# Effect of Rare Earth Elements (La and Ce) on Microstructure and Mechanical Properties of U75V Steel

## TL;DR

Adding rare earth elements like lanthanum and cerium improves the structure and strength of U75V steel, but too much can reduce toughness at very low temperatures.

## Contribution

This study demonstrates how varying rare earth content affects microstructure and mechanical properties of U75V steel, including a reversal in toughness at cryogenic temperatures.

## Key findings

- Rare earth addition modifies inclusions and refines microstructure in U75V steel.
- Sample 2 showed the most balanced improvement in hardness and tensile strength.
- Excessive rare earth content reduces impact toughness at −60 °C.

## Abstract

To investigate the effects of multiple-element rare earth addition on U75V steel, this study produced three types of steel: sample 1 steel without rare earths, sample 2 steel containing 0.0035% La and 0.018% Ce, and sample 3 steel containing 0.02% La and 0.0023% Ce. Microstructural analysis showed that the addition of rare earth elements modified the MnS and silicoaluminate inclusions into RE2O2S and RE2O2S–oxide complexes, which reduced the number and size of inclusions while simultaneously refining the microstructure, including the grain size and the spacing of pearlite layers. Concurrently, RE addition enhanced the steel’s mechanical properties, with the degree of enhancement dependent on RE content; sample 2 exhibited the most balanced improvement. Compared to sample 1, the hardness of samples 2 and 3 increased by 15.3% and 3.6%, respectively, and their tensile strength increased by 7.9% and 6.8%, respectively. Meanwhile, their coefficients of friction decreased significantly, by 69.5% and 22.1%. The impact toughness was also enhanced by RE addition, with both samples 2 and 3 showing higher values than sample 1 at room temperature and moderate low temperatures. Nevertheless, a distinct reversal was observed at −60 °C, where the impact energy of sample 3 was 23.5% lower than that of sample 2. This result implies that while moderate RE addition is beneficial, an excessive amount can adversely affect the toughness under cryogenic conditions.