Drawing Says “Stainless Steel”? 5 Blind Spots on CNC Machined Parts

Drawing Says “Stainless Steel”? 5 Blind Spots on CNC Machined Parts

Written by: Fusort Machinery Engineering Team

Reviewed: October 26, 2026

（Scope note: This is written from the material selection questions we get asked most often in quotes and drawing reviews. It’s meant to help you narrow down the right direction. The standards mentioned are general references. For a specific project, we’d still recommend confirming against your actual environment, drawing requirements and industry standards, and running third-party testing where needed. It doesn’t replace material certificates or an engineering review.）

Almost every week we receive an inquiry where the material field on the drawing just says “stainless steel”, or where the customer says “we want 316, the extra cost is fine.”

Neither is wrong, but both leave a gap. If the drawing only says “stainless steel”, whoever quotes has to guess the grade, and a wrong guess means the price and lead time don’t match. If 316 is specified, sometimes it’s genuinely needed and sometimes it’s just playing safe, in which case the extra money buys nothing.

Choosing stainless steel for machined parts isn’t only about which grade resists corrosion better. It also involves how hard the material is to machine, the total cost, and the surface condition after machining. Here are the blind spots we see people hit most often.

Five blind spots when choosing the grade

One: comparing only the material price, not the machining cost

Why it gets missed: At quoting stage people compare price per kilogram, and the gap between 316 and 304 is easy to see. But on machined parts, machining time and tool wear often make up a large share of the total cost.

Typically, both 304 and 316 work-harden, and 316 is generally gummier and harder to chip-break, so tools wear faster and cycle time or tool changes can go up for the same part. That’s why the real price gap between two grades usually isn’t the same as the raw material price gap.

How to spot it: Ask for the material cost and machining cost to be listed separately. If switching the same part to 304 noticeably lowers the machining cost, material choice is affecting your cost more than you thought. How much it differs depends on the part’s geometry, so we wouldn’t give one universal percentage.

Two: treating 303 as a cheaper 304

Why it gets missed: 303 is also stainless, looks almost identical, and typically machines much more easily, with faster cycle times and an easier-to-achieve surface finish. So it’s tempting to see it as a machining-friendly version of 304.

But 303 has sulfur added to improve machinability, which typically means lower corrosion resistance than 304 and poorer weldability. In humid or chloride environments, or where welding is needed, the risk is higher than with 304.

How to spot it: Start by asking about the end-use environment and downstream processes. For dry indoor parts that aren’t welded and serve a mechanical function, 303 can be a sensible choice. For chloride exposure, welding, or medical and food-contact applications, talk to an engineer before settling on it.

Three: looking only at the environment, not the surface condition after machining

Why it gets missed: The material certificate says 316, but how a part actually holds up isn’t decided by the grade alone. Tooling and fixtures can leave free iron on the surface, and a rough surface tends to hold contaminants more easily.

A common practice is to clean and passivate after machining, to help the surface re-form a stable passive layer. Typical methods are nitric acid passivation and citric acid passivation, and acceptance often refers to general standards such as ASTM A967, or the customer’s own specification. Whether it’s needed, which process to use and what acceptance standard applies depends on the part’s use and the customer’s requirements, so it’s best to state it at the quoting stage.

How to spot it: If parts machined from the same batch of material show local rust spots in service while the material certificate is fine, look at the post-machining surface condition.

Four: using a magnet to identify the grade

Why it gets missed: Many people have heard that 304 and 316 are non-magnetic and use a magnet to check incoming parts.

The catch is that these grades are typically non-magnetic or only weakly magnetic in the annealed condition, but cold working such as turning, cold drawing or cold heading can make the surface slightly magnetic. A part that attracts a magnet doesn’t necessarily have the wrong grade, and a part that doesn’t isn’t proven to be 316.

How to spot it: Don’t use a magnet as an acceptance criterion. To confirm the grade, check the actual chemical composition on the material certificate, and run spectroscopic analysis where it matters.

Five: needing high strength but choosing 304 or 316

Why it gets missed: The requirement is “high strength and rust-resistant”, so 304 or 316 comes to mind.

But austenitic grades like 304 and 316 typically can’t be hardened by heat treatment and gain strength mainly through cold working. If the part carries high loads or needs wear resistance, other stainless types often need to be considered. Under the GB/T 20878 grade system, two groups are common: precipitation-hardening grades such as 05Cr17Ni4Cu4Nb (commonly called 17-4PH), and martensitic grades such as 12Cr13, 20Cr13, 30Cr13 and 95Cr18. Martensitic grades typically have lower corrosion resistance than 304, so there’s a trade-off between strength and rust resistance. Which grade and which heat-treated condition to use depends on the drawing and the application.

How to spot it: Ask about load and wear requirements first, then choose the grade, rather than choosing the grade first and checking whether it’s enough.

How to settle the grade step by step

Step one: write down the use environment. Indoor or outdoor, whether it involves coastal air, de-icing salt, pools, chlorinated cleaners, bodily fluids or disinfectants, and say so if the exposure is only occasional.

Step two: write down the functional requirements. Whether it needs welding, load-bearing or wear resistance, whether magnetism or surface roughness matters, and whether it’s in a field such as medical, food or aerospace that has its own rules.

Step three: put a specific grade on the drawing. Don’t just write “stainless steel”. Name the grade, and if you work to Chinese standards, GB/T 20878 designations can be used: 06Cr19Ni10 corresponds to what is commonly called 304, 06Cr17Ni12Mo2 to 316, 022Cr17Ni12Mo2 to 316L, and Y1Cr18Ni9 to 303. Add the standard requirement if needed. If you’re unsure, ask an engineer to help confirm before you mark it.

Step four: ask for the material certificate and check the actual chemical composition, not just the grade name on top. A material certificate is typically the mill’s quality certificate, and it generally includes the heat number, chemical composition and mechanical properties. For large orders or sensitive applications, run spectroscopic analysis or third-party testing.

Step five: confirm on the first article. For parts with corrosion requirements, we’d suggest verifying the first piece in conditions close to real use before running the batch.

Mistakes we see

Assuming 316 is always better and using it wherever possible, which in an environment without chlorides means extra cost and possibly longer cycle time.

Writing only “stainless steel” on the drawing and letting the shop decide, so the delivered grade doesn’t match the customer’s expectation.

Using a magnet or a spark test to judge the grade as an acceptance criterion.

Ignoring passivation and cleaning after machining and focusing only on the grade, then going back to investigate after rust spots appear in service.

Not confirming industry-mandated material requirements up front on medical or food-contact projects, and only finding the mismatch after delivery.

Questions we actually get asked

Q: The drawing only says “stainless steel”. Should we use 304 or 316?

A: Look at the environment first. If the part will be exposed repeatedly or continuously to chlorides, such as sea air, de-icing salt, pools, chlorinated cleaners, bodily fluids or disinfectants, 316 or 316L is usually the direct choice. If not, 304 is often enough and there’s no need to pay more. Where an industry rule applies, follow the rule.

Q: Is a 316 machined part always much more expensive than 304?

A: Usually it costs more, but by how much depends on the part’s geometry, machining time and quantity, so don’t compare material price alone. Ask the quoting side to list material cost and machining cost separately.

Q: Can we use 303 instead of 304 to save money?

A: It depends on the use. 303 typically machines better but typically has lower corrosion resistance and weldability than 304. It can be considered for dry indoor mechanical parts that aren’t welded, but we generally wouldn’t suggest it for chloride exposure, welding, or medical and food-contact use.

Q: Do machined stainless steel parts need passivation?

A: It’s usually worth considering for parts with rust-resistance requirements. Whether it’s needed, which process to use and which standard to accept against depends on the part’s use and the customer’s requirements, and it’s best confirmed at the quoting stage.

Q: My stainless part attracts a magnet. Is the wrong material used?

A: Not necessarily. 304 and 316 can become slightly magnetic after cold working. To identify the grade, look at the chemical composition, not magnetism.

Q: How are tolerances usually set on stainless machined parts?

A: Dimensions with a stated tolerance follow the drawing. For dimensions without one, drawings in China typically fall back on the general tolerances in GB/T 1804 (equivalent to ISO 2768-1), with common classes f, m, c and v, and we’d suggest stating on the drawing which class applies. Whether a tighter tolerance can be held depends on the part’s geometry, wall thickness, how it’s clamped and how it’s measured. Work hardening and heat-related movement in stainless also play a part, so please confirm with an engineer at the quoting stage with the drawing in hand, rather than relying on a single number.

Q: How should we specify material certificates and inspection reports?

A: You can describe the type you need by reference to GB/T 18253 (equivalent to ISO 10474, the document types commonly known from EN 10204), where a type 3.1 certificate is the one most often requested. Please state in the inquiry or order which certificate you need and whether third-party testing is required, and we’ll confirm against your order requirements.

Further reading

Why Does a Stainless Steel Rivet Rust?

Why Does a CNC Part Fail Tolerance Inspection？

Not sure which grade your part needs? Tell our engineering team about the end-use environment, the functional requirements and your drawing, and we can help you make the call.

Talk to our engineering team