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 (Table 1).
Table 1 Stainless steel types and their Cr and Ni element contents
This is based on the metallographic structure of stainless steel at room temperature. When the mild steel is heated to 1550 ° F, its structure changes from a ferrite phase to an austenite phase at room temperature.
When cooled, the structure of the low carbon steel is transformed into ferrite again.
The austenite structure existing at high temperature is non-magnetic, and its strength is lower and its toughness is better than room temperature ferrite structure.
When the Cr content in the steel is greater than 16%, the ferrite structure at room temperature is fixed so that the steel maintains the ferrite state in all temperature ranges. Therefore, it is called ferritic stainless steel.
When the Cr content is more than 17% and the Ni content is more than 7%, the austenite phase is fixed, so that the austenite state is maintained from the low temperature to the almost melting point.
Austenitic stainless steel is usually called “Cr-Ni” type, and martensitic and ferritic stainless steels are directly called “Cr” type.
Elements in stainless steel and filler metals can be divided into austenite-forming elements and ferrite-forming elements.
The most important austenite-forming elements are Ni, C, Mn, and N, and the ferrite-forming elements are Cr, Si, Mo, and 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 than stainless steel containing less than 5% Ni.
The welded joints of austenitic stainless steel have good strength and toughness, and generally do not require pre-heating and post-weld heat treatment.
In the field of stainless steel welding, austenitic stainless steel accounts for 80% of the total amount of stainless steel, so the focus of this article is the welding of austenitic stainless steel.
How to choose the right stainless steel welding materials?
If the base material is the same, the first criterion is “matching the base material”.
For example, when welding 310 or 316 stainless steel, you need to select the corresponding welding material.
When welding dissimilar materials, follow the guidelines for selecting a base metal with a high content of alloying elements. Such as welding 304 and 316 stainless steel, you have to choose 316 type welding consumables.
However, there are many special cases that do not follow the principle of “matching the base metal”. In this case, remember to “check the welding material selection table”.
For example, type 304 stainless steel is the most common base metal, but there is no 304 electrode.
If you want to match the welding material with the base material, how to choose welding material to weld 304 stainless steel?
When welding 304 stainless steel, use type 308 welding material, because the additional elements in 308 stainless steel can better stabilize the weld area.
308L is also an acceptable choice. L means low carbon content, 3XXL stainless steel means carbon content ≤ 0.03%, while standard 3XX stainless steel can contain up to 0.08% carbon content.
Since L-type welding materials belong to the same type as non-L-type welding materials, manufacturers should especially consider using L-type welding consumables because their low carbon content can reduce the tendency for intergranular corrosion (Figure 1).
In fact, I believe that if manufacturers want to upgrade their products, L-shaped welding materials will be more widely used.
Figure 1 Use of L-shaped welding materials can reduce the tendency of intergranular corrosion
Manufacturers using GMAW welding method can also consider using 3XXSi type welding materials because Si can improve wettability (Figure 2).
In the case where the weldment has a high bulge or the weld pool is poorly connected at the toe of the fillet or lap weld, the use of Si-containing gas shielded welding wire can wet the weld and increase the deposition rate.
Figure 2 In order to improve the wettability of the welding material in GMAW welding, a Si-containing welding wire is used, such as 308L Si or 316L Si
If you consider carbide precipitation, you can choose a 347 type welding material containing a small amount of Nb element (the last question).
How to weld stainless steel and carbon steel?
In order to reduce costs, some structural parts will be welded with a corrosion-resistant layer on the surface of carbon steel.
When welding base alloys containing no alloying elements and base alloys containing alloying elements, a welding alloy with a higher alloy content is used to balance the dilution rate in the weld.
When welding carbon steel and 304 or 316 stainless steel, as well as other dissimilar stainless steels (Table 2), 309L welding consumables are considered in most cases. If higher Cr content is desired, then type 312 is used.
Table 2 Stainless steels with high alloy content of 309L and 312 are suitable for welding stainless steel and carbon steel
It should be noted that the thermal expansion rate of austenitic stainless steel is 50% higher than that of carbon steel.
When welding, the difference in thermal expansion rate will cause internal stress, which will cause cracks.
At this time, you need to choose the appropriate welding material or specify the appropriate welding process (Figure 3).
Figure 3 When welding carbon steel and stainless steel, warpage deformation due to different thermal expansion rates needs greater compensation
What is a suitable pre-weld cleaning operation?
When welding to other materials, first use a chloride-free solvent to remove oil, marks, and dust. In addition, the first thing to pay attention to when welding stainless steel is to avoid contamination by carbon steel and affect the corrosion resistance. Some companies store stainless steel and carbon steel separately to avoid cross-contamination.
When cleaning the area around the groove, use a special sanding wheel and brush for stainless steel. Sometimes secondary cleaning of the joint is required. Because electrode compensation is more difficult when welding stainless steel than when welding carbon steel, therefore, joint cleaning is very important.
What is the correct post-weld cleaning operation? Why do stainless steel weldments rust?
First of all, let’s recall that the reason why stainless steel does not rust: The reaction of Cr and O generates a dense oxide layer on the surface of the material, which plays a protective role.
Stainless steel is rusted because of the precipitation of carbides (see the last question) and heating during the welding process, resulting in the formation of iron oxides on the surface of the weldment.
In the welded state, a perfect weldment may also cause undercuts in rusty areas at the boundaries of the welding heat affected zone within 24 hours.
Therefore, in order to regenerate new chromium oxides, stainless steel needs to be polished, pickled, sanded or scrubbed after welding.
It is important to emphasize that the sander and brush must be proprietary.
Why is stainless steel welding wire magnetic?
Stainless steel with austenitic structure is non-magnetic. However, the higher temperature during welding caused the grains in the structure to grow, and the crack sensitivity increased after welding.
To reduce hot crack susceptibility, welding material manufacturers add ferrite-forming elements to the welding material (Figure 4).
The ferrite phase makes the austenite grains finer, which increases the crack resistance.
Figure 4 Avoiding hot cracking, most austenitic welding materials contain a small amount of ferrite. The picture shows the ferrite phase (gray part) distributed on the austenite matrix in the 309L welding consumable.
The magnet does not attract the austenitic weld metal, but a slight suction force is felt when the magnet is held.
However, this also caused some users to mistakenly believe that the product was labeled incorrectly, or the wrong welding material was used (especially when the label on the package was torn off).
The amount of ferrite in the consumable depends on the service temperature of the application.
For example, excessive ferrite reduces toughness at low temperatures.
Therefore, the ferrite number of type 308 welding materials used for LNG pipelines is between 3-6, while the ferrite number of standard 308 type welding materials is 8.
In short, the consumables may look similar, but small differences in composition can sometimes make a big difference.
How to weld duplex stainless steel more easily?
Generally, the austenite phase and the ferrite phase in the duplex stainless steel structure each account for about 50%.
The presence of a ferrite phase can improve strength and stress corrosion resistance, while the austenite phase can improve toughness.
The combined effect of the two phases makes the performance of duplex stainless steel even better (Figure 5).
The range of duplex stainless steel is very wide, the most common model is 2205: containing 22% Cr, 5% Ni, 3% Mo and 0.15% N.
Figure 5 Duplex stainless steel combines the advantages of ferrite and austenite.
The picture shows the dual phase weld structure of the austenite phase (white part) distributed on the ferrite matrix.
The presence of too much ferrite can cause problems when welding duplex stainless steels (the heat of the arc causes the atoms in the ferrite matrix to reorder).
For this reason, the welding consumables need to provide more austenite forming elements, usually is 2-4% Ni higher than the base metal.
For example, the flux-cored wire used when welding 2205 stainless steel contains 8.85% Ni.
After welding, the ferrite content in the weld is between 25-55% (and possibly higher).
Note: The cooling rate after welding must be slow enough to make austenite re-form, but it can not be too slow, too slow will precipitate intermetallic phase, also can not be too fast, too fast will produce too much ferrite in the heat affected zone.
Always follow the welding procedure and welding material selection manual provided by the manufacturer.
How to control carbide precipitation in austenitic stainless steel?
At 800-1600 ° F, when the carbon content exceeds 0.02%, C will diffuse and migrate to the austenite grain boundaries and react with Cr at the grain boundaries to form chromium carbides.
If Cr is largely fixed by the C element, the corrosion resistance will decrease.
At this time, if exposed to a corrosive environment, intergranular corrosion will occur, causing the grain boundaries to be eroded (Figure 6).
Figure 6 In the water tank filled with corrosive medium, intergranular corrosion occurred in the welding heat affected zone. The use of low-carbon or specially alloyed welding materials can reduce the tendency of carbide precipitation and enhance corrosion resistance.
In order to control the precipitation of carbides, a low carbon content welding material is used to ensure that the carbon content in the weld metal is as low as possible (up to 0.04%).
C can also be fixed by adding Nb and Ti elements. Compared with Cr elements, the elements Nb and Ti have greater affinity with C.
Type 347 consumables are designed for this purpose.
How to prepare for the selection of welding materials?
First, collect information on the end application of the weldment, including the service environment (especially the service temperature, whether there is a corrosive medium and the desired degree of corrosion resistance) and the expected service life.
Information on the mechanical properties required under service conditions is also important, such as strength, toughness, plasticity and fatigue properties.
Most leading welding materials manufacturers provide instruction manuals for selecting the materials.
Here, I will emphasize again: It is recommended to refer to the welding material application manual or contact their technical experts.
They will help us to choose stainless steel welding materials more correctly.