Effects Of Hot Rolling On The Microstructure And Mechanical Properties Of Hastelloy/Steel Composite Panel

Bimetal composite panels have emerged due to the superior performance of both materials,and meet the use requirement of people.Since Q235 steel has good plasticity,weldability and formability,it is often rolled and joined with dissimilar metals,and the obtained composite sheet can combine the excellent properties of both metals.Hastelloy-C alloys can be used as containers for strong oxidation and strong reducing liquids,and play an irreplaceable role in the complex working environment of petrochemical industry.In view of the fact that dissimilar metals are prone to generate brittle intermetallic compounds and oxides during heating,the NiCr-Mo system Hastelloy-C276 and Q235 in Ni-based alloys are chosen and hot roll-bonding after vacuum electron beam welding,and then combined them together to form a bimetal composite panel.Through the adjustment of the rolling process and the subsequent heat treatment process,the relationship between the microstructure and mechanical properties of the composite interface was analyzed by optical microscopy,SEM,EDS and shear,three-point bending and other mechanical tests.Hastelloy/steel can be combined at a press amount of 50% at 1000 °C.The interface is divided into four areas,followed by Hastelloy,diffusion layer,decarburization layer,and base metal.The interface bonding strength increases with the increase of the press amount at the same temperature.When the temperature is increased by 1100 °C and the press amount is 75%,the shear strength of the interface is 432 MPa.The rolling temperature was further increased to 1200 °C at a lower press amount of 75%,the interfacial shear strength was decreased to 368 MPa.The fracture mechanism of the interface shows that the interface shear strength is affected by two factors at the same time.First,the number of mixed oxides at the interface is large,and the shear strength of the interface decreases due to the large size.The second factor is the softening of the matrix at high temperature leading to the decrease of the shear strength.The temperature rises and the oxide content decreases,but the matrix gradually softens.The comprehensive performance of the interface is optimal under 1100 °C rolling,and the bending property of the material can reach the national standard GB/T6396-2008.The results of various bending experiments show that the crack first initiates and expands at the oxide when the interface breaks,and finally causes the interface to debond.The brittle intermetallic compound is not formed at the diffusion interface,and the diffusion layer is an Fe-Ni solid solution formed by the diffusion of Ni into the Fe matrix,and the diffusion layer has a certain hindrance effect on the crack propagation.At the same rolling temperature,different press amount has no obvious influence on the microstructure of the interface.There are not big differences among the width of the interface diffusion layer and decarburization layer and the size of the grains near the interface.The distribution of interface inclusions becomes dispersed and fine as the press amount increases.The main reason is that the number of new-formed oxides are limited.As the press amount increases,the interface area becomes larger,the inclusion distribution range becomes larger,and the inclusions distributed on the interface per unit length become less,which is beneficial to the improvement of the bonding strength of the interface.The combination of Hastelloy and Q235 carbon steel can be completed at 1100 °C and 50% press amount,and the joint surface has no unwelded area.As the press amount increases,the interface inclusions become finer and finer,and the number of inclusion at the interface per unit length decreases,and the interface bonding strength is improved.Heat treatment increases the diffusion distance of the element and thus increases the thickness of the diffusion layer.This effect is especially noticeable when the temperature is raised to 1100 °C,but the heat treatment will generally reduce the shear strength of the interface because the temperature and the holding time will cause the coarseness of crystal grain and the decrease in the matrix strength,wherein the decrease of Hastelloy strength is more pronounced.