Quality Control of Pressure Bearing Forged Parts (Part One)

Abstract: The process and technical requirements of raw materials, forging, heat treatment, welding and inspection of F91 steel forgings are introduced. The common quality defects are analyzed, and improvement measures are proposed.
 
1. Overview
With the popularization of ultra critical units, ASTM A182 F91 is widely used for high temperatures and high pressures. 91 series steel is a modified steel grade formed by adding strengthening elements V, Nb and N on the basis of 9Cr-1Mo type steel, and its metallographic structure is tempered martensite in the heat treatment state. In the American standard, the steel with a Cr content of less than 10% is listed as a ferrite series, so its name in the ASTM A182 standard is still ferrite. ASTM A182 F91 free forging or die forging steel parts are mainly used for pressure-bearing parts such as valve bodies and bonnets of main steam gate valves, superheater blocking valves and main steam traps. The operating temperature of the valve reaches 620°C and the working pressure is about 30MPa.
 
2. Chemical composition and mechanical properties
ASTM A182 F91 steel has excellent high-temperature strength and high-temperature creep resistance below 650°C, low linear expansion coefficient, excellent microstructure stability and processability in long-term operation, and low manufacturing cost. China has carried out pipeline tests on P91 and listed it in the GB5310 standard. The corresponding national grade is 10Cr9Mo1VNb. It is gradually popularized for pressure-bearing forgings such as pipelines, valves and joints, as well as structural parts for the high requirements of heat resistance below 620℃. The chemical composition and mechanical properties of the material are shown in Table 1 and Table 2.
 
Table 1 Chemical composition of ASTM A182 F91  Wt%

 
Table 2 Mechanical properties at room temperatures (JB/T 12000)

 
3. The analysis of forgings
3.1 Process requirements
(1) The forging steel parts should adopt a refining process. In the smelting process, the ingot mold should be dried and the environmental humidity should be controlled. The molten steel is subjected to degassing to control the content of harmful gases such as hydrogen and oxygen to avoid the occurrence of harmful defects such as hydrogen embrittlement and white spots.
(2) When electroslag remelting ingots are used for steel forgings, the electrode blanks used in electroslag remelting should be made of vacuum-treated steel.
(3) The top and bottom of the steel ingots for forgings should have sufficient cutting amount to ensure that the finished forgings are free from shrinkage cavities, porosity, serious segregation and other harmful defects.
(4) For different steel forgings, the forging process should be evaluated separately to verify the quality requirements of the forging process.
(5) Control the forgings within a reasonable range of an initial forging temperature (1200°C) and a final forging temperature (900°C).
(6) Forging ratios of various forging raw materials are shown in Table 3. A higher forging ratio is conducive to the good forging of the core of the steel forging, and the forgings of the valve body can be punched and reamed at both ends. The forgings are cooled in the furnace and annealed at 650℃.
 
Table 3 Minimum forging ratios of main sections

 
3.2 Heat treatment
(1) After the forging is completed, the non-destructive testing is qualified and the rough processing is completed; the appropriate heat treatment process should be selected according to the material of the forging to fully eliminate the forging stress, obtain better microstructure and grain size, and meet the performance requirements. The mechanical properties after heat treatment meet the requirements of Table 2.
(2) Steel forgings must be evaluated for the heat treatment process to verify that the heat treatment process can meet the quality requirements of the steel forgings.
(3) The size of pressure-bearing steel forgings such as valve bodies and bonnets for large-diameter valves is generally large, and should be subjected to rough machining of the middle hole and channel hole, which is conducive to uniform pressure relief on the core and shortens the heating and holding time.
(4) The heat treatment of large steel forgings must be carried out after removing the oxide scale to see the metallic luster. Perform preliminary ultrasonic and magnetic particle inspection, and then heat treatment. The heat treatment of ASTM A182 F91 adopts 1050℃ normalizing plus 750℃ tempering.
(5) When the mechanical properties test or re-inspection of the forgings fails, the batch is allowed to be inspected after re-heat treatment. However, the number of re-heat treatments shall not be performed more than twice. The times for tempering are not counted.
 

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