301 SS loosing strength 1

[scotteekay]

Hi, I have a tactile type dome, which is made from 301 SS. The dome is rated to activate at 800 grams. After limited cycling and exposure to temperatures range of -40 to 140 c the dome looses some of it's strength and now activates at around 500 grams. How can i either, pre "fade" these parts, or prevent the parts from dropping in activation load? My supplier has limited suggestions, and my metallurgist is great for theory, but has limited commercial experience. Thoughts, suggestions? Alternate materials? Best regards, Scott

 

[metman]

At 800 grams, what is the stress?

What is the temper of the material as received? 1/4, 1/2, 3/4, or full hard?

Design for RELIABILITY, manufacturability, and maintainability

 

[TVP]

Also, how is the dome manufactured? Sheet hydroforming? Progressive die stamping? Are the parts heated after forming? Time and temperature? Stress relaxation is one possibility to explain what is happening, but we need more information.

 

[scotteekay]

Hi, and thank you for the reply. I have been away from this project for a bit, but back into it full time now.
The material is .006 thick 301 SS full hard, according to our supplier. Dome is stamped, and there is currently no heat treatment afterwards. We have tried running some heat trils in our model shop furnace. See attached data. First we collected data on sample virgin parts. next we soaked the parts @ 600 F for 30-45 minutes, and allowed to air quench. We next took data on the treated samples. lastly, we ran parts in life test simulation and recorded data when complete.
Findings:
1 The heat treat did cause some relaxation. We noticed a drop in max activation force, loss of total travel ( dome lost some of it's height" and a reduction in the dome differential ( the ability for the dome to invert with applied pressure)as seen by the flattening of the "bell" shape of the curve after treatment.
2. Life testing showed additional drop, although i must say the actual reduction from virgin parts was only about a third of what we were seeing without the heat relaxation, so it definately is helping.
3. Still we desire the nice bell shape curve for the operation of the device, and have created a proposed load curve for our supplier to run to with the hopes of heat soak, and life cycling will drop the curve down to the area we need, such as illustrated by the OEM and import curve.

We have also seen some tactile domes of Brass which do not see the same load loss in part function. Go figure?
Open questions are:
1. proper heat soak temps and times for stress relief?
2. Matrial temper? is full hard where we should be?
3. How would the stress relaxation compare as it relates to temper of material?

If you have any comments, ar additional info, i'd appreciate your time and effort to reply.
Best regards,
Scott

 

[mcguire]

With this material you can get hysteresis effects, which is not found in the literature, in the temperature range you cite. This hysteresis is manifest as a volume expansion on the order of 0.1%. The material grows with a time lag in a sense opposite thermal expansion in the -40 to 140 C range. You can nearly eliminate this effect by tempering at 600 C. It may be a factor in your dome load/deflection performance.
The effect is too complicated to go into here, but I would be happy to share my un-published data privately.

Michael McGuire

http://stainlesssteelforengineers.blogspot.com/