The selection of 4140 and 18CrNiMo7-6 depends on core working conditions including load type, surface wear demand, component size, heat treatment plan and cost budget. Choose 4140 for medium-load shafts with moderate hardness needs if you want lower costs and simple quenching & tempering treatment, while 18CrNiMo7-6 is a carburizing steel with hard wear-resistant surface and longer service life, better suited for heavy-duty equipment and large parts under heavy-load operating environments with higher production costs. And our technical team can suggest suitable material for you once we receive your technical drawing, actual working load and service requirements.