Q: We’ve recently started doing some work that requires some components to be made primarily of grade 304 stainless steel, which is welded to itself and to mild steel.We have experienced some cracking issues on stainless steel to stainless steel welds up to 1.25″ thick.It was mentioned that we have low ferrite counts.Can you explain what this is and how to fix it?
A: This is a good question.Yes, we can help you understand what low ferrite counts mean and how to prevent it.
First, let’s review the definition of stainless steel (SS) and how ferrite relates to welded joints.Black steel and alloys contain more than 50% iron.This includes all carbon and stainless steels and other defined groups.Aluminum, copper and titanium do not contain iron, so they are excellent examples of non-ferrous alloys.
The main components of this alloy are carbon steel with at least 90% iron and SS with 70 to 80% iron.To be classified as SS, it must have at least 11.5% chromium added.Chromium levels above this minimum threshold promote the formation of chromium oxide films on steel surfaces and prevent oxidation formation such as rust (iron oxide) or corrosion caused by chemical attack.
SS is mainly divided into three groups: austenite, ferrite and martensite.Their name comes from the room-temperature crystal structure that makes them up.Another common group is duplex SS, which is a balance between ferrite and austenite in the crystal structure.
Austenitic grades, the 300 series, contain 16% to 30% chromium and 8% to 40% nickel, forming a predominantly austenitic crystal structure.To promote the formation of austenite-ferrite ratio, stabilizers such as nickel, carbon, manganese and nitrogen are added during the steelmaking process.Some common grades are 304, 316 and 347.Offers good corrosion resistance; primarily used in food, chemical service, pharmaceutical and cryogenic applications.Control of ferrite formation provides excellent low temperature toughness.
Ferritic SS is a 400 series grade that is fully magnetic, contains 11.5% to 30% chromium, and has a ferritic predominant crystal structure.To promote the formation of ferrite, stabilizers include chromium, silicon, molybdenum, and niobium during steel production.These types of SS are commonly used in automotive exhaust systems and power plants and have limited high temperature applications.Several commonly used types are 405, 409, 430 and 446.
Martensitic grades, also identified by the 400 series such as 403, 410 and 440, are magnetic, contain 11.5% to 18% chromium, and have martensite as the crystal structure.This combination has the lowest gold content, which makes them the least expensive to produce.They provide some corrosion resistance; excellent strength; and are commonly used in tableware, dental and surgical equipment, cookware, and certain types of tools.
When you weld SS, the type of substrate and its in-service application will determine the appropriate filler metal to use.If you use a gas shielding process, you may need to pay special attention to shielding gas mixtures to prevent certain welding-related problems.
To solder the 304 to itself, you will need an E308/308L electrode.The “L” stands for low carbon, which helps prevent intergranular corrosion.These electrodes have a carbon content below 0.03%; anything above this increases the risk of carbon precipitating to grain boundaries and combining with chromium to form chromium carbides, effectively reducing the corrosion resistance of the steel.This becomes apparent if corrosion occurs in the heat affected zone (HAZ) of SS welded joints.Another consideration for L grade SS is that they have lower tensile strength at elevated service temperatures than direct SS grades.
Since 304 is an austenitic type of SS, the corresponding weld metal will contain most of the austenite.However, the electrode itself will contain a ferrite stabilizer, such as molybdenum, to promote the formation of ferrite in the weld metal.Manufacturers usually list a typical range of ferrite quantities for the weld metal.As mentioned earlier, carbon is a strong austenitic stabilizer, and for these reasons it is critical to prevent it from being added to the weld metal.
Ferrite numbers are derived from the Schaeffler diagram and the WRC-1992 diagram, which utilize nickel and chromium equivalent formulas to calculate the value, which when plotted on the diagram produce a normalized number.The ferrite number between 0 and 7 corresponds to the volume percent of the ferrite crystal structure present in the weld metal; however, at higher percentages, the ferrite number increases at a faster rate.Remember that ferrite in SS is not the same as carbon steel ferrite, but a phase called delta ferrite.Austenitic SS has no phase transformations associated with high temperature processes such as heat treatment.
The formation of ferrite is desirable because it is more ductile than austenite, but must be controlled.Low ferrite counts can produce welds with excellent corrosion resistance in some applications, but are extremely prone to hot cracking during welding.For general usage conditions, the ferrite count should be between 5 and 10, but for some applications lower or higher values ​​may be required.Ferrites can be easily verified on the job using a ferrite indicator.
Since you mentioned that you have cracking issues and a low ferrite count, you need to take a close look at your filler metal and make sure it produces enough ferrite count – around 8 should help.Also, if you are using flux cored arc welding (FCAW), these filler metals typically use a 100% carbon dioxide shielding gas or a 75% argon/25% CO2 mixture, which can cause carbon uptake in the weld metal.You may wish to switch to a gas metal arc welding (GMAW) process and use a 98% argon/2% oxygen mixture to reduce the possibility of carbon pickup.
To weld SS to carbon steel you must use E309L filler material.This filler metal is specially used for welding dissimilar metals and forms a certain amount of ferrite after the carbon steel is diluted into the weld.Since carbon steel will absorb some carbon, ferrite stabilizers are added to the filler metal to counteract the tendency of carbon to form austenite.This will help prevent thermal cracking in welding applications.
In summary, if you want to eliminate hot cracks on austenitic SS welded joints, verify adequate ferrite filler metal and follow good welding practices.Keep heat input below 50 kJ/inch, maintain moderate to low interpass temperatures, and ensure that solder joints are free of any contamination prior to soldering.Use an appropriate gauge to verify the amount of ferrite on the welded joint, aiming for 5 to 10.
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