Because 9Ni steel has good comprehensive properties and price advantages, it is widely used in aerospace, petroleum, chemical industry, shipbuilding, marine engineering, electric power, metallurgy, machinery, nuclear energy and other fields.
This post is based on the construction of the oil and gas module project under salt.
In this project, 9Ni steel not only requires high strength and excellent low-temperature toughness, but also requires SSC (sulfide stress corrosion) resistance under certain oil and gas conditions. Therefore, the welding process of 9Ni steel pipe system is studied.
2. Weldability analysis of 9Ni Steel
9Ni steel was developed by inco in the United States in the 1940s.
It is a medium alloy steel with 9% Ni content (low temperature toughness can reach -196 ℃).
Compared with austenitic stainless steel and austenitic iron nickel alloy, 9Ni steel has lower cost and higher strength;
Compared with aluminum alloy, 9Ni steel has better comprehensive mechanical properties.
However, the material itself has the characteristics of easy magnetization and difficult demagnetization, and the requirements for welding technology are extremely strict.
The weldability of 9Ni steel is mainly analyzed below.
2.1 Cold cracks
Cold cracks generally do not occur when 9Ni steel is welded with high nickel type and medium nickel type electrodes;
When using low nickel and high manganese electrodes, the welding process conditions are improper.
If too small line energy and damp electrodes are used, there is the possibility of cold cracking.
In this case, the generation of cold cracks has three aspects.
2.1.1 Hardened layer appears in the fusion zone.
The carbon content of 9Ni steel itself is not high (≤ 0.10%), and the hardened structure will not be produced during welding, but if the welding material with high carbon content is selected, the carbon content in the fusion zone will increase due to fusion and diffusion, resulting in the hardened layer.
2.1.2. The hydrogen content is too high, and the accumulation of hydrogen in the hardened layer is due to the uncleanness (oil, rust and other impurities) near the weld groove.
2.1.3. The stress concentration of welded joints includes structural stress, thermal stress and restraint stress.
2.2 . Thermal crack
Whether it is high nickel type, medium nickel type, or low nickel high manganese type electrode, there are hot cracks when welding 9Ni steel, among which high nickel type is the most serious.
The reason is that the alloy contains elements such as S and P, which are easy to form low melting point eutectic with nickel, resulting in intergranular segregation;
In addition, C and Si also promote the segregation of elements such as S and P.
Especially in the pure austenite structure, the distribution of impurities on the grain boundary is continuous.
2.3 Low temperature toughness reduction
The reduction of low temperature toughness mainly includes two aspects:
2.3.1 Influence of welding materials:
The chemical composition of the weld metal and fusion zone is related to the welding materials. If the carbon content of the welding materials is high, or the Ni Cr equivalent matching and the Ni Cr equivalent matching of the welding materials and the base metal after fusion fall into the martensite containing area in the stainless steel organization chart, the low-temperature toughness will be reduced.
If the peak temperature is too high, the reverse austenite will be reduced and coarse bainite will be produced, which will reduce the low-temperature toughness.
2.4 Magnetic blow partial
Arc magnetic blow partial will cause poor weld fusion and seriously affect the welding quality.
9Ni steel has high permeability and high remanence induction intensity, so it is easy to produce magnetic blow partial of arc during welding.
In general, when DC method (manual DC arc welding, manual DC argon arc welding, etc.) is used for backing welding of magnetic pipes, especially the magnetic blow partial phenomenon at the initial welding position of backing welding is common, which generally does not exist during filling and cover welding.
3. Preventive measures for welding problems of 9Ni Steel
3.1 Prevention of cold and hot crack tendency
The causes of cold cracks are stress, hardened structure and diffusive hydrogen content of weld metal;
The generation of thermal cracks is related to stress, impurities and chemical composition.
Therefore, the selection of welding materials is very important.
Through analysis, it is found that nicrmo-3 welding material has great advantages in the welding of 9Ni steel.
3.1.1 The linear expansion coefficient of nickel alloy in nicrmo-3 welding material is similar to that of 9Ni steel at room temperature and high temperature, so as to avoid the thermal stress caused by uneven thermal expansion and contraction.
3.1.2 The Ni content of nicrmo-3 welding material is as high as 55% – 65%, and the carbon content is similar to that of 9Ni steel.
They are all low-carbon type.
Considering the dilution effect of the base metal on the weld metal, there is still a high enough austenite structure to avoid the hard and brittle martensite belt on the fusion line.
3.1.3. Nicrmo-3 welding material has the characteristics of low carbon (carbon content ≤ 0.1%), a small “brittle temperature range” in the phase diagram of F-C alloy, high purity (S ≤ 0.03%, P ≤ 0.02%), low hydrogen content, etc.
It can be seen that the use of nicrmo-3 welding material can provide the basic conditions to reduce the tendency of cold and hot cracks in 9Ni steel welds.
Therefore, under the condition of strictly controlling the diffusive hydrogen content, the selection of nicrmo-3 welding material can basically avoid the tendency of cold and hot cracks in the welding of 9Ni steel.
3.2 Guarantee of low temperature toughness of welded joints
Welded joints include weld, fusion line and heat affected zone.
The low-temperature toughness of welded joints generally occurs in weld metal, fusion zone and coarse-grained zone.
The low-temperature toughness of weld metal is mainly related to the type of welding material used.
When 9Ni steel is welded with welding materials with the same composition as 9Ni steel, the low-temperature toughness of the weld metal is very poor, mainly because the oxygen content in the weld metal is too high.
Therefore, Ni based and Fe Ni based electrodes are usually used for welding 9Ni steel.
When 9Ni steel is welded with nicrmo-3 welding material, the chemical composition and metallographic structure of each area are different.
The weld metal is austenitic and has good low temperature toughness;
In the fusion zone, because the carbon content of the welding material is basically the same as that of 9Ni steel, with a Ni content of more than 55%, it can effectively prevent carbon migration and avoid brittle structure in the fusion zone, so as to ensure the low-temperature toughness of the fusion zone;
In the heat affected zone, under the thermal cycle of peak temperature above 1100 ℃, coarse martensite and bainite structures will be produced, which will reverse the reduction of austenite and reduce the low-temperature toughness.
Therefore, the line energy should be controlled as much as possible and multi pass welding should be used to reduce the high temperature residence time.
It can be seen that when 9Ni steel is welded with nicrmo-3 welding material, the low-temperature toughness of the welded joint mainly depends on the welding heat input and the cooling rate of the weld metal crystallization process.
3.3. Methods to overcome magnetic bias blowing
3.3.1. Change the position of the base metal grounding wire:
The grounding wire cannot be connected to the base metal at a long distance, but should be directly led near the groove (or directly placed on the groove) to make the current loop formed by the current on the base metal as short as possible.
3.3.2 Temporarily spot weld several tack welds above the groove (not at the root of the groove), short circuit the magnetic field on both sides of the groove, and grind the tack welds off with a grinder when they are to be primed to this position.
4. Test materials and methods
4.1. Test materials
9Ni steel (355.6mm in diameter and 50.8 mm in wall thickness) produced by Hengyang Valin Steel Pipe Co., Ltd. was used as the base material for the test.
See Table 1 for chemical composition and table 2 for mechanical properties.
Table 1 chemical composition of 9Ni steel pipe (wt%)
Table 2 mechanical properties of 9Ni steel pipe
|Impact energy |
| Yield strength ratio|
4.2 Welding method
According to the actual situation of the product, tungsten argon arc welding (GTAW) is used for backing welding, manual arc welding (SMAW) is used for filling welding and capping welding, and nicrmo-3 welding material is used for welding.
See Table 3 for specific chemical composition.
Table 3 chemical composition of welding materials (wt%)
5. Welding procedure qualification
5.1 Preparation before welding
5.1.1 The cutting and groove processing of 9Ni steel pipe shall adopt the method of mechanical processing as far as possible, and gas cutting or plasma blanking and groove preparation can also be used.
The processed or cut groove shall be polished.
5.1.2 Due to the large wall thickness of the pipe used in this evaluation, a suitable groove type should be designed.
Considering reducing the groove area and welding deformation, improving the welding efficiency and reducing the consumption cost of Ni based welding materials, it is decided to adopt the groove type shown in Fig. 1, with a gap of 2 ~ 4mm and a blunt edge of 0 ~ 2mm.
5.1.3 After the groove processing is completed, the appearance shall be inspected, and there shall be no cracks and delaminations, otherwise it shall be repaired.
5.1.4 Mechanical methods and organic solvents shall be used to clean the surface of the groove and within 20mm on both sides to remove oil, rust, metal chips, oxide film and other dirt on the surface.
Fig. 1 groove details
5.2 Welding sequence and weld bead layout
The backing layer is welded by argon arc welding.
In order to ensure the formation of the root weld bead and the burning through phenomenon of manual arc welding filling, at least two layers of backing welding shall be welded, and the thickness of the weld flesh shall be at least 6mm, which shall be filled by manual arc welding.
The welding layer arrangement sequence is shown in Fig. 2.
Fig. 2 weld bead layout
5.3 Welding process parameters
The heat input is the energy received by the weld per unit length, which is the main factor affecting the welding thermal cycle.
That is to say, controlling the heat input is the key to ensure the mechanical properties and SSC (sulfide stress corrosion) test.
See Table 4 for specific welding parameters.
Table 4 welding parameters
|Weld bead No||Welding method||Welding material model||Specification (mm)||Current (A)||Voltage (V)||Welding speed (mm/min)|
5.3.1 Since the melting point of weld metal welded with nickel based welding materials is about 100 ℃ lower than that of 9Ni steel, it is easy to cause defects such as incomplete fusion between groove edge and weld bead.
Therefore, it is not allowed to strike an arc at will during the welding process, and it is not allowed to strike an arc outside the groove, so as to prevent the arc from damaging the base metal.
5.3.2 When welding the arc, be sure to fill the crater and stay at the arc for a while to avoid crater cracks.
In case of crater cracks, Polish immediately.
5.3.3 In order to ensure the low-temperature toughness and SSC test results of 9Ni steel, the control of welding heat input is very important, and the welding current should not be too large. It is advisable to adopt fast multi pass welding to reduce weld bead overheating, and refine the grain through the reheating effect of multi pass welding.
During multi pass welding, the interlayer temperature should be controlled. Small heat input should be used for welding, and the heat input should be controlled below 20KJ/cm.
The interlayer temperature of multi-layer welding should be lower than 100 ℃ to avoid overheating of the joint.
6. Test results and analysis
6.1 Nondestructive testing
After welding, the test piece was visually inspected, and no undercut, surface pores, cracks, slag inclusions and other defects were found in the weld and heat affected zone.
The weld reinforcement was 0.5 ~ 1.5mm, and the weld and base metal were smoothly transitioned;
No cracks, incomplete fusion, incomplete penetration, slag inclusion and other defects are found in the test piece through radiographic inspection, and the quality of the welded joint meets the standard requirements.
6.2 Tensile test
In tensile test, the tensile specimen is fixed on the WE-100 universal testing machine, and then the tensile stress is applied to it, causing the axial elongation of the specimen until it breaks, which is the main index to measure the strength of materials.
The test results are shown in Table 5.
Table 5 tensile test results
|Test piece No.||Tensile strength (MPA)||Fracture location|
According to the test results, it can be seen that the tensile test results meet the specification requirements.
6.3 Bending test
Bending test is to assess the ability of materials to withstand deformation.
The processed standard bending samples are tested on the WE-100 universal testing machine.
Take 4 side bending samples according to the requirements of the specification, and conduct the bending test with a 63.5mm indenter diameter.
The bending angle is 180 °.
There are no cracks on the surface of the sample after bending and no other defects longer than 3mm in any direction.
The test results meet the requirements of the specification.
6.4 Impact test
Impact test is to put the impact sample on JB-30B impact testing machine, and use the impact load to break the groove of the joint surface, so as to determine the impact performance of the welded joint based on the impact energy consumed per unit area at the breaking point.
This impact test adopts -196 ℃ Charpy impact, and samples are taken at the position 1 ~ 2mm away from the weld surface.
The notch positions are located at the weld center, fusion line, fusion line 1mm, fusion line 2mm and fusion line 5mm respectively.
The test results are shown in Table 6.
Table 6 impact test results
|Notch location||Single impact value (J)||Average impact value (J)|
|Weld center||89, 78, 76||81|
|Meld Line||80, 82, 76||79|
|Meld Line+1 mm||104, 91, 111||104|
|Meld Line+2 mm||78, 99, 85||87|
|Meld Line+5 mm||112, 98, 104||104|
According to the impact results, it can be seen that the impact values meet the specification requirements (-196 ℃ ≥ 41j).
6.5 Macro and hardness test
6.5.1 Through the macro section inspection of the weld, it is found that the weld is completely welded without cracks and other defects. See Figure 3 for the macro sample.
Fig. 3 macro sample photo
6.5.2 Measure the hardness of weld metal, heat affected zone and base metal of welded joints respectively.
The hardness values are shown in Table 7.
Table 7 hardness test results
|Sampling Position||Hardness value (HV10)|
|Heat Affected Zone||253～290|
6.6. SSC (sulfide stress corrosion) test
Take 3 standard welded plate-shaped samples, continuously fill 99.2% CO2 and 0.8% H2S acetic acid solution (initial PH=3) at 25 ℃, and load 80% yield strength with 4-point bending（ σ S=698MPa) and soaked for 720 hours, the samples were not broken.
No cracks were found under the 10x magnifying glass, and the sulfide stress corrosion test of this batch of samples was qualified according to the corresponding standards (see Figure 4).
Fig. 4 surface morphology of compressive stress sample after immersion corrosion
7.1 Using argon tungsten arc welding for backing, manual arc welding for filling and covering, and welding 9Ni steel with ERNiCrMo-3 welding wire and ERNiCrMo-3 welding rod can obtain high-quality welding joints under reasonable welding process conditions.
7.2 All performance indexes of the welding procedure qualification test meet the technical requirements, and have basically mastered the TIG backing, manual arc welding filling and pipe system welding technology of 9Ni steel, which provides valuable experience for guiding production in the future.