The Complete Guide to Stainless Steel Welding Processes

Have you ever wondered why welding stainless steel is considered both an art and a science? This comprehensive guide will demystify the complexities of stainless steel welding, covering the entire process from selecting the right technique to ensuring a quality finish. You will learn about different welding methods, the challenges of working with stainless steel, and essential tips for success. Whether you are a beginner or an experienced welder, this article will provide valuable insights to help you master the intricacies of stainless steel welding.

Understanding Stainless Steel Welded Pipes

Stainless steel welded pipes are formed by rolling stainless steel sheets in a pipe-forming machine and welding them through specialized tooling. Stainless steel has high strength and is susceptible to work hardening due to its face-centered cubic crystal structure. This work hardening characteristic is utilized during the welded pipe forming process.

The forming process presents two main challenges:

• The forming dies must withstand high friction forces, making them prone to wear
• Stainless steel sheets tend to adhere (stick) to the die surface, causing distortion in both the welded pipe and the die surface

Therefore, high-quality stainless steel forming dies must possess excellent wear resistance and anti-adhesion properties. Analysis of imported welded pipe dies reveals that these tools typically feature surface treatments such as hard metal carbide or nitride coatings.

Advanced Welding Technologies

Compared to conventional fusion welding, laser welding and radio frequency welding offer distinct advantages:

• Faster welding speeds
• Higher energy densities
• Reduced heat input

These characteristics result in narrower heat-affected zones, minimal grain growth, reduced welding distortion, and better cold workability. These methods enable automatic welding of thick plates with single-pass penetration. A key feature of I-groove butt welding is that it requires no filler material.

Types of Welding Techniques

Welding techniques are primarily applied to metal substrates. Common welding methods include:

• Electric arc welding
• Argon arc welding (TIG)
• CO₂ shielded welding
• Oxy-acetylene welding
• Laser welding
• Electroslag pressure welding

These techniques can also be adapted for welding plastics and other non-metallic materials. There are over 40 methods of metal welding, which are mainly divided into three categories: fusion welding, pressure welding, and brazing.

Fusion Welding

Fusion welding involves heating the interface of workpieces to a molten state during the welding process, completing the weld without applying external pressure. In this process, a heat source rapidly heats and melts the interface between two materials, forming a molten pool. The molten pool advances with the heat source and, after cooling, forms a continuous weld that joins the two workpieces together.

During fusion welding, when the molten pool comes into direct contact with air, atmospheric oxygen oxidizes the metal, and various contaminants can dissolve into the molten pool. If atmospheric nitrogen or water vapor enters the molten pool, defects such as porosity, slag inclusions, and cracks can form during subsequent cooling, degrading both the quality and performance of the weld.

Pressure Welding

Pressure welding, also known as solid-state welding, involves bonding atoms between two workpieces in a solid state under applied pressure. A commonly used pressure welding process is resistance butt welding. When electric current passes through the connecting ends of two workpieces, the temperature rises due to electrical resistance. When heated to a plastic state, the pieces are joined together under axial pressure.

The common feature of various pressure welding processes is that pressure is applied without using filler metal. Most pressure welding methods, such as diffusion welding, high-frequency welding, and cold welding, do not involve a melting process. This eliminates problems such as the burning of beneficial alloying elements and the intrusion of harmful elements into the weld, simplifying the welding process and improving safety and health conditions.

Additionally, the heating temperature is lower than in fusion welding, and the heating time is shorter, resulting in a smaller heat-affected zone. Many materials that are difficult to fusion weld can often be pressure welded to create high-quality joints with strength equal to the base material.

Brazing

Brazing is a welding method that uses a metal material with a lower melting point than the workpiece as solder. The workpiece and solder are heated to a temperature above the solder’s melting point but below the workpiece’s melting point. The liquid solder wets the workpiece and fills the interfacial gap, achieving welding through atomic diffusion between the solder and workpiece.

Heat-Affected Zone and Weld Quality

The joint between two connecting bodies formed during welding is called a weld. Both sides of the weld are affected by welding heat, causing changes in microstructure and properties. This area is called the heat-affected zone (HAZ).

During welding, due to different workpiece materials and varying welding parameters, the weld and heat-affected zone may experience overheating, embrittlement, hardening, or softening after welding. These changes can reduce weld performance and deteriorate weldability.

To achieve high welding quality, it is necessary to adjust welding conditions appropriately. Weld strength can be improved through:

• Preheating the weld interface before welding
• Maintaining proper temperature during welding
• Post-weld heat treatment

Stainless Steel Pipe Welding Techniques

When adopting welding processes for stainless steel pipes, several key considerations must be addressed:

General Requirements:

• Even small impurities can cause interfacial corrosion
• Welding current should be 20% lower than for low carbon steel
• Use DC reverse polarity to ensure stable arc burning
• In short arc welding, stop the arc slowly and fill the crater
• Ensure the surface that contacts the medium is properly welded
• In multi-layer welding, control inter-pass temperature and consider forced cooling
• Ensure arcing begins within the groove and that the ground connection is secure
• Post-weld distortion can only be corrected through cold working

1. Argon Arc Welding (TIG)

Argon arc welding provides excellent protection for stainless steel, preventing the burning of alloying elements while maintaining high deposition efficiency. This results in well-formed welds with no slag coating and smooth surfaces. Consequently, welded joints exhibit high heat resistance and excellent mechanical properties.

Manual tungsten inert gas (TIG) welding is currently the most widely used argon arc welding method for thin stainless steel plates (0.5 to 3 mm thickness). The welding wire composition should generally match that of the base material. Industrial-grade pure argon is commonly used as the shielding gas.

Welding speed should be appropriately fast, and side weaving should be minimized. For stainless steel thicker than 3mm, MIG welding can be employed.

Advantages of MIG Welding:

• High productivity
• Small heat-affected zone
• Minimal weld distortion
• Excellent corrosion resistance
• Easy automation

2. Gas Welding

Gas welding offers convenience and flexibility, allowing welding in various spatial positions. For some stainless steel components, such as thin plate structures and thin-walled pipes, gas welding may be used even when corrosion resistance is not a primary requirement.

To prevent overheating, welding nozzles should be smaller than those used for welding low carbon steel of similar thickness. Use a neutral flame for gas welding, and select welding wire based on the base material’s composition and performance. For gas welding flux, Gas Flux 101 is recommended.

Technique Guidelines:

• Use left-hand welding method
• Maintain 40-50° angle between welding torch nozzle and workpiece
• Keep flame core at least 2mm from the molten pool
• Ensure welding wire end contacts the molten pool
• Move torch along the weld with the flame
• Avoid sideways torch movement
• Maintain fast welding speed with minimal interruptions

3. Submerged Arc Welding

Submerged arc welding is suitable for medium to thick stainless steel plates (6 to 50 mm thickness). This method offers high productivity and good weld quality but is prone to segregation of alloying elements and impurities.

4. Manual Arc Welding

Manual arc welding is a common and versatile welding method. The arc length is manually controlled by adjusting the gap between the welding electrode and workpiece. The electrode serves both as an arc carrier and as welding filler metal.

This simple welding method can be used for most materials and offers high adaptability for outdoor use, even underwater applications. Most electric welding can be performed using TIG welding principles.

Key Characteristics:

• Arc length depends on manual control
• Typically uses direct current
• Electrodes can be titanium-based or hermetically sealed
• Titanium electrodes provide easy welding with flat, attractive welds
• Welding slag is easily removable
• Extended storage requires electrode re-firing due to moisture absorption

5. MIG/MAG Welding

This automatic gas-shielded arc welding process creates an arc between a current-carrying wire and the workpiece under protective gas. The machine-fed metal wire serves as the welding rod and is melted by its own arc.

Due to its versatility and specificity, MIG/MAG welding remains the most widely used welding process worldwide. It can be used for ferrous and non-ferrous metals, including alloy steels, low-alloy steels, and high-alloy materials, making it ideal for both manufacturing and repair applications.

For steel welding, MAG can accommodate thin steel plates as thin as 0.6 mm. The shielding gas used is typically an active gas such as carbon dioxide or mixed gas. The main limitation is that outdoor welding requires workpiece protection from moisture to maintain gas effectiveness.

6. TIG Welding

TIG welding generates an arc between a refractory tungsten electrode and the workpiece. Pure argon serves as the shielding gas, and the welding wire carries no current. The wire can be fed manually or mechanically, and some specialized applications require no wire feed.

The choice between direct current and alternating current depends on the material being welded. When using direct current, the tungsten electrode is set as the negative pole. This provides deep penetration suitable for various steel grades, though it lacks a “cleaning effect” on the weld pool.

Stainless Steel Welding Inspection Methods

Welding inspection encompasses quality control of materials, tools, equipment, processes, and finished products throughout the entire manufacturing process from design to production. The inspection process is divided into three stages: pre-welding inspection, inspection during welding, and post-welding finished product inspection.

Inspection methods are classified as either destructive or non-destructive, depending on whether the product is damaged during testing.

1. Pre-Welding Inspection

Pre-welding inspection includes verification of:

• Raw materials (base metal, welding rods, flux, etc.)
• Welded structure design

2. Inspection During Welding

This includes monitoring of:

• Welding process specifications
• Weld dimensions
• Fixture conditions
• Quality of structural assemblies

3. Post-Welding Finished Product Inspection

Multiple methods are available for post-welding inspection:

Visual Inspection

Visual inspection of welded joints is a simple and widely used method, forming an important part of finished product inspection. It primarily identifies surface defects and dimensional deviations in welds. Inspection typically uses standard templates, gauges, magnifying glasses, and other tools. Surface defects often indicate potential internal weld defects.

Leak Tightness Testing

For vessels storing liquids or gases, leak tightness testing can detect non-tight weld defects such as through-cracks, porosity, slag inclusions, incomplete penetration, and loose structure. Common methods include kerosene testing, water flow testing, and water impact testing.

Pressure Vessel Strength Testing

Pressure vessels must undergo strength testing in addition to leak tightness testing. Two main types are used:

• Hydrostatic testing
• Pneumatic testing

These tests evaluate weld tightness in vessels and pipes operating under pressure. Pneumatic tests are more sensitive and faster than hydraulic tests, and tested products don’t require wastewater treatment, making them suitable for hard-to-drain products. However, pneumatic testing is more dangerous and requires appropriate safety measures.

Physical Inspection Methods

Physical inspection methods use physical phenomena for measurement or inspection. Non-destructive testing methods are commonly employed to inspect materials or workpieces for internal defects. Current methods include:

• Ultrasonic testing
• X-ray testing
• Penetrant testing
• Magnetic testing

X-Ray Inspection

X-ray inspection detects defects by utilizing material penetration properties and light attenuation within materials. Methods can be divided by light type (X-ray, gamma-ray, high-energy ray) or by defect display method (ionization, fluorescent screen observation, photography, industrial television).

X-ray inspection primarily detects weld defects such as cracks, incomplete penetration, porosity, and slag inclusions.

Ultrasonic Testing

Ultrasonic waves reflect at interfaces between different media when propagating through metals or other homogeneous materials, making them useful for detecting internal defects. Ultrasonic testing can inspect any weld material for defects with high sensitivity, though determining defect nature, shape, and size can be challenging. For this reason, ultrasonic testing is often used in conjunction with X-ray inspection.

Magnetic Testing

Magnetic testing uses magnetic leakage generated by magnetizing ferromagnetic metal parts to detect defects. Methods include magnetic particle testing, magnetic induction testing, and magnetic recording testing, with magnetic particle testing being most widely used.

Magnetic testing can only detect surface and near-surface defects in magnetic metals and provides only quantitative defect analysis. Defect nature and depth must be estimated through experience.

Penetrant Testing

Penetrant testing (including dye testing and fluorescent testing) uses liquid penetration properties to detect and display defects. This method can inspect both ferromagnetic and non-ferromagnetic material surfaces for defects.

Critical Points and Precautions for Stainless Steel Welding

Power Supply and Polarity

1. Use power supply with vertical external characteristics
2. Employ direct current with positive polarity (welding wire connected to negative pole)

Application and Characteristics

1. Generally suitable for welding plates 6mm or thinner
2. Produces attractive welds with minimal welding distortion

Shielding Gas Requirements

1. Use argon with 99.99% purity
2. For welding current 50-150A: argon flow rate 8-10 L/min
3. For welding current 150-250A: argon flow rate 12-15 L/min

Electrode and Positioning

1. Tungsten electrode should extend 4-5mm from gas nozzle
2. In areas with poor shielding, extend to 5-6mm
3. Maintain nozzle-to-workpiece distance under 15mm

Surface Preparation

1. Clean all rust, oil stains, and contaminants from welded parts to prevent porosity

Arc Length Control

1. For ordinary steel: maintain 2-4mm arc length
2. For stainless steel: maintain 1-3mm arc length
3. Excessive length reduces protection effectiveness

Back Side Protection

1. Ensure no back-side oxidation during butt welding
2. Provide gas protection on the back side to prevent weld bead oxidation

Welding Angles

1. Maintain 80-85° angle between tungsten electrode centerline and workpiece
2. Keep filler wire angle with workpiece surface as small as possible (approximately 10°)

Environmental Considerations

1. Provide wind protection in windy areas
2. Ensure adequate ventilation in indoor applications

Requirements for Stainless Steel Welding

1. Determining the Welding Process

There are numerous stainless steel grades, classified by alloy composition into:

• Chromium stainless steels
• Chromium-nickel stainless steels

By metallurgical structure, they are divided into:

• Austenitic type
• Ferritic type
• Martensitic type

The most commonly used in construction are austenitic stainless steels such as 304 (0Cr19Ni9) and 321 (1Cr18Ni9Ti). Austenitic stainless steels are relatively easy to weld, and welded joints maintain high toughness even in the as-welded condition.

However, austenitic stainless steel has thermal conductivity about one-third that of ordinary carbon steel, while its thermal expansion coefficient is 1.5 times that of carbon steel. This combination of low thermal conductivity and high expansion coefficient results in significant deformation and distortion during welding.

Therefore, weld quality depends primarily on whether the welding process suits the base material. When selecting a welding process, consider the following:

Welding Method Selection

Common stainless steel welding methods include:

• Manual arc welding
• Gas shielded welding
• Automatic submerged arc welding

Selection depends on designed medium parameters, construction conditions, service environment, and construction costs. In process pipeline construction, varying pipe diameters and numerous valves and fittings complicate weld joint locations, making manual arc welding generally preferred.

Argon arc welding is typically used for priming pipelines that transport flammable, explosive, or clean-service media, followed by manual arc welding for cover passes to improve internal weld quality.

Welding Material Selection

Stainless steel electrodes are classified as:

• Chromium stainless steel electrodes (prefix “G”)
• Chromium-nickel stainless steel electrodes (prefix “A”)

Chromium stainless steel electrodes are mainly used for welding martensitic stainless steels. Electrode selection considers:

• Base material chemical composition
• Pipeline medium temperature and pressure
• Welding current type (AC or DC)
• Welding method
• Ambient welding temperature

Multiple electrode brands may meet welding requirements through selection and verification. In such cases, choose based on cost-effectiveness.

Weld Groove Design

Weld groove shape depends on the weld’s stress state and should be specified in construction drawings with corresponding specifications or standards. However, standard groove sizes are typically determined solely by base metal thickness and welding method, not accounting for differences in base and welding materials.

Different materials have varying groove size requirements due to different chemical compositions, physical properties, and penetration characteristics. Therefore, adjustments to butt clearance, root face, and groove angles must be made for specific materials during installation.

Oversized grooves increase construction costs and welding stress, potentially causing deformation and cracking. Undersized grooves can lead to incomplete penetration and slag entrapment.

In manual arc welding, stainless steel electrodes have less penetration than carbon steel electrodes, requiring appropriately increased groove angles and butt clearance. This can be controlled using positive deviation values from specifications or determined through test welding.

Welding Current Selection

Austenitic stainless steel resistivity can be nearly five times that of carbon steel, making electrodes prone to overheating and glowing red during welding. High currents cause electrode overheating and coating ingredient burnout, making welds susceptible to defects due to inadequate protection.

Excessive welding current also prevents achieving the desired weld metal composition. Generally, smaller welding currents are preferable.

2. Thorough Pre-Welding Preparation

Comprehensive, targeted preparation is critical for ensuring weld quality. This preparation covers three main aspects:

Welder Qualification Verification

Welders must possess appropriate welding certificates and work strictly within their certified parameters. Welders should have at least two years of experience welding stainless steel or chrome-molybdenum steel.

Welding Material Management

Follow instruction manual requirements for welding material preparation. Electrodes should be baked according to specifications (typically 150-200°C for 1 hour if not specified). Use temperature-controlled ovens for baking, and bake only what will be used immediately.

Store dried electrodes in insulated containers. If exposed for more than two hours, re-bake up to three times maximum.

Groove Preparation

Stainless steel pipe grooves can be machined or plasma cut before welding. Remove oxide films and burrs from grooves. Apply chalk slurry within 50mm of both weld sides to facilitate spatter removal after welding.

Due to “carburization” from stainless steel-carbon steel contact, use dedicated grinding wheels and stainless steel brushes when cleaning stainless steel, weld beads, and spatter.

3. Preventing Deformation and Cracking

Deformation Prevention

Austenitic stainless steel’s high expansion coefficient and low thermal conductivity cause significant welding deformation. Use appropriate deformation prevention fixtures during assembly for different weld positions.

Tack weld and fixed weld locations should be smaller than those for general carbon steel. The welding operator must rationally determine welding sequence. For larger diameter pipes, two people can weld symmetrically in the same direction simultaneously.

For base material thicker than 8mm, use multi-pass welding with low heat input. Use “reverse polarity” connection (workpiece connected to negative electrode) to reduce weld temperature.

Crack Prevention

Keep baked electrodes in heated containers. Maintain welding ambient temperature above 0°C without significant fluctuations. If temperature drops below 0°C, preheat the weld area to 80-100°C.

For out-of-position welding, use the backstep method to start the arc, never starting on the base metal. Transport the electrode in a straight line without weaving.

When horizontal oscillation is necessary, minimize vertical weld oscillation width. Excessive horizontal oscillation promotes thermal cracking and protection failure.

Maintain the shortest possible arc length. Long arcs cause alloy component burning and reduce ferrite content due to atmospheric nitrogen ingress, leading to hot cracking.

Fill all arc craters at arc termination. This is particularly important in root pass welding, where concave craters are difficult to avoid and prone to thermal cracking.