Stainless Steel for Machined Parts: 304, 316L and 17-4PH Explained Stainless is never just stainless. A buyer who writes 'stainless' on a drawing has left the shop to guess between a food-grade austenitic, a chloride-resistant marine grade and a hardenable precipitation grade that machines like a dr

Stainless is never just stainless. A buyer who writes 'stainless' on a drawing has left the shop to guess between a food-grade austenitic, a chloride-resistant marine grade and a hardenable precipitation grade that machines like a dream and then becomes a wear part. The guess works until the part corrodes, seizes or breaks. The grade on the drawing is the specification.

304: The General-Purpose Austenitic

304 is the default stainless. About 18 percent chromium, 8 percent nickel, it is non-magnetic (or weakly magnetic after machining), it resists ordinary atmospheric and food contact, and it polishes well. It machines acceptably, but it work-hardens: push a tool too hard and the surface gets tough, the tool rubs, and the part burns. Sharp tools, moderate feeds and coolant are how a shop keeps 304 under control. It is the right choice for enclosures, fittings, brackets and sanitary hardware in ordinary environments.

316L: Where Chlorides Show Up

316L adds molybdenum, and that addition is the difference between ordinary corrosion resistance and resistance to chlorides. Salt air, wash-down chemicals, seawater and pool chemistry are where 304 starts to pit and 316L holds. The L means low carbon, which reduces sensitization in welded parts. It is the grade for marine hardware, food and dairy processing where aggressive cleaners are used, and medical fittings. It costs more than 304, and it earns it when the environment is wet or chemical.

17-4PH: The Hardenable Strength Grade

17-4PH is a precipitation-hardening stainless. In the condition as supplied it machines like a reasonably free-cutting steel, and after a simple aging heat treatment it reaches around 1000 MPa with good toughness and reasonable corrosion resistance. That is why it is the choice for shafts, pins, valve parts, fixture components and fittings that carry load or see wear. It is magnetic, it is not as corrosion-resistant as 316L in severe service, and the heat treatment is a separate step that has to be scheduled.

Machinability and What It Means for Price

Among these three, none is free to machine. 304 and 316L are gummy and work-harden, so they cost spindle time. 17-4PH machines better in the soft condition but the finish pass still matters. A free-machining variant like 303 exists for high-volume screw-machine work, but it has lower corrosion resistance. Choosing among them is a balance of environment, strength and cost, not a chemistry quiz.

What We Do at KGL

KGL machines 304, 316L and 17-4PH on mills and turning centers, holds tight tolerances on threaded and turned features, and supplies material certificates with the lot. The drawing review matches the grade to the environment: atmospheric, wash-down, chloride, or load-and-wear. The result is the right stainless on the right part, not the one that was cheapest at the bar supplier.

Conclusion

Three stainlesses cover most buyer needs: 304 for general, 316L for chloride and wet service, 17-4PH for strength and wear. Write the grade on the drawing and the part behaves in service the way the metallurgy promises.

FAQ

Q: Is 316L always worth the extra cost over 304?

A: No. In dry, indoor, ordinary environments 304 is fine. 316L earns its price where salt, chloride cleaners or wash-down chemistry exists. Guessing up to 316L everywhere doubles the stainless bill for no benefit.

Q: Can stainless parts be hardened after machining?

A: Only the hardenable grades. 17-4PH hardens by aging after machining; 304 and 316L cannot be hardened by heat treatment. If hardness is the goal, the grade choice has to happen at the drawing stage.

Q: Why do my stainless parts come out slightly magnetic?

A: Machining work-hardens austenitic stainless and induces slight magnetism. That is normal for 304 and 316L and does not mean the material is wrong. If true non-magnetic behavior is required, it should be specified and verified.