Steels are the most widely used construction materials. When building bridges, buildings and many other building structures steel must be welded. The structural strength of a steel structure does not only depend on the strength of the steel, but also on the strength of the welds. That's why weldability of steel is always a very important issue.
Effect of carbon content on weldability of steel
Many mild steels are easy to weld. Welding medium and high carbon steels is more challenging, so during welding the heat affected zone of welding can form martensite, which will lead to a significant reduction in the toughness of the weld.
Various measures are taken to improve the weldability of steels., such as heating the material or minimizing the absorption of hydrogen by the steel. The absorption of hydrogen by steel makes the steel more brittle..
Weldability of low carbon steels
In low carbon steels, the strength of the welded portions is higher, than the base metal. This is due to the fact, that when cooling the heat-affected zone of welding, a finely dispersed pearlite structure is formed in it. Besides, retained austenite along the pearlite grain boundaries inhibits crystallization and therefore contributes to the retention of fine grains, which also contributes to increasing the strength of the welded area.
Transformations of steel in the weld zone
During welding, the steel near the weld is heated above the critical temperature A1 and austenite is formed (figure a). Upon cooling, austenite in this heated zone transforms into a new structure, the type of which depends on the cooling rate and the diagram of the thermokinetic transformation of steel.
Ordinary low carbon steel has such a low hardenability, that at normal cooling rates in air, martensite is almost never formed (figure b).
Alloyed steel is specially heated before welding., to reduce the cooling rate of the weld or subject the welded joint to additional heat treatment to temper the formed martensite (drawing in).
Figure - Transformations of steel in the heat-affected zone of welding:
and) structure of steel at maximum heating temperature in the welding zone;
b) structure of steel with low hardenability in the weld zone after cooling;
in) structure of steel with high hardenability in the weld zone after cooling.
Weldability of hardened steel
Weldability of steel, which was hardened and tempered before welding, has two kinds of problems. First of all, section of heat-affected zone of a weld, which heats up above temperature A1, can form martensite upon cooling. Secondly, section of heat-affected zone of a weld, which has warmed up below temperature A1, may be subject to excessive leave. In a good way, hardened and tempered steel must not be welded.