What Are the Selection Principles of Pressure Vessel Steel?

What is steel for pressure vessels?

Steel for pressure vessels refers to the special steel used for making pressure vessels. Generally refers to strength steel.

In order to meet different design and manufacturing requirements, a series of steel grades can be selected according to the strength level, including carbon and low alloy high strength.

At present, there are five strength grades of 20R, 16MnR, 15MnVR, 15MnVNR and 18MnMoNbR listed in the special steel series for pressure vessels in China.

In the design of pressure vessels, the correct selection of structural materials is crucial to ensure the reasonable structure, safe operation and reasonable economy of the vessel.

The selection of steel shall be based on the design pressure, design temperature and medium characteristics of the equipment.

The selected materials shall have good mechanical properties, corrosion resistance, good welding performance and cold and hot processing performance under the design conditions.

In addition, the most economical materials should be selected to reduce the equipment cost.

1. Steel commonly used in chemical and petrochemical plants

Steel commonly used in chemical and petrochemical plants is classified and defined as follows according to its chemical composition and metallographic structure:

1. Carbon steel

Iron carbon alloy with manganese content less than or equal to 1.2% and carbon content less than or equal to 2.0%, and other alloy elements are not intentionally added.

Low carbon steel generally refers to carbon steel with carbon content less than or equal to 0.25%.

Considering the weldability of steel, the carbon content of steel used for welding structural pressure components shall not be greater than 0.25%.

In other words, carbon steel used for welding pressure vessels is low carbon steel.

The carbon steel mentioned in this material selection principle is low carbon steel.

2. Low alloy steel

Low alloy steel is the general name of low alloy high strength steel and Pearlitic heat-resistant steel.

Among them, low-alloy high-strength steel refers to alloy steel with an alloy content of less than 3.0% for the main purpose of improving steel strength and comprehensive properties.

For example: 16MnR, 15MnV, etc.

3. Pearlitic heat-resistant steel

It refers to low-carbon pearlitic heat-resistant steel with the main purpose of improving the heat-resistant and hydrogen resistant properties of steel and adding chromium, Cr ≤ 10%, molybdenum and other alloy elements.

For example, 18MnMoNb, 15CrMo and other steels.

4. Austenitic stainless steel

Stainless steel whose metallographic structure is mostly austenite at room temperature.

For example: Cr18Ni9, cr17Ni12Mo2.

5. Ferritic stainless steel

Stainless steel with ferritic microstructure at room temperature.

For example: Cr13Al.

6. Martensitic stainless steel

At room temperature, most of the metallographic structure of stainless steel is martensite.

For example: Cr13.

The materials for manufacturing pressure vessels shall comply with the provisions of GBT 150 steel pressure vessels.

The upper limit of the service temperature of the specific steel grade is the maximum temperature of the specific allowable stress value provided in the allowable stress table.

See relevant standards for the chemical composition, normal temperature mechanical properties, supply status, etc. of domestic steel grades similar to ASME-II.

2. General principles for the selection of various steels:

From the perspective of procurement and manufacturing, the steel used for containers should be combined with varieties and specifications as much as possible.

(1) carbon steel

The conditions for selecting Q235-A, F, Q235-A, Q235-B and Q235-C steel grades must comply with the specific provisions of GB150.

When the wall thickness of pressure components is less than 8mm, try to use carbon steel plate.

When the wall thickness of pressure components depends on the stiffness, carbon steel is preferred.

(2) Low alloy steel

When the wall thickness of pressure components depends on the strength, low carbon steel and low alloy steel shall be selected in turn on the premise of meeting the scope of application.

That is, 20R, 16MnR, 15MnVR and other steel plates.

Carbon steel and carbon manganese steel are used at 425 ℃ for a long time.

Due to the decomposition of cementite in the steel, the graphitization tendency of carbide phase is produced, which reduces the strength, plasticity and impact toughness of the material, and the steel is obviously brittle, so it cannot be used.

Low carbon pearlitic heat-resistant steel must be used.

(3) Pearlitic heat-resistant steel

Pearlitic heat-resistant steel is generally used as heat-resistant steel or hydrogen resistant steel with a design temperature higher than 350 ℃.

(4) Austenitic stainless steel

Austenitic stainless steel is mainly used for working conditions where materials are resistant to corrosion or where materials are required to be clean and not contaminated by iron ions.

Austenitic stainless steel is generally not used as heat-resistant steel with a design temperature higher than 500 ℃.

Austenitic stainless steel is generally used as low-temperature steel only when low-alloy steel cannot be selected as low-temperature steel.

When the required thickness is greater than 12mm, austenitic stainless steel composite steel shall be preferred.

(5) Low temperature steel

When the design temperature is less than or equal to -20 ℃, low temperature steel (except low stress) should be generally selected.

If the steel is used below the brittle transition temperature of the material and the stress reaches a certain value, the material will appear brittle failure.

If the material has a certain toughness at the service temperature, brittle failure can be avoided.

In actual production, the toughness of materials is measured by impact test, and the corresponding impact value requirements are specified according to the tensile strength of materials.

In addition to meeting the requirements of tensile strength and yield strength, low-temperature steel must also meet the requirements of impact toughness.

(6) Corrosion resistant steel

Hydrogen corrosion resistant steel – when pearlitic heat-resistant steel is used as high-temperature hydrogen resistant steel, due to long-term use at high temperatures, the methane generated by the chemical reaction between hydrogen dissolved in steel and carbon accumulates, causing the steel to produce internal cracks or even cracking (i.e. hydrogen embrittlement).

Therefore, under the working condition of high temperature containing hydrogen, the Nelson curve should be checked according to the hydrogen partial pressure of the material (design pressure multiplied by the volume percentage of hydrogen) and the design temperature to obtain the steel grade suitable for the working condition.

Nelson curve can be found in HG20581.

(7) Steel for non pressure components

GB150 stipulates the steel for pressure vessels, and there is no written provision for non pressure components.

HG20581 puts forward the following provisions for the selection of steel for non pressure components:

According to the lower limit of service temperature, importance and pressure of the components, the corresponding coefficients K1, K2 and K3 are selected according to the following provisions.

Use high temperature coefficient K1:

T> 0℃,  K1=1;  0℃≤T > -20℃,  K1=2;   -20℃≤T,   K1=3.

Importance coefficient K2:

If there is damage, it will only affect the equipment locally, K2=1;

If there is damage, it will affect the whole equipment, K2=2.

Stress level coefficient K3:

Low stress level, K3=1;

The stress level is less than or equal to 2/3 of the allowable stress, K3=2;

The stress level is greater than 2/3 of the allowable stress, K3=3.

K= K1+ K2 + K3

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