SCC of PH17-4-700H

[GarethMcGrory]

Hi folks, I have been doing a lot of research into using this precipitation hardened stainless steel in a heat treated condition (1150oF)to eliminate hydrogen embrittlment and also to reduce SCC susceptibility in chloride environments (sea salt/water chloride environments). I will be manufacturing the small component using MIM, metal injection moulding.

What i would like is an SCC standard test to determine susceptibility of this particular stainless steel. I have come across ASTM G36 which involves the use of boiling magnesium chloride solution.

However im not sure if this standard covers sintered components.

Does any one know of or could help me determine what is the best standard to use to test for SCC using sintered martensitic stainless steels?

 

[Maui]

I'd suggest reading reference number three from the following paper:

http://www.dshtech.com/PMWC2008174.pdf

Maui

http://www.EngineeringMetallurgy.com

 

[GarethMcGrory]

Many thanks Maui, this is an interesting document you provided me with in terms of the sintering atmosphere and temperature that optimises corrosion resistance of PH17-4.

In terms of SCC of this precipitation hardended stainless steel, do you have any recommendations for a chloride type environment other than ASTMG36 test which involves the use of boiling magnesium chloride solution?

Is there a sodium chloride type immersion test which can also be used (preferentially an ISO, BS or ASTM standard test)? Reason being i have used the sodium chloride + hydrogen peroxide ASTMG110 type test for aluminiums to check for susceptibility to SCC.

Any advice or help is greatly appreciated and acknowledged

 

[EdStainless]

There really is not a standard SCC test. You need to design one appropriate for your material and application.
My first thought is that boiling MgCl is too severe for a PH grade.
You need to be careful with temperature, while raising the test temperature is an easy way to accelerate the testing you can over do it and change the failure mechanism.
You might want to use a less aggressive salt, such as 33% LiCl.
How are you going to apply the stress? Can you load the parts more than they will see in service? This would be a good accelerator also.
Since these are MIM parts what is their density? internal crevice corrosion could happen faster than SCC. This will need to be one of the first things that you look at.

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[GarethMcGrory]

Sorry folks for just replying now, been rather snowed under with work!

Yes i understand that SCC testing can be a difficult thing to guage as other corrosion mechanisms may take place also, making it hard to distinguish unless an expert in the filed of corrosion.

stress will be applied by basically clamping the components as they they will be in service at worst case claping force condition (3000N axial compressive)

As you say, i can clamp them in to extreme load cases/

The density of the of the parts shall be greater than 92% ensuring impermeability to gases & liquids and to prevent porous joins.

Ideally i will want the parts as close to 100% dense as possible however this will involve higher cost.

WHat other materials could i consider which are cost effective, mass manfacturable, yield strength greater than 300MPa, excellent corrosion resistance and that are not susceptible to failure mechanisms such as SCC or hydrogen embrittlement in chloride type environments (sea water air etc). All cosr effective material suggestions would be greatly appreciated. Ideally i do not want secondary processing or plating/coatings on top of base material.

 

[EdStainless]

1. What heat treat aging condition do you plan on using? The H1100 and H1150 have significantly better SCC resistance than lower aging temps.
2. Compressive load? unless there is is tensile or bending load they will not fail by SCC. There is no force to open the cracks.
3. 300MPa is only 44ksi, not very strong.
Is this a very complex part, and hence MIM?
Could it be machined?
There are lots of alloy options, Nitronic 50, any 6%Mo superaustenitic, 2205 or any duplex SS, are three that come to mind quickly.

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[GarethMcGrory]

Hi Ed,

I had planned to use H1150 as i understand this increases the resistance to SCC to high, it also removes failure by hydrogen embrittlement.

Yes i understand tensile forces are required, however even in compression, tensile loading cannot be ruled out.

300MPa is sufficient for the component in question, the axial force experienced is in general 3000N.

Its not extremely complex, however currently i use a die casting process to produce my component. The problem being the alloy undergoes ambient creep which is undesirable. Therefore i have moved my focus towards stainless for corrosion resistance & no ambient creep.

On first thoughts it seems MIM is relatively expensive with an initial quote much higher (10X) than diecast.

The part could be machined, however i will be producing millions of these components, therefore i need a material/process geared towards high volume mass production.

All thoughts welcome folks.

 

[EdStainless]

Could it be stamped in a fineblanking die?
Any of the alloys that I listed will have decent strength and SCC resistance.
I think that you have overkilled both the alloy and the process.

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