Cracked forgings

[arash66]

Recently we heat treated some extruded steel(low alloy steel) tubes (hardening, polymer quench and temper) and they cracked in the bore. refractomere and viscometer tests shows that our polymer concentration is in the range. We think it may be due to the aged polymer. but the thing which has confused me is the shape of the cracks which are not straight (like quench cracks) they are curved and in one occasion the crack had a irregular oval shape.
we have checked all the Heat treatment parameters and they all seem to be ok.
Does anyone have any experience?
Any help will be appreciated.

 

[metengr]

What tube material and size (OD and wall thickness)?

 

[redpicker]

As metengr says, we need more information to give any meaningful advice. In additon to the material chemistry and size, process information (temperatures, times, quench agitation, etc...) will produce better responses.

As the age of the polymer goes, the refractometer and viscosity tests are to adjust for any changes that may occur, within reason. If you are dealing with 20 year old polymer that has been sitting around in a tank and never used, that is another story.

Contact your polymer vendor. Most of them have excellent technical staff and are eager to help customers solve their quenching problems.

rp

 

[arash66]

I will post more details tomorrow morning (UK time), but my main question is can quench cracks be curved?
Regarding polymer, we use them on daily basis. By ageing I mean the loss of effectiveness due to repeated use.

 

[CoryPad]

Yes, quench cracks can be curved.

 

[dbooker630]

Send a sample of your polymer quench to your supplier and have them perform an analysis which typically includes viscosity, concentration, and sediment. They should do the analysis work free of charge.

 

[redpicker]

Yes, quench cracks can be curved.

I agree, unquestionably.

Another thing that comes to mind; many people, since polymer is a substitute for oil, will think they should use it like oil.
This is incorrect.
Most quench oils perform best at 50-60C (120-140F). Polymers, however, will seperate at higher temperatures and begin behaving more like water. They need to be kept below 40C (105F) or so to keep the polymer in solution with the water. It can be difficult to accept, but cracking problems can increase if the temperature of the quench bath gets too high. While too low of a quench temperature is also bad, mostly because it can result in quenching the parts too cold, too high is also bad. If I recall correctly, somewhere between 25 and 40 C (80 and 105F) seems to be where you want to be. Understand that the temperature ranges are based on experience with some of the earlier polymer formulations, you really should check with the manufacturer of the polymer you are using to see what temperature range they recommend for their product.

Finally, you should also monitor the temperature of the parts as they come out of the quench. You want to make sure they are cool enough, but not too cool. The optimum temperature depends on the steel composition, but for 4140, for example, I would target around 95-120C (200-250F) as the temperature to exit the quench. Of course, you also need to make sure the parts don't cool too much before they go into the tempering furnace, or they may crack before they get tempered.


rp

 

[arunmrao]

Polymer quenching is a panacea to most quenching problems,is what most think,(me included). Until about 12 years ago, I realised,that there was more to it, than just adding the right quantity and checking a few parameters.

As like others ,I too agree that quench cracks need not be straight.

_____________________________________
"It's better to die standing than live your whole life on the knees" by Peter Mayle in his book A Good Year

 

[arash66]

Thanks guys for the information.
we check concentration by refractometer on daily basis and by viscometer on weekly basis. I know none of them are accurate enough to tell you if the polymer is in good condition or not so our supplier test the polymer on monthly basis.
RP mantioned a good point regarding temperature of the polymer. our route (which has been written long time ago) asks for temperature to be between 40 to 50 C so I will check this with the supplier to make sre it is still the case.
Normally when we remove the parts from polymer they are around 65C and they go to tempering furnace with maximum 2 hrs delay.

 

[TVP]

I agree with the range that RP mentioned: 25-40 C for water + polymer quenchant. Above 40 C, the polymer generally is not stable over time.

 

[arash66]

If you don t mind some theoretical discussion, I know polymer has reversible solubility but can you please let me know why it is so sensitive to the temperature range? why 25 to 40 can be different from 40 to 50C?