How much do you know about the welding details of stainless steel pipes

Stainless steel can be divided into four types: austenite, martensite, ferrite, and duplex stainless steel.

This is divided according to the metallographic structure of stainless steel pipes at room temperature. When low carbon steel is heated to 1550℉, its structure changes from the ferrite phase at room temperature to the austenite phase. When cooled, the low-carbon steel structure changes back to ferrite.

The austenite structure existing at high temperature is non-magnetic, and its strength is lower than that of the ferrite structure at room temperature, and its toughness is better.

When the Cr content in the steel is greater than 16%, the ferrite structure at room temperature is fixed so that the steel remains ferrite in all temperature ranges. Therefore, it is called a ferritic stainless steel pipe. When the Cr content is greater than 17% and the Ni content is greater than 7%, the austenite phase is fixed so that the austenite state is maintained from low temperature to almost the melting point.

Austenitic stainless steel pipes are usually called the "Cr-Ni" type, and martensitic and ferritic stainless steel pipes are directly called the "Cr" type. The elements in stainless steel and filler metal can be divided into austenite forming elements and ferrite forming elements. The main austenite forming elements are Ni, C, Mn and N, and the ferrite forming elements are Cr, Si, Mo and Nb. Adjusting the element content can control the ferrite content in the weld.

Austenitic stainless steel pipes are easier to weld than stainless steel pipes with a Ni content of less than 5%, and the welding quality is better. The welded joints of austenitic stainless steel pipes are very strong and tough and generally do not require preheating before welding and post-weld heat treatment. In the field of stainless steel pipe welding, austenitic stainless steel pipes account for 80% of the total stainless steel pipe consumption, so the focus of this article is the welding of austenitic stainless steel pipes.

How to choose the right stainless steel pipe welding consumables?

If the parent material is the same, the first criterion is "matching with the parent material". For example, when welding 310 or 316 stainless steel pipes, choose the corresponding welding consumables. When welding dissimilar materials, follow the principle of choosing a parent material that matches the high alloying element content. For example, when welding 304 and 316 stainless steel pipes, choose 316-type welding consumables. However, there are many special cases that do not follow the "matching parent material" principle. In this case, you need to "check the welding material selection table". For example, 304 stainless steel pipe is the most common parent material, but there is no 304 welding rod.

If the welding material matches the parent material, how to choose welding materials to weld 304 stainless steel pipe?
When welding 304 stainless steel pipe, use 308 welding material because the additional elements in 308 stainless steel pipe can better stabilize the weld area.

308L is also an acceptable choice. L means low carbon content, 3XXL stainless steel pipe means carbon content ≤ 0.03%, while standard 3XX stainless steel pipe can contain up to 0.08% carbon content.

Since L-type welding materials belong to the same type of classification as non-L-type welding materials, manufacturers should especially consider using L-type welding materials because its low carbon content can reduce the tendency of intergranular corrosion.

Manufacturers using GMAW welding methods should also consider using 3XXSi welding materials because Si can improve wettability. In the case of high ridges in the weldment or poor weld pool connection at the toe of fillet welds or lap welds, the use of Si-containing gas-shielded welding wire can wet the weld and improve the deposition rate.

If carbide precipitation is considered, 347-type welding materials containing a small amount of Nb can be selected.

How to weld stainless steel pipes with carbon steel?
In order to reduce costs, some structural parts will weld a corrosion-resistant layer on the surface of carbon steel. When welding a base material without alloying elements to a base material with alloying elements, use welding materials with higher alloy content to balance the dilution rate in the weld.

When welding carbon steel and 304 or 316 stainless steel pipes, as well as other dissimilar stainless steel pipes, 309L welding materials are considered in most cases. If a higher Cr content is desired, type 312 is selected.

It should be pointed out that the thermal expansion rate of austenitic stainless steel pipes is 50% higher than that of carbon steel. When welding, the difference in thermal expansion rate will produce internal stress, which will lead to cracks. At this time, it is necessary to select appropriate welding materials or specify appropriate welding processes.

What is the proper pre-weld cleaning operation?
When welding with other materials, first use a chloride-free solvent to remove oil, marks, and dust. In addition, when welding stainless steel pipes, the first thing to pay attention to is to avoid contamination from carbon steel that affects corrosion resistance. Some companies store stainless steel pipes and carbon steel pipes separately to avoid cross-contamination. When cleaning the area around the groove, use a special grinding wheel and brush for stainless steel pipes. Sometimes a secondary cleaning of the joint is required. Since the electrode compensation operation when welding stainless steel pipes is more difficult than when welding carbon steel, its joint cleaning is very important.

What is the proper post-weld cleaning operation? Why do stainless steel pipe weldments rust?
First, let's recall that the reason why stainless steel pipes do not rust is that the reaction of Cr and O forms a dense oxide layer on the surface of the material and plays a protective role. Stainless steel pipes rust because of the precipitation of carbides and the heating during welding that causes iron oxides to form on the surface of the weldment. In the welded state, a perfect weldment may also produce undercuts at the rusty place at the boundary of the weld heat-affected zone within 24 hours. Therefore, in order to regenerate new chromium oxide, stainless steel pipes need to be polished, pickled, ground, or brushed after welding. It should be emphasized that the grinder and brush must be dedicated.

Why is stainless steel pipe welding wire magnetic?
Stainless steel pipes with full austenite structure are non-magnetic. However, the higher temperature during welding causes the grains in the structure to grow, and the crack sensitivity after welding increases. In order to reduce the sensitivity to hot cracks, welding material manufacturers add ferrite forming elements to the welding material. The ferrite phase makes the austenite grains finer, thereby increasing the crack resistance.
Magnets do not attract austenitic weld metal, but a slight suction can be felt when holding the magnet. However, this also causes some users to mistakenly believe that the product is mislabeled or the wrong welding material is used.
The amount of ferrite in the welding material depends on the service temperature of the application. For example, too much ferrite reduces the toughness at low temperatures. Therefore, the ferrite number of 308 welding materials used for LNG pipelines is between 3-6, while the ferrite number of standard 308 welding materials is 8.
In short, welding materials may look similar, but a small difference in composition can sometimes make a big difference.

How to weld duplex stainless steel pipes more easily?

Typically, the austenite phase and ferrite phase in the duplex stainless steel pipe structure each account for about 50%. The presence of the 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 pipes even better. Duplex stainless steel pipes range from a wide range, and the most common type is 2205: containing 22% Cr, 5% Ni, 3% Mo, and 0.15% N.

When welding duplex stainless steel pipes, the presence of too much ferrite can cause some problems (the heat of the arc causes the atoms in the ferrite matrix to reorder). To do this, the welding material needs to provide more austenite forming elements, usually 2-4% higher Ni than the base material. For example, the flux-cored wire used for welding 2205 stainless steel pipes contains 8.85% Ni.

After welding, the ferrite content in the weld is between 25-55% (and may be higher). Note: The cooling rate after welding should be slow enough to allow austenite to reform, but not too slow, which will precipitate intermetallic phases, and not too fast, which will produce too much ferrite in the heat-affected zone. Always follow the welding process and welding material selection manual provided by the manufacturer.

Why do welders need to adjust parameters at any time when welding stainless steel pipes?

The main reason why welders adjust welding parameters (voltage, current, arc length, inductance, pulse width, etc.) at any time when welding stainless steel pipes is mismatched welding material composition. Chemical composition is important, and differences in composition between batches can cause large differences in welding behavior, such as poor wetting and slag removal. Welding material diameter, surface cleanliness, pourability, and spiral shape can all affect welding behavior during GMAW and FCAW.

How to control carbide precipitation in austenitic stainless steel pipes?
At 800-1600℉, 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 fixed in large quantities by C elements, corrosion resistance will decrease. At this time, if exposed to a corrosive environment, intergranular corrosion will occur, causing the grain boundaries to be eroded.
In order to control carbide precipitation, low-carbon welding materials are used to ensure that the carbon content in the weld metal is as low as possible (no more than 0.04%). C can also be fixed by adding Nb and Ti elements. Compared with Cr, the affinity of Nb and Ti with C is greater. Type 347 welding materials are designed for this purpose.

How to prepare for the selection of welding materials?
First, collect information on the terminal application of the weldment, including the service environment (especially the service temperature, whether there is a corrosive medium, and the expected 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 performance.