碳化三铁又称雪明碳铁及渗碳体（Cementite或Iron carbide），化学式为Fe₃C，为具有斜方晶系结构的间隙化合物。硬度很高，但脆性极大。在230℃以下具有弱铁磁性，而在230℃以上则失去铁磁性。不易受硝酸酒精溶液腐蚀，但受碱性苦味酸钠的腐蚀。主要由FeO与CO反应得到。

碳化三铁直接由白铸铁融化形成。在碳钢中，它可由冷却奥氏体（austenite，或称沃斯田铁）或回火（tempering）马氏体（martensite，或称麻田散铁）中获得。其与肥粒铁（奥氏体中另一组分）混合后形成具有层状结构的珠光体和贝氏体。其中的较大薄片曾用来制造大马士革钢。

Cementite, also known as iron carbide, is an interstitial compound of iron and carbon, more precisely an intermediate transition metal carbide with the formula Fe₃C. By weight, it is 6.67% carbon and 93.3% iron. It has an orthorhombic crystal structure. It is a hard, brittle material, normally classified as a ceramic in its pure form, though it is more important in ferrous metallurgy. While iron carbide is present in most steels and cast irons, it is produced as a raw material in the Iron Carbide process, which belongs to the family of alternative ironmaking technologies