Microstructure

Reference to steel microstructure is not always required in a specification. In the case of parts to be stamped or blanked, pretempered strip may be selected in order to maintain uniformly close tolerances and avoid warpage during heat treatment. Pretempered material is usually hardened, quenched and tempered to a martensitic microstructure.

If bends are also required during part production, proper manipulation of bend radius, angle to grain, hardness and analysis will usually provide sufficient formability in pretempered strip.

However, in some cases with severe bends, a higher degree of formability will be required than that offered by martensitic pretempered steel. Rather than automatically selecting annealed stock, the customer should consider strip with bainitic microstructure. Formability will be enhanced without sacrifice in dimensional stability or wear resistance.

The following illustrates the improved formability of bainite over martensite. It indicates the minimum bending radii possible for given hardnesses of bainitic and martensitic AISI 1050 material. For example, bainite hardened to Rc42 can be bent over a radius just twice its thickness (or greater) without breaking, whereas comparable martensite strip requires a radius at least 3.4 times its thickness for safe bending.

Minimum bending radii for AISI 1050 martinsite
and bainite; bend axis transverse to grain direction.

If even greater formability is required for sharp part bending, fully spheroidized annealed strip should be specified. Steels with this microstructure, such as Theis BarcoForm, have a low hardness relative to grade, and therefore maximum ductility for forming operations.

Permissible partial decarburization should be specified in quantitative terms - that is, the depth below the surface to which carbon depletion may extend. A recommended limit for general use is up to 2% of thickness, with "no free ferrite evident".

For some applications, such as cyclic loaded parts, decarburization may affect fatigue strength and shorten life due to lower surface strength. In other applications where the strip edge becomes the working part of the tool, the softness of a decarburized surface would be detrimental. Such cases call for closer limitation by specifying absolute limits of partial decarburization allowed.

Photomicrographs of carbon strip steel at 1000x
           
Pinpoint Spheroidized Martensite Bainite


Back to Contents