What should Suzhou stainless steel welding ventilation pipe manufacturers pay attention to when welding pipes?

In the welding operation, Suzhou stainless steel welded ventilation pipe manufacturers need to take into account three core dimensions: welding quality (ensuring ventilation performance and durability), safety specification (meeting industrial safety standards) and environmental protection requirements (adapting to Suzhou local environmental protection policies), especially for the stainless steel material characteristics (easy oxidation and thermal sensitivity) and the functional requirements of ventilation pipes (sealing and corrosion resistance). The following are the key considerations for sub-modules:

First, before welding: basic preparation and material control (the premise of determining welding quality)

The preparation before welding directly affects the stability of subsequent welding, so it is necessary to focus on three links: material matching, workpiece pretreatment and equipment debugging to avoid welding defects (such as cracks and pores) caused by previous omissions.

1. Accurate matching of stainless steel material and welding material

The commonly used stainless steel material for ventilation pipes is 304/316L(304 is suitable for ordinary environment, 316L is acid and alkali resistant, and suitable for special ventilation scenes such as chemical industry/food), and the selection of welding materials should be strictly matched to avoid the deterioration of corrosion resistance caused by mismatched components:

304 stainless steel pipeline: ER308L welding wire (low carbon content, reducing the risk of intergranular corrosion) or A102 covered electrode is preferred;

316L stainless steel pipeline: ER316L welding wire or A022 covered electrode (containing molybdenum, matching the corrosion resistance of 316L) must be selected;

Do not mix: if 308L welding wire is used to weld 316L pipeline, the corrosion resistance of the weld will be greatly reduced, and it will rust easily in humid/corrosive ventilation environment in the later stage.

2. Pretreatment of pipeline workpiece (to avoid welding defects caused by impurities)

Stainless steel welding requires extremely high cleanliness of the workpiece surface, and impurities such as oil stain, scale and dust will lead to blowholes and slag inclusion in the weld, which should be treated according to the following steps:

Degreasing and degreasing: wipe the pipeline welding groove and the surrounding area of 20mm with acetone or alcohol (avoid using gasoline, the residue is easy to burn), and remove oil stains and fingerprints (sweat contains salt, which will lead to pores during welding);

Remove the oxide scale: polish the groove with an angle grinder and a special grinding wheel for stainless steel to expose the metallic luster (the melting point of oxide scale is higher than that of stainless steel, so it cannot be melted during welding, and slag inclusion will be formed);

Groove processing: groove (commonly used V-groove with an angle of 60-70 and a blunt edge of 1-2mm) shall be processed according to the design requirements, so as to ensure that the edge of groove is free of burrs and cracks, and the butt gap shall be controlled at 1-2mm (too large a gap is easy to burn through, and too small a gap is easy to lack of penetration).

3. Debugging of welding equipment and shielding gas

Stainless steel welding needs to rely on inert gas protection (to prevent high temperature oxidation), and equipment parameters need to be accurately set:

Equipment selection: TIG welding (TIG welding) (suitable for thin-walled pipelines, with beautiful weld formation and good sealing) or MIG welding (TIG welding, suitable for thick-walled pipelines, with high efficiency) is preferred; It is forbidden to weld thin-walled ventilation pipes by manual arc welding (it is easy to cause deformation, uneven weld and affect ventilation resistance);

Protective gas: 99.99% high-purity argon gas must be used (if the purity is lower than 99.9%, the weld will be oxidized and blackened, forming a "blue brittle zone"), and the flow rate should be controlled at 8-15L/min (if the flow rate is too small, the protection will be insufficient, and if it is too large, turbulence will easily occur and air will be involved);

Equipment inspection: Before welding, check the argon flowmeter, welding gun nozzle (nozzle diameter is 5-8mm, which should be replaced in time after wear) and ground wire connection (the ground wire should be fixed with stainless steel special fixture to avoid current instability caused by virtual connection).

2. During welding: Core operation specification (ensuring weld quality and pipeline function)

The core function of ventilation pipeline is "smooth ventilation+no leakage". During the welding process, it is necessary to focus on controlling weld formation, welding parameters and deformation to avoid defects that affect sealing and structural strength.

1. Accurate control of welding parameters (adjusted according to pipe thickness)

Stainless steel pipes with different thicknesses (ventilation pipes usually have a thickness of 0.8-3mm) need to match different parameters. The following are the reference ranges of common specifications:

Pipe thickness (mm) welding method welding current (a) arc voltage (v) welding speed (mm/min) argon flow (L/min)

0.8-1.5 TIG welding 50-80 8-12 80-120 8-10

1.5-3.0 TIG welding 80-120 12-15 100-150 10-12

> ＞3.0 MIG welding 120-180 18-22 150-200 12-15

Key principles: thin-walled pipes (≤1.5mm) need "low current and slow speed" to avoid burning through; Thick-walled pipes shall be welded in layers (backing weld → filler welding → cover welding), and the welding slag shall be cleaned after each layer of welding to prevent slag inclusion.

2. Weld formation and quality control (avoiding leakage and structural risks)

The welding seam of ventilation pipeline shall meet the requirements of "no crack, no air hole, no slag inclusion, penetration and smoothness", with specific requirements:

Backing weld: Give priority to "wire filling" (welding wire is sent into the groove from the inside of the pipeline) to ensure the root penetration (incomplete penetration will lead to dust accumulation on the inner wall of the pipeline, increase ventilation resistance and be easy to corrode);

Cover welding: the residual height of the weld shall be controlled at 0.5-2mm (too high residual height will increase ventilation resistance, and too low will easily lead to insufficient strength), and the width of the weld shall be 2-4mm wider than the groove (to avoid unilateral fusion);

Real-time inspection: observe the state of the molten pool during welding (the molten pool is bright white without blackening and bubbles), stop immediately when finding blowholes, and weld again after cleaning up defects (blowholes will lead to pipeline leakage and affect ventilation effect).

3. Pipeline deformation control (ensuring installation accuracy)

Stainless steel has low thermal conductivity and large thermal expansion coefficient, and is prone to deformation (such as pipe bending and interface dislocation) during welding, which should be controlled by the following measures:

Rigid fixation: before welding, fix the pipe joint with fixtures (such as stainless steel angle iron and fixture frame) to avoid displacement during welding;

Symmetrical welding: for long pipelines or large-diameter pipelines (such as diameter > ＞500mm), adopt "sectional symmetrical welding" (welding alternately from both sides of the pipeline to offset thermal stress);

Control the interlayer temperature: when multi-layer welding, the interlayer temperature should be ≤150℃ (monitored by a thermometer) to avoid deformation caused by overheating of the pipeline (overheating will also reduce the corrosion resistance of stainless steel).

Third, after welding: quality inspection and post-treatment (to ensure compliance and durability)

Inspection and treatment after welding is the key to eliminate hidden dangers and prolong the service life of pipelines. It needs to cover three links: appearance inspection, sealing test and anti-corrosion treatment, especially to adapt to the possible humid and polluted environment in Suzhou (such as industrial areas).

1. Weld quality inspection (mandatory project)

Appearance inspection: check the weld with naked eyes or magnifying glass, and there is no crack, air hole, slag inclusion and undercut (undercut depth ≤0.5mm). If it does not meet the requirements, it needs to be reworked;

Airtightness test: the ventilation pipeline shall be subjected to "airtightness test"-both ends of the pipeline shall be blocked, compressed air (pressure 0.1-0.15MPa) shall be introduced, and the pressure shall be maintained for 30 minutes, and the pressure drop ≤5% shall be deemed as qualified (leakage will lead to the decrease of ventilation efficiency, especially in explosion-proof and anti-corrosion ventilation scenarios);

Non-destructive testing: For pipes in special scenes (such as ventilation pipes in chemical workshops and food workshops), X-ray testing (check for internal incomplete penetration and slag inclusion) or penetrant testing (check for surface cracks) is required, and the detection ratio shall meet the design requirements (usually ≥10%).

2. Anti-corrosion treatment (adapted to Suzhou environment)

Suzhou has a humid climate, and there are slightly corrosive gases in some areas (such as industrial parks), so it is necessary to strengthen the corrosion protection of welds:

Pickling passivation: After welding, the weld is treated with pickling passivation solution (including nitric acid and hydrofluoric acid) special for stainless steel (to remove the oxide scale and the "sensitized zone" of weld bead), and an oxide film is formed after passivation to improve corrosion resistance;

Cleaning and drying: after pickling, wash the inside and outside of the pipeline with clear water, and then dry it with compressed air (residual acid will lead to corrosion);

Special coating: For scenes with strong corrosiveness (such as chemical ventilation), special anti-corrosion coating for stainless steel (such as PTFE coating) can be additionally painted at the weld to enhance corrosion resistance.

3. Pipeline cleaning and acceptance

Internal cleaning: use high-pressure air to purge the inside of the pipeline to remove welding slag, welding wire tip and dust (to avoid impurities falling off during ventilation and polluting the conveyed air);

Size recheck: check the straightness (deviation ≤3mm per meter) and interface size (clearance with flange or other pipes ≤1mm) of the pipeline to ensure the installation and adaptation;

Data record: save welding process parameters and test reports (such as air tightness test records and nondestructive test reports) for subsequent acceptance and traceability (required for environmental protection and safety acceptance in Suzhou).

IV. Safety and environmental protection requirements (in line with Suzhou local codes)

Suzhou has strict requirements on safety and environmental protection in industrial production, so extra attention should be paid to welding operation:

1. Safety operation specifications

Protective equipment: the operator must wear special mask for argon arc welding (anti-ultraviolet), flame retardant work clothes, insulating gloves and non-slip shoes to avoid arc burning and electric shock;

Ventilation and dust removal: local exhaust system (such as welding fume purifier) should be installed in the welding area, and the ozone generated by TIG welding and fume generated by MIG welding should be discharged up to the standard (in line with Integrated Emission Standard for Atmospheric Pollutants (GB 16297-1996);

Fire and explosion prevention: inflammable and explosive materials (such as alcohol and paint) are forbidden to be stored at the welding site, and dry powder fire extinguishers are equipped (stainless steel welding sparks have high temperature and are easy to ignite surrounding objects).

2. Environmental compliance

Solid waste treatment: waste welding wire, welding slag and pickling waste liquid generated by welding need to be classified-waste welding wire/welding slag should be handed over to professional recycling enterprises; The pickling waste liquid should be discharged after neutralization treatment (pH adjusted to 6-9), and direct discharge is prohibited (Suzhou Environmental Protection Department will check it regularly);

Noise control: the noise of welding equipment (such as argon arc welding machine) should be ≤85dB, and sound insulation measures should be taken in factories around residential areas (such as installing sound insulation covers) to avoid noise disturbing people.

V. Precautions for Special Scenes (for common needs in Suzhou)

Ventilation pipe of food/medicine workshop: 316L stainless steel is required, and after welding, it needs to be "passivated+sterile cleaning", and the weld seam should be smooth without dead angle (to avoid bacteria accumulation), and additional "sterility test" is required for testing;

Explosion-proof ventilation pipeline (such as chemical industry park): the weld seam needs to be "strength tested" (the water pressure or air pressure test pressure is 1.5 times of the design pressure), and sparks are prohibited during the welding process ("inert gas protection+low spatter welding wire" can be used);

Large-diameter pipes (such as the main ventilation pipe of the factory building): after welding, stress relief treatment (such as local heat treatment at 600-700℃) is needed to avoid weld cracking caused by stress in long-term use.

To sum up, Suzhou stainless steel welded ventilation pipe manufacturers need to take "quality as the core and safety and environmental protection as the bottom line", and form a closed loop from material matching before welding to inspection and acceptance after welding, which not only meets the functional requirements of ventilation pipes, but also meets the strict safety and environmental protection standards in Suzhou, and finally ensures the long-term stable operation of pipes.