Heat treatment of heavy forging products made of steel 2

[nasarali]

I am trying to do experiments to simulate heat treatment for heavy forging 4340 steel, but the problem is I couldn't get a martensite due to very low cooling rate. my question:
1- Is there any thing can I do to help me to get martensite, Knowing that I do normalizing at 860C, quenching at 860C, and tempering at 640C.
Many thanks

 

[metengr]

You provide little informaiton regarding size of the forging and method of heat treatment. Try oil quenching after austenitizing at 880 deg C.

 

[EdStainless]

Quench media. temperature of media, agitation of media are all issues.
You want to limit quench rate, but keep it as high as possible

Aren't heavy parts in 4340 force air cooled?
Isn't that the point of a 'through hardening' steel.

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Plymouth Tube

 

[nasarali]

Thanks for your answers put the problem with the size, where the real product it's about 8m length and about 1/2m diameter

 

[redpicker]

I think your temperatures are low. I like 950C for normalizing and at least 800C (as mentioned above) for austenitization prior to hardening.
With a 500 mm diameter, there isn't much you can to do form an appreciable amount of martensite at any significant depth below the surface. Very large rotor shafts are quenched using a water spray quench, which approaches an ideal quench, but with 4340, you run the risk of cracking with such a drastic quench. If you are dealing with solid round that has been machined on the OD, and your quench is uniform, you can reduce the cracking, but it will always be a concern.

Even with an ideal quench, you are not going to be much better than the Jominy. That is, the amount of martensite formed 2 inches (50mm) below the surface is going to be about the same as you see at 32/16" (50mm) distance on the Jominy test. At increasing depths, even less.

Generally, with such large sections, high strength is not needed since the stresses are low. If high hardness is needed in localized areas, then local hardening techniques are used (e.g., induction hardening or flame hardening). Generally, heat treatments used for these large sections are a normalize and temper to increase the ductility of the material. In the case mentioned of very large rotor shafts, they will usually ues a highly modified chemistry to increase the heat treat response and rough machine the part to near final shape so the final machining operations do not remove all of the material that did respond to the quenching operation.

But, on the face of it, trying to thorugh-harden a 500MM OD x 8 meter bar of 4340 is not going to be very successfull (except, as they say, for very small values of "through-harden").

rp