Firstly, let's clarify several common heat treatments in steel structure manufacturing:
1. Preheating
Definition: Before welding begins, the area to be welded is heated to a specified temperature (usually 100 ° C-200 ° C).
Main purpose:
Reduce cooling rate: prevent the formation of hard and brittle martensitic structure in the weld and heat affected zone.
Expel moisture: Reducing the source of hydrogen helps prevent hydrogen from causing cracks.
Reduce residual stress: Reduce the temperature gradient in the welding area, thereby reducing thermal stress and shrinkage stress.
2. Post heating/dehydrogenation treatment
Definition: After welding is completed, immediately heat the weld area to 250 ° C-350 ° C and keep it warm for a period of time.
Main purpose:
Accelerating the escape of hydrogen: This is its most core and specialized purpose, used to prevent hydrogen induced delayed cracking.
It is usually considered as part of the welding process, rather than the strict definition of 'final heat treatment'.
3. Post weld heat treatment
This is a broad term referring to the heat treatment carried out after welding to improve the performance of the welded joint. Usually includes the following two types:
A. Stress relief heat treatment
Definition: Heat the entire component uniformly to an appropriate temperature below the steel Ac1 line (usually 595 ° C-675 ° C for carbon steel and low-alloy steel), hold for a period of time (usually calculated based on plate thickness, such as holding for 1 hour every 25mm thickness), and then slowly cool with the furnace.
Main purpose:
Reduce residual stress: By utilizing the creep effect at high temperatures, residual stress can be relaxed, typically eliminating over 80% of welding residual stress.
Improving organizational performance: tempering the hardened microstructure to enhance toughness and plasticity.
Stable size: prevents deformation due to stress release during mechanical processing or use.
B. Normalization/Normalization+Tempering
Definition: Heat the steel to 30 ° C-50 ° C (approximately 900 ° C or above) above the Ac3 line, hold it for a period of time, and then cool it evenly in still air. Afterwards, tempering treatment is usually required.
Main purpose:
Refine grain size and achieve uniform microstructure: Obtain finer and more uniform austenite structure, thereby improving the comprehensive mechanical properties (strength, toughness, and plasticity) of steel.
Mainly used for structures, forgings, and castings that require high mechanical properties, to eliminate their coarse and uneven cast or forged structures.
Step 1: Before welding → decide whether to "preheat" or not
When is it needed?
High material strength/carbon equivalent: When welding high-strength steel (such as Q390 and above), medium high carbon steel, or steel with high carbon equivalent.
When the plate thickness exceeds a certain limit (such as>25mm~30mm, depending on the specifications), preheating is necessary.
High degree of constraint: When the structural rigidity is high and the weld seam cannot contract freely.
Low ambient temperature: When welding in low temperature environments (such as<0 ° C or<5 ° C).
Key points of selection: Preheating is the first line of defense against cracking and one of the most economical and effective means. The selection is mainly based on the carbon equivalent and plate thickness of the steel, and the specific temperature is determined by querying relevant welding standards or empirical formulas.
Step 2: After welding → decide whether to perform "hydrogen removal treatment"
When is it needed?
As mentioned in the previous question, it mainly targets scenarios with high sensitivity to hydrogen induced cracking:
Welding of high-strength steel (especially strength grade ≥ 440MPa).
Thick plate structure (especially thickness>30mm).
The welding joint has high constraint stress.
Key structures with extremely stringent quality requirements, such as offshore platforms and nuclear power equipment.
Key points for selection: Hydrogen elimination treatment is a specialized measure to prevent delayed cracking. It is usually carried out immediately after welding, as a supplement and reinforcement to preheating measures. If preheating can fully control the risk of cracking and does not belong to the high-risk situation mentioned above, specialized hydrogen removal treatment may not be required.
Step 3: Final Stage → Deciding whether to undergo "Stress Relief Heat Treatment"
When is it needed?
This is the most important and complex decision, which requires consideration of the structural usage conditions:
Stress corrosion environment: The structure will operate in environments that may cause stress corrosion cracking, such as marine environments, alkaline environments, hydrogen sulfide environments, etc. Eliminating stress is necessary.
Enduring alternating loads: Structures are used in situations where fatigue loads are significant, such as crane beams, bridges, ships, etc. Eliminating stress can significantly improve fatigue life.
High dimensional stability requirements: The structure needs to maintain extremely high dimensional stability during service, or precision machining is required after welding.
Huge wall thickness: For ultra thick plates (such as those with a thickness greater than 100mm in pressure vessels), huge residual stresses may lead to brittle fracture or deformation.
Extreme usage environment: Structures used in low-temperature environments can improve their resistance to brittle fracture by eliminating stress.
When may it not be necessary?
Ordinary building steel structures (office buildings, residential buildings), under static load, and with small plate thickness.
Lightweight structures such as trusses composed of rolled profiles (H-beams, angle steels).
When the cost of heat treatment is too high, or when the component is too large to undergo overall heat treatment.
Special attention: for quenched and tempered high-strength steel
Caution must be exercised when conducting high-temperature stress relief heat treatment on high-strength steel that has undergone quenching and tempering treatment. Because its tempering temperature may be lower than or close to the stress relief temperature, reheating may result in a significant decrease in its strength. At this time, it is necessary to strictly follow the recommendations of the material supplier, use lower stress relief temperatures, or adopt other alternative methods (such as vibration aging, explosion method, etc.).
Step 4: Special considerations → "Normalization/Normalization+Tempering"
When is it needed?
This is usually not decided on site, but rather completed during the material ordering phase or within the factory.
When the performance of rolled or cast materials cannot meet the design requirements.
Used for manufacturing welded node plates, forgings, or cast steel nodes with extremely high performance requirements.
Usually specified by design drawings or technical specifications.
Final recommendation: The selection of heat treatment process must strictly follow the design documents, relevant technical standards (such as AWS, EN, GB, etc.), and material supplier recommendations. It is a balancing process based on scientific analysis and engineering experience.