TAG’S EXPERIENCE IN DEALING WITH A PARTICULARLY DELICATE PROBLEM.
Over the years, the activities performed by the Dolzago and Cremella metallographic laboratories have gradually intensified and the number of tests carried out has steadily increased.
More specifically, for what concerns the Dolzago laboratory, which follows the part of the production that is dedicated to the aluminium die-casting moulds, the figures provide a clearer picture: in 2016, 6652 metallographic samples were analysed, in 2017 they increased to 7339 (+10.3%), and in the first few months of 2018 this growth trend has already been confirmed.
The testing activities essentially refer to assuring conformity of the tested materials with the requirements of the reference standards.
The use of a specific set of standards for the characterisation of steels is now a consolidated practice among the major end users of die-casting moulds (automotive industries, large foundries); almost all the specifications in use, however, refer to the NADCA # 207 and/or SEP 1614 international standards.
One of the most frequent anomalies encountered during our testing procedures refers to the steel austenitic grain size in the annealed status which, if inadequate, can determine a deterioration in the performance of the die-casting moulds during manufacturing processes.
THE AUSTENITIC GRAIN SIZE STEALS THE SPOTLIGHT.
The size of the austenitic grain is one of the characteristics that are determined by the steel mill during the manufacturing processes and, once set, it will no longer be possible to reduce it.
Therefore, the austenitic grain size is a condition that is related to casting activities and is determined by the manufacturing processes (chemical analysis, forging, reduction module, Poisson coefficient, etc.): in this case the appropriate terms to use are “ex austenitie” or “prior austenite” grain size. This is not helped by fact that there is a demand for increasingly larger raw blocks to meet the requirements of the automotive market that impose the use of increasingly larger mould.
It is not always possible to find microstructural anomalies using optical microscopy to observe the material in the annealed status: in such cases the heat treatment does nothing more than highlight pre-existing defects, or exalts them, making them even more visible, even if they are not related to the heat treatment process and already within the material.
WHAT TO DO HOW TO DO IT
The need to have a fine grain size (according to the reference standard NADCA # 207-2016 which sets a minimum of G7 (ASTM E112-13>=7) as the condition of acceptability) aims to reduce and contain the propagation of pyrocricles/thermal high-cycle fatigue cracks (oligocyclic type) which inevitably “sprout” during the production process.
When an anomaly linked to coarse and/or mixed granulometry is detected, it is necessary to determine the average grain size before the heat treatment, to discover whether the grain size increased during the hardening phase or if the defect was pre-existent.
Paragraph A3 of ASTM E112-13 (“Standard Methods for Determining Average Grain Size”, is the universal standard used to regulate the methods for determining grain size) provides various laboratory methods to conduct the analysis, in addition to the provisions of the latest edition of NADCA#207.
Given the extensive experience gained during so many years in the business, we can state just how important it is to determine the compliance of the purchased steel with the specifications referred to in the applicable standards.
When carried out before starting the mechanical processes at the mould producer, such tests would highlight the unacceptable anomalies, avoiding increases in costs, wasted time and sterile and undesired disputes.