What are the specific measures for controlling the polishing cracks of stainless steel welded pipes

To control the polishing cracks of stainless steel pipes, measures should be taken from multiple dimensions including material selection, welding process, polishing parameters and process control. The specific measures are as follows:

1. Source control of materials

Choose materials with low crack sensitivity

 select 321 and 347 stainless steel that contain Ti and Nb stabilizing elements to reduce the intergranular corrosion tendency.

Control the carbon content to be ≤ 0.03% (such as 316L and 304L)， to avoid the formation of chromium carbide when carbon reacts with chromium at high temperatures， which leads to chromium deficiency at the grain boundaries. 

Material quality verification

Check the quality certificate to ensure that sulfur (S) is ≤ 0.03% and phosphorus (P) is ≤ 0.045% (impurities can reduce the material's toughness).

Perform flaw detection tests on the plates (such as ultrasonic testing) to avoid internal cracks or delamination. 

II. Welding Process Optimization

Control welding heat input

Use low current and rapid welding (such as TIG welding) to reduce the width of the heat affected zone and minimize the risk of coarse grain formation.

Example parameters: When the wall thickness is 3mm, the current is 100-120A, and the welding speed is 15-20cm/min. 

Inter-layer temperature management

During multi-pass welding, the inter-layer temperature should be controlled below 60℃ (monitored using an infrared thermometer) to prevent the weld seams from repeatedly being exposed to heat. 

Weld formation control

Ensure that the weld height is ≤ 1.5mm to avoid stress concentration; adopt a wide and shallow weld shape (width-to-depth ratio ≥ 1.2).

Backside argon gas protection (purity ≥ 99.99%) is used to prevent oxidation and the formation of a brittle layer. 

III. Polishing Process Adjustment

Selection of Abrasive and Grain Size Matching

Coarse Polishing: Use 80-120 mesh grinding wheels to remove the weld height;

Fine Polishing: Gradually increase to 320-600 mesh abrasive belts, avoiding excessive single grinding amount (recommend ≤ 0.05mm per time). 

Polishing pressure and speed control

Pressure: Adjusted according to pipe diameter. Generally, for pipes with a diameter of less than Φ50mm, the pressure should be ≤ 10N/cm²;

Linear speed: The linear speed of the sanding belt should be ≤ 25m/s to prevent local overheating (the surface temperature can be monitored using an infrared thermal imager, which should be ≤ 150℃). 

Cooling and Lubrication

Use water-soluble polishing fluid (such as a coolant containing 10% emulsified oil), with a flow rate of ≥ 10L/min, to reduce friction heat.

Avoid dry polishing and prevent the surface from undergoing martensitic phase transformation due to high temperature (magnetic detection can assist in the judgment). 

IV. Process Quality Monitoring

Early Crack Detection

After polishing 50 tubes, randomly select 1 tube for magnetic particle testing (MT) or penetrant testing (PT), with a focus on inspecting the weld seams and the heat affected zones. 

Surface stress detection

Regularly use an X-ray diffractometer to detect the residual stress on the surface. Ensure that the compressive stress is ≤ 200 MPa (tensile stress is prone to causing crack propagation). 

Metallographic analysis

Take 1 pipe section for metallographic sectioning every week. Observe the grain size (it is recommended that the austenite grain size be ≥ 6 levels) and whether δ ferrite exists (the content should be controlled at ≤ 5%). 

V. Follow-up Handling and Prevention Strain-relief annealing

For high-risk products (such as thick-walled pipes), after polishing, a solution treatment is carried out (for 304/316 materials: air-cooled at 1050-1100℃). 

Surface protection

Immediately perform passivation treatment after polishing (such as immersing in a 20% nitric acid solution for 20 minutes), to form a dense oxide film. 

Packaging and Transportation

Use PE protective film and paper tubes for packaging to prevent scratches; during long-distance transportation, use shock-absorbing pads to reduce vibration and impact. 

VI. Typical Cases and Solutions

Case: A pharmaceutical factory experienced cracks along the weld seam after polishing 316L welded pipes.

Investigation:

The excessive welding heat input (current 180A) resulted in coarse grains in the heat-affected zone;

The coarse polishing process used a low grit number (60) of grinding wheels without cooling and directly performed fine polishing.

Reformulation:

The welding current was reduced to 120A, and the interlayer temperature was controlled at 50℃;

The coarse polishing was replaced with a 100-grit grinding wheel, and an additional 240-grit transition step was added before fine polishing, while using cooling fluid.

Result: The crack occurrence rate decreased from 12% to 0.5%. 

Summary of Key Control Points Welding

Control parameters: Heat input ≤ 1.5 kJ/mm, Inter-layer temperature ≤ 60℃

Testing methods: Infrared thermometer, welding recorder 

Polishing

Control parameters: Sanding belt mesh size ≥ 320, pressure ≤ 10 N/cm²

Testing methods: Surface roughness tester, stress detector 

Final product inspection

Control parameters: 100% visual inspection + 5% magnetic particle/penetrant testing

Testing methods: Magnetic particle testing instrument, penetrant agent 

Through the entire process control, the occurrence rate of polishing cracks can be controlled below 0.1%, meeting the strict requirements of high-end industries (such as medicine, semiconductors).