Effect of Carbon Content on the Processability of Fe-C Alloys Produced by Laser Based Powder Bed Fusion

The present study examines the processability of Fe-C alloys, with carbon contents up to 1.1 wt%, when using laser based powder bed fusion (LB-PBF). Analysis of specimen cross-sections revealed that lack of fusion porosity was prominent in specimens produced at low volumetric energy density (VED), while keyhole porosity was prominent in specimens produced at high VED. The formation of porosity was also influenced by the carbon content, where increasing the carbon content reduced lack of fusion porosity, while simultaneously increasing the susceptibility to form keyhole porosity. These trends were related to an improved wettability, viscosity, and flow of the melt pool as well an increased melt pool depth as the carbon content increased. Cold cracking defects were also observed in Fe-C alloys that had an as-built hardness ≥425 HV. Reducing the carbon content below 0.75 wt% and increasing the VED, which improved the intrinsic heat treatment during LB-PBF, were found to be effective mitigation strategies to avoid cold cracking defects. Based upon these results, a process window for the Fe-C system was established that produces high density (>99.8%), defect-free specimens via LB-PBF without the requirement of build plate preheating.

Comprehensive Analysis of Cold-Cracking Ratio for Flux-Cored Arc Steel Welds Using Y- and y-Grooves

This study investigates various factors that influence the cold-cracking ratio (CCR) of flux-cored arc welds through Y- and y-groove tests. Factors affecting the CCR include the alloy component, diffusible hydrogen content, microstructure, hardness, and groove shape. In weld metals (WMs; WM375-R and WM375-B) of a low-strength grade, the diffusible hydrogen content has a more significant effect on the CCR than the carbon equivalent (Ceq) and microstructure. However, the combined effects of the microstructure and diffusible hydrogen content on the CCR are important in high-strength-grade WM. The CCR of the WM increased upon increasing Ceq and the strength grade because hard martensite and bainite microstructures were formed. Moreover, y-groove testing of the 500 MPa grade WM revealed a more significant CCR than that of the 375 MPa grade WM. Therefore, in high-strength-grade WMs, it is necessary to select the groove shape based on the morphology in the real welds.

Analisis Kegagalan Pembentukan Ellipsoidal Head Pressure Vessel dari Dua Pelat Dilas ASME SA516 Grade 70N dengan menggunakan Metode Fabrikasi Cold Forming

AbstrakHead pada pressure vessel yang berbentuk melengkung, seperti: hemispherical, torispherical, dan ellipsoidal dapat dibuat dari pelat dengan lebar 2.5 m yang mengalami proses metal forming. Namun, pelat yang tersedia di pasaran pada umumnya memiliki lebar 1,6 m. Kondisi ini menjadi batasan apabila ingin menggunakan satu material pelat secara integral sehingga dibutuhkan pelat untuk membuat head dengan lebar yang lebih besar. Oleh karena itu, untuk membuat head dengan lebar 2,5 m dilakukan proses cold forming pada dua pelat yang dilas. Namun setelah proses dilakukan, terjadi kegagalan berupa timbulnya retakan di sekitar area las. Pada paper ini akan dibahas analisis kegagalan proses cold forming yang terjadi pada dua pelat ASME SA516 grade 70N yang digunakan sebagai base metal. Untuk menganalisis penyebab kegagalan, maka dilakukan pengujian kekerasan, tarik, metalografi, dan komposisi kimia. Selain itu juga dilakukan perhitungan untuk mengetahui nilai crack consists of hot (UCS), cold cracking (Pcm), dan carbon equivalent (CE). Hasil perhitungan menunjukkan bahwa material tersebut memiliki nilai UCS di bawah 30, nilai Pcm berada di antara 0,23-0,35%, serta berada di zona II pada diagram Graville dimana nilai tersebut menunjukkan bahwa material memiliki kemampulasan yang baik. Sementara dari hasil pengujian mekanis didapatkan nilai kekerasan dan kekuatan tarik yang lebih besar dari standar, yaitu masing-masing sebesar 300 HBW dan 621 Mpa dengan nilai elongasi yang masih tinggi, yaitu sebesar 21,8%. Hasil pengamatan metalografi menunjukkan terbentuk fase martensit namun dalam jumlah yang sedikit pada area heat affected zone (HAZ) dengan bentuk butir seperti jarum. Fase martensit ini berperan sebagai stress concentration yang menjadi titik awal retak ketika proses cold forming dilakukan. Terbentuknya fasa martensit ini disebabkan oleh proses preheat yang tidak sesuai serta heat input yang terlalu besar. Abstract The head on a pressure vessel with curved shapes such as hemispherical, torispherical, and ellipsoidal is derived from the formed plate. Generally the plates available in the market have a width of 1.6 m, this condition becomes a limitation if you want to use one plate material integrally so that a plate is needed to make a head with a larger width. Therefore, to make a head with a width of 2.5 m, a cold forming process is carried out on two welded plates. However, after the process is carried out, failure occurs in the form of cracks around the weld area. In this paper, we will discuss the failure analysis of the cold forming process that occurred on two ASME SA516 grade 70N plates used as base metal. In order to analyze the causes of failure, hardness, tensile, metallographic, and chemical composition tests were carried out. In addition, calculations were also carried out to determine the value of crack consists of hot (UCS), cold cracking (Pcm), and carbon equivalent (CE). From the calculation results it is evident that the material has a UCS value below 30, the PCm value is between 0.23-0.35%, and is in zone II on the Graville diagram where this value indicates that the material has good weldability. Meanwhile, from the results of mechanical testing, the hardness and tensile strength values are greater than the standard, which are 300 HBW and 621 Mpa, respectively, with a high elongation value, which is 21.8%. The results of metallographic observations showed that the martensite phase was formed but in small amounts in the heat affected zone (HAZ) area with needle-like grain shapes. This martensite phase acts as a stress concentration which is the starting point for cracks when the cold forming process is carried out. The formation of the martensite phase is caused by an inappropriate preheat process and the heat input is too large.

Cold cracking, microstructure, notch toughness, transverse and longitudinal hardness to the weld joint of P460NL1 steel [Fisuracion en frio, microestructura, tenacidad a la entalla, dureza transversal y longitudinal a la union de soldadura del acero P460NL1]

Microstructural development, longitudinal HV10 hardness as transversal to the fusion zone and notch toughness in root and filling pass were evaluated at preheating temperatures of 100 °C, 140 °C, 180 °C, 220 °C and 260 °C in the weld joint of P460NL1 steel, by FCAW in root pass and for filling-finishing by means of the SAW process, the control of the start and interpass preheating temperatures was carried out by thermal blankets and digital pyrometer. The NDT of magnetic particles justify the acceptance or rejection of the specimens for the study. In selected points of the weld joint they showed the dependence of the hardness as a function of the microstructure. At room temperature, immediate cold cracking was observed. In the HAZ, the microconstituents generated areas of high hardness under the cord and areas of low hardness due to overturning, with the presence of Windmastatten ferrite, the increase in preheating determined the decrease and sometimes the absence of hardened structures. The profiles determined a decrease in hardness as the preheating temperature increased. Columnar grains with a high presence of WF ferrite in their different morphologies and FP (C) and refined areas with typical microconstituents were observed in the Fusion Zone. Homogeneity was evidenced with respect to notch toughness at all temperatures and the effect of passes on the longitudinal microstructure, which shows the benefit of the preheating temperature by reducing the hardness in critical areas and homogenizing the welded joint.

Effect of Electrode Waterproof Coating on Quality of Underwater Wet Welded Joints

In this paper, the effects of different hydrophobic coatings on the surface of covered electrodes on the quality of wet welded carbon steel joints were discussed. Commonly available hydrophobic substances used in industrial applications were selected for the research. The aim of using waterproof coatings was to check the possibility to decreasing the susceptibility of high-strength low-alloy S460N steel to cold cracking. During experiments diffusible hydrogen content in deposited metal determination by mercury method, metallographic macro- and microscopic testing and hardness measurements were performed. Investigations showed that waterproof coatings laid on covered electrodes can improve the quality of wet welded joints, by decreasing the Vickers HV10 hardness in heat-affected zone and decreasing the diffusible hydrogen content in deposited metal, which minimalize possibility of cold cracking.

Studies of Kinetic Plasticity Effect in High-Speed Steels under Martensite Transformation

In the paper the authors study how the kinetic plasticity effects the temporary and residual stresses formed in instrumental steels when cooling. They also present the results of temporary stresses relaxation. This phenomenon was applied within the temperature range of the martensite transformation to reduce the cold cracking of the surfaced metal. The paper shows that the superplasticity effect emerging at the moment of martensite transformation plays the crucial role in temporary stresses relaxation.