4330 Heat treat 2

[tpdaniel]

Can some body help me with the holding time in austenising stage as well as tempering. I read an article which you guys recommend for 11 inch bar,Feb 2011

The heat treatment process of this heat of material, 11'' OD:
Hardened 1600F 7.0 Hours Oil Quenched
Tempered 1100F 11.0 Hours Air Cooled

what if the job is thicker ? like 20 inches.
I always had problems with Austenising & tempering these jobs, Can anybody suggest the run times.
Cheers

 

[metengr]

For a solid 20" diameter round

Austenitizing - hold time will be highly dependent on furnace type and loading. Assuming a properly sized furnace with minimal temperature variation and loading, the hold time will be the lag time to assure a uniform temperature in the center of the bar. You should conduct a furnace survey to determine temperature profiles.

For 8" thick plate, I have seen data reported where a uniform temperature is reached after ~ 4.5 hours at 1650 deg F. So, for a thicker round, the 7 hours may not be needed. I would take temperature measurements after 5 hours of hold time, every 15 minutes, and monitor temperature change before removing for quenching. You do not want to spend unnecessary hold time at 1650 deg F because of grain growth and possible surface decarburization concerns, again depending on the type of furnace.

Tempering - 5 hours + 15 min per each additional inch over 5 inches in thickness.

 

[redpicker]

Without knowing the specifics of the furnace you are using, any useful information would have to be a "worse case" scenario.

If you wanted to attach a thermocouple to the surface, you can do pretty good with 15 minutes per inch after the surface reaches temperature, plus an hour, but even that is at best a guess if you don't knowing the equipment you're using.

Considering that the furnace is probably not capable of holding much more than +/- 25F at austenitizing temperatures, probably less if you don't have a recent survey on it, your temperature seems a bit low, too. I think I'd go with 1625-1650F.

Tempering times are typically 1.5 to 2 times the austenitizing times, considering equipment of similar capacity.

If you had a history with the equipment, you would be much better off. Particularly if it covered various sized parts at a range of temperatures.

Good Luck

rp

 

[tpdaniel]

Thanks metenger & redepicker,

We got controlled pit furnace very accurate with a maximum rated load of 10,000kgs. Its very precise with carburising procedures and temperatures.
Furnace fluctatues with in a degree and gives consistent temperatures with thermocouple on job.

Its just the times Iam not getting right for the quench & temper jobs
like you suggested temper to be 1.5 - 2 times austenising.
Is it an hour per inch plus 15 min for evry inch for austenising and double time for temper even at 1100F, Is that how it works ?

Please suggest
Cheers

 

[redpicker]

What do you mean by you are not getting the times right?

Typically an hour per inch will be excessive for large parts. It will work, but for your 20 diameter part, that is a bit much, If you furnace is rated for 10 tonnes, unless you are putting more than 8 tonnes in there, you should be fine.

The 1.5 to 2 times the austenitizing time is just a general rule. Tempering times are generally longer than austenizing times because with tempering, longer times tend to lead to more uniform properties (the longer time allows the temperature to equalize). This is more important with tempering than with austenitizing since with austenitizing, all you have to do is get above the minimum temperature.

If you are trying to reduce furnace times, a thermocouple attached to the surface is the surest way to go. If you are just trying to make sure you don't have to run it twice, your hour per inch will work.

rp

 

[metengr]

Having performed many a shop audit, you need to optimize your heat treatment using coupons of similar part geometry and composition to determine minimum austenitizing hold time and tempering hold time. Rules of thumb are only for a starting point. I am amazed at how many vendors get tripped up because "we need to do it this way, always have done it this way" and never optimizing the process.

 

[tpdaniel]

Thanks redpicker & metengr
Much appreciated !

we have been redoing the quench & temper jobs quite often, never was able to make up a precise tempering temperture propotinate to Brinell hardness. we got good austenising temperature & quench, its the temper which always kills my hardness readings, we never get in a range of 50 Bhn.
I assume that longer tempering times shall help.

Cheers

 

[bhollands]

If you are getting inconsistent results tempering time may not be your problem. Have you tried having the parts normalized and air cooled prior to the specification heat treatment?

Bob

 

[tpdaniel]

Never tried normalising, will try that
Thanks for that
Cheers