Post Forming Heat Treatment(PFHT) of Austenitic Stainless Steel: To be or Not to be

Let's discuss the Post Forming Heat Treatment (PFHT) of the U-Bend section of Stainless Steel Tubes. As outlined in paragraph 9.6.3 of API 660, Heat Treatment after tube forming for the following is required :

1. Carbon steel, Carbon-molybdenum, and low chromium steel U-bends (C-0.5% Mo through 5% Cr steels) in sour or wet hydrogen sulfide service
2. U-bends in brass alloys (e.g. per ASME SB-395/395M)
3. Unstabilized austenitic Stainless Steel with a mean radius smaller than 5 times the nominal outside diameter of the tube.

As evident from this paragraph, Austenitic Stainless Steel tubes require heat treatment. However, there is a slight variation in UHA-44 of ASME Sec.VIII div.1, where the Post Forming Heat Treatment (PFHT) requirement depends on both the design temperature and Forming Strain.

For example, if the Design Temperature falls between 580°C and 675°C, and the Forming Strain (Elongation) exceeds 20%, PFHT is necessary for SS304, as indicated in Table UHA-44.

Furthermore, when the Design Temperature exceeds 675°C, U-bends with Forming Strain greater than 10% must undergo Post Forming Heat Treatment.

For details regarding the Design Temperature and Forming Strain Limitations for other Austenitic Stainless steels, refer to Table UHA-44 in ASME Sec.VIII div.1.

Up to this point, we have explored the requirements of Codes and Standards concerning Post Weld Heat Treatment (PFHT). Now, let's delve into the subject from the perspective of material science. In heat treatment processes, the cooling rate plays a crucial role. If the Austenite material is cooled slowly, it undergoes a phase transformation into Pearlite. Pearlite is a structure composed of alternating layers of Ferrite (87.5 wt%) and Cementite (12.5 wt%), which is not ideal due to its high hardness (approximately 300-500 HV).

This high hardness can be problematic, especially when Stainless Steel tubes are used in services with low MDMT. Tubes with such high hardness may lead to potential disasters or failures in critical applications.

Considering the significance of materials in low MDMT and the implications of high hardness, it becomes imperative to carefully control the cooling rate during the heat treatment process to avoid undesirable phase transformations and ensure the desired material properties for S.S. tubes.

In the context of preventing phase transformation, increasing the cooling rate stands out as the primary solution. For example, cooling in oil has the potential to hinder unwanted phase changes. However, a significant concern arises: How many manufacturers have the capability to effectively control the cooling rate? Can we trust them not to damage our relatively expensive stainless steel tube bundles in the process? This raises the question of whether it's wiser to disregard PFHT considerations for S.S. tubes altogether.

Share your thoughts and decisions in the comment section below; I'd love to hear your insights on this matter.