There are different types of flanges for different welding

1. Raised Face - RF
The gasket receiving surface is raised above the bolt holes, this flange is known as a “Raised face flange.” A combination of a flat ring sheet and a metal gasket ring gasket double jacketed flange type is compatible with gaskets.

A purpose flange facelift is needed, with pressure applied to the smaller gasket area. This allows the joints to bear more force. According to ASME B16.5, the diameter and height dimensions are defined by the pressure class and diameter. The height of the lifted face is defined by the flange pressure rating.

The surface of the flange ASME B16.5 standard is 125 - 250 μin Ra (3 to 6 μm Ra)

The height of the raised face
The following description applies to most types of flange faces. Except for lap joint flange:
For pressure classes 150 and 300, the raised face height is 1.66 mm.
For pressure classes 400, 600, 900, 1500, and 2000, the height of the raised face is 6.4 mm.

2. Flat Face–FF
The face of the flange at the receiving area of the gasket is always smooth as a single surface across the face of the plate. Class 125 cast iron flanges and flanges made of other brittle materials are often referred to as “flat face.” The flange’s face is Flat-Face, which needs the use of a full faceplate gasket. While tightening the nut to avoid torque and cracking. It must not be used with raised flange surfaces. 

3. Tongue and Groove - TG
Tongue-groove surfaces must be compatible. On one flange there is a raised ring or tongue, while the opposite flange has a groove on the face.

Tongue and groove flanges come in a variety of sizes, from large to small. Unlike male and female faces, the tongue-and-groove type’s inner diameter does not extend to the flange base. As a result, the gasket is held both within the diameter. Pump covers and valve bonnets often use flanges with this tongue-and-groove face type.

This tongue-and-groove flange welding has the advantage of being self-organizing and ensuring a close welding. This shrouded joint maintains a good degree of support and does not require any major welding.

Flange faces that are widely used, such as RTJ, T&G, and F&M, should not be welded. Because the flange’s connection surface is incompatible. There was also no form of gasket to aid in the welding process.

4. Male-and-Female–M&F
The male flange’s surface is extended out more than a standard flange. The surface of the female flange has a deep groove for connection.

The female flange surface is 3/16 deep, the male is 1/4 tall, and both have a flat surface. The outer diameter of the female face serves to support and hold the gasket.

Advantages and disadvantages of T&G and M&F flange surfaces
Advantages: Good welding qualities. For a better bonding material, placement and compression are needed.
Disadvantages: Relatively expensive. And is not easily available for purchase. The raised face is easier to buy. Another complexity is that the strict laws are governing the pipe design. This makes it more difficult than ordering the face design of other flange types.