Ferrite requirements

[Venkadesh Lakshmanan]

Dear Experts,

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,

[suraj chavhan]

Dear Sir, 

You may refer  BS EN ISO8249 Standard for Ferrite number testing. refer attachment FYI.

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[Vijay Makwana]

Dear,

Duplex stainless steels are a family of corrosion-resistant alloys, which typically contain 50 % austenite and 50 % ferrite. Phase balance is important in duplex stainless steels as it helps to impart optimum mechanical properties and corrosion resistance. In many applications it is necessary to weld duplex stainless steels. Welding involves a thermal cycle that may alter the phase balance, and so it is necessary to assure that phase balance is still within acceptable ranges in the heat-affected zone and the weld metal. For example, high ferrite content may lead to lower toughness and high austenite content may lead to increased susceptibility to chloride stress corrosion cracking. Quality control of welded duplex stainless steels usually requires confirmation of an acceptable phase balance, by ferrite measurement  

Thanking you,

MTech (Metallurgical & Materials Engineering)

Indian Institute of Technology Kharagpur (IIT)

On Thu, Dec 10, 2020 at 6:34 AM Venkadesh Lakshmanan <venkad...@gmail.com> wrote:

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[james gerald]

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

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[Vijay Makwana]

Dear Vengatesh,

Further to the discussion, the importance of ferrite is explained here in depth:

Duplex austenite-ferrite stainless steels should typically contain ferrite in the range 30-35 % to 60-65 % ferrite but not less than 20 or more than 70 % (approximately 30-110 FN). The phase balance is important because it determines the overall properties of the alloy: the ferrite phase imparts strength, chloride stress corrosion cracking resistance, ferromagnetic properties and solidification cracking resistance and the austenite phase imparts toughness, corrosion resistance and hydrogen cracking resistance and is non-ferromagnetic. The optimum alloy properties are achieved when the ferrite content is around 50 %. This can be achieved routinely during parent material manufacture. However, welding can change the phase balance by differing amounts, depending on the welding parameters, and so it is also important to measure the ferrite content of weld zones.

Generally, ferrite content may be measured in two basic ways: by metallographic or magnetic means. The former is usually destructive and gives results in terms of point fraction, area fraction or volume fraction (% ferrite), whereas, the latter is non-destructive and gives results in terms of a ‘Ferrite Number’ (FN). Unfortunately, it is not necessarily straightforward to perform, interpret and compare ferrite content measurement results, as there are several important factors which affect the results and their accuracy, for example microstructure (very important for welds), composition, size of the region examined, location of measurement, access to the region of interest and measurement method. Moreover, microstructure and composition, in particular, affect the conversion of % ferrite to FN and vice versa, which introduces uncertainties when addressing the common issue of meeting specification requirements in % ferrite when measuring a weld non-destructively using an instrument that measures FN.

MTech (Metallurgical & Materials Engineering)

Indian Institute of Technology Kharagpur (IIT)

[Kannayeram Gnanapandithan]

Is it not 60Austenite & 40ferrite

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[pgos...@rogers.com]

Hello Venkadesh,

Read this publication. Most of your queries are answered here. B 31.3 does not provide any detailed requirements about duplex stainless steels. The other  resource to try would be API-RP-938C. However the basics of Duplex Stainless steels are well explained in this publication.

 

Thanks.

 

 

P.Goswami.P.Eng, IWE.

Independent  & Consulting Welding & Metallurgical Specialist

Linkedin: https://www.linkedin.com/in/pradip-goswami-2999855/

Email:pgos...@rogers.com,pradip....@gmail.com

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[Jignesh Desai]

You may refer to API RP 582 and API TR 938 C for duplex SS ferrite requirement.

This will surely help you to resolve the your query.

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