About contributions of the strain $\gamma \rightarrow \alpha$-transformation and plasticity of main phases into fracture toughness of maraging steel N18K9M5T

A. A. Sukhikh

Institute of Mechanics, Ural Branch of the Russian Academy of Sciences, Izhevsk, Russia

Abstract: The increase of toughness characteristics of maraging steel N18К9М5Т can be achieved by stabilizing of austenite in the martensitic structure. One of the ways of obtaining martensitic-austenitic structure of the steel is a heat treatment, comprising exposure in the inter-critical temperature interval of $A_s - A_f$, quenching with high-speed heating and with a short exposure from temperatures above $A_f$ and aging.
The growth of toughness characteristics of the steel depends on the amount, morphology and properties of the stabilized $\gamma$-phase (mainly its ductility and ability to $\gamma \rightarrow \alpha$-transformation during plastic deformation). Properties and the structure of $\gamma$-phase are determined by the type of austenite and conditions of its formation. Austenite is divided into retained ($\gamma_{ret}$), reverted I ($\gamma_{revI}$) and duplex: retained + reverted II ($\gamma_{revII}$). Reverted austenite I is the $\gamma$-phase that is formed in the martensitic structure under heating to temperatures of inter-critical interval $A_s - A_f$ and persists after cooling to room temperature; retained austenite is $\gamma$-phase that continues after cooling from temperatures above $A_f$. Reverted austenite I is formed within $\alpha$-martensite crystals as well on boundaries of $\alpha$-martensite packets and crystals. Retained $\gamma$-phase in the original structure «$\alpha$-martensite + reverted austenite I» is stored in former volumes of the reverted austenite I. Reverted austenite II is named $\gamma$-phase, that is formed in the structure «$\alpha$-martensite + retained austenite» at the aging temperatures by the way of epitaxial growth on the retained austenite.
This paper presents the mechanical properties of steel N18K9M5T for four types of structural states: $\alpha$-martensite, $\alpha$-martensite + reverted austenite I, $\alpha$-martensite + retained austenite, $\alpha$-martensite + retained austenite + reverted austenite II. It is shown that the two-phase ($\alpha + \gamma$)-structure provides the most significant increase of ductility and toughness characteristics of steel in the presence of strain $\gamma \rightarrow \alpha$-transformation.
According to the results of tests on static crack resistance by means of calculation the estimation of fracture toughness growth of steel N18K9M5T with two-phase ($\alpha + \gamma$)-structure at the expense of strain $\gamma \rightarrow \alpha$-transformation of retained and reverted austenite has been done. Calculations show that the strain $\gamma \rightarrow \alpha$-transformation actually significantly increases the toughness of the steel N18K9M5T. Under considered thermostrengthening modes, the contents of $\gamma$-phase and the accepted assumptions, the fracture toughness growth was 10-30 % of its absolute values.
Additive contributions into the fracture toughness of the steel of martensite and stabilized austenite, caused by their plasticity, are determined. It is found that the contribution of plasticity of retained austenite and duplex $\gamma$-phase in the fracture toughness of N18K9M5T steel is significantly higher than the reverted austenite I, and the contribution increases with the growth of steel strength. The contribution of plasticity of reverted austenite I decreases for two-phase ($\alpha + \gamma_{revI}$)-structure with increasing of the steel strength.

Keywords: maraging steel, structure, fracture toughness, plasticity, martensite, austenite, retained, reverted, strain $\gamma \rightarrow \alpha$-transformation.

UDC: 669.15-194.55/56:539.4.015