Machinability and corrosion resistance
Among stainless steels, sulfur-bearing free-machining grades (for example Type 303) form sulfides that break chips and make machining easier. The sulfides can, however, act as initiation sites for corrosion, so these grades are generally less corrosion resistant than standard austenitic grades (for example Type 304) and less suited to welding.
In chloride environments such as seawater, salt or some cleaning agents, molybdenum-bearing grades (for example the Type 316 family) are often considered for their pitting resistance. The choice depends on whether machining cost or the service environment weighs more.
Leaded and lead-free free-cutting steels
Carbon free-cutting steels improve machinability with additions such as sulfur and lead. Leaded grades (for example SUM24L) help chip control and tool life, but for parts subject to hazardous-substance restrictions or customer environmental standards, lead-free free-cutting steels are compared as well.
Switching to a lead-free grade can change chip form, tool life and cutting conditions, so trial machining is commonly used to confirm the change. The scope of regulations and their exemptions differ by product sector and region and are confirmed against the customer’s requirements.
Low-carbon (L) grades and titanium-stabilized grades
When austenitic stainless steel is welded or exposed to elevated temperature, chromium carbides can form at grain boundaries and deplete the surrounding chromium, making the steel susceptible to intergranular corrosion. This is called sensitization.
- Low-carbon (L) grades
- Grades such as Type 304L and 316L with reduced carbon, leaving less carbon available to form carbides. Widely used in welded construction; room-temperature strength may be slightly lower than the standard grade.
- Titanium-stabilized grades
- Grades such as Type 321 and 316Ti in which titanium ties up carbon as titanium carbide. Often considered for parts in long-term elevated-temperature service.
Lightweight materials: titanium and aluminum
Using the nominal densities of the unit weight table, titanium weighs about 0.57 times and aluminum alloy about 0.34 times as much as carbon steel for the same diameter and length. Because strength and stiffness differ, the weight saving between parts that do the same job can be smaller than this.
- Titanium: high strength for its weight and good corrosion resistance. Its low thermal conductivity concentrates heat at the tool, so machining conditions need care; material price and lead time are also weighed.
- Aluminum alloy: light and easy to machine, but lower in strength and elastic modulus (stiffness) than steel, so a larger section may be needed for the same stiffness. Wear and elevated-temperature strength are further checks.
- Weight alone does not decide the choice; stiffness, fatigue strength, corrosion resistance, machining cost and supply conditions are compared together.
An example selection sequence
- Service environment: temperature, corrosive media, load and wear
- Required performance: strength and hardness, toughness, corrosion resistance, weight
- Processing route: machining, forging, welding, heat treatment
- Regulations and customer requirements: substance restrictions, specified standards
- Supply conditions: applicable standard, dimensions and tolerance, quantity and lead time
- Confirmation: trial machining, sample evaluation, mill certificate check