Microstructure and Properties of Lanthanum-Containing Austenitic Hardfacing Electrode Deposits

Literature Overview and Research Context

The addition of rare earth elements, particularly lanthanum (La), to hardfacing alloys represents an emerging approach to enhancing the microstructural refinement, mechanical properties, and service life of weld overlay deposits. This literature investigates a lanthanum-containing austenitic hardfacing electrode, examining how the addition of lanthanum oxide (La2O3) to the flux coating influences the microstructure, hardness, wear resistance, and corrosion behavior of the deposited layer. The study is particularly relevant in the context of improving the performance of austenitic hardfacing alloys used in mining, cement, and chemical processing industries, where the combination of wear resistance, toughness, and corrosion resistance is often required simultaneously.

Composition Design and Role of Lanthanum

The base composition of the hardfacing electrode is an austenitic system containing approximately 22% Cr, 10% Ni, 3% C, 2% Mo, and 1.5% Mn, with the addition of 0.5% La2O3 to the flux coating. The role of lanthanum in this system is multifaceted and operates at multiple scales — from grain refinement during solidification to modification of inclusion morphology and carbide precipitation behavior.

Lanthanum acts as a powerful grain refiner in molten iron and nickel melts. Its oxide has a high melting point (approximately 2400 °C) and forms stable compounds with oxygen and sulfur, effectively scavenging these elements from the melt. This deoxidation and desulfurization effect reduces the concentration of harmful inclusions such as MnS, which are known to act as crack initiation sites and reduce the ductility of austenitic deposits. Furthermore, La2O3 particles serve as heterogeneous nucleation sites for austenite grains, promoting a finer and more uniform grain structure.

Microstructural Characteristics

Metallographic examination of the lanthanum-containing deposit reveals a refined austenitic matrix with a significantly higher density of fine carbide precipitates compared to the lanthanum-free counterpart. The grain size, measured using the ASTM E112 linear intercept method, is reduced from approximately 120 μm in the base alloy to approximately 65 μm in the lanthanum-modified alloy. This refinement is attributed to the combined effect of La2O3 particle nucleation and the reduced sulfur content, which eliminates the MnS stringers that normally act as preferential crack paths.

The carbide morphology is also modified by lanthanum addition. In the base alloy, carbides form as coarse, irregular M7C3 and M23C6 particles along grain boundaries. In the lanthanum-modified deposit, carbides are finer, more uniformly distributed, and exhibit a more equiaxed morphology. The literature attributes this to the interaction between La and carbon, which promotes the formation of La2C3 particles that serve as additional nucleation sites for carbide precipitation. The volume fraction of carbides increases from approximately 25% in the base alloy to approximately 35% in the lanthanum-modified alloy, contributing to the enhanced hardness and wear resistance.

Mechanical Properties and Performance Comparison

The mechanical and tribological properties of the lanthanum-containing deposit were systematically compared with those of the base alloy under identical welding conditions.