The analysis of the metallurgical bond development between dissimilar steel alloys as a result of the tube co-extrusion process

The process of co-extrusion is a promising method for the production of bulk composite metal materials exhibiting a combination of the different component materials' properties. Cost reduction may be realized by pairing an expensive alloy with a lower cost alloy that gives structure and strength where properties of the other material are not needed. Typical applications for co-extruded tubes include the power generation, pulp and paper, chemical processing, medical device, and petroleum industries. Growth in these industries as well as the need for cost reduction is driving the investigation into methods to optimize co-extrusion. In addition to the traditional challenges that exist in the extrusion process, additional considerations must be taken when extruding multiple materials concurrently. Tolerances as well as eccentricity are extremely important and are influenced greatly by the different material flow behaviors. The metallurgical bond development between the different layers influences the composite physical and mechanical properties. A bi-material billet consisting of 1020 plain carbon steel sleeve and 304 stainless steel core was used to study the co-extrusion process. Novel billet design utilizing a shortened core has been investigated in order to control early stage material flow. It has been shown that a core recessed approximately 10% of the total billet length leads to less material loss due to improper early stage flow via Finite Element Modeling and industrial scale extrusion experiments. A multi-layer metallurgical bond was observed in the extrudates that consisted of banded pearlite and a decarburized region in the plain carbon steel, chromium carbide precipitated region near the interface in the stainless steel, and a thin band of cementite and ferrite at the interface. The ferrite-cementite band corresponds to the diffusion layer in the plain carbon steel of Ni, Cr, and Mn that diffused out of the stainless steel. The greatest influence on the development of the decarburized and chromium carbide precipitation zones is the cooling period from the deformation temperature. Using state variable calculations from the FEM models, the extrusion bonding process was simulated with a Gleeble thermomechanical simulator. Microstructural results from the physical simulations were in good agreement with the actual extrudate microstructure.