Special Metric Materials specialises in stainless steel tubes and fittings, so ensuring our team has an in-depth knowledge of the metal we sell is very important. Understanding the metallurgy of stainless steel is part of this.
Assessing the metallurgy of stainless steel is a critical part of the overall quality control process in many production environments. The many useful properties of stainless steel – corrosion resistance, superior surface finish etc – mean that the metal is used in many industries such as the transportation, chemical, medical, hygienic and food industries.
This blog reviews what the metallurgy of stainless steel is, some of the main tests and how it relates to the use of stainless steel products in ‘real life’.
What is the metallurgy of stainless steel?
When we are looking at the metallurgy of stainless steel, it is important to first ask ‘What is metallurgy’?
Metallurgy is part of materials science and engineering that involves the study of the physical and chemical behaviour of metals. This includes metallic elements, their compounds and their alloys.
The study of metallurgy combines the science and technology of metals and has been extremely useful for many decades because metallurgy involves scientists examining the microstructure of metal. Understanding this microstructure establishes the mechanical properties of the metal and therefore determines how engineers can predict and exploit its behaviours.
Metallurgy has many different definitions:
- It is the science of making metals and alloys in different forms/shapes, with properties suitable for practical use.
- It is the science and study of the behaviours and properties of metals.
- It is the science that deals with procedures used in extracting metals from their ores, purifying and alloying metals and creating useful objects from metals.
- It is an applied science – it links the science of metals to the ‘real-life’ metals industries.
The history of metallurgy can be dated back many thousands of years, when humans first sought metal for its many beneficial properties. The first evidence of extractive metallurgy can also be dated back many thousands of years when basic forms of smelting were used to extract useful ores.
16th Century German scholar, Georg Agricola, is often called the ‘father of metallurgy’ due to the book he published ‘De Re Metallica’ which describes the processes of mining metal ores, and metal extraction.
In the modern era, metallurgists often work in interdisciplinary teams alongside material scientists and engineers.
The basics of metallurgy of stainless steel and other metals
- Metals are inorganic substances which are composed of one or more metallic elements and may also contain some non-metallic elements such as iron, aluminium and nickel.
- Metal has a crystalline structure with the atoms arranged in an orderly manner. They generally have good electrical and thermal properties, are relatively ductile and strong at room temperature. They are important and useful because of their strength, stiffness and toughness.
- Metals are usually subdivided into two categories – ferrous and non-ferrous. Ferrous metallurgy involves processes and alloys based that contain a high percentage of iron (steel and cast irons),while non-ferrous metallurgy involves processes and alloys based on other metals i.e., aluminium, copper, zinc, titanium and nickel.
- A metal element is a single pure substance that cannot be broken into anything simpler.
- A metal alloy is a special mixture of two or more elements mixed together.
Some of the main metallurgy tests are:
- Grain size measurement
- Investigation of general structure, including the content of martensite, ferrite, perlite or austenite
- Identification of delta ferrite and sigma phases
- Evaluation of carbides and their distribution
- Investigation of welds
Composition of Stainless Steel
The metallurgy of stainless steel is related to the composition and the properties of the metal.
Steel is an alloy of iron and carbon. Stainless steels are steels containing at least 10.5% chromium, less than 1.2% carbon and other alloying elements. Stainless steel’s corrosion resistance and mechanical properties can be further enhanced by adding other elements, such as nickel, molybdenum, titanium, niobium, manganese, etc.
On contact with oxygen, a chromium oxide layer is formed on the surface of the material. This passive layer protects it and has the unique ability to repair itself.
Properties of stainless steel:
- Environmental: Given that it is completely and infinitely recyclable, stainless steel is often considered to be the “green material”. Within the construction sector, its recovery rate is close to 100 and this longevity ensures it meets the needs of sustainable construction projects. Taking its environmental benefits further, stainless steel does not leach compounds that could modify its composition when in contact with elements like water.
- Aesthetically appealing: Stainless steel has a wide variety of surface finishes, including matte, bright, brushed, embossed, tinted and even engraved. Because of this, it is often used by architects for building envelopes, interior design and street furniture.
- Hygienic and easy to maintain: Stainless steel items are easy to clean, and standard cleaning products (such as detergents, soap powders) are sufficient and do not damage the surface. This means that it is often used within industrial kitchens, and food and drink processing plants, and it has many medical applications. Here at Special Metric Materials, we have a wide range of stainless steel tube, fixtures and fittings that can be used extensively within the hygienic and food & drink industries.
- Highly durable: Given it has a minimum chromium content of 10.5%, stainless steel is protected by a passive layer of chromium oxide that forms naturally on the surface through the reaction of the chromium with oxygen from air or water. If the surface is scratched, it regenerates itself. This gives stainless steel metals their characteristic corrosion resistance.
There are five main types of Stainless steel:
- Ferritic: Ferritic steels are frequently used in kitchenware, industrial equipment and automotive applications. This is due to its high chromium level and low carbon content which gives it good ductility as well as resistance to corrosion and stress corrosion cracking.
- Austenitic: Austenitic steels are the most widely used grade of stainless steel and contain high levels of chromium and nickel, but low levels of carbon. They have very good formability and resistance to corrosion.
- Martensitic: Martensitic stainless steels are similar in structure to ferritic stainless steels but, due to added carbon, they can be hardened and strengthened by heat treatment which gives them improved toughness, weldability and corrosion resistance.
- Duplex: Duplex stainless steels are the most recent addition to the stainless steel family and were developed in the 1960s. They are called ‘duplex’ as they have approximately equal proportions of ferritic and austenitic phases. This chemical composition gives it better strength, weight saving, greater corrosion resistance and a better price stability.
- Precipitation Hardening (PH): PH stainless steel is created through the addition of copper, molybdenum, aluminium and/or titanium. It can therefore be heat-treated which generates incredible tensile and yield strengths – much more than other grades of stainless steel.
Thanks for reading this Special Metric Materials blog on the metallurgy of Stainless Steel.
