How can you harden workpieces made of structural steel?

In some Do-It-Yourself projects, one often comes up with the embarrassment that the structural steel used has too little hardness for the intended application. Whether and how you can harden structural steel itself, what options are available, and what conditions are necessary for this, you will learn in this article.

Properties of structural steel

Between "mild steel" and stainless steel is no longer meaningful today, since such a declined classification is no longer made today. On the other hand, the respective steel grade is usually given today to describe exact characteristics. However, a certain orientation in the steel grade still offers the leading "S" for the steel designation according to EN 10025. It stands for "structural" - ie a mass steel for steel construction, which is unalloyed.

Structural steel group

As steels for structural steel almost always so-called basic steels are used. Typical grades used for steel construction include:

  • S235JR + AR
  • S355J2 + N

Possibilities for hardening mild steel

In principle, the hardening of structural steel is quite difficult because the carbon content of structural steels is very low (usually less than 0.2%). This means that very little martensite is formed in classical (and simple) hardening processes.

The classical way of heating and then cooling in ice water will bring little success in most of structural steel workpieces. In addition, only the surface layer of the workpiece can be hardened (because of the low carbon content), and the core of the workpiece remains soft and tough even after hardening. In general, however, this is also desirable if you subsequently harden in these cases.

Carburizing and case hardening

A good option is the so-called carburizing. The carburizing process can be carried out in different ways, common are:

  • solid carburizing
  • liquid carburizing
  • gaseous carburizing
  • Carburizing under low pressure

The easiest way is carburizing with carbon powder (solid carburizing). For this purpose, filled with so-called coal powder boxes are used. The temperature during carburizing is approx. 930° C. This is followed, however, usually by further hardening steps - namely the actual hardening and tempering of the steel.

Carburizing merely serves to bring more carbon into the outer layer so that a martensite layer can form there. Hardening takes place simply by quenching (for example in water, hardening oil or molten salt). Tempering the steel (heating to just below the transformation point) relieves internal stresses in the steel, making it even more resistant.

Tips & Tricks

In most cases, you will not be able to carry out such a hardening process yourself if you do not have the necessary equipment (food, coal granules, etc.). As a rule, contract hardening shops usually undertake such work on small parts. Incidentally, gears are hardened the same way to make them more resistant.

Video Board: Rotors Induction hardening machine - Impianto di tempra rotori