Two Main kinds of flanges in China
1.Forged flanges
2.Casting flanges
The reason I take this topic is that our sales market is not in order. Often the seller supplied casting flanges, instead of forged ones, though the buyer ordered forged ones.
The price of casting flange is about 15%-20% lower than forged flanges. Some dishonest suppliers offer very low price for forged flanges to attract the buyer , but they are in fact supply casting flanges.
How to distinguish casting flange and forged one.
From appearance casting flange and forged one look same. It is not easy to tell which is which. Only after you cut them into 2 pieces, one can find that casting one has many tiny cells like a piece of fermentative bread. The construction of forged one is very tight. Needless to say it is because of their different process methods. Forging means using a hammer to heavily hit the hot steel pieces from cylindrical shape to disc shape. The casting way is just pour melting steel into a flange shaped mold. The inside of casting flange is fluffy.
Besides, casting flange is lighter in weight than forged one.

Fatal weak point of Chinese casting flanges
As a Chinese writer, I do not know what foreign workshops make the casting flanges. I clearly know what Chinese workshops do. Like I mentioned above, just pour melting steel into a flange shaped molds and waiting to cool down. That is all. Then the problem appears. We all know the theory of “heat makes something expand and cold makes it contract”. When a hot thing is cooling down its volume will shrinks. After the hot melting steel has poured into the flange mold and filled the mold with steel liquid, it begins to cool quickly. During the cooling, the volume of melting steel shrinks at same time. The mold which is full of melting steel now becomes not so full, because of the cooling steel shrinks. More melting steel needs to be added into the mold, in order to make the mold full. But the worker now will find that it is impossible to refill the molds, because the casting head where the melting steel liquid flow through into the mold is blocked with cooled steel which becomes solid. Now the fatal weak point comes out: the melting steel in mold is shrinking as it cools, but no melting steel to reflow in to remake the mold full. In the end a hollow appears inside the solid flange shape. The hollow is rather big inside the flange body. For small sizes of flanges the hollow can be as big as a peanut, for bigger one the hollow can be same size as an olive. No worker can tell the hollow will be in which part of flange. During machining, the hollow may appears, during drilling bolt holes the hollow may also appears. Usually, when a worker see the hollow appears, he will fill putty into the hollow. So each casting flange’ inside not only has countless tiny cells, but also has a big hollow. So its tensile strength is very limited. It is dangerous to use on pipe line for chemistry liquid or hot steam.
Another weak point: because of the loose surface of casting flange, it is not suitable for zinc coating.