Are You Overlooking These Critical Factors in Welding High-Performance Austenitic Stainless Steels？

Stainless steel weldments (weld and heat affected zone) must be corrosion resistant and structurally sound to meet the requirements of the specific application. In general, the corrosion resistance of the weld should be at least the same as that of the parent metal, and preferably a slightly higher strength grade. Weld quality can be divided into two different aspects - physical weld defects and metallurgical problems. They can reduce corrosion resistance or strength, so they must be avoided. Unacceptable welds must be repaired to ensure the required properties. Most physical defects are already common in carbon steel welding, while metallurgical problems are unique to stainless steel. The following are practical guidelines to help ensure the quality of welds.

Incomplete Penetration Welds 

Incomplete penetration usually occurs in pipe butt welds or other butt welds that are not double-sided. Incomplete penetration refers to the presence of a gap in the weld that can cause corrosion and harbor dirt. The gap reduces the strength and corrosion resistance of the weld and is difficult to sterilize (clean). High quality butt welds require full penetration. Otherwise, the mechanical strength and fatigue resistance of the weld are significantly reduced. To avoid these problems, proper weld design or back-cutting is important, and back-cutting is recommended whenever possible.

Porosity

Surface pores are ideal harbors for dirt and are also a source of corrosion. Surface pores are difficult to clean and disinfect because they attract dirt and bacteria. Porosity is often caused by moisture, which can come from the electrode flux, shielding gas or the surface of the workpiece. To minimize porosity, pay attention to electrode dryness, gas chemistry and cleaning practices. Also, determine a porosity acceptance level to guide radiographic and visual inspection of welds.

Arc Starting and Weld Spatter

Arc starting and weld spatter create crevices and initiate crevice corrosion. To minimize this defect, welders should start the arc within the weld, not next to it. If there is arc starting and weld spatter, it should be removed with a fine grinding wheel.

Avoid Weld Spatter and Uneven Welds ©Outokumpu

Microcracks and Hot Cracks

Microcracks are small, short cracks or fissures that can occur in austenitic stainless steel welds. They rarely propagate and rarely cause weld structural failure, but in certain environments they can cause localized corrosion. Hot cracking, more accurately known as weld solidification cracking, is a serious weld defect that must be avoided.

Adjusting the composition of the commonly used filler metals 308(L) and 316(L) to achieve a ferrite content of 5-10% can significantly improve the resistance to microcracking and hot cracking. Ferrite absorbs shrinkage stresses and has a higher solubility for sulfur, phosphorus and other impurities that cause austenite embrittlement. Standard stainless steel welds with ferrite to the recommended content are generally free of microcracking and hot cracking, but they can occur when heat input is very high, weld restraining stresses are high or the weld is concave. The Ferrite Number (FN) defined by the American Welding Society (AWS A5.4) indicates the amount of ferrite in a weld. The Ferrite Number (FN) is roughly equivalent to the volume percentage of ferrite in an austenitic stainless steel weld. The figure below shows two weld metals with different ferrite contents.

Austenitic Stainless Steel Weld Metals with Different Ferrite Contents ©Lincoln Electric

Filler metals used to weld high-performance austenitic stainless steels do not produce welds containing ferrite, so their welds are susceptible to microcracking and hot cracking. To minimize microcracking and hot cracking, filler metals used to weld high-performance austenitic stainless steels have very low phosphorus and sulfur contents. Heat input and other welding parameters must be carefully controlled during welding. The upper limit of heat input is generally 1.5 kJ/mm (38 kJ/in).

When welding high-performance austenitic stainless steels, avoid any operation that increases the size of the weld pool (such as excessive sway). Large weld pools increase solidification shrinkage stresses. Large weld pools also increase grain size in the weld and heat-affected zone. The grain boundary area of ​​coarse-grained material is smaller than that of fine-grained material. This results in higher impurity concentrations at the grain boundaries, which may reduce corrosion resistance. Excessive shrinkage stresses and high trace element concentrations at the grain boundaries can cause hot cracking.

Other Defects 

Other welding defects, such as inadequate fusion and slag inclusions between weld beads, are unacceptable for both carbon steel and austenitic stainless steels. Surface slag inclusions can cause pitting when the weld is exposed to a corrosive environment. Similarly, a rough surface on the weld will reduce its corrosion resistance. Undercuts can significantly reduce the fatigue performance of the weld. Overstrengthening of the weld root or weld cap can also adversely affect weld performance.

Surface Oxides 

For many applications, temper tint on the inside surface of pipe welds is a concern. There are many ways to eliminate or minimize temper tint oxides. One is to provide adequate inert gas blasting through the pipe. When butt welding using orbital GTAW, the joint must be assembled and purged with inert gas to achieve a weld that is essentially free of temper tint. When manual GTAW root welds are made, some degree of temper tint is usually produced. Depending on the degree of temper tint and the intended use, the oxides are removed by pickling or mechanical polishing.

Temper tint significantly reduces pitting and crevice corrosion resistance, especially for standard austenitic stainless steels. Another potential problem is Microbiologically Influenced Corrosion (MIC). Areas with temper tint are susceptible to MIC, which has been found in untreated water, especially at low flow rates or stagnant flow conditions. Failure to drain and dry the system after hydrostatic testing with normal water can also cause MIC.

There are two challenges in developing acceptance criteria for welds where temper tint cannot be removed. The first is how to quantify the temper tint on the weld. Many industries use a color chart of temper tint to visually determine the grade of temper tint. This chart is found in AWS D18.1 and AWS D18.2. The second is to determine the degree of temper tint that is acceptable in a specific application.

Temper tint of a high-quality GMA weld and heat-affected zone (left), and post-weld pickling restores the corrosion resistance of the weld (right) ©Outokumpu

Temper tint has a more significant impact on the corrosion resistance of standard stainless steels (such as 304L and 316L) than high-performance austenitic stainless steels. When temper tint removal is extremely difficult or costly, designers should consider using more corrosion-resistant grades to better utilize their performance.

Sensitization 

High carbon standard grades become sensitized by short exposure to 480-900°C (900-1650°F) and are susceptible to intergranular corrosion in aqueous and acidic environments. However, advanced standard grades are usually "L-grades" with carbon contents below 0.03%. Therefore, they are resistant to sensitization in normal fabrication welds without subsequent heat treatment. For example, sensitization of 304 stainless steel with 0.042% carbon content takes about one hour at the fastest sensitization temperature. This time is much longer than the time at the welding sensitization temperature. Even so, the exposure time of large weldments in the critical temperature range should be limited.

Using low carbon grades can help avoid sensitization of thick section welds and parts that need to be heat treated after welding. 304L has a long tolerance to the sensitization temperature, so large parts can be cooled safely. With L grades, even mixed stainless and carbon steel components can be stress relieved.

Time-Temperature-Sensitization (TTS) Plot for 304 Stainless Steel with Different Carbon Contents

To improve elevated temperature strength, high temperature grades usually contain at least 0.04% carbon (304H). Fortunately, aqueous corrosion due to sensitization is not a concern for high temperature applications. These grades usually require high carbon filler metals to provide adequate weld elevated temperature strength.

Stabilized 321 and 347 stainless steels are susceptible to narrow knife-shaped corrosion when exposed to temperatures between 480 and 900°C during welding. If knife-shaped corrosion is a concern, a post-weld solution anneal and stabilization heat treatment should be specified. For a discussion of knife-shaped corrosion mechanisms, see the previous chapters in this series.

Most high performance austenitic stainless steels have a lower upper carbon content than conventional "L" grades and will sensitize faster than conventional grades at the same carbon content. However, secondary phase formation is more of a problem for high performance austenitic stainless steel welding than sensitization.

Intermetallic phases

The temperature range for the formation of intermetallic phases, σ and χ, is 500-1050°C. Stainless steels containing σ and χ phases have significantly reduced corrosion resistance and toughness. 5% of σ phase will reduce impact toughness by 50%.

Increasing the chromium and molybdenum content greatly promotes the precipitation of intermetallic phases. At the critical temperature, the formation time of σ and χ phases in high-performance austenitic stainless steels is less than one minute. Therefore, the welding parameters of these materials must include low heat input (less than 1.5 kJ/mm) and interpass temperatures not exceeding 100°C, and the time to the critical temperature should be minimized. The interpass temperature of the weld should be measured at the end of a weld, and the accuracy should be ensured by thermocouple measurements. Temperature-sensitive colored pens are prohibited because they will contaminate the weld.

At non-ideal temperatures, the precipitation time of any intermetallic compound in standard grades usually takes 100 hours or more. Due to their slow kinetics, the precipitation of σ and χ phases is not a problem during the processing and manufacturing of standard grades, but long-term high-temperature service is not optimistic.

Weld Segregation

The weld metal of high-molybdenum high-performance austenitic stainless steels is particularly prone to microsegregation of molybdenum. Microsegregation occurs during solidification because the first solidified metal has a lower molybdenum content and the later solidified metal has a higher molybdenum content, resulting in a micro gradient of molybdenum content. In 6%Mo stainless steel, the low-molybdenum area may have significantly reduced corrosion resistance.

Therefore, in order to compensate for microsegregation, welding high-performance austenitic stainless steel requires the use of overmatching filler metal. When welding 6%Mo stainless steel, it is best to use a nickel-based filler metal with a molybdenum content of not less than 9%, so that the molybdenum content of the first solidified area is not less than 6%, so that the weld metal maintains good corrosion resistance.

Due to microsegregation problems, high-performance stainless steel components that cannot be post-weld annealing cannot be welded by autogenous welding (without filler metal). Autogenous welding is only suitable for weldments that are to be post-weld solution annealed. Solution annealing can homogenize the weld, reduce microsegregation, and restore corrosion resistance.