Effect of Martensitizing Temperature on Creep Strength of Modified 9Cr Steel

Abstract

A modified 9Cr-1Mo steel was cooled to a temperature between the M s point and room temperature from the normalizing temperature and then directly heated to the tempering temperature. It was found that the time to rupture at 650 and 700 C of the steel heat-treated by the new heat treatment increased 2-3 times longer than that of the steel conventionally normalized and tempered. The microstructure of the improved steel was tempered martensite and the size of martensite blocks was larger than for the conventional heat treatment. The hardness of the improved steel was fully recovered after tempering. This strengthening is not caused by fresh martensite, but caused by the refining of M 23 C 6 and V(C,N) particles in addition to the increase in the martesite block size, where M denotes metallic elements.

Phase Transformation Behavior and Microstructural Control of High-Cr Martensitic/Ferritic Heat-resistant Steels for Power and Nuclear Plants: A Review

“…However, it is reported that the retained austenite would be helpful to the creep resistance of weld deposit because of the transformation to un-tempered martensite even after the adequate heat treatment [48] . Tamura et al [49] suggested that the untransformed austenite would evolve into a new microstructure during subsequent tempering, which was related to the peaks in hardness and time to rupture of modified 9 wt% Cr steel. To obtain certain amount of retained austenite in 9 wt% Cr steels, after the austenization treatment, the steels can be cooled to a temperature between the martensite start temperature (M s ) and finish temperature (M f ) for different time [50] .…”

New chromium steel grade for creep applications

“…On the other hand, the innovative thermal treatment (TT2) to be applied on COIN2 increases the normalizing temperature approximately 100 °C, reduces the tempering temperature 15 °C, and adds a martensitization step (intermediate step between normalizing and tempering where the steel after normalizing is cooled down to a temperature below Ms and above RT to retain a certain percentage of austenite). Such a novel processing route pursues an enhanced control of the microstructure [29].…”

“…With TT2, other options may arise. Based on literature [29], in the martensitization, the transformation of austenite into martensite is interrupted at certain temperatures (MT10 or MT30) at which some austenite (10% or 30%) can be retained and stabilized with the martensite formed. Afterwards, in tempering, that austenite may transform into ferrite and carbides/carbonitrides, and martensite is tempered.…”

“…The result of this secondary transformation appears in clear colour and could correspond to fresh martensite [30]. In addition, the possibility of ferrite formation after normalizing (as in TT1) or during tempering (from the austenite retained and stabilized, according to Tamura et al [29]) cannot be disregarded. More retained austenite also implies a lower concentration of martensite formed after normalizing and subsequently tempered (in dark colour).…”

“…In comparison to previous P92 modified steels, the new grade here presented (COIN2) offers improved creep behaviour and other mechanical properties that potentially allow operating in more demanding conditions in terms of temperature and pressure for more efficient power generation (reduced CO 2 emissions). These properties are given by the steel microstructure, which is controlled through the metallurgical strategies here applied [29][30][31].…”