Alloy Segregation - PH Steels 1

[ndtkansas]

I have a company that performs magnetic particle inspection of finished machined parts that are in the hardened condition. Periodically, we identify a condition known as "banding". The principal materials are 17-4 and 17-7. We have conducted metallography and identified after etching, a banded structure that appears to be retained austenite or un-tempered martensite. Most of these conditions are in the center 1/3 of the part. In parallel with the metallography, we conducted hardness tests in both the banded and nonbanded areas of the material. The result was a 50% lower hardness value in the banded area. In a few of the banded structures, we also identified nonmetallic inclusions. All of this was based on etch results.
We have also identified conditions that appear to be ferrite stringers. They are considerably wider in area and sometimes run the entire width of the part. They are typically .001” thick or smaller. From this I have the following questions:
1. What is the band? Is it indeed retained austenite and what would be the chemical composition?
2. Based on review of the material test reports, all materials comply with the applicable AMS material spec. Do any of these conditions have an impact on fatigue strength or fracture toughness?
3. Based on the fact that I am inspecting 17-4 and 17-7, are the metallic inclusions the austenite stabilizers (copper for 17-4 or aluminum for 17-7)?
4. What causes the ferrite stringers in these materials?

 

[JayCarp]

In the case of 17 - 4 PH there is an "adjusted "
chemistry material called 15 - 5 PH which is a lot more likely to be martensitic. 15 - 5 PH has a lot less banding.


Thi primary cause of this is the chemistry ( Cr - Ni )
ratios and a whole bunch of this is due to the fact
that know how and melting practice are a lost art.

 

[EdStainless]

Are you using standard quality material or has it been ESR remelted? We only use remelted material.
Some of the banding is from inclusions, some is variation in microstructure. Yes, during processing this material does not always get the best heat treatment.

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