1. Introduction
Due to its excellent comprehensive properties and cost advantages, 9Ni steel is widely used in various industries such as aerospace, petroleum, chemical, shipbuilding, marine engineering, electrical power, metallurgy, machinery, nuclear power and more.
This post focuses on the construction of the oil and gas module project located under the salt. In this design, 9Ni steel must possess not only high strength and excellent low-temperature toughness, but also SSC (sulfide stress corrosion cracking) resistance under specific oil and gas conditions. Therefore, the welding process of the 9Ni steel pipe system is under study.
2. Weldability analysis of 9Ni steel
9Ni steel was developed by Inco in the United States during the 1940s. It is a medium alloy steel containing 9% nickel, which allows it to exhibit toughness at low temperatures down to -196℃.
When compared to austenitic stainless steel and austenitic iron-nickel alloy, 9Ni steel has greater strength and lower cost. Furthermore, 9Ni steel has better comprehensive mechanical properties than aluminum alloy.
However, the material is prone to magnetization and is difficult to demagnetize. Furthermore, welding technology requires strict adherence to specific requirements.
This article will analyze the weldability of 9Ni steel.
2.1 Cold cracks
Cold cracking is unlikely to occur when welding 9Ni steel with high or medium nickel electrodes. However, when low nickel and high manganese electrodes are used with inappropriate welding conditions such as low line power and wet electrodes, cold cracking may occur. The generation of cold cracks has three aspects:
2.1.1 Appearance of a hardened layer in the fusion zone. Although the carbon content of 9Ni steel is not high (≤ 0.10%), a hardened structure can be produced if a welding material with a high carbon content is selected. This is due to an increase in carbon content resulting from melting and diffusion.
2.1.2. Presence of a lot of hydrogen, which accumulates in the hardened layer due to impurities (such as oil and rust) close to the welding groove.
2.1.3. The stress concentration of welded joints, which includes structural stress, thermal stress, and restraint stress.
2.2. Thermal Crack
When welding 9Ni steel, hot cracking may occur regardless of whether a high-nickel, medium-nickel, or low-nickel, high-manganese type electrode is used. However, the use of an electrode type with a high nickel content may result in more serious cracks.
This is due to the fact that the alloy contains elements such as S and P, which can easily form low-melting point eutectics with nickel. As a result, intergranular segregation may occur. Furthermore, elements such as C and Si can also promote the segregation of S and P.
In particular, when the structure is in a pure austenite state, the distribution of impurities at the grain boundary can be continuous.
2.3 Reduced toughness at low temperatures
The reduction in toughness at low temperatures is mainly influenced by two factors:
2.3.1 Influence of Welding Materials:
The chemical composition of the weld metal and the fusion zone is related to the welding materials used. If the welding materials have a high carbon content, or if the Ni Cr equivalent correspondence of the welding materials and the base metal after melting falls into the martensite-containing area on the stainless steel organization chart, the low temperature toughness will be reduced.
2.3.2 Welding Line Energy and Interlayer Temperature:
The welding line energy and interlayer temperature can change the peak value and temperature of the welding thermal cycle, thus affecting the metallographic structure of the heat-affected zone. If the peak temperature is too high, it can lead to a reduction in reverse austenite and the formation of coarse bainite, which can result in reduced toughness at low temperatures.
2.4 Partial magnetic strike
Partial magnetic blowing can cause poor weld fusion and significantly affect welding quality.
9Ni steel has high permeability and remanence induction intensity, making it susceptible to magnetic partial strikes during welding.
Generally, when using the DC method (manual DC arc welding, manual DC argon arc welding, etc.) for magnetic pipe support welding, partial magnetic blowing is common at the initial welding position of the support weld, but are not normally present during cap filling and welding.
3. Preventive measures for 9Ni Steel welding problems
3.1 Preventing the tendency to cold and hot cracking
The causes of cold cracks in welding are stress, hardened structure and diffusive hydrogen content of the weld metal. The generation of thermal cracks is related to stress, impurities and chemical composition. Therefore, selecting the appropriate welding materials is critical.
After analyzing the properties of different welding materials, it was found that nicrmo-3 welding material is highly advantageous for welding 9Ni steel.
3.1.1 The coefficient of linear expansion of nickel alloy in nicrmo-3 welding material is similar to that of 9Ni steel at both room temperature and high temperature. This similarity helps avoid thermal stress caused by uneven expansion and contraction.
3.1.2 The Ni content of nicrmo-3 welding material is high, ranging from 55% to 65%, and the carbon content is similar to that of 9Ni steel. Both materials belong to the low carbon type. Even with the dilution effect of the base metal, there is still a high enough austenite structure to prevent the formation of a hard, brittle martensite belt at the fusion line.
3.1.3 Nicrmo-3 welding material has the following characteristics: low carbon content (carbon content ≤ 0.1%), a small “brittle temperature range” in the phase diagram of the FC alloy, high purity (S ≤ 0.03%, P ≤ 0.02%) and low hydrogen content. The use of nicrmo-3 welding material can therefore provide the basic conditions necessary to reduce the tendency of cold and hot cracking in 9Ni steel welds.
Therefore, under the strict control of hydrogen diffusive content, the selection of nicrmo-3 welding material can effectively avoid the tendency of cold and hot cracking when welding 9Ni steel.
3.2 Guarantee of toughness at low temperatures of welded joints
Welded joints consist of the weld, the fusion line and the heat affected zone.
The low-temperature toughness of welded joints generally occurs in the weld metal, the fusion zone and the coarse-grained zone.
The toughness of the weld metal at low temperatures is mainly influenced by the type of welding material used.
When welding 9Ni steel with materials that have the same composition as 9Ni steel, the toughness of the weld metal at low temperatures is normally low, mainly due to the high oxygen content in the weld metal.
Therefore, Ni and Fe-Ni based electrodes are generally employed 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 excellent toughness at low temperatures.
In the fusion zone, the carbon content of the welding material is essentially the same as that of 9Ni steel, with a Ni content greater than 55%, effectively preventing carbon migration and avoiding a brittle structure in the fusion zone, thus ensuring the low carbon content of the fusion zone. temperature resistance.
In the heat-affected zone, under the thermal cycle of peak temperature above 1100 ℃, coarse martensite and bainite structures are generated, which reverse the reduction of austenite and decrease the toughness at low temperatures.
Therefore, line power should be controlled as much as possible, and multi-pass welding should be used to minimize high-temperature dwell time.
Therefore, when welding 9Ni steel with nicrmo-3 welding material, the toughness of the welded joint at low temperatures is largely influenced by the welding heat input and the cooling rate of the weld metal crystallization process.
3.3. Methods to Overcome Magnetic Polarization Blowing
3.3.1. Change the position of the base metal ground wire:
To minimize the current loop formed by the current in the base metal, the ground wire should be led directly next to the groove or placed in the groove. It should not be connected to the base metal over a long distance.
3.3.2. Temporarily create several solder points above the groove (not at the root of the groove) to short-circuit the magnetic field on both sides of the groove. When priming in this position, use a grinder to remove the weld spots.
4. Test materials and methods
4.1. Test Materials
9Ni steel (355.6mm diameter and 50.8mm wall thickness) produced by Hengyang Valin Steel Pipe Co., Ltd.
See Table 1 for chemical composition and Table 2 for mechanical properties.
Table 1 Chemical composition of 9Ni steel tube (% by weight)
| Type | W | Yes | Mn | Cr | Mo | Ass | No |
| 9Ni steel | 0.05 | 0.21 | 0.57 | 0.045 | 0.056 | 0.035 | 9.24 |
| Al | s | P | |||||
| 0.02 | 0.004 | 0.006 |
Table 2 mechanical properties of 9Ni steel tube
| Tensile strength R i /MPa |
Yield strength R p0.2 /MPa |
Stretching A/% |
Impactful energy (-195 ℃)KV/J |
Yield resistance ratio % |
| 750 | 698 | 27.5 | 108, 112, 107 | 93 |
4.2 Welding method
Based on specific product requirements, argon tungsten arc welding (GTAW) is utilized for backing welding, while manual arc welding (SMAW) is employed for fill and cap welding. Furthermore, nicrmo-3 welding material is used during the welding process.
See Table 3 for specific chemical composition.
Table 3 Chemical composition of welding materials (% by weight)
| Type | W | Yes | Mn | Cr | Mo | Ass | No | Mo |
| ERNiCrMo-3 | 0.01 | 0.04 | 0.03 | 0.004 | 0.004 | 22.2 | 64.3 | 9.3 |
| ENiCrMo-3 | 0.02 | 0.36 | 0.4 | 0.005 | 0.006 | 22.7 | 63.6 | 8.8 |
5. Qualification of the welding procedure
5.1 Preparation before welding
5.1.1 The cutting and grooving processing of 9Ni steel tubes should preferably use the mechanical processing method. However, gas cutting or plasma cutting and groove preparation can also be used.
The processed or cut groove must be polished.
5.1.2 Due to the large wall thickness of the pipe used in this evaluation, it is necessary to design a suitable type of channel.
Considering the reduction of groove area and welding deformation, improving welding efficiency and reducing the consumption cost of Ni-based welding materials, it was decided to adopt the groove type shown in Fig. 4mm and a blunt edge of 0 2mm.
5.1.3 Once the groove processing is completed, the appearance must be inspected to ensure that there are no cracks or delaminations. If any of these defects are found, they must be repaired.
5.1.4 Mechanical methods and organic solvents should be used to clean the groove surface and the area within 20mm on both sides to remove oil, rust, metal chips, oxide film and any other dirt on the surface.
Fig. 1 groove details
5.2 Welding sequence and weld bead arrangement
The support layer was welded using argon arc welding.
To ensure the formation of the root weld bead and avoid the burning phenomenon during manual filling by arc welding, at least two layers of support welding must be applied, with a minimum weld thickness of 6 mm, and filled by welding manual arc.
See Figure 2 for welding layer arrangement sequence.
Fig. 2 Weld bead layout
5.3 Welding process parameters
Heat input refers to the amount of energy received by the weld per unit length and is the main factor that influences the thermal cycle of welding. Therefore, controlling heat input is essential to ensure mechanical properties and resistance to sulfide stress corrosion cracking (SSC) during testing.
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) |
| 1~2 | GTAW | ERNiCrMo-3 | 2.4 | 110~130 | 15~16 | 50~70 |
| 3~61 | SMAW | ENiCrMo-3 | 3.2 | 80~100 | 19~23 | 110~160 |
5.3.1: Because the melting point of weld metal welded with nickel-based welding materials is approximately 100℃ lower than that of 9Ni steel, it can easily cause defects such as incomplete fusion between the groove edge and the weld bead. solder. Therefore, it is prohibited to form an arc randomly during the welding process, and the arc should not be struck outside the groove to avoid damage to the base metal.
5.3.2: During arc welding, it is important to fill the crater and stay in the arc for a while to avoid cracking the crater. In the case of crater cracks, immediate polishing is necessary.
5.3.3: To ensure the low temperature toughness and SSC test results of 9Ni steel, control of welding heat input is crucial and welding current should not be excessive. It is advisable to use fast multi-pass welding to minimize overheating of the weld bead and refine the grain through the reheating effect of multi-pass welding.
During multi-pass welding, the interlayer temperature must be regulated and a small heat input must be used for welding. Heat input must be controlled below 20KJ/cm. The interlayer temperature of multilayer welding should be kept lower than 100 ℃ to avoid overheating of the joint.
6. Test results and analysis
6.1 Non-destructive testing
After welding, the test piece underwent a visual inspection, which revealed no cuts, surface pores, cracks, slag inclusions or other defects in either the weld or the heat-affected zone.
The weld reinforcement measured between 0.5 and 1.5 mm, and the weld and base metal exhibited a smooth transition.
Radiographic inspection showed no cracks, incomplete fusion, incomplete penetration, slag inclusions or other defects in the specimen, confirming that the quality of the welded joint meets the requirements of the standard.
6.2 Tensile test
During a tensile test, the tensile sample is clamped on a WE-100 universal testing machine. Tensile stress is then applied to the sample, causing axial stretching until it reaches its breaking point. This is the main indicator used to measure the resistance of materials.
The test results are displayed in Table 5.
Table 5 tensile test results
| Test piece no. | Tensile Strength (MPA) | Fracture location |
| 1 | 761 | common metal |
| two | 764 | common metal |
Based on the test results, it is evident that the tensile test meets the specification requirements.
6.3 Flexion test
The bending test evaluates the ability of materials to resist deformation.
Using WE-100 universal testing machine, processed standard bending samples are tested.
To perform the test, four lateral bending samples are collected according to specifications and a 63.5 mm diameter indenter is used. The bending angle is set to 180°.
After the bending test, there should be no cracks or defects greater than 3 mm in any direction on the surface of the samples.
Based on the test results, it meets the requirements of the specification.
6.4 Impact test
The purpose of impact testing is to determine the impact performance of a welded joint by measuring the amount of impact energy consumed per unit area at the point where the groove in the joint surface is broken. To perform this test, an impact sample is placed in a JB-30B impact testing machine, which applies the impact load required to rupture the groove.
For this specific impact test, a Charpy impact is used at a temperature of -196℃. Samples are taken from a position approximately 1 to 2 mm away from the weld surface.
The notch positions are located at various points along the joint, including the weld center, fusion line, 1mm fusion line, 2mm fusion line and 5mm fusion line.
The test results are shown in Table 6.
Table 6 impact test results
| Notch location | Single impact value (J) | Average impact value (J) |
| welding center | 89, 78, 76 | 81 |
| Fusion line | 80, 82, 76 | 79 |
| Fusion line+1mm | 104, 91, 111 | 104 |
| Fusion line+2mm | 78, 99, 85 | 87 |
| Fusion line+5mm | 112, 98, 104 | 104 |
Based on the impact results, it can be seen that the impact values meet the specification requirements of (-196°C ≥ 41J).
6.5 Macro and hardness test
After performing a macro inspection of the weld section, it was determined that the weld is fully welded with no cracks or other defects. Figure 3 shows the macro sample.
Fig. 3 macro sample photo
6.5.2 Measure the hardness of the weld metal, the heat-affected zone and the base metal of the welded joints, respectively.
Hardness values are shown in Table 7.
Table 7 hardness test results
| Sampling position | Hardness value (HV10) |
| weld metal | 219~247 |
| Heat affected zone | 253~290 |
| Base metal | 230~256 |
6.6. SSC (Sulphide Stress Corrosion) Test
Three plate-shaped standard samples were collected and continuously filled with a solution of 99.2% CO2 , 0.8% H2S and acetic acid (initial pH = 3) at 25℃. The samples were then loaded to 80% yield strength using 4-point bending (σS=698 MPa) and soaked for 720 hours. It was observed that the samples did not break.
When examining the samples under a 10x magnifying glass, no cracks were detected. Furthermore, the sulfide stress corrosion cracking test of this batch of samples met the specified standards (see Figure 4).
Fig. 4 Surface morphology of the compressive stress sample after immersion corrosion
7. Conclusion
7.1 With the use of argon tungsten arc welding for support, manual arc welding for filling and covering, and 9Ni steel welding with ERNiCrMo-3 welding wire and ERNiCrMo-3 welding rod, high-quality welding joints can be achieved under appropriate welding process conditions.
7.2 The welding procedure qualification test met all performance indices and technical requirements. We have gained mastery in TIG support, arc welding manual filling and pipe system welding technology for 9Ni steel, which will provide valuable experience to guide future production.
