Turn and Polish surface vs. Centerless Ground 1

[twistedneck]

Help needed, one of our main steel suppliers for raw bar (rods) wants to change from Centerless ground surface to turn and polish surface.

We are worried about fatigue issues. Potential sub surface voids and lapping caused during the turn and polish process, and any other pitfalls of Turn vs. ground surface.

ASTM A108-03 lists the max Ra (micro inches) for level 1,2,3.

1. T&P N/A, CG 40Ra
2. T&P 60Ra, CG 30Ra
3. T&P 40Ra, CG 20Ra.

I believe we are not specifying the grade yet.

Q, are the lapping and void issues in T&P surface real? Some aircraft people here are afraid of T&P for those reasons. The commercial aspect is important too - i.e. steel suppliers want to consolidate this in house (T&P) and eliminate CG surface.

Thanks, Twistedneck.

 

[swall]

Is the T&P or CG the final form of the product, or is there supplemental processing, such as heat treating or plating?

 

[twistedneck]

swall, T&P or CG, both are raw bar (spring) processing methods. We will then hot or cold form it into shape, then shot peen later, finally coat.

I'm worried that shot peen will not remove voids created by T&P.

 

[Goahead]

I may be wrong, but it seems to me that, if you shot peen as you should for spring applications, you are not really concerned with the surface finish.
If this is the case, would fine turning only (to eliminate any possible decarburized layer) without polish, be acceptable for your final product?

http://www.welding-advisers.com/

 

[twistedneck]

Goahead, turning only is a good idea that we will explore. Your right, peening puts about a 200Ra (in) finish on the metal - negating any benefit of a polish.

Now, peening still moves metal around, creates laps and could make the likelyhood of a surface lap even worse if we had turned material vs. ground.

turning creates the sawtooth type profile right? the taller the tooth, more likely it will fold over upon peening and create a fatigue killing sub surface void.

 

[NickE]

Just as an aside to the peening vs surface finish note.

It has been seen in internal studies that the benefits of high residual compressive stresses can be negated by excessively rough surface finish.

I dont have the details infront of me, but I have seen the study and believe the results.

 

[EdStainless]

The function of pealing or turning bar is to remove any surface laps or decarb.
I am more concerned about the polishing operation hiding surface defect. The turned bar should be fine.

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Neither should your protection

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[twistedneck]

NickE - shot peen alone results in a 200Ra surface finish. not to mention, most of the residual compresive stress goes away after the first cycle of fatigue.

Combine that with all the banging and dinging.. and the residual layer is quite uneven - not sure what that does to fatigue.

Ed stainless, your point about avoiding the polishing step is crucial. i'll research that asap.

 

[unclesyd]

Here are two sites with a lot of information on shotpeening. Metal improvement is straight forward while you have to look hard at the other site, click on the proceeding papers.
You might be interested in looking at the Laser Peening process at the Metal Improvement site.

http://www.metalimprovement.com/laser_peening.php

http://www.shotpeening.org/ICSP/confrns.htm#ICSP7

Is it possible to tighten up your specifications for the bar. Where we were we concerned with fatigue we went to great pains to procure bar stock that was specifically manufactured for shafting.

 

[NickE]

twistedneck -- a variety of surfaces finishes can be produced depending on the peening process used, while You may think that the first flex cycle removes the residual compressive stresses (RCS), experimental data does not support this.

I have tested materials in simple beam bending, an increase in residual compressive stresses is accompanied by a significant increase in cycle life, when the peak-peak stress level was increased by the difference in RCS the difference in cycle life disappeared. (surface roughness was the same at ~0.14um Ra)

I have seen results of coil springs tested to failure. Generally an increase in RCS is accompanied by an increase in cycle life. However at some roughness the stress concentrators on the surface begin to initiate failure, even though the RCS are much higher.

Nick
I love materials science!

 

[twistedneck]

More data, the polishing process may be required and i'll look into 'light' turn and polish vs. heavy burnishing.. some people mentioned this would create a harder surface (no RCS) i'm confused here.

Nick, your right.. no question RCS in the right spot helps fatigue. Weather by shot peen or heat treat. In fatiuge, a lot of good surface RCS reamains, just most of it (supported by papers, not experience) gets removed during the first fatigue cycle depending upon the fatgiue regime.

Experimental data showed me that only a small % (up to 30%) gets removed..

Of course w/o that remaining RCS we'd never meet fatigue life.

 

[unclesyd]

You might want to look at the data this site. Even though this is a presentation of the benefits of LPB (Low Pressure Burnishing) they have some good date on RCS (Intensity) both for high and low cycle fatigue test along with mitigation of FOD nucleation sites.

http://www.lambdatechs.com/html/resources/238.pdf#search="carpenter 450 steel"

 

[kenvlach]

unclesyd, thanks for the excellent paper.
Depth profiling was done via incremental electropolishing. twistedneck, you can do this to compare your different finishing techniques. Electropolish away 0.0001" layers at a time and examine. See which method produces best structure (fewest laps, etc.) Also, I've noticed lots of trapped machining oils freed when the smeared surface layer is removed (although mostly with the relatively gummy 316L).

 

[twistedneck]

thanks unclesyd..

kenvlach, that electro polishing thin layer method is how we prep layers for xray to find out what the RCS profile is over a depth.. so we had the perfect surface defect tool sitting there the whole time!

I have a meeting at the mill tomorrow (friday) i'll let you guys know what they propose.. btw, now i'm more ready then ever - oil? who'd a thought?

 

[unclesyd]

Adding some thoughts to kenvlach's post you might want to consider using a UV light source for detection of the oils, if the fluorescent.

You can enhance the bleeding out of the oil by using the Developer used for Dye Penetrant testing. This works for for a fluorescent and nonfluorescent oil.

 

[twistedneck]

unclesyd, that would be an awesome first step.. we could clean w/ a caustic cleaner, then apply the UV light source and the dye developer.. THEN go on to the surface electo polish check.. question, when we remove 0.0005 and look at at, what exactly will we be looking for? I assume it will be done on our normal optical system.

Met with the steel house and the processor

They were very helpful.. some notes

1. with our harder grade (vs. low carbon steel or stainless) chips come of much cleaner and avoid the dreaded tearing and stretching that creates the huge valley's during the turn process.

2. polishing lightly with small amounts of force and low burnish / bending - the surface will not create the voids as bad.

3. Fatigue testing turned only!

 

[unclesyd]

What I've done is to remove the oils on the surface with a suitable solvent and then lightly apply the PT developer to the clean and dry surface. I have used just a visual exam for lightly wetted spots in the developer or if the oil is fluorescent use a UV light. It takes a effort to see the wet spots at times and it may or may not be viable in your case. I have found this extemely useful in the machine shop, especially when a machinest reports something not right in the a process.

I have seen both laps and seams with this process. This can work as some of the cutting oils will inhibit the penetration of the dye normally used in PT testing but will wick out into the developer.

I use this method at other times for looking at parts that have been subject to oil and grease exposure prior to using normal PT or MT after a rigorous cleaning.

This is just a variant on the original PT testing accomplished by using kerosene and whiting or chalk dust.