Stainless steel is a metal widely adopted across industries for its durability, corrosion resistance, and visual appeal. It has properties similar to those of carbon steel, but includes a minimum of 10.5% chromium to enhance corrosion resistance. This article will discuss welding characteristics and methods of different types of stainless steel.
Austenitic Stainless Steel Welding
Austenitic stainless steels have a face-centered cubic (FCC) crystal structure. Austenite is non-magnetic, exhibits moderate yield strength, a high work-hardening rate, and excellent low-temperature toughness. Austenitic stainless steels possess good weldability, allowing them to be fabricated into various complex shapes.
Challenges in Austenitic Stainless Steel Welding
Austenitic stainless steel is the most widely used type of stainless steel. This is attributed to its desirable mechanical and corrosion-resistance properties, and its weldability is considered the best among high-alloy steels. While austenitic steel offers good weldability with no quench hardening or significant grain coarsening in the Heat-Affected Zone (HAZ), the following issues can occur during welding.
Formation of Chromium Carbides
Welded austenitic stainless steel joints can exhibit three types of intergranular corrosion: weld metal intergranular corrosion, sensitization-zone corrosion in the base metal, and knife-line attack. Intergranular corrosion in austenitic steel is generally explained by the chromium depletion theory. In a solution-annealed state, carbon is supersaturated in the austenite. During heating, the supersaturated carbon precipitates along the grain boundaries in the form of Cr23C6. Since the chromium content in Cr23C6 is significantly higher than in the austenitic matrix, the chromium concentration near the grain boundaries drops sharply. Chromium atoms from the grain interior cannot diffuse quickly enough to replenish this loss, resulting in the formation of a chromium-depleted layer (Cr < 11.7%). This depleted layer has a much lower electrode potential than the grain interior, causing the grain boundary to become the anode and dissolve when exposed to a corrosive medium.
Stress Corrosion Cracking (SCC)
Stress corrosion cracking is a delayed fracture phenomenon resulting from the synergistic effect of a specific corrosive environment and tensile stress. Numerous factors influence SCC in stainless steel, including steel composition, microstructure and state, corrosive medium type, temperature, stress magnitude/nature, and structural features. Common austenitic stainless steels susceptible to SCC include 18-8 stainless steel and extra-low carbon stainless steel. Depending on the medium, cracking can be intergranular, transgranular, or a mixed mode.
Hot Cracking
Hot cracking may occur in both the weld metal and the HAZ of austenitic stainless steel. The most common is weld metal solidification cracking, though liquation cracking can occasionally appear in the HAZ or the interpass layers of multi-pass welds.
Austenitic steel has a high susceptibility to hot cracking, primarily determined by the steel’s chemical composition and its microstructural and property characteristics. Austenitic steels contain many alloying elements, especially a certain amount of nickel. Nickel not only improves austenite stability but also easily forms low-melting-point compounds or eutectics with impurities like sulfur and phosphorus, promoting hot cracking. From a physical properties perspective, austenitic stainless steel is characterized by low thermal conductivity and a high coefficient of linear expansion. Under the non-uniform heating conditions of welding, this easily generates large tensile stresses, which contribute to the formation of welding hot cracks.
Embrittlement of Welded Joints
Austenitic steel is widely used under various conditions, including high-temperature, corrosive, and cryogenic environments. Different service conditions impose different performance requirements on the welded joints. Corrosion-resistant steels typically operate at room temperature or below 350°C (662°F) without special mechanical property demands. For high-temperature conditions, short-term applications require the welded joint to have the same strength as the base metal. For long-term applications (e.g., over 10 years), maintaining the joint’s ductility and preventing high-temperature embrittlement is critical. For cryogenic applications, the primary requirement is excellent low-temperature toughness to prevent brittle fracture of the welded joint. Considering these diverse requirements, low-temperature embrittlement and high-temperature embrittlement are significant concerns for austenitic stainless steel welded joints.
Welding Distortion
Due to the poor thermal conductivity and large expansion coefficient of austenitic stainless steel, welding distortion is generally significant, particularly in thin-gauge material. Therefore, appropriate measures should be taken during welding to prevent distortion, such as using suitable fixtures and reducing tack weld spacing.
Welding Methods
Austenitic stainless steel can be welded using Gas Tungsten Arc Welding (TIG), Gas Metal Arc Welding (MIG), Plasma Arc Welding (PAW), and Submerged Arc Welding (SAW).
TIG is used for thin sheet and pipe welding, as well as for root passes in pipe welds. It has a slower travel speed but provides high-quality welds, is free of slag and spatter, and is not restricted by welding position.
MIG allows for rapid deposition of a large amount of weld metal. Compared to TIG, it offers faster welding speeds and lower costs. MIG has 3 main modes of arc transfer:
- Short Circuit Transfer: Requires separate slope and secondary inductance control. Suitable for welding materials up to about 3 mm (approx. 1/8 inch) thick. This mode has a low heat input, making it ideal for thinner materials prone to distortion.
- Pulsed Arc Transfer: Requires a specialized power source to generate pulsed welding current and voltage. It has moderate heat input and is suitable for out-of-position welding.
- Spray Arc Transfer: This mode features a stable arc but high heat input and is generally limited to the flat position. It can weld thin-gauge and plate stainless steel thicker than 3 mm.
SAW is suitable for thick-walled vessels and pipe welding or for horizontal-vertical fillet welds. It produces welds with a high-quality surface finish, often requiring minimal post-weld processing. During SAW, the burn-off of Cr and Ni elements can be compensated for by the transfer of alloying elements from the flux and the wire. Due to the deep penetration of SAW, care must be taken to prevent hot cracking in the weld center and reduce corrosion resistance in the HAZ.
Martensitic Stainless Steel Welding
The microstructure of martensitic stainless steel is predominantly a martensite phase. Martensitic stainless steel is magnetic, has high strength, poor wear resistance, and low toughness. It is difficult to weld and typically requires post-weld heat treatment.
Challenges in Martensitic Stainless Steel Welding
Martensitic stainless steel exhibits higher hardness and lower ductility, a characteristic that is particularly critical for welded joints, as it can easily lead to cracking in the HAZ after welding. Compared to carbon and low-alloy steels, the diffusion rate of hydrogen is slower in martensitic stainless steel (about 4% to 10% of carbon steel’s rate), meaning delayed cold cracking may occur up to five days after welding.
The cold cracking susceptibility of martensitic stainless steel joints increases with higher carbon content. Therefore, grades like 410 and 420 are typically used for welded structures. Higher-carbon grades such as 420J2 and 40Cr13 have an even greater cold cracking tendency and are generally considered unweldable.
Welding Methods
Various fusion welding methods can be used to weld martensitic stainless steel. The common methods are Shielded Metal Arc Welding (SMAW) and Gas Tungsten Arc Welding (TIG).
When using SMAW, low-hydrogen or ultra-low-hydrogen electrodes should be utilized wherever possible. Before welding, electrodes must be baked at a high temperature, typically 300 to 350°C (572 to 662°F), to reduce the diffusible hydrogen content and lower the cold cracking sensitivity.
TIG is mainly used for welding thin-walled components, piping, and critical components. Its advantages include high weld quality, attractive bead appearance, and the ability to achieve a single-sided weld with a double-sided profile (full penetration), which ensures the quality of the internal pipe weld.
Since martensitic stainless steel has poor weldability, all processes (SMAW, Flux-Cored Arc Welding (FCAW), automated welding, or Gas-Shielded Arc Welding) require preheating to maintain the interpass temperature and post-weld heat treatment. The preheat temperature is typically 200 to 300°C (392 to 572°F).
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Ferritic Stainless Steel Welding
The microstructure of ferritic stainless steel is a ferrite phase. Ferritic stainless steels have a very low or zero nickel content, are magnetic, and cannot be hardened by heat treatment. They exhibit high strength and better resistance to chloride and stress corrosion cracking compared to austenitic grades.
Challenges in Ferritic Stainless Steel Welding
Ferritic stainless steel has a coefficient of thermal expansion like carbon steel (and lower than austenitic stainless steel). This makes it less prone to hot cracking than austenitic steel because the solidification process is less likely to form low-melting-point eutectics. Furthermore, very little martensite forms in the HAZ zone that exceeds the critical temperature during the welding thermal cycle, resulting in very low quench-hardening tendency. Therefore, ferritic stainless steel is less susceptible to delayed cold cracking than martensitic stainless steel.
Ferritic stainless steel does not undergo quench hardening (austenite-to-martensite transformation) during post-weld cooling. However, the high welding heat promotes ferrite grain coarsening in the fusion-line region of the HAZ, which reduces the joint’s toughness and cannot be improved by direct heat treatment.
Embrittlement of ferritic stainless steel welded joints is the main factor limiting the application of these steels. The primary issues leading to embrittlement in the weld metal and HAZ are the precipitation of carbon and nitrogen compounds and grain growth during the welding process. Notably, the precipitation of C and N compounds is nearly impossible to eliminate through heat treatment.
Welding Methods
Welding methods that offer concentrated energy and faster welding speeds are preferred for ferritic stainless steel. Plasma Arc Welding (PAW) and Vacuum Electron Beam Welding (EBW) are highly suitable. It is also essential to prevent contamination of the welding materials and to provide effective protection for both the weld pool and the weld root to prevent air infiltration. In addition to using a low heat input, the weld root can be protected with an inert gas shield. Utilizing a water-cooled copper backing plate is recommended to reduce overheating and increase the cooling rate. For multi-pass welding, the interpass temperature should be controlled to approximately 100°C (212°F).
