BH: Bake Hardening
Definition:A strength increase (especially a yield strength increase) due to a decoration and locking of dislocations with C atoms via artificial ageing.
Explanation:BH is one of the phenomena which occur during the industrial paint baking process. The component either with or without pre-strain process is subjected to a heat treatment in order to obtain a high quality paint finish. During such heat treatment at the temperatures between 140 and 200 °C for 10 to 90 minutes, ageing occur in the material. Accordingly, interstitial C atoms (typically 5 to 25 ppm C) diffuse and accumulate in the distorted lattice adjacent to dislocations. Therefore, the dislocations are locked and higher strength is necessary for yielding and further plastic deformation. Such anchoring mechanism is known as the Cottrell effect.

Quantification of the strength increase is via a BH value (shown in Fig. 1a,b as BH0 and BHx), which is defined as the yield strength difference between after and before ageing. In the case of conventional BH steels, the strength increases about 40 to 60 MPa.
Picture /
Figure /

Fig. 1: Schematic tensile curves showing BH values for (a) without prestrain: BH0=ReL-Rp0.2, and (b) with x% prestrain: BHx=ReL-RpX,0. (c) Mechanism of BH: the Cottrell effect.
SFB-Link:Such BH effects can also be observed in advanced high strength steels (→AHSS). Under optimum conditions as in dual-phase steels (→DP steels), BH values can reach as high as 70-120 MPa. However, the underlying mechanism ,needs to be identified due to their complex microstructure, which is likely to result in different diffusion conditions. For high Mn steels, different types of deformation mechanisms must be taken into account.
References:Cottrell, A.H.; Bilby, B.A.: Proceedings of the Physical Society A 62 (1949) S. 49/62.
Elsen, P.; Hougardy : Stahl und Eisen 113 (1993) Nr. 10, S. 101/107.
Anke, T.: TU Clausthal, 2005 (Dr.-Ing. Diss.).
Brühl, S.: RWTH Aachen, 2011 (Dr.-Ing. Diss).