Ferrite in 17-7PH

[DocCB]

It is not clear to me how ferrite content of 17-7PH is controlled. In the mill anneal condition, there is apparently no ferrite. As heat treatments progress to produce martensite and carbides, ferromagnetism increases from RH to TH condition. Is ferrite the residual phase after all this transformation and precipitation?

What are the stringers and globules in the microstructure after the 1950F annealing treatment?
Can sigma form in this material?

 

[EdStainless]

I'll elaborate on magnetics here, I did comment in your thread Mig.
In alloys with clearly defined microstructures you can use magnetics as a good indicator of aging response. You have to play with frequency and field strength, but it can be done.
In 17-7 it may be more difficult. The method might work for individual lots, but it will take some deicate calibration.

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[DocCB]

mighoser:
Yes, I believe you can do this. But you wil have to work up your own standard method, depending on your part geometry. We are learning that Biddle measurements on tube surfaces are dependent on tube diameter(!), and so we are taking care to measure on flat surfaces (tube cross-sections)despite the fact that the Biddle meter has a fairly sharp point. From our measurements, it looks as though a Biddle meter reading of about 1 1/2 to 2 1/2 indicates an HRC value in the low 40's, (17-7) but this needs confirmation. If validated, any reading below 1 would probably mean you are at or above HRC 45.
The 60F step, (or the -50 to -100F step) after 1450 (or 1750F) heating has a major effect on magnetic response, and shows me that on initial cool down (whether AC or Q), there is significant retained austenite in the structure. This period of cooling really has an effect on results.

Just to add to the mysteries of this alloy, I now suspect that there is an aging phenomenon at RT, after HT is completed. But careful experiments are needed to investigate this further. For metallography, care is also needed if you are mounting sample in a press: the thermal cycle is likely to upset the condition of your sample!

 

[unclesyd]

We tried to use magnetic separation to resolve a problem where 17/7 pins with 3 different heat treatments were used in their manufacturing process. These were pointed pins with 1mm dia and 60mm long. The 3 lots of 150,000 pins were commingled after pointing prior to our post processing.
The development group spent considerable time and effort trying to separate the 3 lots to no avail.
They tried our design separator as well as sevral leased varieties. The end result was that these pins were all given the same heat treatment after consulting with ARMCO.

Anecdotal:
Here was 20th century materials formed into wire on modern machinery; heat treating in modern furnaces and ovens. Then were sent to be cut, pointed, and stacked point up in handmade boxes on machines made in the 1870's.

Then shipped by air to us where a stockroom clerk didn't like the colored boxes so he got some modern ones and proceeded to mix and match. All the kings men couldn't figure out how to put the pins back in the proper boxes.