Metallurgy

Metallurgy
Since the birth of the motor car, front axle beams have been made from Forged Steel. Steel is Iron that has had the Carbon content reduced to about 0.5%, and Forging is a hammering process which creates a grain structure aligned with the forces in the component. A Forged steel component is “TOUGH” and “DUCTILE” which means it will bend but not break easily.

Cast iron is the poorest grade of Iron used, and has about 5% of included Carbon. The Carbon lies around in pretty flakes, along with perlite and magnetite, which are Carbon Iron alloys. Cast Iron is weak, and brittle, not suitable for safety related applications.

During the Industrial revolution, many disasters resulted from failure of Cast Iron components. In the years before Steel making was practiced on an industrial scale, the material of choice was ‘Wrought Iron’ in which the Iron is beaten out into long shapes, so that the 5% Carbon is stretched out into long streaks, interspersed with streaks of clear Iron.
Look up ‘Iron Age Man’ to see how these technologies were evolved thousands of years ago, to make better swords and ploughshares.

Steel results from a smelting process, such as the Bessemer Converter, in which most of the excess carbon is burnt out by blasting Oxygen through the melt, leaving a material with around 0.5% carbon. This makes possible the manufacture of a wide range of steels with consistent high strengths
Steel is usually supplied in rolled products, and can truly be called the material on which our modern civilization is built. Steel can also be cast, to create a relatively high strength products with toughness and ductility, many cast steel components are used in motor vehicles. The ultimate material is of forged steel, which has a grain structure aligned to the stress paths in the component, which is why vehicle beam axles are invariably made from forged steel.

The Ford Model T was at first ridiculed for being flimsy and critics said its axles would be sure to bend or break
What they didn’t know was that the axles, and many other parts were made from FORGED VANADIUM STEEL with a yeild strength probably around 500 Mpa.
As the Model T gained a reputation for being practically unbreakable, the critics had to eat their words.

SG or Nodular Cast Iron An improved grade of cast Iron was introduced in the late 1950’s termed “Spheroidal graphite” Iron, or “Nodular Iron”. In this material, the Carbon, perlite and magnetite remain, but the free carbon is made to form round blobs. This greatly increases the strength and even creates some ductility compared to “Grey” or “Flake” cast Iron. The most common use of this material is for Sewer Pipes, Drain covers. In motor vehicles use is usually confined to Engine blocks and Gearbox cases, although it has been used for crankshafts for compressors and low powered, low cost engines.
There is not just one type of SG or Nodular iron, and the manufacturing process can create a wide range of properties and characteristics The metallurgy of SG Iron is tricky, and careful Quality control is always required.

Just examining the material with a microscope, in one spot, may offer some clues about the structure AT THAT SPOT, to an experienced Metallurgist or NDT technician.
Doing what LVVTA have done- promoting a motor mechanic to a metallurgist by hiring him a microscope is a case of a little knowlege being a dangerous thing.