Segregations on forged components: Their effect on defects like cracks and on the microstructure of heat treated parts

Segregations on forged components: their effect on defects like cracks and on the microstructure of heat treated parts Segregation of alloying elements during the ingot solidification process and the subsequent hot forming operations are responsible for the formation of regions exhibiting different chemical composition and therefore different mechanical properties. During the casting of steel the structure of the ingot is characterized by the development of dendrites. The presence of such type of microstructure causes a high lack of homogeneity both in terms of chemical and mechanical behaviour not only in the ingot but also in the finished product obtained from it. In particular chemical heterogeneity develops during solidification, when elements having a low partitioning ratio are ejected into the interdendritic regions causing areas of high solute concentration. Therefore it can be asserted that segregations affect the service properties of the forged parts and often lead to their failure (fig. 1). Several methods for alleviating structural heterogeneity are described in the literature and, in particular, the homogenizing heat treatment carried out at high temperature (usually ranging from 1200°C to 1300°C depending on the chemical composition of the considered steel) after the ingot casting and cooling. In the present paper the effectiveness of different heat treatments in reducing the extent of segregations has been evaluated both using industrial and laboratory furnaces. A 20 Mn5 steel grade has been examined; the choice of such material was done in order to evaluate the effect of the Mn segregation which, along with the C segregation, often causes failures in the forged parts produced using low alloyed Mn steels. In literature the influence of Mn segregation was systematically evaluated by Majka [15] that showed the effect of different heat thermal treatments carried out on a tailored segregated Mn steel and in particular the effect due to the cooling procedures from austenitizing temperature simulating a normalizing heat treatment. The formation of ferrite in low-Mn areas resulted in carbon enrichment in the austenite of adjacent high-Mn zones and caused the formation of perlitic layers or bainitic or martensitic zones depending on the cooling rate and the level of local segregation. Such severe differences in the local steel chemical composition are responsible for the nucleation of microcracks causing the scraps of several forged parts produced starting from Mn low alloyed steel (fig. 2). Starting from such preliminary remarks, in the present paper samples from a scraped forged shaft produced in 20Mn5 steel were cut to be characterized in different heat treatment conditions. The scrap of the shaft was due to the ultrasonic control executed at the end of the production steps; such a control showed cracks having sizes higher than the acceptable limit defect; a microstructural analysis of samples examined in the forged condition showed an important segregation affecting a large part of the shaft (figs. 3-4). Three different set of samples were characterized in the industrial furnace according the following conditions: 1) homogenization at 1320 ° C × 80 min. + air cooling - fig. 5; 2) homogenization + normalization at 900°C × 30 min. + air cooling - fig. 6 3) homogenization + quenching (900°C × 60 min. + water cooling) and tempering (600°C × 1h + air cooling) - fig. 6. A reduction in the level of segregation was obtained after the homogenization treatment even if not all the local differences in the chemical composition were removed. The effect of two different controlled heat treatments carried out in laboratory was also evaluated starting from forged samples heat treated according to: • a heating at 900°C × 60 min+ air cooling (condition A); • a heating at 1320°C × 150 min+ air cooling (condition B). The microhardness distribution obtained on polished samples tested in the condition A and B can be observed in fig. 7 and 8 respectively. The sample treated at higher temperature assured a reduced scatter in the microhardness data (fig. 8) with respect to the condition A (fig. 7) and such a result can be related to a more homogeneous distribution of Carbon and Manganese. Anyway fig. 8 shows the presence of a hardness peak in correspondence of which a high level of Mn was detected by EDS microanalysis as reported in fig. 9; therefore a certain level of segregation is still present inside of the examined sample. From the whole experimental results it can be assumed that although the homogenizing treatment can be used as a reducing Mnsegregation tool, only a proper sequence of casting steps (starting from the casting temperature to the ingot cooling rate) aimed at small dendrite size and controlled Mn ingot distribution can be considered the proper solution to obtain high quality forged components. In fact the absence of severe Mn-segregations inside the ingot is fundament to obtain a forged part without cracks related to martensitic or bainitic brittle areas mainly due to chemical inhomogeneities in terms of carbon and manganese distribution. The extension of such macrosegregation zones are affected from the thermal distribution gradient imposed both during the forging phase and the heat treatment of the forged part and therefore changes in the size and distribution of macrosegregation can be also observed during the forging steps. Unfortunately such variation often cause local high level of stress and therefore can induce cracks nucleation and propagation inside the forged component.