Effect of Hydrogen on Creep Behavior of a Vanadium-Modified CRMO Steel and its Continuum Damage Analysis

Creep properties both in hot hydrogen and in air of a vanadium-modified CrMo steel 2.25Cr1Mo0.25V, widely used in hydroprocessing reactors in petrochemical industry, were investigated to determine the effect of hydrogen on high-temperature creep behavior of the low-alloy ferritic steel. The minimum creep strain rate in hydrogen is higher than that in air, whereas the creep strain at failure in hydrogen is relatively smaller. Many tiny spherical cavities are dispersively distributed in the ruptured specimen under hydrogen, which has relatively higher Vickers hardness. Based on the thermodynamics theory, the pressure of methane generated by the so-called “methane reaction” in the vanadium-modified CrMo steel can be calculated by using corresponding thermodynamic data, assuming that methane can reach its equilibrium state during cavitation. Meanwhile, a creep constitutive model based on continuum damage mechanics (CDM) was proposed, taking methane pressure into consideration. The results show that methane pressure increases nonlinearly with increase of hydrogen pressure while it decreases gradually with increase of temperature. The constitutive model considering the damage induced by methane pressure can be used to predict the effect of hydrogen pressure and temperature on creep life, indicating that the influence of hydrogen at elevated temperatures becomes smaller when increasing temperature or decreasing hydrogen pressure.

Nanostructural Layer and Properties of 35 CrMo Steel Rolled by Cross-Rolling

A nanostructural layer was formed on the surface of 35CrMo steel rods rolled by cross-rolling. The layer microstructure is composed of nano-scale grains and some deformation grains, which contains compressive residual stress of -479Mpa. The surface roughness is decreased from Ra 2.1μm to Ra 0.14μm with improvement of the cylindricity and straightness. The rolling strengthenes the surface but has ignorable influence on the mechanical properties of the rods.

Research on Band Microstructure and Annealing Process of CrMo Steel Used for Precision Ball Screw

The band microstructure has an adverse effect on the comprehensive mechanics performance of Cr-Mo steel used for ball screw. This paper dealt with the different annealing processes combined with the forming mechanism of the banded structure. It shows that the annealing with long time can basically eliminate the band microstructure.