Centro Sviluppo Materiali Research Articles

Powders for Additive Manufacturing: From Design to Certification

Rina Consulting Centro Sviluppo Materiali (CSM) has been involved in the study and development of powder metallurgy for different applications, thanks to its participation in many research industrial and funded projects. The entire metal powder production chain takes place within the company's own researcher and facilities. This allows to produce high quality powders starting from alloy design, VIGA atomization and chemical, rheological and particle size analysis. In recent years, the development has mainly concerned manufacturing processes. Currently only a limited number of metal alloys can be processed by AM. For that reason, the alloy design becomes a really important topic to enlarge AM capabilities to other materials and applications. Starting from commercial Thermodynamic and Kinetic codes and proprietary models on solidification and micro-segregation, the alloy chemical composition can be fine-tuned to optimize the microstructure, considering the target properties of the material and the relevant AM processing windows, taking into account also the post process treatment conditions. Moreover, the knowledge of the production plants allows CSM to have a wide vision on the realization and the characterization of the metal powders focusing to achieve the best powder quality suitable for AM applications. Finally, AM is a relatively “new” process, standardization is still an ongoing activity involving several communities and organizations like ASTM, AWS and ISO; in this contest CSM has already designed the guidelines for qualification and certification processes and has created a dedicated laboratory to qualify powders of AM players.

Online control system for EVO 2+4roll bar reducing and sizing mill of Primetals Technologies

The Bar Reducing and Sizing Technology has a core function in the value-chain for the production of rolled Special Bar Quality products. Primetals Technologies has tested industrially the 4roll ultra-sizing concept as a complement to the 2roll reducing and sizing group of stands. As a synthesis of a consolidated know-how, Primetals developed a new mechatronic unit, the EVO 2+4roll mill, based on the compact combination of a 2roll high-reduction module followed by a 4roll low-reduction module for ultra-precise sizing: it is a patented solution of Primetals and a break-through technology for superior metallurgical and technical features of the rolled product, allowing to gain significant technical and economic advantages in the hot- and post-rolling process routes. A control system for this sizing block has been newly designed. RINA Consulting Centro Sviluppo Materiali SpA supported Primetals in the development of algorithms and guidelines for the process control system implementation. The control logic is based on the definition of “sensitivities” or “system derivatives” used, in a feed forward mode, as reference for the online correction and/or regulation of the control parameters, taking into account the small perturbation of the control variables during the rolling, starting from a working point. The conditions, in terms of control variables values, related to the working and “perturbed” points are evaluated using a mill setup model. On the other hand, a feedback module has been implemented, taking into account the online measurements, so calculating the sensitivities on the base of the actual measured quantity.

Blast Furnace Gas Based Combustion Systems in Steel Reheating Furnaces

The usage of steelmaking process gases in thermoprocessing plants is the most efficient way to reduce the natural gas dependence and therefore both fuel costs and carbon footprint in steelworks. Furthermore NOx emissions can be cut by firing lean gases considering their low adiabatic flame temperatures. A European funded project aiming to enhance the usage of blast furnace gas (BFG) in steel reheating furnaces has been successfully accomplished by a multidisciplinary international consortium of research centers (ArcelorMittal Global R&D Asturias and Maizières, Centro Sviluppo Materiali, Swerea MEFOS, VDEh-Betriebsforschungsinstitut), burner suppliers (Tenova, AGA Linde) and end-users (ArcelorMittal). Three innovative preheated fuel gas burner technologies, namely double regenerative air-fuel, oxy-fuel and flat-flame burners, have been designed and manufactured for 100% BFG firing, so that the inherent constraints of burning very lean gases have been overcome. On the one hand, flameless oxy-fuel combustion with central preheated BFG, so that capital expenditures are limited, allowing easily furnaces retrofitting and potentially carbon capture. On the other hand, for air burners a dual honeycomb regenerator that preheats both air and BFG streams at the burner level, so that operating costs are reduced. Finally, an oxy-regenerative flat-flame burner that transfers homogeneously radiant heat to the load, combining the two beforehand mentioned technologies that are used mainly for high capacity burners (higher than 0.5-1 MW). These systems have been tested (long-term) at both pilot plant and industrial scale in order to define the guidelines for a safe application in the industrial environment, to address technical and economic issues and to put forward guidelines for retrofitting existing furnaces. Moreover, computational fluid dynamics (CFD)modelling has been carried out and validated with the pilot testing results, so that a numerical set-up has been defined for BFG firing. For this purpose a new radiation model for the radiative properties of the products of combustion of BFG has been developed, considering the low water vapor - carbon dioxide ratio. By means of preheating BFG with the waste heat content in flue gases stream, the typical operating temperatures of reheating furnaces (1350ºC) have been achieved without natural gas enrichment, keeping the NOx emissions level below the European regulation threshold.

Modelling approach to support design of new high nitrogen austenitic steel for wind turbines components

The alloy design of a new high nitrogen austenitic steel for bearing used in wind turbine gearboxes has been approached. The alloy design was based on a modelling approach to the different aspects of manufacturing routes. The mechanical strength was predicted by means of empirical models. Nitrogen solubility in liquid steel was estimated on the basis of Sievert’s law and Thermo-Calc calculations. Scheil calculations and models developed by Centro Sviluppo Materiali SpA (CSM) were used for estimating nitrogen solubility. The stability of austenite has been studied by thermodynamic calculations. The results show that amounts of at least N = 0.5% are required to fall in austenite domain approaching the required mechanical strength. Vacuum induction melting + pressure electroslag remelting is required to manufacture alloys with N ⩾ 0.6%, but the addition of elements as Mn can potentially allow to increase N amounts achievable with only vacuum induction melting route processing.

Offshore In-Service Resistance of Double Joints Under Strain-Based Conditions

Tenaris, Centro Sviluppo Materiali, and Saipem launched a cooperative program aimed at evaluating the in-service flaw tolerability of girth welded seamless pipes for offshore applications. Full-scale testing was conducted considering severe biaxial loading scenarios replicating actual offshore in-service conditions. Tearing by an occasional high axial strain was taken into account in order to consider suitability for strain-based conditions. Different notch sizes and sampling positions were also taken into account, thus improving the experimental database, which may assist the designer to evaluate the pipeline residual resistance when it is subjected to specific installation and in-service loading conditions.

Experimental Tests and Analytical Models for Welds and Grade 8.8 Bolts under Heating and Subsequent Cooling

The present article describes experimental tests performed on bolts and welds at the Centro Sviluppo Materiali under heating and subsequent cooling. Force-displacement laws, calibrated on these tests, are proposed afterwards for bolts in tension, bolts in shear and butt welds in shear during a natural fire. These laws can be integrated into models representing the global behaviour of steel and composite joints, based on the Component Method.

Mechanical Performance and Stress-Corrosion-Cracking Resistance of Expanded Casing

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 133382, ’Evaluation of Mechanical Performance and Stress- Corrosion-Cracking Resistance of Post Expanded Carbon Steel and CRA Casing Grades,’ by A. Bufalini, SPE, and R. Morana, Centro Sviluppo Materiali; P.I. Nice, H. Nasvik, SPE, and H. Kjorholt, SPE, Statoil; B.M. Bailey, M.K. Adam, SPE, D.G. Jiral, SPE, R.C. Ross, SPE, and R.C. Smith, Baker Hughes; and M. Ueda, and T. Ohe, Sumitomo Metal Industries, originally prepared for the 2010 SPE Annual Technical Conference and Exhibition, Florence, Italy, 19-22 September. The paper has not been peer reviewed. While use of expandable tubular technology has significantly increased within the oil and gas industry in the last few years, materials-behavior knowledge for expandable casing has not progressed sufficiently. This is because of the lack of a definition of the most appropriate procedures to verify post-expansion material performance and because of the difficulties in performing investigations. These investigations rely on post-expanded material, which is costly and time consuming to obtain, especially for the range of very large plastic deformations applicable to monobore applications. Introduction One of the most important factors that prevent the widespread use of expandable casing is related to the only-partial knowledge of the casing-material behavior once it is subject to the large plastic deformation introduced by the expansion process. Despite an increasing number of applications, involving different levels of expansion, there is not a generally accepted view on how the casing material should be evaluated, nor is there a clear knowledge of the performance of post-expanded material from the point of view of mechanical and stress-corrosion-cracking (SCC) resistance. This lack of knowledge presents a large obstacle for applications of successive monobore sections. Not only do these successive monobore applications involve expansion ratios easily exceeding 20% and possibly reaching 30%, they also depend on expandable tubulars for longer duration than the common expandable drilling liners. An extensive testing program was carried out to evaluate the mechanical performance and the SCC resistance of post-expanded material. The assessment of mechanical performance involved two types of activities. Conventional tensile tests were performed as the main way to define the mechanical behavior after the expansion. Tensile tests were carried out before and after expansion in longitudinal and transverse directions. Tests on carbon-steel grades included the evaluation of possible strain-aging effects, while tests on corrosion-resistant alloys (CRAs) also involved the effect of temperature. Impact tests were performed to gain a basic understanding of post-expanded material toughness. Because collapse resistance is the historical weakness for expandables, full-scale collapse tests were performed on expanded tubes. Additional tests provided enhanced knowledge of the material capability to withstand the large plastic deformation required by monobore applications. A novel test was adopted, involving the use of an optical method for strain measurement to determine extended stress/strain relation for the materials. As part of the evaluation process, the effect of the real pipe configuration was considered by means of finite-element analysis (FEA) of the expansion process made with a 3D model of the casing.

Behaviour of Grade 8.8 bolts under natural fire conditions—Tests and model

Recent full-scale experimental tests performed on steel and composite structures have demonstrated that the presence of tensile forces in axially-restrained beams during the cooling phase of a fire could lead to bolt failures. In order to understand this observation and design structures that are not prone to such a failure mode, it is essential to have a deep knowledge of the material behaviour of all the components, including bolts, during both the heating and cooling phase. In the present article, the test set-ups and the results of the tensile and shear tests performed at the Centro Sviluppo Materiali (Italy) on Grade 8.8 bolts under heating–cooling cycles are described. Then, material laws are defined for characterising the mechanical behaviour of Grade 8.8 bolts under heating and cooling phases. These laws account for the non-reversibility of the mechanical properties of Grade 8.8 bolts.

Comparison between Roll Diffusion Bonding and Hot Isostatic Pressing Production Processes of Ti6Al4V-SiC<sub>f</sub> Metal Matrix Composites

Titanium-metal-matrix composites (Ti-MMC) are materials with very large specific resistance and potential operative temperature up to 800° C. At present these composites are produced by Hot Isostatic Pressing (HIP), a reliable but expensive manufacturing method. To cut production costs, Centro Sviluppo Materiali SpA (CSM) has developed and patented an experimental plant for co-rolling at high temperature sheets of titanium alloy and silicon carbide monofilaments fabrics. The experimental Roll Diffusion Bonding (RDB) pilot plant permits a reduction of process costs of about 40% with respect to the HIP process. This work reports the results of microstructural and mechanical examinations carried out on composites realized by RDB and HIP. The comparison shows that the fibre-matrix interface is stable in both the composites while the mechanical properties of RDB composite are better due to its smaller grain size and high dislocation density.

Development of a Low-Cost Process for Manufacturing of Ti-Metal Matrix Composite by Roll-Diffusion Bonding

Composite materials with titanium-alloy matrix are currently the class of material with the highest specific resistance at temperatures up to 800 °C. The main hurdle to their application is their final cost. Even if it is clear that the costs of constituent materials are decreasing due to volume production effects, the production processing costs remain high due to the batch production approach. Centro Sviluppo Materiali’s (CSM) efforts have focused on the manufacturing process in order to obtain an innovative solution to reduce the manufacturing costs with respect to the hot isostatic pressing (HIP) process that represents the standard production process for this class of materials. The new approach can allow a cost reduction of about 40%; this result was obtained by developing an experimental “diffusion bonding” plant for co-rolling at high temperature in a superplastic rolling regime, sheets of titanium alloy and monofilament silicon carbide fabrics. The experimental pilot plant was proposed for patent with RM2006A000261 in May 2006. This paper describes the manufacturing phases and process results. Moreover, has been shown that the diffusion in the solid state was obtained in a process window that was at least 100 times faster than that of HIP. High-temperature tensile tests were carried out on specimens machined from metallic matrix composite materials produced with the roll-diffusion bonding (RDB) process. The samples produced were also submitted to electrochemical dissolution tests of the metallic matrix in order to verify the geometric integrity of the fibers inside the matrix after the bonding phase. The results achieved as well as the process knowledge acquired with the CSM pilot plant are the base for further development of industrial application of the titanium roll-diffusion bonding.

Development of a Low Cost Process for Manufacturing of Ti-MMC by Roll-Diffusion-Bonding

Titanium-alloy matrix composites (TMC) are nowadays one of the material class with the highest specific resistance from room temperature up to 800° C. Centro Sviluppo Materiali SpA (CSM) efforts have been focused on the developing of an innovative solution to reduce the process costs. The new approach consists in an experimental “diffusion bonding” plant for co-rolling at high temperature sheets of titanium alloy and silicon carbide monofilaments fabrics. The result is a process cost reduction of about 40% respect to HIP process. The experimental pilot plant has been proposed for patent with n° 2006A000261 on may 2006. This paper describes the pilot plant and the process results. The metallographic examination on products shows full bonded samples (100 mm wide and 1500 mm long) obtained in a work field that is at least 100 times faster than that of HIP. High temperature tensile tests have been carried on Roll Diffusion Bonded specimens and the results are reported in comparison with those obtained by Isostatic Pressing (HIP) and Thermal- Spraying (TS) processes on the same composite.

Long-Term Heat Treatments on Ti6Al4V-SiC<sub>f</sub> Composite. Part II - Mechanical Characterization

Ti6Al4V-SiCf composite, manufactured by Hot Isostatic Pressing (HIP) at Centro Sviluppo Materiali, has been submitted to long-term heat treatments (up to 1000 hours) at 400 and 600°C. The mechanical properties of the material, in as-fabricated condition and after heat treatments, have been investigated by instrumented indentation (FIMEC), dynamic modulus, tensile and fatigue tests. For comparison some experiments have been carried out also on the monolithic Ti6Al4V alloy. Results show that heat treatments, also the most severe examined here, do not produce remarkable variations of mechanical characteristics. In agreement with the microstructure examinations presented in part I, this behaviour, quite promising for future aeronautical applications, can be primarily ascribed to the stability of fibre-matrix interface.

Grades 92 and 23: Weldability Assessment and Long-Term Performances for Power Generation Applications

Increased efficiency and emission reduction in modern power plants lead to the use of new advanced materials with enhanced creep strength, with the objective to increase the steam parameters of power plants. With over ten years on market and wide experience related to its use, ASTM Grade 92 is becoming one of the most required materials when high service temperatures are reached (max. 610°C). Its composition, with 9%Cr and 1.5%W, gives rise to martensitic microstructures which offer very high creep strength and long term stability. The improved weldability and creep-strength between 500 and 580°C of the low alloy ASTM Grade 23, as well as a cost advantage over higher Cr materials in this temperature range, make it one of the possible candidates to meet the stringent requirements of modern power plants. Air Liquide Welding (ALW) has optimized and distributes a complete product family for the welding of Grades 23 and 92. TenarisDalmine (TD) focused on the development of Grade 23 tubes and pipes and is working on the development of Grade 92. A deep characterization work of the microstructural evolution and long term creep performances of these high temperature resistant materials was thus undertaken by ALW and TD, in collaboration with the Centro Sviluppo Materiali (CSM). The joint characterization program consisted in the assessment of welded joints creep properties. Welded joints were produced using the gas tungsten (GTAW), shielded metal (SMAW) and submerged arc welding (SAW) processes. Mechanical and creep properties of weldments were measured both in the as welded and post weld heat treated conditions and proper WPS’s were designed in a manner such that industrial production needs were satisfied. Short term creep resistance of cross weld specimens was measured to be within the base material acceptance criteria. Long term base material and cross weld creep performance evaluation are now in progress.

Optimisation of geometry of 185 × 185 mm square billet mould of Sidenor continuous casting machine

Industrial trials conducted at Sidenor Basauri Works, and numerical modelling at Centro Sviluppo Materiali (CSM), have been combined in order to find a square 185 × 185 mm billet peripheral geometry that improves the quality of cast products by eliminating the occurrence of off corner cracks and depressions, and reducing the percentage of rejection. The idea is to change the square shape of the mould section to a new profile having a larger corner radius and outward bulging faces. Experimental trials using moulds with convex faces and larger corner radii resulted in an interesting decrease in the percentage of rejected bars. The presence of longitudinal defects has not been completely eliminated, but the adoption of moulds with larger corner radii has been demonstrated to be successful. Numerical modelling has been used to verify that the larger corner radius is the most important variable that guarantees the improved result, while it is less sensitive to different deflections of the faces. This encourages the use of moulds with flat faces, which produce billets that are much easier to handle with conventional tools for their transfer. It can be concluded that by using moulds with a corner radius of 40 mm, instead of conventional moulds with 10 mm corner radius, and flat faces, the risk of longitudinal off corner defects is significantly reduced.

Control of δ-γ Transformation during Solidification of Stainless Steel Slabs in the Mould

In order to obtain satisfactory workability properties required for defect free slab and strip production, the parameters of the casting process, e.g. cooling rate at the initial solidification for the alloy in question must, on the one hand, be carefully adjusted. On the other hand, controlling the characteristics of the solidification structure by chemical composition then takes on particular significance. The main aims of the work were to find out the influence of the phase transformation δ-γ during the initial solidification of different variants of AISI 304 slabs cast in the industrial process on the ferrite distribution on slab surface, and how this relationship could favour the improvements of the surface slab quality. This report contains the joint contributions of the collaborative ECSC project among Centro Sviluppo Materiali (CSM), Krupp Thyssen Nirosta (KTN) and the Department of Ferrous Metallurgy of Rheinisch Westfälische Technische Hochschule Aachen (RWTH).

Fracture propagation control in very high strength gas pipelines

This paper reports on some aspects of a ENI Group feasibility study of high pressure (> 100 bar) - high strength pipelines designed for transportation of large quantities of natural gas over long distances (> 2500 km). The experimental results obtained in collaboration with Centro Sviluppo Materiali (CSM) and ILVA L.T. clearly showed the full reliability of modern high strength linepipe materials of X70-X80 grade, with respect to fracture behaviour, even at low temperature, in very high pressure regimes.