Classification of chapter - vsb.czkatedry.fmmi.vsb.cz/Opory_FMMI_ENG/AEM/Materials for exacting... · Classification of chapter: 5 ... but the ternary diagram of Fe-Cr-Ni forms the

Subject – Materials for extreme technical applications

Department of Material Engineering, FMMI, VŠB - TUO

Author: Eva Mazancová 66

5. STAINLESS STEELS

Classification of chapter:

5. Stainless steels

5.1. Basic properties of chromium

5.2. Classification of stainless steels

5.3. Ferritic stainless steels

5.4. Austenitic stainless steels

5.4.1. Processing and applications of austenitic stainless steels

5.5. Duplex (austenitic-ferritic) stainless steels

5.5.1. Detected structure changes in duplex steels

5.5.2. Properties of duplex steels and their technical applications

5.6. Martensitic stainless steels

5.6.1. Brittleness development of martensitic stainless steels

5.7. Dispersion-strengthened stainless steels

5.7.1. Processing of dispersion-hardened stainless steels and their

applications

Summarization of chapter terms and questions

Literature

Time necessary for study: 400 minutes

Aim: After studying of this chapter

you will be inform about basic stainless steel types;

you will understand differences among different mentioned steel types;

you gain knowledge of manufacturing/production principles;

you get information about basic properties of given materials, including

their application and you will be able numerous negative influences




control;

Lecture

5. Stainless steels

5.1. Basic properties of chromium

Chromium is a ferrite forming element; it stabilizes the austenitic matrix through free

enthalpy, greatly increases the hardenability of steel. The size of the chromium atom diameter

is similar to that of the iron atom, therefore chromium does not harden the steel matrix

through solid solution, and it does not significantly affect the mechanical properties in the

annealed condition. However, it strengthens the matrix more than e.g. nickel. As a ferrite

forming element, it strengthens the ferritic matrix. It promotes resistance (slowing-down) of

the steel matrix against tempering to a temperature of 150 to 550 °C, which is a prerequisite

for the production of steels with higher strength characteristics.

Chromium thus slows down the decline in hardness when tempering and moves S -

curve to the right. As a result of its strong links to carbon, it forms a variety of carbide phases

with it, which are described in detail in Chapter 6. Besides Cr, highly carbide-forming

elements also include tungsten, molybdenum, vanadium, titanium, and zirconium, while in

contrast to Cr, e.g. Ni and Si strongly enhance the activity of carbon. The corrosion resistance

of stainless steels is due to the formation of a protective surface oxide film (passivation layer)

Cr2O3. Due to the very good links to O (Cr2O3 is formed), Cr also supports increased

resistance of steel against rust and burn-out. Up to the temperature of about 600 to 650 °C,

corrosion resistance of Cr2O3 can be guaranteed, which is associated with a low concentration

of vacancies.

In the binary diagram of Fe-Cr (see Fig. 5.1), the austenitic region is fully closed. Its

lower limit reaches about 820 °C, and on the right, it attacks the maximum limit of the 12 wt.

% Cr level. With the content of almost 25 wt. % of Cr in steel, the formation of brittle

intermetallic phase is supported. This phase generally occurs whenever there are more than

2.5 vacancies in a 3d-sphere. It reaches the peak at about 880 °C with less than 50 wt. % of

chromium, as shown in Fig. 5.1.




Fig. 5.1. Binary Fe-Cr system

Fig. 5.2. Ternary Fe-Cr-C diagram for 8, 12, 15 and 20 wt. % of Cr

5.2. Classification of stainless steels

Steels containing more than 5 wt. % of Cr are the simplest type of materials with

improved corrosion resistance and higher strength at higher temperatures compared to, e. g.

low-alloy steels. Combining Cr, Fe and other alloying elements, results in so called stainless

Tep

lota

°C

Cr wt. %

T

emp

eratu

re °

C

Fe-8 Cr

Fe-12 Cr

Fe-15 Cr

Fe-20 Cr

C wt. %

Tem

per

atu

re

°C




steel or corrosion resistant steels. Many steels of this kind are also used as heat-resistant, or

high-temperature (creep) steels, which are discussed in detail in Chapter 6.

The key alloying element is chromium. Additives other elements such as Ni, Si, Mo, V,

Ti, etc. potentiate its effects. Additive C in the diagram of Fe-Cr-C significantly modifies the

above-mentioned diagram in Fig. 5.1, as can be seen from Fig. 5.2.

The equilibrium diagram of Fe-Ni is in terms of stainless steel of negligible importance,

but the ternary diagram of Fe-Cr-Ni forms the basis of various types of stainless steels, or

heat-resistant steels, because the additive Cr narrows the area of the existence of austenite

(Fig. 5.3).

Fig. 5.3 Ternary Fe-Cr-C diagrams with different chromium contents (in wt. %)

Stainless steels can be divided according to the content of chromium, into steels with

the chromium content of about 5 wt. % and steels with the chromium content above 10 to 12

wt. %. Another division is based on the structure:

1) Ferritic stainless steels

2) Austenitic stainless steels

3) Austenitic-ferritic (duplex) stainless steels

4) Martensitic stainless steels

5) Dispersion hardened stainless steel

Highest corrosion resistance is exhibited by austenitic steels alloyed based on Cr-Ni,

which also have favourable toughness, but lower strength level, while martensitic stainless

Tem

per

atu

re

°C

12 Cr

15 Cr

9 Cr

18 Cr

21 Cr

24 Cr

0 5 10 15/0 5 10 15/0 5 10 15/0 5 10 15/0 5 10 15/0 5 10 15

Cr wt. %




steels are characterized by the lowest corrosion resistance, high strength, and low toughness

parameters. From an economic point of view, martensitic stainless steels are most favourable,

the most expensive steels are dispersion hardened stainless steels.

Steels with chromium content 4.5-6 wt. % are used in case of undemanding demands on

corrosion resistance and creep. Compared to conventional low-alloy steels, they have roughly

four times higher corrosion resistance in the hydrogen sulphide environment, and they are

approximately three times more resistant against oxidation up to 550 °C. The given type of

steel is air hardenable, if it contains more than 0.12 wt. % of C. It is therefore possible for

these steels to achieve high strength properties. Unfortunately, higher levels of C make

weldability characteristics worse. After heating at higher temperature and subsequent rapid

cooling, also martensitic transformation of the matrix in the weld area, or heat affected zone

(HAZ). However, they exhibit good formability (even at cold), which is due to a uniform

dispersion of globulitic carbides. Additives of other elements such as Mo, Si, Ti lead to

increased technical parameters. The additive of 1-1.5 wt. % of Si increases the singe

resistance, about 0.5 wt. % of Mo will support the increased strength properties at elevated

temperatures, and the additive of about 0.5 to 0.75 wt. % of Ti reduces the hardenability of

the given steel type with air cooling so that these can be applied in the state after finishing

rolling, without subsequent heat treatment. Due to significantly strong link of Ti to N and C,

Cr is not bound in carbides (nitrides), and its strong anti-corrosion effect can manifest itself.

Optimum additive of Ti corresponds to 5 times to 8 times the C content in the matrix. The

effect of Nb may be similar to that of Ti, while Nb is added in 10 times the C content for so-

called stabilization.

Fig. 5.4 Microstructure of ferritic stainless steel

5.3. Ferritic stainless steels

They contain 11.5 to 30 wt. % of Cr. Due to this constitution, they do not interfere in

the austenite region and only cover the area of pure ferrite, as shown in Fig. 5.1. The

appearance of the microstructure of ferritic stainless steel with a higher or lower content of

100

µm

----

----

___

___




precipitates of chromium carbides dispersed in the ferritic matrix is shown in Fig. 5.4. To

ensure mono-phase stainless ferritic structures, the following relationship has to apply:

Ferritic stainless steels are processed generally by annealing at the temperature of 760-

830 °C. E.g. stress relief annealing after welding or after cold deformation is applied in this

case. At longer endurance at temperatures above 1000 °C, coarsening of ferrite grain and the

matrix embrittlement occurs.

The given type of steel has the following advantages:

1. Very good corrosion resistance, high heat resistance (depending on the

chromium content) even up to the temperature of 950 to 1100 °C.

2. Reasonable price.

3. Good resistance to stress corrosion in chloride ion environments, to spot and

chipping corrosion.

The disadvantages associated with the type of steel:

Lower levels of notch strength at normal temperature - they are prone to overheating

and the formation of coarse ferritic grains (grain boundaries are not held by anything). In the

welding process, it is necessary to use preheating before welding and reheating after welding

to prevent stress and associated cracking in the weld, or heat influenced zone.

Susceptibility to intergranular corrosion (along the grain boundary) - the

sensitization occurs due to the high diffusion rate of Cr, C and N in ferrite in the grain

boundary region (faster than in austenite), resulting in the formation of high-chrome carbides

and their proximity to the depletion matrix in chromium. If the chromium content in the

depleted region is lower than 12 wt. %, this area is prone to sensitization. Addition of about

1% of Mo has favourable effect on the type of steel in terms of susceptibility to sensitization

after cooling in air, but not in the slow cooling in the furnace. Mo reduces diffusion rate and

slows down nucleation of nitrides. During slow cooling, nitrides are formed at an interval of

about 450 to 550 °C.

Formation of intermetallic phase (see Fig. 5.1) and embrittlement after annealing at

475 °C, i.e. Fragility 475 - this is connected with the formation of hard phase Cr2N with

acicular morphology, which supports the reinforcement of the matrix at the expense of

toughness. It is desirable that the N content in the steel was lower. Eliminating the effect of

Cr2N can be achieved by adding Ti, which has strong links to the N and C, and reduces the

activity of both elements.




For some types of ferritic stainless steel with a lower level of Cr and the upper level of

C or the presence of any other element, which promotes the stability of austenite (e.g. Ni,

Mn), austenite can be formed partially during heating of the matrix above 900 °C, and after

cooling, the austenitic matrix transforms into martensite, which is not further treated

(which may lead to the development of anisotropy of mechanical properties). The result is

increased strength and decrease in toughness.

5.4. Austenitic stainless steels

These are steels of different types, according to the chemical constitution:

Chrome-Nickel (wt. %) – with 12 to 25 of Cr, 8 to 38 of Ni, 0.01 to 0.15 of C, possibly N,

Mo, Cu, or Si-alloyed, or stabilized with Ti and Nb in order to enhance mechanical

properties and corrosion resistance.

Chrome-Mangan-Nickel Steel (wt. %) – with 12 to 22 of Cr, 5 to 12 of Mn, 3 to 8 of Ni,

0.02 to 0.15 of C, N, Mo, or Cu-alloyed, or stabilized with Ti and Nb. Steels exhibit higher

mechanical properties and resistance to corrosion under specific conditions.

Chromium-manganese (wt. %) – with 10 to 18 of Cr, 14 to 25 of Mn, 0.02 to 0.08 of C,

possibly N, Mo, or Cu-alloyed, or Ti and Nb-alloyed.

Cr is a ferrite forming element, but through the free enthalpy it stabilizes the austenite matrix

and provides corrosion resistance in oxidizing environments.

Ni is an austenite forming element and stabilizes austenite matrix even at low temperatures

and also in plastic deformation. It increases corrosion resistance in reducing acids.

Mn is an austenite forming element and if its contents is approximately above 3 wt. %, it

effectively contributes to the suppression of cracking of welds. The role of Mn in higher

contents differs from that of Ni, because at high temperatures it no longer extends the

austenite region (Fig. 5.5). The boundary that separates the austenite region and mixed

regions of ferrite with austenite at 1000 °C and with 13 to 15 wt. % of Cr is not dependent on

the content of Mn. Conversely, higher contents of Ni make it possible to increase the content

of Cr as well, thus achieving a fully austenitic matrix. It is not possible to obtain a purely

austenitic chromium-manganese steel with a Cr content of more than 15 wt. %. However,

after cooling, addition of Mn stabilizes the structure existing while hot, so e.g. steel with 13

wt. % of Cr and 0.01 wt. % of C, which becomes martensitic after cooling from 1000 ° C at

normal Mn content, in the case of 15 wt. % of Mn, it is entirely austenitic. In the presence of




(in wt. %) 3 to 7 of Ni and 0.15 to 0.25 of N, the Mn content of 5.5 to 10 makes it possible to

maintain the austenitic structure with Cr content up to 19. In addition, present Mn increases

the solubility of N.

Fig. 5.5. Part of ternary diagram a) Fe-Cr-Ni at temperatures with the greatest

austenitic area (about 1000 °C) and b) Fe-Cr-Mn with 0.1 wt. % of C at 1000

°C (existence of austenite area in Fe-Cr-Ni system is marked dashed)

C increases the strength levels, stabilizes austenite after cold deformation. It influences

susceptibility to intergranular corrosion, therefore the higher content is undesirable.

N is an austenite forming element, it strengthens the austenite matrix without adverse effect

on the intergranular corrosion in the content of up to 0.2 wt. %, it stabilizes austenite even

during cold forming, and, together with Mo, increases the resistance to crevice and spot

corrosion.

Si is a ferrite forming element inducing cracking of welds. It reduces the resistance of steels

in boiling nitric acid (65 % of HNO3). When the content is about 3-4 wt. %, it eliminates the

susceptibility to intergranular corrosion, it strengthens the overall corrosion resistance in

boiling highly concentrated HNO3 (over 80 %) and in the environment of the acid with the

addition of oxidizing agents.

Mo is a ferrite forming element forming Laves phases (intermetallic phases), its presence

promotes corrosion resistance in all environments except boiling solutions of HNO3,

preferably against spot and crevice corrosion. It improves high-temperature resistance (it does

not apply to fire resistance).

Cu is a weak austenite forming element, it improves corrosion resistance in H2SO4

environment and in contents at about 3 to 4 %, it improves machinability.

Ti and Nb are carbide forming elements suppressing the susceptibility to intergranular

corrosion and increasing high-temperature resistance. Nb causes cracking of welds.

Al is a deoxidizing agent - it is ferrite forming and supports fire resistance.




S, Se, P, Pb are accompanying elements, they increase machinability, but at the expense of

reduction of corrosion resistance.

B is added within about 20 to 40 ppm. It improves formability and improves high-temperature

resistance. Higher levels of these properties deteriorate and promote weld cracking.

Fig. 5.6 Microstructure of austenitic stainless steel

Austenitic steels are paramagnetic. The example of microstructure with twins is shown

in Fig. 5.6. Hardening of the given steel type is possible by cold deformation and hardening

solid solution. Hardening can be realized by using interstitial elements like C, N, and B,

which is more efficient than after the addition of substitutional elements. Addition of N has

an effect on the increase of the yield strength at normal temperature of about twice compared

with addition of C. N has a negligible effect on the process of sensitization. The give type of

steel has the following advantages:

1) it exhibits excellent corrosion resistance, and in many cases, very good high-

temperature resistance as well

2) notch strength reaches a high level even at low temperatures

3) it has a high rate of strain hardening from Ramber-Osgood relation

(associated with high levels of notch strength)

The disadvantages associated with the type of steel:

Higher price (due to the nickel content) - they are susceptible to sensitization (formation of

M23C6 at austenite grain boundaries). The emergence of these coarse carbides leads to the

depletion of austenite matrix of Cr, which may even be lower than 12 wt. % of Cr, and in that

depleted zone, austenitic matrix corrosion begins (see Fig. 5.7).

Susceptibility to sensitization is also due to the lower solubility of carbon in austenitic

stainless steels, which are stabilized with Ti and Nb. Susceptibility to sensitization further

increases when these steels are annealed in the temperature interval of 480-850 °C. In the

100

µm

----

----

___

___




process of welding austenitic steel having a very low content of C, sensitization does not

occur in the case of short thermal cycles, but during stress relief annealing (longer time

interval) in the temperature range from 500 to 850 °C, this can be partly detected. Elimination

of the phenomenon of sensitization is possible by addition of so called stabilizing elements,

such as Ti and Nb. They both show strong binding to carbon, so they are able to bind it and

thereby prevent the possible formation of chromium carbides in case that both elements are

freely available. However, if free Cr and N are available, these elements can form Cr2N.

Fig. 5.7 Schematic depiction of carbon and chromium redistribution in grain boundary

area, where carbides precipitate (contents of C and Cr are in wt. % and

karbid = carbide)

Although stabilized stainless steels are resistant to intergranular corrosion after

annealing in sensitization, they exhibit susceptibility to the development of so-called knife-

edge corrosion. During welding, TiC, or TiCN, which were supposed to stabilize the

austenitic matrix dissolve in the area of the weld, or heat affected zone (HAZ) at high heating

temperatures. The welding process thus results in occurrence of free Ti that has even more

significant bond to O2, therefore, Ti oxide may be preferably formed. This leads to free

movement of carbon in the matrix (in the weld area, or HAZ), which immediately forms

carbides with Cr in the narrow area of the weld, or TOZ. Thus, the matrix in the narrow area

is locally deprived of Cr it can lead to a local reduction of Cr concentration below the safe

level of corrosion resistance, and to localized corrosion in these particular areas. Therefore,

addition of C is also desirable for the welded materials at a level below 0.03 wt. %.

Austenitic steels can exhibit susceptibility to embrittlement by hydrogen or

susceptibility to stress corrosion cracking, in an environment of O or S. A drawback is also

the fact that during welding martensite retroactively converts to austenite in the spot of

C cont. change

in dependence

on carbide

distance

karbidu

level of 12 Cr

18 Cr

Cr

grain 1 grain 2

Change of C

content in

dependence on

carbide distance

arbidu

Solubility level of

0.02 C

C

Intermed. 0.1 C

grain 2 grain 1

gra

in b

ou

nd

ary




heating (above 500 °C). It is also necessary to mind Ni and S. Both elements form the NiS

type of sulphide, which may occur at the grain boundaries of austenite, where it negatively

affects the cohesive strength. When heating to a temperature of 900 °C, NiS disintegrates

(intra-crystalline disruption).

5.4.1. Processing and applications of austenitic stainless steels

Solution annealing is carried out in the temperature range between 950 and 1150 °C

depending on the steel type, followed by rapid cooling, typically in water to prevent

precipitation of carbides. For steels stabilized by Ti and Nb, solution annealing is

supplemented with so called stabilization annealing in the temperature range between 850 and

950 °C/2-4h, possibly followed by special thermal treatment at reduced temperatures in order

to ensure optimal corrosion resistance of the austenitic matrix. The aim of the heat treatment

is to obtain a homogeneous solid solution and optimum corrosion resistance by dissolving

carbides, or other phases in the austenitic matrix, further suppress hardening caused by hot

and cold working, and bind the highest possible proportion of C, or N to permanent carbides,

possibly nitrides, to limit the subsequent long-term heat influence on the structure stability

and corrosion resistance.

Application: in the food industry, power engineering, including nuclear and chemical

industries. The most demanding variants have about 30 to 35 wt. % of Ni and about 20 wt. %

of Cr.

5.5. Duplex (austenitic-ferritic) stainless steel

This type of steel is characterized by 30-50 % of the austenite matrix, the ferritic matrix

always gets the rest of the share. The predominant share of this or that master is crucial in

view of the final properties of the steel. The proportion of the ferritic structure is dependent

on the chemical composition and the heat treatment method. Alloying elements between the

two phases are not distributed evenly. Austenite forming elements are concentrated more in

austenite and vice versa. The value of the partition coefficient depends on the chemical

constitution and the annealing temperature.

The advantages of duplex steels:

1) they show about twice the strength characteristics compared to conventional corrosion

resistant austenitic steels

2) corrosion resistance is almost comparable to corrosion resistance of austenitic steels




3) their susceptibility to sensitization is lower than that of austenitic stainless steels

4) they are also very weldable (no cracks)

5) sufficient resistance to stress corrosion cracking in chloride solutions

Disadvantages of duplex steels:

1) susceptibility to the development of embrittlement after annealing at the

temperature of 475 ° C

2) embrittlement due to possible phase precipitation

3) frequent development of anisotropy of properties

4) significant transition region in the temperature dependence of notch strength

with increasing shares of ferrite

5) susceptibility to sensitization, in particular in the temperature range between 500 and

700 °C. This tendency can be reduced by selecting a lower temperature of solution

annealing than 1100 °C, namely 950 °C.

a) b)

Fig. 5.8 Image of duplex steel (basic material) cooled from temperature of 1020 °C a) in

water, b) with cooling rate of 20 °C.s-1

a) b)

Fig. 5.9 Image of duplex steel in cast state with content of a) 0 wt. % of Mo (Re =

522 MPa, Rm = 730, A = 42 %), b) 4 wt. % of Mo (Re = 699 MPa, Rm =

832 MPa, A = 22.4 %)




Duplex stainless steels have a very low content of C. Figs. 5.8 and 5.9 show an example

of microstructure of duplex stainless steel for two different variants of processing.

5.5.1. Detected structural changes in duplex steels

With austenitic-ferritic steels, carbides form at temperatures below 1000 °C on the

austenite grain boundaries; it is the case of coarse carbides M23C6 rich in Cr. They are

formed on the grain boundary in the phase rich in Cr (in ferrite), which is also characterized

by the highest diffusion rate. The formation of carbides at grain boundaries also does not have

a detrimental effect on susceptibility, e.g. to intergranular corrosion as in the case of

austenitic stainless steels.

The phase occurs in the duplex steels much earlier than with purely austenitic steels,

where the emergence of phase is observed after several hours’ endurances at elevated

temperatures. The phase occurs most frequently in the duplex steels during heating, during

cooling its occurrence is almost unlikely. A temperature above 400 to 500 ° C leads to a

significant hardening of duplex stainless steels, which is connected with the brittleness of

475 ° C of the ferritic phase.

In view of the above-mentioned facts, duplex stainless steels have to undergo

dissolution annealing before use, namely above a temperature of 1000 °C to ensure

dissolution of carbides and intermetallic phases and transition of their elements into solid

solution of both ferrite and austenite. Duplex steels need not be subjected to this heat

treatment, only when welding duplex stainless steel, which is also an advantage.

Deformation processes (forging, forming) pose a greater risk for duplex steels (a higher level

of yield strength) than for purely austenitic steels.

5.5.2. Properties of duplex steels and their technical applications

The strength characteristics of duplex stainless steels are better than in the case of

purely austenitic steels. Presence of ferrite is responsible for that. Strengths is increased with

a higher proportion of ferrite. Ductility is very good even after solution annealing at high

proportions of ferrite. However, the level of notch strength decreases (Fig. 5.10).

In the annealing process, it is advisable to avoid the temperature range of 700 to 1000

°C with elimination of the phase, which increases the strength of the matrix at the expense

of notch strength. Another pitfall is the temperature range from 300 to 550 °C (aging 475 °

C). It can only be used for short-term increase in strength properties. Duplex steels are very




useful for casting purposes because of minimal susceptibility to solidification cracking, and

good fluidity. It allows casting of complex shapes, which is complicated with other types of

stainless steel. Repairs by welding castings can also be easily carried out.

Fig. 5.10 Dependence of rapture toughness (Charpy V) of different steel types on

testing temperature

When alloying dual-phase steels with Mo, Cu, or N, the given type of steel is highly

resistant to inorganic acids (e.g. H2SO4), certain organic media, and seawater. It also resists to

chlorides and it exhibits resistance to intergranular corrosion, but also to spot and crevice

corrosion (especially for variants with higher chromium and molybdenum content). It is also

highly resistant to corrosion cracking. The present stronger ferrite (compared to austenitic

phase) in dual-phase steels, which is the anode towards austenite, can protect austenite

cathodically. Ferrite also contains a higher proportion of Ni, which gives it greater stability.

However, anode character of ferrite may be lost when exposed to stress.

At higher stress, the deformation of ferrite, as well as its susceptibility to cracking is

higher. At low stress, the development of cracks is blocked by ferritic areas. At medium

stress, cracks still do not develop in the ferrite, only along interphase borders. However, at

high stress levels, stress corrosion cracking spreads in both matrices. The above-mentioned

resistance to stress corrosion cracking is significantly associated with the above-mentioned

strength characteristics of the two phases present. Under stress, a higher degree of

deformation localizes preferentially in the “softer” austenite.

Ferritic st.

03Cr18Mo2 Duplex st.

02Cr18Ni5Mo3

Duplex st.

02Cr22Ni5Mo3

austenitic st.

05Cr18Ni9

Temperature °C

C

harp

y V

J.c

m-2




Two-phase steels also have very good resistance against halides and sulphide stress

corrosion cracking. For special purposes, duplex steels are alloyed with copper additive,

which contributes mainly to harden duplex stainless steels, as well as to plastic deformation.

Some types of low-carbon steels are alloyed with titanium, which reduces the tendency

to embrittlement and improves the technological properties. Fig. 5.11 shows the dependence

of the ratio of the corrosion strength (Rmk) and the yield strength (YS) on ductility at 204 °C

and 8-hour exposure in an environment of chloride ions with a pH = 6.0 to 6.5 in the case of

duplex steel, and the content ferrite. The next Fig. 5.12 shows the susceptibility to spot

corrosion of two types of stainless steel (austenitic and duplex) depending on the content of

chloride ions.

5.6. Martensitic stainless steels

The given type of steel finds application e.g. in the production of turbines, armatures,

bolts, and springs. Martensitic stainless steel contain (wt. %) 11.5-18 of Cr, 0.15 to 1.2 of C

(not to get to the phase area), and various alloying additives (e.g. about 1.5 -2.5 wt. % of

Ni, up to 1 wt. % of Mo). From the metallurgical viewpoint, the martensitic stainless steels

are polymorphic, and before quench hardening, they must have an austenitic structure.

A-st. C0.8

Cr18Ni12

Mo3N

A-F -st. C0.2

Cr22Ni5

Mo3N

A-F-st. C0.8

Cr18Ni5

Mo3

A-st. C0.5

Cr18Ni9

Cl- ions content %

Tem

per

atu

re

°C

Ductility %

Nb

T

S/Y

S0

,2

-

28-17 Cr

15-7.7 Ni

a 2.8-0 Mo

Fig. 5.11 TS/YS0.2 dependence on Fig. 12 Critical temperatures of pitting

ductility for Cr-Ni-(Mo) steel in corrosion vs. Chlorides contents for 2

environment of 878 ppm Cl- with austenitic steels (A-steel) and 2 duplex

pH 6-6.5 (A-F) steels

0.01 0.05 0.20 1.0 10

0.2

5

0.5

0.7

5

1

.0

TS

/ Y

S0.2

-




Therefore, their chemical composition is constituted so that the chromium and carbon

contents meet the following condition“.

Advantages of martensitic stainless steels:

1) Compared to other stainless steels, their price is relatively low.

2) The advantage is thermal workability to various strength levels. Most

often, quench hardening from the austenite region (water, oil, or air) followed by

tempering up to around 650 °C not to get into the austenitic region is performed (see

Fig. 5.1).

Disadvantages associated with martensite corrosion resistant steels

1) During welding, quenching in the heat affected zone of a weld joint occurs,

which is not further treated. The result is increased strength, but also a significant

decrease in notch strength.

2) During tempering, this type of steel is prone to the development of temper

brittleness.

3) They exhibit a lower corrosion resistance than the standard austenitic

stainless steels

4) Their applicability is limited.

5) They are prone to embrittlement due to hydrogen and to stress corrosion

cracking in chloride and sulphide environments (effect of higher density of

dislocations in martensitic matrix)

5.6.1. Brittlenes development of martensitic stainless steels

The development of brittleness can be connected with increased density of

dislocations and with fine M3C carbides, which are eliminated along the boundaries of the

original austenite grains. They are formed either in the process of quenching or aging at about

150 °C. Sequential precipitation of M3C carbides and recovery processes during the

tempering process leads to a reduction in dislocation density up to the temperatures of about

320 °C. When tempering at temperatures higher than 400 °C, M3C carbide becomes unstable

and dissolves under simultaneous formation of M23C6 carbide, which preferentially

precipitate on dislocation degrees or at the intersections of intersecting dislocations. The

dissolution phase of M6C is accompanied by deformation of the crystallographic lattice,

thereby increasing the dislocation density. This process is accompanied by embrittlement (up

to formation of cracks) caused by the blocking of dislocations by emerging carbides M23C6.




Higher tempering temperature allows dislocations to climb. Dislocation cells with reduced

internal density of dislocations are formed, accompanied by the growth of carbide phase. In

the temperature range between 650 and 700 °C, matrix recrystallization is fully realized, and

the remnants of morphology of the initial martensitic matrix are destroyed.

Addition of Nb (about 0.10 to 0.15 wt. %) is able to influence the nucleation of M23C6

carbide, limiting its growth and coalescence at higher tempering temperatures. Secondarily,

refinement of the microstructure and reduction of susceptibility to the development of

brittleness occur. Addition of Nb also leads to an increase in resistance to stress corrosion

cracking, the overall uniformity of the tempering process, and to increase in the strength of

the matrix at elevated temperatures. Addition of nitrogen (up to 0.3 wt. %) is able to solidify

the matrix of the given type of steel without adversely affecting corrosion resistance, while

simultaneous increase in hight-temperature resistance.

5.7. Dispersion-strengthened stainless steels

The given type of steels is also known as PH – steels (treated with precipitation

hardening). They are used in aircraft and space industry (e.g. landing gears), and for selected

parts of nuclear reactors. In terms of the chemical composition, their carbon content ranges

from hundredths to tenths of wt. %, about 15 to 20 wt. % of Cr, and about 4-10 wt. % of Ni.

Mo, Cu, Ti, N, Al, or P are typical additional elements for the additional hardening effect,

The advantages of dispersion-hardened stainless steels:

1) easy workability – first, they are processed into the final shape, and the heat

treatment aiming at precipitation hardening is realized in the end.

2) high strength characteristics and at the same time, very favourable

parameters of notch strength

3) higher corrosion resistance than martensitic stainless steels

4) the possibility of application even at relatively high temperatures (about 550

°C)

The disadvantages associated with the dispersion-hardened corrosion resistant steels:

1) these are expensive types of stainless steels

2) they are susceptible to thermal embrittlement




3) the susceptibility to stress corrosion cracking, especially the variants with

higher strength

4) maintaining high levels of strength in welded joints

5) the possibility of occurrence of the anisotropy of properties

Based on structural changes and the realized dispersion hardening, they can be divided

into steels with:

A) basic martensitic matrix

B martensitic matrix after the phase transformation from austenite to

martensite (i.e. semi-austenitic steels)

C) austenitic matrix

5.7.1. Processing of dispersion-hardened stainless steels and their

applications

With steels in which the precipitation hardening takes place in the base martensitic

matrix (type A), heat treatment generally includes the following steps: solution annealing

1050°C/air, which leads to disintegration of austenite to martensite. It is followed by aging at

a temperature of 450-560 °C. It is possible to achieve the strength of about 1500 MPa at 12 %

ductility. An example of chemical composition (wt. %): 0.07 to 0.12 C max. 1 Mn about 17

Cr 7 Ni Cu 2.75 0.7 0.20 Ti Al 0.2 N.

They are widely used in aerospace, shipbuilding, aerospace, and armaments industry.

They are suitable for the manufacture of tools for extrusion of Al, for the manufacture of

moulds for moulding rubber, plastics and compression tools. In comparison with the tool

steels with medium content of C and about 5 wt. % of Cr, they are more resistant to cracking

during grinding and deployment due to thermal fatigue.

Steels undergoing precipitation hardening in the martensitic matrix after phase

transformation from austenite to martensite, i.e. so called semi austenitic type (type B),

can be divided into two groups according to the source of hardening. With one type, the

source of high strength is dispersion hardening, while with the second type, dispersion

hardening is not the primary hardening. They are semi austenitic steels which are hardened

due to the effect of the austenite-martensite phase transformation, and the aging process itself

basically leads to tempering the tempered matrix without significant secondary reinforcing

effect. Freezing after hardening (high degree of disintegration of austenite phase into




martensite), and subsequent tempering at a temperature of about 500 to 550 °C can result in

achieving yield strength of about 1200 MPa, strength of about 1400 MPa with ductility at

around 10 %. Part of a complex heat treatment of both variants of type B is also the solution

annealing at a temperature of about 1050-1080 °C.

For the type of steel, double aging can be applied, which consists of the first heating at

a temperature of about 750 °C/air, and the second heating (always lower temperature than in

the first heating) at a temperature of about 560 ° C/air. Although dual aging leads to a

reduction in strength properties, it increases plasticity.

For a higher degree of hardening of the martensitic matrix, addition of Al, Ti, Cu, Ni

and N is used. Addition of Mo (2-3 wt. %) increases the strength level of steel, as well as

corrosion resistance even at higher temperatures. Mo hardens the matrix both through solid

solution and the dispersion strengthening effect. Hardening stainless steels is also influenced

by Ni3Al, Ni3Ti, NiAl, NiCu, Cr2N type of precipitates and carbides of alloying elements. An

example of chemical composition (wt. %) is: 0.9-0.13 of C max. 1 of Mn, 16 of Cr 7-9 of

Ni, about 1.5 of Al, and 0.10 of N.

Duplex stainless steels with an austenitic matrix are processed by solution annealing at

a temperature of 1050-1150 °C. At high cooling rate, the structure consists of supersaturated

austenite with high parameters of plasticity, followed by aging at a temperature of 650-750

°C, which results in a final strength level of 1200 MPa with ductility of about 15 to 20 %. The

variant of austenitic duplex steels contains about (wt. %) 0.12 of C max. 3.5 of Mn, about 18

of Cr, and 10 of Ni. Other elements are added as well, which can precipitate out in the form

of intermetallic compounds (e.g. Ti, Al) and harden the matrix at a temperature of about 700

°C. For most environments, the corrosion resistance of dispersion-hardened austenitic

stainless steels is comparable to standard stainless steels.

It can be used, e.g. in jet engines, fans, turbine wheels, and at low temperatures, in

petrochemical industry.

In the end of this chapter main terms that you should master are repeated:

Ferritic, austenitic, martensitic, duplex and dispersive strengthened stainless steels, sigma

phase, nitride of chromium, precipitation of M23C6 carbides, corrosion-resistance of Cr2O3

Summarization of chapter terms:




Questions:

1. Could you explain the positive effect of Cr addition in anticorrosion

steels?

2. Which types of anticorrosion steels do you know?

3. Explain the influence of Si, Mo, Ti, Nb and Mo addition?

4. Which Cr contents are typical for ferritic and austenitic anticorrosion

steels?

5. Could you explain positives and negative effects of ferritic anticorrosion

steels?

6. Could you explain brittleness of 475?

7. Which differences divide austenitic anticorrosion steels?

8. What is the core both of negative and positive effects of austenitic

anticorrosion steels?

9. What represents stabilisation of anticorrosion steels?

10. What represents corrosion sensitization? Describe principle of knife

corrosion.

11. Name the basic types of duplex anticorrosion steels.

12. Explain positives and negatives of duplex steels.

13. Do you know the differences in toughness among austenitic, duplex and

ferritic anticorrosion steels?

14. What positives and negatives are connected with martensitic anticorrosion

steels?

15. Could you explain the principle of dispersion hardening of dispersive

strengthened stainless steels?

16. Do you know positives and negatives of dispersive strengthened stainless

steels?

Literature:

AL DAWOOD, M., EL MAHALLAWI, I.S., ABD EL AZIM, M.E., EL

KOUSSY, M.R.: Mat. Sci Tech., 20 (2004) p. 363.




QUESTEDF, P.N., BROOKS, R.F., CHAPMAM, L., MORRELL, R.,

YOUSSEF, R., MILLS, K:C.: Mat. Sci Tech., 25(2) (2009) p. 155.

COATERS, G., CUTLER, P.: Advanced Materials and Processes, 209(4) (2009)

p. 29.

SMUK, O., HÄNNINEN, H., LIIMATAINEN, J.: Mechanical and corrosion

properties of P/M-HIP super duplex stainless steel after different industrial heat

treatments as used for large components. Mat. Sci. Tech., 20 (2004) p. 641.

JANG, Y, KIM, S., LEE, J.: Effect of different Mo kontent on tensile and

corrosion behaviour of CD4MCU část duplex stainless steels. Met. Trans. A, 36

(2005) p. 1229.

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