dnd 5e – Is this homebrew version of Glassteel balanced?

This spell has (almost) no mechanical benefit to balance

The only time that casting this spell would have any mechanical benefit would be when you needed an item made of steel and for some reason only had access to a glass version of that item. Given that such a glass item is at least as difficult and expensive to create or acquire as the steel version and is certainly much rarer in practice, casting this spell will likely never have any mechanical benefit. We typically evaluate “balance” by weighing the cost (in this case, a spell slot, and possibly a spell selection) against the benefit gained. But if the benefit is purely the subjective cool factor of having a glass weapon or tool that’s functionally identical to an ordinary metal version, how can you balance the cost of a spell slot against that? It’s certainly fine to have “flavor” spells, but without any significant mechanical benefit, there is nothing to balance, unless you have some other criterion you want to use to define balance.

A few existing spells and abilities are similar to this

For what it’s worth, there are a couple of spells or other abilities with somewhat similar effects, which you might be able to use or adapt for your purposes. First, there is the druid cantrip Shillelagh, which buffs a single weapon at a time. One approach you could take is to have this spell be a cantrip that works similarly, being limited to a single glass item at a time.

Second, the wizard school of Transmutation includes the Major Transformation feature, which can (with some favorable DM rulings about relative mass and value of glass and steel objects) transmute a glass object to steel permanently. You could allow the ability to instead give the object the physical properties of steel while retaining the appearance of the original glass. While not RAW, this would be a purely flavor change to the ability as written.

There is also the warlock Pact of the Blade:

You can use your action to create a pact weapon in your empty hand. You can choose the form that this melee weapon takes each time you create it (see the Weapons section for weapon options).

The reference to the weapons section of the PHB makes it clear that choosing the “form” of the weapon means that you choose which kind of weapon you want to manifest. But most DMs will also allow the weapon’s appearance to be customized, or base it on the nature of the warlock’s patron, rather than have it simply manifest as a perfectly ordinary-looking weapon of the chosen type. So a glass-looking weapon that functions the same as a steel one is certainly within the realm of possibility here, given the right DM, and the right patron.