Heat treat oven (H900) 17-4 SST 1

[badger2011]

I am inquiring about the H900 heat treat process. We are validating our heat treat oven and running into an issue with getting them up to temperature.

Here's my protocol: 6 thermocouples places on a fixture that record the temp at the level in the oven where our parts sit during H.T.. The ovens are thermo-scientific lab ovens. Kinda small, but we're making due. With no load, the thermocouples record a near perfect cycle as the oven ramps and soaks. Once I run it with a load (20 lbs of 17-4 SST), the thermocouples lag by about 45 minutes. This makes sense that the heat is being absorbed by the load. BUT... even though the oven doesn't reach 900°, the steel is still getting the correct hardness (Rc 40-44).

Do you know how this can be?

I decreased the load, increased the soak time... this helped with the "lag" because the oven appeared to get up to temperature for at least 1 hr. I don't know if this is a waste of time though.

If I "over-soak" the parts at 900°, will the hardness change? i.e. will a 3 hr soak give me a different hardness than the minimum 1 hr soak?

Sorry if these seem like newbie questions. I looked into ASTM, google, etc. and all I could find is that my tolerance for the H900 is +/-10°


Thanks in advance for any help you can offer!

 

[cloa]

How are measuring this hardness? Have you many samples to statistically determine the scatter? It might be luck.

 

[redpicker]

Generally, an H900 treatment is considered to be holding the material at 900F for 1 hour. From what you have written, I cannot tell if you are saying that you are putting the parts in the furnace with the set-point at 900F and, after 45 minutes it reaches 900F and 15 minutes later, you are pulling the parts from the furnace. Or, are you saying that you are putting the parts in the furnace with a 900F set-point and after 45 minutes it reaches set-point and an hour later (1:45 since you loaded the furnace) you are pulling the parts.

For most heat treating processes, the time at temperature is considered the time the furnace thermocouple sees the set-point temperature, since this is usually all that is being controlled. The 1 hour at 900F treatment for H900 is used because it is very reproducible with a wide variety of furnaces using this control method. One hour is typically long enough so that any lag the load is behind the control thermocouple will not be significant in the overall treatment cycle. The furnace uniformity requirement of +/- 10F is there to make sure there isn't much lag to start with. After one hour, the temperature of the load should be very uniform and uniformity is key to peproducibleity with heat treatment.

To answer your question, will a 3 hr soak give a different hardness than a 1 hr soak? Theoretically, yes. Will it be a large difference in hardenss? Probably not.

rp

 

[EdStainless]

Of course you will make hardness, you are underaging. Your elongations may be 0% though. Have you done a tensile?

You need a more powerful oven. Period.
How thick are your parts?
you should be load the furnace at some temp, say 500F. Allow 30 min for everything to stabilize, and then ramp to 900F with a load.

I have looked at old Armco data and the hardness at 900F aging does not decrease with longer aging times. Their curves go out to 48 hours and there is no change from the hardness at 30 min.
Slow your ramp rate some and use a longer hold time.


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