Austenite stability of low-temperature reversion-treated...

Oulun yliopistoUniversity of Oulu – Kerttu Saalasti Institute – Future Manufacturing Technologies

Oulun yliopisto – Kerttu Saalasti instituutti – Tulevaisuuden tuotantoteknologiat (FMT)1

Thesis based on compiled publications on

“Austenite stability of low-temperature reversion-treated microstructures of an AISI 301LN

stainless steel under monotonic and dynamic loading”

Antti Järvenpää / FMT-ryhmäCASR seminaari 8.6.2017



‒ Steel development (3 G)- Higher strength without impairing the ductility

- Better steel without increasing the production costs

‒ Grain size refinement

- Effective grain refinement enhances the strength and ductility may stay

high by TRIP effect

- Reversion treatment

- Strain-induced martensite transforms back to austenite in austenitic steel

- An effective route for grain size refinement increasing strength

- Long research history in University of Oulu (Prof. Pentti Karjalainen)

Background



Thesis - Complex structures obtained at 700–800°C; stability

Cold rolling

Strengthening rolling

Sample preparation

700–800°C

Chemical

composition

Mechanical testing

Fine-grained

reversed austenite,

retained martensite

(and retained

austenite)

70 – 100%

deformation

induced martensite

Metastable

austenite

Gleeble / Induction

OM

XRD

Stress and strain

controlled fatigue tests

Tensile tests

Hardness tests

EBSD

Feritscope

TEM

Cold rolling

Electropolishing

XRD

Oulun yliopisto4

Cold rolling

Strengthening rolling Sample preparation

700–800 °C

Chemical

composition

Mechanical testing

Fine-grained

reversed austenite,

retained martensite

(and retained

austenite)

70 – 100%

deformation

induced martensite

Metastable

austenite

Gleeble / Induction

OM

XRD

Stress and strain

controlled fatigue tests

Tensile tests

Hardness tests

EBSDFeritscope

TEM

Cold rolling Electropolishing

XRD

Annealing

Sample preparation

OM

Feritscope

AISI 301LN



- What is known:

- The stability of austenite against martensitic transformation depends on numerous factors such as temperature, chemical composition,

crystallographic orientation, defect density, surrounding phases, and grain size (GS) /e.g. 1,2/

- It is commonly reported that the highest austenite stability exists with the average grain size (GS) of 1– 2 µm whereas the stability decreases with

both increasing and decreasing average GS /3-6/

1) K. Nohara, Y. Ono, N. Ohashi, Composition and grain size dependencies of strain-induced martensitic transformation in metastable austenitic stainless steels, J. Iron Steel Inst. Jpn2) S.K. Varma, J. Kalyanam, L.E.Murr, V. Srinivas, Effect of grain size on deformation-induced martensite formation in 304 and 316 stainless steels during room temperature tensile testing,

J. Mater. Sci. Lett.3) D. Maréchal, Linkage between mechanical properties and phase transformation in a 301LN austenitic stainless steel. PhD Thesis.4) M.C. Somani, P. Juntunen, L.P. Karjalainen, R.D.K. Misra, A. Kyröläinen, Enhanced mechanical properties through reversion in metastable austenitic stainless steels, Metall. Mater. Trans. 5) A. Kisko, R.D.K. Misra, J. Talonen, L.P. Karjalainen, The influence of grain size on the strain-induced martensite formation in tensile straining of an austenitic 15Cr–9Mn–Ni–Cu stainless

steel, Mater. Sci. Eng. 6) P. Behjati, A. Kermanpur, A. Najafizadeh, H. Samaei Baghbadorani, Effect of annealing temperature on nano/ultrafine grain of Ni-free austenitic stainless steel, Mater. Sci. Eng.

“Austenite stability”

- The focus of the study

- The influence of GS and its distribution on the austenite stability – What explains the inverse trend?



Inverse trend in austenite stability- average GS

Results: “Austenite stability”

The order of the stability of the

reversed structures vary similarly

under monotonic and cyclic

loading!



EBSD study of the details of the structureHeterogeneous microstructures



EBSD study of the details of the structure

Average GS is not a good characteristic of low-T reversed structure

WHY ? Texture no

Low-T reversion structures are unstable

FGA = 900 ºC / 1 s UFGA = 800 ºC / 1 s PRev = 750 ºC / 0.1 s



- Fine-grained stable austenitic structure, FGA obtained at 900°C

- FGA was subsequently heated up 750, 850 and 900 C

Why low-T structures are unstable ?

The effect of precipitation!

PRISMA modeling



Nano-size Cr2N precipitation occurs during annealing at 750–800 °C

STEM




The degree of stability drop follows the predicted kinetics of the precipitation!

Stability of reversed structures




The presence of deformed austenite

and larger austenite grain size

decreases slightly monotonic austenite

stability of structures rolled with low

cold rolling reduction!

No practical differences under cyclic

loading!

The effect cold rolling reduction

On complex structure and its stability

CR and complex structures



Fraction of low-deformed martensite



Some DA grains have the proper orientations to form martensite,

but most of the DA grains are stable

The fraction of DA

Prev-32CR: After 7% tensile straining

Prev-32CR: After 40 000 cycles at

0.4% total strain amplitude



Equal strength can be achieved with cold rolling reductions from 32 to 63%,

both under monotonic and dynamic loading

CR and complex structures



Lower stability in low-T reverted structures can be utilized in strengthening by temper rolling

Cold strengthening



Conclusions – Novel observations

‒ Austenite stability- The ultrafine grain size is not the reason for highly reduced austenite stability in tension and cyclic

loading

- Low-T microstructure consists of submicron, medium size and DA grains- The precipitation is the factor decreasing the mechanical stability of reversion-treated austenitic

structures while formed at 800–700 °C. Especially the medium-size grains are unstable.

- Precipitation at 800–700°C binds the nitrogen from the austenite matrix and reduces the stability

‒ The role of the retained austenite (DA)- Equal mechanical strength can be achieved with partially reverted 32 – 63% cold rolled structures- 63CR: High strength is achieved due grain refinement- 32CR: Lower reduction impairs grain refinement, but the presence of cold-rolled retained

austenite strengthens

‒ Other- Low stability can be utilized in- Cold strengthening (temper rolling)- Cyclic loading by stabilizing the structure - Enhancing fatigue life



Conclusions

‒ Courses: OK!‒ 5th paper under preparation‒ 6th paper and the actual thesis will be finished in 2017‒ Study on reversion structures continues…‒ A lot of papers have been published- 31 reviewed papers (corresponding author in 15 papers) since year 2009

- In addition to the thesis, 7 publications on reversion structures have been published

‒ Presented. A. Järvenpää, P. Karjalainen, M. Jaskari, Effect of grain size on fatigue behavior of type 301LN stainless steel, XVI International Colloquium "Mechanical Fatigue of Metals" Conf. Sept. 24-26, 2012, Brno, Czech Republic

‒ Published. A. Järvenpää, M. Jaskari, L. P. Karjalainen, M. Hietala, Enhancing mechanical properties and formability of AISI 301LN stainless steel sheet by local laser heat treatment, Key Engineering Materials, The Current State-of-the-Art on Material Forming (2013) ISBN-

13: 978-3-03785-719-9

‒ Published. A. Järvenpää, M. Jaskari, M. Hietala, K. Mäntyjärvi, Local reversion of cold formed AISI 301LN, Physics Procedia Vol. 78, 2015, P. 305-311.

‒ Published. A.S. Hamada, A. Järvenpää, E.Ahmed, P. Sahu, A.I.Z. Farahat, Enhancement in grain-structure and mechanical properties of laser reversion treated metastable austenitic stainless steel, Materials and Design, Vol 94, 2016, P. 345–352.

‒ Presented. A. Järvenpää, M. Jaskari, L. P. Karjalainen, Grain size and austenite stability in fatigue of a reversion-treated 301LN type stainless steel, Extended abstract, NT2F16-conference, 2016.

‒ Published. J. Man, I. Kuběna, M. Smaga, O. Man, A. Järvenpää, A. Weidner, Z. Chlup, J. Polák, Microstructural changes during de-formation of AISI 300 grade austenitic stainless steels: Impact of chemical heterogeneity, 21st European Conference on Fracture, ECF21,

20-24 June 2016, Catania, Italy, Structural Integrity Procedia 2, 2016, 2299-2306. http://dx.doi.org/10.1016/j.prostr.2016.06.288

‒ Submitted. J. Man, A. Chlupová, I. Kuběna, T. Kruml, O. Man, A. Järvenpää, L.P. Karjalainen, J. Polák, LCF BEHAVIOUR OF 301LN STEEL: COARSE-GRAINED VS. UFG-BIMODAL STRUCTURE, LCF8 conference



Thesis – Publications

Paper 1: Effect of Grain Size on Fatigue Behaviour of Type 301ln Stainless Steel

Paper 2: Austenite stability in reversion-treated structures of a 301LN steel under tensile loading

Paper 3: Stability of grain-refined reversed structures in a 301ln austenitic stainless steel under cyclic

loading

Paper 4: Demonstrating the effect of precipitation on mechanical stability of austenite in a reversion-

treated 301LN stainless steel

Paper 5: “REVERSED MICROSTRUCTURES AND PROPERTIES AFTER LOW COLD ROLLING

REDUCTIONS IN AISI 301LN STEEL”

Paper 6: “Effect of initial microstructure on mechanical behaviour of cold rolled AISI 301LN”

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