Features of Phase Transformations in Martensitic Class Steel for Oil Grade High-Strength Corrosion-Resistant Pipes

The effect of chemical composition on features of phase transformation in martensitic stainless steels based on 13 wt.% chromium containing 0.04–0.1 wt.% carbon additionally alloyed with nickel (2.0– 5.2 wt.%) and molybdenum (0–1.20 wt.%) is studied by calculation (using a Thermo-Calc program) and experimental approaches. The effect of nickel Ni equ and chromium Cr equ equivalents for test steel chemical compositions on types of crystallization (peritectic or single-phase mechanism with δ-ferrite formation), phase transformation temperatures, and regions of different phases (δ-ferrite, γ -austenite, α -ferrite) is identified. An increase in ferrite-forming element concentration over values of the chromium equivalent up to ≥ 16 wt.% or more leads to steel transition into the martensitic-ferritic class. An increase in the content of austenite-forming elements, primarily nickel, reduces the lower boundary of the temperature range for inverse α → γ transformation and leads to formation of stabilized austenite within the microstructure. Results of modeling phase transformation are compared with the microstructure and phase composition of two grades of industrial steel. It is established that chemical inhomogeneity of the two-phase (δ + γ )-structure formed during crystallization is retained at room temperature. Heat treatment regimes connected with heating in the lower half of the two-phase (α + γ )-region are determined when stabilized austenite is fixed in the steel structure at room temperature affecting mechanical properties. Research facilitates development of new steel compositions at TMK Group plants for the producing seamless casing and pump-compressor pipes of P110 type 13Cr strength group, resistant to carbon dioxide corrosion, including operation in cold macroclimatic conditions.