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Improvement of mechanical properties of laser-welded Trip 700
2002 (English)Independent thesis Advanced level (professional degree), 20 credits / 30 HE creditsStudent thesis
Abstract [en]

High-strength steels combine strength and ductility, and are therefore of great interest for the automotive industry. Recent investigations of welded Trip 700 (steel with strength level 700 MPa) showed a wide spread of the mechanical properties due to brittle cleavage fracture behaviour in peeling test. In this study steel sheets of Trip 700 have been laser welded together in an edge weld. The heat-influence and the fast cooling of laser welding cause a hard martensitic weld. The influence of the weld metal’s hardness on the fracture behaviour in peel loading has been investigated with tensile testing and SEM. First heat-treatments were carried out to reduce the hardness of the weld metal. Their influence on hardness, microstructure and mechanical properties has been analysed. Hardness has a significant influence on the risk for brittle behaviour. A fully ductile behaviour was found for a sample heat- treated in a furnace at 500ºC for 30 minutes. The hardness after this heat- treatment was 300 HV. The calculation program mucg73 was used to calculate TTT-diagram for Trip 700. A cooling curve describing the cooling behaviour after welding was calculated with SYSWELD and superimposed on the TTT-diagram. Thus the existing phases after welding could be predicted. Secondly, the weld metals’ hardness was modified in three different ways. • The laser welding was performed with a lower speed to get a slower cooling of the weld and hence more ferrite, which is softer than martensite, should form according to the calculations of the superimposed cooling curve on the TTT-diagram. • The laser welding was performed with a lower speed and more filler material was added to the weld to decrease the carbon content of the weld metal and hence obtain a softer weld metal. • Re-heating with TIG was performed to obtain an annealed martensite. It was found that method 1 and 2 did not result in a significant decrease of hardness. A sufficient amount of ferrite was not obtained with method 1. The TIG re-heating was the most efficient method to decrease the hardness of the weld and hence improve the steel’s mechanical properties. It is proposed for future work to find a way to realise re-heating with laser.

Place, publisher, year, edition, pages
2002.
Keyword [en]
Technology, materialteknik, Trip-steels, high-strength steels, laser, welding, SYSWELD, mucg73, heat-treatments
Keyword [sv]
Teknik
Identifiers
URN: urn:nbn:se:ltu:diva-56687ISRN: LTU-EX--02/252--SELocal ID: d71164fb-961c-438b-86fb-340187a38a40OAI: oai:DiVA.org:ltu-56687DiVA: diva2:1030074
Subject / course
Student thesis, at least 30 credits
Educational program
Materials Engineering, master's level
Examiners
Note
Validerat; 20101217 (root)Available from: 2016-10-04 Created: 2016-10-04Bibliographically approved

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