Stainless Steel: Its Types And Its Properties That Make It So Useful

Stainless steel is highly desirable material for many many uses, it is based on iron, with the addition of some special alloying elements which stop it rusting. Although iron and steel have been in use for hundreds of years, stainless steel has become used more and more widely in the last hundred years or so since its invention in the early 1900's.

Stainless steel does carry a premium cost compared to 'regular' steel, especially in recent months and years. At the time of writing (spring 2022) we have suffered in recent times from the Covid-19 pandemic, severe disruption to established manufacturing centres in China, escalation of shipping costs, scarcity of some of the alloying elements used in stainless steel, and massive energy price rises, in part due to the recent invasion of Ukraine by Russia. All of these factors have had the effect of a supply chain instability and high volatility (generally meaning in the upward only direction) of the price of stainless steel raw material and stainless steel products. Despite all of these factors stainless steel in its various grades remain the 'go-to' material for many sectors

Graph Showing Normalised Pricing Of Stainless Steel From 2020 To 2022
stainless steel price history though 2020 and 2021

The largest producer of stainless steel today is China, and at Architectural Stainless Fittings the majority of the stainless steel products we provide originate in China (China has some of the words best capability in the process of investment casting of stainless steel, particularly 316 stainless steel), though many of our products are produced in Germany and other parts of the EU.
Some of the key properties of stainless steel that make it attractive for architectural use are as follows:

⦁ High resistance to rust
⦁ Attractive appearance
⦁ Availability and formability in many different forms (sheet, tube, cast, forged, machined)
⦁ Long term durability
⦁ Suitability for welding
⦁ High strength

Grades Of Stainless Steel
In common parlance we often hear of the grades A2 or A4, otherwise known as 304 or 316 grades of stainless steel, with the A4 / 316 grades being understood to be more rust-proof than the A2 / 304 grades. If we look a little deeper we find that there are some over-arching categories of stainless steel comprising:

⦁ Austenitic Stainless Steels
⦁ Ferritic Stainless Steels
⦁ Martensitic & Precipitation Hardening (PH) Stainless Steels
⦁ Duplex Stainless Steels

We will outline the key points about these categories of stainless steel as follows.

Austenitic Stainless Steels
Within the types of austenitic stainless steel we have the grades 316 and 304. 316 is often referred to as Marine Grade, as it has enhanced resistance to corrosion compared to 304, as many feel necessary for use around salt-water.
In practice we could think of the generalised guide that 304 is fine for most exterior uses for a long-lasting rust-proof appearance and durability, but for marine applications the 316 grade is desired. In practice there is some overlap between the uses of the two grades, and often it is down the the customer's or end-user's perception of what level of 'staining' (surface rust) is acceptable in use. The main alloying elements in austenitic stainless steel are Silicone, Manganese, Phosphorous, Chromium and Nickel for 304 stainless steel, with the further addition of Molybdenum for 316 stainless steel. Also of some importance is the carbon content in these various steel grades, there are two related grades known as 304L stainless steel and 316L stainless steel which have reduced carbon content, which leads to an improved resistance to corrosion where welding is involved. At Architectural Stainless Fittings our polished stainless tubing is made from Grade 316L (incorporating a welded seam along the length of the tube), but which polished out after welding to effectively make it invisible.

The chemical composition of the grades of stainless steel we have mentioned is shown in the table below:

composition of 304 and 316 stainless steel grades

Ferritic Stainless Steels
The broad term stainless steel defines a family of corrosion resistant steels which have a minimum of 10.5% chromium. For ferritic stainless steels the main alloying element is chromium, and compared with the austenitic stainless steels the absence of nickel leads to a lower cost. The crystal structure of ferritic stainless steels is body-centred-cubic (in common with pure iron). Compared to austenitic stainless steels, the ferritic stainless steels have some main disadvantages as follows:

⦁ Limited toughness
⦁ Reduced formability
⦁ Poor toughness when welded

Martensitic & Precipitation Hardening (PH) Stainless Steels
Martensitic stainless steels have a body-centred-tetragonal crystal structure, and in their annealed state they have a yield strength of around 275 MPa. They can have varying grades of carbon content, which allow heat treatment to achieve higher strengths, but generally at the expense of reduced ductility and toughness. Special martensitic grades of stainless steel have been developed with the addition of nitrogen and nickel, and lower carbon content which give rise to improved toughness and corrosion resistance as well as improved weldability.

One example grade of a martensitic and precipitation hardening stainless steel is 17/4PH which depending on the level of heat treatment can achieve yield strengths of as high as 1415 MPa.

Duplex Stainless Steels
Duplex stainless steels offer great advantages in terms of higher strength, greater corrosion resistance and lower price, and although their invention goes back to the early 1900's, it is only in recent years that their viability and adoption has risen, thanks to advances in production methods. We have already discussed about the Austenitic and Ferritic types of stainless steel which are made up of mainly one 'phase' of steel - austenite or ferrite. The idea of duplex stainless steel is to have approximately equal amounts of austenitic and ferrite phases giving rise to the following:

⦁ Higher strength compared to Austenitic and Ferritic
⦁ Good weldability, even in thick sections
⦁ High toughness
⦁ Good resistance to corrosion cracking

The products found on the Architectural Stainless Fittings website are generally made from one of the following grades of stainless steel:

316 stainless (sometimes called A4 stainless)

304 stainless (sometimes called A2 stainless)