Nadca 207 and incoming material quality control in Class 2
Materials purchased in Class 2
In this case Nadca 207 says nothing. Even so, the steel should have the same quality as in the case of Class 1, with all levels of its quality assessment, except for impact tests.
Fig. 1 – Nadca 207 – mandatory ratings for die steels
How to deal with it? Here, each tool shop probably has to set its own priorities. If this type of purchase were to apply to critical die parts, we run the risk of sending steel to production from the 18% that represents unsuitable material. So, I would strongly recommend going at least the Class 1 route and with archiving. After all, luck only favours those who are prepared.
Where critical parts are not involved, there are several options.
We cut a sample from each delivered piece and evaluate it for satisfactory/unsatisfactory annealed structures. That’s how Škoda-Auto did it for a long time, when my doctore thesis supervisor, Doc. Ing. Jiří Horyna, CSc. did an initial metallographic assessment for them.
Fig.2 – Part ot the quality report signed by Doc. Ing. Jiřího Horyny, CSc,
If the steel quality is really worthless, it can be recognized. See Figure #3 with the AS10 structure.
Fig. 3 – Microstructure from the laboratory of Doc. Ing. Horyny, CSc. rated AS10.
Today, however, such an analysis can be carried out even without cutting the sample, e.g. using a Keyence digital microscope. The digital camera is portable, with a cable length of up to 20 m. If we grind, polish and etch the test spot on the surface of the block, we can see exactly the same as on the optical microscope. This microscope can easily handle the 500x magnification we need for evaluation, and we can do it anywhere in the tool shop hall.
Fig. 4 – Keyence digital microscope and evaluated structure at 500x magnification
There is other, let’s say hybrid methods. On the back side of the matrix, 3 test rollers are machined with a crown cutter. The first is kept by the tool shop as witness material for immediate or future verification of input quality, the second is evaluated after heat treatment, and the third goes with the die to the foundry.
Fig. 5 – Method of 3 samples for retrospective evaluation of input material quality, heat treatment and foundry behaviour
But the problem arises in the case of evaluation. The image of the structure on the metallographic surface may indicate defects, but it will never say anything about the actual mechanical properties of the steel. So, in some cases it can help, especially when the etched structure will point to unacceptable annealed ASxx structures or structures after hardening HSxx, but often it will only raise a discussion between the interpretation of individual structures without a clear conclusion. Especially when the structures looks OK but the result is not OK.
March 17 , 2023
Jiří Stanislav
