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“With” meaning involving, not in. My coffee maker isn’t draining properly anymore, so the drip basket is overflowing. The spring that lets coffee flow out at the right rate seems to have stiffened over time. But how could that happen? Could the repeated heating (the coffee flows out over the spring) and cooling (after the brew is complete) over three years of daily use have done something similar to quenching, and so stiffened the spring? Or is something else going on?

The question: I do want a working coffee maker, and so troubleshooting and repair suggestions are appreciated, but my question is more from curiosity about what potential physics (if any) could cause a spring to stiffen under repeated heating and cooling over a long period of time.

More details: the plunger that closes the drain when you take the pot out to sneak a cup before brewing ends, and that opens it otherwise to let coffee drip into the pot doesn’t seem to be working properly: it’s not opening enough when the pot is seated under the coffe-maker. That’s what seems to be the problem. This is visibly clear as the basket itself isn’t sitting completely flush into the space it’s supposed to sit down in, as the plunger spring is pushing the basket up a little rather than letting the plunger rise fully and the drain hole open completely. This isn’t normal and is a recent thing (as in the last couple of days).

Update: it’s definitely not clogged. I just held the plunger up with my fingers and ran water in the basket under the sink at a flow rate that is a good bit higher than the fill rate into the basket by the coffee maker, and it ran through just fine without backing up at all. In fact, it took a lot of work to get water to back up. It seems to only happen when the plunger barely rises at all and the drain hole is barely open. Which is what happens when I put the basket in the coffee maker, since the spring seems to be so stiff that instead of opening the drain hole it lifts the whole basket, keeping the drain hole closed or almost totally closed.

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Microstructural changes in metals not subject to stresses above yield are thermally activated, meaning that the reaction rate depends exponentially on the absolute temperature. At the temperature of hot coffee, the rate is so low as to be negligible and therefore the mechanical properties of the spring (which is made from stainless steel so the coffee will not corrode it) will not change with time.

Furthermore, even if the springs was getting annealed by the hot coffee, its strength would then go down, not up.

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It is most likely clogged with grinds or some build up, have you taken the plunger assembly apart to clean it? Can you push the plunger up and down? It will work without the plunger, it is only there to stop the flow when you take the pot out from under it. Make sure the pot still pushes the plunger up when it is under it.

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  • $\begingroup$ I can push the plunger up and down, but when I put the basket down into the top of the coffee maker, instead of the plunger raising and the drain opening, the whole basket gets pushed up by the spring and the drain hole stays closed. With grounds loaded in the basket it weighs it down enough to open the drain hole a little, but water is definitely flowing in faster than it can exit. So I think it’s the spring. We just cleaned the maker thoroughly so it should be a clog. $\endgroup$
    – bob
    Feb 9, 2020 at 2:41
  • $\begingroup$ If nothing else works you can remove the plunger assembly, then just leave the pot under it until it finishes brewing. 20 years ago or so coffee makers didn't have these plungers. $\endgroup$ Feb 9, 2020 at 2:52
  • $\begingroup$ Good point, thanks! Though also just curious about why the spring (apparently) stiffened and whether it could be the heat from the coffee. $\endgroup$
    – bob
    Feb 9, 2020 at 3:02
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    $\begingroup$ Another possibility to investigate: is the lid of the coffeemaker closing completely? If not, the basket won't be pushed all the way down so the drain won't open properly. I would consider all other possible causes first before believing that a spring would get stronger as it ages! $\endgroup$
    – S. McGrew
    Feb 9, 2020 at 3:24
  • $\begingroup$ Generally this type of use might slightly weaken the spring, hardening quenching involves heating and then rapidly cooling to reduce the crystal grain size of the metal. It may be that the lid or the drip arm over the basket is not holding the basket down enough, since this is a recent development. $\endgroup$ Feb 9, 2020 at 3:29

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