Analysis and Development of Weldability of Novel 700 MPa High Strength Steels
Lahtinen, Teemu Samuel (2016)
Lahtinen, Teemu Samuel
2016
Materiaalitekniikan koulutusohjelma
Teknisten tieteiden tiedekunta - Faculty of Engineering Sciences
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Hyväksymispäivämäärä
2016-12-07
Julkaisun pysyvä osoite on
https://urn.fi/URN:NBN:fi:tty-201611214731
https://urn.fi/URN:NBN:fi:tty-201611214731
Tiivistelmä
The present work analyzes the weldability of commercially available modern high strength steels with the yield strength of about 700 MPa, and explores the development of new steel compositions with enhanced weldability. The weldability is improved by two distinct strategies: 1) development of the material composition for enhancing the mechanical properties of the welded components, and 2) modeling of the welding conditions to evaluate and support the extensive analysis of the mechanical and the metallurgical properties of the welds. The weldability of ten different materials was investigated. Three of the materials, comprised of steel plates produced by industrial-scale processes, two of which were produced by a thermo-mechanically controlled process (TMCP) and one by a quenching and tempering (Q&T) method. The remaining seven materials were laboratory heats produced in prospect of compositional development. Materials were welded by a two-pass MAG process with four different heat inputs. All the welds and respective base materials (BM) underwent tensile tests, Charpy-V impact toughness tests and hardness measurements. One industrial-scale, TMCP steel was selected for more extensive analysis including fatigue tests, bending tests and tensile tests with digital image correlation (DIC) measurement. To assess the critical phenomena at the heat-affected zone (HAZ), a thermal model for each heat input was implemented with Sysweld software and the model was experimentally validated.
The results of the microstructural analysis, tensile tests, DIC measurements and hardness profiles reveal that the most problematic domain in the HAZ, in terms of strength and hardness, is the recrystallized zone (reaching peak temperatures of about 950 °C). The accumulative effect of both welding passes lowers the strength and hardness at the root side making the root side more sensitive to weakening, as shown by hardness profiles and bending tests. In addition, increasing the heat input lowered the fatigue strength of the welds. The welds had 150–300 MPa lower max fatigue strength (R=0.1) than the BM. When compared to the Q&T steel, the properties of the TMCP were less prone to degradation caused by welding. In both steels, a reduction in impact toughness energy was observed in vicinity of the fusion line, emphasizing the need for a solution to account for the low toughness in that region. According to the welding tests of the laboratory heats, by increasing the vanadium and molybdenum contents, the softening in the HAZ can be reduced. These elements reduced the grain growth and recrystallization in the domain of the HAZ most prone to softening. However, by adding these elements, the impact toughness deteriorated due to the formation of hard phases at the coarse-grained sub-zone of the HAZ. Increased manganese content improved the impact toughness in the HAZ.
The results of the microstructural analysis, tensile tests, DIC measurements and hardness profiles reveal that the most problematic domain in the HAZ, in terms of strength and hardness, is the recrystallized zone (reaching peak temperatures of about 950 °C). The accumulative effect of both welding passes lowers the strength and hardness at the root side making the root side more sensitive to weakening, as shown by hardness profiles and bending tests. In addition, increasing the heat input lowered the fatigue strength of the welds. The welds had 150–300 MPa lower max fatigue strength (R=0.1) than the BM. When compared to the Q&T steel, the properties of the TMCP were less prone to degradation caused by welding. In both steels, a reduction in impact toughness energy was observed in vicinity of the fusion line, emphasizing the need for a solution to account for the low toughness in that region. According to the welding tests of the laboratory heats, by increasing the vanadium and molybdenum contents, the softening in the HAZ can be reduced. These elements reduced the grain growth and recrystallization in the domain of the HAZ most prone to softening. However, by adding these elements, the impact toughness deteriorated due to the formation of hard phases at the coarse-grained sub-zone of the HAZ. Increased manganese content improved the impact toughness in the HAZ.