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A335 P91 seamless steel pipe welding process

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Because A335 P91 steel has good high temperature comprehensive properties, it has been widely used in the main steam pipeline of large power stations in recent ten years. This paper describes the welding characteristics, welding process, welding quality inspection, welding quality monitoring and development trend of A335 P91 seamless pipe.


In the construction of many large thermal power station units, A335 P91 (P91 steel for short) steel is widely used in the main steam pipeline. This steel is a new type of martensitic heat-resistant steel developed by the United States in the late 1970s and early 1980s, which is equivalent to 9cr-1mov steel produced in China. Compared with domestic steel, this steel has the characteristics of high high temperature strength, good oxidation resistance and creep resistance, relatively high thermal conductivity and low thermal expansion rate. It fills the gap between ferritic steel (such as P22) and austenitic steel, makes the weldment have smaller section size and effectively reduces the wall thickness of the weldment, The material consumption is reduced, the thermal stress of the pipeline is reduced, and the risk of thermal fatigue crack is reduced.

A335 P91 steel pipe was not widely used in thermal power stations in China in the past. The welding procedure qualification and the characteristics of welding and heat treatment of construction units need to be explored in the construction process, and the pipeline assembly sequence also plays a key role in the welding quality. In order to ensure that the welding process and welding quality of P91 steel pipeline meet the requirements, the welding process qualification must be made according to the welding characteristics of P91 steel pipe, and the qualification report shall be used as the basis for on-site installation and welding construction, and the process requirements shall be strictly implemented during on-site construction.

A335-P91-seamless-steel-pipe.JPG

Chemical composition of a335-p91 steel pipe

The chemical composition of a335-p91 steel pipe is as follows:

C 0.08-0.12

Mn 0.30-0.60

P ≤0.02

S ≤0.01

Si 0.20-0.50

Cr 8.00-9.50

Mo 0.85-1.05

V 0.18-0.25

Nb 0.06-0.10

N 0.03-0.07

The number of steel mnn1v91 in France corresponds to the number of steel mnn10crv91 in France, and the number of steel mnn1v91 in Japan corresponds to the number of steel mnn09p9b.

chemical composition

It can be seen from the above table that this steel belongs to medium alloy heat-resistant steel.

A335 P91 steel welding characteristics 

① As A335 P91 is a medium alloy steel, it has a high tendency of cold crack. The welding crack can reach 100% without preheating. When preheating 200 ~ 250 ℃, the generation of cold crack can be avoided.

② In the welding process, the primary task is to strictly control the interlayer temperature of weldments and keep it at the preheating temperature or higher. Secondly, great attention should be paid to the time interval from interlayer temperature cooling to the beginning of post weld heat treatment.

③ The control requirements of welding input heat are relatively high. Practical experience has proved that the impact toughness can be greatly improved by using smaller electrode diameter, lower interlayer temperature and smaller welding line energy.

④ The welded joints are unstable structures with high hardness after welding, and corresponding heat treatment must be carried out after welding.

Selection of welding method for A335 P91 seamless steel pipe

A335 P91 steel has a high hardening tendency and is very sensitive to welding cold cracks. Tungsten argon arc backing, low hydrogen electrode filling and covered electrode arc welding can be used.

Welding process specifications and parameters

Groove type:

Butt clearance: 3 ~ 5mm

Welding parameters

Welding parameters

Preheating mode / temperature: tracking electric heating / 150-200 ℃ (GTAW), 200-250 ℃ (SMAW).

Shielding gas: argon, purity ≥ 99.99%, argon flow: welding gun 8-12l / min, back first pass 10-25l / min, second pass 3-8l / min.

Interlayer temperature: < 250 ℃ (temperature is controlled by far infrared detector).

Post heat treatment: raise the temperature to 300-350 ℃ after welding and keep the temperature for 2h.

Heat treatment: temperature 750 ℃ - 770 ℃, holding time 5min / mm, and not less than 4H; The rise and fall temperature of post weld heat treatment shall be less than 150 ℃ / h.

One horizontal fixing and one vertical fixing are welded according to the above process Φ two hundred and seventy-three × 30 test pieces, after inspection and inspection, meet the specification requirements.

A335 P91 seamless steel pipe Welding construction process

On the basis of qualified process qualification, relevant welding operation instructions are prepared in time according to the process qualification report. The contents of P91 steel welding operation instruction are as follows:

Preparation before welding

1. Selection of welding materials welding wire er90s-b9 welding rod e9015-b9

2. The groove shall be machined. The groove shall be smooth and uniform, and the size shall meet the requirements. It shall be carefully checked before matching. If it is found that it is unqualified, it must be polished with tools such as a grinder until it is qualified. Clean the rust, oil stain and other dirt within 15 ~ 20mm on both sides of the inner and outer walls of the groove until the metallic luster is exposed.

3. Special alignment fixtures shall be used for alignment, and forced alignment is not allowed. During alignment, the inner wall shall be flush. In case of staggered joint, the amount of staggered joint shall not exceed 10% of the wall thickness and shall not be greater than 1mm; Before butt joint, carefully check whether the base metal at the groove is defective, and if so, deal with it separately; The welding materials, welding process and selected welder technical conditions for spot welding shall be the same as those for formal welding; During spot welding of small-diameter thin-walled pipe, the gap is 2 ~ 3mm, and 2 ~ 3 points can be directly fixed in the groove. During spot welding of large-diameter pipe, the gap is 3 ~ 5mm, and the spot welding can be fixed with "positioning block".

A335 P91 seamless steel pipe Preheating before welding

Computer temperature control equipment is used to track and preheat the welded junction. Thermocouples are arranged symmetrically. Thermocouples and pipe fittings should be in good contact. The preheating temperature of argon arc welding is 150 ~ 200 ℃, and that of electrode arc welding is 200 ~ 250 ℃. The preheating width is calculated from the pipe wall thickness of not less than 3 times on each side of the groove center.

Backing welding

WS-400 welding machine with DC positive connection is adopted. When the preheating temperature reaches the specified temperature and is uniform, the large-diameter pipe shall be welded symmetrically by two people, and the welds of the first and second layers shall be argon arc welded. During backing welding, the internal wire feeding method shall be adopted. It shall be noted that the root fusion is good and the thickness shall be controlled within 2.8 ~ 3.2mm. In order to prevent the oxidation of weld metal at the root, the pipe shall be filled with argon for protection.

Argon filling protection can be carried out according to the following requirements:

a. The scope of argon filling protection shall be subject to the groove center, 200 ~ 300 mm on each side. Use soluble paper or other soluble materials and stick them firmly with high temperature resistant tape to make a sealed air chamber.

b. Use the "inflation needle" to insert argon from the groove gap or "flaw detection hole". At the beginning, the argon flow on the back is 10 ~ 25L / min, the second back is 4 ~ 8L / min, and the welding gun is 8 ~ 12L / min.

c. Place the ignited lighter or match near the weld gap. When the flame goes out, it indicates that the internal air has been exhausted, and argon arc welding can be carried out.

Electrode arc welding, filling and capping welding

After argon arc welding, raise the preheating temperature to 200 ~ 250 ℃ and start arc welding; DC reverse connection method and symmetrical welding by two people are adopted. When A335 P91 seamless steel pipe  is welded, the molten iron in the molten pool has high viscosity, poor fluidity and small welding specification, so it is easy to have defects such as slag inclusion and lack of fusion between layers. Therefore, in order to avoid large defects and ensure the comprehensive mechanical properties of the welded joint, multi-layer, multi pass and small specification welding must be adopted. The thickness of the welding layer shall be as thin as possible, and the thickness of each layer shall not exceed 4mm; During swing welding, the swing of the welding rod shall not exceed 4 times of the diameter of the welding rod. Each layer of weld bead must be cleaned, especially the joint and both sides of the weld bead. After the weld appearance is welded, the welder is required to conduct cleaning and self inspection immediately. If it is found that the appearance is not well formed, repair welding immediately. It is strictly prohibited to repair welding directly after the weld cools down.

Post weld heat treatment

When the welded joint cannot be heat treated in time, the post heat treatment with heating temperature of 300 ~ 350 ℃ and constant temperature time of 2 hours shall be carried out immediately after welding.

The post welding heat treatment adopts multi-channel output and multi-point temperature measurement, and the heater corresponds to the thermocouple one by one. The upper and lower temperatures of the welded junction are always the same, so that the rising and cooling speed of the heat treatment welded junction should be ≤ 150 ℃ / h. When the temperature drops to 300 ℃, it can be cooled to room temperature without control. The heating temperature is 750 ~ 770 ℃, and the constant temperature time is 5min / mm, but at least 4 hours.

Weld quality inspection

The content of weld quality inspection and acceptance is as follows:

1. The appearance shall be accepted according to the quality standard of class I weld appearance in DL / t869-2004 technical specification for welding of thermal power plant;

2. NDT: 100% ultrasonic (UT) and magnetic particle (MT) flaw detection shall be carried out for the weld according to JB4730, and it is qualified at level I;

3. Hardness: after heat treatment, conduct 100% hardness measurement, and the hardness value is less than 350hb;

4. Conduct 100% spectral analysis and recheck on the composition of weld metal alloy.

Weld repair

1. The appearance of the welded joint does not meet the standard. The light ones shall be polished and welded, and the serious ones shall be cut off and welded again. 2. The defects of welded joints that fail to pass the NDT can be repaired by welding, but it must be carried out according to the provisions of normal welding process or welding repair process on the basis of confirming that the defects have been completely eliminated. 3 . Generally, the same welded joint shall not be repaired more than twice, otherwise it shall be cut off and welded again. 4. Post weld heat treatment and NDT must be carried out again for the repaired welded joints.

Quality monitoring

Strengthen the control of the welding process of P91 steel. Due to the poor welding performance of P91 steel, poor control of any link in the welding process will lead to fatal defects such as poor mechanical properties of welded joints and cracks. Firstly, all welders shall be trained and obtain the qualification certificate of P91 steel before taking the post. Secondly, they shall be strictly controlled from the aspects of measures, materials, equipment, matching, welding, heat treatment parameters, surface quality of welded joints, nondestructive testing, repair of defective welds and so on.

Conclusion

Through the selection of the above construction process and strict technical and quality management measures, the weld quality is normal from the situation since the hydrostatic test and operation of the main steam pipeline of the power plant. Therefore, the above welding process is effective for P91 steel pipe. In addition, P91 pipe is used for the main steam pipeline of thermal power station, which has higher cost performance than P22 pipe commonly used in the past. At present, the use of P91 steel in large thermal power generating units has become an inevitable trend.


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