Home Forums General Discussion Ask Dr. Peener Shot Peen Before Chrome Plate

I am designing a couple of linear actuators for the aerospace industry with PH steel rods. I will be requiring chrome plate on each for improved wear resistance. My shotpeening experience is paltry, so I want to make sure I am stipulating requirements correctly/appropriately. The materials and heat treats for each rod are as follows:

Rod #1: PH13-8Mo, H1000 (43-47 HRC)
Rod #2: 15-5PH, H1025 (34-42 HRC)

For each, I envision the following process:

1. After grinding OD, stress relieve at 375F for 3 hours

2. Shotpeen per AMS-S-13165 using ceramic bead, .006-.010A (PH13-8Mo) .012-.016A (15-5PH), 100% coverage required.

3. Chrome plate per AMS-QQ-S-320, Class 2, Type I, .003-.006 inch thick.

4. Hydrogen Embrittlement Relieve at 375F for 8 hours (13-8) 3 hours (15-5) (per AMS-QQ-C-320)

5. Final grind

6. Stress Relieve at 375F for 1 hour.

Question #1
Is the shot peen callout above sufficient or is it recommended that I callout a specific bead size?

Question #2
Since PH13-8Mo, H1000 is >200ksi UTS and >.375 in diameter, I chose .006-.010A. Since 15-5PH, H1025 has a UTS of 155ksi and >.375 diameter, I chose .012-.016A. Both intensities are per Table IV of AMS-S-13165. Is this appropriate?

Question #3
Where in AMS-S-13165 is "% coverage" addressed (if at all)? I have used this terminology in the past, but only because I learned from others. I don't see any references to >100% coverage. What is the reason for requiring >100% coverage? What does it even mean? And how does it help/hurt?

Question #4
Do any of the stress relief bakes have an adverse effect on the shotpeen (i.e. roll back the residual stress)? I believe that 375 is a safe temperature for PH steels, but I want to be sure...

Question #5
Is AMS-S-13165 the appropriate spec to use? What's the difference between it and AMS2430, which I am not familiar with?

Thanks for all your help!

Mike Boland

Q1: Yes, you should specify a bead size. I have no experience with ceramic bead so I cannot advise on a size; however, you may not be able to achieve 12-16A on 15-5PH without a lot of media fracture.
Q2: Yes, intensities selected are OK.
Q3: Anything close to 100% coverage will be adequate to your purpose.
Q4: Not only will the stress relief bake at 375 (step 6) not affect the shot peening, it will affect nothing at all and I recommend you eliminate it. You should also eliminate the stress relief bake in step 1. Not so the hydrogen embrittlement relief in step 4. It is necessary for the purpose of hydrogen content reduction and it will not affect the shot peening residual stresses.
Q5: Either AMS 2430 or AM-S-13165 is appropriate to your use in this matter.

John,

Thanks for your advice. The reason I included the stress relief after initial grind was because QQ-C-320 advises so:

QQ-C-320, paragraph 3.2.2:

“All steel parts having an ultimate tensile strength of 150,000 psi and above, which are machined, ground, cold formed or cold straightened, shall be baked at a minimum of 375 +/- 25 deg F for three hours or more prior to cleaning and plating for the relief of damaging residual tensile stresses. When peening is required, thermal stress relief shall be performed prior to shot or rotary flap peening.”

I thought it was a bit curious because it is such a low temperature, but I added it anyway. Thanks again.

Mike,
The idea (and some requirements) for stress relieving after grinding have been around for a long time. It will certainly hurt nothing to stress relieve your parts. My suggestion for elimination in your case stems from work I have done on 4340 alloy steel which showed no effective stress relief will occur below 600F unless the time of exposure is very long, probably hundreds of hours. The temperature threshhold for stress relief in your PH stainless is likely to be much higher than for alloy steel.