What is the welding process for 347H stainless steel pipes

The welding process for 347H stainless steel tubes needs to take into account both its austenitic structure characteristics and the stabilizing effect of niobium elements, in order to prevent intergranular corrosion, hot cracks and other issues, while ensuring high-temperature performance. The following are the detailed welding process points based on the material properties and engineering practice: 

I. Selection of Welding Materials

1. Weld Wire (Filler Metal)

Standard Weld Wire

Use ER347 with niobium (e.g., AWS A5.9 ER347, composition: C ≤ 0.08%, Cr18-21%, Ni9-11%, Nb ≥ 8×C%), to ensure the formation of stable NbC in the weld seam and prevent intergranular corrosion caused by the precipitation of carbides.

Welding Wire without Argon Filler

Such as TGF-347 (flux-cored wire), through the deoxidizers in the flux (such as Al, Ti) to form a back protective slag during welding, no additional argon gas is required, suitable for full-position welding of on-site pipelines (such as 6G weld joints), increasing efficiency by more than 30%.

Weld Rod

For manual welding, E347 (e.g., AWS A5.4 E347-16) can be selected, used for maintenance or thick-walled pipe welding, drying temperature 150-200℃, holding for 1 hour.

2. Weld Materials to Be Avoided

Prohibit the use of ER321 weld wire containing titanium, as titanium is prone to burnout (oxidation temperature approximately 600℃) at high welding temperatures, resulting in insufficient titanium content in the weld seam and inability to effectively fix carbon, increasing the risk of intergranular corrosion. 

II. Welding Methods and Applicable Scenarios

Picture (Figure 1) 

III. Pre-weld Preparation

Groove Processing 

Using mechanical processing (lathe, beveling machine) or plasma cutting (requiring grinding to remove the oxide layer), the recommended bevel shapes are V-shaped (angle 60±5°) or U-shaped (root radius 3-5mm), with a bevel edge of 1-2mm and a gap of 2-3mm. Avoid edge oxidation caused by manual gas cutting.

Surface cleaning 

Remove oil, water and paint from the bevel and the 20mm area on both sides using acetone or alcohol. Use a stainless steel wire brush to remove oxide scale or rust. Do not allow carbon steel tools to come into contact with the base material.

Back protection (non-argon-filled process) 

Before welding, argon gas is filled into the pipeline (flow rate 15-20L/min) until the oxygen content is less than 50ppm (monitored by an oxygen content detector) to prevent backside oxidation and the formation of a chromium-poor layer. 

IV. Welding Process Control

Heat Input Control 

Strictly control the welding current and speed, with the heat input being ≤ 1.5 kJ/mm (for thin-walled tubes, ≤ 1.0 kJ/mm). Adopt the technique of low current, rapid welding, and narrow weld beads to reduce the duration of high temperature and avoid grain coarsening and NbC precipitation. Interlayer temperature

During multi-layer welding, the inter-layer temperature should be controlled at 100 - 150℃. This can be monitored in real time using an infrared thermometer to prevent the weld metal from undergoing excessive aging due to excessively high inter-layer temperatures.

Key operation points 

During TIG welding, keep the tungsten electrode 2-3mm away from the groove, and use a straight line or small oscillation to avoid arc drift that may cause the protection to fail; for MIG welding, maintain the length of the welding wire at 15-20mm to prevent the wire from overheating and oxidizing.

For all-position welding (such as pipe access openings), prefer to use "upward vertical welding" or "downward horizontal welding"， control the shape of the molten pool， and reduce the flow of liquid metal downward. 

V. Post-welding Treatment

Weld Seam Cleaning 

Remove surface welding slag and spatter (using stainless steel chisels or grinding wheels). Do not use carbon steel tools to avoid iron ion contamination.

Heat treatment (as required) 

Solution treatment

For thick-walled pipes (with a wall thickness greater than 15mm) or critical components, after welding, a solution annealing at 1020-1100℃ is required. Water cooling for rapid cooling is necessary to ensure that NbC is fully dissolved and to restore the resistance to intergranular corrosion.

Stress relief

If the workpiece needs to withstand alternating loads, a stress-relieving annealing at 650-700℃ (with a holding time of 1-2 hours for a 25mm thickness) can be performed. The furnace is then cooled to below 300℃ and air-cooled to reduce the risk of stress corrosion. Surface treatment

For components with high corrosion resistance requirements, after welding, perform acid washing and passivation (such as soaking in a 20% HNO₃ + 5% HF solution for 30 minutes), or mechanical polishing (with a roughness Ra ≤ 1.6 μm) to remove the surface oxide film. 

VI. Quality Inspection and Defect Prevention

Non-destructive Testing 

Radiographic Testing (RT)

Performed according to ASTM E155 or GB/T 3323, Grade I is qualified. The focus is on detecting incomplete fusion and pores (single pore diameter ≤ 1.5mm, dense pores ≤ 3 per 100mm).

Penetrant Testing (PT)

Used for surface crack detection, using solvent-based fluorescent penetrant, with sensitivity level B.

Common Defects and Countermeasures 

Thermal cracks:

The cause is the precipitation of NbC during the solidification of the molten pool, which leads to the brittleness of the grain boundaries. The countermeasure is to control the sulfur and phosphorus content (base metal S ≤ 0.030%, P ≤ 0.035%), and reduce the welding stress (avoid forced assembly).

Intergranular corrosion:

It is mostly caused by improper selection of welding materials or excessive heat input. It is necessary to ensure that the Nb/C content of the welding wire is ≥ 8, and perform post-welding solution treatment promptly. Pore

The main reason is that the protective gas is not pure or the groove is not cleaned thoroughly. Therefore, high-purity argon gas (with a dew point of ≤ -40℃) must be used, and the oil should be removed completely before welding. 

VII. Engineering Application Cases

Welding of Boiler Pipes in Power Station

In a 600℃ supercritical unit, the Φ108×12mm 347H pipe was welded using TIG as the base layer (ER347, φ2.4mm, current 120A) and manual welding for the filler and cover layers (E347, φ3.2mm, current 100A). 100% RT inspection was conducted after welding, and the one-time qualification rate was over 98%.

On-site Welding of Thermal Energy Molten Salt Storage Tanks

Full-position welding was carried out using TGF-347 filler wire (φ1.2mm). No argon was required for the backside, and the welding speed was increased to 0.8m/min. Combined with magnetic particle inspection (MT), the weld seam surface was ensured to have no cracks. This was applied to a 50MW thermal energy project in Qinghai, with a cumulative welding length exceeding 10km. 

Summary

The core of welding 347H stainless steel pipes lies in balancing high-temperature strength and corrosion resistance through the use of niobium-containing welding materials, strict control of heat input, and targeted post-welding treatments. In actual operation, the welding method needs to be selected based on the thickness of the workpiece and the service environment. Efficient processes such as those without argon filling should be prioritized. At the same time, attention should be paid to the cleaning of the bevel and the purity of the shielding gas to ensure that the weld properties match the base material, meeting the long-term service requirements of harsh conditions such as power and chemical industries.