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I have a bike frame that has been left outside for years, I tried to remove the stem by hitting the bolt with a hammer, using rust dissolver. I achieved nothing.
We use a torch and water where I work to ease nuts and bolts. I wondered if hot water from a kettle and cold canned air might do the trick for that quill.
Welcome to Bicycle Stack Exchange. A quill stem is typically held in place by the interference fit of a separate wedge-like piece that is attached via a long bolt through the quill stem. See photo below. You can access the head of the bolt on top of the stem. Tightening this bolt draws up the wedge causing it to start sliding by the bottom of the stem. The assembly then becomes "jammed" (fixed in place) in the fork steerer tube. When you loosen the stem bolt, the bolt will begin to rise up out of the stem as it moves up the threads of the wedge. The wedge usually remains fixed in the jammed position and nothing is loosened up but the bolt. When the bolt comes up a centimeter or two above the top of the stem, take a rubber mallet and give the bolt a good whack. Since the bolt is still somewhat threaded into the wedge, this blow will dislodge the wedge and free the stem assembly for removal. See this Park Tool tutorial on quill stems/threaded steer tube systems.
Corrosion in the steer tube involving the stem, wedge or both can increase the difficulty breaking loose the wedge. There isn't direct access to this area for good, direct heat application as it's within the fork steer tube which is within the bike's headtube. It's probably going to be more efficient to spray some penetrating liquid like WD-40 both down the hole where the long stem fixing bolt goes and up into the steer tube accessed via the bottom of it at the fork crown. It may be beneficial to turn the bike upside down to allow more penetrant to work around the stem/wedge/steer tube conjunction. Try to again bang on a partially inserted fixing bolt to try and free the wedge after several minutes of allowing the penetrant to work. Heat might work if you direct a heat gun's output up the bottom hole of the steer tube. Goal would be to expand the metal of the wedge and steer tube which may break the bonds of corrosion. Allowing it to cool for several minutes and reverse the metal expansion then trying to dislodge the wedge is all that should be necessary. It isn't necessary (and it'd be difficult) to heat/expand the steer tube while cooling/contracting the wedge at the same time. Heat can free a seized bolt or wedge when the friction dynamics are changed by the heat and subsequent expansion break loose some corrosive bonding. Sudden cooling isn't necessary because if it's (heat) going to work, it does so on the expansion/heating end of the process. Waiting for it to cool a bit is fine, but the work will be done. A seized bolt (and the surrounding metal) is heated to red-hot. Torque is applied almost immediately after the heat source is removed (and often during the heating process) as a means to further stress the bonds of the expanded metal. I'm not advising you heat anything on the bike red-hot but simply pointing out the way heat can work to free a seized fitting: it's less about changing the size of the components with heated expansion and cool contraction than it is changing friction dynamics by breaking bonds during the heat cycle.
Most forks for quill stems are open from the underside. Remove the wheel and the front brake if its bolt goes through the crown of the fork. Turn the bike upside down and spray your penetrating oil into the steerer from there. Give it plenty of time. Some heat (hair dryer?) might accelerate the process.
Keep water away. Sticking stems are mainly due to corrosion or electrolytic processes which water will only worsen.
My answer to the problem I faced:
First I loose the M6 bolt inside the quill a pair of turns.
Then, I did: 1-Turn the whole bike upside down without the front wheel and sprayed penetrating oil into the fork tube for a day. 2-Apply a lot of force on the handlebar to break the internal rust between the quill and the fork. 3-Hit the bolt's head with an extension and a hammer twice. 4-Repeat from 1 until the quill assembly got loose.
When the handlebar/quill duo got loose, I cleaned and washed everything, then relubed the whole assembly (including inside the quill tube).
