TECHNOLOGIES FOR PRODUCING CRYOGENIC STEEL: FROM MELTING TO HEAT TREATMENT

Abstract

The article presents a systematic review of modern technological processes for the production of cryogenic steels – specialized metallic materials that retain high impact toughness and ductility at ultra-low temperatures down to –269 °C. These alloys are critically important for the construction of storage tanks, transportation pipelines, and transshipment systems for liquefied natural gas (LNG), nitrogen, hydrogen, and other process media. The classification of cryogenic steels by operating temperature is considered: ferritic nickel steels (3.5 % Ni, 5 % Ni, 9 % Ni), austenitic chromium–nickel alloys (type 18–10 and more highly alloyed grades), and cost-effective nitrogen-containing chromium–manganese analogues that partially or completely replace expensive nickel. Particular attention is paid to metallurgical cleanliness: the sulfur and phosphorus content is strictly regulated (less than 0.005–0.010 %), and deep deoxidation with aluminum and calcium, along with synthetic slag treatment, is employed to remove non-metallic inclusions that act as stress concentrators at low temperatures. The stages of hot deformation (rolling in a controlled temperature range) and their effect on the initial grain are described in detail. The analysis of heat treatment as the key technological stage for developing low-temperature toughness occupies a central place in the review. The modes of single quenching and tempering, which lead to undesirable precipitation of carbides along grain boundaries and intergranular fracture, are compared with the advanced technology of double quenching (high-temperature and intercritical), which produces a fine duplex structure consisting of tempered martensite and stable retained austenite (~4 %) and ensures ductile fracture at cryogenic temperatures. Deep cryogenic treatment (DCT) at –196 °C is discussed for the complete transformation of retained austenite and the precipitation of ultrafine carbides. Technological aspects of welding are addressed: the problem of hot cracking in austenitic welds, the requirement to avoid δ-ferrite (brittle at low temperatures), and the potential of laser welding. The directions of technology development are outlined: the development of new high-manganese alloys, optimization of multi-stage heat treatment with computer modeling, import substitution, and digital materials science for predicting the ductile-to-brittle transition.