Introduce: What matters should be paid attention to when processing and welding stainless steel welded ventilation pipes?

Matters needing attention in processing and welding of stainless steel ventilation pipeline

Common materials of stainless steel air duct: 304, 316L, and core difficulties: high temperature sensitization, intergranular corrosion, welding deformation, oxidation blackening and carburizing pollution; Different from ordinary carbon steel duct, direct contact with carbon steel is strictly prohibited in the whole process.

I. Key points of raw materials, blanking and prefabrication

Isolation and protection, put an end to iron ion pollution

Stainless steel plate, coil storage, operation table must be laid with rubber plate, stainless steel sheet or plywood; It is forbidden to put it directly on the carbon steel platform and ground.

The surfaces of cutting, rounding and flanging tools shall be kept clean to avoid carbon steel chips scratching the stainless steel surface; Clean the rust of rollers and cutters before machining.

Soft slings are used for hoisting, and it is forbidden to directly bind wire ropes to prevent scratches and iron infiltration.

Blanking and forming control

Priority is given to plasma cutting (with water purification curtain), and direct cutting of stainless steel is strictly prohibited in flame cutting; After cutting, the oxide slag and carburized layer are removed by grinding.

Stainless steel air ducts with plate thickness ≤1.2mm are mostly formed by bite to minimize welding; Thick plate, high pressure and anti-corrosion conditions use welded air duct.

Roll round and fold to avoid hard folding cracks; After molding, the butt gap is even, and the offset is controlled to be ≤ 1/10 of the plate thickness and ≤2mm.

Groove requirements (welded air duct)

V-groove is recommended for plate thickness ≥3mm; Completely remove oil, water, dust and polish in the range of 20 ~ 30 mm on both sides of the groove. Grease will cause pores and cracks.

Second, the welding process selection (ventilation pipe mainstream scheme)

The first choice for thin-walled air duct (0.8 ~ 3.0 mm): TIG/GTAW.

The inner wall of the air duct must be formed, and condensed water is easy to accumulate inside the ventilation duct. It is strictly forbidden to make the bottom by manual arc welding. Oxidation of the inner wall will quickly produce rust spots.

Thick plate reinforced flanges and supports: argon arc backing+manual arc cover.

Prohibition: ordinary gas welding stainless steel without shielding gas.

Key points: the ventilation pipe belongs to the through cavity, and it is formed by one-sided welding and two-sided, and the protection of the back inside the pipe is the most important.

Three, the key points of welding (the most prone to quality problems)

1. Argon-filled protection on the back (top priority! )

The high temperature oxidation on the back of stainless steel weld will form a loose oxide layer, which will cause intergranular corrosion and short-term weld leakage and rust under humid and ventilated conditions.

Both ends of the air duct section are blocked, and argon is filled inside to exhaust air; Small pipes are directly plugged with sponge and tape; Segmented plugging of long pipe.

Argon flow rate: 8 ~ 12l/min at the back and 8 ~ 10l/min at the front nozzle; Ventilation first, then arc, and delayed air supply after arc stop.

When argon cannot be completely filled due to limited conditions, stainless steel solder paste/anti-oxidation liner on the back can be used as an alternative (long-term replacement is not recommended for exhaust pipes with high anti-corrosion requirements).

2. Heat input control to reduce deformation and intergranular corrosion.

Stainless steel has poor thermal conductivity and large thermal expansion, and is easily deformed and warped;

Use small current, short arc and rapid welding, and prohibit long-term fixed-point burning;

The long weld adopts the method of sectional withdrawal and jump welding to reduce the overall deformation;

The interlayer temperature shall be controlled below 150℃, and cooling shall be stopped when necessary. It is forbidden to directly wash the weld by water cooling (it is easy to produce cold cracks and stress).

304 stainless steel in the range of 450 ~ 850℃ for a long time will be sensitized to precipitate chromium carbide, lose its corrosion resistance and reduce the continuous high temperature residence time.

3. Weld gap, assembly and tack welding

Positioning welding also needs argon protection, and the defects of positioning weld must be polished and removed;

Don't force butt assembly, strong assembly will produce huge welding residual stress, and the vibration in the later operation will easily crack;

Duct flanges and stiffeners should not be fully welded and intensively welded, and intermittent welding should be arranged reasonably to reduce deformation.

4. Welding wire selection and matching

304 plate → ER308/ER308L (low-carbon L-type preferred, intergranular corrosion resistant)

316L plate → ER316L

Low-carbon welding wire is suitable for ventilation system with exhaust air, humidity and condensed water.

Four, the welding process taboo

It is forbidden to polish carbon steel in the welding area to prevent iron dust from splashing on the surface of stainless steel base material (floating rust occurs in the later stage);

Don't clean the weld with carbon steel wire brush! Stainless steel wire brush must be used;

Avoid the stress concentration position at the corner of the air duct when starting and closing the arc; Closing the arc and filling the arc pit to prevent cracks in the arc pit;

The welding seam on the inner side of the air duct should be as smooth as possible to avoid a large number of welding burrs and sharp protrusions-dust accumulation and water accumulation under ventilation conditions, which will accelerate corrosion.

Five, after welding treatment (stainless steel duct is essential)

Cleaning oxide scale

After welding, the golden, blue and black oxide layers on the surface must be removed: stainless steel wire brush polishing → angle grinder impeller polishing; Severe oxidation is treated with stainless steel pickling passivation paste, and then washed thoroughly with clear water.

Correct deformation

Try to correct as cold as possible; If flame correction is needed, strictly control the temperature and local short-time heating, and it is forbidden to bake in a large area.

Protective inspection

Check the weld: no air holes, slag inclusion, cracks and incomplete fusion; Penetrant inspection (PT) shall be carried out for the ventilation duct with pressure/harmful gas according to the design requirements; General ventilation pipe appearance inspection+sealing test.

Stacking of finished products: isolate carbon steel again, and cover it with protection to prevent rainwater from accumulating at the weld position.

Six, supporting components welding points for attention

Stainless steel air duct is connected with carbon steel flange and bracket.

It is forbidden for stainless steel to contact carbon steel directly for a long time, and nonmetallic insulating gaskets (PTFE and rubber) must be added to prevent bimetallic galvanic corrosion;

Welding of reinforcing bars and lifting lugs

The lifting point shall be welded externally as far as possible to reduce the weld seam on the inner wall of the air duct; Avoid the formation of stress points by the intersection of multiple welds;

Dust removal, acid-base waste gas ventilation pipeline: The requirements of welding seam are higher, and the back surface must be fully formed, and full argon arc welding is preferred.

Seven, common fault prevention summary

The inner wall of the weld is black and rusty → argon protection on the back is not in place.

After welding, the air duct is severely warped and deformed → excessive heat input, continuous long welding, and subsection welding is not adopted.

Sporadic rust spots on the surface → iron ion pollution, carbon steel not isolated, and oxide scale not removed after welding.

Weld cracks, pores → groove oil and water vapor are not cleaned, argon gas is impure, and arc pit is not filled.