Development of ultrahigh strength austenitic stainless steels alloyed with nitrogen

It is weIl established that nitrogen in austenitic stainless steels causes a remarkable high strengthening by solid solution as weIl as an increasing in corrosion resistance.For these reasons nitrogen as an aIloying element has been successfully used to develop a number of commercial steels in which it stabilizes the austenitic phase (High Nitrogen Austenitic stainless Steels -HNASS).Presently these steels are manufactured by ingot metallurgy carried out under high nitrogen partial pressure in order to achieve sufficient high nitrogen solubility, or by powder metallurgy.Both production routes are "expensive" and quite "exotic", and this explains why in spite of a good knowledge on favourable effects by nitrogen aIloying in austenitic stainless steels, high nitrogen austenitic stainless steels have not been extensively used and adopted in large quantities yet.This thesis describes the development of new Ultrahigh Strength High Nitrogen Austenitic Stainless Steels with very low nickel content, designed to be made by "ingot metallurgy without overpressure", i.e. using conventional steel making facilities.These new steels are interesting candidates for structural application in car industry, building industry and ocean engineering, due to high mechanical strength, high resistance to corrosion and low cost of production.By means of laboratory experiments combined with thermodynamic calculations suitable alloy compositions were designed in order to increase the nitrogen solubility at atmospheric pressure in the liquid and solid phase, avoiding porosity formation into cast ingots during solidification.On these new alloys the effect of:• solid solution hardening; • grain boundary hardening;• dislocation hardening (during cold working); • dislocation hardening (during hot working); • precipitation hardening (strain ageing); on yield strength, ultimate tensile strength and elongation to fracture were investigated in a systematic way.A high level of strength was achieved combining different strengthening mechanisms following the rule: nitrogen in solid solution as high as possible, grain size as small as possible and cold deformation as high as possible.Solid solution hardening was optimised choosing "the right" amount of carbon and nitrogen taking in account that high nitrogen austenitic stainless steel undergoes to a ductile to brittle transition at room temperature as the sum of nitrogen and carbon content in solid solution approaches one weight percent.The dependence of the ductile to brittle transition upon aIloying elements and grain size was explored.The introduction of strong nitride former elements, mainly vanadium and niobium, able to form nitrides more stable than those based on chromium and molybdenum, was investigated in order to achieve small grains in bulk materials.The role of nitrogen on dynamic recrystallization and dislocation hardening during hot rolling was investigated.Nitrogen in solid solution was found to have profound influence on the temperature of dynamic recrystallization and hence on the thermo-mechanical processes such as hot rolling, where an improved hot workability of the austenite becomes the first target to achieved.