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A high-end medical device company was doing a batch of precision stainless steel parts. The prints themselves stated, "316 stainless steel must be used." In an effort to cut costs, they thought, as they were all stainless, they could substitute with 304 stainless steel. The machining was fine, and the parts came out beautifully finished.

But just as the final customer's incoming inspection was finished, a small magnet derailed the whole order. The components were magnetic, failing the rigorous biocompatibility and non-magnetic environment criteria. It hit them that behind the "stainless" designation was the hidden, invisible "magnetism."

CNC machining usage in aerospace and electronics industries shares the same issues. In this article, we thoroughly uncover the mystery of the magnetism of 304 vs 316 stainless steel for you, and reveal to you how it will influence the success of your CNC machining and product.

Core Answer Summary

Comparative Dimensions	304 Stainless Steel	316 Stainless Steel
Core Microstructure	Austenitic, inherently non-magnetic.	Austenitic, inherently non-magnetic.
Common Magnetic Manifestations	Usually weakly magnetic following processing.	Weaker or non-magnetic following processing.
Source of Magnetism	Cold working (e.g., bending and cutting) converts some austenite to martensite.	Molybdenum (Mo) addition increases structural stability and phase transformation resistance.
Magnetic Permeability (Relative)	High (especially following processing)	Low
Impact of CNC Machining	Chips tend to stick together with one another since they are magnetic in nature, hindering the removal of chips.	Easy removal of chips makes them suitable for precision machining.
General Applications	Kitchenware, domestic appliances, decorative pipes, universal parts for machinery.	Containers, medical machinery, chemical machinery, and seashore environmental machines.

Why is 316 more anti-magnetic than 304? JS Team's Field Test Analysis

Since the last 10 years, the JS team has been offering CNC machining services to high-precision domains like medical, aerospace, and automotive electronics with over 5,000 custom CNC machining manufacturing projects.

We had earlier helped a medical company overcome the non-magnetic machining needs of surgical instruments, increasing the level of product qualification from 60% to 99.8%. We also solved the issue of high magnetic properties of 304 stainless steel parts for an aerospace customer, bringing them in line with standards through process optimization.

This is a summary of our hundreds of practical CNC machining experiences, and it will help you easily grasp the magnetic differences between 304 and 316 stainless steel and become a master in scientific means of identifying them. You are guaranteed that its content will effectively resolve your magnetic problems in CNC machining.

If you need stainless steel magnetic issues resolved in CNC machining, choose our custom CNC machining production services. We will provide you with quality solutions for your project achievement, and you will have full progress updates on ordering.

Resetting the Perception of Stainless Steel's Magnetism

Many individuals have this perception that "stainless steel is nonmagnetic." But this is a common misconception about the magnetism of stainless steel. In our real-world CNC machining activities, we mostly experience customers choosing an inappropriate material due to this misconception.

In fact, the magnetism of stainless steel is not absolute, its magnetism is very dependent on the microstructure of the material.

• Austenitic stainless steels (such as 304 and 316) are inherently nonmagnetic under typical conditions. But during machining and heat treatment, their internal structure may change and induce magnetism.
• Ferritic and martensitic stainless steel are inherently magnetic and can be drawn to magnets freely without machining.
• In addition, stainless steel with a single chromium content, such as 18 stainless steel, is prone to magnetization under normal conditions due to the lack of nickel element to stabilize the austenite structure.

To make it easier for you to understand the magnetism of different kinds of stainless steel, we have presented a table of the magnetism of some general types of stainless steel for your reference in selecting CNC machining parts materials:

Stainless Steel Type	Microstructure	Typical Magnetism	Post-Processing Magnetism	Common Uses
304	Austenitic	Non-magnetic	Prone to Magnetism	Typical brackets, common hardware.
316	Austenitic	Non-magnetic	Slightly Magnetic	Surgical equipment, boat hardware.
430	Ferritic	Magnetic	Minimal Magnetism	Alter Kitchen devices, trim components.
18 Stainless Steel	Ferritic	Magnetic	Minimal Magnetism	Simple hardware, non precision structural components.

304 vs. 316: Magnetic Stability

After understanding the basics of magnetism in stainless steel, you should wonder why 304 and 316, both austenitic stainless steels, would be magnetically different in stability.

Actually, magnetic difference between the two is essentially a battle of microstructural stability, and such stability has a direct impact on product performance after CNC machining.

We've developed a comprehensive table presenting the main differences between the two to help you correctly select the ideal material for your custom CNC machining manufacturing:

Comparison of Dimensions	304 Stainless Steel(stainless steel 18/8)	316 Stainless Steel	Impact on CNC Machining
Main Composition	18% Chromium (Cr), 8% Nickel (Ni)	18% Chromium (Cr), 8% Nickel (Ni), 2-3% Molybdenum (Mo)	316 is easier to maintain non-magnetic following machining due to the presence of the Mo element.
Source of Magnetism	Induced by cold working, e.g., CNC machining.	Magnetism is extremely difficult to induce during machining.	304 requires special magnetic treatment, while 316 reduces subsequent process cost.
Magnetic Stability	Poor, easily magnetized after processing, with large fluctuations in magnetic permeability.	Okay, it can maintain a low magnetic state for a long time, with a magnetic permeability close to 1.	316 is preferable to machine low-magnetic parts that are sensitive, e.g., precision parts.
Suitable Applications	CNC machining low-magnetic parts, e.g., standard brackets.	High-magnetic application, e.g., medical equipment and precision electronic components.	316 is best in the medical and aerospace sectors to reduce quality risk.

Briefly put, the magnetism of 304 stainless steel is "process-induced." Because it lacks the additional strength of molybdenum, its microstructure easily transforms from austenite to martensite after processing, which causes magnetism.

316, on the other hand, with the added strength of molybdenum, does not transform its microstructure and is less susceptible to magnetism even after complex CNC milling and drilling.

In the selection of 304 vs 316 stainless steel,our specialists can recommend the appropriate material based on your CNC machining needs, and top-notch machining techniques to ensure that the CNC machining parts meet your desired quality. If you place an order, you can get an accurate CNC machining price quote immediately, saving you costly material selection errors.

Magnet Test Failure? How to Scientifically Determine Stainless Steel Type

A majority of people use magnets to determine stainless steel type on the premise that "if it sticks, it's not good stainless steel." The process is imprecise.

As 304 stainless steel also becomes magnetically attracted following cold work like CNC machining, magnets can only be used as a quick preliminary screening tool and by no means as the ultimate requirement for distinguishing 304 from 316 stainless steel.

Three such scientific methods we use regularly in real projects are:

Chemical reagent testing

This method is easy and quite accurate and thus perfect for initial analysis on-site. You may use a testing reagent specific to stainless steel, like nickel content testing solution, by placing the reagent on the surface of the material to be tested and observing the change in color after 3-5 minutes.

If it's 304 stainless steel, the surface will be red.If it's 316 stainless steel, there will not be any discernible color change. Consult reagent instructions for individual procedures.

Spectroscopic testing

The most accurate industrial method and a strict material validation process required by the JS team before CNC machining. It checks the chemical composition of the material by comparing its emission spectrum.

It can accurately detect the quantity of elements such as chromium, nickel, and molybdenum with an error margin of less than 0.01%, allowing you to clearly determined that it is 304 vs 316 stainless steel to prevent material misuse from the very source.

Capturing machining sparks

This is an empirical method that uses operators with a minimum of five years of experience in CNC machining. During the CNC machining process, use a high-speed rotating grinding wheel to contact the material and observe the sparks generated:

304 stainless steel produces fewer sparks and with thicker lines and even branching.316 stainless steel produces fewer sparks with thinner lines, very minimal branching, and pale color.

If you need exact identification of the type of stainless steel and need machining, choose our online CNC machining services. We use advanced techniques such as spectral analysis to obtain the correct material and provide CNC machining parts in accordance with your requirements. We also provide material testing reports for further confidence.

Beyond CNC: Complete Factors Affecting Stainless Steel Magnetism

Magnetic properties are not unique to CNC machining but a multi-faceted property that depends on many factors, such as those that include material buying, machining, and additional processing, that have a direct impact on the final magnetic performance of CNC machining parts:

Chemical Composition

It's the underlying factor.As mentioned earlier, molybdenum is the underlying cause of the greater magnetic resistance of 316 compared to that of 304.

Nickel content also affects magnetic characteristics. Increased nickel content stabilizes the austenite structure and renders it resistant to magnetism. Conversely, the absence of nickel content could lead to greater susceptibility to martensitic transformation, which could lead to magnetism.

Cold working

It's the most common factor.Cold working processes such as bending, deep drawing, turning, milling, planing, and grinding can affect magnetic properties. The greater the force and frequency of processing, the greater the stress in the material, the greater the austenite to martensite transformation ratio, and the greater the magnetism.

For example, after multiple bending processes, magnetism in 304 stainless steel becomes gigantic.Processing such as custom gear machining, which requires complex milling and grinding, has a stronger magnetic induction effect due to more pronounced stress concentration.

Heat treatment

Different heat treatments give different effects on magnetic properties. Solution treatment eliminates the martensitic structure from stainless steel and restores the austenitic structure, hence reducing or eliminating magnetism.

For example, after solution treatment, the magnetic permeability of 316L stainless steel is extremely low, almost non-magnetic. But improper annealing has a tendency to enhance magnetism.

Smelting process

Uniformity of composition during smelting will affect magnetic properties.

Variations in material composition in localized areas will occur when elements like chromium and nickel are unevenly distributed while smelting,causing some areas to easily generate magnetism. Impurities acquired during smelting also affect magnetic stability.

Regardless of the reasons behind stainless steel's magnetic properties, we are able to fix them through our CNC machining. We provide you with reliable CNC machining parts to your custom CNC machining manufacturing needs without the hassle of magnetic influences on product quality.

Magnetism: An Invisible Player in CNC Machining and Countermeasures

Does magnetism in stainless steel affect CNC machining directly? Yes, especially for 304 stainless steel. Magnetism acts like a ghostly partner in CNC machining, which could cause issues with chip removal, accuracy, and other objects, so machining becomes more difficult and costly:

1. Chip removal:

During CNC machining of magnetic 304 stainless steel parts, the generated chips will be adsorbed on the surface of the tool or workpiece.

This not only affects proper coolant flow, causing inefficient tool heat dissipation and extra wear, but also prevents timely chip removal, causing scratching on the workpiece surface and affecting quality of surface finish of machined parts.

2. Precision machining issues:

For applications with very high precision in CNC machining tasks, like medical equipment and precision molds, magnetism might interfere with the machinability.

For instance, in milling thin-walled components, magnetism can induce faint attraction between the workpiece and the fixture, which displaces the workpiece position and ends up making the machined size larger than tolerances.

JS has condensed two effective, tested solutions to controlling magnetism in CNC machining below:

1.Selecting 316 stainless steel:

316 stainless steel is extremely magnetically stable and least likely to be magnetized when it is being CNC machining.

This can actually reduce chip removal and accuracy issues significantly, which makes it particularly suitable for magnetism-sensitive industries such as the medical and aerospace industry, aside from reducing the expense of post-processing.

2.Post-processing:

Solution treatment may be performed following CNC machining, if 304 stainless steel is already in use. Treatment eliminates the martensitic structure of the material, reduces magnetism, and adds magnetic specifications to the component.

JS Precision Manufacturing offers surefire solutions to magnetism issues in CNC machining. Contact our online customer service for details on our custom CNC machining manufacturing capacity. As soon as you place an order, we will immediately arrange for production for on-time delivery and avoid rework due to magnetism issues.

Case Study: A High-End Medical Project on the Brink of Collapse Due to Magnetism

Client Background

One startup company produced non-magnetic precision surgical instrument fixtures for a European label. They bought 100 fixtures at around $150 USD per unit. A batch that would have been rejected would have resulted in losses of more than $15,000 USD.

In their first small-batch trial production, they used 304 stainless steel to minimize costs. The components produced using a CNC machine were to their accuracy standards, but upon testing by the customer using a gaussmeter, they discovered weak magnetism in some areas far exceeding the set limit (relative permeability <1.05). The entire batch was rejected.

JS Analysis and Solution

1.Root Cause Analysis

After we got the client's commission, we first tested their scrap material. Spectroscopic analysis proved positive for the reason that the material was 304 stainless steel.

Further investigation revealed that magnetism resulted from localized hardening during CNC milling and drilling processes. Internal stress caused by the excessive depth of cut at high values during machining converted austenite to martensite, producing magnetism.

2.Material Switch

We strongly recommended switching to 316L stainless steel (ultra-low carbon). The "L" signifies ultra-low carbon. This material is not only more magnetism-resistant than 304, but also offers better corrosion resistance, a factor that makes it preferable for medical device use.

3.Process Improvement

Our engineers reviewed their CNC toolpaths and implemented two major changes:

• We employed a "high-speed milling" method with a lowered depth of cut and increased feed rate to minimize the impacts of machining on the material's structure.
• We modified the coolant spray angle to make room for effective evacuation of chips and added a process of stress relief after machining in order to further reduce residual magnetism.

Results

Parts machined with 316L material and with the new parameters were gauged on a gaussmeter and showed that magnetic permeability of all parts remained between the range 0.98-1.02, precisely as demanded by the customer of <1.05.

Ultimately, the customer not only avoided a loss of $15,000 but also got the product three days ahead of time. Our services helped the company establish uniform production standards and achieve long-term trust of European brands.

Lesson learned: Material selection is not just cost and corrosion resistance. The interaction of its physical properties (e.g., magnetism) and machining process is critical to the success of high-end manufacturing.

FAQs

Q1: Does non-magnetic stainless steel exist completely?

In theory, completely stable austenitic stainless steel is non-magnetic. In practice in industrial manufacture, however, it is difficult to achieve an absolute "zero" magnetic permeability. We would normally refer to materials with very low magnetic permeabilities, very close to 1, as "non-magnetic stainless steel." 316L solution treated, for example, would fall into this category.

Q2: Does stainless steel magnetism affect welding?

The magnetism of stainless steel does affect welding. When welding magnetic materials, the magnetic field may disturb the arc and produce "magnetic blow." The effect might cause arc instability and weld bead position deviance, significantly decreasing welding quality. Magnetic workpieces thus have to be demagnetized before welding.

Q3: Why are 304 sometimes attracted by magnets?

The reason why 304 sometimes gets attracted by magnets is that cold working induces magnetism. 304 stainless steel is usually austenitic and non-magnetic in nature. But after cold working processes such as bending, deep drawing, turning, milling, planing, and grinding, its internal structure is changed, some of the austenite transforming into martensite, causing magnetism.

Q4: Is strong magnetism an indication of poor quality?

No.Magnetism is a physical property of stainless steel that is largely governed by microstructure and material processing technology, and it is not a standard for judging the quality of stainless steel. A quality determination of stainless steel depends on critical indices such as composition, corrosion resistance, and mechanical properties.

Summary

Choosing between 304 and 316 is a matter of technical preference impacting the performance of products, regulatory compliance, and even a company's reputation. When it comes to CNC machining, using the right materials and understanding the science of magnetism are essential in guaranteeing the successful completion of custom CNC machining manufacturing projects.

Are you ordering materials for a project that has very specific magnetic requirements? Or must you concern yourself with magnetic components of parts produced by your current supplier? JS can provide you with cost-effective CNC machining prices and high-quality online CNC machining services.

Contact our experienced team, and we'll:

• Provide accurate material identification services to ensure the quality of received material content.
• Recommend and supply the most suitable 316/316L or other specialty non-magnetic stainless steels.
• Develop optimized CNC machining schedules to control magnetism at the point of origin.