How to prevent cracks in pressure vessels?

I. Material Selection and Management – ​​Controlling Crack Risk at the Source

1. Selecting Materials with Excellent Crack Resistance

Prioritize low-carbon equivalent steels (such as SA516GR70), which have good weldability and low cold cracking tendency.

For environments containing corrosive media such as sulfur and chlorine, avoid using materials sensitive to stress corrosion, such as austenitic stainless steel.

Under low-temperature conditions, select materials with good low-temperature toughness to ensure that the impact energy meets the design temperature (e.g., ≥27J at -46℃).

2. Strictly Controlling the Content of Harmful Elements

Limit the sulfur and phosphorus content in the base metal and welding materials (generally ≤0.03%~0.04%) to prevent hot cracking and brittle fracture.

Control the carbon content (generally <0.12% in welding wire) to reduce the tendency for crystallization cracking.

3. Drying and Cleaning Welding Materials

Use low-hydrogen welding electrodes and strictly dry them according to regulations to prevent moisture decomposition and the introduction of hydrogen, reducing the risk of hydrogen-induced cracking. Clean the bevel and both sides to remove oil, rust, moisture, and other impurities, reducing hydrogen sources and the risk of slag inclusions.

II. Optimize Welding Process – Controlling Thermal Stress and Hydrogen Diffusion

1. Preheating and Interpass Temperature Control

For thick-walled or high-strength steel containers, preheat (typically 150–300℃) to reduce cooling rate, slow hydrogen diffusion, and prevent cold cracking.

Maintain the interpass temperature above the preheating temperature to avoid reheating cracks caused by repeated heating of the weld.

2. Rational Selection of Welding Parameters and Sequence

Control welding current, voltage, and speed to avoid excessive heat input leading to grain coarsening or insufficient fusion.

Use symmetrical welding and segmented back-welding techniques to disperse restraint stress and reduce welding deformation and residual stress.

Avoid "mushroom-shaped" welds, improve weld bead formation coefficient, and reduce the tendency for crystallization cracking.

3. Post-weld heat treatment and post-weld hydrogen removal

Perform post-weld heat treatment (e.g., holding at 200–300℃ for several hours) to accelerate hydrogen escape and prevent delayed cracking.

For vessels prone to stress corrosion or made of high-strength steel, perform post-weld stress-relieving heat treatment (PWHT) to reduce residual stress.

III. Structural Design and Stress Management – ​​Reducing Stress Concentration

1. Optimizing Structural Design

Avoid sharp corners and abrupt cross-sections; adopt smooth transition designs to reduce localized stress concentration.

Improve joint types, such as changing protruding nozzles to flush nozzles, to reduce rigid constraints and prevent reheat cracking.

2. Controlling Manufacturing Residual Stress

Eliminate residual stress generated during machining and welding through heat treatment, shot peening, etc.

Avoid excessive cold working to prevent work hardening and microcrack initiation.

3. Use Steel Resistant to Lamellar Tear

For large, thick-walled vessels, select ultra-low sulfur steel (S≤0.005%) or steel with added modifiers to refine grain size, improving resistance to lamellar tearing.

IV. Prevention and Control During Operation and Maintenance – Preventing Crack Propagation During Service

1. Controlling Fluctuations in Operating Conditions

Avoid frequent start-ups and shutdowns, and drastic changes in pressure and temperature to reduce the risk of fatigue cracking.

For vessels under alternating loads, conduct fatigue design and select materials with good plasticity.

2. Preventing Stress Corrosion Cracking (SCC)

Select appropriate materials, avoiding material incompatibility with sensitive media (e.g., avoid using austenitic stainless steel in seawater environments).

Improve the corrosive environment through cathodic protection, coating isolation, or the addition of corrosion inhibitors.

Control the concentration and temperature of alkali solutions; post-weld heat treatment is mandatory when critical values ​​are exceeded.

3. Periodic Inspection and Early Detection

Conduct external inspections, non-destructive testing, and wall thickness measurements according to the "Rules for Periodic Inspection of Pressure Vessels". Focus on inspecting high-risk areas such as welds, nozzles, and transition zones of end caps to promptly detect and address microcracks.