变韧铁，也称贝氏体，是钢在热处理时形成的一种显微组织组成物，是由沃斯田铁在波来铁温度范围以下和马氏体点（麻田散铁转变开始的温度）以上的温度范围内分解而成的肥粒铁和雪明碳铁的混合体。最早由埃德加·贝恩等人描述，故名。

贝氏体分为两种，在较高温度（350～550℃）形成的称“上贝氏体”，其组织在光学显微镜下呈羽毛状；在较低温度形成的称“下贝氏体”，其组织在光学显微镜下呈针状或竹叶状。贝氏体由于碳化物颗粒周围受腐蚀而变得比较粗糙，故在显微镜下呈黑色。

目前贝氏体的转变机制尚存争议，目前主要有“切变学说”和“扩散学说”两个学派。

Bainite is a plate-like microstructure or phase morphology (not an equilibrium phase) that forms in steels at temperatures of 250–550 °C (depending on alloy content). First described by E. S. Davenport and Edgar Bain, it is one of the decomposition products that may form when austenite (the face centered cubic crystal structure of iron) is cooled past a critical temperature. This critical temperature is 1000K (727 °C, 1340 °F) in plain carbon steels. Davenport and Bain originally described the microstructure as being similar in appearance to tempered martensite.