Blog

What is the difference between 304 VS 316 stainless steel?

Stainless steel is a widely used and important type of steel, with 304 and 316 stainless steel being the two most widely used stainless steel grades in the world. Both have excellent corrosion resistance and practical value.

There is no definition of "which is better" for 304 and 316 stainless steel. Because different stainless steel materials have different uses. Today we will learn about the difference between 304 stainless steel and 316 stainless steel.

Processing buddies cannot do without the problem of tool selection every day. Of course, the role of tools is to remove excess surface materials. There are many material choices available, and when it comes to stainless steel, there seem to be many types, such as 304 and 316 stainless steel. How much do you know about these two grades?

304 stainless steel contains 16% to 24% chromium, as well as small amounts of carbon and manganese. The most common form is 18-8 stainless steel. It contains 18% chromium and 8% nickel. 316 grade has almost the same physical and mechanical properties as 304 stainless steel. The main difference is that 316 stainless steel contains about 2% to 3% molybdenum. Additives can improve corrosion resistance, especially for chlorides and other industrial solvents.

1.Chemical composition difference between 304 and 316 stainless steel

From the table, we can see that the biggest difference between 304 stainless steel and 316 stainless steel is that 316 stainless steel has 2-3 more molybdenum than 304 stainless steel. This helps to resist corrosion from chlorides such as seawater and de icing salts.

The maximum molybdenum content that can replace 300 series grades can reach 7%. They have better resistance to chloride. But this type of heavy-duty resistor is only needed under industrial or high concentration exposure conditions.

2.Performance difference between 304 and 316 stainless steel

304 stainless steel is the most common type of steel. As a widely used metal, it has good corrosion resistance, heat resistance, low-temperature strength, and mechanical properties. Good hot processing properties such as stamping and bending. No heat treatment hardening phenomenon (non-magnetic, operating temperature -196 ℃~800 ℃).

316 stainless steel has excellent corrosion resistance, atmospheric corrosion resistance, and high temperature strength due to the addition of molybdenum. Can be used under harsh conditions. Excellent work hardening properties (non-magnetic).

3.Application differences between 304 and 316 stainless steel

The application scope of 304 includes kitchen accessories such as sinks and splash guards, flat bottomed pans, tableware, cutlery, and cabinets. Kitchen utensils such as refrigerators and dishwashers. Indoor pipes, water heaters, boilers, bathtubs, and other household items. Heat exchanger. Commercial food processing equipment, breweries, and pharmaceutical production equipment. Manufacturing nuts, bolts, screws, and nuts. Water pipes, storage tanks, indoor electrical enclosures, and automotive interiors. Decorative strips. 316 application scope: chemical pipelines. Pharmaceutical equipment. Medical equipment and tools. Stainless steel float. Structural steel and components in marine environments. Manufacturing equipment for food, chemical, and petroleum production and processing. Laboratory bench and equipment construction. Architectural paneling in coastal areas. Marine hardware pipes.

4.Differences in Corrosion Resistance between 304 and 316 Stainless Steel

Between 304 and 316 stainless steel, 316 stainless steel has better corrosion resistance than 304 stainless steel. Has good corrosion resistance in pulp and paper production. In addition, 316 stainless steel can also resist the erosion of marine and corrosive industrial atmospheres.

Generally speaking, there is not much difference in chemical resistance between 304 and 316 stainless steel. However, there are differences in certain environments.

The initially developed stainless steel was 304. In some cases, this material is sensitive to pitting corrosion. Adding an additional 2-3% molybdenum can reduce this sensitivity. Thus, 316 was born. In addition, this additional molybdenum can also reduce the corrosion of certain hot organic acids.

316 stainless steel has almost become the standard material in the food and beverage industry. Due to the shortage of molybdenum in the world and the nickel content in 316 stainless steel. 316 stainless steel is more expensive than 304 stainless steel.

Pitting corrosion is a phenomenon mainly caused by the deposition corrosion on the surface of stainless steel. This is because a chromium oxide protective layer does not form during hypoxia.

Especially for small valves, the possibility of sedimentation on the valve plate is very small. Therefore, pitting corrosion is also rare. Among them, various types of water media (distilled water, drinking water, river water, boiler water, seawater, etc.).  

304 stainless steel and 316 stainless steel have almost the same corrosion resistance. Unless the chloride ion content in the medium is high. Now, 316 stainless steel is more suitable.

In most cases, the corrosion resistance of 304 and 316 stainless steel is not significantly different. But in some cases, it may also have significant differences. Specific analysis is required.

Generally speaking, valve users should be aware of this. Because they choose the material of containers and pipelines based on the condition of the medium.

5.heat resistance

316 stainless steel exhibits excellent oxidation resistance when used intermittently below 1600 ℃ and continuously below 1700 ℃. 316 stainless steel cannot be continuous within the temperature range of 800-1575 degrees.

When 316 stainless steel is continuously used outside of this temperature range, it has good heat resistance. Ultra low carbon stainless steel 316L has better resistance to carbide precipitation than SS316. Can be used within the above temperature range.

6.heat  treatment

The annealing process of heat treatment is carried out within the temperature range of 1850 to 2050 degrees. Then perform rapid annealing and rapid cooling. 316 stainless steel cannot be hardened through heat treatment.

7.welding

Between 304 and 316 stainless steel, 316 stainless steel has good welding performance. All standard welding methods can be used for welding. When welding, 316Cb, 316L, or 309Cb stainless steel welding rods or electrodes can be used for welding according to the purpose. In order to achieve the best corrosion resistance, SS316 welded parts need to undergo post weld annealing. If 316L stainless steel is used, post weld annealing is not required.

8.Cost Variance

The cost of stainless steel largely depends on the composition of the alloy.

All stainless steels require a minimum chromium content of 10.5%. They are alloys made of iron. However, various other elements present can have an impact on performance, characteristics, and ultimately cost.

316 contains at least 2.0% molybdenum. It is more corrosion-resistant than 304 stainless steel. Molybdenum is a more expensive element, typically making 316 a more expensive metal grade.

Due to the widespread use of austenitic stainless steel, it has left people with the mistaken impression that all stainless steels have no magnetism. For austenitic stainless steel, it can basically be understood as non-magnetic. Because this is the case with quenched forged steel. But cold formed 304 will have a certain degree of magnetism. For cast steel, if it is 100% austenitic stainless steel, it has no magnetism.

9.Low carbon type

The corrosion resistance of austenitic stainless steel comes from the chromium oxide protective layer formed on the metal surface. If the material is heated to a temperature between 450 ° C and 900 ° C. The structure of the material changes, forming chromium carbide along the edges of the crystal. Therefore, a chromium oxide protective layer cannot be formed at the edge of the crystal, resulting in a decrease in corrosion resistance. This type of corrosion is called "intergranular corrosion".

304L stainless steel and 316L stainless steel were developed to combat this corrosion. Both 304L stainless steel and 316L stainless steel have lower carbon content. Due to the reduction in carbon content, chromium carbide will not be produced and intergranular corrosion will not occur.

It should be noted that a higher sensitivity to intergranular corrosion does not necessarily mean that non low carbon materials are more susceptible to corrosion. This sensitivity is also higher in high chlorine environments.

Please note: This phenomenon is caused by high temperatures (450 ° C-900 ° C). Welding is usually the direct cause of reaching this temperature. For traditional butterfly valves with soft seats, using low-carbon stainless steel does not make much sense because we do not weld it on the valve plate. However, most specifications require 304L stainless steel or 316L stainless steel.