てつきめん[Tetsu-kimen] is correct, as it is written in the furigana.
In a compound word, つ at the end of the first kanji changes to っ when the first consonant of the second kanji is k, s, p or t/ch, since it's easy to pronounce. (When the first consonant of the second kanji is h/f, it changes to p, so つ preceding h/f also changese to っ as a result, as in the last two exapmles below.)
骨格: こつかく(kotsu kaku) --> こっかく
喝采: かつさい(katsu sai) --> かっさい
沸騰: ふつとう(futsu tō) --> ふっとう
血中: けつちゅう(ketsu chū) --> けっちゅう
発表: はつひょう(hatsu hyō) --> はつぴょう(hatsu pyō) --> はっぴょう
熱風: ねつふう(netsu fū) --> ねつぷう(netsu pū) --> ねっぷう
However, when the word is considered to be made of two words, つ is pronounced with no change. For instance, 熱放射 is ねつほうしゃ, not ねっぽうしゃ since it's 熱 + 放射. This is the reason why that's てつきめん. (As for 鉄仮面, it's read てっかめん because it's treated as one word. てっきめん would be probably acceptable when it's actualy pronounced.)