Cutting fluids for alloy steels In practical machining operations on alloy steels (turning, milling, drilling), which cutting fluid is most commonly preferred to provide extreme-pressure lubrication, reduce built-up edge, and control tool wear at elevated temperatures?

Introduction / Context:
Choosing the correct cutting fluid for alloy steels is essential for tool life, surface finish, and process reliability. Alloy steels tend to work-harden and generate high cutting forces and temperatures. Fluids with extreme-pressure additives are often required to prevent adhesion and scoring.

Given Data / Assumptions:

• Material: general alloy steels (not free-machining low-carbon steels).
• Operations: turning, milling, drilling, or tapping.
• Need: high lubricity and boundary film formation to withstand high local pressures and temperature.

Concept / Approach:
The best fluids for alloy steels supply boundary lubrication through chemically active additives. Sulphurised mineral oils react at hot contact zones to form iron sulfide films that reduce friction and welding at the tool–chip interface, improving tool life and finish. Emulsions (soluble oils) are excellent coolants but may offer insufficient EP performance for tougher steels unless fortified. Water or dry cutting typically give poor results in these conditions.

Step-by-Step Solution:

Verification / Alternative check:
Shop practice and tool manufacturer handbooks recommend active or inactive sulphurised oils for alloy steels, especially for slow to moderate cutting speeds and operations like tapping or deep drilling.

Why Other Options Are Wrong:
Water: excellent coolant but zero lubricity; promotes rust and poor tool life.Soluble oil: better cooling, but without strong EP package it may not prevent seizure.Dry machining: generally unsuitable for alloy steels with conventional tooling.Compressed air: aids chip removal and slight cooling, not lubrication.

Common Pitfalls:
Using straight cutting oils without proper sulfur activation for heavy-duty work; ignoring compatibility of sulphurised oils with non-ferrous materials (staining risk).

Final Answer:

Sulphurised mineral oil