Dear,
ASME B31.3 do not specify about the Delta ferrite and Austenite percentage but will be covered in the relevant client specification.
Duplex stainless steel consists of 50% Austenite & 50 % Delta ferrite specifically designed to provide below optimum properties.
1. Increased Tensile strength.
2. Good Pitting and crevice corrosion resistance
3. Good resistance to Chloride stress corrosion cracking.
4. Good ductility
The reason for having dual phase as below.
Advantages of having Delta ferrite.
1. It provides good Pitting and crevice corrosion resistance as achieved by having higher PREN.
2. Good reistance to Chloride stress corrosion cracking.
3. Good corrosion resistance to certain specific acids and environments.
Advantages of having Austenite phase.
1. Good Atmospheric corrosion resistance.
2. Good Ductility and Impact properties (upto -49degC).
The percentage of delta ferrite can be determined by using Ferritoscope (insitu measurements) based on its magnetic properties. Also can be determined by microscopic point counting method (ASTM E562) and also can be determined using Schaffeler Diagram and WRC diagrams from chromium equivalents and Nickel equivalents based on chemical compositions.
Normally ferrite percentage determined after welding and the allowed tolerance is +/- 15% from 35% to 65%.
Below some snap shots from specifications.
Ferrite is BCC phase and Austente is FCC phase.
Thanks & Regards
J.Gerald Jayakumar
On Thursday, 10 December, 2020, 09:04:04 am IST, Venkadesh Lakshmanan <venkadeshmech@gmail.com> wrote:
Dear Experts,
https://materials-welding.blogspot.com/
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Can you please give some guidance regarding ferrite requirements for duplex stainless steel piping as per ASME B31.3. I mean what sub standards to refer, why ferrite test to be performed, either ferrite number or percentage results referring really what is in it. I understood that it is a face (bcc or fcc) , but how to describe it. Can you please give some details to know about this.
Regards,
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