Development of a new generation of creep resistant 12% chromium steels: Microstructure of Z-phase strengthened steels
Doktorsavhandling, 2017

Fossil-fuel fired steam power plants provide more than 60% of the electricity generated worldwide, and account for about one third of the global CO2 emissions. Increasing the steam temperature and pressure leads to a higher thermal efficiency of the power plants and thus lower emissions. The efficiency is limited by the long-term corrosion and creep resistance of economically viable materials used in the critical components of such a power plant, 9–12% Cr steels. Increasing the Cr content from 9% to 11–12% in the best commercially available steels provides sufficient corrosion resistance for an increase from the current maximum steam temperature of 620°C to 650°C for future power plants. However, after a few years of service, formation of coarse Z-phase (Cr(Nb, V)N) precipitates at the expense of a fine distribution of VN precipitates degrades the precipitation hardening and accordingly creep resistance of the steels. In this work a new family of 12% chromium steels is studied, where Ta or Nb is used instead of V to strengthen the steel by forming a dense distribution of Z-phase rather than VN. Z-phase does not nucleate directly as Z-phase, instead it forms through a gradual transformation of existing MX and M2N precipitates. The former leads to the formation of Z-phase with a blade-like morphology and the latter promotes large bulky Z-phase precipitates. As a result of the MX to Z-phase transformation, creep strength comparable to commercially available 9% Cr steels can be achieved. Investigation on the Z-phase precipitates based on Ta or Nb showed that Ta-based Z-phase benefits from a denser distribution and a slower coarsening rate, and thus is recommended for alloy design. Carbon is found to play the most critical role in the precipitation processes of Z-phase strengthened steels. An ultra-low C content and an optimal balance between Ta and N in a model alloy lead to the formation of a fine distribution of TaN in the as-tempered condition, which are transformed to blade-like Z-phase after short-term ageing. Such a low C content leads to very little formation of M23C6 at grain boundaries, which allows for the formation of a continuous film of Laves-phase there and a low impact toughness. Although, the addition of C results in precipitation of Ta(C, N), and hence a slower phase transformation to Z-phase, a low but not ultra-low carbon content is preferred in the new Z-phase strengthened steels.

phase transformation

CrNbN

impact toughness

Creep

CrTaN

Laves-phase

atom probe tomography

precipitation hardening

M2N

MX

transmission electron microscopy

coarsening

PJ Salen, Kemigården 1
Opponent: Peter Mayr, Chemnitz University of Technology

Författare

Masoud Rashidi

Chalmers, Fysik, Materialens mikrostruktur

A new 12% chromium steel strengthened by Z-phase precipitates

Scripta Materialia,; Vol. 113(2016)p. 93-96

Artikel i vetenskaplig tidskrift

Microstructure of Z-Phase Strengthened Martensitic Steels: Meeting the 650°C Challenge

Materials Science Forum,; Vol. 879(2017)p. 1147-1152

Artikel i vetenskaplig tidskrift

Core-Shell Structure of Intermediate Precipitates in a Nb-Based Z-Phase Strengthened 12% Cr Steel

Microscopy and Microanalysis,; Vol. 23(2017)p. 360-365

Artikel i vetenskaplig tidskrift

M. Rashidi, J. Odqvist, L. Johansson, J. Hald, H-O. Andrén, F. Liu,"Coarsening behaviour of Z-phase precipitates in Z-phase strengthened 12% Cr steels

Microstructure and mechanical properties of two Z-phase strengthened 12%Cr martensitic steels: the effects of Cu and C

Materials Science & Engineering A: Structural Materials: Properties, Microstructure and Processing,; Vol. 694(2017)p. 57-65

Artikel i vetenskaplig tidskrift

Tantalum and niobium based Z-phase in a Z-phase strengthened 12% Cr steel

Advances in Materials Technology for Fossil Power Plants - Proceedings from the 8th International Conference on Advances in Materials Technology for Fossil Power Plants; Albufeira, Portugal; 11-14 October 2016,; (2016)p. 1058-1066

Paper i proceeding

M.Rashidi, A. Golpayegani, S. Sheikh, S. Gua, H-O. Andrén, F. Liu, "Transformation processes from pre-cursor phase to Ta-containing Z-phase in 12% Cr Z-phase strengthened steels with varying C content"

Ordna en konsert på ett fartyg på atomär skala, spara miljarder ton koldioxidutsläpp!

Ungefär en tredjedel av de globala koldioxidutsläppen kommer från fossileldade ångkraftverk, som producerar mer än 60% av världens elenergi. Vi skulle spara miljarder ton koldioxidutsläpp varje år redan med en måttlig ökning av kraftverkens verkningsgrad. Men verkningsgraden begränsas av egenskaperna hos tillgängliga stål som måste utstå en svår omgivning bestående av mycket het ånga, 650°C, och mekanisk last.

Att designa ett stål för höga temperaturer liknar mycket att ordna en konsert ombord på ett fartyg. Det är mycket viktigt att fördela passagerarna jämnt ombord så inte en sida överlastas och fartyget får slagsida. När det gäller människor kan man inte förvänta sig att de står stilla ombord och inte flyttar på sig, särskilt inte under en konsert! Naturligtvis umgås de och bildar grupper. Ett sätt att kontrollera passagerarna är att fördela bord jämnt så grupper bildas kring borden. Men under resan kan folk bli uttråkade och flytta sig till ett intressantare bord. Det kan bli dramatiskt om alla de intressanta borden står på den ena sidan av fartyget!

Tillbaks till ståldesignen. Vi kan anta att stålet är fartyget, passagerarna är atomer, och konserten (dansen) är den höga temperaturen (atomer som rör sig). Atomer är lika bra som människor på att bli vänner, om inte bättre. Den största utmaningen i designen av stålet är att fördela atomerna i mycket små grupper genom att använda atomära bord (utskiljningar), som gör stålet starkt nog att utstå de svåra villkoren. Det också viktigt att underhålla borden lika mycket, så atomerna stannar i sin grupp hela vägen till destinationen.

En bra sak med atomer är att de alla lyder lagarna (till skillnad från människor), men den dåliga saken är att atomer följer sina egna lagar, och vi känner inte till dem alla! Vi har visat att i de stål som undersökts i denna avhandling är tantal, krom och kväve bästa vänner, tillsammans kallade Z-fas, och så snart en sådan grupp bildats blir de så underhållna att de tenderar att stanna i gruppen. Vi har också funnit att kol är besvärligt att hantera, det är så attraktivt att det kan bilda stora grupper med tantal innan tantal får chansen att träffa kväve och krom. Det finns förstås lösningar på detta, vi kan till exempel bjuda in kobolt, som uppmuntrar krom att vara tillsammans med tantal och kväve, varefter kol ger sig av. Vi kan också hitta de intressanta ställen som atomer föredrar för att bilda grupper såsom atomära restauranger (korngränser).

Vågorna, vinden, solen, regnet och skuggor kommer så småningom att förstöra de mindre grupperna och stora grupper kommer att bildas på ena sidan för att se land, men det viktiga är att hålla smågrupperna underhållna hela vägen till konsertens slut och till vår destination, vilket tar ett par årtionden för våra stål.

Organize a concert on a ship on an atomic level,
save billion tonnes of CO2 emissions!

About one third of the global CO2 emission comes from fossil-fuel steam power plants, which produce more than 60% of the electricity worldwide. We would save billion tonnes of CO2 emissions every year by even a modest increase in the efficiency of the power plants. But efficiency is limited by the capability of available steels that have to withstand a harsh environment of very hot steam, 650°C, as well as mechanical loading. In this thesis, we try to design stronger steels that can withstand such harsh conditions.

Designing a steel for high temperature application is very much like organizing a concert on a ship. It is very important to distribute people evenly on board so that no side is overloaded and the ship does not tilt towards one side. When it comes to people, you cannot expect them to stay put on board evenly and not move around, especially during a concert! Obviously people socialize and make groups. One way to control people on board is to evenly distribute tables so that groups are formed around the tables. However, during the trip, people may get bored and move to a more interesting table. It will be dramatic if the interesting tables are only on one side of the ship!

Coming back to our steel design. We can assume that the steel is the ship, people are the atoms, and the concert (dance) is the high temperature (moving atoms). Atoms are, if not better, as good as people in making friends. The main challenge in our steel design is to distribute atoms in very small groups using atomic-scale tables (precipitates), which makes the steels strong enough to withstand the harsh condition. It is also important to equally entertain the tables so that atoms stay in their group all the way until our destination.

The good thing about atoms is that they all follow the laws (unlike people), but the bad thing is that atoms follow their own laws, and we do not know them all! We show that in the investigated steels in this thesis, tantalum, chromium, and nitrogen make best friends, called Z-phase, and once the group is formed, they become so entertained that they tend to stay in the group. We also find out that carbon is difficult to handle, it is so attractive that it can form big groups with tantalum before tantalum gets the chance to meet nitrogen and chromium. There are of course solutions to this, for example we can invite cobalt, which encourages chromium to join tantalum and nitrogen, and consequently carbon leaves. We could also spot the interesting locations that atoms would prefer to form groups such as atomic scale restaurants (grain boundaries).

The waves, winds, sun, rain, and shade would eventually destroy the smaller groups and larger ones will form on one side to see the land, but the important thing is to keep smaller groups entertained all the way until the end of the concert and to our destination, which takes a couple of decades for our steels.

Ämneskategorier

Materialteknik

ISBN

978-91-7597-649-5

Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 4330

Utgivare

Chalmers tekniska högskola

PJ Salen, Kemigården 1

Opponent: Peter Mayr, Chemnitz University of Technology

Mer information

Skapat

2017-11-09