The production process of stainless steel necked butt-welding flanges (WN flanges / high-neck flanges) is essentially a continuous chain from "steel billet → forging → machining → welding → heat treatment → inspection". Different manufacturers vary greatly in terms of the degree of automation and equipment selection, but the key processes and quality control points are basically the same. The following will be elaborated step by step according to the processes.
1) Raw materials and cutting (sheeting)
Common materials: 304 (CF8), 304L (CF3), 316 (CF8M), 316L (CF3M), 321, 347 (CF8C) and other austenitic stainless steels are mainly used; duplex steel, nickel-based alloys, etc. may also be employed.
Key points:
Calculate the cutting weight based on the finished size of the法兰， plus the processing allowance, the draft angle (usually about 3-4° on both sides), and the thickness of the skin layer (the allowance for punching holes).
Modern production lines mostly use circular saws for cutting and material processing. The dimensional consistency can be controlled within ±0.2 mm, and the efficiency is much higher than that of traditional band saw machines (there are cases showing a reduction from 300 seconds per piece to approximately 30 seconds per piece).
The consistency of the cutting weight directly determines the consistency of the subsequent forging weight and the yield rate.
2) Heating
Method: Mainly uses medium-frequency induction heating (with less oxidation, fast heating, and stable temperature control).
Control Points:
Stainless steel is more sensitive to oxidation, so greater emphasis is placed on the uniformity of the heating atmosphere and temperature.
By using infrared temperature measurement combined with multi-point closed-loop control, the temperature at the exit is stable, providing a prerequisite for the subsequent automation of forging processes.
The heating temperature should take into account both forgeability and the prevention of overburning/ coarse grains (variations exist for different grades, and this should be carried out in accordance with the material specifications).
3) Forging process (core procedure)
The necked butt-welded flange must generally be manufactured using a forging/forge-welding process (as required by standards such as GB/T, HG/T, ASME, etc.). If it is fabricated from steel plates, it falls under the "non-recommended route", and the butt weld must be fully penetration + post-weld heat treatment + 100% RT/UT (JB4730 Grade II/Grade I), which results in higher costs and risks.
The common forging routes can be divided into two categories:
A. Free forging (single piece / small batch)
Basic deformation process: upsetting → drawing → punching → bending → cutting, etc. The production rate is low and the leftover material is large, but the tools are simple and the versatility is strong.
B. Forging (The mainstream method for mass production, also representing "high quality")
Typical process (taking an automated production line as an example):
Pre-forming: Initial shaping, distribution of metal, control of the direction of the neck and the disc portion.
Final forging (forming): Forming in a closed die once or multiple times to obtain flanges, conical necks, and inner hole blanks.
