Q: We have recently begun doing some work that requires some components to be made primarily from 304 stainless steel, which is welded to itself and to mild steel. We have experienced some issues with weld cracking between stainless steel and stainless steel up to 1.25″ thick. It was mentioned that we have low ferrite levels. Can you explain what it is and how to fix it?
A: That’s a good question. Yes, we can help you understand what low ferrite means and how to prevent it.
First, let’s look at the definition of stainless steel (SS) and how ferrite relates to welded joints. Black steel and alloys contain over 50% iron. This includes all carbon and stainless steels, as well as certain other 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 an iron content of at least 90% and stainless steel with an iron content of 70 to 80%. To be classified as SS, it must have at least 11.5% chromium added. Chromium levels above this minimum threshold promote the formation of a chromium oxide film on steel surfaces and prevent the formation of oxidation such as rust (iron oxide) or chemical attack corrosion.
Stainless steel is mainly divided into three groups: austenitic, ferritic and martensitic. Their name comes from the crystal structure at room temperature of which they are composed. Another common group is duplex stainless steel, which is a balance between ferrite and austenite in the crystal structure.
Austenitic grades, 300 series, contain 16% to 30% chromium and 8% to 40% nickel, forming a predominantly austenitic crystal structure. Stabilizers such as nickel, carbon, manganese, and nitrogen are added during the steelmaking process to help form the austenite-ferrite ratio. Some common grades are 304, 316 and 347. Provides good corrosion resistance; mainly used in food, chemical, pharmaceutical and cryogenic industries. Control of ferrite formation provides excellent toughness at low temperatures.
Ferritic SS is a 400 series grade that is fully magnetic, contains 11.5% to 30% chromium, and has a predominantly ferritic 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 powertrains and have limited high temperature applications. Several commonly used types: 405, 409, 430 and 446.
Martensitic grades, also referred to as the 400 series, such as 403, 410, and 440, are magnetic, contain 11.5% to 18% chromium, and have a martensitic crystal structure. This combination has the lowest gold content, making them the least expensive to produce. They provide some corrosion resistance, superior strength, and are commonly used in tableware, dental and surgical equipment, cookware, and some types of tools.
When you weld stainless steel, the type of substrate and its application in service will determine the appropriate filler metal to be used. If you are using a shielding gas process, you may need to pay special attention to shielding gas mixtures to prevent certain problems associated with welding.
To solder the 304 to itself, you will need an E308/308L electrode. “L” stands for low carbon, which helps prevent intergranular corrosion. The carbon content of these electrodes is less than 0.03%, if this value is exceeded, the risk of carbon deposition at the grain boundaries and chromium bonding to form chromium carbides increases, which effectively reduces the corrosion resistance of the steel. This becomes apparent if corrosion occurs in the heat-affected zone (HAZ) of stainless steel welds. Another consideration for grade L stainless steel is that they have lower tensile strength at elevated operating temperatures than straight stainless steel grades.
Since 304 is an austenitic type of stainless steel, 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 for the amount of ferrite for a weld metal. As previously mentioned, carbon is a strong austenitic stabilizer and for these reasons it is essential to prevent its addition to the weld metal.
Ferrite numbers are derived from the Scheffler chart and the WRC-1992 chart, which use nickel and chromium equivalent formulas to calculate the value that when plotted on the chart gives a normalized number. A ferrite number between 0 and 7 corresponds to the volume percentage of ferritic crystal structure present in the weld metal, however, at higher percentages, the ferrite number increases more rapidly. Remember that ferrite in SS is not the same as carbon steel ferrite, but a phase called delta ferrite. Austenitic stainless steel does not undergo phase transformations associated with high temperature processes such as heat treatment.
Ferrite formation is desirable because it is more ductile than austenite, but must be controlled. The low ferrite content can provide welds with excellent corrosion resistance in some applications, but they are extremely prone to hot cracking during welding. For general use, the number of ferrites should be between 5 and 10, but some applications may require lower or higher values. Ferrites can be easily checked in the workplace with a ferrite indicator.
Since you mentioned that you have problems with cracking and low ferrites, you should take a close look at your filler metal and make sure it’s producing enough ferrites – around 8 should do the trick. Also, if you are using flux-cored arc welding (FCAW), these filler metals typically use a shield gas of 100% carbon dioxide or a mixture of 75% argon and 25% CO2, which can cause the weld metal to absorb carbon. You can switch to the metal arc welding (GMAW) process and use a 98% argon/2% oxygen mixture to reduce the possibility of carbon deposits.
When welding stainless steel to carbon steel, filler material E309L must be used. This filler metal is specially used for dissimilar metal welding, forming a certain amount of ferrite after carbon steel is dissolved in the weld. Because carbon steel absorbs 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 during welding.
In conclusion, if you want to repair hot cracks in austenitic stainless steel welds, check for sufficient ferrite filler metal and follow good welding practice. Maintain heat input below 50 kJ/in, maintain moderate to low inter-pass temperatures, and ensure solder joints are clean before soldering. Use an appropriate gauge to check the amount of ferrite on the weld, aiming for 5-10.
WELDER, formerly called Practical Welding Today, represents the real people who make the products we use and work with every day. This magazine has been serving the welding community in North America for over 20 years.
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