Martensite in 440c stainless Question 5

[sodeen]

While I have plenty of experience with low-carbon steel microstructures, I have only recently begun working with stainless steel (specifically, 440c.)

I have been trying to polish and etch a few samples that based on the heat treatment of the cycle should be mostly martensitic in nature. (Thin Cross-section Induction Heated to 1950-2000 degrees F, then quenched in a water-based quenchant.) However, using Vilella's etch (immersion, 10-15 seconds), I have yet to see anything resembling the light/dark needles usually seen in normal martensite. Instead, depending on how long I leave the etchant on, I am seeing large, light-colored grains (Retained Austenite?) and some darker, ill-defined areas.

I have included a picture of the microstructure (100x). Rockwell Hardness (c) has been consistently between 57 and 60 for this part.

My questions are threefold:

-Is Stainless Steel Martensite different under visual examintaion than regular 1040? Do I need to do something different in order to see the martensitic structure?

-Is there a good web repository that might have a 440c TTT or CCT diagram? Google turns up nothing, and this company has no on-site resources to consult.

- One of the main concerns for these parts is the heat treatment causing the parts to increase their "out-of-roundness." Would the volume change from Austenite to Martensite have an effect on this?

Thank you for your time, if there is anything I can do to clarify please feel free to respond.

-sodeen

 

[unclesyd]

About the quickest and cheapest way is to order diagrams for a single alloy form Kaker.

http://www.kaker.com/std/html/order.ht

 

[TVP]

Answer to Q1: The C content of 440C is ~ 1%, so the as-quenched and tempered microstructure will consist of martensite + retained austenite + undissolved carbides (depending on the austenitizing temperature). The martensite will be in plate form, rather than the lath martensite that occurs when C content is in the range of 0.2-0.5%.

Answer to Q2: None that I am aware of, besides Kaker, which is not free.

Answer to Q3: Yes.

 

[blacksmith37]

The 440 should have a much smaller grain size than a typical 1040. It has very good hardenability, air cooling should harden over one inch thickness. I am looking at a "ferrite start" nose at 5 minutes /1350F (so if the metal gets below 1350 before 5 minutes you will have all martensite), (and carbides and ret aust.).
USSteel used to have a cheap/free paperback of TTT curves (can't find mine).

 

[salmon2]

Interesting question. I got the same background as sodeen. I really hope somebody can comment on the micro a bit. It seems not heat treated properly to me. But I am nothing on stainless steel. Maybe a higher magnification is better for other experts to comment.

Regarding your out of roundness, maybe vertical heat treatment can help you.

 

[metengr]

Information for metallographic examination of various materials;

http://www.cartech.com/techarticles.aspx?id=1450

Try swabbing with Viella's reagent. I have done this and it works well.

 

[lgearhart]

We process a good deal of 440C, and I find Marble's etch to work well. At 57-60, you have a primarily martensitic structure, and I'm certain you also have some retained austenite unless you're deep freezing before any tempering step. Yet if your problem is distortion, examining the microstructure is interesting but doesn't address that issue.

Yes, the size change (generally growth, in my experience) can lead to distortion, but only when you have a non-symmetrical piece: say, a keyway that you have to keep from opening or closing. IF not, and it's just out of round that is bothering you, then look to see where you are generating non-symmetrical stresses.

You say you're water quenching- why? Self quenching should work fine for 440C, and a liquid medium might be applied faster to one side than another. Something else to keep in mind is the possibility of residual stresses from machining. Finally, another source of residual stress (and thereby distortion) are stresses from the raw material manufacturing. We've had some lots of 440C that we COULD NOT keep straight, while other lots worked fine. Later we found out the problem lot (1 inch round!) was annealed as a coil then straightened, and the easy lot was annealed as bars.

Good luck!

 

[sodeen]

Wow, thanks for all the great information!

We are in fact deep freezing before any tempering step. The parts in question are in fact round, and have a maximum cross-section of about 1/2." So I would expect to see mostly martensite.

As I understand it, the water quenching is mostly so the operator can handle the parts for testing immediately after heat treatment. (I'm new to the company and in the process of learning the facilities). The part is currently allowed to air cool to ~1320 F before quench.

One of our outside consultants has also suggested that our stock might be the problem, and that purchasing spherodized 440c (at three times the cost, of course) would solve the out of round issue.

Perhaps a pre-induction hardening anneal (we do have a large furnace on site) would be beneficial in relieving these residual stresses?