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Effects of manufacturing chain on mechanical performance: Study on heat treatment of powertrain components
KTH, School of Industrial Engineering and Management (ITM), Production Engineering. KTH, Centres, XPRES, Excellence in production research. Scania CV, Stockholm, Sweden.
2015 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

The increasing demands for lightweight designs with high strength call for improved manufacturing processes regarding heat treatment of steel. The manufacturing process has considerable potential to improve the mechanical performance and to obtain more reliable results with less variation.

The goal of this thesis is to establish new knowledge regarding improved manufacturing processes in industrial heat treatment applications. Three research questions with associated hypotheses are formulated. Process experiments, evaluation of the mechanical performance, and modelling of the fatigue behaviour assist in answering the questions.

The gas quenching procedure following low-pressure carburising differs from the conventional procedure of gas carburising and oil quenching. It is shown that the introduction of a holding time during the low-temperature part of the quench has a positive effect on mechanical properties, with some 20 percent increase in fatigue strength. This is attributed to increased compressive surface residual stress and stabilisation of austenite.

Tempering is a common manufacturing process step following hardening in order to increase the toughness of the steel. However, the research shows that the higher hardness from eliminating tempering from the manufacturing process is beneficial for contact fatigue resistance. The untempered steel showed not only less contact fatigue damage but also a different contact fatigue mechanism.

Straightening of elongated components is made after heat treatment in order to compensate for distortions. The research shows that straightening of induction hardened shafts may lead to lowering of the fatigue strength of up to 20 percent. A fracture mechanics based model is developed to estimate the effects of straightening on fatigue strength.

Abstract [sv]

Ökande krav på höghållfasta lättviktskonstruktioner kräver förbättrade tillverkningsprocesser för värmebehandling av stål. Det finns stor potential att förbättra mekanisk prestanda och att erhålla mer tillförlitliga resultat med mindre variation genom att förbättra tillverkningsprocessen.

Målet med denna avhandling är att etablera ny kunskap kring tillverkningsprocesser inom industriella värmebehandlingsapplikationer. Tre forskningsfrågor med tillhörande hypoteser formuleras. Processexperiment, utvärdering av mekanisk hållfasthet och modellering av utmattningsbeteende bygger upp besvarandet av frågorna.

Gaskylning som följer lågtrycksuppkolning skiljer sig från det konventionella förfarandet med gasuppkolning och släckning i olja. Resultaten visar att en hålltid i den nedre delen av kylningsförloppet har positiv inverkan på utmattningshållfastheten. Orsaken till förbättringen hänförs till ökade tryckrestspänningar samt stabilisering av austenit.

Anlöpning är en vanlig tillverkningsprocess som efterföljer härdning för att öka stålets seghet. Forskningen visar däremot att den högre hårdheten för oanlöpt stål är fördelaktig för motstånd mot kontaktutmattning. Oanlöpt stål visade mindre mängd kontaktutmattningsskador och även en annan skademekanism.

Riktning av långa komponenter görs efter värmebehandling för att kompensera för de formförändringar som uppstår. Forskningen visar att riktning av induktionshärdade axlar kan leda till sänkning av utmattningshållfastheten med upp till 20 procent. En brottmekanisk modell som uppskattar effekten av riktning på utmattningshållfasthet presenteras.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2015. , 59 p.
Series
TRITA-IIP, ISSN 1650-1888 ; 15-01
Keyword [en]
Heat treatment, case hardening, carburising, induction hardening, gas quenching, tempering, straightening, fatigue strength, contact fatigue, fracture mechanics
National Category
Production Engineering, Human Work Science and Ergonomics
Research subject
Industrial Engineering and Management; Production Engineering; Materials Science and Engineering
Identifiers
URN: urn:nbn:se:kth:diva-163665ISBN: 978-91-7595-501-8OAI: oai:DiVA.org:kth-163665DiVA: diva2:801700
Presentation
2015-05-13, Sal M311, Brinellvägen 68, KTH, Stockholm, 14:00 (English)
Opponent
Supervisors
Funder
XPRES - Initiative for excellence in production researchVINNOVA, MERAVBCKnowledge Foundation
Note

QC 20150410

Available from: 2015-04-13 Created: 2015-04-10 Last updated: 2015-04-13Bibliographically approved
List of papers
1. Gas Quench Rate after Low Pressure Carburizing and its Influence on Fatigue Properties of Gears
Open this publication in new window or tab >>Gas Quench Rate after Low Pressure Carburizing and its Influence on Fatigue Properties of Gears
2013 (English)In: HTM - Journal of Heat Treatment and Materials, ISSN 1867-2493, Vol. 68, no 6, 239-245 p.Article in journal (Refereed) Published
Abstract [en]

Process modifications of the gas quench sequence for low pressure carburized gears can increase the performance of up to 22 %, compared to direct gas quenching. Several test series were made with different interruptions of the gas quenching sequence, near the martensite start temperature Ms. The quench interruption resulted in an increase in magnitude of compressive residual stress which was attributed to temperature homogenization and rearrangement of local stresses. The increased fatigue strength was a result of the combination of enhancement of the compressive residual stress state, and of mechanical stabilization of austenite.

Place, publisher, year, edition, pages
Carl Hanser Verlag GmbH, 2013
Keyword
Steels, martensite, hardening, low pressure carburizing, gas cooling, fatigue strength, fracture surface, residual stress, dilatometer
National Category
Production Engineering, Human Work Science and Ergonomics
Research subject
SRA - Production
Identifiers
urn:nbn:se:kth:diva-137060 (URN)10.3139/105.110203 (DOI)2-s2.0-84893858890 (ScopusID)
Funder
XPRES - Initiative for excellence in production research
Note

QC 20140211

Available from: 2013-12-10 Created: 2013-12-10 Last updated: 2016-04-18Bibliographically approved
2. Influence of tempering on contact fatigue
Open this publication in new window or tab >>Influence of tempering on contact fatigue
Show others...
2011 (English)In: International Journal of Microstructure and Materials Properties, ISSN 1741-8410, E-ISSN 1741-8429, Vol. 6, no 6, 465-478 p.Article in journal (Refereed) Published
Abstract [en]

Most components are tempered after heat treatment operations such as case hardening or induction hardening. The common opinion is that the martensitic structure after heat treatment is too brittle and tempering is necessary to increase toughness.

Tempering is an additional operation which leads to increased costs by energy, handling, and investments. Eliminating tempering from the heat treatment process leads to increased productivity, energy savings, and lowered environmental impact.

Two carburised steels, Ovako 253A (?EN 22NiCrMo12-5F mod. A) and EN 20NiCrMo2 (SAE 8620, SS2506), were tested for contact fatigue resistance in a roller to roller rig. The tested samples were characterised with respect to amount of fatigue damage, residual stress, amount of retained austenite and hardness. The objective was to determine if tempering is always necessary after a heat treatment operation.

The contact fatigue tests show that tempering results in lower contact fatigue resistance. Further, fatigue cracks were found to have initiated in different ways between tempered and untempered steel.

Keyword
tempering, carburising, case hardening, rolling contact fatigue, EN 20NiCrMo2, Ovako 253, heat treatment, productivity, energy savings, environmental impact, carburised steels, contact fatigue resistance, retained austenite, hardness
National Category
Metallurgy and Metallic Materials Tribology
Research subject
SRA - Production
Identifiers
urn:nbn:se:kth:diva-47315 (URN)10.1504/IJMMP.2011.044365 (DOI)2-s2.0-84857243360 (ScopusID)
Funder
XPRES - Initiative for excellence in production research
Note

QC 20111109

Available from: 2011-11-08 Created: 2011-11-08 Last updated: 2015-06-24Bibliographically approved
3. Straightening of induction hardened shafts: influence on fatigue strength and residual stress
Open this publication in new window or tab >>Straightening of induction hardened shafts: influence on fatigue strength and residual stress
2012 (English)In: HTM Journal of Heat Treatment and Materials, ISSN 1867-2493, Vol. 67, no 3, 179-187 p.Article in journal (Refereed) Published
Abstract [en]

Straightening of distorted components after heat treatment is often a necessary operation. The straightening operation leads to local plasticization, which is affecting the residual stress state, the hardness, and ultimately the fatigue strength of the component. The present study evaluates the influence of a straightening operation on fatigue strength and on the residual stress state of induction hardened shafts of steel 42CrMo4. A simplified FEM model was formulated. The model showed that the residual stress state was asymmetric along the circumference of a straightened shaft. Fatigue testing was performed in three point bending and showed that the fatigue strength was reduced by up to some 20 % by heavy straightening. A fracture mechanics model for fatigue crack growth and arrest was developed. The model could be used to predict the fatigue strength of a straightened shaft provided that the residual stress state was known.

Keyword
Straightening, induction hardening, fatigue strength, residual stress, 42CrMo4, fracture mechanics, fatigue crack growth and arrest
National Category
Metallurgy and Metallic Materials Tribology Production Engineering, Human Work Science and Ergonomics
Research subject
SRA - Production
Identifiers
urn:nbn:se:kth:diva-161228 (URN)10.3139/105.110149 (DOI)2-s2.0-84864712982 (ScopusID)
Funder
XPRES - Initiative for excellence in production research
Note

QC 20150317

Available from: 2015-03-11 Created: 2015-03-11 Last updated: 2015-04-13Bibliographically approved

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