What is the Welding Process of DIN2391/94c Precision Hydraulic Seamless Steel Pipes

First, what are the material properties of DIN2391/94c precision hydraulic seamless steel pipes?
1) Standards and Grades of Precision Hydraulic Seamless Steel Pipes
DIN2391: German seamless steel pipe standard, covering various steel grades (such as St37, St45, St52, etc.).
94c: May refer to a specific steel grade (such as St37-2, St45-2) or delivery condition (such as cold-drawn, normalized).
Typical Composition: Low carbon steel or low alloy steel (such as C≤0.2%, Mn≤1.5%), low carbon equivalent (CEV≤0.4%), good weldability.
2) Key Properties
Yield Strength: St37 (235MPa), St45 (355MPa).
Elongation: ≥21% (St37), ≥18% (St45). Hydraulic system requirements: The weld must withstand high pressure (typically >10MPa), with no leakage or deformation.

Secondly, what are the key points of the welding process for DIN2391/94c precision hydraulic seamless steel pipes?
1. Welding method selection
A) TIG welding (tungsten inert gas welding) is the preferred choice.
Advantages: Low heat input, controllable molten pool, high weld quality, suitable for thin-walled precision pipes (wall thickness ≤6mm).
Welding wire selection: Match the strength of the base material (e.g., AWS ER70S-6 or EN ISO 16834-G4Si1).
B) MIG/MAG welding is the secondary choice.
Suitable for thick-walled pipes (wall thickness >6mm), using an Ar+20% CO₂ mixed gas to improve deposition efficiency.
Avoid manual arc welding (SMAW): High heat input, prone to deformation and porosity.
2. Pre-welding Preparation
A) Beveling Design
Thin-walled pipes (≤3mm): Type I beveling, gap 0~1mm.
Thick-walled pipes (>3mm): Type V beveling (angle 60°~70°), blunt edge 1~2mm, gap 2~3mm.
B) Surface Treatment
Remove oil, rust, and scale from the beveling and the area within 20mm on both sides. Clean with acetone or ethanol.
C) Preheating Requirements
Generally, preheating is not required (low carbon equivalent). However, if the ambient temperature is <5℃ or the wall thickness is >12mm, preheating to 50~100℃ is acceptable. 
3. Welding Parameters
Parameters: TIG Welding (Example) MIG/MAG Welding (Example)
Current (A): 80~150 (adjust according to wall thickness) 120~250
Voltage (V): 12~20 18~28
Welding Speed (mm/min): 80~150 150~300
Shielding Gas: Pure Ar (flow rate 8~12L/min) Ar+20% CO₂ (flow rate 15~20L/min)
Wire Diameter: (mm) 1.6~2.4 1.2~1.6
4. Key Operating Points
1) Tack Welding: Tack weld every 100~150mm, weld length 10~15mm, ensuring no defects.
2) Multi-layer Welding: Thick-walled pipes require layer welding, each layer thickness ≤3mm, interpass temperature ≤150℃. 3) Backside Protection: Purge the back of the weld with argon or a mixed gas (Ar + 5% H₂) to prevent oxidation.

Third, what are the post-weld treatment and quality control procedures for DIN2391/94c precision hydraulic seamless steel pipes?
1) Weld Cleaning: Remove weld slag and spatter, grind the weld to be flush with the base material to avoid stress concentration.
2) Heat Treatment: Generally, heat treatment is not required, but if there is a risk of cold cracking (e.g., in high-restraint structures), stress-relieving annealing at 600~650℃ can be performed.
3) Non-destructive Testing: Ultrasonic Testing (UT): Detects internal defects (e.g., lack of fusion, porosity).
4) Penetrant Testing (PT): Inspects surface opening defects (e.g., cracks, incomplete penetration).
5) Hydraulic Test: Hold at 1.5 times the design pressure for 10~15 minutes; no leakage indicates acceptance.
6) Dimensional and Appearance Inspection: Weld reinforcement ≤ 1.5mm, misalignment ≤ 10% of wall thickness, and ≤ 1mm.
No defects such as undercut, dents, or weld beads.

Fourth, Precautions for DIN2391/94c Precision Hydraulic Seamless Steel Pipes?
1) Deformation Control: Use symmetrical welding and segmented back-welding methods; use clamps for fixation when necessary.
2) Environmental Requirements: Welding environment temperature ≥ 5℃, humidity ≤ 80%, wind speed < 2m/s (gas shielded welding).
3) Welder Qualification: Welders must be certified (e.g., ISO 9606 or ASME IX certification).

Fifth, Recommended Standards and Specifications for DIN2391/94c Precision Hydraulic Seamless Steel Pipes?
Welding Procedure Qualification: DIN EN ISO 15614-1 (Metallic Materials Welding Procedure Qualification).
Weld Quality Classification: DIN EN ISO 5817 (Grade B or C, depending on hydraulic system requirements). Hydraulic testing: DIN EN 10297-1 (steel pipes for pressure piping).

Sixth, what are the common problems and solutions for DIN2391/94c precision hydraulic seamless steel pipes?
1) Porosity in precision hydraulic seamless steel pipes is caused by insufficient gas protection or unclean base material. How to solve it? Check the gas flow rate and thoroughly clean the bevel.
2) Cracks in precision hydraulic seamless steel pipes are caused by excessively rapid cooling or stress concentration. How to solve it? Reduce the cooling rate and provide post-weld insulation.
3) Deformation in precision hydraulic seamless steel pipes is caused by excessive heat input. How to solve it? Optimize the welding sequence and use fixtures.
For specific process parameters or special requirements (such as welding dissimilar steels), it is recommended to contact a professional welding engineer for process qualification.