What makes Custom 465 martensitic? 3

[btrueblood]

Recently received an email from Carpenter alloys regarding "Custom 465" alloy, described as a martensitic stainless alloy. Looking at Cr/Ni ratio, though, it puzzles me why this alloy is martensitic? I thought 17/7 15/5 ratios of Cr/Ni were required to keep the alloy from becoming austenitic? The 465 alloy has a roughly 11/10 Cr/Ni ratio, which seems like too much nickel. Wikipedia was of no help, it says that martensitic alloys have to have no nickel at all...

 

[EdStainless]

In PH grades the name 'martensitic' refers to the precipitates that form. In some cases these are not really martensite, but share the same structure.
You have to look at how the material ages and how the element partition in aging. When they are single phase most of these alloys are mostly austenitic. When they age it all changes.

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[mcguire]

465 seems to be an austenitic alloy, but it actually is designed to form all martensite with sub-zero treatment after a normal quench from austenitizing temperatures.

It has no free carbon or nitrogen. These are tied up as carbides and nitrides. It does seem to defy the Delong diagram, but it is really is martensitic. It just is only partially so unless you treat it well below zero. The precipitate is Ni3Ti which precipitates from the martensite upon aging. It is similar to 13-8 PH but the carbon is removed from solution by Nb. Even so 465 has a lower Ms temperature.

Cartech added the extra nickel to allow more Ni3Ti to form which gives it a higher strength potential.

Custom 475 is even stranger. It has to have cobalt to raise its Ms to a level which allows it to become martensitic from quenching.

Wikipedia is wrong about the nickel being absent from martensitics. It is normally minimized because it causes reversion to austenite at the temperatures where you would want to temper the martensite. 431 has a little nickel, but it has to be used with a low temperature tempering which prevents it from gaining useful toughness. All PH grades have nickel.

PH alloys are the strangest and most complicated stainless alloys because of the various manipulations used to maximize alloy content while keeping a reachable martensitic transformation temperature. (Can you tell I just finished researching these alloys for the chapter on PH grades?)

Michael McGuire

http://stainlesssteelforengineers.blogspot.com/

 

[EdStainless]

Mc, I'll bet that you had a tough time editing the chapter down to an acceptable length.
One of things that I find interesting is which condition is best for cold working each grade. Alloys like 17-4 and C455 cold work easiest in the overaged condition. They have good elongations and low stran hardening. The annealed condition works best with 17-7 and C465.
In some grades the strain hardening is so low that we can do two draw passes without an intermediate anneal.

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[metengr]

Nice write-up, McGuire. One other note, if you have access to a Schaeffler diagram (source; Schneider and Climax Molybdenum Co, Foundry Trade Journal, 108,562, 1960), the Cr and Ni equivalent equations for this alloy puts you right in a two phase field of martensite and austenite.

Here are the equations;
Creq = (Cr)+ 2(Si)+ 1.5(Mo) + 5(V) + 5.5 (Al) + 1.75(Nb) +
1.5(Ti) + 0.75(W)

Nieq = (Ni) + (Co) + .5 (Mn) + .3(Cu) + 25(N) + 30(C)

 

[btrueblood]

Michael, thanks, excellent synopsis. Yes, I knew the wikipedia article was junk when it didn't mention the PH series; looking at the PH and similar grades, I couldn't see what the alloy was that made 465 hardenable (no copper, no Nb...). Interesting that Ti can act to create the precipitates required - so the reason that 321 stainless stays martensitic is that is has less Ti? Or could it too form some martensite if rapidly quenched/ sub-zero cooled? Like Ed and you say, the tapdancing that must occur at foundries to keep the PH grades working must rival that riverdancing guy.

Again, thanks.

 

[mcguire]

No, Ti would rather combine with N or C ( or O or S) rather than Ni, so 321 doesn't form any Ni3Ti. 321 has too much Cr+Ni to form martensite. The Ms temperature is very low. You have to deform it to cause martensite to form.

Michael McGuire

http://stainlesssteelforengineers.blogspot.com/

 

[arunmrao]

mcguire
I hope your book comes out soon enough for us novices to know some interesting facts.

 

[mcguire]

You're writing the foundry chapter, aren't you?

By the way, with reference to the PH grades. Plain martensitic grades that have moly udergo precipitation hardening during tempering, a carbide of molybdenum. It's called secondary hardening,but it is PH. Same with any grade that has copper, which some austenitics do for corrosion resistance. They are just seldom heated to the range where the PH occurs.

Michael McGuire

http://stainlesssteelforengineers.blogspot.com/

 

[EdStainless]

...becuse it tends to damage other properties (corrosion resistance and toughness usually).
Yes, I have done it to alloy 20 just to see.

I made a hardenable version of Ni-Resist type 5 a number years back. Some Mo and V and bingo. Ductile austenitic cast iron that will age harden.

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