[MW:16516] Re: Delta Ferrite Test on SS 316L

Delta Ferrite phase is formed when the liquid metal starts cooling and continues as ferrite without getting transformed into austenite. The prefix delta differentiates this ferrite (its formation stage) from the room temperature stable ferrite (alpha). 

As far as measurement of ferrite is concerned, using the Schaeller-Delong Diagrams is one method and using a ferrite meter is another method. Our experience shows that the values obtained by both the methods are quite different esp. when ferrite values in the lower ranges of 0 to 10% are involved. 

Which method is to be used should be specified as per some industry standard or mutually agreed upon by the vendor and the customer. 

Regards

Swamy

Retd. QA Metallurgist

Nuclear Fuel Complex

Hyderabad, India

On Tuesday, January 15, 2013 10:44:52 AM UTC+5:30, sridhar wrote:

Mr. Rudolf,

Please refer to your mail and shall answer your following queries to the best of my knowledge and in a short form. 

No apologies as we  all help each other in one form or other. If you need more info. please feel free to write  to us.

Query:

One of the clients requirement is that Delta Ferrite has to be checked on the weldment as per Schaeffler method.

1. what is this delta ferrite test & how is it conducted

 

a)To understand it, one has to know reason for ferrite formation. Welding metals like 304L, 316L, retain some

amount of delta ferrite formed at grain boundaries, due to composition of weld metal & welding conditions like

high temperature (550⁰ - 950⁰C). Both factors determine the percentage of delta ferrite as well as its size,

morphology and distribution. Though delta ferrite reduces risk of micro fissuring / cracks, if it exceeds 8-10%, corrosion resistance of SS decreases steeply. Ferrite level beyond 7% is unacceptable for high temperature service. Again, Zero or very low ferrite may also create micro-fissures. Hence it has to be checked  /inspected depending

up on theservice conditions.

Ferrite turns to brittle sigma phase (rich in Cr)  at high temperature and affects ductility, toughness and also cause  inter granular corrosion due to Cr depletion from austenite matrix. Hence it has to be maintained depending up on service requirements.

These details were brought by Schaeffler by metallographic  i.e., Ferrite Percentage (shown in the annexure) 

which helps to determine the structure of weldment, like ferrtic, austenitic , percentage of delta ferrite at normal conditions etc. The limitation is the non-equilibrium cooling rate, selection of welding process, shielding gas,

non inclusion of Nitrogen (a strong austenite former)  which has influence on the ferrite morphology. 

De-long has modified the Schaeffler diagram (claimed to be ±4% accurate) improving the accuracy to ± 2%. 

De-long diagram differs with Schaeffler diagram as the  that the values are based on magnetically determined

ferrite content. i.e., the Ferrite No & Nitrogen is included in the calculation of nickel equivalent with a coefficient equivalent of 30.

 

Hence it generally believed that, De-long diagram gives more accurate values compared to Schaeffler diagram.

AWS has switched over to Ferrite No. (FN) instead of percentage ferrite since, FN is derived by a standard measurement and calibration procedure as approved by Welding  Research Council

 

2. Do we buy the instrument and do it ourselves or hire a ndt service company to do this.

   what does this test mean and what does it say.   Is it a destructive test or NDT.   Can we do a PMI and work out

   the Ni (eq) or Cr (eq) to arrive at the Ferrite No. What is the unit of the Delta Ferrite.

a) In Schaeffler diagram, Nickel and Chromium equivalents are calculated and Delta Ferrite percentage is obtained by plotting the values. (Cr Equiv.: % Cr + % Mo + 1.5 % Si + 0.5%Cb  & Ni Equiv. : % Ni + 30% C + 0.5% Mn)

This method involves actually counting the ferrite phase using point counter method and estimating the percentage)

 

In De-long  diagram the Delta Ferrite No.  i.e., FN  is calculated by adding additional 30% N in Ni equiv.)

 

These method is based upon the theory that, ferrite is a magnetic phase in a non magnetic austenitic matrix.

While Nitrogen level in SMAW is 0.06%,  it is 0.02% through GTAW &  0.04% in GMAW process.

 

Magnetic saturation method is a DT to determine the absolute percentage of delta ferrite in the sample and

independent of size, shape, orientation of  the phase etc. Cannot be used to measure ferrite in an actual equipment.

 

Ferrite scope, Magna gauge are other methods, to measure the delta ferrite and their results are said to be accurate and widely used due to its simplicity. They are NDT methods and could be used any one. Both gauges are available easily in the market. No need to have a service company (can be done by yourself) if the no. of checking are frequent and on a long run. 

 

Other  special methods used in a research institute are X-ray diffraction, Electro chemical  etc.

 

Schaeffler and De-long Diagrams are given in the annexure.

 

Trust above information, gathered from various references shall be of help to you.

 

Sridhar.

                                                      

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From: rudolf saldanha <rudol...@yahoo.com>
To: c sridhar <sridh...@yahoo.com>
Sent: Friday, 11 January 2013 6:48 PM
Subject: Delta Ferrite Test on SS 316L

Hi Shridhar,

Greetings. Permit me to introduce myself, Iam Rudolf and I have been reading your responses to various queries raised on Welding Blog. Reading your responses, I felt of approaching you directly, and my apologies if I have circumvented.

We have a project of Constructing Shop fabricated Storage Tanks made of SS 316L

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