What is pressure vessel steel?
Pressure vessel steel refers to the type of steel used in the construction of pressure vessels. Typically refers to high-strength steel.
To meet various design and manufacturing requirements, several types of steel are available based on their strength levels, including carbon and high-strength low alloy.
Currently, there are five types of steel available in China for pressure vessels: 20R, 16MnR, 15MnVR, 15MnVNR and 18MnMoNbR.
In the design of pressure vessels, it is essential to choose the correct structural materials to ensure reasonable structure, safe operation and economical design of the vessel.
The selection of steel for pressure vessels must be based on the design pressure, design temperature and characteristics of the medium that will be stored in the equipment.
The steel chosen must have excellent mechanical properties, corrosion resistance, good welding performance and the ability to withstand cold and hot processing conditions under design conditions.
Additionally, it is important to select the most cost-effective steel to minimize the overall cost of the equipment.
1. Steel commonly used in chemical and petrochemical plants
Steel commonly used in chemical and petrochemical plants is categorized and defined based on its chemical composition and metallurgical structure as follows:
1. Carbon steel
Iron-carbon alloys with manganese content less than or equal to 1.2% and carbon content less than or equal to 2.0% are defined as steel, without intentional addition of other alloying elements.
Low carbon steel refers to steel with a carbon content of less than or equal to 0.25%.
For welding purposes, the carbon content of the steel used in the construction of pressure components should not exceed 0.25% to ensure its weldability.
Therefore, low carbon steel is typically used for pressure vessel welding.
The carbon steel mentioned in these material selection guidelines refers to low carbon steel.
2. Low alloy steel
Low-alloy steel is a term that encompasses both high-strength low-alloy steel and heat-resistant pearlitic steel.
Low-alloy, high-strength steel refers to steel with an alloy content of less than 3.0%, designed to improve its overall strength and properties. Examples of this steel include 16MnR and 15MnV.
3. Heat resistant pearlitic steel
Pearlitic heat-resistant steel refers to low-carbon steel designed to improve its heat and hydrogen resistance properties through the addition of alloying elements such as chromium (Cr ≤ 10%) and molybdenum. Examples of this steel include 18MnMoNb and 15CrMo.
4. Austenitic stainless steel
Stainless steel is a type of steel that has an austenitic metallurgical structure at room temperature. Examples of this steel include Cr18Ni9 and Cr17Ni12Mo2.
5. Ferritic stainless steel
Ferritic stainless steel is a type of stainless steel that has a ferritic microstructure at room temperature. An example of this steel is Cr13Al.
6. Martensitic stainless steel
Martensitic stainless steel is a type of stainless steel that has a martensitic microstructure at room temperature. An example of this steel is Cr13.
Materials used in the manufacture of pressure vessels must meet the regulations described in GBT 150 for steel pressure vessels.
The upper limit of service temperature for a specific steel grade is the maximum temperature at which the specific allowable stress value, as listed in the allowable stress table, can be used.
Consult the relevant standards for information on the chemical composition, normal temperature mechanical properties, availability, and other details of domestic steel grades similar to those specified in ASME-II.
2. General principles for selecting various steels:
From a procurement and manufacturing perspective, it is desirable to use steel with a wide range of varieties and specifications for containers.
(1) Carbon Steel:
The selection of steel grades Q235-A, F, Q235-A, Q235-B and Q235-C must comply with the specific provisions of GB150.
For pressure components with a wall thickness of less than 8 mm, carbon steel plate is preferred.
When the wall thickness of pressure components affects stiffness, carbon steel is the preferred option.
(2) Low alloy steel:
For pressure components where wall thickness affects strength, low carbon steel and low alloy steel should be selected in sequence while ensuring they meet the scope of application.
This includes steel plates such as 20R, 16MnR, 15MnVR and others.
Carbon steel and manganese carbon steel should not be used at 425°C for an extended period as they can result in the decomposition of cementite in the steel, leading to graphitization of the carbide phase. This reduces the strength, plasticity and impact resistance of the material, making it brittle and unsuitable for use.
Instead, heat-resistant pearlitic steel with a low carbon content should be used.
(3) Heat Resistant Pearlitic Steel:
Heat resistant pearlitic steel is commonly used for heat resistant or hydrogen resistant applications with a design temperature above 350℃.
(4) Austenitic Stainless Steel:
Austenitic stainless steel is primarily used in conditions that require corrosion resistance or the need for clean, uncontaminated materials without iron ions.
Austenitic stainless steel should not be used as heat-resistant steel with a design temperature exceeding 500℃.
Austenitic stainless steel is typically only used as a low-temperature steel when low-alloy steel cannot be selected for low-temperature applications.
For thicknesses greater than 12 mm, austenitic stainless steel composite steel should be preferred.
(5) Low temperature steel:
Low temperature steel should generally be selected for applications where the design temperature is less than or equal to -20°C (excluding low stress).
If steel is used below its brittle transition temperature and the stress reaches a certain value, brittle failure may occur.
To avoid brittle failure, the material must have a certain level of toughness at service temperature, which is measured through an impact test. Impact value requirements are specified based on the tensile strength of the material.
In addition to meeting tensile strength and yield strength requirements, low-temperature steel must also meet impact resistance requirements.
(6) Corrosion resistant steel:
Hydrogen Corrosion Resistant Steel – When heat-resistant pearlitic steel is used as high-temperature hydrogen-resistant steel, prolonged use at high temperatures may cause the accumulation of methane from the chemical reaction between the hydrogen dissolved in the steel and the carbon, leading to internal corrosion cracking or even cracking (i.e. hydrogen embrittlement).
Therefore, when working with high-temperature hydrogen, the Nelson curve must be checked according to the hydrogen partial pressure of the material (design pressure multiplied by the hydrogen volumetric percentage) and the design temperature to determine the suitable steel grade .
The Nelson curve can be found in HG20581.
(7) Steel for non-pressure components:
GB150 specifies steel for pressure vessels, but there are no written provisions for non-pressure components.
HG20581 provides the following provisions for the selection of steel for non-pressure components:
Based on the lower limit of service temperature, importance and pressure of components, the corresponding coefficients K1, K2 and K3 are selected as follows:
High temperature coefficient K1:
T> 0°C, K1=1; 0℃≤T > -20℃, K1=2; -20℃≤T, K1=3.
K2 Importance Coefficient:
If damage occurs, it will only affect the equipment locally, K2=1;
If damage occurs, it will affect all equipment, K2=2.
K3 stress level coefficient:
Low level of stress, K3=1;
The voltage level is less than or equal to 2/3 of the allowable voltage, K3=2;
The voltage level is greater than 2/3 of the allowable voltage, K3=3.
K = K1 + K2 + K3
