​Principles of steel selection for pressure vessels

The selection of steel should be based on the design pressure of the equipment, the design temperature and the characteristics of the medium.
The selected material should have good mechanical properties, corrosion resistance, good welding performance and hot and cold processing performance under the design conditions.
In addition, the most economical material should be selected to reduce the cost of the equipment.
A, chemical and petrochemical installations in the commonly used steel according to its chemical composition and metallurgical organization classification is defined as follows:
1. carbon steel
Containing less than or equal to 1.2% manganese, carbon content of less than or equal to 2.0%, without intention to add other alloying elements of iron-carbon alloy. Low carbon steel generally refers to carbon steel with a carbon content of less than or equal to 0.25%. From the steel weldability considerations, for welding structural pressure components with steel carbon content should not be greater than 0.25%. In other words, carbon steel for welded pressure vessels are low carbon steel. The carbon steels referred to in this selection principle are all low carbon steels.
2. Low-alloy steel
Low-alloy steel is a general term for low-alloy high-strength steel and pearlite heat-resistant steel. Low-alloy high-strength steel is to improve the strength of steel and improve the overall purpose of the alloy content of less than 3.0% alloy steel. For example: 16MnR, 15MnV, etc. 3.
3. pearlite heat-resistant steel
Refers to improve the steel heat and hydrogen resistance for the main purpose of adding chromium Cr ≤ 10%, molybdenum and other alloying elements of low carbon pearlite heat-resistant steel. For example: 18MnMoNb, 15CrMo and other steels.
4. Austenitic stainless steel
Most of the metallographic organization at room temperature for the austenitic stainless steel. For example: Cr18Ni9, Cr17Ni12Mo2.
5. Ferritic stainless steel
Most of the metallographic organization at room temperature is ferritic stainless steel. For example: Cr13A1.
6. Martensitic stainless steel
Most of the metallographic organization at room temperature is martensitic stainless steel. For example: Cr13.
The material used in the manufacture of pressure vessels should comply with the provisions of GBT 150 "Steel Pressure Vessels", the upper limit of the use temperature of the specific steel grade is the highest temperature in the permissible stress table to provide specific permissible stress values.
Domestic steel and ASME-II similar steel chemical composition, room temperature mechanical properties, supply state, etc. see the relevant standards.
Second, all types of steel selection general principles:
From the procurement and manufacturing considerations, the steel selected for the container should be combined as far as possible varieties and specifications.
(1) carbon steel
Selected Q235-A, F, Q235-A, Q235-B, Q235-C steel conditions must comply with the specific provisions of GB150.
When the wall thickness of the pressurised element is less than 8 mm, carbon steel plates are used as far as possible. When the wall thickness of the pressurised element depends on the stiffness, carbon steel is preferred.
(2) low-alloy steel
Wall thickness of the pressurised component depends on the strength, in line with the applicable range of premise, in order to choose low carbon steel, low alloy steel. That is, 20R, 16MnR, 15MnVR and other steel plates.
Carbon steel and carbon manganese steel in the long-term use of 425 ℃, because of the decomposition of the carburized body in the steel and the tendency of graphitization of the carbide phase, so that the strength of the material, plasticity and impact toughness decline, steel obviously become brittle, so can not be used. Must use low carbon pearlite heat-resistant steel.
(3) pearlite heat-resistant steel
Pearlescent heat-resistant steel is generally used as a design temperature higher than 350 ℃ heat-resistant steel or hydrogen-resistant steel.
(4) Austenitic stainless steel
Austenitic stainless steel is mainly used for material corrosion resistance or material requirements clean can not be contaminated by iron ions.
Austenitic stainless steel is generally not used as a heat-resistant steel with a design temperature higher than 500°C.
Austenitic stainless steel is generally only used as a low temperature steel when low alloy steel cannot be selected for low temperature use.
When the required thickness is greater than 12mm, priority should be given to austenitic stainless steel composite steel.
(5) Low-temperature steel
When the design temperature is less than or equal to -20°C, low temperature steel should generally be used (except for low stress).
If the steel used in the material brittle transformation temperature below and stress reached a certain value, the material will appear brittle damage. If the material has a certain toughness under the use of temperature, it can avoid brittle damage. In actual production, the impact test is used to measure the toughness of the material and the corresponding impact value requirements are specified according to the tensile strength of the material. Low temperature steel in addition to meet the requirements of tensile strength and yield strength must also meet the requirements of impact toughness.
(6) Corrosion-resistant steels
Hydrogen corrosion-resistant steel - pearlite heat-resistant steel used as high-temperature hydrogen-resistant steel, due to long-term use at high temperatures, dissolved in the steel of hydrogen and carbon chemical reaction generated by methane aggregation, so that the steel produces internal cracks or even cracking (i.e. hydrogen embrittlement). For this reason, under high temperature conditions containing hydrogen, the hydrogen partial pressure of the material (design pressure multiplied by the volume percentage of hydrogen) and the design temperature should be checked against the Nelson curve to find the steel number applicable to the condition. The Nelson curve can be found in HG20581.
(7) Steel for unpressurised components
GB150 provides for pressure vessels with steel, non-pressure components are not expressly provided for, HG20581 on the selection of non-pressure components of steel as follows:
According to the lower limit of the use temperature of the component, the importance and the size of the pressure, the corresponding coefficients K1, K2 and K3 are selected according to the following provisions.
Use the high temperature coefficient K1:
T> 0°C, K1=1; 0°C≤T > -20°C, K1=2; -20°C≤T, K1=3.
Materiality factor K2:
K2=1 if there is damage that affects the equipment only partially; K2=2 if there is damage that affects the equipment as a whole.
Stress level factor K3:
Low stress level, K3=1;
Stress level less than or equal to 2/3 of the permissible stress, K3=2;
For stress levels greater than 2/3 of the allowable stress, K3=3.
K= K1+ K2 + K3
Check the steel grades to be used from the table below according to the plate thickness and K value.
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