Welding materials, mechanical properties and chemical composition requirements

1. For ordinary structural steel, weld metal is usually required with the parent material of equal strength, tensile strength should be used is equal to or slightly higher than the base metal of the Welding Electrode 

2. For the alloy structural steel, structural shapes are usually complex, rigid and large thickness of the weldment, the welding process with great stress, easy to weld cracks, should be used in high-stress joints, weld prone to cracking the case of adverse , you can consider using low intensity level than the base metal electrode
3. When the base metal in the carbon and sulfur, high levels of phosphorus and other elements, the weld prone to cracking, good crack resistance should be used with low hydrogen electrode

Weldment performance requirements and working conditions 1. Withstand the impact of dynamic load and load weldment, in addition to meet the strength requirements, but also to ensure that the weld metal has a high impact toughness and ductility, ductility and toughness index should be used with low hydrogen electrode with higher
2. Exposure to corrosive media weldment should be based on the nature of the medium and corrosion characteristics, choose the appropriate type of stainless steel or other corrosion-resistant electrode rod 3. In work under conditions of high or low temperature solder. Low hydrogen Carbon Steel Electrodes for a good crack resistance of the main requirements of the weld metal and base metal alloy composition the same or similar
In the welded structure rigidity,
3. Difficult to clean up the welding parts of the weldment should be used in oxidizing strong, rust, oxide, oil is not sensitive to the acidity of electrode 3. Conditions imposed can not turn on the welding parts, weld in some non-flat welding position, should be used in all-position welding electrode

Construction conditions and equipment 1. In the absence of DC power, and the welded structure has required the use of low hydrogen electrodes must be the occasion should be used in both AC and DC low hydrogen electrode
2. In small or poorly ventilated situations, the choice of acid electrode (silver electrode) or low-dust electrode
Operational process performance to meet the performance and operation process of the condition, try to use low-cost, high efficiency electrode

Zhongyuan Oilfield third gas processing plant expansion project is a key project of Zhongyuan Oilfield. Projects for the operating temperature (-40 ~ -120 ℃) ??of the process piping first used 0Cr18Ni10Ti (ASTM 321TP) austenitic stainless steel. To ensure the device safe, stable, long-term operation, the stainless steel welding behavior was analyzed, and evaluation process conducted comparative tests to determine the optimal welding performance.

1 0Cr18Ni10Ti behavior of austenitic stainless steel Types Of Welding Wires

0Cr18Ni10Ti austenitic stainless steel is, the microstructure of A (austenite) +3% ~ 5% δ-F (δ-ferrite), and its good weldability, no hardened. But in practice should note the following when the access problem.
1.1 poor resistance to intergranular corrosion
Austenitic stainless steel in the 450 ~ 850 ℃ heating, due to the grain boundary precipitation of chromium carbides, resulting in the formation of chromium depleted zone surrounding the grain, in corrosive media can occur along the grain boundaries so-called intergranular corrosion.
1.2 easy to form hot cracks
Austenitic stainless steel thermal conductivity, linear expansion coefficient, local heating and cooling in the welding conditions, the joints in the cooling process can form a larger tensile stress; other austenitic steel is easy to form a strong direction of columnar grain weld metal is conducive to the segregation of harmful impurities and promote the formation of intergranular liquid interlayer, are prone to weld solidification cracking.
1.3 bad weld
Chrome-nickel austenitic stainless steel welding, due to high content of alloying elements in the weld, weld pool flow is poor, could easily lead to poor weld surface shape. Mainly in the root pass and cover surface deterioration of the back forming the weld surface roughness. Poor weld surface of the forming properties of the weld at room temperature or high temperature condition was not obvious, but in the low-temperature conditions, the shape of the stress concentration caused by poor low temperature properties of the weld seam as much as the internal quality.
Based on the above analysis, in order to solve the condensate due to intergranular corrosion and cracking problems, weld to A + δ phase-phase organization is appropriate. General the number of δ-phase of 4% to 12% is more appropriate.

Prediction 2 weld
According to austenitic stainless steel Cast Iron Welding Rod process, it should select the appropriate soldering materials, the weld metal in a suitable proportion of the δ phase to ensure that the weld microstructure and low temperature mechanical properties.
Scheffler (Schaeffler) phase diagram is a common way to estimate δ, it chrome equivalent "Creq" and nickel equivalent "Nieq" as vertical and horizontal coordinates, in which:
Creq = Cr% + Mo% +1.5 xSi% +0.5 xNb%
Nieq = Ni% +30 xC% +0.5 xMn% +0.33 xCu%
It reflects the chemical composition of impact on the organization. So with the help Schaeffler diagram, first select welding equipment in the line of weld material forecast.
According to JB4708-92 and GB50236-98 recommendation, 0Cr18Ni10Ti manual metal arc welding can choose E347-16 (A123) or E347-150 (A137) electrodes, TIG welding can choose H0Cr18Ni10Ti or H00Cr2Ni10. Recommended welding rod, welding wire and base metal composition and the corresponding calculations predicted results shown in Table 1 to Table 2.
3 mechanical test results and analysis
Welding materials of the above four were the mechanical performance comparison test, the test results in Table 3. The results are the average of six test pieces.
Weld performance is closely related with the organization, on the one hand to prevent the formation of welding hot cracking, the formation of the weld should be A + δ-F phase organization. With the other hand, increased levels of δ-F, will cause embrittlement of σ phase and δ phase selective corrosion, resulting in reduced weld metal toughness; and there is information that, in order to ensure low temperature toughness of the weld, it is best not to take double phase structure, but should obtain single-phase austenite. The use of A137 electrode welding in the weld metal content is lower than δ-F A123 electrode welding weld metal austenite content, and A137 of the electrode rod is shouting in the weld diffusible hydrogen content are more A123 electrode is low, so low-temperature toughness of the weld metal electrode will be higher than A123.
Use H00Cr2Ni10 welding wire can be effective in preventing intergranular corrosion in the weld and heat cracks. As H00Cr2Ni10 ultra-low carbon welding wire is in the grain boundaries is not easy to form Cr23C6, while improving the low temperature toughness, therefore, more than H0Cr18Ni10Ti wire to prevent intergranular corrosion.
For the weld into the form of good, and even welding a weld head and a fixed pass rate is not high, and when the weld heat-affected zone intergranular corrosion, can lead a welding process to be resolved. Rendering the use of TIG welding, the welding wire less energy to control the heat-affected zone in the range of sensitizing temperature range.

4 Conclusion
Based on 0Cr18Ni10Ti welding stainless steel pipe welding materials analysis, the use of Schaeffler diagram calculated mechanical properties of test selection and comparison, select manual metal arc welding using A137 welding, TIG welding using H00Cr2Ni10 wire as an engineering material, the use of tungsten welding backing, manual arc welding fill cap surface obtained good results.
The above order of the selected materials and processes used in practice, welding weld 3200 CPC project, X-ray detection time pass rate 90%, cleanliness of the pipeline wall, greatly reducing the process piping purge time, and fully guaranteed low-temperature stainless steel pipe weld mechanical properties.