Welding construction process of nickel based corrosion-resistant alloy

Nickel based corrosion-resistant alloys are mostly used in high-temperature, high-pressure, and corrosive environments due to their characteristics. They work under harsh conditions, withstand significant environmental pressure, and have high material quality and performance requirements. In practice, the main problems in the welding construction of nickel based corrosion-resistant alloys include crystallization crack welding, joint welding, local welding, etc. Welding construction is a fine process, so these issues must be put on the agenda. Facts have also proven that selecting appropriate welding materials, developing scientific welding plans, clarifying welding processes, and effectively controlling each link and step can meet the needs of welding design and construction. This article discusses the welding construction process of nickel based corrosion-resistant alloys.

0. Introduction

Nickel based corrosion-resistant alloys are a substance with corrosion resistance. Due to this physical property, nickel based corrosion-resistant alloys are mostly used in high-temperature and high-pressure environments. This is its advantage and strength, which is also why they are widely used in fields such as petroleum, chemical engineering, and metallurgy. Nickel based corrosion-resistant alloys can meet some tough engineering corrosion problems, with high and low temperature resistance, strong plasticity, and convenient processing. They are ideal materials for making important engineering parts. However, as for nickel based corrosion-resistant alloys themselves, welding is still difficult, so it is necessary to pay attention to overcoming welding difficulties and focus on solving welding construction processes to utilize the advantages and strengths of the materials fully. There are many considerations to consider when welding nickel based corrosion-resistant alloys.

1. Problems and preventive measures in welding of nickel based corrosion-resistant alloys

During the welding construction process of nickel based corrosion resistant alloys, some problems are prone to occur. If these problems are not solved, the performance of nickel based corrosion resistant alloys cannot be fully utilized. The difficulty and problems of nickel-based corrosion-resistant alloy welding are also greater for offshore engineering construction.

1.1 Welding problems of nickel based corrosion-resistant alloys

The outer steel pipe in the lined stainless steel composite steel pipe is a welded steel pipe produced according to “B/T3091-2001 Welded Steel Pipe for Low Pressure Fluid Transportation”, a seamless steel pipe produced according to “B/T8163-1999 Seamless Steel Pipe for Fluid Transportation”, or a spiral seam welded pipe produced according to “SY/T5037-2000 Spiral Seam Submerged Arc Welded Steel Pipe”.
The outer steel pipe of the lined stainless steel composite steel pipe used for transporting oil and natural gas is produced by B/T9711-1997 “Technical Conditions for Delivery of Steel Pipe for Petroleum and Natural Gas Industry.” The tensile strength of welded steel pipes or seamless steel pipes shall not be less than 335MPa, and the elongation shall not be less than 15%. Galvanized pipes are lined with stainless steel composite pipes, which symbolize the transportation of natural gas and oil. The outer layer of stainless steel composite pipes has an elongation of 25% and has retained pressure. The steel pipes have undergone a water pressure test of more than 3.0MPa, a cyclic decrease, and passed the bending or flattening test specified in the standard. The calculation of gas and oil transmission steel pipes produced according to B9711, galvanized pipes lined with stainless steel composite pipes also requires fracture toughness test, metallographic inspection, tensile test, and high strength pressure test. Compared with ordinary materials, it requires more preliminary test steps and higher requirements for the materials themselves.

1.2 Hot cracking and prevention measures

Nickel-based corrosion-resistant alloys are highly sensitive to welding hot cracks and extremely prone to large cracks during arc crater construction. The intergranular axillary film is the main metallurgical factor that causes solidification cracks in the single-phase structure of nickel based alloys. In the process of using nickel based corrosion resistant alloys, if the composition of nickel based corrosion resistant alloys is not pure or uniform enough, some elements have low melting points and are prone to crystallization. Still, there are many impurities in the crystallization. In addition, non-standard welding and improper heat input will naturally cause cracks. Cracks in the welding of nickel based corrosion-resistant alloys can cause a decrease in their structural strength and render them unusable.
The prevention measures for welding hot cracks in nickel based corrosion-resistant alloys mainly start from the following points.
Firstly, during welding, low current fast welding should be used to fill the arc crater gradually. For weldments with a thickness greater than 6mm, multi-layer and multi-pass welding should also be carried out to ensure the temperature is above the standard during welding. After welding, cleaning and removing impurities on both sides of the weld seam is necessary to prevent excess harmful substances from harming the environment or people during welding. It is necessary to remove them and never take them lightly thoroughly. In addition, the manganese content should also be increased to improve the dissolution of harmful elements in nickel based corrosion resistant alloy welding to achieve healthy and effective nickel based corrosion resistant alloy welding work.

1.3 Problems that may occur during the welding of nickel based corrosion-resistant alloy pipes

Due to the high alloy content of the welding wire, oxidation is prone to occur at the root of the weld seam during the welding process. In response to this problem, argon gas should be filled into the inside of the pipe for protection during backing welding. Especially for both the first and second layers, argon gas injection is required. When using multi-layer and multi-pass welding, attention should be paid to the cleaning work between layers to prevent welding defects and problems.

1.4 Welding porosity and prevention measures

The formation of welding pores is due to the small solid-liquid temperature spacing and low flowability of nickel based corrosion-resistant alloys, resulting in the formation of welding pores under the solidification conditions of rapid cooling during welding. However, from practical analysis, the content of oxygen, nickel, and other alloy elements in the welding process is an important influencing factor. For example, oxygen in liquid metals will react with nickel under high temperatures to form nickel oxide, which can react with hydrogen and carbon elements in liquid metals to generate other harmful substances. The presence of these substances is a potential threat, and once solidified, they cannot escape promptly, Remaining in the weld seam will form pores over time. During the welding process, air holes may also occur if the outer protective skin does not provide protection. In addition, if the welding torch is too high, it is impossible to protect the gas or molten pool, the air volume in the environment is too high, and other reasons are likely to cause welding air holes.
For the issue of welding porosity, it is still necessary to analyze it in a specific manner. Firstly, the construction area of nickel based corrosion-resistant alloys should be as far away from the carbon steel working area as possible, effectively isolated to reduce the moisture content of the welding material, and combined with reasonable and scientific welding techniques and specifications, to ensure that the coating can function normally in the welding process and play a protective role. In addition, it is also necessary to correctly use the flow rate of the welding torch and protective gas, select appropriate nozzles, intact gas transmission belts, qualified purity of protective gas, and the distance between the welding nozzle and the weldment should be appropriate, not too far or too close, and must be strictly controlled within a reasonable range. Exceeding the standard range can cause significant losses. The staff also needs to surround the construction site with a windproof cloth to avoid interference from the external environment during welding construction. They should also promptly clean and manage welding tools and materials, clean up impurities and dirt in the groove and its vicinity, and prevent generating many welding pores.

1.5 Welding slag inclusion and prevention measures

Nickel based corrosion-resistant alloy is a substance with high density. The molten metal has a high nickel content, poor flowability, and low permeability, which is more obvious during welding. Welding rods are subjected to resistance heat during fusion welding. During continuous welding, the latter half of the electrode coating is red hot or semi molten, causing it to lose its arc blowing force and protective effect. The fusion is uneven, and the molten metal directly piles up in the weld seam, causing slag inclusion. During multi-layer and multi-pass welding, poor cleaning of interlayer slag, deep internal interlayer bite, and fast welding speed can all cause slag inclusion.
The problem of welding slag inclusion can be solved from the following aspects. Firstly, welding should be carried out according to standards. During the welding process, the interrupted arc welding method should be used. When conducting multi-layer and multi-pass welding, attention should be paid to the cleaning work between the welds. No residue or impurities should be left to avoid the accumulation of slag, which cannot be removed. In addition, when carrying out welding work, it is necessary to carry out the work in a fixed area to avoid excessive movement and accumulation of impurities, which can be taken advantage of. Moreover, relatively stable power sources should be used, and continuous attention should be paid to improving the welding skills and abilities of welding workers.

2. Welding construction process of nickel based corrosion-resistant alloy

To put it simply, the welding construction process of nickel based corrosion-resistant alloy is the process of welding construction of nickel based corrosion-resistant alloy, including welding plans, welding methods, and other contents:

2.1 Develop a reasonable welding plan

Developing a plan is the first and most important task, as it relates to the progress of subsequent welding work and determines whether the welding work is completed. Therefore, it is necessary to seek truth from facts and carry out welding work orderly based on facts.
When formulating a plan, the first step is to determine the characteristics of the welding material, conduct a comprehensive analysis of the welding material, understand the welding characteristics and characteristics of nickel based corrosion-resistant alloys, and determine the overall direction of the welding plan based on actual needs. Overall, the development of welding work plans for nickel based corrosion-resistant alloys should be realistic and consistent with reality. It is necessary to start with practical needs.

2.2 Choosing the appropriate welding method

The welding performance required for offshore engineering is different, and the specific welding method selection must still be based on reality. For example, most corrugated flexible components in products such as expansion joints and metal hoses are thin plates with a thickness of less than 3mm, and the multi-layer structure is less than 8mm. This requires analysis of the characteristics of nickel based corrosion resistant alloys combined with the inherent characteristics of the enterprise; the welding method with low heat input should be adopted to prevent the grain from being too large, and matching welding tools should be selected to ensure that the welding work meets the needs.

2.3 Cleaning and cleaning of welding work

The surface layer of nickel based corrosion-resistant alloys has a relatively difficult to melt oxide film, which is already present in the production process and cannot be avoided. This layer of oxide film may not matter at certain times, but if the melting point of the welding material is lower than that of this layer of oxide film, the oxide film on the surface of this layer cannot be removed. After the welding work is carried out, impurities are easily generated, causing slag inclusion problems and affecting the welding quality and effect. So before welding, welding work should be done well. A stainless steel wire brush can be used to clean the inner and outer surfaces within a range of 50mm on both sides of the joint until metallic luster is exposed. Then, acetone or alcohol can be used to wipe the brush area, and a clean cloth can be used to dry it. Alternatively, acid washing can be used to remove the oxide film.

2.4 Inspection work

After welding is completed, inspecting and observing the welding interface is necessary. There should be no oil stains, rust, or oxide skin within the 20mm range of the welding interface and both sides. If the above problems occur during testing, polishing treatment is required until the metal luster is exposed.

3. Conclusion

In summary, it is not easy to analyze the welding construction process of nickel based corrosion-resistant alloys, whether in terms of steps or difficulty. However, nickel based corrosion-resistant alloys are a rare material with excellent performance, which can be seen in many fields. The scope of use is very wide, and the angles of use are also different. Therefore, if relevant workers want to truly realize the advantages of nickel-based corrosion-resistant alloys, It is necessary to overcome the difficulties in welding construction, break through bottlenecks, grasp the key points, and achieve reasonable, efficient, and high-quality completion of welding construction work. Only in this way can the use of nickel based corrosion-resistant alloys be laid a solid foundation. The welding construction process of nickel based corrosion-resistant alloy must be taken seriously, and the design of construction plans must be controlled; the construction process and steps must be emphasized, and efforts should be made from multiple angles simultaneously to maximize the advantages of nickel based corrosion-resistant alloy.
Author: Zhang Bin
Source: China Forgings manufacturer – Yaang Pipe Industry Co., Limited (www.epowermetals.com)