Microstructure of steel according to the TTT diagramm

[stanislasdz]

Hi every body

Just a help

I want to perform this heat treatement

1 - Cooling to 350 °C from 850 °
2- Holding at 350 °C four 4H hours
3 - Air coling

What is the final microstructure ?

Thanks in advance

 

[dbooker630]

I would say bainite and ferrite.

 

[redpicker]

You have left out a critical bit of information. In step 1, how long is it taking you to get to 350C from 850C? On the diagram, you have indicated taking 5.5 hours. If this is the case (and the diagram is accurate), you can expect a mixed microsturcture consisting of about 40% primary ferrite, maybe 10% pearlite, and about 50% bainite.

If you can get to below 650C in under 3.5 minutes, you can probably get nearly 100% bainite.

rp

 

[arunmrao]

Since you are now dealing with forgings,is this a bar stock or forged product. Section size,homogenization treatment followed by redpicker's thread concerning cooling rates,will need be considered. I go along with bainite and ferrite.

However, what is it that you are aiming at,let us know too.

I have not failed. I've just found 10,000 ways that won't work." — Thomas Edison
_____________________________________

 

[stanislasdz]

Dbooker630 & redpicker
thanks

redpiceker, yes the time betwen 850 and 350 is about 5.5 hours

My questions for both :

1- How you can know the propotion of each phase, this is not à CCT diagram ?

2- Is this TTT diagram accepatable for the big thickness parts > 600 mm ?

 

[dbooker630]

It appears from the sketch that your cooling curve passes through the top knee of the curve containing austenite, ferrite and some pearlite. The curves in the ASM Heat Treater's guide are similar to what you have presented. The curve you have plotted is similar to what is reported for 25mm from the quenched end. The center of your section will contain more ferrite and pearlite.

 

[arunmrao]

My concerns will be of maintaining adequate cooling rate and being able to hold the temperature precisely at 350C,without going down in some regions.

Perhaps you need to introduce thermocouples at different section thicknesses to understand clearly the expected microstructure.

I have not failed. I've just found 10,000 ways that won't work." — Thomas Edison
_____________________________________

 

[metengr]

stanislasdz;
I would try to locate a CCT diagram for a more accurate depiction during slow cooling. This is a common material for CCT diagrams. The TTT diagram shows austenite reformation at around 530-550 deg C, where the curve displaces to the right for a considerable time interval.

 

[redpicker]

1- How you can know the propotion of each phase, this is not à CCT diagram ?

Just a guess, but at 0.40% C, you should get around 50% free ferrite at equilibrium. At such a slow cooling rate, you should be close to equilibrium, maybe a bit less, hence the 40% number. At the cooling rate you indicated, by the time pearlite can begin to form, you're below the temperature at which peralite will form, so I gave maybe 10% pearlite forming. Remainder will form bainite on cooling and holding at 350C.

2- Is this TTT diagram accepatable for the big thickness parts > 600 mm ?

It's acceptable for isothermal transformation. At slow cooling rates, you approach isothermal transformation, so you can approximate.

For example, on the cooling curve you drew, you hit 650 C at around the 1 hour mark. Your microstructure at that point isn't going to be much different than if you cooled rapidly to 650C and held it there for an hour. Since the only transformation product is ferrite, it's safe to say that you'd be just a little behind equlibrium with continuous cooling at that rate for this steel composition.

But, as metengr points out, CCT diagrams for 4340 are pretty common, so if you need a more exact evalutation, google a bit and you can probably run across one.

rp

 

[stanislasdz]

Thanks for every body

Arunmarao :

Thanks for your advice for thermocouples, my aim was to avoid quench cracking during air cooling for the thikness part > 600 mm

Again Thanks all