Theory Of Marquenching? 2

[tc7]

We recently experienced a serious cracking issue on a weldment made with an aircraft quality 4340. After stress relieving our weldment, the heat treater then normalized it, then went into a subcritical anneal, austenized in endothermic atmosphere and oil quenched. Then double tempered it. Our hardness goal was met at C36-38 but the extensive cracking was hideous and the piece is entirely wasted. It is a complex weldment with heavy sections (2-inch thk) as well as thin sections (1/2-inch thick) and everything in between. It weighs ~325lbs. The components were machined from a variety of shapes - tube, plate and forged rectangular block.

Next time around I hoped to do an aus-bay quench and temper but no one in a 70-mile radius is equipped to do this for me. My next best approach may be a marquench. However my vendors cannot increase the temperature of their quench bath as high as I am asking, due to the volume of their bath. The HT'ers want to marquench at 425ºF whereas the martensite start temperature of 4340 is somewhere around 545ºF.

Questions:
1. what is the theory of marquench? Is it just a stepped quench to reduce the thermal gradients or is there an important significance to quenching the first step above MsubS and holding?
2. If my only available quench temperature is below MsubF, as in my case only 425ºF, is it still a marquench and will I get an acceptable result?
3. given a choice between marquench in salt and a marquench in oil, what are the advantages of either?
4. slightly off topic but, is an outside square corner a point of high stress during quench that can cause crack initiation?


Thankyou.

 

[redpicker]

1) Pretty much as you have assumed; a stepped quench to reduce thermal gradients. Since the Ms temp is well below the nose of the TTT-curve, the marquench process allows slower cooling through the temperature range where the martensite is forming. If the thin and thick sections can undergo the martensite transformation at the same time, the transformation stresses that lead to the cracking are greatly reduced.

2) I really think to answer your question (will I get an aceptable result?) is to try it and see. Have you found a heat treater that is offering to quench your part in 425F oil? Does he have any experience in doing this? This sounds very dangerous and asking for a fire resulting in loss of property and/or life.

3) Salt is used because it does not burn. The flash point of oil is generally too high to use for marquenching, unless the section is thin and uniform enough to allow removal from the quench before the part has cooled below the Ms.

4) Yes.

Just for consideration, have you considered a forced air cool for your part? Essentially, after removal from the austenitizing furnace, the part is placed over a high velocity fan for cooling. Yes, the thicker parts may not fully harden, but often the thick sections are not where the highest stresses are seen. The thin sections may well harden enough to work. It might take some testing to evaluate, but you could use your cracked part as a test piece and cut it afterwards to see what the hardness is at various locations/thickensses.

Another approach would be consider using 4330V material. It will easily harden to 36/38 HRC and with the lower carbon, will be less susceptible to cracking.

rp

 

[EdStainless]

What condition were the various parts in prior to welding? Did you have all of them normalized first? You may have set up some serious mismatch/stress issues by using material in different heat treat conditions to start with.
Sounds like you need to do some testing.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[dbooker630]

At our shop we marquench 4340H steel at 350-375F. The types of parts we heat treat feature significant abrupt diameter transitions (1.5"-2.5"-4.5") and drilled holes that are susceptible to cracking with conventional oil.

Success has been excellent over the years. The flashpoint of marquench oil is typically 525-550F so quenching at 425F is not recommended.

We have also found similar success on these same components using a 'medium' quench oil formulation with operating range between 200 and 300F. Higher for distortion control, lower for improved transformation. Also costs less than marquench oil. Contact Heatbath/Park Metallurgical for details.

 

[tc7]

Redpicker-
Thanks for your thorough reply. I was incorrect about the hot oil temp of my second vendor, both prospective heat treaters have a salt quench. And the designers are looking for a consistent through hardness and that is why they specified 4340, although I will suggest the 4330V idea.

EdStainless-
All material were in the normalized state as delivered, at least that is what the certification papers said.


Dbooker633-
It is curious to me that you are marquenching at 375ºF. If the martensite start temperature is ~540ºF then at 375ºF approximately 90% of the martensite transformation may have already taken place which increases the chance for distortion and cracking. You seem to be simply doing a stepped quench, not a marquench.

 

[TVP]

tc7,

Keep mind that is the temperature of the quench oil, and that the surface of the part does not instantaneously reach 375 F. The term "hot oil quenching" is used almost synonymously with marquenching, even though for some alloys the Ms may technically require salt baths instead of heated oil. Here is an excellent document from Houghton on quenching, with some discussion of hot oil/marquenching:

http://www.houghtonintl.com/images/Hougton on quenching.pdf

 

[Compositepro]

That's a really fascinating link there, TVP.