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#1 - 04/07/10 02:50 PM Conditioning New Cast Steel Shot
Gary J Offline
Member

Registered: 03/24/10
Posts: 5
Loc: Corpus Christi, TX
I work for a U.S. Gov facility that is NADCAP certified, under AMS2430 (latest), and we use cast steel shot certified under AMS2431/1D. We also use "Frozen Processes" to maintain strict control of our process. We are not able to adjust any parameters that affect intensity without multiple levels of authorization. Our CNC machines are about 3 years old. We recently noticed intensity gradually climbing on some of our machines. We were out of our intensity limits in some cases. After looking at the obviuous possibilities, we had no choice but to conclude that we had an issue with the shot. We discovered through in-shop testing that brand new shot had about .0015A less intesity than well used shot. After shooting the new shot against a steel plate for about 2 hours, the intensity started climbing. I am unable to find any information on proper shot conditioning. Perhaps others in the industry simply adjust parameters when intensity climbs out of range, however we are bound by the frozen process controls. Also, the visual inspection in AMS-2430 condemns shot pieces that are considered "marginal" in AMS-2431/1. In other words, once I put brand new shot in my machine, what was acceptable is now unacceptable. I need a way of getting this new shot to acceptable condition visually as well as pre-conditioning to avoid that climb in intesity that comes with normal use. Is there a proper method for conditioning cast steel shot before use in actual production? Is there an issue that needs to be addressed regarding AMS-2430 vs AMS-2431/1?

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#2 - 04/08/10 09:46 AM Re: Conditioning New Cast Steel Shot
Lennart Almqvist Offline
Member

Registered: 08/18/09
Posts: 1
Loc: Sweden
Hi Gary,
We have stopped using cast steel shots since some years back. But looking back to the beginning of the late 80's we realized that we had the variance you're referring to. We simply added a requirement to our purchase specification as follows:

All steel shot shall be subjected to 100 cycles of peening against a hardened steel surface. Shot velocity 60 +/- 5 meters/sec. Blast angle 90 +/- 5 degrees. After cycling the shot shall be size graded, to fulfil sieve analysis requirements of AMS2431/1.

After incorporating conditioning of shots at the shot supplier we got away from that problem.

Best regards

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#3 - 04/08/10 10:00 AM Re: Conditioning New Cast Steel Shot
Walter Offline



Registered: 05/03/99
Posts: 158
Loc: East Hartford, CT, USA
Gary,

I agree with Lennart with regard to the pre-conditioning of new shot prior to use as a best practice. Also, I would suggest that when adding shot to your machine you never add more than 30% of the total hopper capacity. Be sure to add it slowly, mixing it in with the shot already in the machine. You state that you cannot adjust any of the parameters that affect intensity. Don't you have a tolerance range for your air pressure and shot flow? You are permitted to adjust your target point in a frozen process as long as you don't violate the limits. You should be able to adjust within your tolerance band to mitigate a slight rise of .0015". A combination of less air pressure and an increase in shot flow should do the trick. Of course you must run a new 4 point saturation curve to verify you are incompliance.

-Walter

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#4 - 04/08/10 02:57 PM Re: Conditioning New Cast Steel Shot
Gary J Offline
Member

Registered: 03/24/10
Posts: 5
Loc: Corpus Christi, TX
Walter, I am afraid that "frozen" means just that. A change in air pressure and/or media flow can be done but not without going through the channels to change the frozen process plan. Perhaps This issue has not been addressed because most of the industry simply adjusts as necessary to stay on target. The parts we programmed and "froze" back when the CNC machines were new were done with new shot. This means that our T1 was perfect then but is high now that the shot has been conditioned through use. I am wondering if this is an issue that needs to be addressed industry-wide or in the AMS standard. The gentleman from Sweden got away from cast-steel shot completely because of this so it would seem we are not alone. I would like as much input on this as I can get.

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#5 - 04/08/10 03:21 PM Re: Conditioning New Cast Steel Shot
Pete Bailey Offline
Member

Registered: 01/12/05
Posts: 9
Loc: Hamilton, OH
You probably realize that the AMS2431/1 shot is hardening as you use it - that is why intensity is increasing. The two suggestions are correct(1) condition the shot before using it on parts and (2) establish an allowable pressure (and shot flow)range as part of your fixed process.

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#6 - 04/08/10 04:24 PM Re: Conditioning New Cast Steel Shot
Walter Offline



Registered: 05/03/99
Posts: 158
Loc: East Hartford, CT, USA
Gary,

I'm not asking you to change your process only adjust it within the Frozen limits.
Typically PSI has a +/- 5 PSI, a similar tolerance would apply to shot flow in pounds per minute. You machines should have an interface that allows the operator to make these subtle changes. Just to be clear, what I'm saying is of your frozen process calls for 50 PSI +/- 5 you can set your machine to run at 50,51,52..... as long as during the process is never drops below 45 or goes over 55. You have to have this flexibility, your machine is physically changing all the time as all peening machines do. Example your blast hose and nozzles get larger with use. Obviously at some they would need to be replaced as part of your preventive maintenance plan. Also, temperature and barometric pressure can cause variation.

As for the AMS 2431/1 shot in your machine. Your right it does work harden with use thatís why we don't use very much of it at all. Your used shot could possibly now be AMS 2431/ 2 shot. Have you had the suspected shot in the machine hardness tested?

Why did you choose AMS 2431/1 shot? I'm guessing cost, as very few OEM's required regular hardness shot. If at all possible I would switch to AMS 2431/8 Conditioned Cut wire shot. Although it is more costly up front it lasts many times longer. Also, CCW never fractures it just gets smaller with use. It's hardness range by specification is 55-65 just like AMS 2431/2 however I have never seen it vary by more than 3 points. Meaning it's much easier to add to your machine since it hardness variation is considerably tighter than cast steel shot thereby making saturation curves easier to reproduce.

AMS 2430 is currently being revised. The intent is to change figure 1 and 2 to show only acceptable shapes anything else would be bad.

What type of shot classification system does your machine have? If you have just a size classifier your may have problems removing some non-round particles. You need to run the shot through a spiral device to remove these.

Feel free to call me to discuss if you need to. I sent a reply to you via this website with my contact info.

-Walter

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#7 - 04/08/10 04:46 PM Re: Conditioning New Cast Steel Shot
Socrates Offline


Registered: 02/20/06
Posts: 108
Several issues can be identified:
1 A "frozen process" is only effective if all of the parameters remain virtually constant. The "frozen process" parameters should not have been established using new shot. Shot properties change with use. These include a reduction in average shot diameter and 'conditioning' of the particles. The process parameters should be re-established using a shot load that is 'stable'. A stable shot load is one that is screened regularly and has regular, small, additions of new shot trickled in to replenish shot losses. The new shot additions may, or may not, require to be pre-conditioned.
2 There is a lack of information presented about how the 'intensity' changes are being measured. Are the stated changes for complete saturation curves or are they simply changes in the arc height for a fixed peening time? If the changes are detected using complete saturation curves then which method is employed to determine the 'peening intensity' point from the set of data?

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#8 - 04/08/10 04:55 PM Re: Conditioning New Cast Steel Shot
Joe McGreal Offline
Member

Registered: 12/12/08
Posts: 5
Loc: Monticello, Iowa
Gary,
After reading your post, it sounds like a problem related to the size control of your shot. I don't think it's a "cast shot" problem. Most intensity problems can be traced back to shot size control.
You stated, after two hours the intensity increases in some of the machines? The physics of impacting shot particles is a continous reduction of size and mass (intensity). The increase you describe is counter to the known endurance of the shot particle's size.
Preconditioning of shot is a best practice for removing oxide and fugative particles, however I don't think it will assist in solving your problem.
You mentioned shooting shot against a hard steel surface for two hours and noticed the intensity increasing. How many passes is two hours of blasting?
Some machines are having this problem and not others? (This would indicate possibly a size control or velocity issue.)
Don't rule out Almen strips....are they all from the same manufacturer?
Do you do screen analysis of the shot mix and have new shot additions highly controlled.
What controls are there on pressure and shot flow?

Cast Shot does not work harden more than 1.5 Rockwell points, so it would be difficult to to have a measurable intensity gain.
As you know, nozzle wear can change velocities as well.
I hope this is helpful,
Joe

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#9 - 04/09/10 02:41 PM Re: Conditioning New Cast Steel Shot
Gary J Offline
Member

Registered: 03/24/10
Posts: 5
Loc: Corpus Christi, TX
Wow! It is great to hear all this input! Thank you all very much.
With regard to working within the frozen process: We cannot alter the CNC programs because 1) Our programs cannot be changed at all without authorization (even within the +/- 10 psi) 2) The frozen process documentation would have to be altered and that requires alot of red tape. Correct, that frozen plans should not have been made with new shot-I agree 100%. Hind sight is indeed 20/20. I belive this is the root of our problem. With regard to the shot: It is being delivered and screened and checked with the latest technology (computerized eveverything). The machines have built-in screen classifiers and do an excellent job of keeping the shot free from bad particles. My thinking is that new cast steel shot has numerous pieces with nodules stuck to them, hollow pieces that look like spherical welding slag, and twins. As the pieces impact a work surface the nodules break off, the twins separate, and the pieces work harden, the intesity climbs. Like a baseball covered with clumps of mud not having as much impact energy as a clean ball. The hoppers are stocked with an extra 1-200 lbs at all times so the machine can grab some if it needs it. However it never grabs more than about 100 lbs at a time so we should never have to worry about too much new shot being added and more conditioning needed.
The specs we use all call for cast steel shot and getting the specs changed to anything else is not possible. Yes, we are without any wiggle room as you all can see. We have been forced to alter the frozen plans here and there but that is a process.

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#10 - 04/09/10 04:51 PM Re: Conditioning New Cast Steel Shot
Walter Offline



Registered: 05/03/99
Posts: 158
Loc: East Hartford, CT, USA
Gary,

You might find this interesting

http://www.shotpeener.com/library/pdf/1999090.pdf

Are you working to an OEM specifcation such as Sikorksy or are you working to DMWR?

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#11 - 04/09/10 05:10 PM Re: Conditioning New Cast Steel Shot
Walter Offline



Registered: 05/03/99
Posts: 158
Loc: East Hartford, CT, USA
Is your air pressure tolerance really +/- 10 PSI? If so that may very well be your problem.

I would suggest you change your air pressure limits to +/-10% from nominal across the board on all your machines.

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#12 - 04/12/10 09:39 AM Re: Conditioning New Cast Steel Shot
Joe McGreal Offline
Member

Registered: 12/12/08
Posts: 5
Loc: Monticello, Iowa
Gary,
Martinsitic Cast Shot does NOT work harden more than 1.5 HRC. The AMS Range for new shot is 7 HRC points, thus a minimal gain during life is not likely the cause here.
Air pressure, size and flow control limits are the reasonable issue here.
Joe

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#13 - 04/12/10 11:29 AM Re: Conditioning New Cast Steel Shot
Alberto Rodriguez Offline
Member

Registered: 03/18/09
Posts: 1
Loc: Corpus Christi Army Depot
Gary:
Adding small portions of new shot to the existing batch should not have a significant impact in Intensity variation. Even though AMS 2430 and AMS 2431/1 do not cover media conditioning, you are not prevented from having an internal conditioning procedure when a batch is completely replaced with new shot. We will certainly recommend to the committee adding a paragraph to the AMS specs related to this topic. Conditioning of new shot will add quality value and consistency to the process.

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#14 - 04/13/10 02:59 PM Re: Conditioning New Cast Steel Shot
Gary J Offline
Member

Registered: 03/24/10
Posts: 5
Loc: Corpus Christi, TX
Thank you for the link, Walter. I did mistakenly say that we were using a +/- 10 psi range but in fact we are using +/- 10% on air pressure. The machines maintain a very even pressure and are maintained and tested by the manufacturer. I have no reason to belive that air pressure is the problem. The screens kick out any under/over size pieces and our shot checks always turn out very good. It seems that when our new machines were installed we just did not know about this variable.

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#15 - 07/05/10 10:36 AM Re: Conditioning New Cast Steel Shot
Marcel vW Offline
Member

Registered: 07/02/10
Posts: 3
Loc: Schiphol Airport
Gary,
Why not trying to get a substantiation for switching over from Cast Steel Shot towards SCCW shot (high hardness) ? I also had always lot's of problems with my newly bought Cast steel shot and went over to SCCW shot and got full approval of the Aircracft Engine OEM to use it always where-ever Cast Steel shot was mentioned. Since then (1994) we never had any issues anymore around shotmedia quality and related shotpeen-problems with our computerized shotpeen machines and shot sieve systems.
Marcel

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