Iron–nickel based ultra-high strength steel (wt. 18~20% Ni) is characterized by its high strength and low thermal conductivity, and is normally used to make key components by forming and machining processes. The optimization of these processes is based on a deep understanding of the mechanical and dynamic behaviors under high strain, high strain rate, and high temperature. In this paper, the relationship of stress to strain, strain rate, and temperature is systematically investigated by the dynamic compression tests combined with quasi-static compression tests, and the hardening and softening is associated with the transformation in microstructures. According to the analysis, dynamic recrystallization around 600 °C is assumed to be one important influencing factor, hence hot deformation equations are established and the critical strain for dynamic recrystallization and the volume fraction of the dynamic recrystallized grains are defined.

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Dynamic Behaviors and Microstructure Evolution of Iron–Nickel Based Ultra-High Strength Steel by SHPB Testing

This chapter introduces maraging steels and then gives extended background information on the topics of each subsequent chapter. Because of the specialised nature of this research monograph, the first part is relatively brief, assuming that the reader will already have good knowledge of physical metallurgy. The main part of the chapter discusses past literature and poses research questions that are intended to raise the reader's interest in studying the detailed text of subsequent chapters. For this reason, the sections are arranged roughly in the order of the subsequent chapter subjects. The reader should gain an overview of the contents of the book through this chapter.

Introduction to maraging steels

18Ni maraging steel is normally used to make key components by machining processes. It is necessary to understand the fracture mechanism and behaviour of 18Ni maraging steel for the study and furth...

Fracture mechanism and model of 18Ni maraging steel

Maraging steel is a prominent category of ultrahigh-strength steel (UHSS) characterized by excellent comprehensive properties, and it finds wide applications in manufacturing load-bearing structural components. In this study, a novel tungsten-containing maraging steel, C-250W, was designed. The effects of aging treatments on the mechanical properties, microstructure, precipitations, and reverted austenite of C-250W steel were investigated. The results revealed that the optimal combination of strength and toughness could be achieved through an aging treatment of C-250W steel carried out for 5 h at 480 °C after solution treatment at 1000 °C for 1 h. As the aging temperature increased, the proportion of dimples in the impact fracture gradually decreased while that of quasi-cleavage increased, leading to a reduction in Charpy impact energy. The boundary of martensitic lath decomposed gradually as the aging temperature increased, and it disappeared entirely at temperatures higher than 550 °C. Moreover, the aging process resulted in the formation of phases, including spherical Fe2M (M represents Mo, W) and thin strip-shaped Ni3N (N represents Mo, Ti) precipitates. These precipitates coarsened from 5 nm to 50–200 nm with increasing aging temperature. Additionally, the content of reverted austenite increased with the aging temperature. Within the temperature range of 400 °C to 500 °C for aging treatment, the content of film-shaped reverted austenite was approximately 3%, primarily distributed at the boundary of martensite lath. When the aging temperature exceeded 550 °C, the content of reverted austenite reached 20.2%, and its morphology changed from film-shaped to block-shaped, resulting in a decline in strength and toughness.

Effect of Aging Treatment on the Microstructure and Properties of 2.2 GPa Tungsten-Containing Maraging Steel

The influence of heat treatment on the microstructure and mechanical properties of a 2000 MPa grade Co-free 18Ni maraging steel has been investigated, and the strengthening and toughening mechanisms were discussed. The experimental results show that the hardness of the steel in solution condition is independent of both the solution temperature and the time. A combination of high strength and good toughness can be reached under the peak-aged condition, where the ultimate strength is over 2100 MPa, and the elongation and KIC are 9% and 70 MPa· m1/2, respectively. The TEM observation revealed that a great deal of nanometer size precipitates, such as Ni3(Mo, Ti), is distributed in a high density dislocation matrix of the steel treated with peak-aged condition, which realizes a better match of strength and toughness. The precipitation strengthening in the present steel obeys the modified Orowan mechanism.

MECHANICAL PROPERTIES OF ULTRA-HIGH-STRENGTH 18Ni COBALT-FREE MARAGING STEEL

The main damaging elements in 18Ni(350) maraging steel werer removed to exceeding low level (10-5) by ultra-purifying refining process and the influences of ultra-purifying and a little change of some main elements on the strength and fracture toughness of marging steel were examined. It was found that the toughness can be improved up to 61 MPa.m1/2, 10%-30% times higher than that of normal maraging steel with the same strength grade, on the other hand the strength can also be kept at normal grade even though titanium was cut down. It was also found that the category and volume of inclusions are greatly reduced in the ultra-purifying steel, and most inclusions may contain titanium and nitrogen. The improvement of thoughness and the maintenance of strength owe to decreasing the inclusions and increasing the availability of main alloy elements.

STUDY OF ULTRA-PURIFIED 18Ni(350) MARAGING STEEL

Tempering brittleness is an important reason for microcrack formation and cracking in thick wall tube made by steel 1Cr13 which is applied in reactor driving mechanism.This article analysed the quenching effect caused by hot rolling followed by air cooling,tempering brittleness caused by annealing followed by slow cooling,and influence of rolling on the cracking direction.Improper heat treatment condition derives martensite transformation and tempering brittleness,which make the steel cracking when rapidly cold rolling.Tangential tensile stress caused by squashing the tube during rolling is the reason for the formation of longitudinal crack.

SU Guoyue

Papers

MECHANICAL PROPERTIES OF ULTRA-HIGH-STRENGTH 18Ni COBALT-FREE MARAGING STEEL

STUDY OF ULTRA-PURIFIED 18Ni(350) MARAGING STEEL

ANALYSIS OF CRACK FORMATION AT INNER SURFACE OF 1Cr13 THICK WALL TUBE APPLIED IN NUCLEAR REACTOR