Stainless Steel Stamped Parts: 304, 301 and 17-4PH

A stainless steel stamped part looks harmless sitting on the inspection bench. It passes a visual check, the coating is clean, the dimensions are on the print. Then it goes into the field, and eighteen months later a customer sends back a bracket with orange rust blooming at the bend, a spring clip that no longer holds tension, or a housing that cracked through the radius after a few million cycles. These are not three separate problems. They are the same problem viewed from three angles: corrosion, stress relaxation, and fatigue fracture. Pick the wrong grade for a stamped spring, a structural bracket, a sealed housing, or a load-bearing clamp, and one of those three will find you. After thirty years of quoting and troubleshooting precision metal stamping for buyers across automotive, medical, and electronics, I can tell you the grade decision is made in the first ten minutes of a project - and it is almost always made on price instead of on the data that actually predicts failure.

This guide is that data. It walks the grades that dominate stainless stamping RFQs - 304/304L, 301, and 17-4PH - plus the supporting cast (316/316L, 430, and 410) that shows up more often than buyers expect. For each grade you get the numbers that predict behavior under load: strength, hardness, work-hardening response, corrosion limits, relaxation at temperature, and fatigue. You also get the forming, tooling, and inspection consequences, because a grade choice is a process decision, not a chemistry footnote. Our lines run stainless strip from 0.05 to 3.0 mm thick on 21 presses up to 110 tons, with piloted positioning of ±0.005 mm and high-speed runs to 300 SPM, so the numbers below are tied to processes we actually run. For the manufacturing side end to end, the stainless steel stamping guide is the companion read.

The Stainless Family: Structure Sets Behavior

Stainless is not one material. It is a family of alloys held together by one common rule: at least 10.5% chromium, enough to form a self-healing chromium oxide film that stops rust before it starts. Everything else - strength, magnetism, formability, corrosion resistance, price - is set by the crystal structure and the alloying around that chromium floor. That is why the three grades that dominate precision stamping behave nothing alike once you cold-work them and put them under load.

The appeal is obvious: corrosion resistance without a plating line, a clean surface for medical and food contact, and enough cold-work headroom to make thin, springy, dimensionally tight parts at volume. The trap is that buyers assume "stainless" means "won't corrode and won't lose tension." Neither is true across the family. The numbers below are the numbers we quote against, drawn from ASTM A240, ASTM A693, and mill certification data, not from a sales sheet.

Class	Representative grades	Crystal structure	Magnetic in annealed state	Hardening route	Typical stamped part
Austenitic	304, 304L, 301, 316, 316L	Face-centered cubic (FCC)	No (lightly magnetic after heavy cold work)	Work hardening only	Housings, springs, brackets, clips
Martensitic	410, 420	Body-centered tetragonal (BCT)	Yes	Heat treatment	Cutting and wear parts, valve components
Ferritic	430, 409	Body-centered cubic (BCC)	Yes	Limited	Covers, brackets, appliance trim
Precipitation hardening	17-4PH (S17400)	Martensitic, age-hardened	Yes	Solution treat + age (H900, H1025)	Structural clamps, load-bearing brackets

Magnetism matters more than most buyers think. A spring clip for a magnetic sensor, a shield inside a solenoid, or a bracket near a reed switch can misbehave if the grade picks up magnetism or arrives magnetic from the start. Austenitic grades are the answer for non-magnetic duty; martensitic and ferritic grades are not. Put that requirement on the print with the grade, because "stainless" alone does not tell the stamping shop what you need.

Grade 304 and 304L - The 18-8 Workhorse

Type 304 is the classic 18-8 austenitic stainless: roughly 18% chromium and 8% nickel, with carbon capped at 0.08%. The low-carbon variant 304L drops carbon to 0.03% maximum to resist sensitization during welding. In the annealed condition 304 delivers a minimum tensile strength of 515 MPa and a yield of about 205 MPa, with hardness up to roughly 201 HB. Density sits near 8.0 g/cm³, and the alloy is essentially non-magnetic in the annealed state, picking up only light magnetism after heavy cold work.

For stamped parts the real story is corrosion resistance and formability, not strength. 304 is the right call for housings, covers, and non-load-bearing brackets that live in atmospheric, indoor, or mildly chemical environments. It forms beautifully - it bends, draws, and pierces without cracking in the annealed condition - it welds, and its chromium oxide passive film self-heals in normal air. The weakness is mechanical: annealed 304 is too soft to act as a spring, and cold-working it for strength pushes you toward 301, which work-hardens far more aggressively. Use 304/304L where the part's job is to enclose, shield, or separate - not where it must hold a calibrated force.

Formability is where 304 earns its keep in the press. In the annealed condition it bends flat on itself with the grain and tolerates a minimum bend radius of roughly one times the material thickness against the grain, which is why deep-drawn housings, one-piece shield cans, and convoluted covers are almost always 304. It also takes a draw ratio that the harder grades cannot. If your part is a drawn or heavily formed shape, start with annealed 304 and only move to a stronger grade if the service load demands it.

Grade 301 - Work-Hardened for Springs and Clips

301 is the spring-grade cousin of 304. Same basic chemistry family, but a lower nickel content and a much steeper work-hardening curve. That is what lets a thin strip reach real strength through the stamping operation itself. In the half-hard (1/2 hard) temper, 301 typically shows a tensile strength around 930 MPa, yield near 515 MPa, elongation about 25%, and a hardness near Rockwell B 88. Push to full hard and tensile climbs past 1275 MPa with yield around 965 MPa. That is why 301 is the default for stamped springs, leaf springs, battery contacts, and the stamped clips that hold a panel or a connector under constant deflection.

The half-hard condition matters because it balances springback control with fatigue life. A part stamped from full-hard 301 holds more force but cracks more easily at the bend radius, especially below 0.3 mm thickness. Half-hard gives you a predictable forming window and a fatigue curve you can actually design around. For a spring clip that must survive ten million cycles at moderate deflection, 301 half-hard is usually the answer - provided you respect the relaxation behavior covered below.

Because 301 reaches its strength by cold work, the temper is a specification, not an accident. Half-hard, three-quarter hard, and full-hard strip form differently, spring back differently, and deliver different spring rates, and the mill certificate is the only place the temper is written down. We verify incoming hardness on every coil and reject coils that drift, because a "301" coil that arrives at the wrong temper will stamp perfectly and fail in service. If you are designing stamped springs and clips, the spring stamping guide covers the beam geometry, grain direction, and bend radius rules that decide whether the temper survives the part design.

Grade 17-4PH - Precipitation-Hardened Strength for Load-Bearing Parts

When a stamped bracket or structural clamp must carry real structural load and still resist corrosion, 17-4PH (UNS S17400, Type 630) enters the picture. It is a martensitic precipitation-hardening stainless: about 15-17.5% chromium, 3-5% nickel, 3-5% copper, with niobium-columbium for grain control. The defining condition is H900 - solution treated and aged at 900°F (about 482°C). In H900, 17-4PH delivers a minimum tensile strength of 1310 MPa (190 ksi), yield strength of at least 1170 MPa (170 ksi), and hardness of 40 HRC or higher, typically landing in the 40-47 HRC band. That is roughly 2.5 times the strength of annealed 304 from the same thickness of strip.

The trade is twofold. First, 17-4PH is more expensive and requires the aging heat treatment after forming, which means you cannot cold-work your way to final strength the way you do with 301. The part is stamped in the soft solution-treated condition, then aged, which adds a heat-treat step and a second set of dimensional checks to the program. Second, its corrosion resistance is good but not as forgiving as 304 in chloride environments - it is the right choice for structural clips and brackets in industrial and marine-adjacent service, not for long-term immersion in salt. Where a stamped part must simply not yield, 17-4PH H900 is the grade that buys you margin.

There is a third condition worth knowing: H1025, aged at 1025°F, trades a little strength (about 1070 MPa minimum tensile) for noticeably better toughness and stress-corrosion resistance. If a part is both highly loaded and exposed to chlorides, H1025 is often the safer call than H900 even though the name is less familiar. The age condition belongs on the drawing exactly like the temper does on 301, because H900 and H1025 parts are not interchangeable.

316, 430, and 410: The Supporting Cast

Three more grades appear in stamping RFQs often enough that they deserve a section, and each one solves a problem the big three do not.

316 and 316L add molybdenum (2-3%) to the 18-8 base, which lifts resistance to pitting and crevice corrosion in chlorides dramatically. In the annealed condition 316 runs about 515 MPa minimum tensile with a yield near 205 MPa, similar to 304, but its pitting resistance equivalent number is meaningfully higher, which is why it is the default for marine-adjacent hardware, chemical processing, and anything that sees salt or repeated cleaning chemistry. For medical work, vacuum-melted 316L VM with controlled inclusions and carbon at or below 0.030% is the implant-contact standard, and the medical device stamping guide covers the cleanroom, electropolishing, and traceability chain that surrounds it.

430 is the ferritic utility grade: 16-18% chromium, almost no nickel, magnetic, and priced well below the austenitic grades. It is the right call for appliance trim, covers, and brackets in indoor environments where the corrosion demand is modest. It cannot be hardened by heat treatment, it forms less ductilely than 304 at the same thickness, and it will not match 304 in a chloride environment - but for a painted or lightly exposed part, 430 can cut material cost noticeably.

410 is the martensitic grade: 11.5-13.5% chromium, hardenable by heat treatment to far beyond its annealed strength. It is chosen for stamped parts that need an edge, a hard surface, or wear resistance - valve seats, latch components, cutting and scoring elements. The compromise is corrosion resistance that is the weakest of the group in wet service, which is why 410 parts are often plated or coated.

Grade	Class	Typical tensile (condition)	Corrosion profile	Typical stamped use
304 / 304L	Austenitic	≥ 515 MPa (annealed)	Good in atmospheric and mild chemical	Housings, covers, shields, brackets
301	Austenitic	~930 MPa (1/2 hard) to >1275 MPa (full hard)	Good, similar to 304	Springs, clips, contacts, washers
316 / 316L	Austenitic	≥ 515 MPa (annealed)	Best of the group in chlorides	Marine hardware, medical, process parts
430	Ferritic	~450-520 MPa (annealed)	Moderate, indoor duty	Trim, covers, appliance parts
410	Martensitic	~480 MPa annealed; hardenable	Weakest in wet service	Latch parts, wear edges, valve components
17-4PH	PH martensitic	≥ 1310 MPa (H900)	Good, below 316 in chlorides	Structural clamps, load brackets

Corrosion and Surface Finish: Where Stainless Stops Being Stainless

Stainless resists corrosion because of a chromium-rich oxide film a few nanometers thick, and every process step either preserves that film or damages it. Three failure modes are worth naming, because each one has a distinct cause and a distinct fix.

Pitting and crevice corrosion start where the film breaks down locally - at a chloride, at a crevice under a washer, at a scratch across a bend. The pit grows into the metal and, under load, becomes a crack starter. Grade selection is the first defense (316 for chlorides, 304 for indoor), and surface quality is the second: a smutty, heat-tinted, or contaminated surface pits sooner than a clean passivated one. This is why we specify passivation per ASTM A967 (nitric or citric acid) after forming and any heat treatment, and verify with salt-spray testing per ASTM B117 on the finished surface.

Intergranular corrosion follows sensitization: carbon precipitates at grain boundaries when the alloy spends time in the 425-875°C range, which happens in welding and in some forming and stress-relief cycles. The low-carbon grades (304L, 316L) exist precisely to resist this, which is why welded assemblies and thin, welded-then-formed parts default to the L grades.

Stress corrosion cracking (SCC) is the one that kills loaded parts in chlorides: tensile stress plus chloride plus temperature, and a normally ductile bracket cracks through with almost no warning. Austenitic grades are the most susceptible; 17-4PH in the overaged H1025 condition resists it better than H900; and 316 is more resistant than 304 but not immune. If a part is loaded, warm, and chloride-exposed, say so on the RFQ - the grade and condition decision changes completely.

Surface finish closes the corrosion loop. Electropolishing per ASTM B912 removes a controlled layer of material and leaves a smooth, chromium-enriched surface that sheds contaminants and resists pitting, which is why medical and food-contact parts specify it. Mechanical finishing, brushing, and bead blasting remove heat tint and scale but leave a mechanically worked surface that may still need passivation afterward. For a stainless part that must stay clean in service, the finish spec is not cosmetic - it is corrosion control, and it belongs on the print.

Stress Relaxation and Fatigue: The Two Silent Failures

Stress relaxation is what kills stamped springs and clips long before fatigue does. A spring is held at a fixed deflection, the load stays applied, and over time the material creeps microscopically so the force drops. Room-temperature relaxation in 301 is small but not zero; at elevated temperature it accelerates sharply. At 150°C a 301 spring held at moderate stress can retain only about 85-90% of its initial load after 1000 hours, and the number falls faster as you approach 200°C. 17-4PH H900 relaxes far less because of its precipitated microstructure, which is one reason it is specified for high-temperature clamps and fastener-style brackets.

The practical rule: if your stamped spring or clip runs hot or must hold tension for the life of the product, specify 301 with a conservative stress design and validate with a relaxation test, or step up to 17-4PH. Do not bury the issue with thicker material and hope - thicker material just stores more stress and relaxes from a higher starting point.

Fatigue fracture starts at a stress concentrator: a sharp bend radius, a pierced hole edge, a burr left by a worn punch. Grade sets the ceiling, but geometry sets where you hit it. 301 half-hard carries a strong fatigue limit for its thickness and is the natural choice for cyclically loaded clips and contacts. 17-4PH H900, with its higher strength, tolerates higher alternating stresses before crack initiation. 304, being softer, is the weakest of the three in fatigue and should not be asked to flex repeatedly. Every fatigue-resistant stamped part we run gets a radius review at the tooling stage and a deburring spec - because the material grade cannot save a part that was stamped with a knife edge.

The interaction between the two failure modes is what most drawings miss. A relaxation test alone does not predict fatigue life, and a fatigue test at room temperature does not capture what happens at 150°C, where both mechanisms accelerate. If the part is a spring that runs hot, the qualification plan needs both tests at the service temperature, not one or the other.

Forming Stainless: Springback, Bend Radii, and Tool Wear

Stainless does not form like carbon steel, and the difference shows up in three places: springback, bend cracking, and tool wear.

Springback scales with yield strength, so a half-hard 301 part springs back two to three times as far as an annealed 304 part of the same geometry. The die must be designed with over-bend compensation from the start - typically several degrees on a 90° form, more on a U-bend - and tuned against the actual coil hardness. A die cut for annealed strip will throw a half-hard coil's parts out of angle on the first hit. This is why temper and hardness belong on the drawing and the mill certificate, not just the grade.

Bend radii are the fatigue and cracking boundary. Annealed 304 bends flat on itself with the grain and to about one times the thickness against the grain. Half-hard 301 needs roughly 1-2 times the thickness with the grain and more against it; full-hard 301 and 17-4PH H900 need generous radii or the outer fiber cracks on the press. For a spring, the rule is stricter still: the bend should be formed across the strip grain where possible, and the burr must sit on the inside of the bend, never on the tension face, or the bend becomes a crack starter. A spring with a knife-edge burr on the outside radius will fail the fatigue test even with a perfect grade.

Tool wear and galling are the production cost of stainless. Austenitic grades work-harden at the cut edge, so punches and dies see higher loads and faster wear than on carbon steel, and the work-hardened slivers that break off can gall the die surface. We run stainless in tooling built from D2 or M2 tool steel with proper clearances, keep punches sharp (a dull punch raises burr height and edge work-hardening), and use lubricants formulated for stainless rather than general-purpose oils. The toolroom - wire EDM, CNC, and grinding - is where that tooling gets built and reconditioned in-house, because outside tooling vendors do not know your springback data.

Tolerances, Capability, and Process Control

A grade decision is worthless if the part ships out of tolerance. For precision stamped components in automotive and medical supply chains, we hold a process capability of Cpk ≥ 1.33 on critical dimensions. A Cpk of 1.33 means the process spread sits comfortably inside the spec limits with roughly four-sigma centering, translating to a defect expectation on the order of tens of parts per million rather than per thousand. For a stamped spring whose free length or contact force is critical, that capability is the difference between a stable line and a quarterly complaint. We hit it through progressive die design, in-die sensing, and SPC on the features that actually affect function - not by inspecting defects out at the end.

The equipment side is the enabling assumption. Our fleet is 21 presses from Aida and Zhenli Micron, 25 to 110 tons, with positioning repeatability of ±0.005 mm on the pilots, strip thickness from 0.05 to 3.0 mm, and strip width up to 650 mm. High-speed lines sustain 300 SPM on terminal and small-part work. Stainless runs at the lower end of the speed range because the material is stronger and the tooling wears faster, but the dimensional capability is the same. Quality systems run to IATF 16949:2016 with ISO 14001:2015 for environmental management, and the QC lab uses CMM, optical measurement, and in-line vision to keep the checks independent of the operator. The quality and capability page documents the system in more detail.

Three dimensions deserve specific callouts on a stainless print. Flatness, because residual stress from the strip and the cut edges curls thin parts. Burr height, because a burr is a corrosion site and a fatigue starter, and stainless burrs are harder to remove than carbon steel burrs. And thickness, because spring force scales with the cube of beam thickness - a 0.05 mm drift on a 0.5 mm beam is not a 10% force error, it is closer to 30%.

Choosing the Grade by Part Type

Springs and clips: 301 half-hard is the default. Use full-hard only when space is tight and the bend radius is generous. Reach for 17-4PH H900 when the clip runs hot or carries structural preload.

Brackets and supports: 304/304L for non-load or light-load brackets in benign environments. 17-4PH H900 for brackets that must carry structural load or resist deformation under assembly torque.

Housings and covers: 304/304L for corrosion resistance and formability, where the part's job is enclosure and shielding rather than load. 316/316L when the enclosure sees salt, cleaning chemistry, or marine air.

Clamps and retaining elements: 301 half-hard for spring clamps; 17-4PH H900 for heavy-duty or high-temperature clamps where relaxation cannot be tolerated.

Wear and cutting elements: 410, hardened after forming, when the part needs an edge or a hard surface; 316 when the same function must also survive chlorides.

Property	304 / 304L (annealed)	301 (1/2 hard)	17-4PH (H900)
Cr - Ni content	18% Cr, 8% Ni	~17% Cr, 7% Ni	15-17.5% Cr, 3-5% Ni
Tensile strength	≥ 515 MPa	~ 930 MPa	≥ 1310 MPa
Yield strength	~ 205 MPa	~ 515 MPa	≥ 1170 MPa
Hardness	≤ 201 HB	~ RB 88	≥ 40 HRC
Stress relaxation	Low (soft, low load)	Moderate, worsens above 150°C	Low, heat resistant
Relative cost	Baseline	Similar to 304	Highest of the three
Best use	Housings, covers	Springs, clips	Structural brackets, clamps

Grade Selection FAQ

Can I substitute 304 for 301 on a spring clip to save money? Only if you redesign the beam. Annealed 304 at roughly 205 MPa yield delivers a fraction of 301 half-hard's spring rate, so the clip will hold far less force at the same geometry. Substituting the grade without changing the geometry is a field failure in the making.

Why does my 304 bracket show light magnetism? Cold work transforms part of the austenite to martensite in 18-8 grades, so stamped edges and bends pick up measurable magnetism. If the part must stay non-magnetic under a sensor, either limit the forming severity or choose a stabilized grade and verify with a permeability check on the finished part.

Is 17-4PH worth the price for a bracket? Run the numbers: it is roughly 2.5 times the strength of annealed 304 from the same thickness, which can let you drop a gauge, shrink the footprint, or eliminate a reinforcing rib. The premium pays for itself when the alternative is a thicker, heavier 304 part or a second support.

Do I need plating on stainless? Usually not - the point of stainless is the passive film. Plating appears mainly on 410 parts (for corrosion) and on stainless contacts (for conductivity), and our reel-to-reel lines can apply selective gold, silver, tin, and nickel at 2-8 µm or zinc at 5-12 µm where a specific interface demands it. But if the drawing calls stainless, ask first whether the plating is solving a real problem or just habit.

How do I verify the temper on the coil? Ask for the mill certificate with hardness and temper, and have the stamper verify incoming hardness on each coil. A coil that arrives at the wrong temper will stamp perfectly and fail in service, and it is the cheapest quality check in the whole program.

Make the Grade Call Early

The cheapest time to choose a stainless grade is on the front end of the program, when a one-line note on the print settles whether you stamp from 304, 301, or 17-4PH. Change it after the tool is built and you are buying a second die. The same logic applies to the temper, the finish, and the qualification tests: each one is cheap to write on the drawing and expensive to discover in the field.

If you are sourcing precision stamped springs, brackets, housings, or clips and the grade decision is still open, send us the application - operating temperature, required force or load, environment, and annual volume - and we will quote the right material, the right temper, and a Cpk ≥ 1.33 process so the field returns never start. The fastest way in is a drawing plus those four numbers on the quote request page, and we will come back with the grade call and the reasons behind it.