We give you some data about the samples next to the exhibit’s microscope.
Ferrite. It is almost pure iron with silicon and phosphorus impurities. It is the essential component of steel and is the softest, most ductile, malleable and magnetic of steels.
Perlite. Compound formed by alternative sheets of ferrite and cementite. Composed of 88% ferrite and 12% cementite, it contains 0.8% carbon. More challenging and stronger than ferrite but softer and more malleable than cementite. Its name is due to the irisations it acquires when illuminated, similar to pearls.
Ledeburite: It is not a component of steels but of foundries. It is formed by cooling a liquid carbon melt that is stable up to 723°C, decomposing from this temperature into ferrite and cementite.
Austenite. It is the densest constituent of steel and is formed by a solid solution of iron carbide, which is ductile, tenacious, soft and resistant to wear. It’s not magnetic. It has excellent plasticity and is easy to work with (forging, stamping, etc.).
Cement. It is the hardest component of steels with hardness greater than 60HRC with very crystallized molecules and, therefore very fragile. It is magnetic up to 210ºC, in which it loses this property.
Martensite. It is a metastable phase that does not appear in the diagram. It is the most critical component produced by heat treatments designed to produce ideal mechanical properties. When tempered, it is the constituent of steels obtained by quenching steels from their austenitic state at high temperatures. It is magnetic and cementite is the hardest component of steel. It comes in the form of needles. It’s magnetic. Slatted martensite formation has low carbon content, produces higher toughness and higher ductility but lower strength, and, in plate, has high carbon content and produces much higher strength but can be quite brittle and not ductile.
Widmanstätten: It’s meteorite iron. What if the word siderurgia (iron and steel industry) comes from sidereal, “coming from the stars”? Approximately 90% of the composition of the core of our planet is iron which is kept in a liquid state by its very high temperature. Earth’s geomagnetism, which determines the magnetic poles, is generated by those deposits of liquid iron that are inside the Earth.
Let’s talk about meteorites. The fall of meteorites has been interpreted as divine messages by many cultures since prehistoric times, and in some cultures, they are still worshipped as celestial bodies. Stone meteorites have been used to carve hunting weapons and domestic instruments, with a hardness that is almost indestructible.
Siderites, or iron-containing metal meteorites, are derived from the cores of ancient planets that were destroyed about 3.9 billion years ago by catastrophic impact during the formation of our Solar System. They have a pattern of crossed lines called Widmanstätten lines. Octahedrites are the most common crystallization of iron meteorites, which are composed primarily of various alloys of iron and nickel, among them kamacite (α-Fe-Ni), also known as ferrite, it is body-centred cubic iron with <6% by weight nickel, and taenite (γ-Fe-Ni) also called austenite, face-centred cubic iron with >25% by weight nickel.
Iron meteorites are classified into eight subgroups based on metal structure, an intergrowth of two Fe metals with Ni. It was once believed that in order for Fe and Ni to form these beautiful crystalline patterns, the rate of cooling had to be very slow, incredibly slow, let’s say, between 1°C and 10°C per million years. Today, there is research in metallography that has achieved these patterns synthetically in the laboratory. So, in the future, separate lines of evidence should be sought to support cooling rate data.
