Three grades, three different jobs
304, 316 and 17-4PH are the stainless steels most often requested for machined parts. They are not three strengths of the same material. 304 and 316 are austenitic grades valued for corrosion resistance and formability, while 17-4PH is a precipitation-hardening grade chosen for strength. Picking the right one is mainly a question of environment and load.
This article compares them by environment, strength, magnetism, machinability and weldability, using typical handbook values. For turned shaft drawings specifically, see the separate shaft drawing guide; here the focus is the grade decision itself.
Grade at a glance
| Topic | 304 | 316 | 17-4PH |
|---|---|---|---|
| Family | Austenitic | Austenitic (with molybdenum) | Martensitic precipitation hardening |
| Key alloying | ~18% Cr, ~8% Ni | ~17% Cr, ~10% Ni, ~2% Mo | ~17% Cr, ~4% Ni, ~4% Cu |
| Typical yield, annealed or H1150 | ~215 MPa | ~205 MPa | ~725 MPa (H1150) |
| Typical yield, highest condition | Raised only by cold work | Raised only by cold work | ~1170 MPa (H900) |
| Hardenable by heat treatment | No | No | Yes |
| Magnetic | Essentially no when annealed | Essentially no when annealed | Yes |
| Chloride resistance | Moderate | Good | Moderate, similar to 304 |
| Machinability | Fair; work hardens | Fair; work hardens, slightly gummier | Good in Condition A and H1150 |
Choosing by environment
Indoor, dry or mildly humid
304 is usually enough. It resists ordinary atmospheres, fresh water and many food products. Most kitchen equipment, enclosures and general hardware use it.
Outdoors near the coast, de-icing salts, swimming pools
Chlorides cause pitting and crevice corrosion. 316 contains molybdenum, which noticeably improves resistance to chloride pitting. It is the usual choice for marine hardware, coastal installations and chemical process equipment.
Seawater immersion and hot chlorides
Even 316 can pit in stagnant seawater or warm chloride solutions. Duplex stainless steels or higher-alloy grades may be needed. Treat this as a specialist decision and get materials advice.
Medical and pharmaceutical
316L, the low-carbon version of 316, is widely used for its corrosion resistance and weldability. Implant applications use specifically certified grades such as 316LVM; normal 316 bar is not interchangeable with those.
Highly loaded parts with moderate corrosion exposure
17-4PH provides strength several times that of annealed 304 while keeping corrosion resistance broadly similar to 304. It suits shafts, valve parts, pump components and aerospace fittings.
Understanding 17-4PH conditions
17-4PH is supplied in Condition A (solution annealed) and is then aged at a chosen temperature to reach its final properties. The condition name gives the aging temperature in Fahrenheit.
| Condition | Typical character |
|---|---|
| H900 | Highest strength and hardness, lowest toughness |
| H1025 | High strength with improved toughness |
| H1075 | Balanced strength and toughness |
| H1150 | Lower strength, best toughness and stress-corrosion resistance |
| H1150M | Softest aged condition, often used for further cold work |
Parts are often machined in Condition A and aged afterwards. Aging causes a small, predictable dimensional change, typically a slight contraction, so tight features may be finish-machined after aging. Machining pre-aged H1150 bar is also common when tolerances are tight. Specify the final condition on the drawing; the shop decides the route with you.
H900 has the lowest toughness and is more sensitive to stress-corrosion cracking. Many designers choose H1025 or H1150 unless the highest strength is truly needed.
Magnetism
Annealed 304 and 316 are essentially non-magnetic, but cold work, such as machining, bending or drawing, can make them slightly magnetic. Welds can also become slightly magnetic. If a part must be non-magnetic, for example near sensors or in MRI environments, state a magnetic permeability limit and expect possible extra annealing.
17-4PH is always magnetic. That is not a defect and does not indicate a wrong grade.
Machining considerations
304 and 316 work harden quickly. If a cutter rubs instead of cutting, the surface hardens and the next pass becomes harder. Sharp tools, steady feeds and rigid setups are essential. 316 tends to be slightly gummier than 304. Free-machining 303 machines far better than either and is a common substitute for small, non-welded parts where its somewhat lower corrosion resistance is acceptable.
17-4PH in Condition A or H1150 machines reasonably well. Machining in H900 is possible but slower and harder on tools.
Welding
304L and 316L, the low-carbon versions, are preferred for welded assemblies because they resist sensitization, a loss of corrosion resistance near welds caused by chromium carbide formation. 17-4PH can be welded but usually needs post-weld heat treatment to restore properties; it is less often used in welded structures.
Surface treatment
All three grades benefit from passivation after machining to remove free iron and restore the protective oxide film. Electropolishing gives a smoother, cleaner surface and is common on 316L for medical and food equipment. These treatments are covered in a separate finishing article.
Common grade substitutions
- 303 for 304 in small machined parts that are not welded
- 316L for 316 where welding is involved
- 304L for 304 in welded assemblies
- 15-5PH for 17-4PH where better transverse toughness is needed
- 316 for 304 when moving equipment closer to the coast
Substitutions should be agreed in writing. A drawing that simply says "stainless steel" invites the cheapest acceptable grade, which may not suit the environment.
A quick decision path
- Non-corrosive indoor use, no special strength: 304, or 303 for easier machining
- Chlorides, marine air or chemical exposure: 316 or 316L
- Welded construction: 304L or 316L
- High strength with moderate corrosion resistance: 17-4PH, usually H1025 or H1150
- Non-magnetic requirement: 316 with a permeability limit noted
Cost and availability
304 is the most widely stocked stainless grade and usually the least expensive of the three. 316 typically costs more because of its nickel and molybdenum content. 17-4PH is stocked mainly as round and flat bar; heat treatment adds a step and some lead time. On small, heavily machined parts, machine time often outweighs the material difference.
Galvanic and crevice issues
Stainless steel can still corrode when the design creates the wrong conditions. Tight crevices, such as under washers, gaskets or overlapping plates, trap stagnant water and allow local corrosion even in 316. Where possible, seal crevices, open them up so they drain, or choose a more resistant grade.
When stainless is fastened to aluminum or carbon steel in a wet environment, the less noble metal corrodes faster. Insulating washers, sealants or coatings reduce the effect. Mixed-metal assemblies outdoors deserve a quick review at the design stage.
Cleaning and maintenance also play a part. Chloride-containing cleaners, bleach and steel wool can leave residues or embedded iron that start rust spots on otherwise sound stainless parts. Good passivation and proper cleaning instructions prevent most of these complaints.
Heat and temperature limits
304 and 316 keep good toughness down to very low temperatures, which makes them useful for cryogenic equipment. At high temperatures they resist oxidation well, although long service in a certain intermediate temperature range can reduce corrosion resistance in non-L grades. 17-4PH should not be used above its aging temperature for long periods, because its properties will change. If service temperatures are well above room temperature, mention them in the request.
What to put on the drawing
State grade, condition and any certificate needs. Examples: "316L per ASTM A276, passivate per ASTM A967" or "17-4PH, H1150, hardness to be verified". If the part is safety-critical, ask for mill certificates and, for aged 17-4PH, a hardness check on each heat-treatment lot.
Getting a second opinion
Corrosion depends on many details, including crevices, galvanic couples with other metals, cleaning chemicals and temperature. If the environment is unusual, describe it in your quote request. An engineer can suggest a grade and flag design details, such as tight crevices, that cause corrosion regardless of grade.