Stainless steel properties and how they change with nickel

Stainless steel is basically a family of iron-based alloys known for their corrosion and heat resistance. It weighs nearly three times as compared to aluminum. Like steel, stainless steel is also an alloy. An alloy is made up of various components, nickel being one of the most important of them. It is represented by the symbol Ni and its atomic number is 28. Nickel is a transition metal in the d-block with a high boiling point (14530C). It is ductile and exhibits magnetic properties at room temperature. First discovered in the 1700s, nickel is used as a catalyst to various chemical reactions and is one of the main alloying elements in stainless steel. Nickel in Stainless Steel is a widely used constituent.

Overview of nickel-containing stainless steel

Nickel-containing stainless steels have an inherent resistance to corrosion in addition to being easy to shape and weld, ductile at very low temperatures and suitable for high-temperature applications. The austenite region of steel is widened and the ferrite region is contracted by nickel, an austenite stabilizer. At high temperatures, nickel increases resistance to corrosion and oxidation. The hardness and strength of nickel are increased by reducing the grain size. Additionally, they are not magnetic, as compared to conventional steel and stainless steel without nickel. This implies that they can be transformed for diverse material applications such as the chemical industry, health industry, as well as home interior and appliances

Nickel is a critical constituent for steel making so much so that 75% of stainless steel manufacturing constitutes of grades that contain nickel. The grades with the maximum nickel component are Type 316, which has 11% nickel, and Type 304, which has 8% nickel.

Nickel provides these properties to steel by changing its crystal structure to an austenitic (face-centred cubic crystal) structure at almost all temperatures. At room temperature, conventional steel has a ferritic (cubic crystal with a body-centred structure) structure. These distinctive qualities are only given by the addition of enough nickel, usually 8–10%.

How does nickel impact the properties of stainless steel?

1. Formability –  The austenitic structure of stainless steel provides it with excellent formability and ductility.  Particularly, Grade 304, which contains 8% nickel and 18% chromium, exhibits good ductility.  A slightly higher nickel concentration would improve the austenite's stability even further. This also makes it more suitable for deep drawing. These alloys are not prone to delayed cold cracking, unlike low-nickel, high-manganese alloys. Due to their outstanding formability, 300-series austenitic alloys are frequently employed for products like cooking pots and sinks.

2. Weldability - Welding is a common method of fabrication for stainless steel equipment. In general, nickel austenitic alloys, especially Grade 304 and 316 are the most often fabricated stainless steels in the world. They are also better for welding than other alloys. They don't tend to become brittle like ferritic alloys do because of high-temperature grain development, and the welds have great bend and impact qualities. Both thick and thin portions can be easily welded together.

3.  High temperature properties - The austenitic alloys of stainless steel that contain nickel have much stronger high-temperature strength than other alloys, especially when it comes to their capacity to withstand creep, or the tendency to move slowly or permanently deform under mechanical forces. Additionally, these alloys are far less likely to develop destructive brittle phases when subjected to temperatures higher than 300 degrees Celsius.  During thermal cycling, nickel decreases spalling and stabilises the protective oxide covering. For applications requiring high temperatures and fire resistance, austenitic alloys are used.

4.  Corrosion resistance - The chromium-rich oxide layer is primarily responsible for stainless steels' resistance to corrosion. However, this layer is prone to damage, especially when chlorides are present, and such damage can trigger localised corrosion such pitting and crevice corrosion. In the presence of chlorides, molybdenum and nitrogen both increase resistance to pit initiation. Although nickel does not influence this in the initial phase, but it is crucial in reducing the rate at which both crevice corrosion and pitting spread. Additionally, nickel affects how resistant stainless steel is to chloride stress-corrosion cracking, a different type of localised corrosion. However, there is minimal resistance in these circumstances at nickel concentrations of about 8%. At nickel levels, both lower and higher than this, stress corrosion-cracking resistance increases noticeably.

5. Lustre and Finish - All stainless steel grades appear similar at first glance. Comparing identically polished surface finishes side by side does reveal variations in colour and lustre, though. The 200-series grades typically appear darker and the ferritic grades cooler-looking than the nickel austenitic grades; nonetheless, appearance and aesthetic attributes will always be a matter of preference. Although customers prefer a brighter, whiter metal, the popularity of the 300-series is evidence that there are some architectural applications where a greyer colour may be desirable. Due to their innate work-hardening qualities, the 200- and 300-series stainless steels are also more scratch-resistant.

It is clear that the common nickel-containing austenitic grades are excellent all-around performers.  They are easily available, well-known, adaptable, and simple to use. Additionally, they work well and are extensively recyclable. All of this means that they frequently offer the most useable, low-risk solution.