440 C SS corrosion 4

[Indicator1]

This part has been passivated per AMS-QQ-P-35, TYPE II.
It is a shaft .025" OD. Hardness: 580 to 820 HV under 500g load.
It appears it has some surface corrosion. Microscope observation shows small pits. Corrosion product is black.
This shaft works together with a bearing, which is made of brass, CDA 36000, half hard.
What can be done to eliminate/decrease this corrosion, please?

 

[metengr]

What are the mating conditions? Lubricant? Temperature? Are the corrosion pits in a line or random and where in proximity to the bearing?

 

[Indicator1]

There is NO lubricant.
The distance between the bearing and the shaft is .001" each side. However, the load is very low.
Temperature: mostly room but can be from -65C to +125C.
See the picture attached.

 

[israelkk]

440C is a martensitic stainless steel and have very low corrosion resistance compared to austenitic stainless steel such as 302, 304, 303. The contact with a copper alloy makes a bad match. I would use a heat cured coating per MIL-PRF-46010G

http://everyspec.com/MIL-PRF/MIL-PRF-030000-79999/MIL-PRF-46010G_17101/

8-12 Micron thick which is a dry lubricant. If you can not use a coating from some reason I would make a pin from Custom 455/465 H1000 or H950. 580HV is 53.7 RC so the Custom will do. 820HV is 63.3RC which makes the pin too brittle.

The range of temperatures you mention indicate, Military, Aerospace or Space use.

 

[EdStainless]

My hunch is that the pits were there from HT, and just hidden by the surface finish.
This material will rust in a humid environment.
You might even get the strength that you need from 17-4PH, but custom 455 would also be a viable option.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[israelkk]

EdStainless

The reason I offered Custom 455/465 is to get the minimum hardness hardness and toughness together with corrosion resistance. I am not sure you can have both hardness and toughness with 17-4PH. The selection of 440C at 63 RC is too brittle to have toughness.

 

[EdStainless]

C360 half hard will have a UTS of about 50-60 ksi, this will only be RB80 or so.
I cant see any reason to mate it something harder than about RC40.
455/465 don't really have great corrosion resistance, hence my preference for 17-4PH H1025.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[Indicator1]

My problem is this shaft is a part of a magnetic circuit.
I have no data for 440 C, or alloys you recommended.
Are they comparable in regards of BH curve? Or comparable in regards of Coercivity and Residual flux density?

 

[Indicator1]

To EdStainless

I understand HT is heat treatment, correct?
If this material is tempered above 370C, Chromium carbide will precipitate, and it is not Stainless any more.
Is that what you mean?

 

[EdStainless]

Indicator, speaking of 440C, yes. A lot of the Cr ends up as carbides in the final condition and not helping corrosion resistance.
How magnetic do you need it? AC or DC? What saturation level? All of these alloys will have fairly high coercivity, after all Cr steels similar to 440 used to be used for permanent magnets.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[Indicator1]

Edstainless,

It is a DC application. No, it will be not saturated. It is a part of the coil core.
There is another part of the coil core, which plays a major role. Shaft plays a minor role in the magnetic circuit.
Sorry, I have NO data on 440C magnetism.
If two materials you suggested are somewhat close to 440C in regards of coercivity and residual flux density, then I will try them.
Otherwise I will use dry lubricant, suggested by israelkk.

 

[israelkk]

Indicator1

The magnetic issue complicates things. In one hand you use the 440c as bearing shaft which needs high hardness 53-63RC. On the other hand, stainless steels for magnetic use are martensitic stainless steels from the 400 series. Therefore, are not really good for corrosion as austenitic stainless steels that are non-magnetic. It seems the designer wanted two properties that doesn't go together for a robust design.

 

[Indicator1]

Israelkk,

Yes, I have the same understanding of this situation.
I am reading the MIL spec you recommended for the lubricant.

 

[btrueblood]

Could you replace the C36000 bearing element with a sintered (porous bronze or "oilite") bearing with oil or ptfe impregnation?

 

[EdStainless]

I have rough magnetic info, stress rough.
It looks like Br for both would be roughly 5-7kGauss.
Hc is another matter, I find values for similar alloys ranging from a low of 20 Oe up to 100 Oe.


= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[Indicator1]

Israelkk,

You are right about Military, Aerospace application.
MIL-PRF-46010G, p.6.9 reads:
6.9 Aerospace Components – Types I and II lubricants previously found under MIL-L-0046010E are now under SAE Aerospace Standard AS5272. The SAE document still allows and use lead material to meet aerospace application requirements. For other than aerospace applications products under MIL-PRF-46010 shall be used. (Use of MIL-PRF-46010 in aerospace applications must first be validated.)
It looks like this dry lubricant is NOT for Aerospace application.

 

[israelkk]

I just wanted to guide you to solid film lubricant mainly MoS2 type. They change the specs all the time. There are other dry lube coatings applied by sputtering etc. I am not sure if you product is for space or aerospace. Each has its rules such as outgassing. If it is for optical systems heat cured MoS2 dry lube may be an issue.

 

[Indicator1]

My product is for aircrafts.
Do I start with MoS₂ application? Any documents/MIL standards on that, please?

 

[israelkk]

Look for Molykote dry lube products from Dow corning. Try here:

http://www.globalspec.com/search/products?page=mi#sqid=20233279&comp=4305&vid=178779

You can call them.

 

[EdStainless]

I have also used PVD WS2 (tungsten di-sulfide). This worked very well.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube