Heat treatment and hardening of steel

Steel properties are not fixed — they can be changed by controlling how hot the material gets and how quickly it cools. This is the basis for both hardening of tools and for understanding what happens in a weld seam where the steel is in practice heat-treated uncontrollably by the arc's heat.

§Why heat changes steel

When steel is heated above a certain temperature, its internal structure changes. How the structure looks afterward depends on cooling: fast cooling 'freezes' a hard but brittle structure, while slow cooling gives a softer and tougher material. It's this interplay between temperature and cooling that all heat treatment is built on.

§Hardening and tempering

With hardening the steel is heated and cooled quickly — for example in water or oil. The result is very hard but also brittle and stressed. Therefore hardening is almost always followed by tempering where the material is heated again to a lower temperature to remove some of the brittleness and stresses for a small loss of hardness. Together they give a hard and at the same time usable tool steel.

§Annealing and normalising

• 01Annealing: makes steel softer and easier to work
• 02Normalization: standardizes the structure and provides a predictable starting point
• 03Stress relief anneal: reduces internal stresses after e.g. welding or machining

§The heat-affected zone in welding

With welding the seam itself melts but right next to it is a zone that becomes heat-treated without melting — the heat-affected zone (HAZ). Here the steel's structure is changed by the welding heat and it is precisely here that many cracks and soft or brittle points arise. Understanding heat treatment helps the smith understand why rapid cooling or wrong welding sequence can weaken a joint.

§Preheating

When welding thick or hardenable steel the workpiece is often preheated so cooling after welding is slower. This reduces the risk of hard brittle zones and cracks. Whether preheating should be done and to what temperature is set out in the welding procedure — it is not a free choice.

§PLACEHOLDER_102

The classic blacksmith reads the temperature of the iron's glow colour and it is still a valuable skill. But for controlled heat treatment, sight alone is not enough — here temperature measurement and fixed times are used because a few degrees difference can determine whether the result is usable or not.