In the world of pressure vessels (like those governed by ASME Section VIII), the terms “stress relieved during fabrication”, Cold-Formed Heads, and Hot-formed Heads relate to heat, but they are very different techniques used in PV construction. PWHT is about treating the material after it’s been worked, while “hot formed heads” is about shaping the material in the first place.
First, let’s see what CGA 2.1 says about NH3 storage tanks:
Any pressure vessel used as a storage tank shall have:
1) been stress relieved during fabrication in accordance with the code in effect at the time of fabrication;
2) cold-formed heads that have been stress relieved; or
3) been fabricated with hot-formed heads.
These requirements are a critical safety measure designed to prevent a phenomenon known as Stress Corrosion Cracking (SCC). In the world of pressure vessels, particularly those that hold hazardous materials such as anhydrous ammonia, “residual stress” is the enemy.
The code essentially mandates that the vessel’s metal be in a “relaxed” state to prevent unexpected cracking under pressure or in corrosive conditions.
- Stress Relieved During Fabrication
When a vessel is welded together, the intense heat causes the metal to expand and contract unevenly. This creates residual tensile stresses—locked-in energy that is trying to pull the metal apart even before you add any internal pressure.
To “stress relieve” (technically called Post-Weld Heat Treatment or PWHT), the entire vessel — or at least the welded seams — is heated to a high temperature (typically around 1100∘F to 1200∘F for carbon steel) and held there for a specific duration before cooling slowly.
This allows the atoms in the metal to rearrange, “relaxing” those internal tensions.
- Cold-Formed Heads (Must be Stress Relieved)
“Heads” are the end caps of the vessel (the curved parts). Cold-forming means the metal was bent or pressed into shape at room temperature. Bending thick steel plates at room temperature causes work hardening. This significantly increases the internal stress and makes the metal more brittle at the curve of the head.
Because cold-forming introduces significant mechanical stress, the code requires these heads to undergo a separate stress-relief heat treatment to restore ductility and relieve internal stresses.
- Hot-Formed Heads
This is an alternative to cold-forming. Hot-forming involves heating the steel plate to a glowing red temperature (well above its transformation range) before pressing it into the shape of a head.
Since the metal is shaped while hot, internal stresses don’t build up as they do during cold forming. The cooling process after hot-forming acts as a natural stress relief, so these heads generally do not require an additional heat treatment cycle after they are shaped.
Why does this matter?
If a vessel has high residual stress and is exposed to certain chemicals (like ammonia with low water content), it can develop Stress Corrosion Cracking (SCC). These cracks can be microscopic at first but can lead to a catastrophic, “unzipping” failure of the vessel. By following these three rules, you ensure the vessel is chemically and mechanically stable for long-term storage.