The Structure and Properties of a Weld-Deposited Layer onto Steel Hardox 450 Using a Boron-Containing Wire

Abstract

By using the methods of modern physical material science, the structure-phase states and properties of the layers formed on low-alloyed steel Hardox 450 with the use of welding wires with a boron content amounting to 4.5 and 6.5 wt % have been analyzed. In the initial state, steel Hardox 450 has the structure of tempered martensite in the bulk and along the boundaries of crystals in which cementite particles are located. The particles located in the bulk are needle-shaped and those located along boundaries are mainly round-shaped. The revealed extinction bend contours indicate a torsion curvature of the crystal lattice in this area of the material. They begin and finish at the interfaces of the martensite crystals. The scalar density of chaotically arranged dislocations forming a reticulate-type substructure is 6.2 × 1010 cm–2. The weld-deposited layer onto steel Hardox 450 has more than two-fold microhardness than that of the base metal. The analysis of state diagrams for the Fe–C, Fe–B, and B–C systems and of polythermal cross-sections in the Fe–C–B system has shown that a rapid cooling of Fe23C6–Fe23B6 alloys from the liquid state facilitates the formation of multiphase structural states. It has been established by means of transmission electron diffraction microscopy that the reasons for the high microhardness level of the surface layers consist in the following factors: the formation of iron borides and the crystals of ultrafine packet martensite (up to 100 nm) having a high level (~1011 cm–2) of scalar dislocation density; the presence of nanosized particles of iron and boron carbides in the bulk and along the boundaries of martensite crystals; and a high torsion curvature level in the crystal lattice of iron borides and α-phase grains caused by internal stress fields along interphase boundaries (the interface between iron boride crystals and α-phase grains) and intraphase boundaries (the interface between iron borides and martensite crystals in the packet). Increasing boron concentration from 4.5 to 6.5% is accompanied by a sufficient (1.2–1.5-fold) increase in the hardness of the weld-deposited layer. It is caused by a 1.5–2.0-fold increase in the size and relative content of iron boride areas.