Welding technology of 9Ni Steel

9Ni steel has been widely used in aerospace, petroleum, chemical industry, shipbuilding, marine engineering, electric power, metallurgy, machinery and nuclear energy due to its good comprehensive performance and price advantage.

This paper takes the construction of oil and gas module under salt as the background. 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.

Weldability analysis of 9Ni Steel

9Ni steel was developed by inco company in 1940s. It is a medium alloy steel with 9% Ni content (low temperature toughness can reach – 196 ℃). Compared with austenitic stainless steel and austenitic Fe Ni alloy, 9Ni steel has lower cost and higher strength; Compared with aluminum alloy, 9Ni steel has better comprehensive mechanical properties. But the material itself has the characteristics of easy magnetization and difficult demagnetization, so the requirements of welding process are very strict. The weldability of 9Ni steel is analyzed.

Cold crack

When welding 9Ni steel with high Ni type and medium Ni type electrodes, there is no cold crack; When using low nickel and high manganese type electrode, the welding process conditions are improper, such as using too small line energy and wet electrode, there is the possibility of cold cracking. In this case, there are three aspects of cold crack.
There is a hardening layer in the fusion zone. The carbon content of 9Ni steel itself is not high (≤ 0.10%), which will not produce hardening structure during welding. However, if the welding material with high carbon content is selected, the hardening layer will also be produced due to the increase of carbon content in the fusion zone due to fusion and diffusion.
Hydrogen content is too high, hydrogen accumulation in the hardened layer is due to unclean (oil, rust and other impurities) near the weld groove.
The stress concentration of welded joint includes microstructure stress, thermal stress and restraint stress.

Hot crack

No matter high nickel type or medium nickel type, or low nickel and high manganese type electrode, there are hot crack problems when welding 9Ni steel, among which the high nickel type is the most serious. The reason is that the alloy contains s, P and other elements, and it is easy to form low melting point eutectic with nickel, resulting in intergranular segregation; In addition, C and Si also promote the segregation of S, P and other elements. Especially in pure austenite, the distribution of impurities on the grain boundary is continuous.

Low temperature toughness decreases

There are two main reasons for the decrease of low temperature toughness:
Influence of welding material: the chemical composition of weld metal and fusion zone is related to welding material. If the carbon content of welding material is high, or the Ni Cr equivalent matching and the Ni Cr equivalent matching of welding material and base metal after fusion fall in the martensite containing area of stainless steel microstructure, the low temperature toughness will be reduced.
Welding line energy and interlayer temperature will change the peak value and temperature of welding thermal cycle, thus affecting the microstructure of heat affected zone. If the peak temperature is too high, the reverse austenite will be reduced and the coarse bainite will be produced, thus the low temperature toughness will be reduced.

Magnetic bias blowing

Arc magnetic bias blowing will cause poor weld fusion and seriously affect the welding quality. 9Ni steel has high permeability and high remanence induction, so the phenomenon of arc magnetic bias is easy to occur in the welding process. In general, when the magnetic pipe adopts DC method (manual DC arc welding, manual DC argon arc welding, etc.) for backing welding, especially in the initial welding position of backing welding, the phenomenon of magnetic bias blowing is relatively common, which does not exist in filling and cover welding.

Welding problems and preventive measures of 9Ni Steel

Prevention of cold and hot crack tendency

The causes of cold cracks are stress, hardened structure and diffusive hydrogen content in weld metal; The hot crack is related to stress, impurity and chemical composition. So the choice of welding material is very important. It is found that nicrmo-3 welding material has great advantages in welding 9Ni steel.
The coefficient of linear expansion of Ni alloy in nicrmo-3 welding material is close to that of 9Ni steel at room temperature and high temperature, so as to avoid the thermal stress caused by uneven thermal expansion and cold contraction.
The content of Ni in 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 base metal on weld metal, there is still enough high austenite structure to avoid the occurrence of hard and brittle martensite Zone in fusion line.
Nicrmo-3 welding material has the characteristics of low carbon (carbon content ≤ 0.1%), small “brittleness temperature range” in F-C alloy phase diagram, high purity (s content ≤ 0.03%, P content ≤ 0.02%) and low hydrogen content.
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 weld. Therefore, under the condition of strictly controlling the diffusive hydrogen content, the cold and hot cracking tendency of 9Ni steel can be basically avoided by using nicrmo-3 welding material.

Guarantee of low temperature toughness of welded joint

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 grain zone. The low temperature toughness of weld metal is mainly related to the type of welding material used. When welding 9Ni steel with welding material with the same composition as 9Ni steel, the low temperature toughness of weld metal is very poor, which is mainly due to the high oxygen content in weld metal. Therefore, Ni based and Fe Ni based electrodes are usually used for welding 9Ni steel.
When welding 9Ni steel with nicrmo-3 welding material, the chemical composition and microstructure of each zone are different. The weld metal is austenite with 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, and the Ni content is more than 55%, it can effectively prevent the carbon migration and avoid the brittle structure in the fusion zone, so as to ensure the low-temperature toughness of the fusion zone; In the heat affected zone, the coarse martensite and bainite will be produced under the action of thermal cycle at the peak temperature above 1100 ℃, and the reverse austenite will decrease, resulting in the decrease of 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 welding 9Ni steel with nicrmo-3 welding material, the low temperature toughness of welded joint mainly depends on welding heat input and cooling rate of weld metal crystallization process.

Methods to overcome magnetic bias blow

Change the position of the base metal grounding wire: the grounding wire should not be connected to the base metal at a long distance, but should be directly led to the groove (or directly placed on the groove), so that the current loop formed by the current on the base metal is as short as possible.
Temporary spot welding is applied to several positioning welds above the groove (not at the root of the groove) to short-circuit the magnetic field on both sides of the groove. The positioning welds are ground off with a grinder when backing welding to this part.

Test materials and methods

Test materials

9Ni steel (355.6 mm in diameter and 50.8 mm in wall thickness) produced by Hengyang Valin Steel Pipe Co., Ltd. is used as test base material. See Table 1 for chemical composition and table 2 for mechanical properties.
Table 1 chemical composition (wt%) of 9Ni steel pipe

Type	C	Si	Mn	Cr	Mo	Cu	Ni
9Ni	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 pipe

Tensile strength  Rm/MPa	Yield strength Rp0.2/MPa	Elongation A/%	Impact energy（-195℃）   KV/J	Yield strength ratio  %
750	698	27.5	108， 112，107	93

Welding method 

According to the actual situation of the product, GTAW is used for backing welding, SMAW is used for filling welding and covering welding layer, and nicrmo-3 welding material is used for welding material. The specific chemical composition is shown in Table 3.
Table 3 chemical composition (wt%) of welding materials

Type	C	Si	Mn	S	P	Cr	Ni	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

Welding procedure qualification

Preparation before welding

The cutting and groove processing of 9Ni steel pipe shall adopt the mechanical processing method as far as possible. Gas cutting or plasma cutting and groove preparation can also be adopted. The groove after processing or cutting shall be polished.
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 Figure 1, with a gap of 2-4mm and a blunt edge of 0-2mm.
The appearance inspection shall be carried out after the groove processing, and there shall be no cracks and delamination, otherwise it shall be repaired.
The surface of groove and its two sides within 20 mm shall be cleaned by mechanical method and organic solvent to remove oil, rust, metal chips, oxide film and other dirt on the surface.

Fig. 1 groove details

Welding sequence and bead arrangement

The backing layer is welded by argon arc welding. In order to ensure the formation of root pass and the burn through phenomenon of manual arc welding filling, at least two layers of backing welding should be welded, and the thickness of welding flesh should be at least 6 mm. Manual arc welding filling is adopted. The welding layer arrangement sequence is shown in Fig. 2.

Figure 2 weld bead layout

Welding process parameters

Heat input is the energy received per unit length of weld, which is the main factor affecting welding thermal cycle. In other words, controlling heat input is the key to ensure mechanical properties and SSC (sulfide stress corrosion) test. See Table 4 for specific welding parameters.
Table 4 welding parameters

Number of weld bead	Welding method	Type of welding material	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

Because the melting point of weld metal welded by nickel based welding material is about 100 ℃ lower than that of 9Ni steel, it is easy to cause defects such as non fusion between groove edge and weld bead. Therefore, it is not allowed to strike the arc at will during welding, let alone arc outside the groove, so as to avoid arc damaging the base metal.
When the arc is stopped, the crater must be filled and stay for a while to avoid crater cracks. If there are crater cracks, they should be polished immediately.
In order to ensure the low-temperature toughness and SSC test results of 9Ni steel, the control of welding heat input is very important. The welding current should not be too large. Fast multi pass welding should be used to reduce the overheating of weld bead, and the grain size should be refined by reheating effect of multi pass welding. The interlaminar temperature should be controlled during multi pass welding. Small heat input should be used. The heat input should be controlled below 20kj / cm. The interlaminar temperature of multi-layer welding should be lower than 100 ℃ to avoid overheating of the joint.

Test results and analysis

Nondestructive testing

After the welding, the appearance of the specimen was inspected, and no undercut, surface porosity, crack, slag inclusion 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 smooth transition; No cracks, incomplete fusion, incomplete penetration, slag inclusion and other defects were found in the specimens by radiographic inspection, and the quality of welded joints met the standard requirements.

Tensile test

In tensile test, the tensile specimen is fixed on we-100 universal testing machine, and then tensile stress is applied to it, which causes the specimen to extend axially until it is broken, which is the main index to measure the material strength. The test results are shown in Table 5.

Table 5 tensile test results

Specimen number	Tensile strength（MPa)	Fracture location
1	761	Base metal
2	764	Base metal

According to the test results, it can be seen that the tensile test results meet the specification requirements.

Bending test

Bending test is to examine the ability of materials to bear deformation. The processed standard bending samples are tested on we-100 universal testing machine. According to the requirements of the specification, four side bending specimens were taken, and the bending test was carried out with the diameter of 63.5mm indenter, and the bending angle was 180 °， After bending, there is no crack on the surface of the sample and no other defects with length greater than 3mm in any direction. The test results meet the requirements of the specification.

Impact test 

The impact test is to put the impact sample on the JB-30B impact testing machine, and use the impact load to break the groove on the joint surface, so as to determine the impact performance of the welded joint based on the impact energy consumed on the unit area of the breaking place. Charpy impact test at – 196 ℃ is adopted in this impact test. Samples are taken at the position of 1-2mm from the weld surface. The notch positions are respectively located at the weld center, fusion line, fusion line 1mm, fusion line 2mm and fusion line 5mm. 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
Fusion line	80、82、76	79
Fusion line+1 mm	104、91、111	104
Fusion line+2 mm	78、99、85	87
Fusion 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).

Macroscopic and hardness test

The macro section inspection of the weld shows that the weld is completely penetrated without cracks. The macro sample is shown in Figure 3.

Fig. 3 photos of macro samples
The hardness of weld metal, heat affected zone and base metal of welded joint are measured respectively, and the hardness values are shown in Table 7.
Table 7 hardness test results

Sampling location	Hardness value（HV10）
Weld metal	219～247
Heat affected zone	253～290
Base metal	230～256

SSC test

Three standard welded plate specimens were continuously filled with 99.2% CO2 and 0.8% H2S acetic acid solution (initial pH = 3) at 25 ℃ and loaded with 80% yield strength at 4-point bending（ σ The samples were not broken after 720 hours of immersion. No cracks were found under the 10 times magnifying glass. According to the corresponding standards, the batch of samples were qualified in sulfide stress corrosion test (see Figure 4).

Figure 4 surface morphology of compressive stress specimen after immersion corrosion

Conclusion

High quality welding joints can be obtained under reasonable welding conditions by using argon tungsten arc welding as backing, manual arc welding as filling and capping, ERNiCrMo-3 welding wire and enicrmo-3 welding rod to weld 9Ni steel.
All the performance indexes of welding procedure qualification test meet the technical requirements. We have basically mastered the pipe welding technology of 9Ni steel, such as TIG backing, manual arc welding filling and cover, which provides valuable experience for guiding production in the future.

Source: China Large Diameter Flange Manufacturer – Yaang Pipe Industry (www.epowermetals.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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