I was quoted this morning by my local machine shop to replace the corrupted 5/8"
chrome plated steel piston rod. The machine shop confirmed it was not bronze. $380 plus tax. Holy crap, chrome plated steel. No wonder the pitting
@Agro ! Anyway, I said ok, please replace the piston rod, but only if you can do it with stainless steel as opposed to the original chome plated steel.
I like your local machine shop already. Although few of us will actually declare on a public forum the material specification of a gas piston rod for obvious reasons your machine shop is giving you sound advice.
However chrome plating of piston rods is a process better suited for hydraulic oil lubricated applications such as you see on hydraulic rams on construction machines hydraulic fork lifts and such like.
The hydraulic oil being an excellent lubricant for the contact area between a slow moving piston against a static polymer plastic sealing ring and despite the corrosion pitting of your piston rod in question the parent metal corrosion protection of the exposed ram in say a construction environment is of upmost importance.
Now the rod loading is another headache and as the highest loads are on these big construction diggers so much so that stainless is totally out of the question. So for the parent metal a higher yield strength is required say 350 to 700 Mpa together with a high'ish tensile strength say 500 to 800 MPa
Surface hardness is also a consideration say 45 to 50 HRC as is hardness 200 to 300 HB
So I've limited your 3500 available steels now to around half a dozen
42CrMo4, 4821H, and 280X for good measure as its American are 3 of them
Then you add a corrosion resistance with say 20 microns of chrome as long as you don't have any fine threads.
Out of interest I designed a back pressure regulator for oxygen use offshore on deck By application 316 stainless was out due to Oxygen but bronze was approved by corrosion and oxygen inertness however oxidation consideration went to a nickel bronze while cost and the bean counters went for a controlled 3 micron nickel plating due to the required fine M20 x 1.5mm control thread. Balance is key.
However in your application although a thickness of 20 microns is fine while the surface finish or roughness is more critical hence your local machine shop advocating a chrome rod where the surface roughness will be around > 0.2 Ra (μm) micron and with a Chrome hardness of around 850
you can clearly see these guys are proper engineers and know what they are taking about.
You be contrast don't have that lovely oily lubricant sliding stuff over a mirror smooth finished rod at high loads but at very low velocity speeds.
No your out in the back yard with oxygen as dry as a bone jamming gas at the temperature of the Sun and at a speed that light would be proud of. Creating friction a Boy Scout on crack would envy and compressing a gas known to kick off with a light show of sparks smoke and noise the Grateful Dead would enjoy.
In a nutshell your screwed ring Kim and get that specification. Then come back or go back to your local engineering team explain fully what your doing dry gas oxygen no lubricant high temperature 5/8 low rod loading and pay them $380 dollars and add $50 for a couple of beers. You may be pleasantly surprised
I do hope so.
