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Magnaflux Testing and Ferrite Stringers in 410 Stainless

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Maui

Materials
Mar 5, 2003
1,937
I have a customer who is manufacturing a turbine blade from 410 stainless steel. They are inspecting these parts using magnetic particle inspection (magnaflux testing). They have been complaining that they are picking up indications on approximately 30% of the parts, and are rejecting them as a result. They claim that the rejections are being caused by delta ferrite stringers. I obtained several of the rejected parts, and sent them to an outside lab for metallurgical analysis. The parts are in the process of being analyzed, but the testing so far has identified what appears to be very light chatter marks on the surface of one blade. I suspect that this is what has been causing the indications during magnaflux testing. After taking a photograph of the chatter mark, the lab polished the surface very lightly in stages to find out if there was any underlying metallurgical anomaly in the same area such as delta ferrite. None has been found. Other areas on the blades that were marked by the magnaflux inspector did not even reveal evidence of scratching. And polishing these areas did not reveal any delta ferrite or other identifiable cause for the testing indications. I suspect that the inspectors may have the equipment set up to be too sensitive, and are over inspecting these parts. By the way, the delta ferrite content of this steel is evaluated prior to shipment, and is well below the maximum 1% content that is allowed. My question is this: can anyone provide any evidence that delta ferrite stringers produce visible indications on 410 stainless steel during magnetic particle inspection? My suspicion is that this is extremely unlikely, but I would like to hear from those who are well versed in this testing procedure (since I am not). Thank you.

Maui

 
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Check the amount of S and ask why they aren't using 403.

"You see, wire telegraph is like a very long cat. You pull his tail in New York and his head is meowing in Los Angeles. Do you understand this? Radio operates the same way: You send signals here, they receive them there. The only difference is there is no cat." A. Einstein
 
The 410 stainless can be used for stationary vanes of diaphragms or blade rings (depending on the turbine OEM) in large steam turbines or in the rotating blades of the compressor section of gas turbines. Normally, I have not seen it used on rotating buckets or blades. My guess is that the material is probably 403 stainless, and someone may not have typed it correctly.

With that said, I have been involved with many large steam turbine overhauls and gas turbines and have not seen delta ferrite stringers cause problems with wet fluorescent MT. I have seen faint indications on the surface that were removed by surface polishing and did not result from microstructural anomalies.
 
Agree with metengr...shouldn't be an anomalous issue with MT.

The chatter marks will definitely give an irrelevant indication.
 
MAUI,
Delta ferrite stringers on the matrix of tempered martensite give a MT indications and are mainly placed in the centre of blade corresponding to the center of hot rolled H+T flat bar if the blade was obtained by machining / milling.
Same situation could appear if turbine blade is obtained by drop forging of round bars or billets In this last case, the probability of delta ferrite stringers is poor because the centre of blade is not the centre of bars or billets.
This kind of problem is well known by steelmakers and turbine /powerplant company.
The reason why of delta Ferrite in martensitic grades depends on (1) chemical composition and (2) temperature of rolling or forging even if its distribution (3) depends on steel making process and kind of ingot ( EAF+AOD or ESR or VAR).
Te first one (1) depends on chemical balance of Cr/Ni equivalent ( there are some formula applied to obtaine a structure with no or poor delta ferrite suggested by experience of steelmakers or imposed by metallurgits of power plant makers), the secon one (2) dependes on temperature. The higher temperature of hot working, the higher amount of delta ferrite. (Just a look a diagram of Cr-Fe-C to verify what could happen). About (3) , it's well know that ESR and VAR ingots have less segregations than standard one. However, this doesn't mean that a ESR ingot could supply a final product free of delta ferrite if the point (1)and (2) are not well evaluated.
About MAUI's investigations in order to understand the reason why of MT indications, I think that the a Laboratory is able to identify if the indication is caused by Ferrite,non metallic inclusion or scratches.
Assuming is right what the Laboratory said ( no ferrite, no scratches , no not metallic inclusions as Si-Al,Si, oxide ans so on), I suggests to verify if the indications coming from carbide stringers or large segregations ( by a right etching).Metalguy suggested to evaluate Sulphur content because large MnS could cause indications. This could be right but I never see a steelmaker who process his martensitic SS steel for turbine blade with S% close to maximun(0,03%) Usually S% is lower than 0,005%. Therefore, it it wll be very difficult to generate large MnS inclusion.


 
Old guy footnote : There are several modified 13 Cr alloys used for steam turbine components with various mfgr claims for advantages, also hardness makes a great difference. GE had one alloy (417 ?) that they claimed had better internal dampening : It was supposed to resist corrosion fatigue-but we still had failures.
As mentioned , polish and repeat the MT should resolve the ferrite issue.
 
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