Stainless steel is a very important material in chemical plants.
Chemical plants handle many corrosive substances.
It is a dangerous liquid that easily dissolves iron.
A material commonly called stainless steel exists in the world as a material that can withstand such liquids.
There are various types of stainless steel. The number is very large.
For batch chemical plants, it is sufficient to pass the test by knowing only two types, SUS304 and SUS316L.
After reading this article, you will be able to understand the differences between the two types of materials and be able to use them effectively in the field.
It will also be easier to gain the trust of the manufacturing department.
- Types of stainless steel used in batch chemical plants
- How to use SUS304 and SUS316L in batch chemical plants
- Related article
Types of stainless steel used in batch chemical plants
Let’s introduce the types of stainless steel used in batch chemical plants.
Adding elements Ni and Cr to carbon steel
Stainless steel is carbon steel with added elements.
The basic material is carbon steel, and stainless steel is an advanced material that has certain elements added to it.
Elements include nickel (Ni), chromium (Cr), and molybdenum (Mo).
There are different types depending on the degree of this addition.
Fe-Cr type (ferrite/martensite)
Adding chromium Cr to iron Fe increases corrosion resistance.
This is because an oxide film called chromium oxide forms on the metal surface.
barrier around iron
An image like this would be fine.
Ordinary iron (Fe), which has almost no Cr, rusts when exposed to the atmosphere.
However, when the Cr content exceeds 12%, corrosion hardly occurs in the atmosphere.
Stainless steel systems are grouped by the type of additives.
The basic group consisting of Fe and Cr added is called the Fe-Cr system.
Simply put, this understanding is fine.
- The more carbon C there is, the stronger it becomes.
- If there is a lot of chromium Cr, the corrosion resistance will increase.
Depending on the ratio of Cr to carbon steel, it is divided into martensite and ferrite.
Martensite has less Cr
Martensite sacrifices corrosion resistance in favor of mechanical strength.
According to JIS, the Cr content is 11.5-18% and the C content is 1.1% or less.
Rapid cooling after welding may cause martensite to precipitate as foreign matter.
Ferrite has a lot of Cr
Ferrite has a lot of Cr.
We place emphasis on corrosion resistance.
According to JIS, the Cr content is 11-32% and the C content is 0.12% or less.
Even if it is rapidly cooled after welding, crystals that become foreign substances will not precipitate.
It is also resistant to stress corrosion cracking caused by chloride ions.
Fe-Cr-Ni system (austenite)
The Fe-Cr system with Ni added is called the Fe-Cr-Ni system.
Generally, it is called austenitic stainless steel.
The reason for adding Ni is to expand the range of corrosion resistance and improve mechanical properties.
In the Fe-Cr system, there is a trade-off between corrosion resistance and mechanical properties, but in the Fe-Cr-Ni system, Ni is added to improve this.
According to JIS, the Cr content is 16-26% and the Ni content is 4-26%.
SUS304 is the basic austenitic stainless steel.
First, consider 304 for liquids that corrode iron-based materials.
I think 304 is often used for process liquid delivery piping in batch systems.
Batch deals with various types of production, so discussing corrosion resistance here requires considerable effort.
If that’s the case, it would be better to choose a material that can be used for anything.
From that point of view, iron is not used.
I use it in batch systems, thinking, “Let’s keep the process liquid at 304 or higher.”
Deciding which material to choose “for now” is a matter of different design philosophies.
If SUS304 may cause stress corrosion cracking, use SUS316L.
316L is resistant to stress corrosion cracking, so it feels safer than 304.
I think there are many batch-type enthusiasts who think that 304 is a bit unsettling, so they go up one rank to 316L .
In the past, SUS304L was sometimes chosen, but now its distribution volume is decreasing.
If you are concerned about stress corrosion cracking, you will need to upgrade from SUS304 to SUS316L.
duplex stainless steel
Duplex stainless steel consists of two phases: austenite and ferrite.
The percentage of ferrite phase is about 40-70%.
Applicable to SUS329 series.
This can be said to be the only drawback of austenitic stainless steel, which seems to be versatile. Stress corrosion cracking.
- Ferrite is resistant to stress corrosion cracking, but its corrosion resistance and mechanical strength are uncertain.
- Austenite is susceptible to stress corrosion cracking, but has strong corrosion resistance and mechanical strength.
The idea is to do good things.
Duplex stainless steel is a candidate for high-grade metals that cannot be used with 316L.
This is a very special system with high salt concentration and high temperature.
Naturally, they are very expensive, so if they are to be adopted, they are usually carefully considered during process development.
The chemical components of the four main materials are shown below.
Let’s compare SUS304 and SUS316L.
L is less carbon C
SUS316L is Low Carbon, which lowers the amount of C and alleviates sensitization.
SUS316 has molybdenum Mo
A molybdenum checker is sometimes used to distinguish between SUS304 and SUS316L.
As a person in charge of the field, just understanding this is sufficient.
SUS329J4L focuses on Ni and Cr
Let’s focus on Ni and Cr for SUS329J4L.
Ni is 0 in SUS430, which is a ferrite, 8 in SUS304, which is austenite, and 5.5 in SUS329J4l, which is a duplex stainless steel.
The amount of Cr is higher than that of ferrite and austenite.
This seems to be a necessary factor to create a two-phase structure.
Duplex stainless steel is characterized by a finer metal structure than single-layer ferrite or austenite.
Therefore, its strength is also relatively high.
However, a finer metal structure means a wider grain boundary area.
This increases the risk of sensitization, which is probably why a lot of Cr is added.
How to use SUS304 and SUS316L in batch chemical plants
If you are a process engineer or mechanical engineer, can you tell me how to use SUS304 and SUS316L differently in your factory?
This is a fairly basic matter in the field of materials.
I would like to be able to answer it.
If you input only textbook content and are not conscious of the output, it will be difficult to answer.
Verbalization is important.
That’s why we will explain how to use SUS304 and SUS316L in batch chemical plants.
SUS316L is used in areas with a slightly higher risk.
SUS316L is used in higher risk locations than SUS304.
- Not a liquid with low pH or strong acidity
- No need to use glass lining or fluororesin lining
- SUS304 isn’t bad either.
Situations like this are very common in batch chemical plants.
The point is that even SUS304 is not bad.
If SUS304 is okay, just use SUS304 instead of SUS316L.
Still, SUS316L has a longer lifespan, so let’s choose SUS316L.
SUS316L was chosen in this conflict.
In a sense, you might say it’s shocking.
If you are not aware of this, you may misunderstand that stress corrosion cracking does not occur with SUS316L.
High-risk locations in batch chemical plants
We know that SUS316L is used in high-risk locations, but where exactly are the high-risk locations?
The following locations apply:
- Reactor (piping is SUS304, equipment is SUS316L)
- High temperature locations (reactor/gas line)
- Places with high salt or slurry concentrations
- Equipment that has a higher load than other equipment, such as high pressure or high speed equipment
I will explain each.
I often see cases where the reactor is made of SUS316L even if the piping is SUS304.
Compared to piping, reactors contain a large amount of hazardous materials.
If the material is SUS304 and the equipment malfunctions due to corrosion, it will leak into the surrounding area and cause a catastrophe.
In order to avoid this as much as possible, it is a reasonable idea to use SUS316L, which lowers the risk.
Although it is slightly disadvantageous in terms of cost and delivery time compared to SUS304, it has the great advantage of providing peace of mind.
Exceptions include the following:
SUS304 may be used at the storage tank or buffer tank level. This is an example of thinking of equipment as a substitute for piping.
Stirring tanks that do not involve complicated reactions are also made of SUS304.
Unless you understand the process, it’s difficult to judge how to use them properly.
high temperature place
I would also like to use SUS316L instead of SUS304 in high temperature areas.
Specifically, reactors and gas lines.
It is said that if the temperature increases by 10°C, the reaction rate doubles .
In a sense, it is natural that you would want to use SUS316L, which lowers the risk in processes that involve high temperatures.
High salt concentration/slurry concentration
SUS316L may also be used when the salt concentration or slurry concentration is high.
This completely depends on the process fluid, so it’s a case by case basis.
This is easy to imagine if you remember that textbook corrosion data depends on temperature and concentration.
If you can use it when selecting materials in practice, you can say that you have graduated from being a beginner and have become an intermediate.
High pressure/high speed equipment
As with high-temperature equipment, it is better to use SUS316L for high-pressure and high-speed equipment.
This also aims to reduce risk.
High pressures include high pressure gas and first-class pressure vessels, and high speeds include centrifugal separators.
Under normal conditions, corrosivity is not that high, but as a result of the disturbance element of pressure becoming stronger, the corrosion rate may increase.
It will be difficult to obtain data, but if you are worried about it, I think you should use SUS316L.
Linking with erosion and corrosion, it is OK if you keep in mind that speed and pressure are also one of the corrosion factors.
If SUS316L is not available
SUS316L is not available.
What should we do in such a case?
Determine the timing of replacement while monitoring trends with SUS304.
Let’s go with this idea.
This lowers the initial cost, but increases the level of monitoring during production and increases maintenance costs.
This balance will be disrupted, so we need to be aware of the pros and cons and tackle them accordingly.
If you have been using SUS304 since the factory was built, this kind of thinking will not work, but if you are using SUS304 in an environment that originally uses SUS316L, it is important to change your way of thinking.
The number of cases where materials cannot be procured is increasing in various situations, so it is better to think about it in terms of assuming the worst case scenario.
To be extreme, I feel like all piping should be SUS304.
Materials is an essential skill for chemical plant engineers.
There are so many types, and the amount of stainless steel alone is huge.
The following books are recommended to get a complete picture.
If you want to know more, check out the article below.
We explained stainless steel used in batch chemical plants.
We explain the classification of ferrite, martensite, and austenite, as well as SUS304, SUS316L, and SUS329J4L.
In general, you should select SUS316L for areas of high importance, and SUS304 for other areas.
Please feel free to post any concerns, questions, or concerns you may have regarding the design, maintenance, and operation of chemical plants in the comments section. (The comment section is at the bottom of this article.)
*We will read all comments received and respond seriously.