AISI S7 help

[mrzoom]

We have been using this type of material treated to 42RC for a cylinder end cap used on a manual 17.5 ton riveter.

I'm having a hard time looking for material properties of this type.

any input is greatly appreciated

thanks,

 

[btrueblood]

AISI S7 is a shock-resisting tool steel, capable of air-hardening. From

http://www.matweb.com,

for Allegheny-Ludlum S7 Tool Steel (UNS T41907):

Component Wt. %
C 0.5
Cr 3.25
Fe 94
Mn 0.7
Mo 1.4

Machinery's Handbook lists same info., plus shows hardening temperature 1700-1750 F, Tempering range 400-1150F, Approx. tempered hardness 57-45 Rc.

Yahoo search gives:

http://www.edro.com/edro7.pdf

and

http://cartech.ides.com/datasheet.aspx?i=103&e=114&c=TechArt

This last datasheet gives a chart of ultimate & yield strengths as functions of HRC.

 

[metengr]

Crucible provides some technical information on their S7 tool steel, see the web site below;

http://www.crucibleservice.com/eselector/prodbyapp/tooldie/crus7t.html

Are you looking for an alternative to using S7 tool steel or more information on this alloy?

 

[mrzoom]

Yes I'm looking for an alternative.

we use this material for an end cap of a 17.5 ton cyl.
they break at the threads even with a radii on the minor dia.

I was thinking of changing the heat treat higher than 42rc

what would you guys suggest? We put through 4500 psi some pressure spikes of 4900 psi.


Thanks to all for the links.

 

[metengr]

mzroom;
Based on your description, it sounds you have a notch sensitivity problem with your current material. I would suggest a quenched and tempered AISI Type 4340 alloy steel. This material has excellent hardenability, good fatigue resistance and fracture toughness in a quenched and tempered heat treat condition.

If you don't need the hardness for any kind of wear surface application for your end cap, I would consider a hardness range of 35 to 38 HRc for this material to achieve optimum fatigue strength and fracture toughness.

 

[rd400guy]

When it comes to tools (dies, punches, etc.), breakage or chipping is ususally a toughness issue. Deformation (mushrooming) at the working surface is a hardness isssue. If the breakage is a result of deformation, then it's hardness. S7 is just about the best tool steel you can buy in terms of toughness. At 42Rc, you should have excellent toughness.

Here comes the barrage of questions:
-Is this a recurring problem, or an anomaly?
-Is this riveter a new design or something that has been in use for a while?
-Is the riveter unique to your company/application, or are there similar machines in use elsewhere, if so do they see this type of failure?
-Did you do any metallurgical investigation of the failure?
-Is the part double tempered?

Here's the biggie:
-Are you cutting threads after hardening, and if so did you do a stress relief?

Depending on the metal producer and manufacturing process used, there may be non-metallic inclusions from which failure originated (the anomaly). There may also be unusual stress risers inherent in the design, or unpredicted forces acting on the part. Perhaps the configuration has to be "beefed up" to deal with unknown/unconsidered forces. Do you have any pictures of the failure?

 

[mrzoom]

4340 isn't that p20? if so, tried that in the past and did not last as long as the s7.

We break these caps every three months.(avg.)

riveter is of old design. as much as I would like to go to the new and current design politics and cost involved just ain't going to work. We have been using this design since the start of the product line.

I have four of these in manual and four of these in auto.
all fail in the same area but different intervals as the manual sees more.


The use of this mat'l (s7) is due to the mat'l investigation.

Double tempered? I will need to find out. common term used is "drawn twice" here. definition of double tempered is ?....


this cyl end cap is purchased item. So I will need to find out about the stress relief after thread cutting

I don't have a picture to offer. but before the S7 mat'l they would just snap at the threads. this S7 cap shows a crack inside where the piston rod goes through and where the break point is.

 

[israelkk]

To add to this thread, I suspect that the cylinder cup is suffering of impact loads (from the piston?) add to this the stress concentration from the thread and you probably has very high concentrated stresses that leads to fatigue. A metalurgical inspection can reveal it and some times by naked eye if you can see the beach lines. Did you do a stress analysis? From such stress analysis you can estimate life cycle to see if it matches the three month of operation.

For a stress analysis you have to know the actual dimensions of all parts involving and the forces/impacts acting on the cup.

 

[metengr]

P20 is a tool steel, and is not the same as AISI Type 4340. In reviewing your additional information, the design of the end cap is like an end cap on a small pressure vessel. The tool steel may have the toughness and hardness but the notch sensitivity I was alluding to earlier related to fatigue notch strength not impact strength or toughness.

You need to consider a material that exhibits good fatigue notch strength because of the number of pressure cycles that the end cap is exposed to in your rivet gun design. The confirmation of the failure mechanism thru metallurgical analysis is an absolute must, as pointed out by rd400guy.

 

[mrzoom]

just to add to this thread.

We have suspect rivets. They are coming in harder than what they should be.

thanks for all the input