It can be said that pre weld heat treatment is one of the key links to ensure the safety, reliability, and service life of welded structures, and its importance cannot be overemphasized.
It is not an "optional" option, but a "must-have" for many materials (especially medium high carbon steel, high-strength steel, alloy steel, certain non-ferrous metals, etc.) and important structures.
Below, I will explain in detail its importance from several aspects:
1、 The main purpose of pre weld heat treatment
The core purpose of pre weld heat treatment is to create a favorable condition for welding, prevent welding defects, and ensure that the performance of the post weld joint meets the requirements. Specifically, it includes the following common processes and their functions:
1. Preheating
This is the most common and important form of pre weld heat treatment.
Main purpose:
Reduce cooling rate: After welding, the molten pool will cool rapidly. Excessive cooling can lead to the formation of a hard and brittle martensitic structure in the heat affected zone (HAZ). Preheating can slow down the cooling rate of welds and heat affected zones, effectively preventing the formation of such hardened structures.
Reduce welding stress: Preheating can reduce the significant temperature difference between the weld area and other parts of the base metal, thereby significantly reducing the thermal stress caused by uneven thermal expansion and contraction.
Moisture Removal: Preheating can help remove moisture and dampness from the welding area and its vicinity, thereby reducing the source of hydrogen gas and preventing hydrogen induced cracking (cold cracking). This is one of the most dangerous defects in high-strength steel welding.
Improving welding processability: Preheating can improve the fluidity of the molten pool, facilitate the escape of gas and the floating of slag, and reduce the possibility of porosity and slag inclusion.
2. Hydrogen removal treatment
Sometimes classified as post weld heat treatment, but strictly speaking, it is carried out immediately after welding.
Main purpose: Immediately heat the welded part to a certain temperature (usually 250 ° C-350 ° C) after welding and keep it warm for a period of time. The purpose is to accelerate the diffusion and escape of hydrogen atoms in the weld seam, "drive them away" before hydrogen induced cracking occurs, and completely eliminate the hidden danger of cold cracking. This is crucial for welding ultra-high strength steel.
2、 Why is it important? What if we don't do it or can't do it well?
Neglecting or incorrect pre weld heat treatment can directly lead to catastrophic consequences:
Cracking: This is the most direct and dangerous consequence. Especially hydrogen induced cracks, which often have a delay and may only appear hours or even days after welding is completed, like a "time bomb", posing a great threat to structural safety.
Joint embrittlement: The formation of a hard and brittle martensitic structure leads to a sharp decrease in the impact toughness of the welded joint, making it unable to withstand impact loads and prone to brittle fracture.
Excessive residual stress: Excessive welding residual stress can:
Promote stress corrosion cracking (SCC): In corrosive environments, high stress areas are preferentially corroded and cracked.
Impact on dimensional stability: During mechanical processing or use, stress release can cause deformation of the component.
Reduce fatigue strength: High stress concentration points can become the origin of fatigue cracks.
Defects such as incomplete penetration and slag inclusion: In some cases, if not preheated, the fluidity of the molten pool is poor, making it more prone to process defects.
3、 Practical application examples
Ships and offshore platforms: High strength thick steel plates are widely used, and preheating and hydrogen removal treatment must be strictly carried out, otherwise they are prone to fracture in harsh marine environments.
Pressure vessels and pipelines, especially pressure equipment, involve safe operation, and relevant standards (such as ASME, GB 150) have mandatory and detailed regulations on preheating temperature and interlayer temperature.
Aerospace: The performance requirements for materials are extremely high, and thermal control (including preheating) during the welding process is the lifeline to ensure the reliability of parts.
Heavy machinery: Key structural components such as large excavators and crane booms, welding cracks can cause the entire structure to fail.
Conclusion
Pre weld heat treatment (especially preheating) is not an optional process, but an essential core component of the welding process specification (WPS). Its importance is reflected in:
Prevent defects: directly prevent the occurrence of fatal defects such as cracks and pores.
Guarantee performance: Ensure that the welded joint has mechanical properties that match the base metal, especially toughness and strength.
Control stress: reduce residual stress and deformation, improve the dimensional stability and service life of the structure.
Ensuring safety: For critical structures that bear weight and pressure, it is the first and most important line of defense to ensure overall safety.
Whether preheating is required and what is the preheating temperature, it needs to be comprehensively evaluated based on factors such as material chemical composition (carbon equivalent), thickness, structural confinement, hydrogen source, etc., and strictly follow relevant design standards and welding process specifications. Never operate arbitrarily based on experience.