Dear Eng-Tips people
We have some problems by hardening/quenching AISI 4140. We do some test with a Ø2" x 3" blank.
First we austenitize the material at 840ºC for two hours at this temperature, then this material is quenched in oil, with not agitation. The material reached only 30 HRC.
What I should better in my process to reach at least 45 HRC after tempering?
How much time should the blank stay at 840ºC?

how sure are you that you really got 4140?

Are you using one of those infrared-laser guided non-contact thermometers to set the temperature? We recently ran into some big problems when a shop was using one for a hot forming fabrication processes with temps up in your range. the standard emissivity was way wrong

4140 will not fully harden in the section size you have. You need a steel with higher alloy content, such as 4340 or H-11.

So, let me explain this better.
The hardness is 30HRC on the surface of the blank, I really thought I wanna achieve something like 40 - 45 HRC on surface.
The guys do something, that results in better hardness, when the hardening oven reach 840ºC, they let this for some 30 min at this temperature, then this is quenched in oil and after tempering they achieve 45 HRC.
I take a look at some ASM and other books, that for the hardening process you need for every 25mm, 1 h at the austenitizing temperature, so 2" gives me 2 hours. But this process didnt work for me.
About the temperature I have no laser to verify this, but I wanna take a look.

Matbol,

There are numerous things that you need to inspect, verify, etc. Is the surface of the bar/part decarburized? Decarburization could be present in the original bar or could occur because of poor atmosphere control in the furnace. You should measure the hardness at the surface and through the cross section. You should also etch the cross section and evaluate the microstructure for % martensite, % pearlite, presence of decarburization, intergranular oxidation, etc.

What are the furnace conditions, specifically, does the furnace reach 840 C when the steel bars are placed within it? Do the bars ever reach 840 C? Just because one pyrometer or thermocouple within a furnace shows 840 C does not mean that the steel bar has the same temperature.

As explained in the second answer above, AISI 4140 lacks the required hardenability to provide the through hardness you seem to need.
If you need only surface hardness of 30 HRC (and you are not interested in core hardness) you might consider surface hardening by induction heating, followed by quenching and tempering.

http://www.welding-advisers.com/

There are a few reasons you aren't getting your target hardness.

Perhaps the main one is your austenitizing temp is a bit low. Depenging on the prior microstructure of the 4140, 840C (1544F) may not austenitize in a reasonable amount of time. A normalizing treatment at 925F may help.

As for time, the old rule of thumb would be an hour per inch, so 2 hours should be fine. Actually between 1.5 and 2 hours would be fine.

Not agitating the oil is also not helping you either. What is the temperature of the oil? If you are using cold, non-agitated oil, even if everything else is fine, that could cause the problems.

Finally, you need to look at the surface condition of the material. It is probably OK, but if there is a thick forging scale present, that will be a problem with quenching.

I would first normalize the material, then quench in agitated oil.

rp

I would remove any forging/rolling ...scale.
I Would go under vacuum for that HT
I would not preheat the furnace
I would control the furnace temperature
I would warm the oil
I would agitate the oil

If all these do not work, I will do
surface hardening (induction,laser).

Keep in mind that different heats of 4140 can vary widely in response to HT. A heat that is high in Mn, Mo, and Cr might make it, while one that's on the low side won't. Look up hardenability and "H" factor.

"You see, wire telegraph is like a very long cat. You pull his tail in New York and his head is meowing in Los Angeles. Do you understand this? Radio operates the same way: You send signals here, they receive them there. The only difference is there is no cat." A. Einstein