The effect of four types of stainless steel and alloy elements
There are four types of stainless steel: austenitic, martensitic, ferritic and duplex stainless steel (as shown in Table 1).
Table 1 Types of stainless steel and their Cr and Ni element contents
| Types | Cr/% | Ni/% | Note |
|---|---|---|---|
| Austenite | 16-30 | 8-40 | 200,300 |
| Martensite | 11-18 | 0-5 | 403,410,416,420 |
| Ferrite | 11-30 | 0-4 | 405,409,430,422,446 |
| Duplex | 18-28 | 4-8 | 2205 |
This is based on the metallographic structure of stainless steel at room temperature. When carbon steel is heated to 1,550°F, its structure transforms from a ferrite phase to an austenite phase. After cooling, the low carbon steel structure returns to ferrite.
The austenite structure at high temperatures is non-magnetic and has lower strength but better toughness compared to the room temperature ferrite structure.
If the chromium (Cr) content in the steel is greater than 16%, the room temperature ferrite structure is stabilized, causing the steel to remain in the ferrite state in all temperature ranges. This type of steel is called ferritic stainless steel.
If the Cr content is greater than 17% and the nickel (Ni) content is greater than 7%, the austenite phase is stabilized, allowing the steel to remain in the austenite state from low temperatures until close to its melting point. This type of steel is called austenitic stainless steel and is generally referred to as the “Cr-Ni” type. Martensitic and ferritic stainless steels are called “Cr” type.
Elements in stainless steel and filler metals can be classified as austenite-forming or ferrite-forming. The most important austenite-forming elements are Ni, carbon (C), manganese (Mn) and nitrogen (N), while the main ferrite-forming elements are Cr, silicon (Si), molybdenum (Mo) and niobium (Nb). The element content can be adjusted to control the ferrite content in the weld.
Austenitic stainless steel is easier to weld and has better welding quality compared to stainless steel with less than 5% Ni. Austenitic stainless steel welded joints exhibit good strength and toughness and typically do not require preheating or post-welding heat treatment.
In the field of stainless steel welding, austenitic stainless steel accounts for 80% of the total amount of stainless steel used, therefore this article will focus on the welding of austenitic stainless steel.
How to choose the right materials for stainless steel welding?
When welding the same base material, it is important to match the base material to the welding material. For example, when welding 310 or 316 stainless steel, you must use the corresponding welding material.
In the case of dissimilar materials, it is recommended to choose a base metal with a high content of alloying elements. For example, when welding 304 and 316 stainless steel, you should choose 316 type welding consumables.
However, there are exceptions to the base material matching principle. In these cases, it is important to consult a welding material selection table. For example, although type 304 stainless steel is a common base material, there is no 304 electrode available.
If you want to match the welding material with the base material, how to choose the welding material to weld 304 stainless steel?
When welding 304 stainless steel, it is recommended to use type 308 welding material, as the additional 308 stainless steel elements can effectively stabilize the weld area. 308L is also an acceptable alternative. The “L” in 308L stands for low carbon, with a carbon content of 0.03% or less. In comparison, standard 308 stainless steel can contain up to 0.08% carbon.
L-type welding materials, such as 308L, belong to the same type as non-L-type welding materials, but have the advantage of a lower carbon content, reducing the risk of intergranular corrosion (Figure 1).
It is believed that the use of L-type welding consumables will increase as manufacturers aim to improve the quality of their products.
Figure 1 The use of L-shaped welding materials can reduce the tendency of intergranular corrosion
Manufacturers using the GMAW welding method may consider using type 3XXSi welding materials as the addition of silicon (Si) can improve wettability (Figure 2).
In situations where the weldment has a high bulge or the weld pool is poorly connected at the tip of the fillet or overlap weld, the use of Si-containing gas shielded welding wire can improve wettability and increase the deposition rate.
In GMAW welding, to improve the wettability of the welding material, a Si-containing welding wire such as 308L Si or 316L Si can be used (Figure 2).
When considering carbide precipitation, a type 347 welding material with a small amount of niobium (Nb) can be chosen as a solution.
How to weld stainless steel and carbon steel?
To reduce costs, some structural parts may have a corrosion-resistant layer added to their surface by welding carbon steel.
When welding base alloys without alloying elements and base alloys with alloying elements, a welding alloy with a higher alloy content is used to balance the dilution rate in the weld.
When welding carbon steel with 304 or 316 stainless steel, as well as other different stainless steels (Table 2), 309L welding consumables are commonly used. If a higher chromium (Cr) content is desired, then type 312 is used.
Table 2 High alloy stainless steels 309L and 312 are suitable for welding stainless steel and carbon steel
| No | Yes | W | Mn | Cr | FN WRC-92 | N | Mo | |
|---|---|---|---|---|---|---|---|---|
| 309L | 13.4 | 0.4 | 0.02 | 1.8 | 23.2 | 10 | 0.05 | 0.1 |
| 312 | 8.8 | 0.4 | 0.1 | 1.6 | 30.7 |
It is important to note that the thermal expansion rate of austenitic stainless steel is 50% greater than that of carbon steel.
During welding, the difference in the rate of thermal expansion can result in internal stress and cause cracking.
To mitigate this problem, it is necessary to select the appropriate welding material or specify the appropriate welding process (Figure 3).
Figure 3 highlights the need for greater compensation when welding carbon steel and stainless steel due to warping deformation caused by their different rates of thermal expansion.
What is a proper pre-weld cleaning operation?
When welding other materials, it is crucial to first clean the area using a chloride-free solvent to remove oil, marks and dust. One of the main considerations when welding stainless steel is to avoid contamination by carbon steel, which can compromise corrosion resistance. To avoid cross-contamination, some companies store stainless steel and carbon steel separately.
When cleaning the area around the groove, use special sandpaper and a brush designed specifically for stainless steel. In some cases, secondary cleaning of the joint may be necessary. Because electrode compensation is more challenging when welding stainless steel compared to carbon steel, proper joint cleaning is crucial.
What is the correct post-welding cleaning operation? Why do stainless steel weldments rust?
To begin with, it is important to highlight that stainless steel does not rust due to the protective oxide layer formed by the reaction between chromium (Cr) and oxygen (O).
However, stainless steel can rust as a result of carbide precipitation and heating during the welding process, leading to the formation of iron oxides on the welding surface. Furthermore, an apparently perfect weld can result in cuts in rusty areas at the edges of the weld heat-affected zone within 24 hours.
To regenerate new chromium oxides and prevent rust, it is necessary to polish, pickle, sand or rub the stainless steel after welding. It is important to highlight that the sandpaper and brush used must be specific for stainless steel.
Why is stainless steel welding wire magnetic?
Austenitic stainless steel is non-magnetic in nature. However, high temperatures during welding can cause grain growth in the structure, leading to an increase in sensitivity to cracking after welding.
To mitigate susceptibility to hot cracking, welding material manufacturers add ferrite-forming elements to the welding material (Figure 4). The presence of the ferrite phase helps to refine the austenite grains, thus increasing resistance to cracking.
Figure 4 illustrates the use of ferrite to prevent hot cracking in austenitic welding materials. Most austenitic welding materials contain a small amount of ferrite, as can be seen in the image of the 309L welding consumable, where the ferrite phase (gray part) is distributed throughout the austenite matrix.
Austenitic weld metal is not attracted to a magnet, but a slight suction force is felt when a magnet is held close to it. However, this has led some users to mistakenly believe that the product was incorrectly labeled or that the wrong welding material was used, particularly when the label is missing from the packaging.
The amount of ferrite in the consumable depends on the service temperature of the application. For example, an excessive amount of ferrite can reduce toughness at low temperatures. This is why the ferrite number of type 308 welding materials used for LNG pipelines is between 3-6, while the ferrite number of standard type 308 welding materials is 8.
In conclusion, although consumables may appear similar, small differences in composition can have a significant impact.
How to weld duplex stainless steel more easily?
Typically, duplex stainless steel structure is composed of approximately 50% austenite phase and 50% ferrite phase. The ferrite phase contributes to improved strength and stress corrosion resistance, while the austenite phase increases toughness. The combination of these two phases results in even better performance for duplex stainless steel (Figure 5).
The range of duplex stainless steel is quite wide, with 2205 being the most common type. 2205 contains 22% chromium, 5% nickel, 3% molybdenum and 0.15% nitrogen.
Figure 5 Duplex stainless steel combines the advantages of ferrite and austenite.
The image shows the two-phase weld structure of the austenite phase (white part) distributed in the ferrite matrix. However, excessive amounts of ferrite can pose challenges when welding duplex stainless steels, as the heat from the arc can cause the atoms in the ferrite matrix to rearrange.
To solve this problem, welding consumables need to provide more austenite-forming elements, which generally have a 2 to 4% higher nickel content than the base metal. For example, the flux-cored wire used in welding 2205 stainless steel contains 8.85% nickel. After welding, the ferrite content in the weld is typically between 25-55% (and can be even higher).
It is important to note that the cooling rate after welding must be slow enough to allow the austenite to reform, but not too slow as this may result in precipitation of the intermetallic phase. Likewise, cooling too quickly can result in excess ferrite in the heat-affected zone.
To ensure the best results, always follow the welding procedure and welding material selection manual provided by the manufacturer.
How to control carbide precipitation in austenitic stainless steel?
At temperatures between 800-1600°F, if the carbon content exceeds 0.02%, carbon (C) will diffuse and migrate to the grain boundaries of austenite and react with chromium (Cr) to form chromium carbides.
If too much chromium is fixed by carbon, corrosion resistance will decrease, leading to intergranular corrosion if exposed to a corrosive environment. This corrosion will result in erosion at grain boundaries (Figure 6).
Figure 6 illustrates the intergranular corrosion that occurred in the zone affected by the welding heat of a water tank filled with corrosive medium. To reduce the likelihood of carbide precipitation and improve corrosion resistance, low-carbon welding materials or special alloys can be used.
To control carbide precipitation, a low carbon welding material is employed to ensure that the carbon content in the weld metal is as low as possible, down to 0.04%. Furthermore, the addition of Nb and Ti elements can also fix carbon, as these elements have a greater affinity for carbon than chromium. Type 347 consumables are designed specifically for this purpose.
How to prepare for the selection of welding materials?
To select the appropriate stainless steel welding material, it is important to gather information about the final welding application. This includes details about the service environment, such as the service temperature, the presence of a corrosive medium and the desired level of corrosion resistance, as well as the expected service life.
Information on the mechanical properties required under service conditions, such as strength, toughness, plasticity and fatigue properties, is also important.
Most major manufacturers of welding materials provide instruction manuals for material selection. It is highly recommended to consult these manuals or consult the manufacturer's technical experts for assistance in choosing the correct welding material. This will ensure that the correct material is selected for the specific application and requirements.
