Weldability Analysis and Application of Hastelloy C-276 Pipeline
What is Hastelloy C-276?
Hastelloy C-276 (abbreviated: Hastelloy) is a nickel-based alloy, which is mainly used in strong corrosive media. In order to improve the corrosion resistance and cold and hot processing performance of Hastelloy C-276, Hastelloy C-276 has undergone three major improvements, namely, it is divided into three series of B, C and G, and has been widely used in many fields abroad, such as petroleum, chemical industry and environmental protection. In the paper, combined with the actual engineering, the introduction of Hastelloy C-276 alloy pipe welding technology.
1. Application background
A large domestic petrochemical plant to expand 450,000 tons / year purified terephthalic acid (PTA) plant, the device “Hastelloy” pipeline design in the “oxidation unit”, “catalyst preparation and recovery unit “catalyst preparation and recovery unit” and “catalytic combustion unit”.
- Transport medium: mainly for the catalyst and its combustion exhaust gas;
- Pipe material code: Hastelloy C-276;
- Design maximum pressure:2.0MPa;
- Design maximum temperature: 215℃;
- Pipe diameter and wall thickness are shown in Table 1. 2.
2. Characteristics of Hastelloy C-276 alloy
2.1 Chemical composition
Hastelloy C-276 alloy belongs to nickel-molybdenum-chromium-iron-tungsten system nickel-based alloy, the chemical composition of the material certificate is shown in Table 2.
2.2 Mechanical properties
The mechanical properties of Hastelloy C-276 alloy are outstanding, it has the characteristics of high strength and high toughness, so it is difficult to machine, and its strain hardening tendency is very strong. When the temperature is high, Hastelloy C-276 alloy tends to absorb harmful elements and its mechanical properties and corrosion resistance are reduced.
The material is annealed at 1150°C and rapidly cooled with water. Its mechanical properties test values are shown in Table 3.
2.3 Corrosion resistance
Hastelloy C-276 alloy is mainly produced for chemical process environment, especially in the case of mixed acid, such as flue gas desulfurization system discharge pipe, mainly resistant to wet chlorine, various oxidizing chlorides, chloride salt solution, sulfuric acid and oxidizing salt, in low and medium temperature hydrochloric acid have very good corrosion resistance. The high content of Ni and Mo in the alloy makes it very resistant to chloride ion stress corrosion fracture.
3. Analysis of the weldability of Hastelloy C-276 alloy
Hastelloy C-276 alloy welding, more sensitive to water, grease, oxides and other pollutants, especially to prevent pollution of carbon steel, so the cleanliness of the bevel and construction isolation requirements are higher.
Through the choice of welding method and bevel shape, the solidification organization of the weld metal is controlled, and the fusion penetration and fusion of the alloy weld is improved.
In the alloy, sulfur and nickel can form a low-melting eutectic to the grain boundary of the weld metal segregation; silicon will cause thermal embrittlement cracking of nickel alloy. Therefore, when welding Hastelloy C-276 alloy, it is necessary to control the carbon content of the weld metal; keep the sulfur, silicon and other elements in a low content. At the same time, choose a good welding method and matching bevel shape, and remove contaminants. Also control the welding heat input.
Table.1 Steel pipe diameter and wall thickness
|Diameter of steel pipe||1″||2″||3″||4″|
|Wall thickness of steel pipe/mm||6. 35||5.54||5.49||6.02|
Table.2 Chemical composition of Hastelloy C-276 alloy (mass fraction)
Table.3 Mechanical properties of Hastelloy (C276) alloy
|Temperature T/° C||Yield strength σ0.2/MPa||Tensile strength σb/MPa||Elongation of section (%)|
Table.4 Welding process parameters
|Welding level||Diameter of welding wire/mm||Current/A||Voltage/V||Argon flow/L/min|
|Filling and covering||85-95||12-14||11-12|
4. Hastelloy C-276 alloy welding process
4.1 The choice of welding method
Choose a welding method with low welding heat input.
Combined with the diameter of the pipe, and taking into account the practicality and versatility of the construction site, we chose to use tungsten gas shielded welding (GTAW), welding equipment selected pulse type welding machine.
4.2 Selection of welding materials
Design documents clearly require: must ensure the corrosion resistance of the weld metal. Combined with the choice of welding method, the choice of composition and the base material is comparable to the ERNiCiMo-4 welding wire.
4.3 Selection of welding bevels
Referring to the ASME Boiler and Pressure Vessel Code for the successful experience of C-276 alloy welded joints, the design of the bevel form for the external bevel, the specific dimensions are shown in Figure 1.
4.4 Welding process parameters
Before welding construction, we did the welding process evaluation test, qualified welding process parameters are shown in Table 4.
4.5 Post-weld heat treatment
Post-weld heat treatment of C-276 alloy, in fact, is a solid solution treatment of the material, that is: in 1040 ℃ -1150 ℃ heating; subsequently, within two minutes of rapid cooling to about 400 ℃ (black state).
According to the small diameter of the site, we choose the rope type electric heater for heating the weld channel, the axial fan for forced cooling, and the whole process is monitored by thermocouple for temperature.
5. Hastelloy C-276 alloy construction process points
5.1 Material inspection
5.2 Cutting and beveling
Use mechanical or grinding wheel machine and choose stainless steel special grinding wheel to cut and grind.
Figure.1 butt bevel form diagram
Pre-weld grinding of the mechanically processed bevels.
5.3 Pre-weld cleaning
Pipe section grouping before the bevel with a grinder will be cleaned within the bevel and its inner and outer sides within 25mm of the surface of the base material oxide film, until the metal luster.
Sulfur-free acetone or trichloroethylene solution should be used to clean the base material, bevels and blunt edges within 50 mm on each side of the joint before welding to remove grease and other impurities from the surface of the welded parts.
After processing, dry the water or impurities with a clean cotton cloth.
5.4 Assembling, welding
Welding with argon gas purity of 99.99% or more, and back flushing argon protection until the weld is cooled to below 400 ℃ (weld metal color is black state).
Grouping to avoid strong grouping, so as not to produce stress.
Spot welding welding must be strictly enforced formal welding process requirements.
Welding to strictly enforce the process discipline, strict welding order, filling and cover when the multi-channel welding welding joints should be staggered.
The welding process should be strictly controlled welding heat input. The use of short arc, not swing, multi-layer multi-channel welding method; while strictly controlling the interlayer temperature, a layer of welding after the use of water cooling method for cooling, in order to prevent the weld grain growth and thermal cracking. Welding interlayer if there are pores, cracks, not fused and other defects, must be thoroughly removed after the defect, before continuing welding.
Welding arc, the arc pit should be filled.
After the welding arc, it is strictly forbidden to immediately hot molten pool and wire directly exposed to the air, the torch should continue to both argon protection to prevent oxidation.
5.5 Solid solution treatment
Before the heat treatment to clean the surface of the alloy oil and other dirt that may be generated during the heat treatment of all the carbon elements.
Heating up to 1130 ℃, stop heating, and quickly remove the external insulation cotton, with multiple axial fans, in 2min, the weld inside and outside the forced cooling. The number of fans depends on the cooling effect.
Pickling and passivation treatment after heat treatment.
5.6 Construction protection
The welding area should be kept clean, dry and dust-free.
During construction, all relevant materials must be isolated from the carbon steel to prevent carburization contamination.
6. Hastelloy C-276 alloy weld rework
- (1) The rework should be carried out by welders who have passed the examination for the corresponding project, and the rework process should be evaluated and carried out in strict accordance with the rework program.
- (2) The same part of the number of rework should not exceed two times in general, after two rework still can not meet the requirements of the weld, must be cut out and in accordance with the construction program to re-construction.
In a large domestic petrochemical plant expansion of 450,000 tons / year purified terephthalic acid (PTA) plant “Hastelloy” pipeline construction, in strict accordance with the above-mentioned welding construction process, achieved a welding pass rate of 100%, the spectral inspection of 100% qualified results.
Author: Sun Guiying, Tang Chongfeng
Source: China Hastelloy C-276 Pipeline Manufacturer – Yaang Pipe Industry Co., Limited (www.metallicsteel.com)