Evolution of A-Type Macrosegregation in Large Size Steel Ingot After Multistep Forging and Heat Treatment
- PDF / 2,807,941 Bytes
- 10 Pages / 593.972 x 792 pts Page_size
- 18 Downloads / 194 Views
esence of chemical heterogeneities and nonuniform distribution of alloying elements produced during solidification of metallic alloys is called segregation. Depending on its scale,[1] segregation can be observed at three levels: microsegregation takes place
ABDELHALIM LOUCIF, EMNA BEN FREDJ, NATHAN HARRIS, DAVOOD SHAHRIARI, and MOHAMMAD JAHAZI are with the De´partement de Ge´nie Me´canique, E´cole de Technologie Supe´rieure, 1100, rue Notre-Dame Ouest, Montre´al, QC, H3C 1K3, Canada. Contact email: [email protected] LOUISPHILIPPE LAPIERRE-BOIRE is with Finkl Steel-Sorel, 100 McCarthy, Saint-Joseph-de-Sorel, QC, J3R 3M8, Canada. Manuscript submitted September 9, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS B
at the scale of dendrites, macrosegregation occurs at the scale of the product, and mesosegregation is produced at an intermediate scale such as primary grains. Macrosegregation is commonly observed in large size cast ingots. The typical macrosegregation patterns that could be formed in cast ingots are presented in Figure 1.[2,3] Depending on their position, morphology, and concentration, different patterns can be revealed: positive at the top, negative conic at the bottom, A-type bands (channels) between the centerline and the external surface, and V-type bands along the centerline of the ingot.[1,3] The A-type segregation bands appear as linear regions,[4] which present an important enrichment in alloying elements.[5] They are caused by the movement of enriched interdendritic fluid through the mushy zone as a result of thermosolutal convection.[6]
Fig. 1—Typical macrosegregation patterns that could be formed in steel ingots: (a) schematic representation and (b) sulfur print of a 10-ton ingot.[2,3] (‘‘Mechanisms of Formation of A- and V-Segregation in Cast Steel,’’ J.J. Moore and N.A. Shah: J. Int. Met. Rev., copyright Institute of Materials, Minerals and Mining, reprinted by permission of Taylor & Francis Ltd., www.tandfonline.com on behalf of Institute of Materials, Minerals and Mining.).
Often, the as-cast steel ingots undergo open die forging operations at high temperatures for processing to final (shafts, rolls, and slabs) or semifinished products used as preforms for closed die forging. The ingots are generally preheated at high temperatures for long durations, often resulting in reduction or rarely in elimination of macrosegregation patterns. However, in large size high-strength steel ingots that contain significant amounts of alloying elements, some segregation, particularly macrosegregation, may still remain after the homogenization heat treatment.[7] On the other hand, it is well known that macrosegregation zones with their different chemical compositions modify the thermophysical properties[8] and microstructure[6] and may induce changes in the mechanical properties of the affected zones.[5] Higher homogenization temperatures or longer holding times are not viable alternatives because they increase the cost of the finished product and may also induce local melting, which leads to cracking
Data Loading...
