Engineering with fine-grained structural steels

As materials with high resistance to brittle fracture, fine-grained structural steels are very important for high-grade weldments in steel construction, mechanical engineering and the fabrication of apparatus and vessels. Besides good weldability they are distinguished by good toughness behaviour in the residual welding stress range. Through fixing the annealing limit at 30 mm initially (today 38 or 50 mm as the case may be) and dimensioning according to the yield strength, development during the course of recent years has led to the high-strength hardened and tempered grades. Finally the gap between 400 and 500 N/nm² yield strength was closed by the microalloyed steels, some of them air-hardening.Today a large number of grades are available also for hot or cold-working components, throughout the strength range. In view of their various merits, the designer does not always find it easy to make the correct choice. Despite extensive literature, setbacks are repeatedly sustained. Often insufficient allowance is made for the properties of the material in the design, fabrication, inspection and trials. There have been cases of the wrong grade being selected for the conditions in question.The following article is therefore intended to provide the designer with some additional fundamentals for dimensioning, choice of material, fabrication and testing.