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FI Test- und Messtechnik GmbH

6734 views
09:56 Mar-05-2010

anjafo79

NDT Inspector
Norway,
Joined Aug 2009
204
super 13% chrome MPI or DPI

is it possible to do MPI on super 13% chrome?

 
13:10 Mar-05-2010

Michel Couture

NDT Inspector,
consultant,
Canada,
Joined Sep 2006
806
Re: super 13% chrome MPI or DPI In Reply to anjafo79 at 09:56 Mar-05-2010 (Opening).

A qwick way to find out would be to take a small magnet and see if it is attracted to your part. If the magnet is attracted to the part it can be inspected by MT.

Another way is to measure the tangential-field strength using a Hall effect probe gaussmeter. The field strength on the part surface should be between 30 - 60 Gauss.

A third option is to use a Magnetic Field Indicator or Shims with artificial defects in them (QQI) while magnetising your part. For more info your can refer to ASTM E709 and E1444.

 
13:57 Mar-05-2010

P V SASTRY

R & D, NDT tecniques metallurgy
TAKEN VRS FROM THE POSITION OF SR. DEPUTY GENERAL MANAGER BHEL CORPORATE R&D,
India,
Joined Jan 2003
195
Re: super 13% chrome MPI or DPI In Reply to anjafo79 at 09:56 Mar-05-2010 (Opening).

Dear Sir,

It is very much possible to do MPI on this steel.This is a Martensitic stainless steel. In the annealed condition this is highly ferromagnetic and you will not notice any difference between this and another carbon steel and this.

In the heat treated condition this is a shade less ferromagnetic than the carbon steel but as far as MPI is concerned not a problem at all and the field achieved is of a few thousands of Gauss.

We regularly test a similar type of steel which is used for steam turbine blades and hydroturbine runners etc. In fact I am posting this reply from the site where we are in the middle oftesting turbine blades by Wet Flouresscent Magnetic Particle Testing(WFMPT). (These blades have a Chromium content of about 12 to 13 %).

We magnetize by coiling and give about 800 to 900 Ampere turns(HWDC) for a pack of about 10 to 15 blades depending on their size. We use octagonal field indicator (old ASTM A-275) for verifying the adequacy of the field.

Hope this information helps.

With Best wishes

P V SASTRY

 
15:16 Mar-05-2010

anjafo79

NDT Inspector
Norway,
Joined Aug 2009
204
Re: super 13% chrome MPI or DPI In Reply to P V SASTRY at 13:57 Mar-05-2010 .

it was mentioned to me that the material might give false indications due to chemical make up almost like scaling?

 
16:37 Mar-05-2010

Michel Couture

NDT Inspector,
consultant,
Canada,
Joined Sep 2006
806
Re: super 13% chrome MPI or DPI In Reply to anjafo79 at 15:16 Mar-05-2010 .

Anjafo,

I'm not sure if it would be the result of the chemical composition. If you obtain scale like indication on such metal, it could be attributed to segregation. One would have to do chemical and grain structure analysis to ensure the material qualities were met.

 
17:21 Mar-05-2010
Derek
Re: super 13% chrome MPI or DPI In Reply to Michel Couture at 16:37 Mar-05-2010 .

I have performed MPI on several materials with similar chrome content and have had very good success but keep in mind that not only chrome will effect the magnetic properties of the material. It may be a good idea to find the complete chemistry of the material or post the grade on here and one of the experts will definately have an answer for it.

 
23:04 Mar-05-2010

P V SASTRY

R & D, NDT tecniques metallurgy
TAKEN VRS FROM THE POSITION OF SR. DEPUTY GENERAL MANAGER BHEL CORPORATE R&D,
India,
Joined Jan 2003
195
Re: super 13% chrome MPI or DPI In Reply to anjafo79 at 15:16 Mar-05-2010 .

My Dear Mr. Anjfo,

It is not true that this steel gives any more false indications than other carbon or low alloy steels.

But any welding on this steel gives a distinct indication at weld junction (HAZ) due to permeability changes caused by alloy segregation and not attributed to any defect. This effect is a bit more accentuated than in carbon and low alloy steels.

But if the part is subjected to a full quality heat treatment after welding (involving solutionizing at 1050 degrees centigrade and subsequent tempering and stress relieving) this permeability difference almost vanishes and the weld junction is not shown distinctly in subsequent MPI. If the part is only tempered or stress relived after welding this effect is only partially alleviated.

A fully heat treated steel of this composition consists of predominantly tempered martensitic phase with scanty amounts of ferrite and retained austenite. In annealed state this steel shows predominantly ferritic phase with some fine to coarse carbide. This steel has relatively higher Permeability and lower Retentivity in annealed condition. As I mentioned earlier, even in the fully heat treated condition ( consisting of predominantly Martensitic phase) this steel is highly ferromagnetic and no practical difference is noticed while conducting MPI.

However there is another indication shown by these steels particularly in the bar stock. During the original solidification process a small part of the liquid phase solidifies into delta ferrite while the bulk of the liquid phase is converted to austenitic phase which subsequently undergoes transformation to other phases like martensite or ferrite etc depending on the heat treatment.

The small amounts of delta ferrite formed at the high temperatures is not easily transformed in any of the subsequent treatments and stays there. It will undergo physical transformation in any of the mechanical treatments like rolling etc. For example if the steel is subsequently drawn into bar stock of either square or round sections, this delta ferrite is also drawn into streaks. These streaks stay mostly towards the centre of the section but may stray towards the outer side.

Since these delta ferrite streaks have a distinctly different permeability compared to the bulk of the section they show up in the MPI and are often mistaken as inclusions. Many battles were fought on this count.

It is really difficult to make out what they actually are and components were rejected after costly processing. It took lot of persuasion and technical convincing (by showing the microstructures etc.) to put the wheels back on track. A very minor presence of delta ferrite streaks is not harmful for most applications while inclusions (of certain size and composition) are not acceptable.

You can safely go ahead with MPI of this steel. Our philosophy is not to do PT if we can do satisfactory MT. PT requires more surface preparation/cleaning and testing time.But we often verify MT indications by PT for not only seggregating the false indications but mainly for assessing the depth of the sure cracks. (true only for shallow cracks)

With Best wishes

PV SASTRY

 
15:43 Mar-06-2010

Carlos Correia

R & D, - -
UCV & EGROUP,
Venezuela,
Joined Oct 2008
120
Re: super 13% chrome MPI or DPI In Reply to P V SASTRY at 23:04 Mar-05-2010 .

Mr. Sastry has posted one of the most clear and elegant answers that I already seen here. Congratulations Mr. Sastry !!!!

 
12:02 Jul-30-2015
Rakesh Bekkam
Re: super 13% chrome MPI or DPI In Reply to Carlos Correia at 15:43 Mar-06-2010 .

Dear Satry sir,

Nice to meet you in this blog.

I have a question about same thing that we have done FMPI on Chrome SS Steel bars. So we have observed linear indications, then we have verified in Macro in metallography , Bu there is no defects found in metallography. My Question is in this ind of condition i have got an a statement from one our customer that on ''martensite steels we will get indications but then we need to conform by usinf FPI method. If No indications oberved after FPI , Clearly accept it''.

Please clarify me why it will be happen on martensite..

Rakesh Bkkeam

 
13:18 Jul-30-2015

P V SASTRY

R & D, NDT tecniques metallurgy
TAKEN VRS FROM THE POSITION OF SR. DEPUTY GENERAL MANAGER BHEL CORPORATE R&D,
India,
Joined Jan 2003
195
Re: super 13% chrome MPI or DPI In Reply to Rakesh Bekkam at 12:02 Jul-30-2015 .


Dear Rakheshji,

I have given a detailed explanation for this in one of my previous postings in March 2010. You may go through the link.

However I am trying to give a short answer to your question here.

Some times you may get some linear indications in bar stock of 13% Chrome steels ( martensitic steels). These are not necessarily due to physical defects like inclusions or cracks but due to metallurgical reasons.

These indications may be due to retained austenitic or delta ferrite stringers. Most probably due to retained austenite stringers as the delta ferrite is usually confined towards the center of the bar.

You may not see them in Macro but certainly see them in Micro.

These stringers exhibit different magnetic permeability compared to the martensitic matrix, giving rise to the indications during MPI.

Since these are due to metallurgical reasons they do not give indications in Penetrant testing. Your Customer is right about this.

Best wishes

P V SASTRY

 


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