Features and Forging Processes of Flat Welding Flanges

1. Features of flat welding flanges
Flat welding flanges save space, reduce weight and have good sealing performance due to no leakage design in joint area. The reason why dimension of compact flanges is small is that diameter of sealing element is reduced and sealing surface thereof is thus reduced. Besides, flange gasket is replaced by sealing ring to ensure the match between two sealing surfaces. With the decrease of required pressure, dimension and number of bolts are correspondingly reduced. Thus, as a new kind of flanges, flat welding flanges with small size and light weight are finally designed, and their weight decreases by 70%~80% compared with traditional flanges. Therefore, flat welding flanges with good performance, namely saved space and light weight, are applicable for industrial purposes.

2. Forging processes of flat welding flanges
At the initial stage of flat welding flange forging, density increases at a greater rate due to easy deformation and relatively small deformation force of perform with pores. At the last stage of forging, deformation force needed for eliminating residual pores increases rapidly due to large closed pores and increasing deformation resistance. Deformation resistance is closely related to deformation temperature. Relatively high deformation temperature contributes to compactness and the decrease of deformation resistance, which is also applicable for compactness of flat welding flange.
Flat welding flange forging process has higher requirement on equipment than traditional process. Displacement characteristics of plunger chip should match well with deformation and compactness features of deform. Contact time of billets and moulds should be reduced as short as possible. Forging press should have good rigidity. Movable crossbeam and piston should good guide precision to ensure precision of forging parts. Hence, mechanical presses, such as crankshaft press and relatively high precision friction press, are usually used for flat welding flange forging.

Forging processes of flat welding flanges can be classified in two ways:
(1) The forging processes of flat welding flanges can be categorized into open die forging, upsetting, extrusion, die forging, closed die forging and closed upsetting. The finish machining of complex forgings may require only one process or just several processes. Closed die forging and closed upsetting have high utilization rate of materials because of no burr. Due to no burr, stressed area of forgings decreases and required loads also decrease. However, it should be noted that volume of billets should be strictly controlled and so are forging dies. Besides, forgings should be measured and wear of forging dies should be reduced.
(2) From the perspective of movement mode of forging dies, forging processes of flat welding flanges can also be divided into swing forging, rotary swaging, roll forging, cross wedge rolling, ring rolling, skew rolling, etc. Rotary forging, swaging and skew rolling processes can be used for precision forging machining. To improve utilization ratio of materials, roll forging and cross wedge rolling can be used for former procedure of long and thin materials. As with open die forging, rotary swaging can be also used for partial forming, except one difference, namely rotary swaging requires relatively little forging force. To ensure precision of rotary swaging and open die forging, movement direction of forging dies and rotary swaging process are controlled by computer so as to form forgings with complex shape and high precision through relatively little forging force, such as steam turbine blade with big size and many varieties.