Technical Guide for the Whole Process of Welding 15CrMoG Thick-walled High-pressure Steel Pipe

First, in-depth analysis of welding pain points: the "difficult" characteristics of welding 15CrMoG thick-walled high-pressure steel pipes
15CrMoG is a chromium-molybdenum heat-resistant steel. When the wall thickness is ≥16mm, there are three major technical difficulties in welding:
- Risk of thermal cracking: Cr and Mo elements are prone to form carbide segregation at high temperatures, and the shrinkage stress concentration of the weld during cooling is prone to cause microcracks;
- Hardening tendency: The heat-affected zone (HAZ) produces martensitic structure during rapid cooling, and the hardness can reach more than 350HV, resulting in increased joint brittleness;
- Porosity sensitivity: The pipe contains Mo elements, and the molten pool has poor fluidity. If the welding material is damp or the groove is not cleaned properly, CO pores are easily generated.

Second, the three-step method before welding 15CrMoG thick-walled high-pressure steel pipes
▶ 1. Groove and surface treatment
- Groove processing: mechanical cutting (such as lathe turning) or plasma cutting, V-shaped groove (angle 60°±5°), blunt edge 1-2mm, gap 2-3mm;
- Surface cleaning: Wipe the 20mm range on both sides of the groove with acetone to remove oil and oxide scale to reveal the metallic luster (it is strictly forbidden to use a wire brush to clean to avoid iron filings).
▶ 2. Selection of welding materials and equipment
The welding material model of the manual arc welding method: is R307 (E5515-B2), and the applicable wall thickness is ≤30mm. The advantage is good wind resistance and suitable for on-site operations.
The welding material model of argon arc welding + arc welding method: TIG-R30 (φ2.5mm) + R307, and the applicable wall thickness is >30mm. The advantage is excellent root forming and reduced interlayer defects.
▶ 3. Preheating process control
Preheating temperature: 200-250℃ for wall thickness ≤25mm, 250-300℃ for wall thickness >25mm (monitored by infrared thermometer, the temperature measuring point is 100mm away from the edge of the groove);
Preheating range: uniform heating of the 150mm area on both sides of the groove to avoid local overheating.

Third, the welding process and key operations of 15CrMoG thick-walled high-pressure steel pipes
▶ 1. Precise control of welding parameters of 15CrMoG thick-walled high-pressure steel pipes
- Manual arc welding: current 110-140A (φ3.2mm welding rod), voltage 22-24V, welding speed 150-200mm/min;
- Argon arc welding: current 80-100A, voltage 10-12V, argon gas flow 8-12L/min (flow 5-8L/min when back-filled with argon protection).
▶ 2. Interlayer temperature and weld formation
- Interlayer temperature: maintain 200-250℃ (preheating is required if it is lower than 150℃), and monitor in real-time with a temperature measuring pen;
- Weld bead control: single-bead weld thickness ≤4mm, width ≤12mm, use a wire brush to clean the slag after each layer of welding (grinding with a grinding wheel is strictly prohibited to avoid carbon increase).
▶ 3. Special process points
- Arc extinguishing treatment: fill the arc pit when the arc is extinguished to avoid shrinkage;
- Back root cleaning: use carbon arc air planing to clean the root during multi-layer welding, and use a grinding wheel to grind to metallic luster to prevent interlayer infusion.

Fourth, post-weld heat treatment of 15CrMoG thick-walled high-pressure steel pipe: stress elimination
1. Heat treatment process:
Heating rate: ≤200℃/h (after 300℃)
Insulation temperature: 670-700℃, insulation time = wall thickness (mm) × 2min (at least 30min)
Cooling rate: ≤150℃/h (air cooling after 300℃)
2. Key purpose: reduce HAZ hardness (≤250HV), reduce residual stress by more than 60%, and improve joint impact toughness (AKV≥34J at -20℃).

Fifth, quality acceptance and safety warning of 15CrMoG thick-walled high-pressure steel pipes
Non-destructive testing: 100% radiographic testing (RT) or ultrasonic testing (UT), II level qualified;
Safety tips: set up a warning area during heat treatment to prevent personnel from contacting high-temperature pipes; wear a dust mask during welding (Mo oxide dust can easily cause respiratory discomfort).

Core tips: preheat in place to prevent cracks, accurately control the forming parameters, and heat insulation after heating to improve performance - welding of 15CrMoG thick-walled high-pressure steel pipes, details determine success or failure!