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An Experimental Study to Understand the Localized Corrosion and Environment-Assisted Cracking Behavior of AISI 420-Martensitic Stainless Steel
KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Chemistry, Surface and Corrosion Science.
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Motivation and aim: Currently steel molds are designed with cooling channels to reduce the solidification time of molten plastic within the mold to improve the productivity. As water is generally used as the cooling medium, corrosion and environment-assisted cracking (EAC) leading towards the dysfunction of mold, can increase the production downtime. This was observed in some cases. Hence the primary aim of this thesis is to study the corrosion and EAC behavior of a martensitic stainless steel (MSS) in Cl containing environment to further the current understanding thereby to optimize the existing alloy/s and to design and develop new steel grades.

Methods: The MSS had been austenitised at 1020°C, and subsequently quenched in nitrogen gas at fast (3°C/s), and slow quenching rates (0.6°C/s). Then tempering was done at 250°C, and 500°C, respectively, twice for two hours. Microstructure was predicted and characterized using Thermocalc simulation, dilatometry, light optical microscopy, scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction, atomic force microscopy (AFM). Localized corrosion behavior was characterized using standard salt spray test, electrochemical experiments, scanning Kelvin probe force microscopy, in-situ AFM. Stress relaxation associated with 250°C, and 500°C tempering was characterized by a new method for both fast (FQ) and slow quenched (SQ) conditions. Based on the %stress relaxation, initial loading levels were altered and the corresponding environment-assisted cracking behavior was investigated at two different loading levels.

Results: Samples tempered at 250ºC exhibited higher corrosion resistance than samples tempered at 500ºC in both FQ and SQ conditions. FQ samples exhibited higher corrosion resistance with an ability to passivate than SQ samples when tempered at 250ºC. However, when tempered at 500°C, the corrosion resistance was poor for both FQ and SQ samples. These observed differences clearly indicate the strong influence of microstructure on the corrosion behavior of the material. There are preferential active sites in the microstructure, which dictate the sequence of corrosion events. Secondary Cr-rich carbides formed during 500ºC tempering apparently deteriorate the corrosion resistance in spite of their smaller sizes as compared to undissolved Cr-rich carbides.  Stress relaxation increased with increasing tempering temperature. In the FQ condition, 250°C temper exhibited superior EAC resistance than 500°C temper in both loading scenarios, indicating the dominant role of corrosion resistance in delaying the failure. Whereas in SQ condition, 500°C temper exhibited superior EAC resistance than 250°C temper in both loading scenarios, indicating the dominant role of applied stress in delaying the failure. The pitting susceptibility increased with increasing applied stress on both FQ and SQ conditions. The fractographic features suggest that the mechanism of failure was mixed mode involving both active path dissolution and hydrogen embrittlement, which could have been operative during the failure in varying magnitude in respective scenarios. 

Conclusions: Based on this research work, it can be concluded that, in order to have a longer service life, both the localized corrosion behavior and the residual stresses are to be considered while recommending tempering temperature to mold makers.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2018. , p. 128
Series
TRITA-CBH-FOU ; 2018:56
National Category
Corrosion Engineering
Research subject
Chemistry
Identifiers
URN: urn:nbn:se:kth:diva-239324ISBN: 978-91-7873-007-0 (electronic)OAI: oai:DiVA.org:kth-239324DiVA, id: diva2:1264656
Public defence
2018-12-14, F3, Lindstedtsvågen 26, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20181121

Available from: 2018-11-21 Created: 2018-11-20 Last updated: 2018-11-26Bibliographically approved
List of papers
1. Corrosion Behavior of a Martensitic Stainless Steel AISI 420 Modified From a Mold Size Point of View
Open this publication in new window or tab >>Corrosion Behavior of a Martensitic Stainless Steel AISI 420 Modified From a Mold Size Point of View
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2016 (English)Conference paper, Published paper (Refereed)
National Category
Corrosion Engineering
Identifiers
urn:nbn:se:kth:diva-239318 (URN)
Conference
10th International Tool Conference
Note

QC 20181126

Available from: 2018-11-20 Created: 2018-11-20 Last updated: 2018-11-26Bibliographically approved
2. Correlative Microstructure Analysis and In Situ Corrosion Study of AISI 420 Martensitic Stainless Steel for Plastic Molding Applications
Open this publication in new window or tab >>Correlative Microstructure Analysis and In Situ Corrosion Study of AISI 420 Martensitic Stainless Steel for Plastic Molding Applications
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2017 (English)In: Journal of the Electrochemical Society, ISSN 0013-4651, E-ISSN 1945-7111, Vol. 164, no 4, p. C85-C93Article in journal (Refereed) Published
Abstract [en]

In this work, the corrosion behavior of tempered AISI 420 martensitic stainless steel (MSS) was studied by in-situ atomic force microscopy (AFM) in 0.1M NaCl and correlated with the microstructure. Thermocalc simulation, dilatometry, and X-ray diffraction (XRD) were performed to investigate phase transformation which showed the formation of M3C, M7C3, and M23C6 type of carbides and also retained austenite. Optical microscopy, scanning electron microscopy (SEM), and AFM characterization revealed undissolved carbides and tempering carbides in the martensitic matrix. Volta potential mapping measured by scanning Kelvin probe force microscopy (SKPFM) indicated higher electrochemical (practical) nobility of the carbides with respect to the martensitic matrix whereas regions adjacent to carbides showed lower nobilities due to chromium depletion. Open circuit potential and cyclic potentiodynamic polarization measurements showed metastable corrosion activities associated with a weak passive behavior and a risk for localized corrosion along certain carbide boundaries. In-situ AFM measurements revealed selective dissolution of certain carbide interphases and martensitic inter-lath regions indicating higher propensity to localized corrosion.

Place, publisher, year, edition, pages
Electrochemical Society, 2017
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-208588 (URN)10.1149/2.0531704jes (DOI)000400958600073 ()2-s2.0-85020244401 (Scopus ID)
Note

QC 20170612

Available from: 2017-06-12 Created: 2017-06-12 Last updated: 2018-11-26Bibliographically approved
3. In situ AFM study of localized corrosion processes of tempered AISI 420 martensitic stainless steel: Effect of secondary hardening
Open this publication in new window or tab >>In situ AFM study of localized corrosion processes of tempered AISI 420 martensitic stainless steel: Effect of secondary hardening
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2017 (English)In: Journal of the Electrochemical Society, ISSN 0013-4651, E-ISSN 1945-7111, Vol. 164, no 13, p. C810-C818Article in journal (Refereed) Published
Abstract [en]

The effect of secondary hardening of tempered AISI 420 martensitic stainless steel on the corrosion behavior in aqueous 0.01 M NaCl has been studied, in-situ, using atomic force microscopy (AFM) to monitor real-time localized corrosion processes. Scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy, and X-ray diffraction analyses confirmed the presence of undissolved and secondary carbides (Cr23C6, Cr7C3, Cr3C2, Cr3C, Cr2C, and CrC) as well as retained austenite, all finely dispersed in the tempered martensitic matrix. Electrochemical measurements, consisted of monitoring of the open-circuit potential vs. time and cyclic polarization in 0.01 M NaCl solution, were performed to evaluate the passivity and its breakdown, and it was seen that initiation sites for localized corrosion were predominantly peripheral sites of carbides. In-situ AFM measurements revealed that there was a sequence for localized corrosion in which the neighboring matrix next to secondary carbides dissolved first, followed by corrosive attack on regions adjacent to undissolved carbides. Tempering at 500◦C reduced the corrosion resistance and the ability to passivate in comparison to tempering at 250◦C.

Place, publisher, year, edition, pages
Electrochemical Society, 2017
National Category
Corrosion Engineering
Identifiers
urn:nbn:se:kth:diva-218312 (URN)10.1149/2.1261713jes (DOI)000418409800096 ()2-s2.0-85033663829 (Scopus ID)
Note

QC 20171127

Available from: 2017-11-27 Created: 2017-11-27 Last updated: 2018-11-26Bibliographically approved
4. Experimental and modelling study of the effects of tempering on the susceptibility to environment-assisted cracking of AISI 420 martensitic stainless steel
Open this publication in new window or tab >>Experimental and modelling study of the effects of tempering on the susceptibility to environment-assisted cracking of AISI 420 martensitic stainless steel
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(English)Manuscript (preprint) (Other academic)
National Category
Corrosion Engineering
Identifiers
urn:nbn:se:kth:diva-239345 (URN)
Note

QC 20181126

Available from: 2018-11-21 Created: 2018-11-21 Last updated: 2018-11-26Bibliographically approved
5. Effect of residual stress on environmentally assisted cracking behavior of slow quenched AISI 420martensitic stainless steel tempered at 250°C and 500°C
Open this publication in new window or tab >>Effect of residual stress on environmentally assisted cracking behavior of slow quenched AISI 420martensitic stainless steel tempered at 250°C and 500°C
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(English)Manuscript (preprint) (Other academic)
National Category
Corrosion Engineering
Identifiers
urn:nbn:se:kth:diva-239346 (URN)
Note

QC 20181126

Available from: 2018-11-21 Created: 2018-11-21 Last updated: 2018-11-26Bibliographically approved

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