O desempenho de serras de fita bimetálica depende principalmente das características da junção de suas extremidades, executada pelo processo de soldagem topo-a-topo por centelhamento. Este é um processo de soldagem por resistência, constituído por um aquecimento elétrico, seguido por um rápido forjamento dos materiais. O presente trabalho tem como objetivo, estudar a influência da variação da corrente elétrica, força de recalque e tratamento térmico de revenimento nas microestruturas e propriedades mecânicas das juntas soldadas topo-a-topo por centelhamento. Neste estudo, foram utilizadas amostras de serras de fita bimetálica de 27 mm de largura e 0,9 mm de espessura. O material é um composto bimetálico constituído por aço rápido M42 para os dentes, soldado a um corpo de aço carbono de baixa liga e alta resistência D-6a. As microestruturas e as propriedades mecânicas das amostras soldadas foram investigadas. Foi observado que os parâmetros de soldagem e o revenimento afetam as propriedades mecânicas. A dureza na seção transversal à linha central da solda apresentou valores levemente superiores à respectiva dureza na seção longitudinal. Amostras não revenidas resultaram em maior resistência à tração, associada ao parâmetro de menor temperatura de revenimento. Ficou evidenciado que o revenimento reduziu a dureza e a resistência à tração de todas as amostras. / The performance of bimetal band saw blades depends mainly on the features of joining their ends, carried out by flash butt welding process. This is a resistance welding method, which consists of electrical heating followed by a fast material forging. The present work has the objective of studying the influence of changing the electrical current, upset force and tempering heat treatment on the microstructures and mechanical properties of the flash butt welded joints. In this study, samples of bimetal band saw blades having width of 27 mm and thickness of 0.9 mm were employed. The material consists of a composite M42 high speed steel for the teeth welded together with a D-6a ultra-high strength carbon low alloy steel for the backer. The microstructure and mechanical properties have been investigated. It is pointed out that welding parameters and tempering affected the mechanical properties. The transversal hardness to the welding center line has shown values slightly higher than the respective longitudinal hardness. Non-tempered samples gave the highest tensile strength with the parameter of lowest tempering temperature. It is also found that the tempering reduces the hardness and the tensile strength of all samples.

Sulfide Stress Cracking (SSC) resistance of the newly developed API-X100 High Strength Low Alloy (HSLA) steel was investigated in the NACE TM0177 "A" solution. The NACE TM0177 "A" solution is a hydrogen sulfide (H2S) saturated solution containing 5.0 wt.% sodium chloride (NaC1) and 0.5 wt.% acetic acid (CH3COOH). The aim of this thesis was to study the effect of microstructure, non-metallic inclusions and alloying elements of the X100 on H2S corrosion and SSC susceptibility. The study was conducted by means of electrochemical polarization techniques and constant load (proof ring) testing. Microstructural analysis and electrochemical polarization results for X100were compared with those for X80, an older generation HSLA steel. Uniaxial constant load SSC testing was conducted using X100 samples and the results were compared with those reported for older generation HSLA steels. Addition of H2S to the NACE TM0177 "A" solution increased the corrosion rate of X100from 51.6 to 96.7 mpy. The effect of H2S on the corrosion rate was similar for X80. The corrosion rate for X80 increased from 45.2 to 80.2 mpy when H2S was added to the test solution. Addition of H2S enhanced the anodic kinetics by forming a catalyst (FeHSads) on the metal surface and as a result, shifted the anodic polarization curve to more current densities. Moreover, the cathodic half cell potential increased due to the decrease in pH, from 2.9 to 2.7, which shifted the cathodic polarization curve to more current densities. The increase in both the anodic and cathodic currents, after H2S addition, caused the rise in the corrosion current density. In H2S saturated NACE TM-0177 "A" solution, the X100 steel corrosion rate was higher than the X80 steel by 20%. Longer phase boundaries and larger nonmetallic inclusions in the X100 microstructure generated more areas with dissimilar corrosion potentials and therefore, a stronger driving force for corrosion. Higher density of second phase regions and larger nonmetallic inclusions acted as an increased cathode area on the X100 surface which increased the cathodic current density and consequently, increased the corrosion current density. Proof ring tests on the X100 gave a threshold stress value, C5th, of 46% YS, 343.1 MPa(49.7 ksi). The main failure was caused by SSC cracking. SSC nucleated at corrosion pits on the metal surface and microcracks in the metal body and propagated perpendicular to the applied stress. Hydrogen Induced Cracking (HIC) was observed in the X100. HIC cracks nucleated at banded martensite-ferrite interfaces and propagated along the rolling direction parallel to the applied tensile stress through the softer ferrite phase. When compared to older HSLA grades, the X100 tested in this study had a high SSC susceptibility and therefore, is not be recommended for H2S service applications. The high X100 SSC susceptibility was caused by the material high corrosion rates in H2Smedia which formed corrosion pits that acted as crack initiation sites on the metal surface and provided more hydrogen that migrated into the steel. In addition, the X100 inhomogeneous microstructure provided a high density of hydrogen traps in front of the main crack tip which promoted SSC microcrack formation inside the metal. Microcracks in the metal body connected with the main crack tip that originated from corrosion pits which assisted SSC propagation. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

The object of this thesis is to assess the structural stability of heterogeneous weld joint, which consists of two different base materials (16,537 steel and 16,236 steel) and weld metal NiCrMo2.5-IG with buttering layer P24-IG. All these materials belong to the group of low-alloyed creep-resistant steels that are to be used mainly in the power industry. The weld joint was made by application of the TIG hot wire welding method. Structural stability of this joint was evaluated by carbon redistribution and microstructural changes after annealing in temperature 300 °C and 400 °C during 500 hours. For modelling the phase composition of particular steels was used the ThermoCalc software. All gained results should be employed to appraise whether the heterogeneous weld joint of the steam turbine´s rotor is eligible for long-term operation in hard working conditions.

The microstructure of steel is well known to affect hydrogen-induced cracking (HIC) susceptibility by having certain heterogeneities serving as effective hydrogen trap sites. A consensus on whether or not fine-scale niobium carbide (NbC), nitride (NbN) and carbonitride (Nb(C,N)) precipitates can behave as effective hydrogen traps has yet to be established. The H-trapping capacity of Nb precipitates in a Fe-C-Mn-Nb model steel was investigated with the goal of minimizing embrittlement effects and improving the design of X70 pipeline grade steel for sour service oil and gas applications. First, a heat treatment was applied to the model steel to change the Nb-based precipitate size distribution, which was subsequently characterized via transmission electron microscopy, electron energy loss spectroscopy, and atom probe tomography. The experimental heat treatment increased the number of fine-scale precipitates (<15 nm) that are ideal for APT characterization. NbN and NbC precipitates of various stoichiometries were confirmed within the steel. Further, a custom electrolytic H-charging device was designed, fabricated, and validated using thermal desorption spectroscopy. Additionally, the extent of galvanic corrosion between NbC and NbN and the steel matrix was determined using custom scaled-up particle matrix specimens. Potentiodynamic polarizations conducted using active and passivating electrolytes revealed the relative nobility of the materials. Both NbC and NbN particles were more noble than the steel matrix; thus, possessing driving force for galvanic corrosion, with the particles serving as cathodes. Future studies involving electrolytic charging of the steel in a D-based electrolyte coupled with atom probe tomography will facilitate the direct observation of H-trapping sites relative to various Nb-based precipitates and contribute to an improved understanding of the mechanisms governing HIC. / Thesis / Master of Science in Materials Science and Engineering (MSMSE)

Atlas Copco Secoroc AB are searching after new bulk materials for drill heads that are used in percussive drilling in order to improve their strength and durability. The aim of this project is to assist Atlas Copco in this search and provide them with further information regarding material properties, alloying elements, suppliers, etc. A literary study was carried out in order to identify materials that had UTS and KIC more than or equal to 1700 MPa and 70 MPa*m1/2, respectively. Materials that fulfilled these criteria were T250 grade maraging steel, Cobalt free maraging steel, High cobalt maraging steel, 300 grade maraging steel, AerMet 100, AF1410, S53, M54, 300M, 4340M and PremoMet. These were categorized into maraging steels, high alloy secondary hardened steels, and low alloy steels, and were then further researched.  The material with the highest combination of UTS and KIC was M54 followed by AerMet 100; while AF1410 had the highest KIC but a low UTS, and PremoMet had the highest UTS but a low KIC. Maraging steels and HASH steels have a similar price range, while low alloy steels are much cheaper.

Atlas Copco Secoroc AB are searching after new bulk materials for drill heads that are used in percussive drilling in order to improve their strength and durability. The aim of this project is to assist Atlas Copco in this search and provide them with further information regarding material properties, alloying elements, suppliers, etc. A literary study was carried out in order to identify materials that had UTS and KIC more than or equal to 1700 MPa and 70 MPa*m^1/2, respectively. Materials that fulfilled these criteria were T250 grade maraging steel, Cobalt free maraging steel, High cobalt maraging steel, 300 grade maraging steel, AerMet 100, AF1410, S53, M54, 300M, 4340M and PremoMet. These were categorized into maraging steels, high alloy secondary hardened steels, and low alloy steels, and were then further researched. The material with the highest combination of UTS and KIC was M54 followed by AerMet 100; while AF1410 had the highest KIC but a low UTS, and PremoMet had the highest UTS but a low KIC. Maraging steels and HASH steels have a similar price range, while low alloy steels are much cheaper.

A aplicação de aços de alta resistência está fortemente difundida na indústria mecânica, principalmente em aplicações onde a redução de peso é importante, como por exemplo, a indústria de implementos rodoviários e maquinários agrícolas. Esta dissertação visa analisar a influência de diferentes aportes térmicos no comportamento microestrutural e mecânico de juntas soldadas através do processo MAG de modo a obter juntas com propriedades otimizadas. Para este estudo foram utilizadas chapas metálicas com espessura de 3 mm do aço de alta resistência e baixa liga USI LNE700. Os parâmetros nominais de energia de soldagem foram variados de acordo com os valores descritos pelo fornecedor SSAB. O processo de soldagem foi realizado com o auxílio de um sistema robotizado para manter a homogeneidade ao longo da junta, distância e posicionamento correto no comprimento total da junta soldada. Os ensaios para verificação do comportamento microestrutural e mecânico foram realizados com auxílio de microscopia ótica, perfis de microdureza e ensaios de tração. Buscaram-se utilizar aportes térmicos reduzidos para minimizar os efeitos de revenimento da martensita presente no metal de base, um dos principais responsáveis pela redução das propriedades mecânicas da junta soldada. Os resultados não mostraram significativa variações na microestrutura e propriedades de tração do material, porém o preenchimento da junta soldada formada apresenta um comportamento diretamente proporcional à energia de soldagem. / The application of high strength steels is strongly diffused in the engineering industry, especially in applications where weight reduction is important, such as the industry of agricultural machines and trailers industry. This investigation aims to analyze the influence of different heat inputs on microstructural and mechanical behavior of joint welded by GMAW in order to obtain joints with optimized properties. Sheet metal with a thickness of 3 mm high strength low alloy steel LNE700 (supplier Usiminas) were used. The welding energy was varied around the nominal value informed by the steel supplier SSAB for this study. The welding process was made using robotic system to maintain homogeneity along the joint, right distance and position during the total weld joint length. The microstructural and mechanical behaviors were performed with the optical microscope, microhardness profile and traction test. We attempted to use lower heat inputs to minimize the effects of tempering of martensite present in the base metal, a major contributor to the reduction of the mechanical properties of the welded joint. The results did not show significant microestructural and tensile properties variation, however, when the welding energy is increased, the welded joint penetration formed was increased as well.

Fiber laser welding (FLW) was used to join advanced high strength steel (AHSS) and high strength steel (HSS); specifically two dual-phase (DP) steels, with ultimate tensile strengths above 980 MPa and with different chemistries (DP980 Rich and DP980 Lean), and a high strength low alloy (HSLA) steel, with an ultimate tensile strength of 450MPa (HSLA450). The welding speed and power were varied to develop a process envelope for minimizing weld concavity. In order to attain welds free of weld concavity a balance of speed and power was required; weld concavity could be reduced by lowering power and increasing speed. Welds with amounts of concavity ranging from 15 % to 35 % were characterized with respect to hardness, tensile and fatigue testing. Tensile results revealed that DP steel was sensitive to weld concavity while HSLA450 was not. At stress amplitudes enduring beyond 1000 cycles, welded specimens exhibited lower fatigue resistance compared to the base metal. Concavity reduced the fatigue life of DP980 steels, where increasing the amount of concavity further reduced the fatigue resistance, while the fatigue resistance of HSLA steel welds was not sensitive to weld concavity. Hardness profiling of the welds revealed that HAZ softening was present in the DP980 steel welds. The amount of HAZ softening was normalized; allowing for comparison of different steels. Welds made by FLW demonstrated reduced softening compared other laser welding types because FLW was capable of welding with lower heat input. A difference in the FZ hardness was observed between the DP980 steels because of the difference in carbon content of the steels; where higher carbon content resulted in higher FZ hardness. Additionally the high cooling rate in FLW created higher fusion zone hardness than the values predicted by Yurioka’s model based on arc welding. Examination of the microstructure revealed that the soft zone of DP980 Lean steel possessed severely tempered martensite and untransformed ferrite while DP980 Rich generated a structure with a mixture of tempered martensite, untransformed ferrite and a small fraction of non-tempered martensite. This difference in HAZ softening was attributed to the alloying content of the DP980 Rich steel the higher alloying content of DP980 Rich steel formed a stable austenite that could exist near the Ac1 temperature and enabled the formation of new martensite in the soft zone. The effects of HAZ softening were apparent in tensile testing where the DP980 Lean steel, which exhibited higher softening, demonstrated by a severe reduction in elongation while the DP980 Rich steel, which had higher resistance to softening, attained elongation comparable to its base metal. HSLA450 exhibited a slight reduction in elongation due to the hardening of the fusion zone. The welded DP980 Rich and HSLA450 steels consistently failed within the base metal, while the DP980 Lean steel failed in the soft zone. The welded DP980 Rich steel also demonstrated limiting dome heights comparable to the base metal while the severe softening in the DP980 Lean led to premature fracture in the soft zone, yielding a larger reduction in the limiting dome height.

Fiber laser welding (FLW) was used to join advanced high strength steel (AHSS) and high strength steel (HSS); specifically two dual-phase (DP) steels, with ultimate tensile strengths above 980 MPa and with different chemistries (DP980 Rich and DP980 Lean), and a high strength low alloy (HSLA) steel, with an ultimate tensile strength of 450MPa (HSLA450). The welding speed and power were varied to develop a process envelope for minimizing weld concavity. In order to attain welds free of weld concavity a balance of speed and power was required; weld concavity could be reduced by lowering power and increasing speed. Welds with amounts of concavity ranging from 15 % to 35 % were characterized with respect to hardness, tensile and fatigue testing. Tensile results revealed that DP steel was sensitive to weld concavity while HSLA450 was not. At stress amplitudes enduring beyond 1000 cycles, welded specimens exhibited lower fatigue resistance compared to the base metal. Concavity reduced the fatigue life of DP980 steels, where increasing the amount of concavity further reduced the fatigue resistance, while the fatigue resistance of HSLA steel welds was not sensitive to weld concavity. Hardness profiling of the welds revealed that HAZ softening was present in the DP980 steel welds. The amount of HAZ softening was normalized; allowing for comparison of different steels. Welds made by FLW demonstrated reduced softening compared other laser welding types because FLW was capable of welding with lower heat input. A difference in the FZ hardness was observed between the DP980 steels because of the difference in carbon content of the steels; where higher carbon content resulted in higher FZ hardness. Additionally the high cooling rate in FLW created higher fusion zone hardness than the values predicted by Yurioka’s model based on arc welding. Examination of the microstructure revealed that the soft zone of DP980 Lean steel possessed severely tempered martensite and untransformed ferrite while DP980 Rich generated a structure with a mixture of tempered martensite, untransformed ferrite and a small fraction of non-tempered martensite. This difference in HAZ softening was attributed to the alloying content of the DP980 Rich steel the higher alloying content of DP980 Rich steel formed a stable austenite that could exist near the Ac1 temperature and enabled the formation of new martensite in the soft zone. The effects of HAZ softening were apparent in tensile testing where the DP980 Lean steel, which exhibited higher softening, demonstrated by a severe reduction in elongation while the DP980 Rich steel, which had higher resistance to softening, attained elongation comparable to its base metal. HSLA450 exhibited a slight reduction in elongation due to the hardening of the fusion zone. The welded DP980 Rich and HSLA450 steels consistently failed within the base metal, while the DP980 Lean steel failed in the soft zone. The welded DP980 Rich steel also demonstrated limiting dome heights comparable to the base metal while the severe softening in the DP980 Lean led to premature fracture in the soft zone, yielding a larger reduction in the limiting dome height.

The objective of this thesis is to further understand the austenite Abnormal Grain Growth (AGG) phenomenon in relation with precipitation state in a low alloy steel. The abnormal grain growth is addressed from both experimental and numerical modeling point of view. Prior austenite grain size distribution, precipitation volume fraction and size distribution evolution of the different heat treated states are experimentally determined for two different industrial alloys (steel-A and steel-B) in different heat treated states and experimental results are compared with model predictions. A two-step modeling technique is adopted in this study: precipitation modeling and abnormal/normal grain growth modeling. The abnormal/normal grain growth modeling is done using a simplified analytical model where the grain growth is assumed to be driven by the decrease in interfacial energy. Both the conventional Zener pinning and corner pinning by precipitate is considered as boundary movement retarding forces. The precipitation model is based on the Classical Nucleation and Growth Theories. The assumption of homogeneous precipitate nucleation and growth gave a good prediction of volume fraction, mean radius and size distribution in comparison with the experimental results. Two coupled modeling approaches of abnormal grain growth and precipitation model: ①Soft coupling and ②Dynamic coupling; shed light on the different physical parameters controlling the grain growth condition in a particular material's state. A reasonable prediction of AGG and NGG is obtained from both approaches. The dynamic coupled modeling enabled us to paint a comprehensive time-temperature mechanism map of grain growth conditions. It is found that AGG in the austenitic state depends strongly on the initial grain size distribution and precipitation state. The modeling and the experimental results showed that the precipitation state evolution (increasing or decreasing volume fraction) also impact normal/abnormal grain growth. Plausible explanations in relation with the mean austenite grain size and the precipitation state are derived for the AGG phenomenon from the present work.