what is 'heavy-duty steel', please?

Answer 1

It's just an advertising blurb, intended to mean, "We used thick steel for this, instead of cheap thin steel! Our product is better!"

There's no term like "heavy-duty steel" in structural or mechanical engineering. There's lots of different grades and alloys of steel, but "heavy-duty" steel isn't one of them.

Answer 2

Steel is an alloy whose major component is iron, with carbon content between 0.02% and 1.7% by weight, depending on grade. Carbon is the most cost effective alloying material for iron, but many other alloying elements are also used.[1] Carbon and other elements act as a hardening agent, preventing dislocations in the iron atom crystal lattice from sliding past one another. Varying the amount of alloying elements and their distribution in the steel controls qualities such as the hardness, elasticity, ductility, and tensile strength of the resulting steel. Steel with increased carbon content can be made harder and stronger than iron, but is also more brittle. The maximum solubility of carbon in iron is 1.7% by weight, occurring at 1130 degrees Celsius; higher concentrations of carbon or lower temperatures will produce cementite which will reduce the material's strength. Alloys with higher carbon content than this are known as cast iron because of their lower melting point.[1] Steel is also to be distinguished from wrought iron with little or no carbon, usually less than 0.035%. It is common today to talk about 'the iron and steel industry' as if it were a single thing; it is today, but historically they were separate products.

Currently there are several classes of steels in which carbon is replaced with other alloying materials, and carbon, if present, is undesired. A more recent definition is that steels are iron-based alloys that can be plastically formed (pounded, rolled, etc.).

Iron alloy phases
Austenite (γ-iron; hard)
Bainite
Martensite
Cementite (iron carbide; Fe3C)
Ledeburite (ferrite - cementite eutectic, 4.3% carbon)
Ferrite (α-iron; soft)
Pearlite (88% ferrite, 12% cementite)


Types of Steel

Plain-carbon steel (up to 2.1% carbon)
Stainless steel (alloy with chromium)
HSLA steel (high strength low alloy)
Tool steel (very hard; heat-treated)


Other Iron-based materials

Cast iron (>2.1% carbon)
Wrought iron (almost no carbon)
Ductile iron