Superbainite

Structure of superbainite. Inset is a same-scale image of a carbon nanotube. [1]

According to archaeologists, the Iron Age began in 1300 BC and lasted for around two millennia. Today, steels (alloys of iron and carbon) comprise 95% of global metal consumption and this trend shows no sign of declining.

Glancing at the media, however, one would be forgiven for assuming that steel is now a has-been. We are bombarded with stories of novel materials: carbon nanotubes, metallic glasses, graphene, carbon fibre, nickel superalloys. . . all of which are “stronger than steel”.

“Now we can construct space elevators!” claim the articles. “Let’s build a climbing frame to the moon! We’ll use this stuff to make everything!”

The observant among us, however, will note that most cars, trains and buildings still don’t feature superalloys, metallic glass or magic nanotubes. Neither are they invisible; nor do they fly; nor do they do any of the other things that journalists tend to ‘predict’.

Instead, steels somehow remain the best — and cheapest — materials for the job. Also, they are stronger than steel. This is because ‘steel’ is a vague construct used by sensationalists, with an unspecified strength guaranteed to be less than that of a novel material. Metallurgists rarely refer to ‘steel’, just as the Inuit have fifty words for snow, not one of which is ‘snow’.

Audi’s audacious aluminium advertising artifice

Audi’s advert for their A6 is really beautifully made…

Suppose you could make metal do anything you wanted,
use it in ways no one thought possible,
at Audi that’s what we do,
the new Audi A6 with Aluminium-hybrid body,
engineered for a lighter touch.

The way the metal forming is done in the advert is really nice, just shaping the parts by hand, just like the clay model can be formed when producing models of the car.

The technology is interesting, and challenging, a combination of aluminium and steel parts are used to make the car body. About 20% of aluminium by weight of the car body is aluminium, that means about 40% by volume. Non-load bearing parts such as body panels are aluminium (which benefit from good stiffness/weight ratio). All of the car body is made from cold formed and warm formed steels as in conventional car body. Interesting, aluminium sections seem to be present as side impact bars and bumper. From the advert you might be left with the impression that the whole body is aluminium, or that this is something that would be desirable, especially confusing since ‘hybrid’ is also now commonly used to refer to automobiles which use combinations of different power sources for the engine.

This video shows which parts of the car body are aluminium and steel.

However these cars overall are not much lighter due to the use of aluminium. From the previous model of A6 the weight saving is 30 kg, the weight if the total car is 1575 kg unladen or 2,155 kg gross weight (figures for 4 door 2.0 diesel). I want to look up the weight of Audio A6 since they are first introduced, that’s because in all cars there has been a trends towards increasing weight, despite all the advances in decreasing the weight of the car body.

Why does my Stainless Steel Knife Corrode?

My metallurgist colleague was very disappointed in this knife because she believed that stainless steel wouldn’t corrode. Unfortunately if left in still water oxidation pitting will occur in all stainless steels. My colleague was very surprised to get corrosion in this knife because it is usually hand washed and then just left with a little water on it, similar to other utensils, but after six months it had started to rust. The corrosion may be the result of the relatively high carbon content in the knife, which means it is likely to form carbide particles which act to sensitise the steel.

X50CrMoV15 is a German standard with around 0.5 wt% carbon (percentage by weight) and total around 15 wt% chromium, molybdenum and vanadium. This is regarded as being highly stain resistant. Usually a steel is regarded as stainless it is possible to maintain a chromium level above 12 wt% at the surface, since this has been found to form a stable oxide layer.

C	Si	Mn	P	S	Cr	Mo	V
0.45 – 0.55	max 1	max 1	max 0.04	max 0.015	14 – 15	0.5 – 0.8	0.1 – 0.2

Z-knives have previously found that rusting occurred on X50CrMoV15 as a result of using dishwasher on otherwise unused knife.

`Japanese’ Cooks Knife

New York Times announcement regarding the discovery of stainless steel.

Severe Tempering of Bainite Generated at Low Transformation Temperatures

These are some pictures of the beautiful microstructure of super-bainite. The fine structure of bainite plates and austenite thin films is formed by isothermal transformation at 200°C (or alternatively 473.15 K, or 392 F). With this fine microstructure (or more accurately nanostructure) it is possible to reach very high strength in steel (more than 2 GPa ultimate tensile strength).

Before

Super Bainite Before Tempering (as transformed at 200°C), as seen in transmission electron microscope

Super bainite as transformed at 200°C, as seen in scanning electron microscope.

After

‘Super bainite’ after severe tempering.

This is what happens if you attempt to temper at the highest possible temperature. As expected the microstructure approches equilibrium and coarsens. We found that the temperature calculated using thermodynamic software is not applicable to this heat treatment. Rather than an equilibrium structure of ferrite and carbides as we expected at this temperature, austenite, ferrite and carbide phases were all stable. On cooling, the austenite transformed, usually to pearlite as seen here, but in some cases bainite and martensite were also observed.

Read more: Severe tempering of bainite generated at low transformation temperatures (mathewpeet.org) or Severe tempering of bainite generated at low transformation temperatures (phase transformations)

Steels in car bodies

“Boron Extrication dot com” is a site by a fire-fighter with automotive engineering experience (he’s from Detroit – autocity). There is a large collection of schematics and photographs of car bodies showing use of different materials.

UHSS B Pillars