1.5507 | 23MnB3 +QT (8.8)

The screw material 23MnB3 is available in the following heat treatment states and strength classes:

• [434 HV 1] hardened, not tempered (tetragonal martensite)
• [374 HV 1] Strength class 11.9 (lowest tempering temperature)
• [348 HV 1] Strength class 10.9 (medium tempering temperature)
• [238 HV 1] Strength class 8.8 (highest tempering temperature)

1st number x 100 = Rm
1st x 2nd number x 10 = Rp0.2.

In the purely hardened state, the microstructure consists of tetragonal martensite. You can see a nice comparison of the needle structure if you compare the tetragonal martensite with the strength class 11.9.

The strength class is set via the height of the tempering temperature. The higher this is, the lower the strength values. In the images taken at 1000x optical magnification, a clear difference can be seen between the individual strength classes in the proportions of fine epsilon carbides that form during tempering. The higher the tempering temperature, the more epsilon carbides can precipitate. This in turn leads to greater relaxation of the martensite, accompanied by lower strength and higher elongation values.

Images of etching on the former austenite grain boundaries with detailed etching information can be found in the separate case study under the same material designation.