TECHNICAL INFORMATION

WHY IT IS IMPORTANT TO MEASURE THE RESISTANCE!! As we stated before, it is fundamental to obtain an adequate microstructure both to extend the life of the mould and to realize successfully other surface treatments like: ion nitriding and PVD coating. In order to obtain an adequate microstructure we must use a homogenous steel, as most uniform as possible, to produce the figure of the mould. Figure 2 and 3 show respectively a good and bad starting situation. This is not an exception to the rule, it usually happens to work with moulds whose structures are totally compromised at the beginning!! The first and primary step is, therefore, to check the steel to get good performances!!!

	Fig.1: GOOD structure at annealed status: carbides on ferritic matrix Fig.1: BAD structure at annealed status: strong ferrite ghosts, scar superior bainite with rough martensite and carbides on the surface We after analyze the microstructure after tempering, where a good structure presents fine martensite with traces of low bainite and carbides on the matrix (see fig.2), while a bad one is made up of rough martensite with a relevant presence of superior bainite and sometimes also pearlite and with carbides on the surface (see fig.3). Also an inexperienced can understand the reasons why the first microstructure is better than the second one: it is enough to analyze the morphology of the inferior bainite with the one of the superior bainite. A tempering with gas quenching at high pressure allows to arrive to such cooling rates as to obtain adequate microstructures but this has a disadvantage: the non homogeneity of the cooling. Different studies showed that, if the pressure used in the cooling is the same, the tempering speed is different according to the charge composition and, therefore, for what we stated before, it is possible to get different metallurgical structures for different charges, being the pressure a not completely determined parameter on the cooling rate. Furthermore, due to the not uniformity of the cooling rate, it is possible that a big piece has a faster cooling rate only in some parts, this causes variations in the structure and deformations. To reduce the deformations it is possible to reduce the quenching pressure, as it is normally done, with the result of obtaining inadequate metallurgical structures. This causes pyrocracks and cracks caused by thermal shocks to which the mould undergoes during the use. Different is the tempering in a salt bath. During the cooling in a salt bath, the charge is plunged into basins, which at T.T.N. have a volume ratio of 1:10, granting a total cooling homogeneity. This reduces the deformations but, above all thanks to the high coefficient of thermal exchange of quenching salts, in a higher cooling rate than the one we may get, with the same deformities, with gas quenching.
	Fig.4 1 - mechanical load-unload 2 - no decarburization on the surface 3 - good resistance 4 - reduced deformations The possibility of the T.T.N. factories (see. fig.4) to cool also big pieces (until to 1000X1000X1500 mm - max. 2300 Kg.) in two different baths of melted salts (the first at 500°C and the second at 200°C) allows to unify the cooling rate and uniformity at the controlled temperature to obtain enough cooling rate to give the desired tempering structure but not too high, to avoid distortions of the item. Using the salt bath it is possible to use the bainitic "window", which is own by steels for the hot-working (see fig.1), so as to avoid any structural transformations, the difference of temperature between core and surface is unformed, fundamental thing to reduce deformations, while in the salt bath from 500°C to 200°C the rate is high enough to avoid the creation of the superior bainite and therefore obtaining an adequate structure according to the mechanical properties, first of all the resistance (see fig.5). At T.T.N. we performed a great quantity of tests inserting resistance standard specimens without notch, with dimensions of 10X10X55 mm, in cooling holes of big matrix for die-casting, putting them to such a depth to simulate the behavior of the core in the mould and we then cooled the matrix in salt baths (cycle 1). At the same time similar specimens have undergone the same test conditions but cooled in gas at 6 bar pressure (cycle 2). Those specimens, after being tempered and hardened, have undergone resistance tests at the University of Trento with a Charpy 300 J machine, which underlined the different behavior of the specimens cooled in gas as compared to those cooled in salt baths.
	Fig.5: Cooling curves of a specimen in 1.2343 in salt bath at double stage with core thermocouples and on the surface of the specimen.