“Stainless steel” is a large family that includes hundreds of specific grades. Choosing the wrong grade may cause parts to corrode, crack or fail too early. This can lead to huge economic losses or even safety risks.
For key parts, choosing the right material from the casting stage is the first step to ensure products are reliable for a long time.
QD is a professional stainless steel casting manufacturer. We will explain the properties of the most commonly used 304 and 316 stainless steel. We will also provide clear suggestions for material selection.
Why Doesn’t Stainless Steel Rust?
The reason stainless steel doesn’t rust comes from a clever scientific principle: passivation.
The core secret of stainless steel lies in Cr in its alloy composition. When the chromium content reaches about 10.5% or more, after contacting oxygen in air or water, a very thin but very dense and stable transparent oxide film forms on its surface automatically. This film is called the “passivation film”.
The functions of this passivation film:
- Barrier function: It adheres firmly to the surface of the steel. It effectively blocks the contact between corrosive substances (like water and air) and the inner metal.
- Self-repair: If the passivation film on the surface has local damage due to mechanical or chemical reasons, chromium will quickly combine with oxygen under conditions with oxygen. It will form a new passivation film again to achieve self-repair.
Note: The passivation film does not make stainless steel completely free from rust. When in a strong corrosive environment, or when the passivation film cannot be repaired in time after damage, stainless steel may still have local corrosion. Different grades of stainless steel have obvious differences in the stability and corrosion resistance of their passivation films.
Grade Analysis – 304 Stainless Steel
304 stainless steel is the most well known all purpose grade among austenitic stainless steels. It accounts for more than 40% of QD’s stainless steel casting orders.
Composition Features
High chromium and nickel content: Typical 304 stainless steel contains about 18% Cr and 8% Ni. Cr provides corrosion resistance. Ni stabilizes the austenitic structure and improves the material’s toughness, formability and weldability.
Advantages
- Good overall corrosion resistance: It has good resistance to air, fresh water, food media and many chemicals.
- Excellent formability and weldability: It is easy to perform cold working such as stamping, stretching and bending. Its weldability is good.
- Good low temperature performance: It is not easy to become brittle in low temperature environments.
- Attractive appearance: It is easy to polish and maintains a bright and clean look.
Limitations
Sensitive to chloride ions: In environments containing chloride ions, the passivation film is easily broken locally. This causes pitting corrosion and crevice corrosion.
Typical Applications
- General mechanical parts and structural components
- Food processing equipment (not for extremely harsh corrosive environments)
- Architectural decorations and household items
- Kitchen equipment and sinks
Grade Analysis – 316 Stainless Steel
When 304 stainless steel cannot meet corrosion resistance requirements, 316 stainless steel becomes a natural upgrade option. It achieves a huge improvement in corrosion resistance through a key change in its composition.
Key Composition Difference
Adding Mo: Based on 304 stainless steel, 316 stainless steel adds about 2% to 3% Mo. This is the core reason for its performance improvement.
How Does Molybdenum Improve Performance?
The addition of Mo greatly enhances the stainless steel’s ability to withstand harsh environments:
- Enhancing passivation film stability: Mo mixes into the passivation film, forming a denser “Cr-Mo-O” composite passivation film. This film is much more resistant to chloride ions than the pure chromium passivation film of 304. It can effectively prevent the adsorption and penetration of chloride ions.
- Inhibiting pitting corrosion spread: Even if the passivation film has small damage by accident, Mo can slow down the speed of chloride ions gathering at the corrosion point. It prevents pitting corrosion from deepening further and buys time for the self repair of the passivation film.
Advantages
① Extremely strong resistance to pitting corrosion and crevice corrosion. Suitable for seawater, salt water and chlorine containing chemical medium environments.
② Higher high temperature strength than 304. It can maintain stable performance below 800℃ and can be used in medium and high temperature corrosion scenarios.
③ Better corrosion resistance to organic acids (such as acetic acid, citric acid) and weak alkalis than 304.
Limitations
① Higher cost than 304. The raw material price is usually 15% to 25% higher. The casting cost increases accordingly.
② Because it contains molybdenum, its formability is slightly worse than 304. The casting process for complex shaped castings requires more precise control.
Typical Applications
- Marine environments: Marine equipment, coastal building components
- Chemical and petrochemical equipment: Containers, pipelines, heat exchangers
- High end food and pharmaceutical industries: Equipment that comes into contact with high salt and acidic media
- Medical implants and surgical instruments
304 vs. 316
To show the differences between the two more clearly, please refer to the table below:
| Comparison Dimension | 304 Stainless Steel | 316 Stainless Steel |
| Core Composition | Cr 17-19%, Ni 8-10.5%, C ≤ 0.08% | Cr 16-18%, Ni 10-14%, Mo 2-3%, C ≤ 0.08% |
| Corrosion Resistance | Good. Suitable for most mild environments. | Excellent. Especially resistant to chloride ion pitting and crevice corrosion. |
| Formability and Weldability | Excellent | Good (process optimization needed) |
| Raw Material Cost | Lower (benchmark value 100) | Higher (about 115-125) |
| Application Scenarios | General machinery, non-high-chlorine food equipment, construction | Marine, chemical industry, high-chlorine food and pharmaceutical, medium-high temperature |
| Key for Material Selection | Avoid use in water or chemical environments with chloride ions | First choice for chlorine-containing environments and chemical contact |
| QD Casting Order Proportion | About 40% | About 30% |
Meaning of the “L” Version: Born for Welding
You may often see grades like 304L or 316L. The “L” here stands for Low Carbon. The core difference is in carbon content: 304L has a carbon content of ≤ 0.03% (while 304 is ≤ 0.08%), and 316L has a carbon content of ≤ 0.03% (while 316 is ≤ 0.08%).
This seemingly small difference in carbon content is very important for the corrosion resistance of welded parts.
Why Is Low Carbon So Important for Welding?
When stainless steel castings need welding, the welding area will go through high temperatures. At this time, if the carbon content in the stainless steel is relatively high, carbon will react with Cr at the grain boundaries. It forms chromium carbide and precipitates.
This precipitation leads to two serious problems:
- Chromium depletion at grain boundaries: A large amount of chromium near the grain boundaries is consumed. This makes it impossible to form a passivation film at the grain boundaries, and the corrosion resistance drops sharply.
- Intergranular corrosion: Corrosive media will first attack the grain boundaries with chromium depletion. This causes the loss of bonding force between grains. When the part is under stress, it is easy to crack along the grain boundaries. This kind of corrosion is called “intergranular corrosion”. It has strong concealment and great harm — the surface of the part seems intact, but the inside has already “rotted”.
But, low carbon stainless steels like 304L and 316L have very low carbon content (≤0.03%). When welding, the amount of reaction between carbon and chromium is greatly reduced. They hardly form chromium carbide precipitates, which fundamentally avoids the risk of intergranular corrosion.
Selection Recommendations for 304L/316L
1. Situations where the “L” version is required:
- The casting needs welding, and it will be used in a corrosive environment after welding.
- The welded part needs pickling and passivation treatment (pickling will worsen intergranular corrosion).
2. Situations where the “L” version is not needed:
- The casting does not need welding, or is only welded in a non corrosive environment.
- It is sensitive to cost, and there is no risk of intergranular corrosion in the working conditions (304 and 316 have better cost performance in this case).
QD’s Material Guarantee
At QD, we know well that materials are the foundation of product quality.
Accurate composition control: We are equipped with direct reading spectrometers. We conduct fast and accurate on site composition analysis for each batch of molten steel. The content of key elements such as Cr, Ni and Mo strictly meets the requirements of target standards like ASTM and GB.
Complete material traceability: We provide material certification reports issued by authoritative third party organizations for each batch of materials. These reports usually include Grade 3.1 or the more strict Grade 3.2.
Technical support from metallurgical experts: Our team can recommend the most economical and efficient grade for you according to your specific needs. This is done on the premise of ensuring performance. It helps avoid the wrong selection of “excessive performance” or “insufficient performance”.
Contact Us
Are you unsure between 304 and 316? Or do you have needs for other special stainless steels? The team of metallurgical and casting experts at QD Casting Factory is willing to provide you with free material selection consultation.
Please briefly describe the working conditions your parts will face, such as:
- Contacted media (types and concentrations of water, acids, alkalis, salts)
- Operating temperature (normal temperature, high temperature, low temperature or alternating changes)
- Force bearing conditions (load bearing, wear resistance, impact resistance)
- Special industry standards (food grade, classification society, pressure vessel certification, etc.)
We will provide you with professional grade recommendations and casting process plans based on this key information.
FAQ
1. What is the maximum temperature resistance of 316 stainless steel? Can it be used in high temperature working conditions?
316 stainless steel has good high-temperature strength and oxidation resistance below 800℃. It can be used in medium and high-temperature scenarios. If the temperature exceeds 800℃, its mechanical properties will decrease. You need QD experts to recommend a more suitable high-temperature stainless steel grade based on the specific temperature and force conditions.
2. How to decide whether to choose 304 or 316 for my parts?
① If it comes into contact with seawater, salt water, chlorine containing detergents, etc., choose 316 first.
② If it comes into contact with strong acids, strong alkalis, or is used in marine or chemical environments, choose 316.
③ If it is only used in fresh water, air, or weak acid and alkali environments (such as general machinery, construction), choosing 304 is sufficient.
When you are unsure, you can provide the working conditions to QD experts. We will provide free material selection suggestions.
3. What type of stainless steel is usually used for food grade applications?
“Food-grade” does not only refer to the grade. It also emphasizes surface condition (easy to clean, no dead corners) and compliance with hygiene standards. Both 304 and 316 are often used in the food industry. For equipment that comes into contact with high-acid or high-salt foods, 316/L is a better choice. QD can provide precision castings and surface treatments that meet food hygiene requirements.
4. Do stainless steel castings need heat treatment?
They usually do. After casting, there are residual stresses inside the castings. Their structure may also be uneven. Through solution treatment, the precipitated carbides can be dissolved. This eliminates stresses and restores the corrosion resistance and toughness of austenitic stainless steel.
5. Can QD provide material certification for the grade I need?
Absolutely. We promise to provide standard-compliant material certificates for all key components. This ensures the castings you purchase have clear material composition, traceable source and reliable quality.