1. Introduction
Chromium-molybdenum steel, also known as medium temperature hydrogen resistant steel, refers to the type of steel that is significantly improved in high temperature strength and creep limit by adding alloying elements such as Cr (≤10%) and Mo.
It also has excellent hydrogen corrosion resistance and high temperature performance, making it widely used in petroleum refining, hydrogen chemical devices and high temperature equipment.
It is one of the common types of steel used in pressure vessels.
This article discusses the characteristics of chrome-molybdenum steel materials and considerations in design, manufacturing, non-destructive testing, heat treatment and startup/shutdown operations in the context of the Jiutai Methanol Synthesis Project.
2. Basic characteristics of chrome molybdenum steel
2.1 Heat Resistance
The addition of elements such as chromium, molybdenum and alum improves the steel's high-temperature oxidation resistance and high-temperature strength.
The mechanism of action is as follows: Chromium exists mainly in cementite (Fe3C), and chromium dissolved in cementite increases the decomposition temperature of carbides, preventing the occurrence of graphitization, thus increasing the heat resistance of the steel.
Molybdenum has a solid solution strengthening effect on ferrite and can also increase the stability of carbides, which benefits the strength of steel at high temperatures.
The inclusion of an adequate amount of vanadium allows the steel to maintain a fine-grained structure at higher temperatures, increasing the thermal stability and strength of the steel.
2.2 Hydrogen corrosion resistance
Elements such as chromium and molybdenum increase the stability of carbides, preventing their decomposition, thus reducing the chance of methane formation due to the reaction of carbides and precipitated carbon with hydrogen.
The addition of vanadium allows the steel to maintain a fine-grain structure at higher temperatures, significantly increasing the steel's stability under high temperature and pressure conditions.
2.3 Tempering Embrittlement
Temper embrittlement of chrome-molybdenum steel refers to the phenomenon in which the impact resistance of steel decreases when operated for a long period in the temperature range of 370°C to 595°C.
This is the exact temperature range within which our commonly used hydrogen equipment operates. Experimental studies have shown that in chrome-molybdenum steel for pressure vessels, temper embrittlement is more severe when the chromium content is between 2% and 3%.
Elements such as phosphorus, antimony, tin, arsenic, silicon and manganese have a significant impact on quench embrittlement. The embrittlement is reversible; Materials that have been severely embrittled can be defragmented through appropriate heat treatment.
2.4 High tendency to brittleness, which can generate delayed cracks
Due to the addition of alloying elements such as chromium, molybdenum and vanadium, the critical cooling rate of the steel is reduced, increasing the stability of supercooled austenite.
If the welding cooling rate is fast, the transformation of austenite to pearlite in the superheated zone of the heat-affected zone is unlikely to occur.
Instead, it transforms into martensite at lower temperatures, forming a tempered structure.
Under the combined action of complex residual stress in the welded joint and diffused hydrogen, the tempered structure in the welding area and heat-affected zone is highly susceptible to hydrogen-induced delayed cracking.
3. Design Considerations
3.1 Choice of Materials
Under specific operating conditions, the selected materials must not only have superior resistance to hydrogen corrosion, but also effectively control the tendency to temper brittleness.
They must also have good weldability. Chemical composition determines structure, structure determines performance, and performance determines use. Ultimately, the key is in controlling the chemical composition.
3.1.1 Measures Against Hydrogen Corrosion
Chrome-molybdenum steel does not suffer from hydrogen corrosion, even under high pressure and lower temperatures (~200°C). However, it may suffer from hydrogen corrosion when operating in high-temperature, high-pressure hydrogen environments.
Typically, we select chrome molybdenum steel materials for specific operating conditions based on the Nelson curve, which corresponds to operating temperature and hydrogen partial pressure.
As can be seen from the Nelson curve, the higher the chromium and molybdenum content, the greater the resistance to hydrogen corrosion.
On the curve, if the vessel's operating conditions are above the solid line, it indicates the occurrence of hydrogen corrosion. If they are below the solid line, it indicates that hydrogen corrosion will not occur.
3.1.2 Measures to control the tendency to temper brittleness
By regulating the content of elements such as P, Sb, Sn, As, Si, Mn in the material, the tendency of temper brittleness can be controlled.
The temper embrittlement sensitivity coefficient J of common steel and the temper embrittlement sensitivity coefficient x of the weld metal are typically used for this purpose. For commonly used 2.25Cr-1Mo, the following control indices are used:
- J=(Si+Mn)x(P+Sn)x10≤150; Elements are replaced by weight percentage.
- X=(10P+5Sb+4Sn+As)/100≤15ppm; Elements are replaced by x10 (ppm).
In practical engineering applications, it is also necessary to control the content of trace elements Cu and Ni. The Cu content should not exceed 0.20% and the Ni content should not exceed 0.30%.
3.1.3 Determination of Cracking Sensitivity
Crack sensitivity is related to carbon equivalent, the value of which must be determined by the manufacturer based on the evaluation of the welding process.
The calculation method is: Ceq=C+Mn/6+(Cr+Mo+V)/5+(Ni+Cu)/15.
As the carbon equivalent value increases, the weldability of the steel deteriorates. When the Ceq value is greater than 0.5%, the sensitivity to cold cracking increases and the welding and heat treatment processes become more rigorous.
For commonly used Cr-Mo steel materials with 485Mpa ≤ UTS <550Mpa, Ceq is generally limited to approximately 0.48%.
When simulated welding and post-welding heat treatment are performed on product weld test plates, the maximum carbon equivalent can be increased to 0.5%.
3.2 Structure
Due to the high hardening tendency of Cr-Mo steel, it is prone to cracking and delayed cracking at corner welds.
Therefore, the structural design must pay attention to the following points:
3.2.1 Reduce the degree of restriction and reasonably design the joint structure.
3.2.2 The welding surface must not have a recess.
3.2.3 Hole reinforcement must be implemented as a whole, and ring reinforcement structures must not be used.
3.2.4 Internal extension type nozzles must not be used.
3.2.5 The connection with accessories must adopt a full double penetration structure, and corner welds must not be used.
3.2.6 The cylinder butt joint should preferably use a U-shaped groove.
3.3 Welding
Cr-Mo steel has a higher carbon equivalent value and generally has a tendency to cold crack to varying degrees. This can be avoided by the following measures:
3.3.1 Strictly control the hydrogen content in the welding rod and use a basic electrode with low hydrogen content.
3.3.2 Preheating must be done before welding the equipment assembly. Through preheating, the cooling rate of the welding material can be reduced to prevent the formation of hard and brittle structures.
The preheating temperature is determined by evaluating the welding process. Before evaluating the welding process, a welding crack test must be carried out on the sample to determine the preheating temperature, which must not be lower than the preheating temperature during the entire welding process.
At the same time, the interlayer temperature must be controlled not to be lower than the preheating temperature. Post-heating measures must be taken immediately after welding.
3.4 Non-Destructive Testing
Each Cr-Mo steel sheet used in the coating must undergo ultrasonic testing.
For high-temperature, high-pressure, thick-walled reaction vessels, after 100% radiographic inspection of butt joints, ultrasonic testing and additional magnetic particle testing shall be performed on welded joints permitted for ultrasonic testing after heat treatment and hydrostatic testing.
Ultrasonic tests are more sensitive to cracks and defects than radiographic tests, therefore they must be carried out carefully, considering the timing of non-destructive tests.
3.5 Post-welding heat treatment
During the container manufacturing process, hydrogen gas can infiltrate the metal, causing small cracks in the steel, a phenomenon known as hydrogen embrittlement.
To avoid hydrogen embrittlement, post-weld dehydrogenation treatment must be carried out immediately.
Dehydrogenation treatment involves heating the weld and surrounding parent material to a high temperature immediately after welding, thereby increasing the diffusion coefficient of hydrogen in the steel.
This stimulates the release of supersaturated hydrogen atoms into the weld metal, thus inhibiting the occurrence of cold cracks. Dehydrogenation treatment may be considered unnecessary if post-weld heat treatment (PWHT) is performed immediately after welding.
Vessels of any thickness made from Cr-Mo must undergo general post-welding heat treatment. Post-welding heat treatment of Cr-Mo steel not only eliminates residual stress but also improves the mechanical properties of the steel, which is advantageous in resisting hydrogen corrosion.
3.6 Startup and Shutdown Procedures
Cr-Mo steel can succumb to brittle failure when its operating temperature is low or close to the ductile-to-brittle transition temperature, and the stress reaches a certain level.
However, such failure is almost avoidable when the actual stress in the vessel is less than one-fifth of the yield strength of the Cr-Mo steel.
Therefore, for pressure vessels made of Cr-Mo steel, a procedure of increasing the temperature before pressure during startup and reducing the pressure before temperature during shutdown must be adopted to avoid brittle failures.
4. Choosing allowable stress
By implementing international standard Cr-Mo steel materials
Due to discrepancies in safety factor determination and calculation methods between national and international standards for allowable stress of material, when using Cr-Mo steel materials of international standards, national rules for calculation of allowable stress should be applied.
Taking SA387Cr.11G1.2 as an example, the calculation of its allowable stress is as follows:
First, obtain the tensile strength and yield strength at various temperatures for the material from ASME.
The allowable stress at room temperature is the lesser of the room temperature tensile strength divided by 3.0 and the yield strength divided by 1.5.
As there is no data on tensile strength at high temperatures in the domestic market, the allowable stress at high temperatures is obtained by dividing the yield strength at high temperatures by 1.6.
If the calculated value is greater than the allowable voltage at room temperature, adopt the room temperature value. Otherwise, use the calculated value.
The ASME allowable stress of this material reveals that when the temperature exceeds 450°C, the allowable stress drops rapidly, at which point the creep limit governs the allowable stress.
Since ASME does not provide creep limit data above 450°C, and the safety factors for creep limit in national standards and ASME are consistent, we directly adopt the ASME allowable stress. The specific allowable stress at the design temperature can be obtained using interpolation.
5. Conclusion
This article describes some specific requirements for Cr-Mo steel materials. In detailed design work, it is necessary to consider all aspects in accordance with standard specifications, carry out comprehensive analysis, so as to obtain a safe, economical and rational design.
