Fyzikální metalurgie supermartenzitických ocelí

Abstract

Supermartensitic steels represent a cheaper alternative to austenitic and duplex stainless steels used in oil and gas industry, against which they have the advantage of greater strength and acceptable corrosion and stress corrosion cracking resistance. In order to achieve the optimum material properties of these steel, the intercritical annealing at temperatures above Ac1 is performed after cooling from the austenizing temperature, when a certain proportion of stable so-called "reverted" austenite arises in the structure and improves toughness. After 2000, new types of precipitation hardenable martensitic stainless steels were developed with a unique combination of high strength, toughness, fatigue and corrosion resistance was obtained after hardening at temperatures above 500 C, which leads to precipitation of fine particles of minor phases, especially intermetallics. The first part of the thesis is then focused on laboratory simulation of heat treatment of steel 13Cr6Ni2,5MoTi, which involved single-stage or two-stage intercritical annealing and testing of a wide variety of material properties and structural characteristics including intergranular and pitting corrosion resistance. The proposed regime of optimized heat treatment was then successfully applied in practice on the base material and the welded joint of the pipe. In the second part, the effect of long-term aging (1000, 2000 and 3000 hours at 475°C) on the properties, microstructure and substructure of two grades of precipitation hardenable martensitic stainless steels, Custom 465 and MLX 17, was analysed. After aging there was a slight decrease in strength and impact energy, which was the result of the additional precipitation of very fine particles of intermetallic phases and also decomposition of solid solution  to form chromium-rich particles of nanometric size. Aging at 475 °C was in both steels accompanied by stabilization of significant proportion of reverted austenite and its influence completely compensate the effect of the additional precipitation hardening of martensite.