I filed two W2 forms. There's no federal income tax withheld on either, but I have social security tax and medicare tax withheld on both of them. Don't I get a refund?

Social security and medicare taxes deducted from your paychecks are not withholdings, but are your contributions to Social security and Medicare. They are not refundable.

If you do not have federal tax withholdings, you do not get a refund, unless you have a refundable tax credit such as the Earned income tax credit.

And if you don't have any federal withholding taken out you might have a tax due if you made over $12,200 total income.