Paulo Roberto Mei

Relationship between thermomechanical treatment, microstructure and α′ martensite in stainless Fe-based shape memory alloys

Abstract This work presents some preliminary results relating training treatment, training temperature and the formation of α′ martensite to the shape recovery effect of stainless shape memory alloys. For the composition tested, the sample shows some mechanical memory (constant tensile stress at 4% strain and constant yield stress throughout the training cycles) with a very good shape recovery (95% after 4% tensile strain) at a training temperature of 873 K. Its residual strain is related to the generation of perfect dislocations only. For the sample trained at 723 K, the residual strain could be attributed to incomplete reversion of stress-induced e martensite, in part due to the blocking effect of α′ martensite and also to the generation of perfect dislocations. The influence of α′ martensite on shape recovery is relative and is dependent on training temperature, and the preferential growth of α′ martensite is shown to occur for large grain size.

EFEITO DA ADIÇÃO DE VANÁDIO NO DESGASTE POR ROLAMENTO COM DESLIZAMENTO EM RODAS FERROVIÁRIAS

The railway industry seeks to increase the volume transported to reduce transport-related costs. Increasing the load ́s volume, increases the stress applied to the wheel / rail system, causing further damage to the components, which increases maintenance costs, whether for repair or replacement. The present work deals with the addition of vanadium to the steel of the railway wheels and tries to evaluate the wear resistance by rolling contact fatigue of the microalloyed steel with vanadium, comparing with the steels normally used in the class railroads wheels of AAR (American Association Railroads). In this way, a disc-on-disk machine was used to simulate wheel-rail contact and to quantify rolling wear with sliding on railway wheels. The load is applied to the disks by a servomotor connected to a spindle and monitored by a load cell. Temperature and speed meters make the acquisition of data in real time, which is stored on a computer. The steel used to perform the tests (disc-on-disc) was 0.7% C steel with (7V) and without (7C) the addition of 0.13% vanadium. The characteristics of the discs tested were analyzed by measures of mass loss, surface roughness and hardness. The integrity of the discs surface and subsurface was analyzed by optical microscopy, scanning electron microscopy (SEM) and optical profilometry. It was verified that the 7V (microalloyed) steel showed lower mass loss and that the presence of debris accelerates the wear, acting mainly on the delamination/shelling mechanism, with formation of lamellar debris. 7C steel without debris showed mainly surface cracks and soft undulations, while 7C steel showed more surface cracks, delamination and undulations. The disc-to-disc machine acted efficiently and effectively on the tested parameters, providing reliability to the results. Key Word: Disc-on-disc. Vanadium. Rolling contact fatigue.

Guide for Recommended Practices to Perform Crack Tip Opening Displacement Tests in High Strength Low Alloy Steels

Fracture mechanics approach is important for all mechanical and civil projects that might involve cracks in metallic materials, and especially for those using welding as a structural joining process. This methodology can enhance not only the design but also the service life of the structures being constructed. This paper includes detailed consideration of several practical issues related to the experimental procedures to assess the fracture toughness in high strength low alloy steels (HSLA) using the crack tip opening displacement (CTOD) parameter, specifically pipeline steels for oil and gas transportation. These considerations are important for engineers who are new in the field, or for those looking for guidelines performing different procedures during the experimentation, which usually are difficult to understand from the conventional standards. We discuss on topics including geometry selection, number of replicate tests, fatigue precracking, test procedure selection and realization, reports of results and other aspects.

Effects of Vanadium on the Continuous Cooling Transformation of 0.7 %C Steel for Railway Wheels

To understand the effect of vanadium on the austenite decomposition of a 0.7 %C steel used in railway wheels the Continuous Cooling Transformation (CCT) diagrams were obtained and the microstructures analyzed with optical, SEM, TEM and XRD techniques. Vanadium refined the austenitic grain (12 and 6 μm for 7C and 7V, respectively), what can be explain by the presence of fine (10 nm in diameter) V4C3 precipitates, which restricts the austenitic grain growth. In addition, vanadium, in solid solution, reduced the pearlite interlamelar spacing (0.13 and 0.11 μm for 7C and 7V, respectively) by depressing the initial temperature pearlite formation (644 and 639 °C for 7C and 7V, respectively). He increased the ferrite volume fraction from 1 to 3 % at cooling rate of 1 oC/s, due the fact that vanadium is a ferrite stabilizer. Vanadium addition did not affect the initial temperature for martensite formation, but increased the hardenability with martensite formation at slower cooling rates (10 and 5 oC/s for 7C and 7V, respectively). For higher cooling rates (20 to 100 oC/s), the austenite transformation to martensite at room temperature was incomplete and all steels presented martensite and retained austenite, which volumetric fraction was near the same for both steels varying from 20 to 40 %.

Friction stir welding of steels for the oil and gas industry

Friction stir welding (FSW) has become a viable and important manufacturing alternative in several industries. This solid-state welding process offers considerable improvements in the mechanical properties of the joint. At the beginning it was developed as a joining alternative for light Al and Mg alloys and it eventually evolved to higher melting temperature alloys, such as steels, stainless steels, titanium alloys and Ni-based alloys. Most of these alloy systems are widely used in the oil and gas industry, where dissimilar joining among them is not uncommon. Conventional arc welding processes and more recently hybrid combinations with laser welding are widespread or under development to be used in these industries. However, metallurgical issues associated with the solidification process and hydrogen embrittlement impair the weldability of most of these high melting temperature alloys. Therefore, FSW is an interesting alternative to overcome some of the challenges associated with the similar and dissimilar joining of structural steels, stainless steels, and Ni-based alloys, especially for circumferential joining of pipelines. Recent developments on weldability studies of pipeline steels seeking the technology deployment will be presented ranging from parameters development, microstructural characterization, and fracture toughness to process robustness evaluation.