Martensite in 440c stainless Question
Martensite in 440c stainless Question
(OP)
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
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





RE: Martensite in 440c stainless Question
http://www.kaker.com/std/html/order.ht
RE: Martensite in 440c stainless Question
Answer to Q2: None that I am aware of, besides Kaker, which is not free.
Answer to Q3: Yes.
RE: Martensite in 440c stainless Question
USSteel used to have a cheap/free paperback of TTT curves (can't find mine).
RE: Martensite in 440c stainless Question
Regarding your out of roundness, maybe vertical heat treatment can help you.
RE: Martensite in 440c stainless Question
http://www.cartech.com/techarticles.aspx?id=1450
Try swabbing with Viella's reagent. I have done this and it works well.
RE: Martensite in 440c stainless Question
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!
RE: Martensite in 440c stainless Question
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?