Abnormal Hardness in Welding Heat Affected Zones of F91 Forged Valves (Part Two)

3. Measures
To improve the hardness and metallographic structure of F91 low alloy steel after welding, combined with the above welding process analysis, the following measures are proposed.
 
(1) Welding processes
According to DL/T 869-2012 "Welding Technical Regulations for Thermal Power Plants", the welding process and evaluation are compiled, and the welding operation is carried out according to the welding process; the preheating of welding and the temperature between welding layers are controlled.
a. Welding methods
The welding method of argon arc welding bottom welding plus manual arc welding filling and covering surface is adopted.
b. Selection of welding materials
According to the chemical composition and mechanical strength of F91, ER90S-B9 is adopted for the welding wire, and the welding rod is E9015-B9.
c. Requirements for the welding environment
It is necessary to have an effective windproof device during welding. The wind speed during gas shielded welding is not greater than 2m/s, and the wind speed during other welding is not greater than 8m/s; the ambient temperature is greater than or equal to 5°C.
d. Welding parameters
 
Table 2 Welding parameters

 
The bottom layer is argon arc welding, and the other layers and the cover surface are manual arc welding.
e. Argon filling protection
Argon: 8 to 12 L/min for the front, 10 to 25 L/min for the first layer on the back, and 3 to 8 L/min for the second layer.
Argon gas protection is required in the primer, the first layer, and the second layer to prevent oxidation of the welding seam root.
f. During welding, if it is necessary to interrupt the welding, at least the thickness of the welding layer must be 9mm or 25% of the welding groove must be filled before the welding can be interrupted, whichever has the smaller value. If it is unavoidable to interrupt the welding, it is necessary to carry out welding heat treatment and then re-welding.
 
(2) Heat treatment processes
According to DL/T 819-2019 "Technical Regulations for Welding Heat Treatment of Thermal Power Plants", the heat treatment process of F91 is compiled, and the corresponding heat treatment is carried out according to the process and pipe wall thickness.
a. Heating methods
Using flexible ceramic resistance heating or far-infrared radiation heating.
b. Post-weld heat treatment parameters
 
Table 3 The parameters of post-weld heat treatment

 
The post-weld heat treatment parameters are shown in Table 3. The temperature rise rate and temperature drop rate shall not exceed 150°C/h. When the temperature is decreased to 300°C, the air cools to room temperature.
c. When the metallographic or hardness test results are abnormal, normalizing plus tempering heat treatment is required. The normalizing temperature is 1040 to 1080℃, and the tempering temperature is between 730 and 800℃.
 
(3) Length of groove positions
The length of the two ends of the valve from the position of the welding groove shall be carried out according to the standard, and the length of one side shall not be less than 350mm.

(4) Metallographic inspection and hardness testing
To ensure the metallographic characteristics of F91, after the welding is completed, perform the inspection; the metallographic inspection and hardness test are carried out within a minimum of 100mm from the welding area. The specific length is determined according to the length of the welding heat treatment area to ensure that the metallographic abnormality and abnormal hardness of base materials of valves are avoided. After manufacturing according to the above process, the hardness value of the abnormal area of ​​the valve is measured, and the F91 low alloy forged steel plug valve is connected to the pipe made from P91 and with a length of 350mm after welding and heat treatment. According to the requirements of the "Technical Supervision Regulations for Metals of the Factory", the hardness value of the test is shown in Table 4 below.
 
Table Measuring values of the hardness

 
4. Conclusion
After the implementation of the treatment measures, the same type of F91 valve has been used for 3 years in the electric tower project 2 × 660MW project, Zhuneng 2 × 300MWCFB project, Jining 2 × 350MWCFB project and Dianta 2 × 600MW project. No abnormal reduction occurs for hardness values again. The valve is in good operating condition, and feedback from the user is satisfying. Therefore, the new processing method of welding and heat treatment for the F91 valve proposed in this article is feasible and can be used to guide the welding and manufacturing of valves made from the same material.
 

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