Stainless steel is renowned for its many beneficial attributes that have profoundly affected a wide variety of industries. Since its development more than a hundred years ago, stainless steel has been prized for its high strength and corrosion resistance at an affordable price point. These benefits include many welding applications as well.
If you are already familiar with stainless steel you know that despite its many useful properties, it is actually more difficult to weld than traditional carbon steel. Stainless steels are defined as iron-based alloys that contain at least 10.5% chromium. The name originates from the thin layer of chromium oxide film that offers a high level of corrosion resistance, acting as a barrier to limit the access of oxygen and water to the underlying metal; this film also has the ability to heal itself.
Because of these differences, there are several things to keep in mind when welding stainless steel in comparison to traditional carbon steel.
AN INTRODUCTION TO WELDING WITH STAINLESS STEEL
The first aspect of stainless steel that welders must be aware of is that there are five families of alloys, each of which has its own unique characteristics. Number one is austenitic stainless steel, which includes the 200 and 300 series. This family includes 304, the most popular alloy available today. The key alloying agents of this group are chromium and nickel.
Next, there are ferritic stainless steels, which are known for being non-hardenable. They have a higher chromium, lower carbon content and frequently have molybdenum, aluminum and titanium added. These alloys generally fall into the 400 series.
While the martensitic stainless steels can seem similar in their composition when compared to ferritic steels, they also fall into the category of 400 series grades. They contain a relatively higher amount of carbon and allow for hardening via heat treatment. Martensitic alloys include 403, 410, 416 and 420.
Duplex stainless steels get their name because they have similarities to both ferritic and austenitic alloys. They are made up of approximately 24% chromium and 5% nickel. Despite using the same alloying agents that austenitic stainless steels use, duplex grades offer higher yield strength and better resistance to stress corrosion when exposed to chlorides.
The final family, known as precipitation-hardening stainless steels, rely on alloying agents that include aluminum, copper and titanium. They are able to be hardened using an age hardening heat treatment process. The precipitation-hardening stainless steels can be grouped into three types, all of which fall into the 600 series.
HOW TO CHOOSE A STAINLESS STEEL ALLOY FOR WELDING
While stainless steels are generally considered to have good weldability, this certainly does not apply to all alloys. That’s why you must pay particular attention to the requirements of your specific application when selecting an alloy to be welded.
In general, welders need to keep a number of things in mind. For instance, you must ensure that the joint surfaces and the filler metal are not exposed to oxide, organic material, or any other kind of contamination that might impair their weldability. It is also necessary that the filler materials you are using make for a good match with the base metal.
When it comes to the austenitic family, these alloys can be welded using a number of different welding processes. You must keep in mind that the high carbon content of these alloys means they are susceptible to intergranular corrosion. This is the result of the chromium in the stainless steel combining with the carbon instead of forming the chromium oxide layer.
Fortunately, this can be avoided by using the low carbon form of an alloy. Grades such as 304 and 316 also come in 304L and 316L versions more suitable for welding. These will be less likely to suffer from intergranular corrosion. Furthermore, certain grades such as 321 or 347 are known to be more stable when welding.
The family of stainless steels that are most readily weldable are in the ferritic group. Thanks to the lower chromium content, corrosion will not be of great concern and the chance of hot cracking occurring during welding is reduced. However, it is possible that there will be a loss of toughness and ductility in the heat affected zone. This can be seen when peak temperatures from 900 to 1600F are achieved.
The best way to prevent this sensitivity is to choose an alloy with a very low carbon base or to use a grade that has titanium or niobium added.
The family that is hardest to weld are the martensitic stainless steels. This is because of the high carbon content, which can result in weld cracking. Several steps must be taken to avoid this, including maintaining a low hydrogen level during welding, a preheat process and post-weld heat treatment to lower the brittleness of the weld joint. In fact, it is impossible to successfully weld many martensitic alloys. With careful preparation and the correct filler material it is possible to weld with some grades such as 403, 410 and 420.
As for the other families, welding is possible, but several steps must be taken to avoid mistakes. For instance, precipitation hardening stainless steels gain their strength and hardness from intermetallic precipitates that are formed during heat treating. Welding is likely to negatively affect these precipitates, so they must be heat treated again after welding while paying special attention to the filler used.
Clinton Aluminum is not just a supplier. Our number one priority is to make sure our customers succeed, that’s why we strive to be partners at every step in the production process to ensure that your finished product meets all of your specifications. Our experienced team is dedicated to helping you save both time and money so that you can be more competitive.
Please call today to speak with a member of our knowledgeable and friendly sales staff to learn more about how Clinton Aluminum can help support your stainless needs.