4130 Austemper - 1mm sheet structure

[twistedneck]

Hi this is my first post here!

I'm planning to Austemper a rectangular structure of 4130 sheet steel, 1mm thick, appox 10" long, 1.5" box. I'm creating some small stampings and dimples on this structure as well, so there will be a lot of stresses built up.

Goals are to obtain toughness, hardness and reduce warping. I plant to normalize first, then fix any warping, then Austenetize.

I'm shooting for Austemper in Nitric / Nitrite salt at 720F (MS = 710F). Goal is 45 RC.

Austenetize in a foil envelope at 1600F..

How long do i hold 4130 at 720F? I can't find a good 4130 TTT.

Is there any way i can dip the temp to 710F (MS Temp) and retain the lower bainite toughess?

Please comment isothermal transformations and reduced warping. Thanks.

 

[swall]

This will be a marginal proposition, since you will have only about 1.3 seconds to miss the nose of the curve, according to my ancient isothermal transformation chart for 4130. The bainite finish point on the chart is about 2 minutes, for an article with infinitly small section size. Your 1mm section will require much more time.If distortion is your main concern, you could look at doing a standard quench and temper interrupted with a 1000F quench into salt and hold until the temperature equalizes.

 

[TVP]

twistedneck,

I am probably looking at the same TTT curve to which swall is referring, and my conclusion is the same-- ferrite starts to form almost immediately, so it will be very difficult to austemper 4130 and have a uniform bainite microstructure. Austemper is usually performed on higher carbon grades like 1055. I also agree with swall that martempering (interrupted quench into salt above Ms) is a very good idea for reducing distortion. ASM Handbook Volume 4 or ASM's Heat Treater's Guide are good references for this.

 

[twistedneck]

Swall and TVP, thanks for that excellent information.

I was worried about this 1.5 second nose, I thought my TTT might be a mis-print.

I don't see how you can quench 4130 in anything but Brine and expect it to miss the A+F nose, even with this 1mm thin material.

A bigger question, what is the effect of Austempering 4130 considering you will have Austenite + Ferrite + carbides in the mix?

With this TTT nose time limitation i dont see how a Martemper or interrupted quench can help. Frusration is kicking in.. Jeff

 

[redpicker]

You are going to have trouble getting decent transformation in a foil envelope.

The key to low distortion is even cooling, which would be hard to accomplish with a foil envelope.

I think I'd look into austenitizing in a protective atmosphere, oil quench, and temper to the desired hardness. If distortion was an issue, consider fixturing during temper.

I don't think the difference in properties between Q&T 4130 and austempered 4130 at the same hardness would be significant. I would think that being able to fixture the part during tempering would give you better dimensional control.

rp

 

[twistedneck]

rp, thanks for that information. Upon researching it further.. you are correct, 40+ RC hardness is the minimum to get the benefits of Austemper.

If i agitate the salt bath i can increase the cooling rate by up to 25%. 4140 is well known as Austemper steel, max section thickness from AISI is 2.5mm. The 4130 TTT nose comes in at 2.5 seconds. Interpolating.. a 1.0mm section of 4130 would just squeak by with the 1.2 second nose. Worst case i'd have some fine pearlite.

Its VERY difficult to fix these parts after tempering, so i'm going for it. My resulting hardness might be lower than planned, about 35RC, but that's OK.

Thoughts on this? I'm taking it to the lab and I can't wait to see what the microstructure and hardness look like. Hopefully distortion will be minimal due to the Austemper.

 

[twistedneck]

RP, one more thing.. I will cut the foil just before placing the part into salt pot. Part will slide out of the foil on a ceramic plate into the salt. It will then hang in a vertical orientation.

 

[twistedneck]

How ever much i'd like, I cant marquench this 4130 thin part. I would need to figure out a way to quench the part and pull it of salt again within two seconds, not very easy, even or repeatable.

Modified Marquench could work, but i'd be stuck with the warping due to uneven phases.

Water quench wont work even with normalizing first.. warp city with 1mm cross section box.

My plan, since I need to pull heat out quick, down to 1000F in 1.2 seconds (easy with salt, water, agitation and 1mm thin sheet) is temp offset. The transformation to Bainite begins in 5 seconds (Bainite comes quick with 4130) i will simply offset (lower) the temp of my salt bath so that adding the part brings bath temp up to working temp (Ms + 10 degrees, 720F) within five seconds, then kick the PID back on to maintain 720F for the 20 or 30 min to convert to 100% Bainite. This will help cool the part fast, and make sure its not transformed into Martensite.. Thoughts?

 

[twistedneck]

I guess i should have realized, what i was talking about is an accepted modification of normal austempering called 'upquenching' and it creates a small amount of tempered martensite, may warp a bit more but will give the desired hardness, lower Bainite and help beat the pearlite nose!!!

 

[unclesyd]

Have you considered preheat treated material for you component. Though there forte is CS they have other alloys available in the heat treated condition.

http://www.theis-usa.com/carbon.htm

 

[twistedneck]

To All on this thread, thanks so much for your help on Austempering 4130.

I did manage to beat the 1.2 second 4130 pearlite nose - I believe easily (no Pearlite). Agitation and water addition to salt bath not required.

Microstructure is a Bainite / Tempered Martensite duplex structure, mostly Bainite as planned, 43.5Rc.

http://home.comcast.net/~jcheck10/1-200X.jpg

Yes, i've got a lot of decarb issues to solve, however look at the Bainite, and the fine grains - i'm very happy.

 

[TVP]

Congratulations on the success of your project. Do you think the decarb occurs during the austentizing process, or during the transfer to the salt bath? The former should be quite easily fixed, but the latter may require some modifications to your already difficult process. From the scale that was included on the image the decarb is quite deep, so I am guessing it occurs during the austenitizing step. Did you use a foil bag?

 

[swall]

I see a significant amount of total decarb in that micro. I would guess that it was present on the sheet prior to the heat treat process. Hard to do that when austenitizing if you have any sort of furnace atmosphere at all and keep the heat time short.

 

[twistedneck]

TVP / Swall, I believe most the decarb is occurring in the kiln as i can't quite get a positive pressure due to leaks, but i'm not sure how much. No foil, just argon - next time i'll use a lot more volume.

i've significantly improved the transfer via a blanket over the brick to hold in argon from the wand, and hold air o2 out. I will run more trials in the future after i get this improved.

There are more pictures that tell a story about the Ferrite grains. See below. Sample #1 is the first sample, its got much more ferrite - the consensus so far is its due to cooling of the part too much before quenching, dipping in to the litte ferrite part of the iron carbon phase diagram. its even because the secion is so thin, only 1mm.

that layer you see isn't scale, its bakelite between the two sheet samples. i combined to one mount - bad idea, bleed over, polishing compound coming out, etc. despite the jewlery cleaner.

see more pics

http://home.comcast.net/~jcheck10/1-200X.jpg

http://home.comcast.net/~jcheck10/2-200X.jpg

http://home.comcast.net/~jcheck10/2over1-100x.jpg

http://home.comcast.net/~jcheck10/1-1000X.jpg

http://home.comcast.net/~jcheck10/2-1000X.jpg