Stainless Steel CNC Machined Parts — 303, 304 & 316L

Grades we machine

We machine stainless steel machined parts from nine grades on our own floor in Qidong, Jiangsu Province: 301, 303, 304, 304L, 316, 316L, 410, 416, and 17-4PH. Each grade is a distinct material with a different balance of machinability, corrosion resistance, weldability, and strength, so we recommend the grade against your drawing instead of defaulting to one. The requirement comes from the drawing; our job is to tell you which grade meets it and how it will machine.

• 301 stainless steel is the high-work-hardening austenitic grade. It gains strength as it is cold worked rather than from heat treatment, so it suits clips, spring-tempered parts and formed components where the drawing calls out a spring condition rather than a hardness.
• 303 stainless steel is the free-machining grade. The added sulfur lets it cut cleanly with less tool wear, so it is the practical default for high-volume turned parts such as stainless steel screws, nuts, and threaded fittings. That machinability is the point of the grade, and it comes at a cost: 303 gives up corrosion resistance compared with 304 and welds poorly, so it is the wrong choice for welded joints or parts that will sit in chloride or chemical service.
• 304 stainless steel is the general-purpose austenitic grade. It welds well and resists corrosion in most environments, which makes it the workhorse for brackets, housings, and fluid fittings. It cuts a little harder than 303 and work-hardens if a pass is too light, but it is still a routine material for us.
• 304L stainless steel is the low-carbon version of 304. The lower carbon level prevents chromium carbides from forming at the weld, so the heat-affected zone keeps its corrosion resistance. Choose 304L when the part is welded and then returned to corrosive service without a post-weld treatment.
• 316 stainless steel adds molybdenum for better resistance to chlorides, salt, and many process chemicals. It is the standard choice for marine hardware, pump parts, and chemical fittings where 304 would pit too quickly.
• 316L stainless steel pairs the molybdenum of 316 with the low-carbon weldability of the L grade. It is the default for welded parts in marine and chemical service, because the weld zone stays resistant after joining.
• 410 stainless steel is a martensitic grade: hardenable by heat treatment, magnetic, and used where moderate corrosion resistance plus a hardness requirement is enough. It is the grade to reach for when 304 would be too soft and 17-4PH more than the part needs.
• 416 stainless steel is the free-machining martensitic grade. Like 303 it uses added sulphur to break the chip, but unlike 303 it can also be hardened and tempered, so it suits turned parts that need both a fast cut and a wear-resistant surface.
• 17-4PH stainless steel is a precipitation-hardening grade. Solution treated and aged, it delivers far higher strength than any austenitic grade while keeping useful corrosion resistance. It is specified for shafts, high-strength fasteners, and load-bearing parts where the drawing calls out the heat-treated condition.

The L grades exist to solve one problem: sensitisation. When standard 304 or 316 is welded, chromium carbides can form at the grain boundaries and pull chromium out of the surrounding metal, which weakens corrosion resistance exactly where the weld heat touched. The low-carbon versions keep carbon low enough that this does not happen, so the L grades are the safe choice for welded assemblies. What 17-4PH buys over the austenitic grades is strength: after solution treating and aging it approaches the strength of alloy steel while keeping corrosion resistance far better than carbon steel, at the cost of a harder, more abrasive cut. If the part is welded or sits in salt water, an austenitic grade is usually the better answer; if the part carries load and the drawing calls out a heat-treated condition, 17-4PH is the grade to reach for. Solution treating and aging are arranged through our vetted partner network.

Machining characteristics of stainless steel machined parts

Stainless is tougher to machine than aluminum or free-machining carbon steel, and each grade behaves differently. The austenitic grades work-harden under the cutter, so we keep tools sharp and feeds consistent; a light rubbing pass can harden the surface and dull the edge faster than a proper cut. Stainless also conducts heat poorly, so heat concentrates at the cutting edge rather than spreading through the part. We run controlled speeds and coolant to protect tool life and hold ±0.01 mm on precision features, with ISO 2768-m as the general tolerance unless your drawing says otherwise.

303 is the exception to the work-hardening rule. The sulfur that makes it free-machining also breaks the chip cleanly and reduces built-up edge, so it turns and threads faster than the other grades and keeps tool costs down on long runs. That is why it is the practical default for stainless steel bolts and nuts in high volume. 304 and 316 cut more slowly, and 17-4PH in the aged condition is the hardest cut of the group, so its cycle time and tooling are planned around the higher strength.

Thin walls and long slender sections deflect under the cutter, so we plan the cut sequence and add support before quoting rather than chasing the tolerance later. All of this means stainless steel machined parts cost more per piece than the same geometry in aluminum or carbon steel: the material costs more, cuts slower, and wears tools faster. We return DFM feedback with your quote within 24 hours so you can see where the tolerance drives the cost before you commit, and orders start at 50 pieces per style and material.

Typical parts made from this material

Stainless suits threaded hardware, fluid-path components, and anything that must resist corrosion without a coating. It covers most of the fastener families we machine, and the grade choice follows the service conditions. Common orders include stainless steel bolts and nuts, stainless steel screws and nuts, threaded rods, shafts, bushings, brackets, and fittings.

• Stainless steel bolts and nuts in 303 for volume runs or 316/316L for marine service, with washers to match.
• Stainless steel screws and threaded rods and studs for assembly hardware, in 303 where speed matters and 304 or 316L where the joint is welded or exposed.
• Shafts and bushings and sleeves in 316L where corrosion resistance is required, or in 17-4PH where the drawing calls out strength.
• Fittings and adapters and valve bodies for fluid and process connections, in 316/316L for chloride service.
• Brackets, housings, and food-contact hardware in 304 or 304L where the part is welded after machining.

If you are unsure which grade a part needs, send the drawing through the request a quote form and we will recommend the grade with DFM feedback within 24 hours.

Surface finishes that pair well

Stainless parts often ship as-machined, but we can add finishes through our vetted partner network, managed under one quality process. The finish is chosen for corrosion performance, cleanliness, or appearance, depending on where the part lives:

• Passivation removes free iron from the surface and restores the natural corrosion resistance of 303, 304, and 316. It is the standard protective step after machining.
• Electropolishing smooths and brightens the surface for clean, low-contamination environments and improves corrosion resistance on wetted surfaces.
• Black oxide gives stainless a uniform matte black appearance for cosmetic and low-reflectance parts.
• Sand blasting and polishing and brushing create matte textures and directional finishes for covers and housings.
• 17-4PH parts are typically specified in the heat-treated condition, and the surface finish is chosen to suit the final service rather than to replace heat treatment.

Every batch ships with a material certificate (MTR) and pre-shipment photos, so you can confirm the grade before the parts reach your line.