cracking/checking induced by turning operations

[Tmoose]

I'm well aware of the possibility of creating "heat checks" when grinding iron and steel. I've even created a few.

How possible or even likely is it to create cracks/checks/indications in a steel shaft made only using turning type operations?

thanks

Dan T

 

[swall]

It's certainly possible. I have seen surface damage, but not quite cracking, on peeled and burnished SAE 5160 bar. I have seen surface damage (burn)from milling, as revealed by nital etch inspection,on aircraft landing gear components. On spring wire, I have seen cracks due the frictional heat generated by the wire being pulled past a capstand.

 

[Maui]

Tmoose, that is strongly dependent upon the grade of steel, it's microstructure (annealed versus hardened for example), the type of tooling used and its condition, rigidity of the set-up, etc. If you are concerned about creating adverse surface conditions that can lead to a degradation in the performance of the part in service, the information contained in the following paper may be relevant:

http://www.sciencedirect.com/science/article/pii/004316489606927X

Maui

http://www.EngineeringMetallurgy.com

 

[mfgenggear]

Tmoose

depending on the configuration of the parts, heat treat condition & so on.

cracking from to high of jaw pressure, excessive feeds & speeds.

I seen parts crack because the jaw pressure was to high & the parts where thin & fragile.

Parts turned with to high of speed cracked.

rarely seen any parts over tempering but it's possible.

Mfgenggear

 

[Tmoose]

sorry - should have said solid shafts with diameters from about 3.5 to 14 inch. Carved from normalized 1045 and 4140, possibly "quenched and tempered" depending on availability, but we are only asking for 75 KSI/112 KSI UTS on the bigger shafts

 

[Maui]

Are these press forged prior to turning? Hot rolled scale can hide fine cracks that are created during hot forging. I've seen this many times.

Maui

http://www.EngineeringMetallurgy.com

 

[Tmoose]

Deep in the previous millenium it looks like we used to specify starting with stepped forgings, then cylindrical forgings. Nowadays we don't prohibit using a forged blank but they have been hard to get. I've heard in the big sizes we generally have better (but still bad) luck finding hot rolled blanks. The mag particle inspection behind this question occurs only near the very end of processing, after many sections have had their diameters reduced several inches. I am trying to understand the real potential risk of inducing cracks with the processes used to carve inches the diameters. First reports are it is all turning, and no grinding.

thanks

Dan T

 

[mfgenggear]

Tmoose

look at the parent material as said above.
raw stock may have cracks.

Mfgenggear

 

[gearcutter]

If coolant is being used (particularly liquids) then yes, any machining operation on carbon steel will run the risk of surface cracks developing due to the untempered martensite that can be produced.
Along with environmental considerations, this is one of the reasons why industry is beginning to turn away from using coolants............particularly on grinding operations.


Ron Volmershausen
Brunkerville Engineering
Newcastle Australia

http://www.aussieweb.com.au/email.aspx?id=1194181

 

[Tmoose]

thanks all.
Yes, we UT the shaft blanks pretty early in the process.

The current quest is to justify whether we need mag particle at the end.
Detecting induced cracking was originally considered one (theoretical) concern.

It remains legitimate, possibly. There were some bearing seats that suppliers used to finish grind.
Reports are they now turn them to within 0.002", and finish polish to size with something like this.

http://www.abs-products.com/abs_ima.../specialty-tools/bicycle-handled-polisher.jpg

My first reaction is to be mighty suspicious of the results, especially taper, whether used on crankshafts or our bearing seats, but if the QC guys say they measure OK I guess I have no real complaint.

 

[mfgenggear]

Tmoose

Yes it does. I have seen for what ever reason, The UT NDT not detect fine defects. be it operator error or the procedure it self.
I would highly recommend Mag & Demag after finish machining. yes there could be cases where the mfg may final grind parts, straighten or could open up defects during final machining. I am on the conservative school of thought when quality is concerned.
The cost is well worth the piece of mind.

Mfgenggear