heat treating 4130 steel

[eds026]

I tried to heat treat a piece of 4103 steel .125 thick 6 inches long shaped like a "u". I put the piece in the furnace at 1600 degrees for 2 hours (the oven was preheated), then oil quenched, and anealed at 700 degrees for 1 1/2 hours. The rockwell hardness I achevied was only 22 on the c scale. I would like to achive 45-50.

thank you

 

[metengr]

Two questions;
1. Where and how did you perform the hardness testing?

2. Did the furnace contain a protective (reducing) or inert atmosphere to prevent surface decarburization during heating and soaking at 1600 deg F?

 

[eds026]

I was in our lab. We have a rockwell hardness tester.

Yes, it did. I meant 4130 not 4103

 

[metengr]

I would suggest you remove a sample and have a metallographic examination performed to verify your heat treatment process. This examination will determine if you had achieved a thru-hardened, quenched and tempered microstructure. I would also confirm the chemical composition of the stock to make sure it was 4130 alloy steel.

 

[eds026]

I was looking for the proper times and tempatures for this process. I have the certs that it is 4130.

 

[metengr]

Your austenitizing temperature of 1600 deg F is ok, the furnace soak at temperature should only be 30 minutes ( at 1600 deg F for 30 minutes) not 2 hours. Immediately quench in oil (this is most important to perform as rapidly as possible).

Place back in the furnace at your original tempering temperature of 700 deg F for 30 minutes (at temperature), remove from the furnace and cool to room temperature. I don't have immediate access to my reference material to confirm the 700 deg F tempering temperature, but off the top of my head it appears to be about right for your required hardness range.

 

[eds026]

Thank you, I'll try this tomorrow.

 

[swall]

I'd say that a 700F temper is a tad high for your required hardness. I would suggest 450F for one hour.

 

[israelkk]

Instead of quessing you can use MIL-H-6875 which can be found at

http://assist.daps.dla.mil/quicksearch/

 

[Carburize]

An additional source of this data - not free however - is the ASM Handbook Volume 4 Heat Treating.
This gives an austenitize recommended temperature of 1500 - 1600 F
Water Quench
Temper at 400F gives around 47 Rc
Temper at 500F gives around 45 Rc

 

[Tmoose]

When they did the Rockwell testing was the sample lying flat, in direct and complete contact with the anvil?
Any bow or gap will produce a wildly soft reading. Testing hollow sections is tricky

 

[swall]

Tmoose--good job. I was going to bring up that subject myself, but forgot.

 

[NickE]

It looks liek his testing may have been fine. Its the quench and temper that were wrong. Oil quench isn't fast enough to get a high enough as quenched hardness. And the temper temp is almost double that given by the ASM hbk. (and long at 1.5hrs.)

I have a related question though. Austenitizing at the top of the reccommended temp range for over 4times the reccommended time seems like it would cause extensive grain growth. What is the effect of this grain growth on the as quenched hardness? Tempered hardness?


Nick
I love materials science!

 

[metengr]

Normally, coarse grains increase hardenability in carbon and low alloy steels because you have fewer nucleation sites to induce phase transformation upon cooling. As far as tempering, I don't believe you would see any effects other than lower toughness and increased risk of reheat cracking (aka as stress relief cracking).

 

[metengr]

Forgot to answer your question - no increase in hardness. The hardness of martensite is a function of carbon content not grain size.

 

[swall]

NickE--In my experience, oil quenching will be fine for a 4130 part that is only 1/8 inch thick.

 

[swall]

NickE--That is, unless you had a heat of steel that was right at the minimum jominy.
eds026--can you give us the chemistry from your cert?

 

[NickE]

Thanks guys,

swall- oh I forgot to look at his geometry... [voice=HomerSimpson](DOH!)[/voice]

metman- ya know I keep forgetting that.

So the over temper reduced his hardness by 20some odd points?

 

[swall]

NickE--I looked at geometry first thing. His 1/8 inch thickness gets you to J3 (eqivalent round for a flat is 1.5t). Now, unless you had a heat of 4130 right at the low end, you should be able to get better than the J3 hardness of Rc42.

 

[metengr]

Swall/Carburize;
As a side note, I was evaluating the temper cycle of 700 deg F for 30 minutes earlier today for this material after you guys had confirmed the 1 hour tempering temperature for this material.

My curiosity got the best of me because I had remembered that you could temper at higher temperatures with reduced time to achieve similar hardness results because tempering is a thermal activation process (time and temperature dependent). I was able to locate a H-J tempering parameter plot for AISI Type 4130 Q&T. To achieve a hardness of ~ 45 HRC, you can temper at 700 deg F for 18-20 minutes versus the standard temper of 1 hour @ 450 deg F.