C15 | 1.0401 +QW +T Hardened and tempered (quenchant: water)

This hypoeutectoid steel (bar material with ⌀ = 28 mm) with 0.14 % carbon (measured with OES; nominal: 0.12 – 0.18 %) was austenitised at 950 °C for 30 min, quenched in water and then tempered at 250 °C for 2 hours. The austenitisation temperature is slightly above the temperature range actually used for this material (780-820 °C for edge hardening; 880-920 °C for core hardening). Due to the too low carbon content, the C15 is not martensitically hardenable. Classically, one would case harden this steel to produce a hard, martensitic surface layer.
The core structure consists of bainite, martensite and ferrite.
The edge region of the sample was slightly decarburised, where the microstructure consists only of bainite and ferrite.
The area adjoining the decarburised edge layer also contains – like the core – bainite, martensite and ferrite, whereby the martensite content is noticeably higher here than in the core. This is also reflected in the hardness: in the edge area it is about 190 HV5, towards the core it drops to about 170 HV5. This can be explained by the fact that the quenching at the edge of the sample is naturally higher than in the core, so that more martensite could form here.