Material substitution

Note the following :Material substitution regulations

In the process of equipment design and manufacturing, it is often difficult to purchase and select materials or due to insufficient economic consideration and analysis. Material substitution is widely used in the design of pressure vessels. The “Safety Technical Supervision Regulations for Stationary Pressure Vessels” has made clear provisions in the substitution of pressure vessel materials. In its design and manufacture, the main requirement is that the material to be substituted is not only used in the selection of pressure-bearing parts of the pressure vessel. The appearance quality is similar, and the chemical composition, size standard, performance index and monitoring method of the material should be fully considered for reasonable substitution. The most basic principle of material substitution is: in substitution, we must eliminate the application of materials that do not meet various indicators, strictly ensure the quality of materials and achieve absolute guarantees. Technically, the technical requirements of substitute materials cannot be lower than those of the substitute materials, and more A variety of material selection methods to detect and test materials.

The procedure requirements for material substitution are:

(1) The substitution of the pressure-bearing parts of the container must be strictly carried out, and it must be approved by the technical department of the substitute unit and submitted to the re-inspection report or quality certificate of the substitute material, and the person in charge shall approve the approval;

(2) Only after obtaining the permission of the original design unit and obtaining the certification documents, can material substitution be used in the manufacture of pressure vessels;

(3) The design drawings, construction drawings, and quality certificates of the pressure vessel at the time of delivery should carefully indicate the specifications, parts, materials and specifications of the substitute materials.

Replace the bad with the good

All metal materials used in pressure vessels must have excellent properties, including mechanical properties, corrosion resistance, high temperature resistance, and manufacturing processes. The performance of each material is fixed. From the perspective of performance comparison, there are often problems of “good” and “bad” between materials. However, each type of pressure vessel has different requirements for material performance under different circumstances. Therefore, the judgment of “good” and “bad” in material substitution should be based on reality, and specific problems should be analyzed in detail. Below, based on my own work experience, the author mainly discusses several typical problems of “substituting good for bad”.

2.1 In the manufacture of pressure vessels, in terms of mechanical energy such as strength and mechanical characteristics, although the commonly used low-alloy steel is significantly better than carbon steel, its cold workability and weldability are inferior to carbon steel. Generally speaking, if the strength level is high, the cold workability and weldability are poor, and the two are negatively correlated. Therefore, when substituting this aspect, the welding process should be adjusted accordingly, and there may be corresponding changes during the heat treatment, so full attention should be paid.

2.2 Make careful and thorough consideration when replacing materials, otherwise various safety hazards may occur in the actual use of pressure vessels. For example, in a wet hydrogen sulfide environment and equipment with stress corrosion cracking risk, the sensitivity of the container to stress corrosion cracking increases with the increase of the strength level of the steel used in the container, and the two are positively correlated. At this time, if low-alloy steel such as 16MnR is used for 20R and Q235 and 20R series steels, it is very easy to cause problems. Therefore, such “substituting the good for the bad” behavior is not feasible in principle and should be prohibited. In many aspects of performance, killed steel is more advantageous than rimmed steel. However, in the manufacture of glass-lined vessels, the enamel effect of killed steel is not as good as that of rimmed steel.

2.3 Generally speaking, the corrosion resistance of stainless steel is excellent, but its corrosion resistance is not as good as low alloy steel and carbon steel in an environment containing chloride ions.

2.4 Compared with ordinary stainless steel, although ultra-low carbon stainless steel has a price advantage and good corrosion resistance, the former has better high-temperature thermal strength. In general, in order to improve the corrosion resistance, the carbon content needs to be reduced, and in order to improve the high temperature resistance, the carbon content must be increased. Therefore, in this case, the temperature of the equipment should be specially designed precisely to “replace the inferior with the good”, and recalculate if necessary.

Replace thin with thick

“Substituting thickness for thinness” often transforms the stress state of a plane stress-like shell into a plane strain state, which is harmful to the stress state of the container. Generally, thick-walled containers are better than Thin-walled containers are more prone to three-way tensile stress, which in turn causes plane strain brittle fracture.

3.1 For a vessel with equal thickness welded between the head and the cylinder in the original design, if the thickness of the individual parts of the vessel shell is replaced by thinner, it is easy to increase the geometric discontinuity of the shell, so that the gap between the head and the simplified The local stress on the connecting part increases. At this time, it will cause great damage to the container with stress corrosion tendency. It may cause fatigue cracks, and severely may cause fatigue fractures.

3.2 When a thick plate replaces a thin plate, the connection structure often changes accordingly. For example, when the simplified and thickened head are connected, it is usually necessary to chamfer the head. For equipment with a pipeline as the main simplified structure, if the thickness of the cylinder wall is increased, the inner side of the simplified side must also be trimmed at the connection part between the head and the simplified. When the thickness increases greatly, it is often related to the change of the welding process.

3.3 The “thickness instead of thinness” on the overall level of the container shell will not cause the local stress of the simplified joint and the head to increase, but it will inevitably lead to adverse effects. (1) After the thickness is increased, the flaw detection method and welding process in the original shell design must be changed accordingly, which increases the difficulty; (2) The increase in the thickness of the shell will inevitably increase the weight of the container. When it is large, it will inevitably have an adverse effect on the foundation and support of the container; (3) For containers that have a heat transfer effect on the shell at the same time, the increase in the thickness of the shell will definitely affect its heat transfer effect.

Other considerations

When substituting materials, appropriate adjustments should be made to the welding process according to the actual material used. The general adjustment principle is: when high-grade materials are used to replace low-grade materials, the standard of low-grade materials can still be used for experiment and acceptance, without increasing the standard; the resistance of different materials When the properties such as high temperature and toughness are different, the corresponding temperature may also change when the lowest water pressure test is carried out. At this time, it must be strictly implemented in accordance with the relevant regulations of GB150; when the plate thickness increases beyond the cold coil thickness specified by GB150 When the tube body must be subjected to stress-relieving heat treatment; when the thickness of the steel plate reaches a certain level, ultrasonic testing is also required, and if necessary, the pressure of the water test is increased.

Conclusion The design and manufacture of pressure vessels for the main material body just now is the basis of current vessel applications, and it is also the most difficult point in the material substitution of pressure vessels. In terms of the mechanical performance requirements of the material, the toughness of the material should also be considered when the strength of the material is tested twice. Under the condition that the toughness is satisfied, the strength should be increased as much as possible. From this point of view, it is necessary to correctly define “good” and “bad” in the selection of pressure vessel materials, not simply considering the thickness and strength of the material, but comprehensive analysis and consideration.