DHT Heat Treatment
Dehydrogenation Heat Treatment (DHT) is a thermal process used primarily in welding to remove residual hydrogen from weld metal and the heat-affected zone (HAZ) to prevent hydrogen-induced cracking, also known as cold cracking or delayed cracking. Below is a detailed explanation of DHT, its purpose, process, and applications:
What is DHT?
DHT involves heating a welded component to a specific temperature, typically between 300–350°C (570–650°F), and holding it for a minimum duration (e.g., 1–4 hours) to allow hydrogen to diffuse out of the weldment. This process is critical for materials prone to hydrogen embrittlement, such as low-alloy steels (e.g., Cr-Mo, Cr-Mo-V), high-strength steels, and quenched-and-tempered (Q&T) steels. Hydrogen can be introduced during welding through moisture, flux, or the welding environment, and if not removed, it may cause cracking as it accumulates in the microstructure.
Why is DHT Required?
Prevent Hydrogen Cracking: Hydrogen trapped in the weld metal or HAZ can lead to cold cracking, which may occur hours or days after welding (delayed cracking). DHT accelerates hydrogen diffusion out of the material, reducing this risk.
Material Sensitivity: Certain materials, like Cr-Mo-V steels, have a high susceptibility to hydrogen-induced cracking due to their microstructure and slow hydrogen diffusion rates. DHT is often mandatory for these alloys.
Welding Process: Flux-based welding processes (e.g., FCAW, SAW, SMAW) introduce hydrogen into the weld, necessitating DHT if preheat is not maintained or post-weld heat treatment (PWHT) is delayed.
Fabrication Requirements: DHT is used when welding is paused, preheat is stopped, or PWHT cannot be performed immediately, ensuring hydrogen is removed from partially completed welds.
How Does DHT Work?
Mechanism: At elevated temperatures (e.g., 300–350°C), hydrogen’s diffusivity increases, allowing it to escape from the weld metal and HAZ into the atmosphere. This reduces the hydrogen concentration below critical levels (e.g., <1.8 ml/100g for high-strength steels) that could cause cracking.
Temperature and Time: Typical DHT conditions are 300–350°C for 1–4 hours, depending on the material, thickness, and fabrication code (e.g., ASME BPVC Section IX). For example, Cr-Mo-V steels often undergo DHT at 350°C for 4 hours, though lower temperatures (e.g., 280°C) over longer periods can achieve similar results.
Process: The weldment is heated in a controlled oven or using localized heating methods, held at the target temperature, and then allowed to cool slowly. This is distinct from PWHT, which focuses on stress relieving and microstructural changes at higher temperatures (e.g., 620–760°C).
Key Applications
Materials: Commonly applied to low-alloy steels (e.g., SA 387 Gr 12 Cl 2, Cr-Mo-V), high-strength steels, and Q&T steels used in pressure vessels, reactors, and piping for oil refineries or hydrogen-related services.
Weld Types: DHT is typically used for less constrained welds, such as shell welds or main reactor seams, rather than highly restrained joints, which may require intermediate stress relieving (ISR) at higher temperatures (e.g., 620–660°C).
Standards: Governed by codes like ASME BPVC Section IX (where it’s referred to as post-heating) and API 934C for specific applications. It’s often a recommended practice, though not always mandatory, depending on client specifications or material thickness.
DHT vs. Other Processes
DHT vs. PWHT: DHT is specifically for hydrogen removal and occurs at lower temperatures (300–350°C) than PWHT (620–760°C), which aims to relieve residual stresses and modify microstructure. DHT does not restore toughness in advanced materials.
DHT vs. ISR: Intermediate stress relieving (ISR) is performed at higher temperatures (e.g., 620–660°C) for thicker or highly restrained welds to reduce stresses and hydrogen, often replacing DHT in critical applications.
DHT vs. Hydrogen Bake-Out: Hydrogen bake-out (outgassing) is similar but may refer to different temperature ranges or processes, depending on the context. DHT is a specific post-weld treatment, while bake-out can apply more broadly.
Requirements and Considerations
When Required: DHT is typically required if preheat is not maintained until PWHT, or if welding is paused during fabrication. It’s critical for thick sections (>2 inches) or hydrogen-related services.
Code Compliance: ASME Section IX considers DHT (post-heating) a non-essential variable, meaning it may not require requalification of welding procedures if omitted, unless specified by standards like API 934C.
Challenges: Maintaining consistent temperatures (e.g., 350°C for 4 hours) can be difficult in low ambient conditions, but studies show lower temperatures (e.g., 280°C) over longer durations can be effective.
Limitations: While DHT removes hydrogen, it does not address other issues like toughness restoration in advanced materials, which may require ISR or PWHT.
Practical Notes
Temperature Control: Exact temperature and duration depend on the material, thickness, and code. For example, ASME B31.3 for process piping may specify requirements based on control thickness.
Client Specifications: Some clients or licensors may impose stricter DHT requirements than standard codes, especially for critical applications.
Post-Arc Gouging: DHT may be required after arc-air gouging (e.g., for weld repairs) if preheat is interrupted, as gouging can introduce additional hydrogen or stresses.