treatment is essentially a post weld heat treatment. The principle is:
Provide energy to accelerate hydrogen diffusion: Heat the weldment to a certain temperature (usually 250 ° C-350 ° C) and hold it for a period of time. This temperature is much lower than the phase transition point of steel and will not change its metallographic structure.
Reduce hydrogen supersaturation: At higher temperatures, the solubility of hydrogen in steel increases, and the diffusion coefficient also increases sharply. This allows supersaturated hydrogen atoms to have enough energy to diffuse outward from within the metal.
Let hydrogen escape: During the insulation process, hydrogen atoms can fully diffuse to the surface of the workpiece and combine to form hydrogen molecules, which are released into the atmosphere.
Through this process, the hydrogen content in the weld seam is reduced below the safety critical value, thereby fundamentally eliminating the risk of hydrogen induced delayed cracking.
4、 Key points of hydrogen elimination treatment process
Temperature: Typically between 250 ° C and 350 ° C. If the temperature is too low, the hydrogen diffusion rate may not be sufficient; Excessive temperature may have adverse effects on the properties of certain materials or cause unnecessary tempering effects.
Time: The insulation time is determined based on the thickness and steel type of the workpiece, usually calculated as 1 hour for every 25 millimeters of thickness, but not less than 1 hour. Thick plate workpieces require longer insulation time to ensure that hydrogen in the core can also escape fully.
Timeliness: Hydrogen removal treatment must be carried out immediately after welding, preferably before the weld cools to room temperature. Because hydrogen induced cracks may begin to develop during the cooling process to room temperature. The standard usually requires that hydrogen removal treatment must be carried out within 4 hours after welding.
5、 Under what circumstances is hydrogen elimination treatment necessary?
Hydrogen elimination treatment is not necessary for all welding, but it is mandatory in the following key situations:
High strength steel welding: The higher the strength grade of the steel, the higher its sensitivity to hydrogen embrittlement and the easier it is to produce hydrogen induced cracks.
Thick plate structure: The thicker the plate, the faster the cooling rate of the weld seam, and the easier it is for hydrogen to supersaturate; Meanwhile, the constraint stress of thick plates is greater, and the path for hydrogen diffusion from the core to the surface is also longer.
Harsh working environment: structures used in low-temperature environments, subjected to dynamic or alternating loads, are highly sensitive to cracks.
Structures with high rigidity: The structure has high rigidity and welding residual stress, which can easily cause cracking when combined with hydrogen.