Urea stainless steel pipes are the main material used in the production process of urea chemicals in China. With the expansion of the scale of chemical production, production and construction in some low-temperature environments are inevitable.
In pipe welding, the fluids inside the pipe extract heat, and the temperature difference between the low-temperature environment also reduces the temperature of the weld, which increases the quality of welding defects. In control aspects
Welding environment temperature, welding equipment protection,
Carrying out heat preservation and slow cooling, and selecting the most suitable
Welding materials, welding The work can be completed successfully and the
Low Urea Stainless Steel Pipe Welding Quality Control Problem
Temperature can be resolved.
1. Low temperature condition on factors welding quality
1.1 Low temperatures affect welding quality
Firstly, temperature can affect the quality of welding work on stainless steel structures, since the material is preheated during welding and controlling the temperature of this heating affects differences in the strength of the steel pipe.
At low ambient temperatures, the tightness of the weld may not reach the expected values. Even though a weld can be completed in this environment, it is very difficult to consistently control the temperature between layers of weld. Immediately after the completion of the welding work, the quality of the weld seam becomes very fragile due to the lower temperature, as the welding stresses are very high, the crystallization of the weld seam leads to segregation and the stress field of the Weld seam leads to increases hardening and increases susceptibility to cracking in cold seams.
Lower temperatures also represent a major challenge for welders and the costs of welding technology are rising. Low temperatures also affect the normal operation of construction machines and welding tools. Welders cannot adapt well to the ambient temperature and cannot achieve the set welding pass rate.
1.2 The phenomenon of permafrost subsidence affects the quality of welding
If it is an alpine plateau or a low temperature zone, we will provide support by welding the underground columns. Some pipes are supported by permafrost, and permafrost is supported by unstable factors such as solar radiation over time as the permafrost thaws and becomes softer. This phenomenon of unequal forces supporting the pipeline support columns will affect the quality of the welding work.
1.3 The layout properties of the material itself influence the welding results
The quality of welding also depends on the material of the steel structure itself, as there are some differences in the arrangement of the crystals within each steel structure. When these materials are brought to a low temperature state, the results of material welding and room temperature welding have different effects. Therefore, we need to select the welding material according to the welding properties of each material when welding at different environmental temperatures.
2. Control of pipe welding temperature measurements
2.1 Set up a cold insulation tent at the welding site
Based on the first chapter of the analysis, the temperature of the welding environment can directly affect the welding quality. Suppose the welding process is carried out in a low temperature environment. In this case, you can build a cold insulation tent so that technicians and welders can stay in a closed, insulated environment and use heaters to reach the desired temperature. Welding quality and efficiency can be effectively improved without being affected by the outside world.
2.2 Welding preheating for good results
When welding in a low temperature environment, it is really difficult to maintain a stable preheating temperature. Preheating before each welding process is an important part of the entire pipe welding process. This early preheating allows hydrogen to escape quickly, reducing cold crack formation. The welding voltage can be reduced during the preheating period. To ensure the preheating temperature during welding, for example, the upper ring of the frequency heater and the flame preheater can be preheated.
Flame preheating is mainly used for thicknesses less than 40mm, electric heating is mainly used for thicknesses greater than 40mm.
2.3 Welding between layers must guarantee good temperature control
Welding is an important factor in welding. Quality control in steel pipe welding is very high because the correlation between cooling rate and stress intensity is very high. Furthermore, the temperature of the actual welding process must be controlled.
For example, whenever a weld works well, the temperature of the entire steel structure should be checked. Suppose the temperature of the middle layer is below 60°C. In this case, it is necessary to control the preheating temperature and wait until the temperature of the intermediate layer reaches the standard value before proceeding with welding.
2.4 Selection of suitable welding materials
The cracking resistance of the welding material is an important selection criterion. The welding material should be selected to meet the strength requirements of the steel structure of the pipeline welding material. The molten liquid metal that occurs during welding also affects the strength of the weld. A welding material with low hydrogen content and effective insulation measures is the best choice.
2.5 Further modernization and optimization of temporary support for gas pipelines
Chapter 1.2 mentions that the permafrost environment on highlands is a special welding environment. When carrying out welding work, the pipe support mount must be in stable condition. The permafrost in the high plateaus is frozen, and direct sunlight, warming, and thawing cause the ground to become soft. Soil settlement affects the stability of the steel structure and the welding quality of the group. Welding time on frozen ground can be extended by using woven sandbags and piles of dirt. Artificial mounds of earth can increase stability.
2.6 Insulation and slow cooling after welding
In Chapter 1.3 it is mentioned that after welding work is completed, cold cracks appear and an internal crystalline structure forms in the steel structure. A drop in temperature leads to an abnormal arrangement of the crystalline phase within the welding channels, which is not conducive to welding. The insulation can be heated in time after welding. The heating area is circular in the middle of the weld, the radius of the insulation area is 100mm, the radius is 200mm, to reduce the temperature gradient changes as much as possible.
3. Welding Urea Grade Stainless Steel 310 MoLN (UNS S31050)
3.1 Properties of urea grade stainless steel 310 MoLN (UNS S31050)
The chemical properties of urea stainless steel 310 MoLN (UNS S31050) are shown in Table 1. This is an improved steel structure with the widest range of applications and is most commonly used in the urea industry. For example, in the urea synthesis tower, the high-pressure condenser, high-pressure injector and other main equipment are most commonly used.
The performance must have the best corrosion resistance, adapt to high pressure and resist intergranular corrosion. Compared to other types of stainless steel, this stainless steel has good welding properties, but welding is a complex metallurgical process. The mechanical properties and corrosion resistance of the welding head reach the level of the base material. Much corrosion near the weld leads to corrosion and equipment damage. The thermal cracking scenario is mainly due to the fact that the linear expansion coefficient of stainless steel is large and the thermal conductivity is low.
At this time, welding in the high temperature zone will produce crystalline segregation for a long period of time, which will harden a crystallized liquid film. Intergranular cracks are created under tensile stress.
This section of stainless steel is introduced into the iron during the manufacturing process; Iron in the aminomethane medium of urea is more susceptible to corrosion. Therefore, the iron content must be strictly controlled and not exceed 0.6%.
Using appropriate welding materials and welding methods to control ferrite content and reduce corrosion and thermal cracking, you can use electrode arc welding, urea stainless steel surface welding, and high-pressure heat exchanger welding of stainless steel tube plate joints.
Table 1 Chemical composition of 310 MoLN (UNS S31050)
| standard | Steel type | ||||||||
| Chemical composition % | |||||||||
| W: | Mn: | Yes: | P: | S: | RC: | Mon: | Ni: | N: | |
| PT | 1.4466 –X1CrNiMoN25-22-2 | ||||||||
| <0.02 | <2.0 | <0.7 | <0.025 | <0.010 | 24.0 – 26.0 | 2.0 – 2.5 | 21.0 – 23.0 | 0.10 – 0.16 | |
| ASTM | UNS S31050 – AISI 310MoLN – 25.22.2 | ||||||||
| <0.025 | <2.0 | <0.5 | <0.020 | <0.030 | 24.0 – 26.0 | 1.6 – 3.0 | 20.5 – 23.5 | 0.09 – 0.16 | |
| NF | Z2CND25-22Az | ||||||||
| <0.02 | <2.0 | <0.75 | <0.025 | <0.010 | 24.0 – 26.0 | 2.0 – 2.5 | 21.0 – 23.0 | 0.10 – 0.16 | |
| B.S. | 725LN | ||||||||
| <0.02 | <2.0 | <0.7 | <0.025 | <0.010 | 24.0 – 26.0 | 2.0 – 2.5 | 21.0 – 23.0 | 0.10 – 0.16 | |
| SEW 400 | 1.4465 – X1CrNiMoN 25-25-2 – X1CrNiMoN25-25-2 | ||||||||
| <0.02 | <2.0 | <0.7 | <0.020 | <0.015 | 24.0 – 26.0 | 2.0 – 2.5 | 22.0 – 25.0 | 0.08 – 0.16 | |
| GOST | 02Ch25N22AM2 – 02 X25Н22АМ2 | ||||||||
| <0.02 | 1.5 – 2.0 | <0.4 | <0.020 | <0.015 | 24.0 – 26.0 | 2.0 – 2.5 | 21.0 – 23.0 | 0.10 – 0.14 | |
3.2 Welding process measurements for stainless steel grade 310 MoLN (UNS S31050)
When larger grains and residual stresses arise during linear power welding, the ferrite content also increases. This, in turn, affects the corrosion resistance of the welded joint, such as resistance to methylamine fluid and the corrosion effects of urine. A multi-layer narrow road welding process can be used, in which the energy input is low and linear, a short arc and a fast, non-oscillating welding process.
At the welding point on the back of welding some metallic material Welding process with argon protection. Argon arc welding can be used for the primer layer and the second weld layer. The exact situation depends on the weld wall thickness. If the wall thickness is less than 8 mm, argon arc welding must be used.
If the wall thickness is more than 8 mm, argon arc welding must be used. When the welding fill height is greater than 10 mm, high-speed multi-layer and multi-channel welding should be used.
The temperature between the solder layers must not be exceeded during welding. Make sure the layer temperature is 60°C. When welding, you should also pay attention to cleanliness and protection, and carefully avoid carburizing. When a layer of weld is finished, it must be polished, cleaned and inspected before proceeding with welding. An unused wire must be cleaned and oxidized before it can be used again.
If the pipe diameter is ≤ 40 mm, welding test can be carried out. The exterior of the weld seam is checked, the ferrite penetration content is checked, and leakage defects are detected. Finally, standards can be a surface cleaning to continue welding, the second layer of welding after the penetration test, e.g. B. RT Test and finally you can check appearance and penetration test. Use the probe detector, clean the probe and solder with acetone before testing, and remove the oil and rust in time.
If unqualified welds are detected, the defective mechanical grinding and chipping areas must be reworked. When reworking, use argon arc welding and minimize the number of reworks. Any rework damages the metal structure. If there are several reworks, the technician must analyze and approve the operation.
Commonly used wire 25.22.2LMN, electrode BM310Mol can perform low temperature welding of urea grade stainless steel 310 MoLN (UNS S31050), using argon arc welding as primer and choosing arc welding method as coverage.
With good preparation, it is necessary to strengthen management, control the welding process, use advanced and reasonable welding process, improve the technical quality of welders, and ensure a strict welding management system to ensure welding quality and avoid rework .
The development of national industrialization to improve the quality and quantity of the direction of continuous deepening of the temperature factor in the welding process needs to be addressed.
To ensure the smooth progress of welding work, it is necessary to take specific measures, carry out insulation at room temperature, carry out good preheating, good insulation after welding and slow cooling of the weld, as much as possible, to minimize the cold crack defects and ensure the quality of pipeline welding.
